Silicon Photonic Crystal Slab Research Articles

Toroidal dipole bound states in the continuum in asymmetric dimer metasurfaces

Structural symmetry plays a pivotal role in the emergence of symmetry-protected bound states in the continuum (BICs), often observed at the Γ-point within the first Brillouin zone. However, structural symmetry is not an absolute requirement for the formation of BICs at the Γ-point. In this work, we demonstrate that all-dielectric metasurfaces and photonic crystal slabs, made of dimer nanostructures with different sizes and shapes, can sustain BICs at the Γ-point. We show that the nature of these BICs is well preserved, irrespective of the size mismatch/difference, as long as the center-to-center distance between two nanodisks is equal to half of the lattice constants of a superunit cell. The BICs are transformed into quasi-BICs (QBICs) with finite quality (Q) factors by varying the interspacing of dimer nanodisks. Multipole decomposition indicates that this BIC is primarily governed by a toroidal dipole, with a secondary contribution from a magnetic dipole and magnetic quadrupole. Furthermore, we establish that such a BIC is robust against the shape of nanodisks. Notably, we observe that the Q-factor of QBICs for right nanodisks displaced along the y-axis is three orders of magnitude higher than those along the x-axis, suggesting an effective approach to realizing ultrahigh-Q resonances. Finally, we present an experimental demonstration of such a BIC by fabricating silicon dimer metasurfaces and photonic crystal slabs with dimer nanoholes. The trend of measured Q-factors and resonant wavelengths of QBICs shows good agreement with theoretical predictions. The maximum Q-factor is up to 22 633. These results not only advance our understanding of BICs within compound metasurfaces but also hold great promise in enhancing light–matter interactions.

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.

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.

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.

Experimental Realization of Topologically‐Protected All‐Optical Logic Gates Based on Silicon Photonic Crystal Slabs (Laser Photonics Rev. 17(8)/2023)

Experimental Realization of Topologically‐Protected All‐Optical Logic Gates Based on Silicon Photonic Crystal Slabs (Laser Photonics Rev. 17(8)/2023)

Faraday rotator based on a silicon photonic crystal slab on a bismuth-substituted yttrium iron garnet thin film

We report the design of an ultrathin Faraday rotator consisting of a silicon photonic crystal (PhC) slab on a bismuth-substituted yttrium iron garnet (Bi:YIG) thin film. By directing light into guided modes in the Bi:YIG layer via diffraction in the PhC layer, we numerically demonstrate a Faraday rotation angle of ∼45° at a telecom wavelength with a Bi:YIG layer thickness of only ∼500 nm. This structure permits a high light transmittance of about 70%, enabled by electromagnetically induced transparency. The proposed design only requires nanopatterning of the Si layer, providing a viable route to practical ultrathin Faraday rotators.

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.

Large-area silicon photonic crystal supporting bound states in the continuum and optical sensing formed by nanoimprint lithography.

Optical bound states in the continuum (BIC) are found in various dielectric, plasmonic and hybrid photonic systems. The localized BIC modes and quasi-BIC resonances can result in a large near-field enhancement and a high-quality factor with low optical loss. They represent a very promising class of ultrasensitive nanophotonic sensors. Usually, such quasi-BIC resonances can be carefully designed and realized in the photonic crystal that is precisely sculptured by electron beam lithography or interference lithography. Here, we report quasi-BIC resonances in large-area silicon photonic crystal slabs formed by soft nanoimprinting lithography and reactive ion etching. Such quasi-BIC resonances are extremely tolerant to fabrication imperfections while the optical characterization can be performed over macroscopic area by simple transmission measurements. By introducing lateral and vertical dimension changes during the etching process, the quasi-BIC resonance can be tuned over a wide range with the highest experimental quality factor of 136. We observe an ultra-high sensitivity of 1703 nm per RIU and a figure-of-merit of 65.5 for refractive index sensing. A good spectral shift is observed for detecting glucose solution concentration changes and adsorption of monolayer silane molecules. Our approach involves low-cost fabrication and easy characterization process for large-area quasi-BIC devices, which might enable future realistic optical sensing applications.

High-speed tunable optical absorber based on a coupled photonic crystal slab and monolayer graphene structure.

Reconfigurable metasurfaces have been pursued intensively in recent years for the ability to modulate the light after fabrication. However, the optical performances of these devices are limited by the efficiency, actuation response speed and mechanical control for reconfigurability. In this paper, we propose a fast tunable optical absorber based on the critical coupling of resonance mode to absorptive medium and the plasma dispersion effect of free carriers in semiconductor. The tunable absorber structure includes a single-layer or bi-layer silicon photonic crystal slab (PCS) to induce a high-Q optical resonance, a monolayer graphene as the absorption material, and bottom reflector to remove transmission. By modulating the refractive index of PCS via the plasma dispersion of the free carrier, the critical coupling condition is shifted in spectrum, and the device acquires tuning capability between perfect absorption and total reflection of the incident monochromatic light beam. Simulation results show that, with silicon index change of 0.015, the tunable absorption of light can achieve the reflection/absorption switching, and full range of reflection phase control is feasible in the over coupling region. The proposed reconfigurable structure has potential applications in remote sensing, free-space communications, LiDAR, and imaging.

Active quasi-BIC optical vortex generators for ultrafast switching

The Pancharatnam–Berry phase induced by the winding topology of polarization around a vortex singularity at bound states in the continuum (BIC) provides a unique approach to optical vortex (OV) generation. The BIC-based OV generators have the potential to outperform their counterparts that rely on spatial variations in terms of design feasibility, fabrication complexity, and robustness. However, given the fact that this class of OV generators originates from the topological property of the photonic bands, their responses are generally fixed and cannot be dynamically altered, which limits their applications to photonic systems. Here, we numerically demonstrate that a silicon photonic crystal slab can be used to realize optically switchable OV generation by simultaneously exploiting the vortex topology in momentum space in conjunction with silicon’s nonlinear dynamics. Picosecond switching of OV beams at near-infrared wavelengths are observed. The demonstrated nontrivial topological nature of the active generators can significantly expand the application of BIC toward ultrafast vortex beam generation, high-capacity optical communication, and mode-division multiplexing.

A novel 8-channel DWDM demultiplexer on silicon photonic crystal slab: Design and analysis

In this work an 8-channel DWDM demultiplexer is proposed with multiple designs and numerical analysis. Photonic crystal ring resonators of airholes on a 220 nm thick silicon slab are used as wavelength selective elements. Demultiplexers of three different designs are considered and their characteristics are analyzed. For all the three designs, the resolved wavelengths are in the range of 1530–1540 nm, where Erbium Doped Fiber Amplifiers are applicable. For the optimized device, the average coupling efficiency, cross talk are found to be 70.4% and − 16.76 dB, respectively. Maximum and minimum coupling efficiencies of 79% and 56% are achieved. Photonic bandgap computation is performed by using plane wave expansion method and spectral characteristics are obtained by applying 3D finite difference time domain method.

High‐Harmonic Optical Vortex Generation from Photonic Bound States in the Continuum

AbstractStemming from bound states in the continuum (BICs), momentum‐space polarization vortices observed in photonic structures provide an attractive approach to generating optical vortex (OV) beams. On the other hand, dominated by the selection rules, the harmonic generation from nanostructures exhibits a nonlinear geometric phase that depends on both the harmonic orders and the handedness of circularly polarized harmonic signals. Here, the third‐ and fifth‐harmonic optical vortex generation from an amorphous silicon photonic crystal slab, supporting the guided resonance associated with BICs at near infrared wavelengths, is numerically demonstrated. The results show that, determined by the nonlinearity phase, the topological charge (lnω) associated with the nth‐harmonic OV beams follows σ(n∓1)q, where q is the polarization charge of the BIC and the ∓ sign represents the opposite or same polarization of the nth‐harmonic signal relative to the circular polarization state (σ) of the fundamental waves. Exploiting harmonic multiplexing, this approach can significantly improve the channel capacity of OV generators based on topologically protected optical BICs.

Modeling the optical properties of twisted bilayer photonic crystals

We demonstrate a photonic analog of twisted bilayer graphene that has ultra-flat photonic bands and exhibits extreme slow-light behavior. Our twisted bilayer photonic device, which has an operating wavelength in the C-band of the telecom window, uses two crystalline silicon photonic crystal slabs separated by a methyl methacrylate tunneling layer. We numerically determine the magic angle using a finite-element method and the corresponding photonic band structure, which exhibits a flat band over the entire Brillouin zone. This flat band causes the group velocity to approach zero and introduces light localization, which enhances the electromagnetic field at the expense of bandwidth. Using our original plane-wave continuum model, we find that the photonic system has a larger band asymmetry. The band structure can easily be engineered by adjusting the device geometry, giving significant freedom in the design of devices. Our work provides a fundamental understanding of the photonic properties of twisted bilayer photonic crystals and opens the door to the nanoscale-based enhancement of nonlinear effects.

Photonic Bound States in the Continuum in Si Structures with the Self‐Assembled Ge Nanoislands

AbstractGermanium self‐assembled nanoislands and quantum dots are very prospective for CMOS‐compatible optoelectronic integrated circuits but their photoluminescence (PL) intensity is still insufficient for many practical applications. Here, it is demonstrated experimentally that the PL of Ge nanoislands in silicon photonic crystal slabs (PCS) with hexagonal lattice can be dramatically enhanced due to the involvement in the emission process of the bounds states in the continuum. These high‐Q photonic resonances allow to achieve PL resonant peaks with the quality factor as high as 2200 and with the peak PL enhancement factor of more than two orders of magnitude. The corresponding integrated PL enhancement is demonstrated to be more than one order of magnitude. This effect is studied theoretically by the Fourier modal method in the scattering matrix form. The symmetry of the quasi‐normal guided modes in the PCS is described in terms of group theory. This work paves the way toward a new class of optoelectronic components compatible with silicon technology.

Double-layer metasurface for enhanced photon up-conversion

We present a double-layer dielectric metasurface obtained by stacking a silicon nanodisk array and a silicon photonic crystal slab with equal periodicity on top of each other. We focus on the investigation of electric near-field enhancement effects occurring at resonant excitation of the metasurface and study its optical properties numerically and experimentally. We find that the major difference in multi-layer metasurfaces when compared to conventional single-layer structures appears to be in Rayleigh–Wood anomalies: they are split into multiple different modes, which are themselves spectrally broadened. As a proof of concept, we cover a double-layer metasurface with a lanthanide-doped up-conversion particle layer and study its interaction with a 1550 nm photoexcitation. We observe a 2.7-fold enhanced up-conversion photoluminescence by using the stacked metasurface instead of a planar substrate, although only around 1% of the up-conversion material is exposed to enhanced near fields. Two mechanisms are identified explaining this behavior: First, enhanced near fields when exciting the metasurface resonantly, and second, light trapping by total internal reflection in the particle layer when the metasurface redirects light into high angle diffraction orders. These results pave the way for low-threshold and, in particular, broadband photon up-conversion in future solar energy and biosensing applications.

4-Channel DWDM demultiplexer on silicon photonic crystal slab

4-Channel DWDM demultiplexer on silicon photonic crystal slab

Integrable all-optical switch for photonic integrated circuits

A completely new model of an all-optical switch (AOS) is proposed using a silicon photonic crystal (PhC) slab based nonlinear Mach–Zehnder interferometer (NMZI). Both arms of the NMZI are designed using p-toluene sulfonate (PTS) based slotted PhC waveguides (PTS-SPCWs). Extreme optical confinement in SPCW is used here to intensify optical nonlinearities of PTS. The intensified nonlinearity produces significant phase changes in optical pulses within a small waveguide length, even when the optical power is sufficiently small. One arm of the NMZI is coupled with a control wave to incur additional phase shift compared to the other due to cross-phase modulation, in addition to self-phase modulation, which is a common phenomenon for both. The differential phase shift originates destructive/constructive interference, leading to a successful switching operation of the AOS. Switching characteristics of the AOS are evaluated for a variety of parameters. A typical example shows that the AOS can switch off 12.55 mW probe power with an extinction ratio of ≈ 50.5 d B by applying 5.65 mW control power, while the device footprint is merely 259 µ m 2 . The low-power and high-speed operation makes this small footprint AOS device suitable for applications in photonic integrated circuits.

Topologically protected beam splitters and logic gates based on two-dimensional silicon photonic crystal slabs.

The beam splitters are essential optical components that are widely used in various optical instruments. The robustness of beam splitters is very necessary to all-optical networks. Here we report the design of the topologically protected beam splitter, whose splitting ratio can change flexibly to an arbitrary ratio, such as 50:50, 33:67, 25:75, based on the two-dimensional silicon photonic crystal slab. By using the 50:50 beam splitter, all major logic gates (OR, AND, NOT, XOR, NAND, XNOR, and NOR) are suitably designed with the linear interference approach. Additionally, these devices exhibit robustness even though some disorders exist. It is expected that these robust and compact devices are potentially applicable in optical computing and signal processing.

Metasurface Enhanced Sensitized Photon Upconversion: Toward Highly Efficient Low Power Upconversion Applications and Nanoscale E-Field Sensors.

Large-scale nanoimprinted metasurfaces based on silicon photonic crystal slabs were produced and coated with a NaYF4:Yb3+/Er3+ upconversion nanoparticle (UCNP) layer. UCNPs on these metasurfaces yield a more than 500-fold enhanced upconversion emission compared to UCNPs on planar surfaces. It is also demonstrated how the optical response of the UCNPs can be used to estimate the local field energy in the coating layer. Optical simulations using the finite element method validate the experimental results and the calculated spatial three-dimensional field energy distribution helps us to understand the emission enhancement mechanism of the UCNPs closely attached to the metasurface. In addition, we analyzed the spectral shifts of the resonances for uncoated and coated metasurfaces and metasurfaces submerged in water to enable a prediction of the optimum layer thicknesses for different excitation wavelengths, paving the way to applications such as electromagnetic field sensors or bioassays.

Numerical modeling of integrated electro-optic modulators based on mode-gap shifting in photonic crystal slab waveguides containing a phase change material

This paper presents novel designs and simulation results of electro-optic modulators in integrated silicon photonics platforms. The electro-optic modulators described in this paper are based on the modulation of the photonic bandgap in a silicon photonic crystal slab waveguide, consisting of a phase change material (germanium selenide) sandwiched in between silicon layers. Three-dimensional finite difference time domain (FDTD) modeling is employed to simulate two configurations of electro-optic modulators based on mode-gap shifting in photonic crystal slab waveguides—one that is designed for operation with the transverse electric (TE) polarization of light and the other for the transverse magnetic (TM) polarization. When the electric field is applied across the germanium selenide layer surrounded by doped silicon layers, there is a change of phase of the germanium selenide layer. A large refractive index contrast between the two phases of the germanium selenide layer—i.e., between the amorphous phase and the crystalline phase—on application of the electric field is responsible for shifting of the mode gap in the dispersion characteristics of the PhC slab waveguide. Our results show excellent performance of the electro-optic modulators in terms of the high On/Off extinction ratio ( > --> 37 d B in the TE configuration and > --> 28 d B in the TM configuration), broadband switching capability ( ∼ 100 n m ), low insertion loss in the On state ( < 2 d B ), high range of tunability (by changing the lattice constant and radius of air holes), and low operating voltage ( < 4 V ). The proposed devices can be used as electro-optic modulators in future nanoscale photonic integrated circuits.