Photonic Crystal Slab Research Articles

MULTISTABILITY IN A CHIRAL SEMICONDUCTOR MICROCAVITY

The features of the bi- and multistability effects in the semiconductor Bragg microcavity with chiral photonic crystal slab on the upper mirror are investigated theoretically. It is shown that the response of such a chiral structure under a linearly polarized coherent resonant pump demonstrates sharp multistable transitions with abrupt jumps of the exciton intensity and degree of circular polarization. It is shown that of the thresholds of bistable transitions in the system with different sense of circular polarization differ slightly, i.e. in case of a non-optimized structure, we can expect to obtain even a larger amplitude of the jumps of the degree of circular polarization of the excitonic response due to the multistability than in a specially optimized chiral structure with a high degree of circular polarization at low pump intensity.

A lightweight neural network compatible with high-correlation broadband optical filters in computational spectroscopy

Computational spectrometer is powerful for portable in situ applications. To achieve better signal to noise ratio (SNR), broadband optical filters are more commonly introduced. As for conventional sparse signals reconstruction algorithms like gradient projection, its performance relies on the low correlation property of optical filters, which poses severe challenge for filters design and fabrication. In this study, we propose a lightweight neural network(NN) named HBOF-Net that can be compatible with high-correlation broadband optical filters to renovate the situation. The spectra reconstruction performs very well under a group of high correlated optical filters and multiscale training datasets. Firstly, we propose a group of various structure design of photonic crystal(PC) slabs and select every 16 to construct a spectrometer megapixel randomly. Then we extract different percentage of spectra from CAVE and ICVL dataset to build up multiscale training dataset groups. Afterwards, the training of the network has been carried out and the testing curve indicate there is no over fitting. A total of 16 trainings have been carried out and the results have been compared and discussed. Lastly, the experiment demonstrates that the mean square error(MSE) of the reconstructed spectra by HBOF-Net can reach 1E-6 by maximum correlation coefficient of 0.93 under multi-scale datasets which is 90%, 10%, 5%, and 3% of the entire dataset. This study successfully prove that NN can be compatible with high correlated optical filters, which is a significant leap in the development of computational spectrometers. It provides a good reference for computational spectrometers based on broadband and high-correlation optical filters.

Topological band gaps and edge modes materialized by symmetric silicon-on-insulator photonic crystal slabs in the mid-IR range

We successfully fabricated silicon-on-insulator (SOI) photonic crystal (PhC) slabs in which electromagnetic topological band gaps and edge modes were materialized for symmetric transverse-electric-like modes. Because the structure of our specimens can be regarded as symmetric about the horizontal middle plane of the PhC slab, their symmetric and antisymmetric eigenmodes were rigorously separated, so we could achieve genuine photonic band gaps for the former. We fabricated those specimens by electron beam lithography of the top Si layer of SOI wafers and successive plasma-enhanced chemical vapor deposition of a ${\mathrm{SiO}}_{2}$ capping layer. We confirmed the complete common band gap of topologically trivial and nontrivial PhCs and the topological edge modes on the boundary between them by angle-resolved reflection spectroscopy in the mid-infrared range. This is an observation of the common band gap and edge modes that materialized in symmetric PhC slabs without a membrane structure, that is, without the etching of a sacrificial layer under the PhC to increase the refractive-index contrast and the width of the common band gap.

Label-free multiplex sensing from buffer and immunoglobulin G sensing from whole blood with photonic crystal slabs using angle-tuning of an optical interference filter.

Direct detection of biomarkers from unpurified whole blood has been a challenge for label-free detection platforms, such as photonic crystal slabs (PCS). A wide range of measurement concepts for PCS exist, but exhibit technical limitations, which render them unsuitable for label-free biosensing with unfiltered whole blood. In this work, we single out the requirements for a label-free point-of-care setup based on PCS and present a wavelength selecting concept by angle tuning of an optical interference filter, which fulfills these requirements. We investigate the limit of detection (LOD) for bulk refractive index changes and obtain a value of 3.4 E-4 refractive index units (RIU). We demonstrate label-free multiplex detection for different types of immobilization entities, including aptamers, antigens, and simple proteins. For this multiplex setup we detect thrombin at a concentration of 6.3 µg/ml, antibodies of glutathione S-transferase (GST) diluted by a factor of 250, and streptavidin at a concentration of 33 µg/ml. In a first proof of principle experiment, we demonstrate the ability to detect immunoglobulins G (IgG) from unfiltered whole blood. These experiments are conducted directly in the hospital without temperature control of the photonic crystal transducer surface or the blood sample. We set the detected concentration levels into a medical frame of reference and point out possible applications.

Observation of terahertz transition from Fano resonances to bound states in the continuum.

Bound states in the continuum (BIC) in metamaterials have recently attracted attention for their promising applications in photonics. Here, we investigate the transition from Fano resonances to BIC, at terahertz (THz) frequencies, of a one-dimensional photonic crystal slab made of rectangular dielectric rods. Simulations performed by an analytical exact solution of the Maxwell equations showed that symmetry-protected, high-quality factor (Q), BIC emerge at normal incidence. For non-normal incidence, BIC couple with the freely propagating waves and appear in the scattering field as a Fano resonance. Simulations were verified by realizing the photonic crystal slab by 3D-printing technique. THz time-domain spectroscopy measurements as a function of the incidence angle matched the simulation to good accuracy and confirmed the evolution of Fano resonances to high-Q resonances typical of BIC. These results point out the design of highly sensitive and low-cost THz devices for sensing for a wide range of applications.

Superimposition of topological charges between vortex beams and singular points of photonic crystal slabs

The bound state in the continuum (BIC) is a singular point of polarization in the momentum space of a periodic structure. It has been demonstrated that vortex beams can be generated by utilizing the polarization vortex around BIC based on the geometric phase in momentum space. In this letter, we propose a reflective photonic crystal (PhC) to generate a vortex beam and demonstrate the superimposition of topological charges in the momentum space between PhC and the vortex beams. In addition to BICs, we demonstrate that other singularities like degenerate points can also be used to generate vortex beams.

Experimental Realization of Topologically‐Protected All‐Optical Logic Gates Based on Silicon Photonic Crystal Slabs

AbstractAs the field of topological photonics has developed over the past 10 years, it has been proven that the combination of topology and photons is very beneficial to the design of robust optical devices against some disturbances. However, most of the work on robust optical logic devices stays at the theoretical level. There are very few topologically‐protected logic devices fabricated in experiments. Here, the experimental fabrication of a series of topologically‐protected all‐optical logic gates is reported. Seven topologically‐protected all‐optical logic gates (OR, XOR, NOT, XNOR, NAND, NOR, and AND) are fabricated on silicon photonic platforms, which show strong robustness even if some disorders exist. These robust logic devices are potentially applicable in future optical signal processing and computing applications.

Design of a room-temperature topological exciton-polariton laser in a ZnO/TiO2 photonic crystal slab

We propose theoretically a scheme to get a room-temperature 2D topological exciton-polariton laser with propagating topological lasing modes. The structure uses guided modes in a photonic crystal slab. A ZnO layer provides strong excitonic resonances stable at room temperature. It is capped by a TiO$_2$ layer pierced by a triangular lattice. The exciton-polariton modes of the 3D structure are computed by solving numerically Maxwell's equations including the excitonic response. The designed triangular lattice shows a transverse electric gap. The triangular lattice is shown to be the limit of a staggered honeycomb lattice when one of the sub-lattices vanishes. Its topology can be characterized by symmetry indicators. The interface between two shifted triangular lattices supports two counter-propagating modes lying in the gap of the bulk modes. The interface states are analogous to quantum pseudospin Hall interface states. These modes show orthogonal polarizations. They can be selectively excited using polarized excitation and are well-protected from back-scattering. These modes can benefit from the exciton-polariton gain at room temperature because of their sufficiently large exciton fraction and favorable position in energy. The strong localization of these propagating modes makes them suitable to host topological lasing triggered by a non-resonant pump localized on the interface.

Visualization of photonic band structures via far-field measurements in SiNx photonic crystal slabs

Band structures of the photonic crystal slabs play a significant role in manipulating the flow of light and predicting exotic physics in photonics. In this Letter, we show that the key features of photonic band structures can be achieved experimentally by the polarization- and momentum-resolved photoluminescence spectroscopy utilizing the light emission properties of SiNx. The two-dimensional spectra clearly reveal the energy-momentum dispersion of band structures, which is in perfect agreement with the simulation results. The isofrequency contours can be measured easily by adding a bandpass filter with a desired photon energy. Furthermore, it is convenient to observe clearly and directly the optical singularity—the optical bound states in the continuum featured by dark point in three-dimensional photoluminescence spectra. The polarization-resolved isofrequency contours clearly show that this dark point is the center of an azimuthally polarized vortex. Finally, the helical topological edge states can be easily observed in photonic topological insulators with deformed hexagonal lattices. Our work provides a simple and effective approach for exploring topological photonics and other intriguing phenomena hidden in the photonic crystal slabs.

Modulation of the Multiple Bound States in the Continuum of the All-Dielectric Metasurface

Based on the ideas of modulation of multiple BICs, ultrahigh Q-factor resonance has been realized by array of planar metasurfaces with H-shaped nanoholes, perforated on a Si3N4 photonic crystal slab surrounded by a silica medium. Multiple BICs of both at Γ BICs and off Γ BICs are obtained. Two BICs at the Γ point show up in the lower band of the visible light, and they can be tuned by the center position and hole size of each part of the H-shaped, the refractive index of the surrounding medium, and the lattice constant. In detail, they move apart from each other or shift together with a fixed distance, with the structure parameters changing. On the other hand, the off Γ BICs behave only in the form of monotonously shifting. The at Γ BICs transform from BICs to quasi-BICs obviously with the broken symmetry of the H-shaped nanohole; however, its influence on off Γ BICs is insignificant. BICs in both types are ultra-sensitive to the surrounding medium, and the Q-factor of the BICs reaches a value more than 106. The results here are helpful in the design of photonic filters and sensors.

Twist-tunable moiré optical resonances

Multilayer stacks of twisted optical metasurfaces are considered as a prospective platform for chiral nanophotonic devices. Such structures are primarily used for the realization of circularly polarized light sources, artificial optical rotation, and circular dichroism. At the same time, the behavior of their hybrid photonic modes is strongly affected by the moir\'e-pattern of superimposed periodic constituents. In this work, we show that moir\'e-periodicity in bilayer dielectric photonic crystal slabs leads to an arise of unlimitedly narrow optical resonances, which are very sensitive to the relative twist and gap width between the sublayers. We demonstrate the structure providing twist-tuning of the hybrid mode wavelength in the range of 300--600~nm with quality factor varying from~$10^2$~up~to~$10^5$ correspondingly. The obtained results pave the wave for the utilization of moir\'e-assisted effects in multilayer photonic crystal slabs.

Experimental Observation of Vector Bound States in the Continuum

AbstractBound states in the continuum (BICs) provide an important way for creating photonic devices reliant upon optical modes with high Q‐factors. However, current experimental implementations of BICs in photonic crystal (PhC) slabs only possess a certain polarization, which fundamentally restricts their use to surface‐enhanced applications assisted by scalar‐polarized near fields. In this work, vector BICs in PhC slabs are theoretically designed and experimentally realized. In theory, the symmetry‐protected vector BICs can be constructed by tuning geometric parameters of the PhC slab, where the coalescence of eigenfrequencies for a pair of TE‐ and TM‐like modes appears. It is found that the homogeneous superchiral fields with three orders of magnitude larger than circular polarized lights can be achieved, assisted by vector quasi‐BICs. Furthermore, Si3N4 PhC slabs are experimentally fabricated and their band structures are measured. It is shown that a pair of symmetry‐protected BICs with TE‐ and TM‐like polarizations can coalesce with each other at a suitable value for the diameter of array holes, indicating the realization of vector BICs. This work can possess important applications in the field of ultrasensitive detections of molecular chirality.

A Terahertz Circulator Based on Magneto Photonic Crystal Slab

In this paper, a terahertz circulator based on a magneto photonic crystal slab is envisaged. The triangular lattice photonic crystals with a line defect waveguide were constructed on an Al2O3 ceramic slab. Two cylindrical ferrites and two copper-clad plates in the junction of the Y-shaped wave-guide worked as a magneto-optical cavity resonator to approve the nonreciprocal function. In the working frequency range, 0.212–0.238 THz, the isolation of the circulator was better than 20 dB, and the insertion loss was better than 1 dB. The designed circulator based on the magneto photonic crystal slab experienced low loss and a wide bandwidth that satisfied its use in the THz application.

Arbitrarily polarized bound states in the continuum with twisted photonic crystal slabs

Arbitrary polarized vortex beam induced by polarization singularity offers a new platform for both classical optics and quantum entanglement applications. Bound states in the continuum (BICs) have been demonstrated to be associated with topological charge and vortex polarization singularities in momentum space. For conventional symmetric photonic crystal slabs (PhCSs), BIC is enclosed by linearly polarized far fields with winding angle of 2π, which is unfavorable for high-capacity and multi-functionality integration-optics applications. Here, we show that by breaking σz-symmetry of the PhCS, asymmetry in upward and downward directions and arbitrarily polarized BIC can be realized with a bilayer-twisted PhCS. It exhibits elliptical polarization states with constant ellipticity angle at every point in momentum space within the vicinity of BIC. The topological nature of BIC reflects on the orientation angle of polarization state, with a topological charge of 1 for any value of ellipticity angle. Full coverage of Poincaré sphere (i.e., - {{pi over 4}} le chi le {{pi over 4}} and - {{pi over 2}} le psi le {{pi over 2}}) and higher-order Poincaré sphere can be realized by tailoring the twist angles. Our findings may open up new avenues for applications in structured light, quantum optics, and twistronics for photons.

Quasi-guided modes resulting from the band folding effect in a photonic crystal slab for enhanced interactions of matters with free-space radiations.

We elucidate that guided modes supported by a regular photonic crystal slab structure composed of a square lattice of air holes in a silicon slab will transition into quasi-guided (leaky) modes when the radius of every second column of air holes is changed slightly. This intentional geometric perturbation will lead to a doubling of the period in one direction and the corresponding shrinkage of the first Brillouin zone. Because of the translational symmetry in the k-space, leaky waves inheriting the spatial dispersion of the original guided modes, which do not interact with external radiation, will appear with the dispersion curves above the light cone. Our results show that ultrahigh Q-factor resonances with large operating bandwidth can be achieved. Interestingly, the perturbation in only one direction of the photonic lattice will lead to an in-plane wave number-dependent resonance characteristic in both directions. Our numerical results demonstrate a local enhancement of the electric field magnitude by the order of 102, which is even more significant than those in most plasmonic structures. These quasi-guided modes with superior properties will provide a new platform for efficient light-matter interactions.

Mode analysis of GaN two-dimensional photonic crystal nanocavities undercut by photo-electrochemical etching

GaN two-dimensional (2D) photonic crystal nanocavities with a single embedded InGaN quantum well are undercut by photo-electrochemical (PEC) etching and optically characterized to investigate the fundamental mode. The PEC etching selectively removes an InGaN-based sacrificial layer to form air-suspended GaN photonic crystal cavity slabs. We investigated the resonant modes of the photonic crystal nanocavities by micro-photoluminescence spectroscopy measurement at room temperature. The wavelengths of the measured resonant peaks and their dependence on the photonic crystal period agreed well with numerical analysis, allowing us to determine the fundamental mode in the measured spectra. The highest quality factor for the fundamental mode reached 3400 at blue wavelengths. This work would contribute to the improvement of GaN 2D photonic crystal nanocavities using PEC etching as well as their applications towards integrated light sources in visible wavelengths.

Dual-wavelength chiral metasurfaces based on quasi-bound states in the continuum

In this paper, we numerically demonstrate a dual-wavelength chiral metasurface with the chirality only existing at two resonant wavelengths, making use of the excitation of two quasi-bound states in the continuum (quasi-BICs). This chiral metasurface is constructed on the basis of photonic crystal slabs with square lattice of circular holes by introducing rectangular holes as perturbations. The two quasi-BICs supported by chiral metasurface are orthogonal to each other, thus the chirality at the two resonant wavelengths can be controlled independently by changing the geometry of the perturbations. Furthermore, a chirality-switchable metasurface is demonstrated taking advantage of the sensitivity of quasi-BICs to the variation of out-of-plane symmetry caused by changing refractive index of surroundings. The proposed chiral metasurface could find potential applications in nonlinear optics, sensing, imaging and near-to-eye displays.

Exfoliated Flexible Photonic Crystal Slabs for Refractive Index and Biomolecular Sensing

Using the optical transduction properties of one-dimensional photonic crystal slabs is a promising approach for label-free biosensing in the field of flexible and wearable biosensing. In this work we demonstrate the fabrication of flexible photonic crystal slabs (f-PCS) via an exfoliation method from rigid photonic crystal slabs. We investigate the processing effect on the optical properties and show that the optical resonances are maintained, but affected. The quality factor changes from 50 to 31 for the TE mode and from 187 to 136 for the TM mode after the process. We demonstrate use of f-PCS for refractive index sensing and achieve a limit of detection (LOD) of 3.6 E-4 refractive index units (RIU). Furthermore, we introduce vapor-phase functionalization of the f-PCS and show first results of biosensing of antibodies from diluted feline serum.

Suppressing the mechanochromism of flexible photonic crystals.

Photonic crystal slabs (PCS) are a promising platform for optical biosensing. Yet, flexible applications based on PCS for biosensing have been limited, as the mechanical properties influence the optical ones. Here, we show the suppression of the mechanochromism effect for flexible PCS. We obtained flexible photonic crystal slabs by sputtering of a dielectric 100 nm Nb2O5 high refractive index layer onto a flexible nanostructured polydimethylsiloxane (PDMS) substrate with 370 nm grating period. The PCS exhibit a guided mode resonance at around 650 nm. We demonstrate that these flexible photonic crystal slabs show less than 0.5 nm resonance shift for 4% strain and call them stabilized PCS (sPCS). We compare this to a resonance shift of ∼21 nm for ∼4% strain of a flexible photonic crystal with a flexible nanoparticle high index layer (mechanochromatic PCS, mPCS). This high resonance shift is expected from the Bragg equations, where 4% grating period change correspond to approximately 4% change of the resonance wavelength (i.e., ∼26 nm at a resonance wavelength of 650 nm), if changes in the mode effective refractive index are neglected. In a stretch series we obtain color-to-strain dependencies of 4.79 nm/% strain for mPCS and 0.11 nm/% strain for our stabilized sPCS. We analyze the suppression of the mechanochromism with detailed microscopy results. We observe that fissures and fractures form in the rigid waveguiding layer of the sPCS upon mechanical stress. An algorithm based on Holistically-Nested Edge Detection (HED) is used for automated counting of cracks. Rigid photonic crystal cells with sizes on the order of 10 µm to 100 µm are formed that explain the stable optical properties. Even more stable optical properties with less than 0.03 nm wavelength shift per 1% strain are demonstrated for sPCS with an additional dielectric 100 nm SiO2 low index layer beneath the Nb2O5 waveguide layer decoupling the waveguide further from the flexible PDMS substrate.

Controllable Patterning of Metallic Photonic Crystals for Waveguide-Plasmon Interaction.

Waveguide-plasmon polaritons sustained in metallic photonic crystal slabs show fascinating properties, such as narrow bandwidth and ultrafast dynamics crucial for biosensing, light emitting, and ultrafast switching. However, the patterning of metallic photonic crystals using electron beam lithography is challenging in terms of high efficiency, large area coverage, and cost control. This paper describes a controllable patterning technique for the fabrication of an Ag grating structure on an indium-tin oxide (ITO) slab that enables strong photon-plasmon interaction to obtain waveguide-plasmon polaritons. The Ag grating consisting of self-assembled silver nanoparticles (NPs) exhibits polarization-independent properties for the excitation of the hybrid waveguide-plasmon mode. The Ag NP grating can also be annealed at high temperature to form a continuous nanoline grating that supports the hybrid waveguide-plasmon mode only under transverse magnetic (TM) polarization. We tuned the morphology and the periodicity of the Ag grating through the concentration of silver salt and the photoresist template, respectively, to manipulate the strong coupling between the plasmon and the waveguide modes of different orders.