KEYWORDS: Oscillators, Photons, Signal detection, Silicon, Modulation, Surface plasmons, Crystals, Electrooptic modulation, Near field optics, Laser frequency
Silicon one-dimensional optomechanical cavities offer a cost-effective and highly scalable solution for the study and implementation of non-linear phenomena. By modifying the refractive index of silicon through thermal or free-carrier effects, it becomes possible to optically drive these resonators into a state of high-amplitude and coherent self-sustained mechanical oscillation. The nonlinearity stemming from this amplification mechanism provides significant adaptability in adjusting the frequency of mechanical resonators, enabling experiments such as injection locking, synchronization, and the study of chaotic dynamics. In this work, we show different novel configurations for the synchronization between mechanical flexural modes of silicon nanobeams and their locking to an external reference signal. The results hold great promise for applications in the distribution of clock signals in future photonic integrated circuits, as well as for establishing extensive networks of optomechanical resonators for studying complex non-linear dynamics.
The physics taking place in optomechanical cavities can be used to operate on microwave signals in the optical domain, which belongs to the realm of microwave photonics. Here we review our recent work on the processing of microwave signals via optomechanical cavities built on silicon chips. Using a silicon optomechanical crystal cavity with a mechanical mode in the phonon lasing regime, we show that the generated microwave tone displays a phase noise low enough to be used in practical applications. We also demonstrate frequency conversion of realistic data streams making use of the inherent nonlinear behavior of the cavity. We also discuss how the long-term stability of the resulting optomechanical microwave oscillator can be improved by locking two different mechanical modes and bringing them to the phonon lasing states. These results suggest that ultra-compact optomechanical cavities show potential application in photonics-enabled next-generation wireless networks and satellite communications.
Silicon optomechanical (OM) cavities have been presented as relevant elements in microwave photonics and optical RF processing, particularly in applications requiring low-weight and compactness. In this work, we introduce and demonstrate a new functionality by employing a silicon OM crystal cavity operated in the phonon lasing regime for optical upconversion of a radio-frequency data signal employing orthogonal frequency division multiplexing (OFDM) modulation. The OM crystal cavity is created on suspended silicon nano-beams with one-dimensional (1D) periodicity with <10μm2 foot-print. The proposed OM crystal cavity operates as an optoelectronic oscillator at the GHz regime, with a low phase noise for the first harmonic at 3.9 GHz in the self-sustained oscillation regime. The OM crystal cavity characterization indicates that the optical resonance is centered at 1541.2±0.3 nm with a loaded optical quality factor Qo ≈ 4×103. Using such cavity we demonstrate successful upconversion of full-standard IEEE 802.16e WiMAX signals employing OFDM with QPSK modulation per-carrier over different bandwidths.
The realization of high-Q optomechanical cavities in silicon photonic crystals enables the coupling of near-infrared photons and GHz-frequency phonons in ultra-small volumes. Such coupling enables myriads of novel phenomena (resolved sideband cooling, optomechanically induced transparency, or phonon lasing) both in the linear and nonlinear regimes. Here, we report recent advances in silicon photonic crystal optomechanical cavities beyond the linear regimes. First, we show that by combining thermal effects, free-carrier-induced refraction and optomechanical coupling is it possible to attain different states in the cavity, ranging from chaos [1] to phonon lasing [2], being easy to switch from one state to another by using an external optical source [3]. We also show that using think mechanical links between adjacent optomechanical cavities enables to synchronize their mechanical motion via weak mechanical coupling [4]. These results can be easily upscaled to more than two cavities and are thus the first step towards realizing integrated networks of synchronized OM oscillators, which promise an enhancement of the performance of single nano-mechanical oscillator systems and enable a novel architecture for neuromorphic computing applications
References
[1] D. Navarro-Urrios et al.,” Nature Commun. 8, 14965 (2017).
[2] D. Navarro-Urrios et al, “A self-stabilized coherent phonon source driven by optical forces,” Sci. Rep. 5, 15733 (2015).
[3] J. Maire et al., “Optical modulation of coherent phonon emission in optomechanical cavities,” Arxiv pre-print on https://arxiv.org/abs/1802.01146
[4] M. F. Colombano et al., “Synchronization of optomechanical cavities by mechanical interaction,” submitted.
Dielectric and metallic nanostructures can be tailored to provide unusual interaction with light waves. For example, they support localized resonances highly sensitive to the surroundings, complex scattering patterns and ultrafast nonlinearities. When arranged in 2D or 3D lattices, they form metasurfaces and metamaterials that enable to manipulate free-space light beams at will. However, the properties of such nanostructures also manifest when isolated, which could be used to advance in the miniaturization of photonic integrated circuits beyond the diffraction limit as well as to achieve new functionalities not attainable in conventional integrated optics. This could lead to the paradigm of hybrid plasmonicphotonic circuits consisting of subwavelength processing units linked by lossless dielectric waveguides. Here, I will show efficient ways to integrate dielectric and metallic nanostructures with silicon waveguides. The resulting structures could be useful in biosensing, Raman spectroscopy or ultrafast all-optical switching. In addition, I will show that when the nanostructure is placed asymmetrically with respect to the waveguide axis, it gives rise to spin-orbit interaction. This effect enables new functionalities such as polarization synthesis or Stokes nanopolarimetry, which can be implemented on a silicon chip in ultra-small foot-prints.
Measuring the state of polarization (SoP) of light beams is of paramount importance in many scientific and technological
disciplines, including chemistry, biosensing, astronomy and optical communications. Commercial polarimeters are built
by using bulky and expensive optical elements, including half-wave plates or grid polarizers, with little prospect for
miniaturization. Inspired by the concept of spin-orbit coupling, here we introduce a nanophotonic polarimeter that
measures the full SoP – Stokes parameters – of a light beam over an ultrabroad wavelength range. The active region of
the device, formed by a metallic nanoantenna on top of a silicon waveguide crossing, is less than a square wavelength,
one order of magnitude smaller than polarimeters based on metasurfaces and many orders of magnitude smaller than
commercial devices. Our approach is universal and therefore applicable to any wavelength regime and technological
platform, opening a new route for miniaturized polarimeters.
Phoxonic crystals are periodic structures affecting simultaneously the propagation of light (photons) and sound (phonons) of similar wavelengths. For instance, by introducing periodicity of the order of the micron on semiconductor membranes, a phoxonic band gap for near infrared light and sound at GHz frequencies appears. The insertion of defects can give rise to the simultaneous localization of photons and phonons in cavities and waveguides. Moreover, new structures can be tailored to enhance the light-sound interaction in such small volumes. In this work, the last advances in phoxonic crystal structures (including the so-called optomechanical cavities) will be reviewed. Techniques to inject light and sound in phoxonic structures will be described. Future possible applications of phoxonic crystals, ranging from ultrasensitive sensing to all-optical information storage, will finally be introduced.
We study the gain in an acousto-optical cavity made of an indirect bandgap semiconductor with simultaneous
confinement of photons and photons. We obtain that the phonon confinement in such a cavity can give rise to optical
gain at room temperature.
We study the electromagnetic behavior of a structure consisting of coupled aluminum nanodisks on a silicon waveguide
at telecom wavelengths. Numerical simulations show that the fundamental TE-like waveguide mode excites a localized
magnetic plasmon resonance between adjacent nanodisks with suitable dimensions, leading to transmission dips. For a
sufficient number of disks (periodically distributed along the propagation direction), the structure supports a magnetic
mode arising from a magneto-inductive coupling between neighboring nanodisks, as revealed by an Eigenmode analysis.
The transmission response of the samples was measured for both polarizations through an end-fire set-up, confirming
that the strong resonances are only present for TE polarization. Measurements and simulations are in good agreement,
showing that the resonances strength is maximized for three coupled nanodisks.
We study the behavior of a two-level system inside a photonic cavity in which mechanical oscillations are induced. We
distinguish four different regimes: a) no interaction, b) optomechanical, c) atom-cavity-field coupling and d)
"competition". We observe how the revival and collapse processes become chaotic when the coupling constants of these
regimes are sufficiently high. Changes of the Rabi frequency and collapse and revival times as a consequence of the
injection of mechanical oscillations are discussed.
In this work, we present a Jaynes-Cummings model of an indirect bandgap semiconductor engineered to confine
simultaneously photons and phonons (acousto-optical cavity). From our theoretical analysis, the typical collapse-revival
behavior is obtained. Finally, we get an analytical approximate expression of Rabi frequency in such a system.
In this work, we show that closely-spaced gold nanohoops periodically distributed in a square lattice can provide
a strong magnetic response in the near infrared regime when illuminated under normal incidence (perpendicular
to the structure plane). Therefore, just a single metallic layer is needed to achieve the magnetic activity. A key
point to achieve this response is that the aspect ratio must be higher than 1. Transmission and reflection spectra
taken by means of a Fourier-Transform Infrared spectrometer show a strong absorbance peak at a wavelength that
can be tuned by modifying the hole radius of the nanohoops or the underlying dielectric substrate. Numerical
simulations show that at the resonance wavelength a virtual current loop is created, giving rise to a strong
magnetic moment and a large magnetic field enhancement in the space between nanohoops.
Sub-micron waveguides and cavities have been shown to produce the confinement of elastic and optical waves in the
same devices in order to benefit from their interaction. It has been shown that square and honeycomb lattices are the
most suitable to produce simultaneous photonic and phononic band gaps on suspended silicon slabs. The introduction of
line defects on such "phoxonic" (or optomechanical) crystals should lead to an enhanced interaction between confined
light and sound. In this work we report on the experimental measurements of light guiding through waveguides created
in these kinds of two-dimensional photonic crystal membranes. The dimensions of the fabricated structures are chosen to
provide a "phoxonic" bandgap with a photonic gap around 1550 nm. For both kinds of lattice, we observe a high-transmission
band when introducing a linear defect, although it is observed for TM polarization in the honeycomb lattice
and for TE in the square. Using the plane-wave expansion and the finite element methods we demonstrate that the guided
modes are below the light line and, therefore, without additional losses beside fabrication imperfections. Our results lead
us to conclude that waveguides implemented in honeycomb and square lattice "phoxonic" crystals are a very suitable
platform to observe an enhanced interaction between propagating photons and phonons.
Periodic media offer impressive opportunities to manipulate the transport of classical waves namely light or sound.
Elastic waves can scatter light through the so-called acousto-optic interaction which is widely used to control
light in telecommunication systems and, additionally, the radiation pressure of light can generate elastic waves.
Concurrent control of both light and sound through simultaneous photonic-phononic, often called phoxonic, bandgap
structures is intended to advance both our understanding as well as our ability to manipulate light with
sound and vise versa. In particular co-localization of light and sound in phoxonic cavities could trigger nonlinear
absorption and emission processes and lead to enhanced acousto-optic effects. In the present communication,
we present our efforts towards the design of different phoxonic crystal architectures such as three-dimensional
metallodielectric structures, two-dimensional patterned silicon slabs and simple one-dimensional multilayers,
and provide optimum parameters for operation at telecom light and GHz sound. These structures can be used
to design phoxonic cavities and study the acousto-optic interaction of localized light and sound, or phoxonic
waveguides for tailored slow light-slow sound transport. We also discuss the acousto-optic interaction in onedimensional
multilayer structures and study the enhanced modulation of light by acoustic waves in a phoxonic
cavity, where a consistent interpretation of the physics of the interaction can be deduced from the time evolution
of the scattered optical field, under the influence of an acoustic wave.
A simple configuration for achieving a radio frequency transparent 90° hybrid, for broadband QAM wireless systems
using silicon photonics is proposed. The device consists of a high Q ring resonator which induces an optical 90° phase
shift between two adjacent resonant wavelengths. When these optical carriers are modulated by an RF carrier the
resulting device behaves as an RF 90° hybrid. Numerical simulations of the phase shift were performed on a 40 GHz
carrier, and to demonstrate the frequency transparency phase shift simulations was also performed at a carrier frequency
of 60 GHz. One of the main applications of such a device is the generation of millimeter wave 10 Gb/s wireless based on
quadrature amplitude modulation.
We prove theoretically that it is possible to build embedded reflectionless squeezers/expanders using transformation
optics. We illustrate the potential of this finding by proposing an application in which the squeezer is a key element: an
ultra-short perfect coupler for high-index nanophotonic waveguides.
Exploiting the concept of internal surface plasmon polariton (I-SPP) resonances, which appear at non-single metallic
film stacks, we have designed a metamaterial showing a negative effective refractive index within a large bandwidth.
The designed structure consists of an arrangement of several fishnet layers. By properly adjusting the lattice constant and
the thickness of the dielectric slab of the fishnet, an I-SPP mode can be excited at a certain frequency giving rise to a
negative effective refractive index. Thus, the combination of several fishnet layers, each one of them configured to
excite an I-SPP at a different frequency, enables us to extend the bandwidth at which a negative effective refractive
index is achieved, as long as the selected resonances are close enough. Specifically, from a subwavelength chain of two
fishnet layers, the retrieved effective parameters show a negative index behavior in a frequency span of about 44THz
centered at 210THz, owing to the fact that an I-SPP is excited in each fishnet layer at slightly different frequencies.
In this work, we report theoretical and experimental results on the use of Cadmium Telluride (CdTe) doped with Zinc
(Zn) as core material for the development of all-optical photonic devices. We include the design of optical waveguides
for strong field confinement, technological processes to grow CdTe on 6" or 8" wafers (suitable for high-volume
manufacturing) as well as the fabrication and optical characterization of optical waveguides with a CdTe core.
In this work, several building blocks for high-performance all-optical switching on silicon are addressed. The FP6-PHOLOGIC approach is based on exploiting the nonlinear properties of silicon nanocrystals embedded in slot
waveguides, in which propagating light is highly confined.
Transmission of light through linear defects in two-dimensional (2D) photonic crystals has been already successfully demonstrated in two ways: numerical simulations and experimental measurements. Recently, novel waveguides have been proposed in which the propagation of photons is performed via hopping due to overlapping between nearest-neighbors defect cavities. These waveguides are commonly referred to as coupled-cavity waveguides (CCW). In this work, we present a comprehensive analysis of the light transmission (TM modes) in CCW's created in hexagonal 2D photonic crystals made of high-index dielectric rods. Numerical simulations of the transmission are performed using a 2D Finite-Difference Time-Domain method. A plane wave algorithm and a simple one-dimensional (1D) tight-binding model are employed to describe the miniband which allows the light transport. It is shown that modifying the individual cavities along the CCW one can control the average frequency and the dispersion relation of the miniband. The results also show that this novel guiding method can be used to develop 1310nm/1550nm Coarse-WDM optical demultiplexers employing bended waveguides.
When rows of cylinders are periodically removed from a hexagonal array of dielectric cylinders, a new two-dimensional (2D) photonic crystal (PC) arises. The new structure consists of a lattice of vacancies embedded in the initial hexagonal lattice. We called it Suzuki Phase because it remains similar structures discovered in the 60's by K. Suzuki studying alkali halides. A plane-wave algorithm as well as a 2D finite difference-time-domain method has been employed to study the photonic properties of this PC as a function of the filling fraction (f) in the case of high dielectric cylinders ((epsilon) equals 13.6) in air. For TM- modes, it is shown that in a certain range of f an isolated miniband appears in the gap of the initial hexagonal lattice. The miniband, which is created by the coupling of defect states, is described by a tight-binding formalism with two parameters. Also, the frequencies of the two possible vacancy defects in the SP have been obtained and their symmetry analyzed.
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