Photonic Crystal Slab Research Articles

Phase noise matching in resonant metasurfaces forintrinsic sensing stability.

Interferometry offers a precise means of interrogating resonances in dielectric and plasmonic metasurfaces, surpassing spectrometer-imposed resolution limits. However, interferometry implementations often face complexity or instability issues due to heightened sensitivity. Here, we address the necessity for noise compensation and tolerance by harnessing the inherent capabilities of photonic resonances. Our proposed solution, termed "resonant phase noise matching," employs optical referencing to align the phases of equally sensitive, orthogonal components of the same mode. This effectively mitigates drift and noise, facilitating the detection of subtle phase changes induced by a target analyte through spatially selective surface functionalization. Validation of this strategy using Fano resonances in a 2D photonic crystal slab showcases noteworthy phase stability (). With demonstrated label-free detection of low-molecular-weight proteins at clinically relevant concentrations, resonant phase noise matching presents itself as a potentially valuable strategy for advancing scalable, high-performance sensing technology beyond traditional laboratory settings.

Directional spontaneous emission in photonic crystal slabs

Abstract Spontaneous emission is one of the most fundamental out-of-equilibrium processes in which an excited quantum emitter relaxes to the ground state due to quantum fluctuations. In this process, a photon is emitted that can interact with other nearby emitters and establish quantum correlations between them, e.g., via super and subradiance effects. One way to modify these photon-mediated interactions is to alter the dipole radiation patterns of the emitter, e.g., by placing photonic crystals near them. One recent example is the generation of strong directional emission patterns – key to enhancing super and subradiance effects – in two dimensions by employing photonic crystals with band structures characterized by linear isofrequency contours and saddle points. However, these studies have predominantly used oversimplified toy models, overlooking the electromagnetic field’s intricacies in actual materials, including aspects like geometrical dependencies, emitter positions, and polarization. Our study delves into the interaction between these directional emission patterns and the variables mentioned above, revealing the untapped potential to fine-tune collective quantum optical phenomena.

Observation of gapless corner modes of photonic crystal slabs in synthetic translation dimensions

Second-order topological photonic crystals support localized corner modes that deviate from the conventional bulk-edge correspondence. However, the frequency shift of corner modes spanning the photonic band gap has not been experimentally reported. Here, we observe the gapless corner modes of photonic crystal slabs within a parameter space by considering translation as an additional synthetic dimension. These corner modes, protected by topological pumping in synthetic translation dimensions, are found to exist independently of the specific corner configuration. The gapless corner modes are experimentally imaged via the near-field scanning measurement and validated numerically by full-wave simulations. We propose a topological rainbow with gradient translation, demonstrating the ability to extract and separate specific frequency components of light into different spatial locations. Our work contributes to the advancement of topological photonics and provides valuable insights into the exploration of gapless corner modes in synthetic dimensions.

Chiral Light in Twisted Fabry–Pérot Cavities

AbstractFundamental studies of the interaction of chiral light with chiral matter are important for the development of techniques that allow handedness‐selective optical detection of chiral organic molecules. One approach to achieve this goal is the creation of a Fabry–Pérot cavity that supports eigenmodes with a desired electromagnetic handedness. This paper theoretically studies chiral Fabry–Pérot cavities with mirrors comprising 1D photonic crystal slabs made of van der Waals As2S3, a material with one of the highest known in‐plane anisotropies. By utilizing the anisotropy provided by As2S3, Fabry–Pérot cavities with constitutional and configurational chiralities are designed. It is demonstrated that in cavities with constitutional chirality, electromagnetic modes of left or right handedness exist due to the chirality of both mirrors, often referred to as handedness preserving mirrors in the literature. At the same time, cavities with configurational chirality support modes of both handednesses due to the chiral morphology of the entire structure, set by the twist angle between the optical axes of the upper and lower non‐chiral anisotropic mirrors. The developed chiral Fabry–Pérot cavities can be tuned to any distance between the mirrors by properly twisting them, making such systems a prospective platform for the coupling of chiral light with chiral matter.

Directing valley-polarized emission of 3 L WS2 by photonic crystal with directional circular dichroism.

The valley degree of freedom that results from broken inversion symmetry in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) has sparked a lot of interest due to its huge potential in information processing. In this experimental work, to optically address the valley-polarized emission from three-layer (3 L) thick WS2 at room temperature, we employ a SiN photonic crystal slab that has two sets of holes in a square lattice that supports directional circular dichroism engendered by delocalized guided mode resonances. By perturbatively breaking the inversion symmetry of the photonic crystal slab, we can simultaneously manipulate s and p components of the radiating field so that these resonances correspond to circularly polarized emission. The emission of excitons from distinct valleys is coupled into different radiative channels and hence separated in the farfield. This directional exciton emission from selective valleys provides a potential route for valley-polarized light emitters, which lays the groundwork for future valleytronic devices.

Exploiting Topological Darkness in Photonic Crystal Slabs for Spatiotemporal Vortex Generation.

Spatiotemporal optical vortices (STOVs) with swirling phase singularities in space and time hold great promise for a wide range of applications across diverse fields. However, current approaches to generate STOVs lack integrability and rely on bulky free-space optical components. Here, we demonstrate routine STOV generation by harnessing the topological darkness phenomenon of a photonic crystal slab. Complete polarization conversion enforced by symmetry enables topological darkness to arise from photonic bands of guided resonances, imprinting vortex singularities onto an ultrashort reflected pulse. Utilizing time-resolved spatial mapping, we provide the first observation of STOV generation using a photonic crystal slab, revealing the imprinted STOV structure manifested as a curved vortex line in the pulse profile in space and time. Our work establishes photonic crystal slabs as a versatile and accessible platform for engineering STOVs and harnessing the topological darkness in nanophotonics.

Polarization mixing, bound states in a continuum, and exciton-polaritons in photonic crystal slabs by a guided-mode expansion approach

Photonic crystal (PhC) slabs, or patterned multilayer waveguides, are known to support truly guided modes with no losses, as well as quasi-guided modes that lie in the continuum of far-field radiation. In this contribution, we present a guided-mode expansion approach – and the corresponding free software named legume – that allows calculating a number of features of PhC slabs: (a) symmetry properties and the issue of polarization mixing in coupling to far-field radiation; (b) the occurence of bound states in a continuum, which have infinite Q-factor and give rise to topological singularities of the far-field polarization; (c) the description of active two-dimensional layers through a suitably formulated light-matter coupling Hamiltonian, allowing to describe the regime of strong coupling leading to photonic crystal polaritons. Comparison with rigorous coupled-wave analysis, and the insurgence of non-hermitian features in the optical properties, are also addressed.

Photonic approach in stacked slabs having periodic holes for enhancing photocatalytic activities.

Photonic approaches can improve the efficiencies of photo-electrochemical devices towards CO2 reduction and fossil fuel-free societies. In a system consisting of stacked dielectric slabs having periodic holes with each slab coated by photocatalyst layers at both sides, immersed in water, we show that an incident electromagnetic field is effectively confined in the photocatalyst layers, resulting in the enhancement of the photocatalytic activities. In addition, the antireflection effect was engineered by adjusting the distances between the photonic crystal slabs. Numerical results reveal an enhancement factor of 3 for the absorption of electromagnetic fields at the operation frequency in the 3rd band of the dispersion diagram, compared to the bulk photocatalyst. Our system has the feature of periodic holes allowing the movement of reaction products. An analytical model is developed using the revised plane wave method and perturbation theory, which captures the trends observed in numerical results.

Polarization Field in Momentum Space of Two-Dimensional Photonic Crystal Slabs (Invited)

Polarization Field in Momentum Space of Two-Dimensional Photonic Crystal Slabs (Invited)

Effective Index Approximation Method for Graded Photonic Crystals Slabs: Precise Design of A Mikaelian Lens

Effective Index Approximation Method for Graded Photonic Crystals Slabs: Precise Design of A Mikaelian Lens

Realizing Ultrahigh‐Q Resonances Through Harnessing Symmetry‐Protected Bound States in the Continuum

AbstractHarnessing the power of symmetry‐protected bound states in the continuum (SP BICs) has become a focal point in scientific exploration, promising many interesting applications in nanophotonics. However, the practical realization of ultrahigh quality (Q) factor quasi‐BICs (QBICs) is hindered by the fabrication imperfections. In this work, an easy approach is proposed to achieve ultrahigh‐Q resonances by strategically breaking symmetry. By introducing precise perturbations within the zero eigenfield region, QBICs with consistently ultrahigh‐Q factors, beyond conventional limitations are achieved. Intriguingly, intentionally disrupting symmetry in the maximum eigenfield region leads to a rapid decline in QBIC's Q‐factors as the asymmetry parameter increases. Leveraging this design strategy, ultrahigh‐Q modes with a high Q‐factor of 36,694 in a silicon photonic crystal slab are experimentally realized . The findings establish a robust and straightforward pathway toward unlocking the full potential of SP BICs, enhancing light‐matter interactions across diverse applications.

On polarization of Smith-Purcell radiation from a photonic crystal slab

We analyze the polarization of radiation arising during the pass of a relativistic electron over a 2D photonic crystal slab consisting of subwavelength particles arranged at the nodes of a flat rectangular lattice. It is shown that the contribution of the parallel and perpendicular polarizations depends on the direction of propagation of the radiation, the bifurcation of the maxima is due to the contribution of the perpendicular polarization, while the asymmetry of the angular distribution is due to the contribution of the parallel polarization.

Design and Observation of Topological Band Gaps and Edge Modes of SOI Photonic Crystal Slabs in the Mid-Infrared Range

Design and Observation of Topological Band Gaps and Edge Modes of SOI Photonic Crystal Slabs in the Mid-Infrared Range

Reflectionless graphene perfect absorber based on parity symmetric unidirectional guided resonance.

We propose a type of reflectionless graphene perfect absorber (GPA) in which the reflection channel is forbidden, while the transmission channel is open. Peak absorption of 99.97% in the near-infrared is numerically demonstrated for monolayer graphene loaded on a one-dimensional silicon photonic crystal slab with rhomboid cross sections that supports parity symmetric unidirectional guided resonances (UGRs). Based on the proposed GPA, a transmissive optical modulator with a modulation depth of about 28 dB and an insertion loss of 0.31 dB by varying the Fermi energy level graphene from 0.3 eV to 0.7 eV is numerically presented. Remarkably, the design strategy can be straightforwardly applied to other two-dimensional (2D) materials. Our study may find promising applications in 2D material-based optical modulators and filters.

Ultra-compact dual-purpose photonic crystal slab waveguide for both generating and isolating optical frequency comb

In this article, we present a novel approach to achieve a highly compact Si3N4 photonic crystal waveguide by incorporating air holes. This structure is developed in two functional applications that simultaneously perform the functions of generating optical frequency combs and demultiplexing them. The presence of air holes enhances control and precision in the dispersion engineering process. At the same time, the light conduction, based on the refractive index contrast between the core and cladding, along with the conduction occurring in the photonic bandgap region of the structure, leads to enhanced trapping of light within the core of the structure. As a result, a nonlinear coefficient of 4.2 m-1W−1 is obtained, which originates from the small effective mode area of 0.67 µm2. By employing four-wave mixing, a total of 40 comb lines are generated with a frequency spacing range of 10 GHz, 50 GHz, 60 GHz, and 100 GHz over a 6 mm length waveguide. The same waveguide structure is also developed for demultiplexing the comb lines, with four filters at the end which, individually extract the optical combs possessing wavelengths of 1317.8 nm, 1318.5 nm, 1319.2 nm, and 1319.9 nm. This demultiplexer has a Q-factor, average transmission efficiency, channel spacing, and spectral linewidth of 7209.35, 99.2 %, 0.7 nm, and 0.185 nm, respectively. The suggested design can significantly advance integrated photonic circuits, optical communications, and remote sensing applications requiring simultaneous information delivery, such as remote surgical operations and sensing, because of its substantial compactness.

Photonic crystal slabs with maximal chiroptical response empowered by bound states in the continuum

To enhance the strength of chiral light–matter interaction for practical applications, the chirality and quality factors (Q-factors) of current methods need to be strengthened simultaneously. Here, we propose a design of photonic crystal slabs (PhCs) supporting chiral bound states in the continuum (BICs) of transverse electric (TE) and transverse magnetic (TM) modes, exhibiting maximal chiroptical responses with high Q-factors and near-unity circular dichroism (CD=0.98). Different from the past, the PhCs we employed only have reduced in-plane symmetry and can support simultaneously chiral quasi-BICs (q-BICs) of TE and TM mode with two-dimensional ultra-strong external and internal chirality. Based on the temporal coupled-mode theory, two analytical expressions of CD of chiral q-BICs response are revealed, which are consistent with the simulation results. Furthermore, we elucidate these results within the charge-current multipole expansion framework and demonstrate that the co-excitation of higher-order multipole electric/magnetic modes is responsible for near-perfect CD. Our results may provide more flexible opportunities for various applications requiring high Q-factors and chirality control, such as chiral lasing, chiral sensing, and enantiomer separation.

Purcell effect in two-dimensional photonic crystal slabs with triangular lattice

Purcell effect in two-dimensional photonic crystal slabs with triangular lattice

Active control of mid-wavelength infrared non-linearity in silicon photonic crystal slab.

Natural materials' inherently weak nonlinear response demands the design of artificial substitutes to avoid optically large samples and complex phase-matching techniques. Silicon photonic crystals are promising artificial materials for this quest. Their nonlinear properties can be modulated optically, paving the way for applications ranging from ultrafast information processing to quantum technologies. A two-dimensional 15-μm-thick silicon photonic structure, comprising a hexagonal array of air holes traversing the slab's thickness, has been designed to support a guided resonance for the light with a wavelength of 4-μm. At the resonance conditions, a transverse mode of the light is strongly confined between the holes in the "veins" of the silicon component. Owing to the confinement, the structure exhibits a ratio of nonlinear to linear absorption coefficients threefold higher than the uniform silicon slab of the same thickness. A customised time-resolved Z-scan method with provisions to accommodate ultrafast pump-probe measurements was used to investigate and quantify the non-linear response. We show that optically pumping free charge carriers into the structure decouples the incoming light from the resonance and reduces the non-linear response. The time-resolved measurements suggest that the decoupling is a relatively long-lived effect on the scale comparable to the non-radiative recombination in the bulk material. Moreover, we demonstrate that the excited free carriers are not the source of the nonlinearity, as this property is determined by the structure design.

Observation of Dual‐Polarization Topological Photonic States at Optical Frequencies

AbstractTopological protection in photonics offers new prospects for confining and guiding light waves. Up to now, most reported works on topological photonics have focused on the realization of topological phases originally found in condensed matter systems. However, photonic systems can possess a distinguishing topology that differs from their quantum electronic counterparts due to the vectorial nature of light. Here, dual‐polarization topological photonic states are theoretically predicted and experimentally demonstrated at optical frequencies on the all‐dielectric silicon‐on‐insulator platform. The topological band gaps opening at valleys for both TE‐ and TM‐polarizations in the properly designed breathing Kagome photonic crystal slabs are demonstrated, leading to dual‐polarization topological edge states below the light cone. The topological edge states are directly observed by imagining the near fields with scattering scanning near‐field microscopy. In addition, the significant improvement of the quality factor of topological states via engineering the transition near the boundary along with the robustness of edge states is presented. The proposed dual‐polarization topological states and the improvement of quality factor via boundary profile engineering paves the way toward novel applications of topological photonics for realizing integrated polarization‐dependent applications such as polarization multiplexing photonic devices and on‐chip chiral quantum optics.

Polarization states and far-field optical properties in dielectric photonic crystal slabs.

We study the role of topological singularities like Bound States in a Continuum (BICs) or Circularly Polarized States (CPSs) in determining ellipticity of the far-field polarization in dielectric metasurfaces. Using finite-difference time-domain as well as rigorous coupled-wave analysis simulations, we determine the behavior of the Stokes parameter S3 in the whole k space above the light cone, with special regard to the region close to the singularities. Moreover, we clarify the relation between the topological singularities and the circular dichroism in reflectivity.