Photonic Crystal Slab Research Articles

Anisotropy-induced evolution of topological half vortices in a dielectric photonic crystal slab.

Circular polarization points (C points), which evolve from bound states in the continuum and feature topological half vortices, are important in manipulating chiroptical effects and controlling valley exciton emission in photonic systems. The methods for generating and manipulating C points usually rely on symmetry breaking of structures, but new strategies are required for more refined control. In this work, we investigate the generation and evolution of C points in photonic crystal slabs (PCSs) composed of anisotropic media while keeping the geometrical structure unchanged. By adjusting the optical axis of the anisotropic media, we introduce two degrees of freedom including azimuth and rotation angles, leading to the V point to split into two C points with opposite handedness. Our approach offers flexible control of C points by tuning material properties, presenting new, to the best of our knowledge, opportunities for direction- and spin-dependent optical devices.

Optical chromatography of ultrasmall particles by Brownian motion in a tilted optical potential induced by bound states in the continuum

We investigate sorting Rayleigh optical particles up to several nanometers in size during Brownian motion in a tilted periodic potential with multiple deep wells. The wells are induced by optical bound states in the continuum within a system of parallel photonic crystal slabs immersed in a liquid. The Brownian dynamics of the particles is significantly altered by resonant optical forces, leading to the complete spatial separation of particles with a size difference of approximately 1% during the diffusion process. In addition, the possibility of creating an integrated platform for continuous optical sorting is discussed.

All-polarization asymmetric silicon nitride photonic crystal slab waveguides by combining a single-polarization bandgap and birefringence

Abstract Photonic crystal (PhC) waveguides on the silicon nitride (SixNy) platform currently are capable only of transmission of a single light polarization, severely limiting polarization diversity applications. In this work, we propose a scheme that combines a single-polarization photonic band gap (SPBG) formed by the first two bands with the same polarization, along with large birefringence between the first two bands with different polarizations. In simple terms, one polarization in the PhC slab waveguide is guided by the SPBG, while the other polarization is index guided, and overlap between the frequency bands for the two polarizations in which propagation is forbidden, dubbed the no-polarization-mode region (NMR), tuned by the aspect ratio of PhC slabs. This approach theoretically achieves PhC waveguides on asymmetric SixNy slabs that guide both polarizations within the NMR. Three-dimensional (3D) planewave-method calculations show that through simple adjustment of the aspect ratio of the PhC slab, the first two bands with different polarizations can be adjusted to be shifted in frequency to form the NMR, where neither TE- nor TM-like modes can propagate along the in-plane directions. The NMR is then systematically investigated and optimized for various refractive indices and slab thicknesses, and using the optimized-NMR design, line-defect waveguides supporting propagation of both polarizations are theoretically demonstrated. A 3D finite-difference time-domain simulation shows that an all-polarization bandwidth as large as 180 nm can be obtained in an optimized PhC waveguide with SixNy refractive index of 2.4, which can be tuned by x and y. In addition, our investigation shows that an NMR can also be obtained in an asymmetric rod-type PhC slab even when the refractive index is as low as 1.8, offering significant flexibility in engineering of the structure and refractive index of all-polarization devices on SixNy and, by extension, in other moderate-index platforms.

Integrated photonic crystal beam splitters based on thin film lithium niobate.

We propose and demonstrate integrated photonic crystal (PhC) beam splitters based on X-cut thin film lithium niobate (TFLN). Its working principle is based on bandgap guidance and total reflection in the PhC slab. We designed two structures: one features a triangular lattice, while the other exhibits a tetragonal lattice. These structures have compact footprint, with size of 14.857×4.982 and 15.275×5.295 µm2 for triangular and tetragonal lattices, respectively. We fabricate these devices by directly etching the TFLN. The measured insertion losses are 3.482 (triangular lattice) and 2.181 (tetragonal lattice) dB. Our proposed PhC beam splitter provides a novel approach for the realization of compact photonic devices on TFLN.

Topology of far-field signals for photonic crystal slabs

The study of band topology in photonic crystals was primarily focused on near-field effects, including edge states and high-order corner states. However, this work investigated the polarization distribution of radiated fields for photonic crystal slabs to get their far-field properties of band topology. We introduced a topological invariant—the winding number of far-field polarization around the Brillouin zone boundary and confirmed a robust correspondence between it and the Chern number of energy bands from the perspective of symmetry, which can be used to analyze the process of topological phase transition. It is found that changes in the winding number and Chern number, associated with the exchange of far-field polarization singularities, especially for bound states in the continuum, will emerge during phase transition. These findings offer insights for further understanding the intriguing properties of topological materials.

Tunable bound states in the continuum with loss compatibility.

Dynamic control of bound states in the continuum (BICs) is usually achieved by engineering structural geometries of lossless optical systems, leading to a passive nature for most current BIC devices. Introducing materials with tunable permittivity, i.e., refractive index and loss, may offer a new degree of freedom in designing reconfigurable BIC metadevices with active functionalities. However, achieving loss-accompanied or loss-driven BIC manipulation while preserving its ultrahigh Q factor is extremely challenging. Here, we report a loss-compatible BIC manipulation mechanism based on far-field interference in a mirror-assisted photonic crystal slab, wherein the loss of tunable material not only harmoniously coexists with ultrahigh Q factor, but also serves as a pivotal joystick of BIC dynamics in momentum space. By modulating loss and refractive index of tunable material through the amorphous-crystalline phase transition, simulation results show the active switching of topological charge for BICs, as well as the multidimensional control of chiroptical effect for quasi-BICs, including steerable response/emission direction and chirality continuum with far-field ellipticity ranging from -0.944 to +0.943. Our findings suggest a distinct route to construct BIC metadevices with active functionalities and foster deeper exploration of intrinsic loss applications within the ultrahigh-Q photonic system.

Origins and conservation of topological polarization defects in resonant photonic-crystal diffraction.

We present a continuative definition of topological charge to depict the polarization defects on any resonant diffraction orders in photonic crystal slab regardless they are radiative or evanescent. By using such a generalized definition, we investigate the origins and conservation of polarization defects across the whole Brillouin zone. We found that the mode crossings due to Brillouin zone folding contribute to the emergence of polarization defects in the entire Brillouin zone. These polarization defects eventually originate from the spontaneous symmetry breaking of line degeneracies fixed at Brillouin zone center or edges, or inter-band coupling caused by accidental Bloch band crossings. Unlike Bloch states, the polarization defects live and evolve in an unbound momentum space, obeying a local conservation law as a direct consequence of Stokes' theorem, but the total charge number is countless.

Magnetically tunable bound states in the continuum with arbitrary polarization and intrinsic chirality

Bound states in the continuum (BICs), which are exotic localized eigenstates embedded in the continuum spectrum and exhibit topological polarization singularities in momentum space, have recently attracted great attention in both fundamental and applied physics. Here, based on a magneto-optical (MO) photonic crystal (PhC) slab placed in external magnetic fields with time-reversal symmetry (TRS) breaking, we theoretically propose magnetically tunable BICs with arbitrary polarization covering the entire Poincaré sphere and efficient off-Γ chiral emission of circularly polarized states (C point). More interestingly, by further breaking the in-plane inversion symmetry of the MO PhC slab to generate a pair of C points spawning from the eliminated BICs and tuning the external magnetic field strength to move one C point to the Γ point, an at-Γ intrinsic chiral BIC exhibits chiral characteristics on both sides of the PhC slab with near-unity circular dichroism exceeding 0.99 and a high-quality factor of 46,000 owing to the preserved out-of-plane mirror symmetry. Moreover, the chirality of the chiral BICs can be inverted by flipping the magnetic bias. Our work opens an unprecedented avenue to explore the unique topological photonics of BICs with broken TRS and promises multiple applications in chiral-optical effects, structured light, and tunable optical devices.

Observation of spatiotemporal dynamics for topological surface states with on-demand dispersion

Dispersion management in guided wave optics is of vital importance for various applications. Topological photonics opens new horizons for manipulating unidirectional guided waves utilizing edge states. However, the experimental observation of spatiotemporal dynamics for guided waves with on-demand dispersion in topological photonic crystal is an important issue awaiting exploitation. Herein, we experimentally investigate the spatiotemporal properties of topological surface states with on-demand dispersion, where they are supported by a truncated valley photonic crystal with surface modulation. We observe the electromagnetic dynamics of surface states with typical dispersions, where dynamical trapping of an electromagnetic pulse mediated by the unidirectional surface state with flat dispersion and backward beam routing using reversed dispersion properties are achieved in photonic crystal slabs. Both numerical and experimental results substantiate the ultimate dispersion management for topological surface states, which could pave new ways for the manipulation of electromagnetic waves on the surface of photonic devices.

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

Design and implementation of broadband optical vortices in photonic crystal slabs

We propose and discuss a simple method to generate broadband optical vortices, utilizing the inherent topological vortex structures of polarization around bound states in the continuum supported by a photonic crystal slab in the momentum space to induce the generation of vortex beams. Since the proposed structure is composed of silicon pillars arranged periodically, it lacks a true optical geometric center. It is insensitive to the position of the incident light and does not require a specific optical alignment process compared to traditional spiral phase plates. Furthermore, because it is composed of dielectric pillars, it can achieve vortex beam generation at any desired working wavelength. We also discuss the robustness of its structure, showing that it can be immune to certain manufacturing defects. Therefore, the proposed structure not only provides a new method for manipulating the angular momentum of photons but also has potential new applications in integrated optical information processing and optical tweezers.

Efficient Thermo-Optic Switching and Modulating in Nonlocal Metasurfaces.

High-Q optical resonances in nonlocal metasurfaces, benefiting from significantly enhanced light/matter interactions, feature strong responses even under a weak external stimulus. In this work, we leverage the high-Q resonances of quasi-guided modes (QGMs) supported by a photonic crystal slab (PCS) structure to achieve efficient optical switching/modulation. The QGMs with an experimentally measured Q-factor of ∼2200 are realized by shifting every second column of air holes in a rectangular lattice within a silicon slab. At a weak illumination intensity of less than 4.0 W/cm2 from a 532 nm continuous-wave pump laser, the QGM resonance around 1550 nm experiences a pronounced spectral shift, with modulation depth exceeding 55%. This is attributed to the thermo-optic response caused by photothermal heating of the metasurface triggered by the absorption of the pump laser in silicon, which is further verified by the electrical heating approach. Our reported results showcase a simple yet effective way of tailoring light propagation in nonlocal metasurfaces.

Resonance Wavelength Stabilization of Quasi-Bound States in the Continuum Constructed by Symmetry Breaking and Area Compensation.

Dielectric metasurface supported symmetry-protected bound states in the continuum (SP-BIC) provide an important platform for enhancing light-matter interactions. However, the conversion of SP-BIC into quasi-BIC (QBICs) through symmetry breaking is often accompanied by a shift in the resonance wavelength. In this work, we present a generalized viewpoint aimed at achieving the wavelength stability of QBICs through symmetry breaking and area compensation (SBAC). Three SBAC schemes we propose are equal proportional compensation along both x- and y-directions and equal proportional compensation along x-direction only or y-direction only. The QBICs resonance wavelengths stabilized at about 1200, 1520, and 1434 nm are achieved in monomer, dimer, and tetramer metasurfaces, respectively. Finally, we perform an experimental demonstration in a nanohole dimer photonic crystal slab. The QBICs resonance wavelength stabilized at 1658 nm for SBAC. Our approach provides new routes for realizing QBICs with a stable wavelength and tunable Q-factor.

Quasi-bound state in the continuum enhancing background limited infrared detectivity

Quasi-bound states in the continuum (quasi-BIC) have tunable high Q factors and thus have many potential applications in optoelectronics. Here, we propose to construct a quasi-BIC with a perturbated elliptical-cylinder photonic crystal slab, and then develop a new way based on the quasi-BIC to address the background noise problem of infrared detectors. By integrating the photonic crystal slab with a quantum well infrared photodetector (QWIP), and by pushing the quasi-BIC system with absorption to the critical coupling state, an ultra-narrow photoresponse band (Q factor equal to 1150) with a high peak quantum efficiency (over 60%) is achieved in the long-wavelength infrared range. The critical coupling state is realized by matching the radiation Q factor, which is mainly controlled by the quasi-BIC, to the absorption Q factor, which is mainly controlled by the doping of the quantum wells (QWs). This device rejects most background radiation and thus significantly suppresses the background noise, resulting in a background limited specific detectivity (DBLIP∗) 11.85 times that of a conventional ideal photoconductor. A detailed formula is employed for calculation of DBLIP∗, incorporating the wavelength and incident angle dependence of the quantum efficiency. The ultra-narrow photoresponse band with a high peak responsivity can be tuned over the photosensitive wavelength range of the QWs by simply modifying the in-plane structural parameters. This work opens a new avenue to greatly enhance the specific detectivity for infrared detectors based on the joint effect of quasi-BIC and critical coupling.

Optical moiré bound states in the continuum

Trapping electromagnetic waves within the radiation continuum holds significant implications in the field of optical science and technology. Photonic bound states in the continuum (BICs) present a distinctive approach for achieving this functionality, offering potential applications in laser systems, sensing technologies, and other domains. However, the simultaneous achievement of high Q-factors, flat-band dispersions, and wide-angle responses in photonic BICs has not yet been reported, thereby impeding their practical performance due to laser direction deviation or sample disorder. Here, we theoretically demonstrate the construction of moiré BICs in one-dimensional photonic crystal (PhC) slabs, where high-Q resonances in the entire moiré flat band are achieved. Specifically, we numerically validate that the radiation loss of moiré BICs can be eliminated by aligning multiple topological polarization charges with all diffraction channels, enabling the strong suppression of far-field radiation from the entire moiré band. This leads to a slow decay of Q-factors away from moiré BICs in the momentum space. Moreover, it is found that Q-factors of the moiré flat band can still maintain at a high level with structural disorder. In experiments, we fabricate the designed 1D moiré PhC slab and observe both high-Q resonances and a slow decrease of Q-factors for moiré flat-band Bloch modes. Our findings hold promising implications for designing highly efficient optical devices with wide-angle responses and introduce a novel avenue for exploring BICs in moiré superlattices.

Multiple Circular Polarizations Coexisting with Bound States in the Continuum Without Breaking Symmetry

AbstractBound states in the continuum (BICs) have been engineered in periodic photonic systems to achieve diverse polarization behavior in momentum space. By breaking C2z symmetry of photonic crystal slabs (PhCS), purely circular polarization, which hold significant potential for applications in topological physics and chiral optics, can be achieved near the BICs. In this study, the intriguing phenomenon of BICs on the degenerate band of PhCS with a triangular lattice featuring cylindrical holes are investigated. Unlike previous studies that mainly focused on BICs on non‐degenerate photonic bands, this research reveals a sophisticated interplay between BICs and Dirac points on the more intricate degenerate photonic band. This interaction gives birth to two pairs of purely circular polarizations with opposite chirality, even without breaking any symmetry of the PhCS. Additionally, It is find that by further breaking the σz mirror symmetry of the PhCS, these purely circular polarization states can be significantly amplified. This findings not only enrich the polarization responses of high‐Q photonic devices but also enables the modulation of chiral light, laying the groundwork for the creation of high‐quality optical devices with precisely engineered polarization properties.

Lasing from Doubly Degenerate Bound States in the Continuum.

This paper reports dual-mode vortex lasing with a topological charge of -2 via doubly degenerate bound states in the continuum. We patterned a photonic crystal slab with hexagonal TiO2 nanoparticles arranged in a hexagonal lattice to support doubly degenerate bound states in the continuum at the Γ point, and the finite array was truncated to have a square boundary. Because emission from bound states in the continuum at the Γ point is forbidden, lasing emerges at off-Γ points, where the double degeneracy is lifted, resulting in two lasing peaks close in wavelength. The coherence time of vortex lasing is about 1.4 ps under pulsed pumping with a width of 75 fs. Polarization- and angle-resolved spectral measurements reveal that the two lasing peaks show orthogonal polarization states.

Giant magneto-optical Kerr effects governed by the quasi-bound states in the continuum.

The magneto-optical Kerr effect (MOKE), as one of the magneto-optical effects, exhibits polarization change upon reflection that can be used to explore the internal information of magnetic materials with broad applications in modern information technology. However, typically, MOKE is quite weak due to the lower magneto-optical interaction. To tremendously enhance the MOKE, quasi-bound states in the continuum in a one-dimensional Ce- doped Y3Fe5O12 (CeYIG) film photonic crystal slabs (PCS) are proposed to improve the magneto-optical interaction in this work. A giant enhancement in the rotation angle and ellipticity of the longitudinal MOKE, which is about 93.4 and 136.8 times stronger than a pure uniform CeYIG, can be realized. Almost circularly polarized reflected beams with different chiralities are obtained with the CeYIG film. By tuning the geometric parameters of the PCS and the applied external magnetic field, dynamic control of polarization states of the reflected beams with different wavelengths can be realized. This magneto-optical metasurface provides a convenient way for the implementation of magneto-optical devices such as information memory devices, sensors, polarizers, and chiral devices.

Optical trapping using quasi-bound states in the continuum of photonic crystal slab.

This paper explores efficient and stable optical trapping using quasi-bound states in the continuum (quasi-BICs) in photonic crystal slabs. By breaking inversion symmetry by transforming square holes into isosceles trapezoidal holes, we create quasi-BICs with finite quality factors. Calculations show that optical forces at enhanced electric field locations effectively trap particles, with significant potential wells at these sites. Multipole expansion analysis indicates that particles at sharp corners suppress radiation and enhance trapping stability. Moreover, an increased number of trapped particles causes a redshift in resonance frequency and strengthens optical forces, especially at sharp corners. These insights are crucial for designing photonic crystal slabs for practical optical trapping applications, where positioning more particles at sharp corners improves quasi-BIC mode and trapping efficiency.

Modulation of optical force by adjusting the distance between three-layer photonic crystal slabs

Optical forces induced by polarization singularities from photonic crystal slabs (PCSs) have gained widely attention in recent years. To enhance the degree of freedom in controlling far-field optical forces, we propose and design a three-layer PCSs structure that simultaneously possesses multiple optical forces with reconfigurability. By modulating the interlayer spacing, the coupling strength between the slabs is regulated, resulting in a band transition from the nodal rings to the nodal ridge, which facilitates the generation of dual polarization vortices. These dual polarization vortices induce multiple phase singularities at their boundaries, leading to multiple optical force singularities, offering the possibility of capturing and dispersing nanoparticles. Interestingly, tuning the spacing among the slabs can transform the converging force into the diverging force at the center, therefore achieving particle screening. This work not only expands the application of nodal ridges to optical force in photonic structures but also provides a feasible approach for the generation of double-ring vortex beam in the nano-optics field.