This PDF file contains the front matter associated with SPIE Proceedings Volume 10541, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Plants are remarkable optofluidic devices that function almost exclusively through the interaction of two fluids (water and air) and optics (sunlight) [1]. We will describe some of the optofluidic mechanisms that have evolved in plants and discuss implications for optofluidic devices for solar harvesting. [1] Optofluidics of plants. Demetri Psaltis, Andreas Vasdekis and Jae-Woo Choi, APL Photonics 1, 020901 (2016)

Atomically thin layered semiconductors reveal unique properties such as strong excitonic emission and valley degree of freedom, which are expected for exotic physics and valleytronics applications at 2D limit. To achieve 2D valleytronics and enable new functionality, efforts from both property understanding and device engineering are in need. In this talk, I will present recent progress from my group aiming for realization of 2D valleytronics. To begin with, I will discuss our work on valley selection rules for nonlinear optical process in monolayer WS21. The finding of such nonlinear optical selection in a 2D valleytronic system is crucial for potential valley optoelectronic device applications such as 2D valley-polarized THz sources and coherent control for quantum computing. I will then focus on device engineering to achieve first electrical generation and control of valley polarization in 2D materials2. With unique spin–valley locking property in monolayer WS2, valley carrier injection was achieved via a diluted ferromagnetic semiconductor with efficiency up to 45%. In the last, our discovery of new functional 2D materials will be overviewed3,4, which may not only boost the development of valleytronics but inspire new interdisciplinary topics. 1. Xiao, J. et al. Nonlinear optical selection rule based on valley-exciton locking in monolayer ws2. Light Sci. Appl. 4, e366 (2015). 2. Ye, Y. et al. Electrical generation and control of the valley carriers in a monolayer transition metal dichalcogenide. Nat. Nanotechnol. 11, 598–602 (2016). 3. Lu, A.-Y. et al. Janus monolayers of transition metal dichalcogenides. Nat. Nanotechnol. 12, 744–749 (2017). 4. Gong, C. et al. Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals. Nature 546, 265–269 (2017).

In 1929, von Neumann and Wigner showed that Schrödinger’s equation can have, somewhat surprisingly, bound states above the continuum threshold [1]. These bound states represent the limiting case of quasi-bound states with an infinite lifetime, i.e., resonances that do not decay. It was recently realized that bound states in the continuum (BICs) are intrinsically a wave phenomenon and are thus not restricted to quantum mechanics. Since then, they have been shown to occur in many different fields of wave physics such as acoustics and photonics. In photonics’ terminology, BICs are eigenmodes of an open system with an infinite radiation quality factor, Qrad. To take advantage of this unique property to design high quality resonant cavities, most investigations have focused on dielectric structures that, unlike their plasmonic counterparts, are not limited by their material quality factor, Qmat [3-5]. To investigate the properties of BICs, various platforms have been used such as 1D gratings [3], waveguide arrays [4], and 2D photonic crystal slabs [5]. In this contribution, we have designed a high quality cavity based on a BIC and harnessed its novel properties to achieve a compact low-threshold nanophotonic laser. [1] J. von Neumann and E. Wigner, “On some peculiar discrete eigenvalues” Phys. Z, 465 (1929). [2] C. Linton et al., “Embedded trapped modes in water waves and acoustics” Wave Motion 45, 16 (2007). [3] D. C. Marinica et al., “Bound states in the continuum in photonics” Phys. Rev. Lett. 100, 183902 (2008). [4] Y. Plotnik et al., “Experimental observation of optical bound states in the continuum” Phys. Rev. Lett. 107, 183901 (2011). [5] C. W. Hsu et al., “Observation of trapped light within the radiation continuum” Nature 499, 188 (2013).

In a waveguide-QED system, under certain condition, the spontaneous emission rate of an atom cloud with a single excitation can be enhanced. Single-photon superradiance refers to the case when the enhancement attains its maximum. We show that an atom cloud exhibiting single-photon superradiance can be described by an effective two-level system. We also adopt a real space numerical approach to validate the understanding of single-photon superradiance using such an effective mapping picture. We further numerically investigate the spontaneous emission of superradiant state and dark state.

OLED lightings are getting more attention from industry since characteristic of surface lighting gives design flexibility and energy saving. Transparent conductive electrodes (TCEs) and internal out-coupling layer are essential components to determine OLED performance. Typical out-coupling layer is combination of scattering structure and planarizing layer. ~m planarizing layer smooths rough topology causing from the scattering structure so that sufficiently flat TCE can be coated on it. However undesirable gap between the TCE and the scattering structure occurs inevitably by the insertion of ~m planarizing layer. This leads to diminish out-coupling effect because the scattering structure is located optically too far from TCE to reduce wave guiding loss around TCE. Thus, it is important to bring the scattering structure close to TCE so as to reduce the wave guiding loss around TCE. Here, we present an innovative manufacturing process named “Reverse Layered Transfer process” that makes TCE directly contact with scattering structure, resulting in high out-coupling efficacy. The flexible OLED applied with Reverse Layered Transfer showed 1.8 times higher luminous efficacy than a rigid OLED based on normal process using ref ITO TCE. TCE integrated with out-coupling layer using “Reverse Layered Transfer process” and its device property will be described, together with experimental results and theoretical explanation.

Miniaturized optical systems with planar form factors and low power consumption have many applications in wearable and mobile electronics, health monitoring devices, and as integral parts of medical and industrial equipment. Flat optical devices based on dielectric metasurfaces introduce a new approach for realization of such systems at low cost using conventional nanofabrication techniques. In this talk, I will present a summary of our recent work on dielectric metasurfaces that enable precise control of both polarization and phase with large transmission and high spatial resolution. Optical metasurface components such as high numerical aperture lenses, efficient wave plates, components with novel functionalities, and their potential applications will be discussed. I will also present the results of our efforts on optimizing and increasing the diffraction efficiency of metasurfaces. Furthermore, by using metasurface cameras and planar retroreflectors as examples, I will discuss a vertical on-chip integration platform that introduces a new architecture for the on-chip integration of conventional and novel optical systems and enables their low-cost manufacturing.

We designed semiconductor metasurfaces comprised of rectangular arrays of all-Si antennas exhibiting sharp transmission dips. The sizes and periodicities of the antennas were shown to control the spectral positions and linewidths of the corresponding antenna resonances. The designed resonant metasurfaces were fabricated on a 600-nm-thick silicon-on-sapphire substrate using electron beam lithography, hard mask deposition, and inductively coupled HBr plasma etching. Multiple areas with different antenna sizes were defined on the same substrate. The samples were characterized with a FTIR system under normal incidence and paraxial beam configuration. The spectra revealed transmission dips in the spectral range of 3–4 μm, with the central wavelengths corresponding to the local simulated field enhancements of Eloc = 10–15. The metasurfaces were irradiated by a train of femtosecond laser pulses from a supercontinuum-based mid-IR optical parametric amplifier (OPA) pumped by a Ti:Sapphire amplifier. The incoming 200 fs mid-IR pulses, centered at 3.6 μm, had a maximum fluence of 60 mJ/cm2. The transmitted third harmonic radiation was refocused on and detected by an InGaAs detector based short wave-IR spectrometer. For pump–probe experiments, the residual near-IR 782 nm beam was split off from the output of the OPA, sent through a delay line, and focused at the sample collinearly to the mid-IR beam. Using a home-built calibrated mid-IR spectrometer based on a blazed diffraction grating and a mid-IR camera (Electrophysics PV320), we observed a pump-dependent blue-shifting and eventual disappearance of the resonant dips, accompanied by the reduction of the third harmonic generation.

In the last several years, metasurfaces have demonstrated promise as both flat optical elements to replace conventional three-dimensional components (prisms or lenses) as well as to access functions that are unachievable in conventional optics. To date, the functional performance of metasurfaces have typically been encoded at the time of fabrication, which fixes the achievable phase and amplitude for each elements in an array. However if actively controlled metasurface elements can be designed to dynamically control the phase shift and amplitude change imposed by each metasurface element, we could realize phased arrays to enable complex spatio-temporal wavefront engineering. We report here design and experimental demonstration of a tunable conducting oxide metasurface that achieves such active control by incorporating materials with voltage-tunable optical permivitties, such as indium tin oxide (ITO), into a metasurface [1]. We design a metasurface that consists of an aluminum back plane, HfO2 gate dielectric followed by a 14 nm thick ITO active layer, and a periodic array of aluminum patch antennas. We choose the dimensions of the Al antennas so that the antenna magnetic dipole resonance occurs at 1550 nm. By applying a gate bias between the Al antenna and ITO active layer, charge accumulation or depletion occurs at the ITO/HfO2 interface. This results in modulation of the ITO complex permittivity, thus altering the metasurface reflection phase and amplitude. The designed metasurface is capable of >270° phase shift. Our design enables independent control of each metasurface element enabling electrical control of the metasurface phase profile, which is an essential requirement for demonstration of continious beam steering. [1] Y.-W. Huang et al., “Gate-Tunable Conducting Oxide Metasurfaces”, Nano Letters 16, 5319-5325 (2016).

Engineering the wavefront of light in random media allows the control of wave propagation in space and time by exploiting the spatial and spectral degrees of freedom introduced by multiple scattering (M. Mounaix et al, Phys. Rev. Lett. 116, 253901 (2016)). To apply this far-field control strategy and focus electromagnetic energy at the nanoscale, it is necessary to introduce scatterers that feature strongly enhanced and confined optical fields such as plasmonic nanoantennas. In particular, semi-continuous gold films close to the percolation threshold feature high local field enhancements (S. Gresillon et al, Phys. Rev. Lett. 82, 4520 (1999)) but also propagating surface plasmon waves that can be controlled using a spatial light modulator (P. Bondareff et al, ACS Photonics 2, 1658 (2015)). In this presentation, we demonstrate how controlling the phase of an incoming pulsed laser on a chosen 10 µm x 10 µm area of a random plasmonic metasurface allows us to optimize the two-photon luminescence (TPL) of gold at a given position of the sample. The optimized TPL intensities, that are associated with strong local field enhancements, are increased by a factor of 50 for semi-continuous films that are close to percolation compared to samples far from it, demonstrating that the morphology and randomness of the plasmonic film play an essential role in the control of nonlinear luminescence. Furthermore, we show that TPL intensities can be enhanced at any position of a percolated film, opening exciting perspectives for the wavefront engineering of local field enhancements in random plasmonic metasurfaces.

Phase change media utilize a remarkable property portfolio including the ability to rapidly switch between the amorphous and the crystalline state, which differ significantly in their properties. This property combination makes them very attractive for photonic applications ranging from data storage to fast optical switches, employing the pronounced difference of optical properties between the amorphous and crystalline state. This talk will discuss the origin of the unique material properties, which characterize phase change materials. In particular, it will be shown that only a rather small group of materials utilizes ‘metavalent bonding’, a novel, yet fundamental bonding mechanism, which can explain many of the characteristic features of phase change materials. This insight is employed to predict systematic property trends and to explore the limits in stoichiometry for photonic applications of this material class. It will be demonstrated how this concept can be used to tailor fast optical switches. Yet, the discoveries presented here also force us to revisit the concept of chemical bonding and bring back a history of vivid scientific disputes about ‘the nature of the chemical bond’.

Optical phase change materials (O-PCMs) are a unique class of materials which exhibit extraordinarily large optical property change (e.g. refractive index change > 1) when undergoing a solid-state phase transition. These materials, exemplified by Mott insulators such as VO2 and chalcogenide compounds, have been exploited for a plethora of emerging applications including optical switching, photonic memories, reconfigurable metasurfaces, and non-volatile display. These traditional phase change materials, however, generally suffer from large optical losses even in their dielectric states, which fundamentally limits the performance of optical devices based on traditional O-PCMs. In this talk, we will discuss our progress in developing O-PCMs with unprecedented broadband low optical loss and their applications in novel photonic systems, such as high-contrast switches and routers towards a reconfigurable optical chip.

We demonstrate a highly-integrated, subwavelength, and reconfigurable nanoscale spatial light modulator capable of modulating the amplitude, phase, and polarization of impinging light both spatially and spectrally. These properties are enabled by integration of plasmonic metasurfaces with phase-change materials. Owing to the ultrafast switching speed, considerable scalability, high switching robustness, good thermal stability, adaptability with complementary metal oxide semiconductor (CMOS) technology, and large refractive index change contrast between its amorphous and crystalline phases, germanium antimony telluride (GST), a well-known PCM, is used for these miniaturized dynamic metadevices. To show the unprecedented capability of this hybrid plasmonic-PCM material platform for practical applications, we investigate a plasmonic-GST gradient metasurface comprising of a patterned array of gold nanostrips that is separated from an underlying reflecting gold plate by a thin layer of GST. While the plasmonic inclusions support enhanced short-range surface plasmons, which are highly coupled to both the electric and magnetic components of the incident optical field, the real-time structural transition between the states of GST constituent, upon excitation with an external electric stimulus, provides a remarkable refractive index contrast for reconfiguration. Such dynamic metasurfaces could lead to new avenues for realization of reconfigurable, fast, and energy-efficient miniaturized photonic components such as multifocus lenses, all-optical switches, vortex beam generator, and grayscale holograms in a reversible and nonvolatile fashion.

Interest in negative refraction has been motivated by the possibility of creating a “superlens” as proposed by Pendry (Phys. Rev. Lett. 85, 3966 (2000)). This theoretical work showed that a material capable of negative refraction amplifies evanescent waves and allows this material to act as a lens with a resolution not limited by working wavelength. Although theory and some experiments have shown that certain metamaterials and photonic crystals (PhCs) can act as superlenses, realistic demonstration of negative refraction in the optical and infrared range remains a challenge. This is because most metamaterials employ lossy metal elements and most PhC structures found to exhibit negative refraction are made of positive index dielectric materials and are two-dimensional. Subwavelength imaging of a 3D object requires a 3D PhC capable of negative refraction. Inspired by the numerical simulations of Luo, et. al. (Appl. Phys. Lett. 81, 2352 (2002)), we demonstrate the fabrication and characterization of a 500nm-diameter polymer core, 250nm-thick Germanium shell 3D photonic crystal lattice that exhibits negative refraction in the mid-infrared, centered around 8µm. This 3D photonic crystal resembles a BCC lattice of air cubes in dielectric media and was fabricated using two-photon lithography direct laser writing of an acrylic polymer resin scaffold followed by RF sputtering of Ge. The band structure of the lattice was mapped using FTIR spectroscopy reflectance measurements, and negative refraction was observed using far-field IR transmission imaging.

The central correlations between the geometrical and topological characteristics of structured photonic materials and the photonic functionality they enable is of fundamental importance. Here, we introduce a new metric, local self-uniformity (LSU) as a measure of the structural order of photonic network structures. The LSU characterizes the intimate connection between uniformity of the local environments and the overall photonic band gap properties and provides a new design strategy for non-periodic materials. LSU can be employed to rank photonic networks with the SRS-gyroid and diamond networks reaching a maximal unity value of LSU. We then explore the connection between the LSU concept and the photonic band gap formation and introduce a novel architecture, the amorphous gyroid network or triamond. Moreover, we demonstrate all architectures displaying large photonic band gaps, be they periodic or disordered, are characterized by large values of the LSU metric. We also show that LSU is significant metric beyond the formation of photonic band gaps and apply it to characterise the wing-scale structuring in the butterfly Pseudolycaena marsyas. We fabricate the first prototypes of amorphous gyroid at a centimetre length scale in high index alumina ceramic. To achieve this, we employ a novel lithography-based ceramic manufacturing (LCM) process which can and achieve sub-millimetre feature resolution with a minimum of post-processing steps. Microwave transmission measurements are in good agreement with FDTD simulations and confirm the existence of large and robust band gaps.

We demonstrate a non-mechanical on-chip optical beam steering device using the photonic crystal waveguide with a double periodic structure that repeats the increase and decrease of hole diameter. Guided slow light in this waveguide is radiated to be a light beam. Slow light shows strong dispersion, which allows a deflection angle of approximately 10 times that of a normal diffraction grating. We fabricated the device using complementary metal oxide semiconductor process and observed a beam deflection angle of 24° in the longitudinal direction with maintaining the divergence angle of 0.3° when the wavelength was changed by 27 nm. Four such waveguides were integrated, and one of them was selected by a Mach-Zehnder optical switch. Then, the lateral beam steering was obtained when a cylindrical lens was placed above these waveguides. By combining the longitudinal and lateral beam steering, the collimated beam was scanned two-dimensionally with 80 × 4 resolution points.

The ability to manipulate light-matter interactions using complex, aperiodic electromagnetic media is at the heart of current nanoplasmonics and metamaterials technologies. Efficient approaches for multiscale electromagnetic field enhancement, concentration and manipulation of fields with designed spatial-frequency spectra in complex media enable the control of propagating and non-propagating electromagnetic modes in optical nanostructures with broadband/multi-band enhanced responses. Besides its fundamental interest, photonic-plasmonic coupling in complex aperiodic environments is also of great importance for a number of device applications such as nano-antennas, ultrafast optical switchers, nanoscale energy concentrators, laser nano-cavities, and optical biochemical sensors. In this talk, I will discuss our work on the engineering of light scattering and resonance phenomena in low-loss dielectric nanostructures with designed aperiodic geometries for active devices integrated atop the widespread and inexpensive silicon platform. In particular, I will discuss applications to the optical beam shaping of partially coherent radiation and absorption enhancement in thin-film optical photodetectors and solar cells. To this purpose, a new class of aperiodic media generated from prime numbers in complex quadratic fields will be introduced, and their distinctive scattering properties will be discussed within the rigorous Green's matrix spectral method. The presented work will focus on the design, fabrication and characterization of novel photonic nanostructures that enables the control of anomalous light transport phenomena in silicon-compatible low-loss materials, such as the recently demonstrated logarithmic photon sub-diffusion, thus defining a novel approach to tailor light-matter interactions for technologically relevant applications to optical sensing, light emission, and energy conversion on a chip.

We discuss the unusual scattering features enabled by confining electromagnetic energy and enhancing the interactions of light and matter in nanostructures, based on the concept of embedded eigenstates within the radiation continuum. We discuss how metasurfaces and metamaterials may be able to trap light in plain sight, and how lossless structures may be able to store energy in the transient by engaging complex zeros in the scattering response of the system. We also shed light on the role that reciprocity plays in the response of these systems, and how non-reciprocal or non-linear systems may enable unusual functionalities, playing a pivotal role in low-energy nanophotonic opto-electronic and bio-sensing devices.

Suppressing loss mechanisms in plasmonic structures is critical for the demonstration of high-quality-factor resonances with narrow linewidths. An extensively explored approach for loss reduction in these structures is the implementation of lattice plasmons (LPs) in which the primary loss mechanisms (i.e., Ohmic and radiation losses) can be simultaneously reduced. LPs take advantage of in-plane dipolar coupling of the scattered light from plasmonic arrays to provide narrow resonance linewidths at wavelengths approaching inter-particle distances. Here, we report numerical design and experimental demonstration of ultra-sharp (FWHM ≈ 6 nm) and tunable LP resonance modes in an array of gold (Au) nanopatches separated from a backside metallic film via a thin alumina (Al2O3) spacer layer. We show that oblique excitation of the array induces out-of-plane electric dipoles, which enable diffractive coupling of the incident light to the array, thus, exciting the LP mode. Furthermore, the excitation angle can be controlled to precisely tune important attributes of the LP lineshape including the resonance linewidth and the spectral position. Using spectroscopic ellipsometry measurements and finite-difference time-domain modeling, we show that the LP modes are only achievable through TM-polarized excitations, as a TE-polarized light lacks an out-of-plane electric-field component. The structure reported here holds a great promise for applications seeking strong light-matter interactions.

Efficient control of light-matter interactions requires simultaneous manipulations of both electric and magnetic field components of light. For plasmonic nanoparticles, the electric interactions dominate while the magnetic counterparts are relatively weak. In addition, dissipative losses in plasmonic nanoparticles are large, making the realization of high-efficiency devices difficult. In order to solve these problems, high-index dielectric nanostructures arise as promising low-loss candidates with equally strong electric and magnetic resonances in the visible spectrum. In this presentation, based on numerical simulations, we will demonstrate our recent results on spectral tuning of the electric and magnetic dipole (ED and MD) resonances by arranging amorphous silicon nanoparticles into a periodic array. By forming a rectangular array with different periodicities in the x- and y-axis, the ED and MD resonances are separately coupled and effectively engineered. With this method, a variety of optical applications and devices can be accomplished, e.g. to improve the detection performance of a biosensor based on the MD resonance of a silicon nanoparticle and to realize a full 2π phase control in dielectric metasurfaces. Interestingly, it is also possible to achieve a MD resonance with sub-10 nm linewidth and an enhancement factor for the magnetic field intensity larger than one order of magnitude compared to a single isolated particle. Furthermore, based on this arrangement we will show a record high emission enhancement of a magnetic dipole source in an array of hollow silicon nanocylinders. An enhancement factor by three orders of magnitude can be obtained compared to a MD emitter in free space.

The ability to engineer metamaterials with tunable nonlinear optical properties is crucial for nonlinear optics. We theoretically and experimentally demonstrate a novel approach to create and/or enhance large second-order nonlinear optical susceptibilities (χ(2)) in electronic metamaterials consisting of dielectric-semiconductor-dielectric (DSD) multilayers. The generated fixed charges (Qf) at dielectric/semiconductor interfaces are exploited to engineer a non-zero built-in electric field within a semiconductor layer asymmetrically cladded with different dielectric layers that create opposite signs of Qf. The asymmetry of these charges extends the depletion region into the entire semiconductor layer and consequently, the induced high internal electric field interacts with the third-order nonlinear susceptibility (χ(3)) of the semiconductor resulting in an enhanced, prominent effective χ(2) in the bulk of asymmetric DSD metamaterials. We investigate this composite effect by studying free-space second-harmonic generation via Maker fringes analysis technique, simultaneously calculating the components of the effective χ(2) tensor in various DSD composite metamaterials. The highest component, χ(2)zzz, is 2 pm/V for the as-fabricated single period silicon dioxide (SiO2) / amorphous silicon (a-Si) / aluminum oxide (Al2O3) multilayer stack. The magnitude of χ(2)zzz is further enhanced to 8.5 pm/V after thermal annealing processes. Also, metals have been employed to enhance nonlinear optical interactions through field localization. Here, inspired by the electronic properties of materials, we introduce and demonstrate experimentally an asymmetric metal-semiconductor-metal (MSM) metamaterial that exhibits a large and electronically tunable effective second-order optical susceptibility (χ(2)). The induced χ(2) originates from the interaction between the third-order optical susceptibility of the semiconductor (χ(3)) with the engineered internal electric field resulting from the two metals possessing dissimilar work function at its interfaces. We demonstrate a five times larger second-harmonic intensity from the MSM metamaterial, compared to contributions from its constituents with electrically tunable nonlinear coefficient ranging from 2.8 to 15.6 pm/V. Spatial patterning of one of the metals on the semiconductor demonstrates tunable nonlinear diffraction, paving the way for all-optical spatial signal processing with space-invariant and -variant nonlinear impulse response. Finally, the constituents of this nonlinear composite DSD and MSM metamaterials, and the deposition technique make its manufacturing compatible with CMOS process, enabling their application for chip-scale silicon photonic integrated circuits and free space applications.

Zero-index metamaterials exhibit exotic optical properties such as uniform spatial phase and infinite wavelength. These extreme properties can be utilized for integrated-optics applications. However, practical implementation of zero-index-based photonic devices requires compatibility with complementary metallic-oxide-semiconductor (CMOS) technologies. Zero-index metamaterials have been previously demonstrated in both out-of-plane and integrated configurations by taking advantage of a photonic Dirac-cone dispersion at the center of the Brillouin zone. Such metamaterials feature a square matrix of high aspect-ratio pillars and offer matched impedance through simultaneously zero effective permittivity and permeability. However, these configurations are inherently incompatible with integrated devices due to out-of-plane excitation, metallic inclusions, or high aspect-ratio structures. This work demonstrates a CMOS-compatible zero-index metamaterial consisting of a square array of air-holes in a 220-nm-thick silicon-on-insulator wafer. To experimentally verify the refractive index, we measure the angle of refraction of light through a triangular prism consisting of the metamaterial. The index is extracted using Snell's Law to verify a refractive index of zero at a wavelength of 1625 nm. Through the air-hole in silicon configuration, the proportion of silicon is increased as compared to designs based on high aspect-ratio silicon pillars. This enables a platform with low-aspect-ratio features, improved confinement of transverse electric polarized light, as well as the original benefit of matched impedance. Featuring a trivial monolithic fabrication and capacity for integration with the expansive library of existing silicon photonic devices, this metamaterial enables implementation of proposed zero-index devices and offers a powerful platform for exploring the future applications of zero-index materials.

With the development of various recent tools to control electromagnetic wave propagation, such as transformation optics, the long-sought dream of rendering objects invisible has become a matter of practical implementation. However, the required index profile derived with such techniques leads to material properties that are not readily available in nature and, hence, various experimental simplifications and performance scarifications are inevitable. Therefore, it has been a widespread belief that perfect cloaking cannot be achieved with conventional materials. Here, we follow a different direction and provide a unique method based on scattering cancellation rather than conventional coordinate transformations, and show that perfect invisibility can be indeed achieved for any specified angular range and operational bandwidth by employing merely all-dielectric materials. The presented method is based on our recently proposed generalized Hilbert-like transform [1] that is able to eliminate the undesired scattered waves for any type of object, regardless of its shape/size, by directly tailoring the object’s scattering potential. In this direction, we show that the impinging wave on an object can be perfectly restored owing to the effective cancellation of the scattered waves emanating from the object and the surrounding index profile. We demonstrate this effect by experimental analyses conducted at the gigahertz regime. The proposed method represents an important step towards the ultimate goal of cloaking arbitrarily large objects at various wavelength regimes and may have profound implications especially in noninvasive near-field probing applications, where conventional transformation optics based cloaks fail to provide the interaction of the wave with the object.

Energy conservation techniques have been widely explored in recent years for several applications: IR camouflage, solar absorbers and for IR thermal harvesting as well. While many absorbers have been demonstrated using plasmonic metal nanoparticles, surface texturing and low density broad band absorbers, they still encounter inevitable drawbacks. The state of art absorbers are either suffering instability over time or bulkiness which limit their practical application. Metamaterials have provided a significant improvement overcoming the aforementioned challenges through introducing ultra-broad band absorbers. However, the urge for CMOS compatible sub-wave length absorber that can be integrated for opto/electronic devices is still a major challenge. We demonstrate a mid IR silicon absorber using doped Silicon/Silicon Hyperbolic Metamaterial (HMM) integrated with sub-wave length Si grating. HMMs are characterized by their hyperboloid dispersion momentum space that provides large density of photonic states. By applying sub-wavelength grating on HMM, light from free space can be coupled to high propagation wave vectors of the hyperbolic modes upon breaking the momentum mismatch restriction, leading to noticeable absorption. We are able to show that an all Si based designed HMM is capable to achieve absorption across the mid IR wavelength range reaching absorption (A) of value 0.9.This proposed CMOS compatible Si-based absorber serves as good candidate for IR thermal harvesting application for on chips purposes.

We demonstrate a hybrid material platform for high-speed integrated optical modulation through integration of graphene with silicon-on-insulator (SOI) substrates after adding a thin layer of an oxide material. The modulation is performed by charge accumulation in the graphene and Si layers of the resulting capacitor to change the index of refraction of both layers (through free-carrier plasma dispersion effect). The advantages of graphene layer include stronger free-carrier plasma dispersion effect, and larger carrier mobility (to achieve smaller device resistance and thus, higher operation speed). We also report solving some of the major challenges in achieving high-quality hybrid platform, especially avoiding the tearing of the graphene layer during the mechanical transfer through adding a layer of hexagonal boron nitride (h-BN) on the two sides of the graphene layer. The h-BN layer also works as an isolation layer to maintain the intrinsic carrier mobility of graphene. We demonstrate reduced graphene resistance by a factor of 3 through h-BN encapsulation. The potential performance measures of the resulting structure along with its extension to double-layer graphene modulators will be discussed. The hybrid graphene modulator has the potential for applications including optical interconnection, optical signal processing, and optical computing.

Alloying has served as a powerful means for tuning the non-vanishing optical bandgap of two-dimensional (2D) transition-metal dichalcogenides (TMDs), a family of 2D materials with optoelectronic properties covering a wide spectral window ranging from visible to near-infrared. In addition to the bandgap engineering, ‘spatial’ modulation of the composition ratio (i.e., x) in a ternary TMD alloy (e.g., MX2xX2(1-x)’; M: transition metal, X, X’: chalcogens) enables formation of lateral heterostructures with complex functionalities within the plane of 2D materials, a new asset that expands the realm of applications in which 2D materials can be incorporated. Despite several demonstrations of alloying in 2D TMDs, the phenomenologically important issue of strain development and its effect on the optical and structural properties of 2D TMD alloys is still missing. Here, we show that alloying processes induce a biaxial tensile strain that acts on the lattice of 2D TMD alloys and affect their optical properties. In addition, we show that such strain inflicts sever fracture of the alloys via formation of sub-micron-sized cracks. Our experimental characterization combined with detailed theoretical modeling suggest the important role of the Van der Waals interaction between the 2D material and the substrate in formation of the alloying-induced strain. Furthermore, we demonstrate the critical role of crystal defects in cracking of the TMD alloys, which further emphasizes the importance of high quality synthesis of 2D TMD crystals for practical applications.

Thermophotovoltaics (TPVs) are a potential technology for waste-heat recovery applications and utilize IR sensitive photovoltaic diodes to convert long wavelength photons (>800nm) into electrical energy. The most common conversion regions utilize Gallium Antimonide (GaSb) as the standard semiconductor system for TPV diodes due to its high internal quantum efficiencies (close to 90%) for infrared radiation (~1700nm). However, parasitic losses prevent high conversion efficiencies from being achieved in the final device. One possible avenue to improve the conversion efficiency of these devices is to incorporate metallic photonic crystals (MPhCs) onto the front surface of the diode. In this work, we study the effect of MPhCs on GaSb TPV diodes. Simulations are presented which characterize a specific MPhC design for use with GaSb. E-field intensity vs. wavelength and depth are investigated as well as the effect of the thickness of the PhC on the interaction time between the e-field and semiconductor. It is shown that the thickness of MPhC has little effect on width of the enhancement band, and the depth the ideal p-i-n junction is between 0.6μm and 2.1μm. Additionally, simulated results demonstrate an increase of E-field/semiconductor interaction time of approximately 40% and 46% for a MPhC thickness of 350nm and 450nm respectively.

Plasmonic systems exhibit a host of intriguing optical properties. In this talk I shall assert that the underlying source of this diversity is geometry: that is to say the shape of a plasmonic particle or the structure of a particular surface. I shall show how geometric complexity can be generated starting from simple structures such as waveguides and transforming them into more complex structures, then turning to transformation optics to understand how the properties of the complex structure relate to the simple one. Particular attention will be paid to singular structures which are know to act as harvesters of light and are responsible for the enhanced spectroscopic response that occurs in SERS experiments.

In this work, we demonstrate superchiral light generation based on achiral plasmonic surfaces. At resonance, the symmetric cavity-coupled plasmonic system generates single-sign chiral near-field whose helicity is determined solely by the handedness of the incident light. We elucidate the mechanism for such unique superchiral near field generation and find its origin in coherent and synergetic interactions between plasmonic and photonic cavity modes. The cavity-coupling enhances otherwise weak plasmonic chiral near-field by many folds. Furthermore, the system in a unique way suppresses the far field chirality due to its totally symmetric geometry providing a route for surface-enhanced chiroptic spectroscopy on a single surface.

We present a planar subwavelength spectral light separator, which sorts light by separating different spectral and polarimetric components into different channels in a snapshot, efficient, and angularly robust way. The device is composed of subwavelength-size rectangular aperture pairs, where each aperture pair consists of two perpendicularly-oriented identical apertures, in a metal film having deep-subwavelength-size thickness. The device is planar and ultrathin. It has subwavelength-size cross-section and deep-subwavelength-size thickness. Different aperture pairs simultaneously collect different spectral components of light, and different apertures of aperture pairs simultaneously collect different linear polarization components of light. The device operation is based on Fabry-Pérot-like localized resonances in the apertures and it does not rely on any periodicity or grating effect. Hence, the device can be used in an individual, nonperiodic, subwavelength-size configuration as well as in an array configuration composed of subwavelength-size unit cells. When aperture pairs are used as detecting elements, different spectral components of light can be detected independent of the polarization of light. When apertures instead of aperture pairs are used as detecting elements, different linear polarization components of light can be detected in addition to different spectral components. The operation of the device is largely independent of the incidence angle of light, which results in an angularly robust, wide-angle device. All these features are attractive for efficient, compact, snapshot spectral imaging systems, especially for multispectral imaging purposes. We show the operation of the device by examining its interaction with electromagnetic waves with the finite-difference frequency-domain (FDFD) method.

We describe performance characterization of spectrally tunable nano-engineered filters operating in the longwave infrared (LWIR) from 8 to 12 micron using quantum cascade lasers (QCLs) tunable over the full spectral range. The filter design is based on using the guided mode resonance (GMR) phenomenon. The device structure consists of a subwavelength dielectric grating on top of a planar waveguide using high index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) with refractive index of 2.4. The filters are designed to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest of the light. The tuning of the reflection wavelength is achieved by changing the angle of incidence of light by mechanically tilting the filter. Filters based on one dimensional (1D) gratings are polarization dependent and those based on two dimensional (2D) gratings are close to polarization independent. To design the filter with a strong narrow band reflectance, we used the rigorous coupled wave (RCW) algorithm to simulate the filter. Here we will describe design and characterization of prototype filters with 1D grating. Anti-reflection coatings were applied to improve transmission over the entire spectral region. Our experimental setup consists of a QCL system operating at room temperature, nanoengineered filter and an uncooled broadband sensor. We will present the filter design, detailed characterization experiment and compare the theoretical and experimental results.

Sub-wavelength arrays have garnered significant interest for many potential optoelectronics applications. We fabricated sub-wavelength silicon nanopillar arrays with a ratio of radius, r and a center-to-center distance, a, of r/a ≈ 0.2 that were fully embedded in SiO₂ for narrow stopband filters that are compact and straightforward to fabricate compared to conventional Bragg stack reflectors. These arrays are well-suited for hyperspectral filtering applications in the infrared. They are ultra-thin (<0.1λ), polarization-independent, and attain greater efficiencies enabled by low loss compared to plasmonic-based designs. The choice of Si as the nanopillar material stems from its low cost, high index of refraction, and a band gap of 1.1 eV near the edge of the visible.

These arrays exhibit narrow near-unity reflectivity resonances that arise from coupling of an incident wave into a leaky waveguide mode via a grating vector that is subsequently reradiated, also known as guided mode resonances (GMRs). Simulations reveal reflectivities of >99% with full width at half maxima (FWHM) of ≈0.01 μm. We demonstrate a fabrication route for obtaining nanopillar arrays that exhibit these GMRs. We experimentally observed a GMR with an amplitude of ~0.8 for filter arrays fabricated on silicon on insulator (SOI) substrates, combined with Fabry-Perot interference that stems from the underlying silicon layer.

We present a computational study of two-photon scattering process in an atom-ring resonator-waveguide QED system. By properly manipulating the operating frequency of incoming photons, we show that two-photon bound state and photon antibunching statistics are generated through resonator-mediated atom-photon interactions. Numerically, we find that mild backscattering and dissipation enhance the quality of generated photonic correlations. In addition, we also report the quantum photonic halo effect and the dissipation-induced photonic correlation transition phenomenon.

Resonant four-wave-mixing in microcavities has recently proven to be particularly interesting for obtaining ultra-efficient nonlinear wavelength conversion, parametric and frequency combs generation. Contrarily to the commonly used microring or whispering gallery mode cavities, photonic crystal nanocavities have not revealed yet their full potential in this direction. Despite their high-Q and ultra-small modal volume, they are not evidently suited for resonant four wave mixing as they do not naturally exhibit modes at equally spaced frequencies, a necessary condition for energy conservation. In this work, we designed and fabricated 1D photonic crystal nanobeam cavities which exhibit ultra- high Q modes around 1.55µm equally spaced in frequency. These nanocavities are made of InGaP material bonded on top of a SOI waveguide optical circuitry. The evanescent wave coupling between the cavities and the waveguides can be controlled at will by changing the SOI waveguide width. The large electronic bandgap of InGaP inhibits 2 photon absorption at 1.55µm and allows us to exploit pure Kerr nonlinearity. The electromagnetic potential inside the cavity is shaped to be spatially parabolic by engineering the hole position along the cavity. Thus, by construction the resonant modes supported by the cavity are equispaced in frequency. The measured loaded Q factors exceed 105 and the free spectral range (FSR) goes from 150GHz to 1THz depending on the size of the cavity. We demonstrate that the FSR remains quasi constant (flat dispersion). Four wave mixing and parametric generation is observed using CW pump power of few mWs.

Core-satellite nanostructures are meta-atom candidates that have strong magnetic modes at optical frequencies and their characterization is important for the development of metamaterials and metafluids. We utilize an electrodynamics-multipolar analysis (ED-MA) method to perform a detailed study of the electric and magnetic modes of a core-satellite nanostructure composed of silver nanoparticles decorated on a dielectric core. In addition to excitation with linearly polarized scalar beams, we utilize radially and azimuthally polarized cylindrical vector beams to selectively excite and enhance the multipolar modes of the core-satellite nanostructure. The refractive index of the dielectric core is altered to better understand the role of retardation in the unique multipolar modes that arise from these vector beam excitations. A more complex Ag-core silica-shell nanostructure decorated with Ag-satellites is also introduced and is shown to diminish the role of magnetic modes and introduce new electric modes selectively excited by a radially polarized beam.

Currently, many nanophotonic structures are designed based on heuristic principles, for example the design of gratings based on wavevector matching, or the design of a nanoantenna to have a length equal to a half-wavelength. In contrast, automated iterative optimization-based approaches can yield complex nonintuitive structures that provide better performance than heuristically-designed structures. This is especially true for 3D photonic structures, whose behavior can be more dificult to conceptualize than that of 2D structures. The challenge is that multi-parameter optimization routines often require hundreds or thousands of iterations, but a typical full-wave nanophotonic simulation may take hours per iteration. One type of optimization approach that has been applied to nanophotonics is topology optimization (TO). While TO works well for top-down fabrication where structures are fabricated from continuous media, it is not as well-suited for bottom-up, or additive-type fabrication methods based on building blocks of non-negligible size. Here we present a design methodology based around iterative object placement optimization, where the iterative loops are rapidly computed using a coupled dipole approach. The coupled dipole approach cannot capture all of the same physics as full-wave simulation approaches, however it can provide approximate results much more rapidly. We compare the accuracy of the coupled dipole approach to finite-difference time domain simulations. Two application designs will also be presented: compact 3D waveguide couplers and superresolution imaging structures.

The design of optical resonator structures usually processed by proposing the structure and then through some numerical or experimental process, determining the supported states. This is often a hit and miss approach as the desired properties of the state do not match the application requirements. A numerical design approach is presented in which the input is the desired optical resonator state and the output is the geometrical and material properties of the resonator structure that will support the state. The technique is presented for cylindrically symmetric structures using the Fourier-Bessel numerical mode solver.

Photonic crystals (PhCs) are artificial wavelength-scale periodic structures that enable the manipulation of light propagation and possess intriguing dispersive characteristics such as negative refraction, self-collimation, slow/fast light and super-prism. Among these properties, here we present compact low-symmetric PhCs exhibiting S-vector super-prism effect with high diffraction ability. Each unit-cell of square lattice PhC structure includes two dielectric rods in air background and it provides a high-resolution super-prism effect for transverse-magnetic bands. Theoretical calculations of band structures as well as equi-frequency contours are conducted by solving Maxwell’s equations with plane-wave expansion method to investigate the superprism effect of the proposed PhCs. Such asymmetric PhC configuration has a wide wavelength sensitivity from a/λ = 0.610 to a/λ = 0.628. Its operating frequency range provides also a huge angle magnification from 20.6° to 59.9°. In terms of diffraction ability, the proposed PhC structure overcomes the problems of irregular beam generation and irregular beam divergence in usual PhC structures. According to finitedifference time-domain calculations with 3° angle of incidence, it is obtained that the light is diffracted in a range between 20.6° and 59.9° inside the structure with a high wavelength sensitivity. That effect could be used for wavelength demultiplexing applications. Moreover, the numerical time-domain calculations are made to verify the theoretical analyses. Depending on the incidence angle, the light propagating inside the PhC medium steers up/down perfectly with a collimated behavior. Such highly wavelength sensitive self-collimated light diffraction property can be used to separate propagating beam at the output channels with low cross-talks in compact photonic integrated systems. Experimental verification of the intended superprism effect is also conducted in the microwave frequencies and a quite well wavelength sensitivity effect is observed.

In this paper, a highly sensitive surface plasmon photonic crystal fiber (PCF) biosensor is reported and studied to monitor glucose concentration. The suggested design is based on a well-known large mode area (LMA) single mode PCF infiltrated by a plasmonic material. Additionally, an etching process is applied to increase the biosensor sensitivity. The numerical analysis is obtained using a full vectorial finite element method (FVFEM). The suggested biosensor based on a commercial PCF with plasmonic rod achieves sensitivity as high as 7900 nm/RIU with corresponding resolution of 1.26 × 10^-5RIU^-1. The analysis also reveals that the proposed biosensor has a linear performance which is needed practically. Therefore, the reported biosensor has advantages in terms of fabrication feasibility and high linear sensitivity

The Mid Infrared MIR wavelength range offers many advantages in different applications. Chemical and biological detection are one of these applications, as it contains the absorption fingerprints of many gases and molecules. In addition integrated plasmonics are suitable platform for high sensitivity on chip sensors. In this paper we propose plasmonic Mach-Zehnder Interferometer (MZI) working as a gas sensor near the absorption fingerprints of many gases in the mid-infrared region. The proposed MZI contains a vertically stacked metal-insulator-metal (MIM) and metalinsulator (MI) waveguide. The sensitivity of MI waveguide is lower at higher wavelengths and also lower for gaseous medium than for liquid medium. In addition the losses of the MIM waveguide with oxide layer as insulator are much larger than the losses of the MI waveguide with gas as insulator which will result in poor visibility interferometers. Using a high index layer above the metal of the MI waveguide the sensitivity of the waveguide to gaseous in the mid infrared has been significantly enhanced. This layer also balances the intrinsic losses of both MI and MIM waveguides. The thickness and the refractive index of this layer have been optimized using finite difference modal analysis. Using this layer high sensitivity and high figure of merit (FOM) have been achieved for our MZI. This structure offers simple fabrication and low cost sensor that is suitable for rapid, portable and high throughput optical detection using multiplexed array sensing technique.

We demonstrate that conductometric gas sensing at room temperature with SnO2 nanowires is enhanced by visible and supra bandgap UV irradiation when and only when the metal oxide nanowires are decorated with Ag nanoparticles (r < 20 nm); no enhancement is observed for the bare SnO2 case. We combine spectroscopic techniques with conductometric gas sensing to study the wavelength dependency of the sensors response, showing a strict correlation between the Ag loaded SnO2 optical absorption and its gas response as a function of irradiation wavelength. Our results lead to the hypothesis that the enhanced gas response under UV-vis light is the effect of plasmonic hot electrons populating the Ag nanoparticles surface. Finally we discuss the chemoresistive properties of Ag loaded SnO2 sensor in parallel with the theory of Plasmon-Driven Catalysis, to propose an interpretative framework that is coherent with the established paradigma of these two actually separated fields of study.

Low-symmetric photonic crystals (PCs) have a high level of tunability in terms of their iso-frequency contours or refractive indices via angular orientations as well as spatial variations of unit cell elements. This phenomenon brings along emerging extraordinary dispersion properties dependent to rotation and position of unit cells. In this study, we show that the electromagnetic waves propagating inside the low-symmetric waveguides can be exposed to phase delays by tuning the angular orientations of unit elements. In this way, low-symmetric PC waveguides based Mach-Zehnder Interferometers (MZIs) exhibiting controllable phase properties are designed. The investigated all-dielectric tunable interferometric systems are numerically analyzed in both frequency and time domains. Furthermore, by exploiting spectral sensitivity of proposed MZIs, conceptual demonstration of gas sensor systems is performed. The designed optical gas sensors have minimum selectivity of 200 nm/RIU even in the case of refractive index variation on the order of 10^-4 RIU in analytes. Having these advantages, proposed interferometric configurations based on low-symmetric periodic structures may offer a significant alternative for photonic applications that require controllable output power or sensing of gaseous substances.

Stochastic resonance is a paradoxical phenomenon whereby a weak signal can be amplified by application of noise. Stochastic resonance occurs in a number of nonlinear systems, in neurobiology, mesoscopic physics, photonics, atomic physics, mechanics,... The classical picture of stochastic resonance involves the stochastic synchronisation of the motion of a fictious particle (representing the system's state) in a bistable potential subjected to a weak amplitude harmonic modulation (the input signal) and to amplitude noise. Stochastic amplification of the weak signal is revealed in the spectral amplification at the signal frequency for a non zero input noise strength. We report on the observation of phase stochastic resonance in a nanomechanical, photonic crystal membrane with integrated electrical actuation. The nanomechanical oscillator is forced by a coherent driving signal which results in a bistable behavior. Bistability occurs in a bidimensional phase space since the system has a response in amplitude and in phase. We subject the oscillator to an additional slow phase modulation and to phase noise. We evidence a stochastic resonance phenomenon with amplification of the phase or amplitude response of the system for a non-zero input noise. Moreover, a theoretical analysis reveals that phase noise acts in a multiplicative fashion. This has important consequences on the optimal parameters for stochastic resonance to occur and explains the observed noise-induced detuning in the system. Phase stochastic resonance may have impact on several domains, including signal transmission telecommunication with coherent protocols such as Phase Shifting Keying, or metrology with improved detection.

Optical activity is a well-established phenomenon. It means that the eigenstates are chiral, leading to a polarization rotation for incident linearly polarized light. Here we investigate the counterpart of optical activity in mechanics, namely “mechanical activity”(or phononic activity) in tailored 3D microstructured metamaterials. In the static regime, the simple-cubic chiral metamaterials exhibit a twist upon pushing onto them – a degree of freedom that is forbidden in ordinary (Cauchy) continuum mechanics. In the dynamic regime, we find a splitting of the lowest two bands, which is most pronounced near the middle of the first Brillouin zone, and chiral eigenstates.

To exploit phononic crystals/metamaterials to various applications including functional devices, it is useful to monitor the propagation of acoustic waves in the structures in a spatiotemporal manner. In this talk, our recent development of the time-resolved two-dimensional gigahertz acoustic field imaging technique, which is suitable for such investigations, will be presented. It utilizes the optical pump-probe method in which the acoustic waves/vibrations are generated by the absorption of ultra short light pulses (pump light pulses) with temporal width of the picosecond regime through the thermoelastic effect, and the surface displacement caused by the acoustic waves/vibrations is monitored with delayed light pulses (probe light pulses) using an optical interferometer. By varying the delay time and scanning the probe light spot position across the imaging area, the spatiotemporal evolution of the acoustic field is obtained. The technique is applied to study two-dimensional phononic crystals consist of regularly aligned holes in a silicon (100) substrate. From the spatiotemporal images of the acoustic field, we can retrieve the dispersion relation of the acoustic modes in two-dimensional k-space. The specific phonon-focusing patterns as well as the mode pattern of the nearly zero-group velocity modes around the phononic band gap are observed. We also clarify the details of the dispersion relation of the wave-guide mode for the one-dimensional wave guide formed in the two-dimensional phononic crystal, with the newly developed arbitrary frequency measurement technique. These results show the advantage of applying spatiotemporal imaging technique to investigate the phononic crystals/metamaterials and their derivatives.

Phononic crystals, artificial materials with periodically arranged scattering centers, were introduced more than two decades ago as the elastic waves analogue of photonic crystals. These materials, either in two or three dimensions, can exhibit large frequency regions of prohibited propagation of elastic waves, the so-called phononic band gaps (PBGs). On the other hand, typical elastic wave propagation in random structures is associated with diffusion, or in extreme situation with localization, and random structures do not exhibit band gaps. Here, we introduce a new class of structurally disordered phononic structures, hyperuniform disordered phononic structures (HDPS) that exhibit large elastic band gaps. These structures are created from initially arbitrary point patterns by imposing hyperuniform correlations among the points and finally decorating them with a specific scatterers, so that the structure factor becomes isotropic and vanishes for all k-vectors within a specific radius. The disorder can smoothly be tuned to produce structures ranging from totally random to fully periodic by adjusting a single parameter. Such amorphous structures exhibit large band gaps, comparable to the ones found in the periodic counterparts, ballistic and diffusive propagation depending on the modes frequency and a large number of localized modes near the band edges. We discuss the formation of high-Q cavity modes and waveguides with 100% transmission in these disordered structures in the GHz regime. Such phononic-circuit architectures are expected to have a direct impact on integrated micro-electro-mechanical filters/modulators for wireless communications and acoustic-optical sensing devices.

Publisher's Note: One of the authors listed on the first published version of this paper (original publication date 21 March 2018) was removed on 4 February 2019 at the request of that author. The paper has been updated to reflect this change. Self-assembled synthetic opals are suitable for integration into solution-processed thin film solar cells. In this work, finite-difference time-domain simulations are carried out to tailor optical properties of monolayer and multilayers of semiconductor spheres to trap light when these structures are incorporated into thin film solar cells. In particular, architectures in which spheres are filled with a photoactive material and embedded in a lower refractive index medium are examined. Based on spectra and field intensity maps, this study demonstrates that opal-like photonic crystals obtained from colloidal templates and filled with light-absorbing material can significantly harvest light by exploiting photonic band resonances.

The spontaneous emission of an excited molecule can be tailored by its environment. Modifications of the spontaneous emission rate using plasmonic structures are widely investigated for applications ranging from the near-field optics, nanophotonics, to biomedical imaging. It is possible to track the spontaneous emission rate of a dipole emitter which responds to spatial changes of the environment and therefore reflect the morphology of surface of interest. In this work, we model the fluorescence lifetime imaging of gold nanorod dimers by utilizing a single dipole emitter as a sensitive probe scanning along one dimension above the metallic nanostructures. The fluorescence lifetime is spatially mapped out as an attempt to reconstruct the corresponding images. However, it is found that the lifetime imaging is not always consistent with the real morphology of nanostructure. Artifacts in lifetime imaging may arise due to the strong coupling fields in the resonance structures. The sharpness of nanorod dimers could make spontaneous emission rate of a dipole emitter change dramatically and play a key role in artifacts. The operation frequency of a dipole emitter can also influence the lifetime and contribute to artifacts. Here, we will investigate the relation between orientations of dipole emitters and spatial profile of the image. In addition, we will address strategies to distinguish these artifacts from the real morphology and present a theoretical model based on the waveguide geometry to examine possible origins of artifacts.

Due to the relatively weak birefringence of natural materials in terahertz regime, metasurfaces have been proposed for compact terahertz phase modulators since they show effectively strong birefringence only with ultrathin structures. However, previous designs of metasurface show limited phase modulation reaching only up to the quarter-wavelength phase, and there has been no single metasurface design that works for a terahertz half-waveplate. Here, we present a metasurface that modulates the phase variably up to 180 degrees. The phase modulation is achieved by a hyperbolic metasurface composed of periodically arrayed rectangular metal rings with different periods for horizontal and vertical axis. By controlling each period, we show that our hyperbolic metasurface can possess large positive and negative permittivity values for horizontal and vertical axis and the phase shift can reach up to the 180 degrees. To check the validity of our design, we fabricate reconfigurable metasurface films and demonstrate the phase modulation 90 to 180 degrees. All results show good agreement with numerical simulation results.

We develop a method based on the reciprocity and Green function to efficiently obtain the far-field pattern of dipole emitters around plasmonic nanostructures. Applying this method to air hole arrays fabricated on metal films, we reveal their plasmonic characteristics in the near-field scanning optical microscopy. Modeling scanning-probe tips as surface plasmon launchers, we clarify the orientation effect of their equivalent dipoles and also how these effective dipoles contribute to the excitation of different plasmonic modes, resulting in distinguishable characteristics in the far-field imaging. The outcomes of our calculations are validated with the experimental data from a high-resolution raster scanning nano-focusing plasmonic tip. Satisfactory agreements between the model and measurements are demonstrated.

To overcome the classic sensitivity vs selectivity trade-off often associated with sensors used in diagnostic applications, signature spectroscopic information that is characteristic of the molecules to be sensed can be exploited. Raman spectroscopy offers such information and is suitable for biological fluids. It is considered a label-free sensing method that inherently has excellent specificity. Sensitivity on the other hand is generally low unless amplification of the generally weak Raman signal is achieved. Surface enhanced Raman Scattering (SERS) employs localized surface plasmons on metallic nanoparticles to amplify this signal by several order of magnitude. In this work, SERS substrates were prepared by growing silver nanoparticles using electrodeposition on silicon nanowires that were prepared using metal assisted chemical etching. Experimental results agree with finite difference time domain (FDTD) simulation results. Using pyridine as a probe molecule, Raman signal intensity was found to correlate well with the pyridine concentration in the range 10^-6 M to 10^-9 M, indicating its applicability as a quantitative sensor. Very low concentration of pyridine, 10^-11 M, was detected although at this low concentration the detection is only qualitative. The enhancement factor was calculated to reach 10¹¹. Spot-to-spot, sample-to-sample, and batch-to-batch variation was studied to ensure repeatability, which had been a long-standing issue of low-cost SERS substrates. In addition, experiments over several days highlight the robustness of these SERS substrates. This work bolsters the use of SERS as a low cost sensing method with good sensitivity and specificity for a plethora of applications without compromising on repeatability or robustness.

Polarization dependence has been considered undesirable for most photonic devices, as it degrades the performance of photonic systems employing the devices. On the other hand, if the polarization dependence can be all-optically controlled and strongly enhanced at modest optical powers, it would be an attractive means for all-optical polarization control with a large dynamic range and/or polarization extinction ratio.

In this paper, we show that forward stimulated Brillouin scattering (FSBS) can be highly polarization-selective in silicaglass subwavelength elliptical-core optical waveguides suspended in the air, which may be used as a novel way of efficient all-optical polarization control. By using the full-vectorial finite element analysis, we find that at certain core ellipticities FSBS for one polarization mode mediated by a specific phonon mode is almost eliminated, while FSBS for the other polarization mode is rather enhanced. For example, the strong suppression of FSBS is observed for only the polarization mode along the major core axis when the scattering process is mediated by the TR₂₁-like phonon mode. Such the polarization selectiveness is not observed in the case of conventional (backward) stimulated Brillouin scattering. The origin of the intriguing phenomena can be explained in terms of the dielectric perturbation induced by the interplay between electrostriction and radiation pressure. The polarization-selective FSBS is feasible and may be experimentally demonstrated by using microstructured optical fibers with high air-filling fractions or air-suspended slab waveguides fabricated on on-chip platforms. Our study provides a new opportunity of simple waveguide design for engineering boundary-enhanced optical forces and photon-phonon interactions.

Phase-change materials (PCMs) provide a route to adding dynamic tunability and reconfigurability to many types of photonic devices by changing the phase-state of the PCM itself. In this work we discuss the use of the phase-change alloy GeSbTe (GST) in the design of dynamically tunable filters operating in the infrared. GST is used to manipulate the extraordinary optical transmission of a periodic hole-array in a metallic layer, so creating ultra-thin, tunable band-pass filters. We discuss the use of such filters for multispectral imaging, suggest some approaches to overcome various practical challenges, and, finally, show that through the use of appropriate post processing algorithms this tunable filter could provide a cheap, ultra-thin, real-time, and relatively high performance multispectral imaging device.

Chiral metamaterials have attracted great interests in recent years owing to fascinating properties such as negative refraction, strong optical activity, and circular dichroism, which can be applied in many optoelectronic devices. Helix is especially suitable for studying chiral responses as the helical geometry well resembles the feature of circularly polarized light. In this study, we use a helix structure as a model system to demonstrate the general response of circularly polarized light in a three-dimensional dielectric helix nanostructure. The optical characterization is performed based on finitedifference time-domain (FDTD) method and our results show that the helix structure, consisting of dielectric helices arranged in a square lattice, exhibits multiple resonant peaks. The retrieved effective parameters from the complex transmittance and reflectance will be presented and the dispersion characteristics will be discussed based on various geometric parameters. The resonant frequencies can be tuned by structural parameters, and negative permeability can be achieved when the resonances are adjacent in frequencies. Depending on the geometrical arrangements, we will demonstrate the unique optical properties in an anisotropic helix nanostructure. Our analysis yields physical insight into the interaction of circularly polarized light with a three-dimensional chiral nanostructure, and provides design guidelines towards the implementation of all-dielectric photonic metamaterials.

Group-velocity dispersion (GVD) engineering is vital to many nonlinear optical phenomena and has been widely used for nonlinear optics. Aiming at different applications, one need to control and engineer the sign, value, slope of dispersion and the number of zero-dispersion wavelengths (ZDWs). In this work, we demonstrate generation of 5 ZDWs in a new type of bilayer waveguides. Outer layer of this waveguide can be formed by depositing without etching. Material combinations are Ge₂₃Sb₇S₇₀ (n≈2.2) and Ge₂₈Sb₁₂Se₆₀ (n≈2.6). In this waveguide, an extremely wideband-low and flat dispersion can be obtained from 2.6 to 15.5 μm (2.6 octaves).

A novel sensor for detecting particulate matter 2.5 (PM_2.5, particles with a diameter smaller than 2.5 μm) concentration in environment air is presented by using modal interference in a photonic crystal fiber (PCF). The sensor is composed of a single mode-multimode-PCF-multimode-single mode optical fiber, and the corresponding polypyrrole (PPy) sensing nanofilm with a light-inducing characteristic is synthesized onto the outside surface of a PCF in-situ by an interfacial ploymerization method. The experimental result shows that the thickness of the sensing film is within the range of 100~150 nm by a scanning electron microscope. When the sensor is placed in PM_2.5 air flow, the PM_2.5 particles are absorbed onto the surface of a PPy sensing film due to a light-inducing electrostatic effect, resulting in the refractive index (RI) change of a sensing film. For PM_2.5 air flow with a concentration of 55 μg/m³ and a sampling time of 30 min, the characteristic wavelength of the interference spectra has a blue shift with 1 nm. After turning off the light source, the characteristic wavelength of the sensor is back to the initial value owing to no light-inducing electrostatic effect and the PM_2.5 particles desorbing. The sensor has a good reversibility.