We introduce multilayer structures with the phase-change material germanium-antimony-tellurium (GST) for use as broadband switchable absorbers in the infrared wavelength range. We use a memetic optimization algorithm coupled with the transfer-matrix method to optimize both the material composition and the layer thicknesses of the multilayer structures. We show that in the optimized structures near perfect absorption can be switched to very low absorption in a broad wavelength range by switching GST from its crystalline to its amorphous phase. Our results could pave the way to a new class of broadband switchable absorbers and thermal sources in the infrared wavelength range.
We introduce non-Hermitian plasmonic waveguide-cavity structures based on the Aubry-Andre-Harper model to realize switching between right and left topological edge states using the phase-change material germanium-antimony- tellurium (GST). The structure unit cells consist of a metal-dielectric-metal (MDM) waveguide side-coupled to MDM stub resonators with modulated distances between adjacent stubs. In such structures the modulated distances introduce an effective gauge magnetic field. We show that switching between the crystalline and amorphous phases of GST leads to a shift of the dispersion relation of the optimized structure so that a right topological edge state for the crystalline phase, and a left topological edge state for the amorphous phase occur at the same frequency. Thus, we realize switching between right and left topological edge states at that frequency by switching between the crystalline and amorphous phases of GST. Our results could be potentially important for developing compact reconfigurable topological photonic devices.
We introduce a nanoplasmonic isolator consisting of a cavity coupled to a metal-dielectric-metal (MDM) waveguide. The waveguide and cavity are filled with a magneto-optical (MO) material, and the structure is under a static magnetic field. We show that, when MO activity is present, the cavity becomes a traveling wave resonator with unequal decay rates into the forward and backward directions. As a result, the structure operates as an isolator. We also introduce non-Hermitian plasmonic waveguide-cavity systems with topological edge states (TESs) at singular points. The structure unit cells consist of an MDM waveguide side-coupled to MDM stub resonators with modulated distances between adjacent stubs. In such structures the modulated distances introduce an effective gauge magnetic field. We show that such structures achieve extremely high sensitivity of the reflected light intensity. TESs at singular points could lead to singularity-based plasmonic devices with enhanced performance.
Device size has now reached the nanoscale range due to advancements in technology and scaling in the fields of very large-scale integration. The single-electron transistor (SET) is a promising solid-state device that can provide an extension for Moore’s law and is suitable for next-generation nanoelectronics design and application. Due to the Coulomb oscillation properties of the SET in addition to the high gain and ultra-low power consumption of the tunnel field effect transistor (TFET), the implementation of the hybrid SET/TFET will primarily benefit high density (nanoscale), low-power integrated circuits (ICs), and fast switching devices. In this study, we present a hybrid model of a graphene-based single electron transistor [1] with an n-type double-gate graphene nanoribbon TFET structure [2] utilized as an integrator. For simplicity, the TFET is used in the shorted gate configuration by connecting both the front and back gates. Following this, we design a fourth order analog low pass filter using the integrator circuit of SET/TFET. With the implementation in SPICE and Matlab, we analyze the transfer function of our proposed filter from its frequency characteristics (Bode plot). Our findings reveal significant roll-off and, as a consequence, increased filtering functions with low power consumption. This study adds to the realization and implementation of SET/TFET into applications where high frequency contributes to the reliability, performance, and low power required for nanoscale devices and designs.
Resonant propagation of light is important for building novel light source and chip-scale optical interconnects. Here, we introduced an optoplasmonic amplifier which is operating in the visible range and generating Raman signal internally with injection seeding. We introduced the microspheres as a chain with different arrangements such as – single sphere; two spheres with equal and unequal sizes; three spheres with equal sizes and multi spheres with different sizes. We analyzed the effect of excitation and polarization with respect to different spheres and position of excitation. We noticed a shift of mode position with respect to different sizes of microspheres. We also had different kind of underlying substrates such as silicon nanopillar, polymer nanopillar, pyramid polymer and nanohole polymer and investigate the effect of these substrates on various chains of microspheres.
We investigate the exceptional points in a two-layer cylindrical waveguide structure consisting of absorbing and nonabsorbing dielectrics. We show that by tuning the parameters of the structure the complex effective indices of two waveguide modes can coalesce so that an exceptional point is formed. We show that the sensitivity of the effective index of the waveguide mode is enhanced at the exceptional point. We also investigate using phase-change materials in multilayer structures to switch between singular points. We show that in multilayer structures consisting of phasechange, lossless dielectric, lossy, and gain materials, absorbing or spectral singularities can be switched to exceptional points, and self-dual spectral singularities can be switched to unidirectional spectral singularities by switching the phasechange material from its crystalline to its amorphous phase. Our results could be important for developing new compact reconfigurable singularity-enhanced optical devices.
Molybdenum disulfide (MoS2) is a two-dimensional material which has demonstrated semiconducting behavior [1]. Different kinds of irradiations create the defects in molybdenum disulfide (MoS2) structure, different types of irradiations modulate the density of sulfur vacancies in MoS2[1]. MoS2 based and other 2-D materials-based devices and sensors are used in harsh environments [1]. In [2] authors have demonstrated studies of gamma irradiation on mono layer graphene. To develop and fabricate the MoS2-based devices and sensors, nanoelectronics instrumentation such as Transmission Electron Spectroscopy (TEM), Scanning Electron Microscopy (SEM), Raman Spectroscopy, X-ray Photo-Electron Diffraction (XPS) techniques are required for characterization of MoS2. Moreover, these radiation techniques, have huge impacts on electronic and optical properties of MoS2 [3]. So, it is important to study irradiation effects on the crystal structure and properties of MoS2. In this work, Co-60 source was used for the irradiation, which has nominal irradiation dose 2.07 Gy/min (207 rad/Min) (±5%). We have irradiated gamma-rays on four samples of single-layer molybdenum disulfide over copper substrate. We exposed the irradiation dose of 1.0 kGy (100 krad), 1.75 kGy (175 krad), 2.65 kGy (265 krad) and 3.0 kGy (300 krad) of irradiations on sample number one, two, three and four respectively. Through the Raman Spectroscopy, we studied E12g, A1g peaks. A1g peak is at 403.6 cm-1 and E12g peak is at 384.7 cm-1 in pristine MoS2 Raman spectroscopy. Raman spectroscopy is nondestructive tool for characterization of S vacancies in MoS2.
Automated detection of orbital angular momentum (OAM) can tremendously contribute to quantum optical experiments. We develop convolutional neural networks to identify and classify noisy images of Laguerre–Gaussian (LG) modes collected from two different experimental set ups. We investigate the classification performance measures of the predictive classification models for experimental conditions. The results demonstrate accuracy and specificity above 90% in classifying 16 LG modes for both experimental set ups. However, the F-score, sensitivity, and precision of the classification range from 57% to 92%, depending on the number of imperfections in the images obtained from the experiments. This research could enhance the application of OAM light in telecommunications, sensing, and high-resolution imaging systems.
We numerically design and experimentally test a SERS-active substrate for enhancing the SERS signal of a monolayer of graphene in water. The monolayer is placed on top of an array of silver-covered nanoholes in a polymer and is covered with water. Here we report a large enhancement of up to 200000 in the SERS signal of the graphene monolayer on the patterned plasmonic nanostructure for a 532 nm excitation laser wavelength. Our numerical calculations of both the excitation field and the emission rate enhancements support the experimental results. We also propose a highly compact structure for near total light absorption in a monolayer of graphene in the visible. The structure consists of a grating slab covered with the graphene monolayer. The grating slab is separated from a metallic back reflector by a dielectric spacer. The proposed structure could find applications in the design of efficient nanoscale visible-light photodetectors and modulators.
While several approaches have been proposed to optimize the geometrical dimensions of multilayer photonic nanostructures with a given material composition, very few works have considered simultaneously optimizing the material composition and dimensions of such nanostructures. Here, we develop a hybrid optimization algorithm as a method to design optimal multilayer photonic structures. Leveraging recent progress in metaheuristic optimization, we develop an optimization method consisting of a Monte Carlo simulation, a continuous adaptive genetic algorithm, and a pattern search algorithm. We first perform a Monte Carlo simulation over the entire design space. Structures are ranked according to the chosen fitness function. We find that this method yields viable material compositions. The material compositions of the best structures are used to parameterize the genetic algorithm in the next stage. A number of genetic algorithm populations are generated, one for each material composition, to optimize the thicknesses. These populations are run in parallel for a number of generations, evaluating the structures of each generation and using the characteristics of those that best satisfy the fitness function to improve other structures. The resulting populations converge towards the optimum of their solution space typically after a few thousand generations. The genetic algorithm used is continuous because parameters are treated as real numbers rather than bit strings as in classical genetic algorithms, and adaptive because the algorithm uses characteristics of the population pool to guide optimization. Finally, we apply a pattern search local optimization algorithm to the best result from each population to find the exact optimum.
We introduce a non-parity-time-symmetric three-layer structure, consisting of a gain medium layer sandwiched between two phase-change medium layers for switching of the direction of reflectionless light propagation. We show that for this structure unidirectional reflectionlessness in the forward direction can be switched to unidirectional reflectionlessness in the backward direction at the optical communication wavelength by switching the phase-change material Ge2Sb2Te5 (GST) from its amorphous to its crystalline phase. We also show that it is the existence of exceptional points for this structure with GST in both its amorphous and crystalline phases which leads to unidirectional reflectionless propagation in the forward direction for GST in its amorphous phase, and in the backward direction for GST in its crystalline phase. Our results could be potentially important for developing a new generation of compact active free-space optical devices. We also show that phase-change materials can be used to switch photonic nanostructures between cloaking and superscattering regimes at mid-infrared wavelengths. More specifically, we investigate the scattering properties of subwavelength three-layer cylindrical structures in which the material in the outer shell is the phase-change material GST. We first show that, when GST is switched between its amorphous and crystalline phases, properly designed electrically small structures can switch between resonant scattering and cloaking invisibility regimes. The contrast ratio between the scattering cross sections of the cloaking invisibility and resonant scattering regimes reaches almost unity. We then also show that larger, moderately small cylindrical structures can be designed to switch between superscattering and cloaking invisibility regimes, when GST is switched between its crystalline and amorphous phases. The contrast ratio between the scattering cross sections of cloaking invisibility and superscattering regimes can be as high as ~ 93%. Our results could be potentially important for developing a new generation of compact reconfigurable optical devices.
We introduce non-parity-time-symmetric plasmonic waveguide-cavity systems, consisting of resonators with unbalanced gain and loss side-coupled to a waveguide. We show that the design parameters of such systems can be tuned to obtain exceptional points, and realize unidirectional reflectionless light propagation. We then propose a compact perfect absorber unit cell, based on unidirectional reflectionlessness at exceptional points. We show that with proper design, light can propagate into the perfect absorber unit cell with reflection close to zero in a broad wavelength range. By cascading multiple unit cell structures, near total light absorption can be achieved in a wide range of frequencies.
Orientation of plasmonic nanostructures is an important feature in many nanoscale applications such as catalyst, biosensors DNA interactions, protein detections, hotspot of surface enhanced Raman spectroscopy (SERS), and fluorescence resonant energy transfer (FRET) experiments. However, due to diffraction limit, it is challenging to obtain the exact orientation of the nanostructure using standard optical microscope. Hyperspectral Imaging Microscopy is a state-of-the-art visualization technology that combines modern optics with hyperspectral imaging and computer system to provide the identification and quantitative spectral analysis of nano- and microscale structures. In this work, initially we use transmitted dark field imaging technique to locate single nanoparticle on a glass substrate. Then we employ hyperspectral imaging technique at the same spot to investigate orientation of single nanoparticle. No special tagging or staining of nanoparticle has been done, as more likely required in traditional microscopy techniques. Different orientations have been identified by carefully understanding and calibrating shift in spectral response from each different orientations of similar sized nanoparticles. Wavelengths recorded are between 300 nm to 900 nm. The orientations measured by hyperspectral microscopy was validated using finite difference time domain (FDTD) electrodynamics calculations and scanning electron microscopy (SEM) analysis. The combination of high resolution nanometer-scale imaging techniques and the modern numerical modeling capacities thus enables a meaningful advance in our knowledge of manipulating and fabricating shaped nanostructures. This work will advance our understanding of the behavior of small nanoparticle clusters useful for sensing, nanomedicine, and surface sciences.
It has been shown that surface enhanced Raman spectroscopy (SERS) has many promising applications in ultrasensitive detection of Raman signal of substances. However, optimizing the enhancement in SERS signal for different applications typically requires several levels of fabrication of active plasmonic SERS substrates. In this paper, we report the enhancement of SERS signal of a single layer of graphene located on a plasmonic nano-Lycurgus cup array after placing water droplets on it. The experimental data shows that addition of water droplets can enhance the SERS signal of the single layer of graphene about 10 times without requiring any modifications to the nano-Lycurgus cup array. Using fullwave electromagnetic simulations, we show that addition of water droplets enhances the local electric field at the graphene layer, resulting in stronger light-graphene interaction at the excitation pump laser wavelength. We also show that the addition of water droplets on the graphene layer enables us to modify the band diagram of the structure, in order to enhance the local density of optical states at the Raman emission wavelengths of the graphene layer. Numerical calculations of both the excitation field enhancement at the location of the graphene layer, and the emission enhancement due to enhanced local density of optical states, support the experimental results. Our results demonstrate an approach to boost the SERS signal of a target material by controlling the band diagram of the active nanostructured SERS substrate through the use of fluidic dielectrics. These results could find potential applications in biomedical and environmental technologies.
We design a non-parity-time-symmetric plasmonic waveguide-cavity system, consisting of two metal-dielectric-metal stub resonators side coupled to a metal-dielectric-metal waveguide, to form an exceptional point, and realize unidirectional reflectionless propagation at the optical communication wavelength. We also show that slow-light-enhanced ultra-compact plasmonic Mach-Zehnder interferometer sensors, in which the sensing arm consists of a waveguide system based on a plasmonic analogue of electromagnetically induced transparency, lead to an order of magnitude enhancement in the refractive index sensitivity compared to a conventional metal-dielectric-metal plasmonic waveguide sensor. Finally, we show that plasmonic coaxial waveguides offer a platform for practical implementation of plasmonic waveguide-cavity systems.
Achieving active control of the flow of light in nanoscale photonic devices is of fundamental interest in nanophotonics. For practical implementations of active nanophotonic devices, it is important to determine the sensitivity of the device properties to the refractive index of the active material. Here, we introduce a method for the sensitivity analysis of active nanophotonic waveguide devices to variations in the dielectric permittivity of the active material. More specifically, we present an analytical adjoint sensitivity method for the power transmission coefficient of nanophotonic devices, which is directly derived from Maxwell’s equations, and is not based on any specific numerical discretization method. We show that in the case of symmetric devices the method does not require any additional simulations. We apply the derived theory to calculate the sensitivity of the power transmission coefficient with respect to the real and imaginary parts of the dielectric permittivity of the active material for both two-dimensional and three-dimensional plasmonic devices. We consider Fabry-Perot cavity switches consisting of a plasmonic waveguide coupled to a cavity resonator which is filled with an active material with tunable refractive index. To validate our method, we compare it with the direct approach, in which the sensitivity is calculated numerically by varying the dielectric permittivity of the active material, and approximating the derivative using a finite difference. We find that the results obtained with our method are in excellent agreement with the ones obtained by the direct approach. In addition, our method is accurate for both lossless and lossy devices.
Recently, unidirectional reflectionlessness was demonstrated in classical photonic structures at optical exceptional points. Here we introduce a non-parity-time symmetric plasmonic waveguide-cavity device consisting of two metaldielectric- metal stub resonators side coupled to a metal-dielectric-metal waveguide. We tune the geometric parameters of the structure to obtain the exceptional point and realize unidirectional reflectionlessness at the optical communication wavelength. We investigate the properties of the plasmonic exceptional point as well as the associated physical effects of level repulsion, crossing and phase transition. We also show that by properly cascading the plasmonic waveguide-cavity structures we can design a wavelength-scale unidirectional plasmonic waveguide perfect absorber.
The magneto-optical effect has been used to control the propagation of surface plasmon polaritons in plasmonic waveguides. Here we investigate single-interface metal-dielectric and metal-dielectric-metal plasmonic waveguides in which either the dielectric or the metal is a magneto-optical material. We derive the dispersion relation of these waveguides, and investigate the effect of an externally applied static magnetic field. We find that in metal-dielectric-metal waveguide structures in which the dielectric is a magneto-optical material, the symmetry of the structure prohibits any non-reciprocal propagation in the system. Moreover, the induced change in the propagation constant of the supported modes in the presence of an externally applied static magnetic field is relatively small. In addition, we find that using a magneto-optical metal in a single-interface metal-dielectric plasmonic waveguide results in non-reciprocal propagation of the plasmonic modes along the interface. We also find that in metal-dielectric-metal plasmonic waveguides in which the metal is a magneto-optical material, the propagation constant of the supported modes is dependent on the relative direction of the applied magnetic fields to the upper and lower metal regions. If the applied magnetic fields to the two metal regions are equal and in the same direction, the induced changes in the propagation constants of the modes propagating in the positive and negative directions are the same. On the other hand, if the directions of the applied external magnetic fields are opposite, the propagation constants of the modes propagating in the positive and negative directions are different. We finally investigate Fabry-Perot cavity magneto-optical switches.
We present optimized aperiodic structures for use as broadband, broad-angle thermal emitters which are capable of drastically increasing the efficiency of tungsten lightbulbs. These aperiodic multilayer structures designed with alternating layers of tungsten and air or tungsten and silicon carbide on top of a tungsten substrate exhibit broadband emittance peaked around the center of the visible wavelength range. We investigate the properties of these structures for use as lightbulb filaments, and compare their performance with conventional lightbulbs. We find that these structures greatly enhance the emittance over the visible wavelength range, while also increasing the overall efficiency of the bulb.
Waveguide-resonator systems are particularly useful for the development of several integrated photonic devices, such as
tunable filters, optical switches, channel drop filters, reflectors, and impedance matching elements. In this paper, we
introduce nanoscale devices based on plasmonic coaxial waveguide resonators. In particular, we investigate threedimensional
nanostructures consisting of plasmonic coaxial stub resonators side-coupled to a plasmonic coaxial
waveguide. We use coaxial waveguides with square cross sections, which can be fabricated using lithography-based
techniques. The waveguides are placed on top of a silicon substrate, and the space between inner and outer coaxial
metals is filled with silica. We use silver as the metal. We investigate structures consisting of a single plasmonic coaxial
resonator, which is terminated either in a short or an open circuit, side-coupled to a coaxial waveguide. We show that the
incident waveguide mode is almost completely reflected on resonance, while far from the resonance the waveguide mode
is almost completely transmitted. We also show that the properties of the waveguide systems can be accurately described
using a single-mode scattering matrix theory. The transmission and reflection coefficients at waveguide junctions are
either calculated using the concept of the characteristic impedance or are directly numerically extracted using full-wave
three-dimensional finite-difference frequency-domain simulations.
Bulk thermal emittance sources possess incoherent, isotropic, and broadband radiation spectra that vary from
material to material. However, these radiation spectra can be drastically altered by modifying the geometry of
the structures. In particular, several approaches have been proposed to achieve narrowband, highly directional
thermal emittance based on photonic crystals, gratings, textured metal surfaces, metamaterials, and shock waves
propagating through a crystal. Here we present optimized aperiodic structures for use as narrowband, highly
directional thermal infrared emitters for both TE and TM polarizations. One-dimensional layered structures
without texturing are preferable to more complex two- and three-dimensional structures because of the relative
ease and low cost of fabrication. These aperiodic multilayer structures designed with alternating layers of silicon
and silica on top of a semi-infinite tungsten substrate exhibit extremely high emittance peaked around the
wavelength at which the structures are optimized. Structures were designed by a genetic optimization algorithm
coupled to a transfer matrix code which computed thermal emittance. First, we investigate the properties of the
genetic-algorithm optimized aperiodic structures and compare them to a previously proposed resonant cavity
design. Second, we investigate a structure optimized to operate at the Wien wavelength corresponding to a
near-maximum operating temperature for the materials used in the aperiodic structure. Finally, we present a
structure that exhibits nearly monochromatic and highly directional emittance for both TE and TM polarizations
at the frequency of one of the molecular resonances of carbon monoxide (CO); hence, the design is suitable for
a detector of CO via absorption spectroscopy.
In this paper, we introduce slow-light enhanced nanoscale plasmonic waveguide devices for manipulating light at the
nanoscale. In particular, we investigate nanoplasmonic metal-dielectric-metal (MDM) waveguide structures for highsensitivity
sensors. Such plasmonic waveguide systems can be engineered to support slow-light modes. We find that, as
the slowdown factor increases, the sensitivity of the effective index of the mode to variations of the refractive index of
the material filling the structures increases. Such slow-light enhancements of the sensitivity to refractive index variations
lead to enhanced performance of active plasmonic devices such as sensors. We consider Mach-Zehnder interferometer
(MZI) sensors in which the sensing arm consists of a slow-light waveguide based on a plasmonic analogue of
electromagnetically induced transparency (EIT). We show that a MZI sensor using such a waveguide leads to
approximately an order of magnitude enhancement in the refractive index sensitivity, and therefore in the minimum
detectable refractive index change, compared to a MZI sensor using a conventional MDM waveguide.
We explore an approach to enhance the efficiency of solar cells using photonic nanostructures for solar ther-mophotovoltaics. Our focus is on designing photonic nanostructures that can provide broadband absorption in a narrow angular range for solar thermophotovoltaic systems which do not employ sunlight concentration. We consider structures consisting of an aperiodic multilayer stack of alternating layers of silicon and silica on top of a thick tungsten layer. The layer thicknesses are optimized to maximize the angular selectivity in the absorp-tivity for both TE and TM polarizations. Using such an approach, we design structures with highly directional absorptivity for both polarizations.
We show that the space-mapping algorithm, originally developed for microwave circuit optimization, can enable the efficient optimization of nanoplasmonic devices. Space-mapping utilizes a physics-based coarse model to approximate a fine model accurately describing a device. The main concept in the algorithm is to find a mapping that relates the fine and coarse model parameters. If such a mapping is established, we can then avoid using the direct optimization of the computationally expensive fine model to find the optimal solution. Instead, we perform optimization of the computationally efficient coarse model to find its optimal solution, and then use the mapping to find an estimate of the fine model optimal. In this paper, we demonstrate the use of the space mapping algorithm for the optimization of metal dielectric- metal plasmonic waveguide devices. In our case, the fine model is a full-wave finite-difference frequency domain (FDFD) simulation of the device, while the coarse model is based on the characteristic impedance and transmission line theory. We show that, if we simply use the coarse model to optimize the structure without space mapping, the response of the structure obtained substantially deviates from the target response. On the other hand, using space mapping we obtain structures which match very well the target response. In addition, full-wave FDFD simulations of only a few candidate structures are required before the optimal solution is reached. In comparison, a direct optimization using the fine FDFD model in combination with a genetic algorithm requires thousands of full-wave FDFD simulations to reach the same optimal.
Resonant subwavelength plasmonic apertures can efficiently concentrate light into deep subwavelength regions, and
therefore significantly enhance the optical transmission through the apertures, or the absorption in the apertures. In
addition, grating structures, consisting of periodic arrays of grooves patterned on the metal film on both sides of a metal
aperture, are commonly used to enhance the coupling of incident light into the aperture through the excitation of surface
plasmons. For efficient surface plasmon excitation, however, the period of the grating has to be equal to the surface
plasmon wavelength, and several grating periods are required. Thus, such structures need to be several microns long. In
this paper, we show that a compact submicron structure consisting of multiple optical microcavities on both the entrance
and exit sides of a subwavelength plasmonic slit filled with an absorbing material can greatly enhance the absorption
cross section of the slit. We show that such microcavity structures can increase both the coupling of incident light into
the slit mode, as well as the resonant absorption enhancement in the slit by fine tuning the reflection coefficients at the
two sides of the slit. An optimized submicron structure consisting of two microcavities on each of the entrance and exit
sides of the slit leads to ~9.3 times absorption enhancement compared to an optimized slit without microcavities at the
optical communication wavelength of 1.55 microns. Finally, we show that multiple microcavity structures can also be
used to greatly enhance the coupling of free-space radiation into subwavelength plasmonic waveguides.
Plasmonic devices, based on surface plasmons propagating at metal-dielectric interfaces, have shown the potential to
manipulate light at deep subwavelength scales. One of the main challenges in plasmonics is achieving active control of
optical signals. In this paper, we introduce active plasmonic devices enhanced by waveguide dispersion engineering. We
consider plasmonic waveguide systems consisting of a metal-dielectric-metal waveguide (MDM) side-coupled to arrays
of MDM stub resonators. The MDM waveguide and stubs are filled with an active material whose absorption coefficient
can be modified with an external control beam. Such plasmonic waveguide systems can be engineered to support slowlight
modes. We find that, as the slowdown factor increases, the sensitivity of the effective index of the mode to
variations of the refractive index of the active material increases. Such slow-light enhancements of the sensitivity to
refractive index variations lead to enhanced performance of active plasmonic devices such as switches. To demonstrate
this, we consider absorption switches based on Fabry-Perot cavity structures, consisting of slow-light plasmonic
waveguide systems sandwiched between two conventional MDM waveguides. We find that increased slowdown factor
leads to increased induced change of the propagation length of the slow-light mode for a given refractive index variation,
and therefore to increased modulation depth. Compared to conventional MDM absorption switches, slow-light enhanced
switches achieve significantly higher modulation depth with moderate insertion loss. We use a scattering matrix theory
to account for the behavior of the devices which is in excellent agreement with numerical results obtained with the finitedifference
frequency-domain method.
Plasmonic devices, based on surface plasmons propagating at metal-dielectric interfaces, have shown the potential to
guide and manipulate light at deep subwavelength scales. In addition, slowing down light in plasmonic waveguides leads
to enhanced light-matter interaction, and could therefore enhance the performance of nanoscale plasmonic devices such
as switches and sensors. In this paper, we introduce slow-light subwavelength plasmonic waveguides based on a
plasmonic analogue of electromagnetically induced transparency (EIT). Both the operating wavelength range and the
slowdown factor of the waveguides are tunable. The structure consists of a periodic array of two metal-dielectric-metal
(MDM) stub resonators side-coupled to a MDM waveguide. The two cavities in each unit cell have a resonant frequency
separation which can be tuned by adjusting the cavity dimensions. We show that in the vicinity of the two cavity
resonant frequencies, the system supports three photonic bands, and the band diagram is similar to that of EIT systems.
The middle band corresponds to a mode with slow group velocity and zero group velocity dispersion in the middle of the
band. Decreasing the resonant frequency separation, increases the slowdown factor, and decreases the bandwidth of the
middle band. We also find that metal losses lead to a tradeoff between the slowdown factor and the propagation length of
the supported optical mode. We use a single-mode scattering matrix theory to account for the behavior of the
waveguides, and show that it is in excellent agreement with numerical results obtained with the finite-difference
frequency-domain method.
We theoretically investigate the effect of fabrication-related disorders on subwavelength metal-dielectric-metal
plasmonic waveguides. We use a Monte Carlo method to calculate the roughness-induced excess attenuation coefficient
with respect to a smooth waveguide. We find that the excess attenuation is mainly due to reflection from the rough
surfaces. For small roughness height (δ<4nm), the excess optical power loss due to disorder is small compared to the
material loss in a smooth waveguide. However, for large roughness height (δ>4nm), the excess attenuation increases
rapidly and the propagation length of the optical mode is severely affected. We also find that the disorder attenuation due
to reflection is maximized when the power spectral density of the disordered surfaces at the Bragg frequency is
maximized. Finally, we show that increasing the modal confinement or decreasing the guide wavelength, increase the
attenuation due to disorder.
We introduce extremely compact all-optical nonlinear switches based on Y-shaped plasmonic waveguides. We consider
a Y-shaped structure, consisting of a subwavelength metal-dielectric-metal input waveguide branch connected to two
metal-dielectric-metal output waveguide branches. The Y-shaped channel is embedded in a metallic film and filled with
a Kerr nonlinear material. We show that such a device can be designed to function as a switch between the two output
branches, controlled by the intensity of the incident light. We also show that the Y-shaped plasmonic structure can be
used as a tunable optical splitter.
A metallic slot waveguide, with a dielectric strip embedded within, is investigated for the purpose of enhancing
the optics-to-THz conversion efficiency using the difference-frequency generation (DFG) process. To describe
the frequency conversion process in such lossy waveguides, a fully-vectorial coupled-mode theory is developed.
Using the coupled-mode theory, we outline the basic theoretical requirements for efficient frequency conversion,
which include the needs to achieve large coupling coefficients, phase matching, and low propagation loss for both
the optical and THz waves. Following these requirements, a metallic waveguide is designed by considering the
trade-off between modal confinement and propagation loss. Our numerical calculation shows that the conversion
efficiency in these waveguide structures can be more than one order of magnitude larger than what has been
achieved using dielectric waveguides. Based on the distinct impact of the slot width on the optical and THz
modal dispersion, we propose a two-step method to realize the phase matching for general pump wavelengths.
We introduce a periodic plasmonic waveguiding structure which supports a guided subwavelength optical mode with
slow group velocity at a tunable wavelength range and with a tunable slowdown factor. The structure consists of a metal-dielectric-metal (MDM) waveguide side-coupled to a periodic array of MDM stub resonators. Both the MDM waveguide
and MDM stub resonators have deep subwavelength widths. We show that such a structure supports a guided optical
mode with slow group velocity. The wavelength range in which slow light propagation is achieved can be tuned by
adjusting the MDM stub resonator length and the periodicity of the structure. We also show that the slowdown factor
increases as the periodicity of the structure decreases, and that light can be slowed down by several orders of magnitude.
We find that there is a tradeoff between the slowdown factor and the propagation length of the supported optical mode.
In addition, for a given slowdown factor and operating wavelength, the propagation length of the optical mode in the
periodic plasmonic waveguide is much larger than the propagation length of the mode supported by a conventional
MDM waveguide, in which the slowdown factor can be tuned by adjusting the dielectric layer width. Finally, we show
that light can be coupled efficiently from a conventional MDM waveguide to such a periodic plasmonic waveguide. Such
slow-light plasmonic waveguides could be potentially used in nonlinear and sensing applications. We use a characteristic
impedance model and transmission line theory to account for their behavior.
We theoretically investigate the properties of absorption switches for metal-dielectric-metal (MDM) plasmonic
waveguides. We show that a MDM waveguide directly coupled to a cavity filled with an active material with tunable
absorption coefficient can act as an absorption switch, in which the on/off states correspond to the absence/presence of
optical pumping. We also show that a MDM plasmonic waveguide side-coupled to a cavity filled with an active material
can operate as an absorption switch, in which the on/off states correspond to the presence/absence of pumping. For a
specific modulation depth, the side-coupled-cavity switch results in more compact designs compared to the directcoupled-
cavity switch. Variations in the imaginary part of the refractive index of the material filling the cavity of
Δκ=0.01 (Δκ=0.15) result in ~60% (~99%) modulation depth. The properties of both switches can be accurately
described using transmission line theory.
A multi-layer photonic crystal can be used to suppress coherent thermal conductance below the vacuum conductance
value, over the entire high-temperature range. With interlacing layers of silicon and vacuum, heat can only be carried by
photons. The thermal conductance of the crystal would then be determined by the photonic band structure. Partial
photonic band gaps that present over most of the thermal spectrum, as well as the suppression of evanescent coupling of
photons across the vacuum layers at high frequencies, would reduce the amount heat conducting photon channels below
that of the vacuum. Thus such multi-layer structures can be very efficient thermal insulators. Besides, the thermal
conductance of such structures can exhibit substantial tunability, by merely changing the size of the vacuum spacing.
Plasmonic waveguides have shown the potential to guide subwavelength optical modes, the so called surface
plasmon polaritons, at metal-dielectric interfaces. In particular, a metal-dielectric-metal (MDM) structure supports
a subwavelength propagating mode at a wavelength range extending from DC to visible. Thus, such a
waveguide could be important in providing an interface between conventional optics and subwavelength electronic
and optoelectronic devices. Nonlinear processes such as second-harmonic generation (SHG) are important
for applications such as switching and wavelength conversion. In this paper, we show that field enhancement in
MDM waveguides can result in large enhancement of SHG. We first consider a structure consisting of a MDM
waveguide filled with lithium niobate, which is sandwiched between two high-index-contrast dielectric waveguides.
Such a structure forms a Fabry-Perot resonant cavity and can be designed to have a resonance at both
the first and second harmonic. We show that this doubly resonant device results in more than two orders of
magnitude enhancement in SHG compared to a uniform slab of lithium niobate. We also consider structures in
which multisection tapers are used to couple light in and out of the MDM waveguide. We optimize the tapers so
that their transmission efficiency is maximized at both the first and second harmonic. For such structures the
field enhancement is due to the squeezing of the optical power from the wavelength-sized dielectric waveguide to
the deep subwavelength MDM waveguide.
We investigate the properties of the modes supported by three-dimensional subwavelength plasmonic slot waveguides.
We show that the fundamental mode supported by a symmetric plasmonic slot waveguide, composed of
a subwavelength slot in a thin metallic film embedded in an infinite homogeneous dielectric, is always a bound
mode. Its modal fields are highly confined over a wavelength range extending from zero frequency to the ultraviolet.
We then show that for an asymmetric plasmonic slot waveguide, in which the surrounding dielectric
media above and below the metal film are different, there always exists a cutoff wavelength above which the
mode becomes leaky.
The authors show that the incorporation of gain media in only a selected device area can annul the effect of material loss,
and enhance the performance of loss-limited plasmonic devices. In addition, they demonstrate that optical gain provides a
mechanism for on/off switching in metal-dielectric-metal (MDM) plasmonic waveguides. The proposed gain-assisted plasmonic switch consists of a subwavelength MDM plasmonic waveguide side-coupled to a cavity filled with semiconductor material. In the absence of optical gain in the semiconductor material filling the cavity, an incident optical wave in the plasmonic waveguide remains essentially undisturbed by the presence of the cavity. Thus, there is almost complete transmission of the incident optical wave through the plasmonic waveguide. In contrast, in the presence of optical gain in the semiconductor material filling the cavity, the incident optical wave is completely reflected. They show that the principle of operation of such gain-assisted plasmonic devices can be explained using a temporal coupled-mode theory. They also show that the required gain coefficients are within the limits of currently available semiconductor-based optical gain media.
We analyze the spatial coherence of thermal field emitted from a lossy dielectric slab using fluctuation-dissipation
theorem.1 For a given wavelength λ, the coherence property varies drastically with the distance from the slab surface.
The coherence length is roughly
λ / 2 in the far-field zone, but in the extreme near-field zone, it is many orders of
magnitude smaller than λ, due to spatially fluctuating surface charges at the air-dielectric interface. On the other hand,
in the intermediate near-field zone, the coherence length can be much longer than
λ / 2 if the loss is small, because of
the presence of waveguide modes of the slab. Such long-ranged coherence falls off approximately as
1/√x , in contrast
to
1/x for a blackbody radiator, where x refers to displacement parallel to the slab surface. Furthermore, at a point of
fixed distance from the slab surface, the frequency spectrum of the local energy density exhibits distinct fluctuation
pattern, which is shown to be closely related to the waveguide dispersion relation.
We theoretically investigate the properties of compact couplers between high-index contrast dielectric slab waveguides and two-dimensional metal-dielectric-metal subwavelength plasmonic waveguides. We show that a coupler created by simply placing a dielectric waveguide terminated flat at the exit end of a plasmonic waveguide can be designed to have a transmission efficiency of ~70% at the optical communication wavelength. We also show that the transmission efficiency of the couplers can be further increased by using optimized multisection tapers. In both cases the transmission response is broadband. In addition, we investigate the properties of a Fabry-Perot structure in which light is coupled in and out of a plasmonic waveguide sandwiched between dielectric waveguides. Finally, we discuss potential fabrication processes for structures that demonstrate the predicted effects.
We propose to exploit the unique properties of surface plasmons to enhance the signal-to-noise ratio of mid-infrared
photodetectors. The proposed photodetector consists of a slit in a metallic slab filled with absorptive semiconductor
material. Light absorption in the slit is enhanced due to Fabry-Perot resonances. Further absorption enhancement is
achieved by surrounding the slit with a series of periodic grooves that enable the excitation of surface plasmons that
carry electromagnetic energy towards the slit. Using this scheme, we design and optimize a photodetector operating at
lambdao = 9.8 microns
with a roughly 250 times enhancement in the absorption per unit of volume of semiconductor material
compared to conventional photodetectors operating at the same wavelength.
In a recent paper [Phys. Rev. Lett. 94, 197401 (2005)], we introduced a mechanism for creating artificial high refractive index metamaterials by exploiting the existence of sub-wavelength propagating modes in metallic systems. We showed that a perfect metal film with a periodic arrangement of sub-wavelength cut-through slits can be regarded as a dielectric slab with a frequency-independent effective index. Here, we discuss the optical properties of such a system when the perfect metal condition is no longer valid, e.g., in the visible and near infrared wavelength regimes. If the metal obeys a
plasmonic dispersion model, we find that films with a periodic arrangement of sub-wavelength slits support two distinct types of guided modes: a surface mode and a set of effective dielectric slab modes. We show how the behavior of both modes is affected by film thickness and surface properties.
We demonstrate the existence of a bound optical mode supported by an air slot in a thin metallic film deposited on a substrate, with slot dimensions much smaller than the wavelength. The modal size is almost completely dominated by the near field of the slot. Consequently, the size is very small compared with the wavelength, even when the dispersion relation of the mode approaches the light line of the surrounding media. In addition, the group velocity of this mode is close to the speed of light in the substrate, and its propagation length is tens of microns at the optical communication wavelength. We also investigate the performance of bends and power splitters in plasmonic slot waveguides. We show that, even though the waveguides are lossy, bends and splitters with no additional loss can be designed over a wavelength range that extends from DC to near-infrared, when the bend and splitter dimensions are much smaller than the propagation length of the optical mode. We account for this effect with an effective characteristic impedance model based upon the real dispersion relation of the plasmonic waveguide structures.
Several problems in nanophotonics are uniquely suitable for frequency domain modeling methods. We first present a new method for sensitivity analysis of nanophotonic devices. The algorithm is based on the finite-difference frequency-domain method and uses the adjoint variable method and perturbation theory techniques. We show that our method is highly efficient and accurate and can be applied to the calculation of the sensitivity of transmission parameters of resonant nanophotonic devices. Frequency-domain methods are also essential in modeling of plasmonic devices due to the complicated dispersion properties of metals at optical frequencies. Here we demonstrate the existence of a bound optical mode supported by a slot in a thin metallic film deposited on a substrate, with slot dimensions much smaller than the wavelength. The modal size is almost completely dominated by the near field of the slot. Consequently, the size is very small compared with the wavelength, even when the dispersion relation of the mode approaches the light line of the surrounding media. In addition, the group velocity of this mode is close to the speed of light in the substrate, and its propagation length is tens of microns at the optical communication wavelength.
We present a new method for sensitivity analysis of photonic crystal
devices and nanophotonic devices in general. The algorithm is based on the finite-difference frequency-domain method and uses the adjoint variable method and perturbation theory techniques. We show that our method is highly efficient and accurate and can be applied to the calculation of the sensitivity of transmission parameters of resonant nanophotonic devices.
We use coupled optical and electronic simulations to investigate design tradeoffs in electrically pumped photonic crystal light emitting diodes. A finite-difference frequency-domain electromagnetic solver is used to calculate the spontaneous emission
enhancement factor and the extraction efficiency as a function of
frequency and of position of the emitting source. The calculated
enhancement factor is fed into an electronic simulator, which solves the coupled continuity equations for electrons and holes and Poisson's equation. We simulate a two-dimensional structure consisting of a photonic-crystal slab with a single-defect cavity, and investigate different pumping configurations for such a cavity.
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