Subwavelength Silicon Research Articles

High-harmonic generation from subwavelength silicon films

Abstract Recent years have witnessed significant developments in the study of nonlinear properties of various materials at the nanoscale. Often, experimental results on harmonic generation are reported without the benefit of suitable theoretical models that allow assessment of conversion efficiencies compared to the material’s intrinsic properties. Here, we report experimental observations of even and odd harmonics up to the 7th, generated from a suspended subwavelength silicon film resonant in the UV range at 210 nm, the current limit of our detection system, using peak power densities of order 3 TW/cm2. We also highlight the time-varying properties of the dielectric function of silicon, which exhibits large changes under intense illumination. We explain the experimental data with a time domain, hydrodynamic-Maxwell approach broadly applicable to most optical materials. Our approach accounts simultaneously for surface and magnetic nonlinearities that generate even optical harmonics, as well as linear and nonlinear material dispersions beyond the third order to account for odd optical harmonics, plasma formation, and a phase locking mechanism that makes the generation of high harmonics possible deep into the UV range, where semiconductors like silicon start operating in a metallic regime.

Silicon Sub-wavelength Grating based Scalable Selective Higher Order Mode Pass Filter for Dual-band Operation

Silicon Sub-wavelength Grating based Scalable Selective Higher Order Mode Pass Filter for Dual-band Operation

Enhancing stimulated Brillouin scattering in suspended silicon waveguides through subwavelength nanostructuration [Invited

Brillouin optomechanics is playing a key role in the development of groundbreaking devices and novel functionalities in integrated silicon photonics, such as narrow linewidth filtering and lasers, tunable frequency, non-reciprocity, etc. Most silicon-based optomechanical waveguides, which use anchoring arms or perforated slabs to ensure mechanical stability and operate for transverse-electric polarized light, face challenges with acoustic mode leakage into the lateral Si slab, limiting the photon-phonon overlap and the Brillouin gain. Here, we propose new waveguide designs based on subwavelength nanostructuration to tailor near-infrared photons and GHz phonons and maximize the Brillouin gain. We introduce six different geometries suitable for both membrane or fully suspended configurations (i.e., without transversal arms anchoring the core to the Si slab). Our three-dimensional optomechanical simulations predict that subwavelength silicon membranes with strip, slot, and SWG slot core waveguides achieve gains up to 12257 W-1m-1 at mechanical frequencies of 12-13 GHz. Moreover, suspended silicon waveguides with SWG slots achieve a high gain of 43542 W-1m-1 at 4.45 GHz, with the ability to adjust the mechanical frequency from 4 to 9 GHz. Further enhancements in the Brillouin gain are studied by integrating side arms to amplify the moving boundaries effect in the suspended SWG slot waveguides and leveraging the slow light regime, which can significantly increase the Brillouin gain up to 17 × 106 W-1m-1 for a mechanical mode at 11.18 GHz.

Silicon Subwavelength Grating Coupler With Ultra-High Reproducibility, Ultra-Wide Bandwidth, and Ultra-Low Back Reflection

Silicon Subwavelength Grating Coupler With Ultra-High Reproducibility, Ultra-Wide Bandwidth, and Ultra-Low Back Reflection

Liquid Crystal Integrated Dynamic Terahertz Metalenses

AbstractMetasurfaces can freely manipulate the wavefront of electromagnetic waves with the merits of minimization and integration. Dynamic metadevices have continued to be urgently pursued to realize on‐demand functionalities. Among them, dynamic metalenses have attracted particular attention due to their widespread applications, especially in the promising terahertz region. Here, a liquid crystal integrated terahertz metalens with dynamic focusing properties is proposed. Subwavelength silicon columns are specifically designed and set to realize the same lens phase profile for orthogonal linear polarizations, while the rotation of columns additionally introduces a spin‐selective conjugated geometric phase. A photopatterned liquid crystal layer is added to compensate for the geometric phase; therefore, a polarization‐independent single focus is achieved. With a saturated bias applied to the liquid crystal, the phase compensation vanishes; therefore, a spin‐selective bifocal function is realized. When lens and polarization grating profiles are encoded to the geometric phase separately, longitudinal and lateral spin‐dependent focus separations are demonstrated. The metalens can be electrically switched between single‐focus and bifocal functions. The dynamic metalens can be utilized in depth‐of‐field imaging and telecom port selection. The design endows various existing metadevices with dynamic functions.

Angular momentum modulation of vortex Cherenkov radiation in optical waveguides.

Vortex free-electron radiation has attracted considerable interest because of its promising potential for applications in communication, high-density radiation sources, and particle detection. Here, we reveal angular momentum modulation of vortex Cherenkov radiation using subwavelength silicon waveguides. The topological charge of vortex radiation field can be controlled by the position parameters of two electron beams based on the rotational symmetry. Besides, the spin angular momentum is accompanied by the excited orbital angular momentum due to the spin-orbit interaction of light. In particular, the periodic evolution of spin and orbital angular momenta are demonstrated by breaking the symmetry of the waveguide. Our results provide a novel mechanism for flexibly regulating vortex electron radiation and exploring electro-optical interaction.

Influence of discontinuities on photonic waveguides.

Fabrication-induced imperfections in photonic wire waveguides, such as roughness, stitching errors, and discontinuities, degrade their performance and thereby lower the yield of large-scale systems. This degradation is primarily due to the high insertion losses induced by imperfections, which scale nonlinearly with the index contrast in wire waveguides. Here we investigate the influence of discontinuities in photonic waveguides and later show a platform that is robust to fabrication imperfections. Our platform is based on an array of silicon nano-pillars, arranged to form a sub-wavelength (SW) grating waveguide. We focus on investigating the robustness by considering an abrupt break in the waveguide, as an extreme case of discontinuity. We show that sub-wavelength silicon waveguides are robust against unwanted large discontinuities relative to the operating wavelength. We measure a transmission loss of <2.2 dB at 1550 n m, for a discontinuity of length 2.1 μ m, when compared to more than 7 d B of loss in conventional silicon wire waveguides for the same discontinuity. Our results show that this mode of protection is broadband, covering the entire telecommunication band (λ =1500-1600 nm). We believe that this investigation of the influence of discontinuities in photonic waveguides could be a step toward the realization of low-loss optical waveguides.

Large effective length and high efficiency by embedding L-shaped radiating blocks in subwavelength grating slot waveguide

Integrated optical antennas featuring large effective lengths and high radiation efficiency (RE) are essential to chip-scale light detection and ranging technology. However, there are challenges in simultaneously achieving both large effective length and high RE in silicon photonics platforms with high refractive index contrast. For traditional silicon waveguide grating antennas based on silicon photonics, the RE is relatively low and the antenna effective lengths are constrained to several hundred micrometers because of the high refractive index contrast. In this article, an embedded grating waveguide antenna which is L-shaped radiating blocks embedded in the slot gap of a silicon subwavelength grating slot waveguide is proposed and investigated numerically. Simulation results indicate the antenna’s effective length is above 4.25 mm, and the far-field divergence angle is near 0.0197°. Using L-shaped radiation blocks can break the diffraction’s perpendicular symmetry and increase the antenna’s RE to near 0.75 at 1550 nm compared to traditional design. Meanwhile, the antenna’s field-of-view reaches around 17.5° × 48.15° (θ× ϕ) to satisfy the needs of conventional optical phased arrays.

Metasurface optical trap array for single atoms

Metasurfaces made of subwavelength silicon nanopillars provide unparalleled capacity to manipulate light, and have emerged as one of the leading platforms for developing integrated photonic devices. In this study, we report on a compact, passive approach based on planar metasurface optics to generate large optical trap arrays. The unique configuration is achieved with a meta-hologram to convert a single incident laser beam into an array of individual beams, followed up with a metalens to form multiple laser foci for single rubidium atom trapping. We experimentally demonstrate two-dimensional arrays of 5 × 5 and 25 × 25 at the wavelength of 830 nm, validating the capability and scalability of our metasurface design. Beam waists ∼1.5 µm, spacings (about 15 µm), and low trap depth variations (8%) of relevance to quantum control for an atomic array are achieved in a robust and efficient fashion. The presented work highlights a compact, stable, and scalable trap array platform well-suitable for Rydberg-state mediated quantum gate operations, which will further facilitate advances in neutral atom quantum computing.

Coherent energy transfers between orthogonal modes of a dielectric cavity bridged by a plasmonic antenna

Here, we demonstrate a strategy that two orthogonal modes in a dielectric cavity can efficiently couple with each other through the bridging effect of a plasmonic antenna. In such a dielectric-antenna hybrid system, a plasmonic antenna can coherently interact with both modes of the dielectric cavity, which brings sufficient coherent energy transfers between the two orthogonal modes. Specifically, a broad electromagnetic mode and a narrow whispering gallery mode (WGM) in a subwavelength silicon disk are considered, where they cannot directly interact with each other through near-field couplings. By introducing a plasmonic antenna, coherent energy transfer between the above two modes occurs, which is confirmed by both far-field spectra and near-field distributions. More investigations show that spectral and spatial overlaps between the involved modes can largely affect energy transfer behaviors. Those overlaps are highly dependent on various parameters of the system. The WGM response in the hybrid system can even exceed that of an individual disk. Our proposed strategy can be extended to other similar systems and the modified optical responses can find applications in enhanced light-matter interactions.

Brillouin lasing in microspheres coupled with silicon subwavelength grating waveguides

Brillouin lasing has been widely applied to microwave oscillators, optical gyroscopes, optical sensors, and other fields owing to its strong and flexible dynamics. Microspheres with ultrahigh quality factors and small mode volume are very suitable for generating Brillouin lasing, but it is difficult to integrate them on a chip. Here, we utilize subwavelength grating waveguide as a photonic coupler to connect the silica microsphere with a silicon chip and realize on-chip backward Brillouin lasing. It is experimentally indicated that when the on-chip pump power reaches 24.8 mW, the backward Brillouin lasing with an on-chip slope efficiency of 5% will be excited. Our method makes it easier to realize on-chip Brillouin acousto-optic interaction and greatly expands the application range of microspheres.

Recent advances in metamaterial integrated photonics

Since the invention of the silicon subwavelength grating waveguide in 2006, subwavelength metamaterial engineering has become an essential design tool in silicon photonics. Employing well-established nanometer-scale semiconductor manufacturing techniques to create metamaterials in optical waveguides has allowed unprecedented control of the flow of light in photonic chips. This is achieved through fine-tuning of fundamental optical properties such as modal confinement, effective index, dispersion, and anisotropy, directly by lithographic imprinting of a specific subwavelength grating structure onto a nanophotonic waveguide. In parallel, low-loss mode propagation is readily obtained over a broad spectral range since the subwavelength periodicity effectively avoids losses due to spurious resonances and bandgap effects. In this review we present recent advances achieved in the surging field of metamaterial integrated photonics. After briefly introducing the fundamental concepts governing the propagation of light in periodic waveguides via Floquet–Bloch modes, we review progress in the main application areas of subwavelength nanostructures in silicon photonics, presenting the most representative devices. We specifically focus on off-chip coupling interfaces, polarization management and anisotropy engineering, spectral filtering and wavelength multiplexing, evanescent field biochemical sensing, mid-infrared photonics, and nonlinear waveguide optics and optomechanics. We also introduce a nascent research area of resonant integrated photonics leveraging Mie resonances in dielectrics for on-chip guiding of optical waves, with the first Huygens’ metawaveguide recently demonstrated. Finally, we provide a brief overview of inverse design approaches and machine-learning algorithms for on-chip optical metamaterials. In our conclusions, we summarize the key developments while highlighting the challenges and future prospects.

Features of a Gaussian beam near-field diffraction upon variations in the relief height of subwavelength silicon optical elements

Features of the near-field diffraction of Gaussian beams and Laguerre-Gauss modes by silicon subwavelength optical elements with varying relief height were studied in this paper using a finite-difference time-domain method. Diffractive axicons and subwavelength ring gratings were considered as the optical elements with varying relief height. It was shown that the height of individual rings of the ring grating relief can be selected in such a way that the focal spot size is reduced to 0.26λ, an extended intensity line of length up to 4.88λ is generated and optical traps are formed.

Flexible transmission control of mode conversion in a TiO2 cladding silicon subwavelength waveguide by engineering inter-waveguide mode coupling

Mode conversions can be effectively accomplished in coupled mode silicon photonic waveguide systems when specific phase-matching conditions are fulfilled. However, conventional device using such principles is optimized to a certain coupling length, which can only achieve a fixed energy transmission. Moreover, the conventional method to tune the refractive index is insufficient for effective transmission manipulation. In this paper, we systematically analyze the coupling strategy in the silicon-based dual-waveguide coupled-mode system, and propose an effective transmission tuning method by engineering the inter-waveguide mode coupling. We introduce titanium dioxide as cladding material to a subwavelength waveguide-based directional coupler to change the refractive index variable, which is both CMOS compatible and has negative thermo-optical coefficient. By implementing different temperature on waveguide, the dispersion curves of stripe waveguide and subwavelength waveguide can be tailored independently, thus the coupling coefficient can be changed without changing the geometry of waveguide. We demonstrate prototype of TE0-TE1 multi-mode variable optical attenuator. The output can be tuning 0.05%–96.4% of input energy when the temperature rises 0-400 K. We extend the device to TE2 and TE3 with output intensities of 0.04-91.4% and 0.04-88.8% over the temperature rise range of 317 K and 214 K, respectively. Cascading mode converters for different mode-order, we can arbitrarily mix the mode percentage in output waveguide to generate different interference patterns. We demonstrate a reconfigurable optical tweezer using this setup, which is capable of manipulating nanoscale particles. We envision that such methodology can be implemented in flexible multimode optical networks and labs on chip.

Photonic management of silicon nanocylinder arrays to enhance photovoltaic performance

The light–matter interaction of subwavelength and periodic silicon (Si) nanostructures strongly correlates with their geometrical features, resulting in them being highly unsuitable for the practical development of Si-based photovoltaic applications. In this study, the concepts of effective medium and retrieval methods are needed to deal with the subwavelength periodic dielectric structure. Using finite-difference time-domain simulations, we study the interactions of electromagnetic radiation with a square array of dielectric rods parallel to the incident light, and the effective optical properties such as refractive index, permittivity, and permeability are calculated. Furthermore, the electric field distributions are also plotted for a deeper understanding of the energy changes within Si nanocylinder arrays (SiNCAs) under different incident wavelengths of radiation. By employing calculated optimized SiNCAs for the construction of hybrid solar cells, improved cell performances showing a conversion efficiency of 13.79% are demonstrated, with further estimation by electrical chemical measurements for a better understanding of the carrier transition. These are numerically and experimentally interpreted by the involvement of excellent light-trapping effects, delivering a method to design correlated photovoltaic devices.

All-dielectric metasurface refractive index sensor with high figure of merit based on quasi-bound states in the continuum

All-Dielectric metasurfaces are widely used in the field of sense due to their small size, freedom of design, and low radiation loss. However, the optical sensor still has the problem of wide full width at half maximum (FWHM), which leads to a low figure of merit (FOM). In order to solve this problem, an all-dielectric metasurface based on quasi-bound states in the continuum (q-BIC) was proposed for refractive index sensing. The sensor structure consisted of a prism/metasurface/analyte, where the metasurface was composed of periodically arranged subwavelength asymmetric silicon gratings. The asymmetry of the structure led to the coupling of the tangential and radiative field components in the superficial layer at the resonance position, which formed a q-BIC resonance peak with a narrow FWHM in the reflection spectrum. We explained the causes of the q-BIC resonance peak generation by finite element analysis, and analyzed the factors affecting the FWHM, and also simulated the machining errors. The results showed that by reducing the asymmetry of the sensor, it could have a high FOM, with a FOM of 1.35 × 107 RIU−1. In addition, simulated machining errors within ±5 nm showed that small machining errors did not lead to the disappearance of q-BIC, which proved that the sensor had good robustness. These findings had provided ideas for the accurate detection of trace substances.

Silicon Subwavelength Grating Slot Waveguide based Optical Sensor for Label Free Detection of Fluoride Ion in Water

Recently, Subwavelength Grating Waveguides (SWG WG) attract a lot of attention for sensing applications because it offers better sensitivity than conventional WGs. The sensitivity of SWG WG is greatly improved due to the interaction between the analyte and guiding mode in the SWG segments. In this study, a highly sensitive sensor based on a silicon SWG slot waveguide (SWGSW) is proposed for detecting low-concentration fluoride contaminations in water. The gap called as “slot” between the silicon slab along with cladding and the spaces between the segmented silicon rail waveguide is filled with fluoride ion-contaminated water samples as analyte for strong light–matter interaction between guiding modes of the SWG structure, resulting in greater sensitivity of the proposed sensor. The sensitivity of the proposed structure is estimated by numerical simulations as S = 615.38 nm/RIU which is comparable to the existing reported optical sensor structures based on optical waveguide and optical fibre with a smaller footprint. Additionally, the effect of variation of geometrical parameters on the transmission resonances of the proposed structure is also analyzed for the optimization of a SWG WG structure. The obtained results will be helpful to realize and design highly sensitive sensors based on the SOI platform.

Terahertz bandpass filter with Babinet complementary metamaterial mirrors and silicon subwavelength structure.

We fabricated a rigid bandpass filter with a broad far-infrared wavelength range of high transmission using a silicon subwavelength structure with a Babinet complementary metamaterial half-mirror pair, despite its apparent light-blocking structure. The rigid one-piece filter was produced by a simple process involving photolithography, dry etching, and deposition, each performed only once. The transmission principle relies on the Fabry-Perot resonance with a metamaterial half-mirror pair that exhibits extraordinary optical transmission due to spoof surface plasmon polaritons. The transmission center wavelength was successfully predicted by the basic equation of Fabry-Perot resonance with an effective medium approximation. In contrast, a narrower bandwidth and a lower minimum transmittance than those predicted from the basic equation were provided by the subwavelength Si structure between the metamaterial mirrors, resulting in enhanced bandpass filter characteristics.

Tunable nonlocal metasurfaces based on graphene for analogue optical computation

Meta-optical devices have recently emerged as ultra-compact candidates for real-time computation in the spatial domain. The use of meta-optics for applications in image processing and wavefront sensing could enable an order of magnitude increase in processing speed and data throughput, while simultaneously drastically reducing the footprint of currently available solutions to enable miniaturisation. Most research to date has focused on static devices that can perform a single operation. Dynamically tunable devices, however, offer increased versatility. Here we propose graphene covered subwavelength silicon carbide gratings as electrically tunable optical computation and image processing devices at mid-infrared wavelengths.

Nanoantennas for selectively switching on and off under different input phase distributions and wavelengths

Optical nanoantennas, as devices connecting free-space light and on-chip light, are important components of integrated photonic circuits. They are often used as optical couplers, and in optical routine of intermediate layers of IC chips. To have more degrees of freedom to manipulate the light scattering, we design a novel optical nanoantenna composed of subwavelength silicon pillars. The antenna can exhibit on and off scattering states under different input optical modes and exhibit opposite on-off characteristics at different wavelengths. The antenna is small in size and flexible in design, and has the potential to build a more flexible 3D optical link for photonic integrated circuits.