Photonic Crystal Geometry Research Articles

Broadband transparent Huygens' spaceplates

Spaceplates have emerged in the context of nonlocal metasurfaces, enabling the compression of optical systems by minimizing the required empty space between their components. In this work, we design and analyze spaceplates that support resonances with opposite symmetries, operating under the so-called Huygens’ condition. Using the temporal coupled-mode theory, we demonstrate that the spatial compression provided by Huygens’ spaceplates is twice that of conventional single-resonance counterparts. Additionally, they can support broader operational bandwidths and numerical apertures, facilitating the reduction of chromatic aberrations. Moreover, Huygens’ spaceplates maintain nearly full transparency over a wide frequency and angular range, allowing their straightforward cascading for multi-frequency broadband operation. Finally, we propose a physical implementation of a Huygens’ spaceplate for optical frequencies based on a photonic crystal slab geometry.

Experimental demonstration of a nanobeam Fano laser.

Microscopic single-mode lasers with low power consumption, large modulation bandwidth, and ultra-narrow linewidth are essential for numerous applications, such as on-chip photonic networks. A recently demonstrated microlaser using an optical Fano resonance between a discrete mode and a continuum of modes to form one of the mirrors, i.e., the so-called Fano laser, holds great promise for meeting these requirements. Here, we suggest and experimentally demonstrate what we believe is a new configuration of the Fano laser based on a nanobeam geometry. Compared to the conventional two-dimensional photonic crystal geometry, the nanobeam structure makes it easier to engineer the phase-matching condition that facilitates the realization of a bound-state-in-the-continuum (BIC). We investigate the laser threshold in two scenarios based on the new nanobeam geometry. In the first, classical case, the gain is spatially located in the part of the cavity that supports a continuum of modes. In the second case, instead, the gain is located in the region that supports a discrete mode. We find that the laser threshold for the second case can be significantly reduced compared to the conventional Fano laser. These results pave the way for the practical realization of high-performance microlasers.

Suppression of filamentation in refraction index modulated Kerr media

In this work, the possibility of using periodic refractive index modulation – photonic crystals – to counteract the spatial Kerr effect, thus suppressing filamentation, is shown. Numerical simulations by beam propagation method indicate that fine control of the spatial dispersion can be achieved by tuning the photonic crystal geometry and projecting to the appropriate Bloch mode branch by chirping the longitudinal period. Controlling the spatial dispersion allows management of the strength of diffraction. Furthermore, it is shown that photonic crystals can be engineered with such geometry that the designed diffraction compensates for self-focusing, effectively delaying, or entirely eliminating beam filamentation.

Symmetry Criterion and Far-Field Control of Photonic-Crystal Surface-Emitting Lasers

Photonic-crystal surface-emitting lasers (PCSELs) have led to amazing results in overcoming the divergence limitation of semiconductor lasers. However, so far, the physical mechanism behind this promising control of far-field characteristics is still unclear. Here, we perform a theoretical study of the mechanism of the influence of photonic crystal (PhC) geometry on the far field of PCSELs. The perspective from group theory is adopted in our analysis. We explore the function of symmetry in the formation of the far-field pattern (FFP) and clarify the roles played by various PhC parameters in this process. Through our analysis, a symmetry criterion to design PCSELs with single-lobed narrow beams is shown, where an asymmetric in-plane PhC structure, a large vertical confinement factor, and a sufficient number of periods are required. Our results reveal the physical origin of the narrow beam of PCSELs, which can even reach 0.1° with a PhC cavity size of over 1000 μm at a lasing wavelength of 940 nm.

CMOS-compatible compact optical isolator based on space-time modulated coupled resonators.

This work shows how phase-shifted time-modulations applied to two or more near-field coupled resonators can be engineered to achieve magnet-free optical isolation in a compact footprint. Because of the strong light-matter interaction afforded by high quality factor resonant cavities, only modest modulation amplitudes are required. A coupled mode theory model is developed which is subject to simulated annealing to optimize device performance. Then it is shown via finite-difference time-domain simulation how the device may be implemented in a one dimensional photonic crystal geometry etched in a silicon ridge waveguide. These devices can be implemented in native silicon with standard electrical contacts thereby maintaining CMOS-compatible fabrication without the need for additional specialized materials. Isolation ratios of over 40 dB with insertion loss less than 1 dB using modulation frequencies under 25 GHz are achievable in this device platform.

Computational Modelling of Doubly‐Photopolymerized Holographic Biosensors

AbstractHolographic sensors are optical devices capable of tuning reflection wavelength, dependent upon nanostructured variations in refractive index. Computational modelling is utilized to simulate the recording and swelling characteristics of developed doubly photopolymerized (DP) holographic sensors. The holographic devices simplify fabrication processes, reduce financial costs, and improve biocompatibility. A holographic grating is achieved through in situ photopolymerization of a highly crosslinked polymer to produce nanostructured refractive index modulation. The unique swelling characteristics DP holographic sensors possess necessitate the development of system‐specific computational modelling. Hydrogel parameters, including film thickness, refractive index change, layer number, and external medium refractive index are examined for their effect on reflection spectra. Optimized computational models are utilized to study the effect of differential swelling rates of individual layer spacings on sensor response, indicating an idealized reduction in a swelling of 50% for the highly crosslinked region. A 2D photonic crystal geometry with additional periodicity is developed, to inform further sensor design opportunities. Optimized parameters for both 1D and 2D photonic structures will assist the further development of DP holographic sensors.

Bio-Molecular Applications of Recent Developments in Optical Tweezers.

In the past three decades, the ability to optically manipulate biomolecules has spurred a new era of medical and biophysical research. Optical tweezers (OT) have enabled experimenters to trap, sort, and probe cells, as well as discern the structural dynamics of proteins and nucleic acids at single molecule level. The steady improvement in OT’s resolving power has progressively pushed the envelope of their applications; there are, however, some inherent limitations that are prompting researchers to look for alternatives to the conventional techniques. To begin with, OT are restricted by their one-dimensional approach, which makes it difficult to conjure an exhaustive three-dimensional picture of biological systems. The high-intensity trapping laser can damage biological samples, a fact that restricts the feasibility of in vivo applications. Finally, direct manipulation of biological matter at nanometer scale remains a significant challenge for conventional OT. A significant amount of literature has been dedicated in the last 10 years to address the aforementioned shortcomings. Innovations in laser technology and advances in various other spheres of applied physics have been capitalized upon to evolve the next generation OT systems. In this review, we elucidate a few of these developments, with particular focus on their biological applications. The manipulation of nanoscopic objects has been achieved by means of plasmonic optical tweezers (POT), which utilize localized surface plasmons to generate optical traps with enhanced trapping potential, and photonic crystal optical tweezers (PhC OT), which attain the same goal by employing different photonic crystal geometries. Femtosecond optical tweezers (fs OT), constructed by replacing the continuous wave (cw) laser source with a femtosecond laser, promise to greatly reduce the damage to living samples. Finally, one way to transcend the one-dimensional nature of the data gained by OT is to couple them to the other large family of single molecule tools, i.e., fluorescence-based imaging techniques. We discuss the distinct advantages of the aforementioned techniques as well as the alternative experimental perspective they provide in comparison to conventional OT.

Topology-optimized dual-polarization Dirac cones

We apply a large-scale computational technique, known as topology optimization, to the inverse design of photonic Dirac cones. In particular, we report on a variety of photonic crystal geometries, realizable in simple isotropic dielectric materials, which exhibit dual-polarization and dual-wavelength Dirac cones. We demonstrate the flexibility of this technique by designing photonic crystals of different symmetry types, such as ones with four-fold and six-fold rotational symmetry, which possess Dirac cones at different points within the Brillouin zone. The demonstrated and related optimization techniques could open new avenues to band-structure engineering and manipulating the propagation of light in periodic media, with possible applications in exotic optical phenomena such as effective zero-index media and topological photonics.

Enhanced geometries of macroporous silicon photonic crystals for optical gas sensing applications

A macroporous silicon photonic crystal is designed and optimized theoretically for its use in gas sensing applications and IR optical filters. Light impinges perpendicularly onto the sample surface (vertical propagation) so a three-dimensional (3d) structure is used. For gas sensing, a sharp resonance is desired in order to isolate an absorption line of the gas of interest. The high Q-factors needed mandate the use of a plane defect inside the PhC to give rise to a resonant mode inside the bandgap tuned to the gas absorption line. Furthermore to allow gas passage through the device, an open membrane is required. This can affect the mechanical resilience. To improve the strength of the photonic crystal the pores are extended after the “active” 3d part. The number of modulations, and the extension length have been optimized to obtain the largest Q-factor with reasonable transmitted power. These proposed structures have been experimentally performed, probing an enhancement of almost an order of magnitude in the Q-factor in respect with the basic case. Simulations considering CO2 have been performed showing that the proposed structures are promising as precise optical gas sensors.

Design of nanowire-induced nanocavities in grooved 1D and 2D SiN photonic crystals for the ultra-violet and visible ranges.

Nanowire-induced SiN photonic crystal (PhC) nanocavities specifically designed for the ultra-violet and visible range are investigated by three-dimensional finite-difference time-domain calculations. As opposed to their silicon PhC counterpart, we find that the formation of nanowire-induced two-dimensional (2D) SiN PhC nanocavities is more challenging because of the low refractive index of SiN. We thus discuss optimization strategies to circumvent such difficulties and we investigate the influence of critical design parameters such as PhC geometry, as well as nanowire geometry and position. We also propose a novel nanowire-induced cavity design based on one-dimensional (1D) nanobeam PhCs. We finally report on nanowire-induced nanocavity designs in 1D (resp. 2D) PhCs presenting quality factors as high as Qc = 5.1 x 104 (resp. Qc = 2.5 x 104 with a mode volume Vm=1.8(λ/nrNW)3 (resp. Vm=5.1(λ/nrNW)3), which show good prospects for light-matter interaction in the near-ultraviolet and visible ranges.

One-dimensional photonic crystals with highly Bi-substituted iron garnet defect in reflection polar geometry

It is represented the results of modelling of magnetooptical properties in reflection polar geometry of one-dimensional photonic crystal, in which highly Bi-substituted iron garnet defect of composition Bi1.0Y0.5Gd1.5Fe4.2Al0.8O12 / Bi2.8Y0.2Fe5Oi2 is located between the dielectric Bragg mirrors (SiO2 / TiO2)m (were m is number of layer pairs) and buffer SiO2 and gold top layers of different thicknesses is placed on structure. The modification of spectral line- shapes of microcavity and Tamm plasmon-polariton modes depending on m is found.

Conical photonic crystals for enhancing light extraction efficiency from high refractive index materials.

We propose, analyze and optimize a two-dimensional conical photonic crystal geometry to enhance light extraction from a high refractive index material, such as an inorganic scintillator. The conical geometry suppresses Fresnel reflections at an optical interface due to adiabatic impedance matching from a gradient index effect. The periodic array of cone structures with a pitch larger than the wavelength of light diffracts light into higher-order modes with different propagating angles, enabling certain photons to overcome total internal reflection (TIR). The numerical simulation shows simultaneous light yield gains relative to a flat surface both below and above the critical angle and how key parameters affect the light extraction efficiency. Our optimized design provides a 46% gain in light yield when the conical photonic crystals are coated on an LSO (cerium-doped lutetium oxyorthosilicate) scintillator.

Stress sensing characteristics of two-dimensional photonic crystal cross-waveguide geometry

A two-dimensional photonic crystal cross-waveguide consisting of dielectric rods array immersed in air is designed. The optical field pattern and transmission spectra are analyzed by finite difference time domain method. The propagation characteristics and stress sensing behavior of photonic crystal are emphasized through analyzing the linear shift of central wavelength. The results show that optimized cross-waveguide can be used as a high performance dual-channel stress sensing device based on tunable photonic crystal structure.

Nanoimprinted superlattice metallic photonic crystal as ultraselective solar absorber

A two-dimensional superlattice metallic photonic crystal (PhC) and its fabrication by nanoimprint lithography on tantalum substrates are presented. The superior tailoring capacity of the superlattice PhC geometry is used to achieve spectrally selective solar absorption optimized for high-temperature and high-efficiency solar-energy-conversion applications. The scalable fabrication route by nanoimprint lithography allows for a high-throughput and high-resolution replication of this complex pattern over large areas. Despite the high fill factor, the pattern of polygonal cavities is accurately replicated into a resist that hardens under ultraviolet radiation over an area of 10 mm2. In this way, cavities of 905 nm and 340 nm width are achieved with a period of 1 μm. After pattern transfer into tantalum via a deep reactive ion-etching process, the achieved cavities are 2.2 μm deep, separated by 85–95 nm wide ridges with vertical sidewalls. The room-temperature reflectance spectra of the fabricated samples show excellent agreement with simulated results, with a high spectral absorptance approaching blackbody absorption in the range from 300 to 1900 nm and a steep cutoff. The calculated solar absorptivity of this superlattice PhC is 96% and its thermal transfer efficiency is 82.8% at an operating temperature of 1500 K and an irradiance of 1000 kW/m2.

The polaritonic spectrum of two-dimensional photonic crystals based on uniaxial polar materials

We investigate the dispersion relations of two-dimensional photonic crystals made of cylindrical rods of uniaxial polar materials that exhibit transverse phonon-polariton excitations. The rods are considered to be embedded in a dielectric background. The photonic properties are obtained with the use of the finite-difference time domain (FDTD) method and the auxiliary differential equation (ADE) technique. The anisotropy of the dielectric function is explicitly considered using an empirical approach that assigns different weights to contributions of the parallel (z) and transversal (t) polaritonic relations. The effective dielectric function is then expressed as a weighted combination of the longitudinal and transversal components: ε(ω)=αzεz(ω)+αtεt(ω). Different sets of values of the coefficients αz and αt have been considered. The frequencies of the allowed electromagnetic modes are determined as the local maxima of the spectral analysis using a fast Fourier transform (FFT). The particular case of a square photonic crystal superlattice geometry is analyzed, and input data corresponding to phonon frequencies of wurtzite nitride semiconductors is used. It is shown that larger values of the quantity |νz,T-νt,T| are desirable if the associated dielectric anisotropy is used as a tool for tuning photonic properties in the system.

Design of a high-transmission waveguide bend for Kagome photonic crystal lattice

In this paper, a 60° photonic crystal bending geometry for TM modes of a Kagome lattice is proposed. A two dimensional array of dielectric rods with an effective refractive index equal to 3.25 is used for this purpose. The proposed bending structure has a tapered topology which increases the transmission and decreases the reflection to the input waveguide. A bandwidth of 4nm, where the central wavelength is at 1330nm is obtained. Bandwidth is defined as the region where the transmission ratio is higher than 0.8 or equally 1dB. The simulation is performed with the help of two numerical algorithms which are called finite difference time domain (FDTD) and plane wave expansion (PWE) methods.

InP-based photonic crystal electro-optic modulator

In this article, we designed a Mach – Zehnder Interferometer (MZI) based modulator based on photonic crystal at 1.55μm wavelength. The layer stack of the slab waveguide consists of the InP/InGaAsP/InP clad/film/substrate layers. The geometry of photonic crystal is pillar-based in the air background with a square lattice. The radius of pillars is determined 110nm at a lattice constant of 580nm by determining the gap map and band diagram. Band diagram shows that only transverse magnetic (TM) mode can propagate through the waveguides. The designed Y-junction is based on the directional coupler. The optimized structure of the Y-junction power splitter shows a 40% shorter coupling length and about 10% improvement in spectral transmission at 1550nm central wavelength.The MZI-based modulator is consisted of two optimized Y-junction splitter and combiner with two arms. The driving reverse bias voltage is applied on an arm of the MZI. The reverse bias voltage of 7V creates a change in refractive index of about 1.1×10−3. This change is sufficient to obtain a total phase difference of π between both arms. The field distributions reveal that the wave from the output port can interfere constructively or destructively.

Bloch Modes and Evanescent Modes of Photonic Crystals: Weak Form Solutions Based on Accurate Interface Triangulation

We propose a new approach to calculate the complex photonic band structure, both purely dispersive and evanescent Bloch modes of a finite range, of arbitrary three-dimensional photonic crystals. Our method, based on a well-established plane wave expansion and the weak form solution of Maxwell’s equations, computes the Fourier components of periodic structures composed of distinct homogeneous material domains from a triangulated mesh representation of the inter-material interfaces; this allows substantially more accurate representations of the geometry of complex photonic crystals than the conventional representation by a cubic voxel grid. Our method works for general two-phase composite materials, consisting of bi-anisotropic materials with tensor-valued dielectric and magnetic permittivities ε and μ and coupling matrices ς. We demonstrate for the Bragg mirror and a simple cubic crystal closely related to the Kelvin foam that relatively small numbers of Fourier components are sufficient to yield good convergence of the eigenvalues, making this method viable, despite its computational complexity. As an application, we use the single gyroid crystal to demonstrate that the consideration of both conventional and evanescent Bloch modes is necessary to predict the key features of the reflectance spectrum by analysis of the band structure, in particular for light incident along the cubic [111] direction.

Low-cross-talk and high-contrast all optical bistable switching based on coupled defects in a nonlinear photonic crystal cross-waveguide geometry

We present a photonic crystal all-optical bistable switch configuration consisting of crossing perpendicular waveguides geometry with instantaneous Kerr nonlinearity. A microcavity is added to the signal waveguide which is coupled with the defect at the waveguide intersection to increase the transmission contrast. We demonstrate with the finite-difference time-domain method that such a configuration can function as a low crosstalk and high contrast all-optical switch.

Superlattice photonic crystal as broadband solar absorber for high temperature operation.

A high performance solar absorber using a 2D tantalum superlattice photonic crystal (PhC) is proposed and its design is optimized for high-temperature energy conversion. In contrast to the simple lattice PhC, which is limited by diffraction in the short wavelength range, the superlattice PhC achieves solar absorption over broadband spectral range due to the contribution from two superposed lattices with different cavity radii. The superlattice PhC geometry is tailored to achieve maximum thermal transfer efficiency for a low concentration system of 250 suns at 1500 K reaching 85.0% solar absorptivity. In the high concentration case of 1000 suns, the superlattice PhC absorber achieves a solar absorptivity of 96.2% and a thermal transfer efficiency of 82.9% at 1500 K, amounting to an improvement of 10% and 5%, respectively, versus the simple square lattice PhC absorber. In addition, the performance of the superlattice PhC absorber is studied in a solar thermophotovoltaic system which is optimized to minimize absorber re-emission by reducing the absorber-to-emitter area ratio and using a highly reflective silver aperture.