Silicon Photonic Crystal Slab Research Articles

Adjustable large-area dielectric metasurfaces for near-normal oblique incident excitation

We present large-area (25 cm2) dielectric metasurfaces based on silicon photonic crystal slabs. Adjustment of the slab thickness allows to systematically shift the metasurface resonances over several hundreds of nanometers. We compute the three-dimensional field energy density near the surface and determine optimum slab thicknesses for selected near-infrared excitation wavelengths applied in biophotonics. Our simulations reveal up to 17-fold enhanced near-field energy densities at normal incidence, but over 500-fold enhancement at 4° incident excitation. We explain this behaviour via the coupling of external radiation with symmetry-protected bound states in the continuum. These results enable metasurface-enhanced spectroscopy on large areas and underline the benefit of slight oblique incidence excitation conditions.

Design of on-chip optical logic gates in 2D silicon photonic crystal slab

We have demonstrated the design of two types of on-chip optical logic gates in 2D silicon PC slab, which can support AND and XOR logic gate functions at different input frequencies. Different shapes of the center directional emitting cavity and different input/output directions can lead to different logic operations without nonlinearity or magnetism. More complicated logic networks can be built based on these integrated basic logic gates, which may lead us to construct on-chip large-scale logic circuits.

High temperature, experimental thermal memory based on optical resonances in photonic crystal slabs

We present an experimental thermal memory with direct optical control and readout. Information is stored in the internal temperature of the device, while laser illumination is used to read, write, and erase stored bits. Our design is based on an absorptive optical resonance in a silicon photonic crystal slab. When the slab is illuminated by a laser with a wavelength close to the resonance, the optical absorption is nonlinear with power, resulting in thermo-optic bistability. We experimentally demonstrate bistability in a fabricated device and show the reading, writing, and erasing of a single memory bit. A hybrid optothermal model shows good agreement with the experiment. Time dependent measurements show that the experimental write/erase times are less than 500 µs. We demonstrate that memory reliability is maintained over 106 cycles, with less than 3% change in the transmission values for the memory ON and OFF states. Our approach allows operation in high temperature and/or highly fluctuating temperature environment up to 100 °C or greater.

Fano-resonant silicon photonic crystal slab for efficient third-harmonic generation.

Strong light-matter interactions in resonant photonic nanostructures open up opportunities for enhancing nonlinear responses. In this work, by applying Fano resonances, we experimentally demonstrate efficient third-harmonic generation (THG) obtained with 2D silicon photonic crystal slabs (PCSs) thanks to the field enhancement in the dielectric layer. A 160-fold enhancement of THG is observed in the silicon PCS compared to the unpatterned silicon film. Through slightly changing the radius of the PCS, tunable THG on a single chip is obtained, paving a way for the optical manipulation of harmonic generation.

Perfect light absorption of monolayer MoS2 with cross-shaped groove air resonator

A novel configuration of a monolayer MoS2 covered on a silicon photonic crystal slab with cross-shaped groove of air resonator, supported by a gold back reflector, has been proposed to achieve perfect absorption in monolayer MoS2 both theoretically and numerically. The perfect absorption of monolayer MoS2 is explored by the critical coupling with guided resonance. Simulated results demonstrate that the absorption wavelengths of monolayer MoS2 depend on the geometrical parameters of the structure, exhibiting advantageous tunability of operating wavelength in monolayer MoS2. These results indicate that the designed optical structure might open promising technologies for improving the light–matter interaction in two-dimensional transition-metal dichalcogenides (TMDCs) and has potential applications in wavelength-sensitive optoelectronic devices.

Nanophotonic-Enhanced Two-Photon-Excited Photoluminescence of Perovskite Quantum Dots

All-inorganic CsPbBr3 perovskite colloidal quantum dots have recently emerged as promising material for a variety of optoelectronic applications, among others for multi-photon-pumped lasing. Nevertheless, high irradiance levels are generally required for such multi-photon processes. One strategy to enhance the multi-photon absorption is taking advantage of high local light intensities using photonic nanostructures. Here, we investigate two-photon-excited photoluminescence of CsPbBr3 perovskite quantum dots on a silicon photonic crystal slab. By systematic excitation of optical resonances using a pulsed near-infrared laser beam, we observe an enhancement of two-photon-pumped photoluminescence by more than one order of magnitude when comparing to using a bulk silicon film. Experimental and numerical analyses allow relating these findings to near-field enhancement effects on the nanostructured silicon surface. The results reveal a promising approach for significant decreasing the required irradiance levels for multi-photon processes being of advantage in applications like low-threshold lasing, biomedical imaging, lighting and solar energy.

Spectral characterization of silicon photonic crystal slab using out-of-plane light coupling arrangement

Fano resonance is analyzed by measuring the transmission and reflection from a photonic crystal slab containing a hexagonal symmetry of holes in silicon fabricated by electron beam lithography. The spectral features in the measured transmission and angle dependent reflection are in good agreement with those calculated using rigorous coupled wave analysis method (using DiffractMOD module of RSoft $$^{\mathrm {TM}}$$ ). This provides a convincing proof to this versatile and convenient out-of-plane measurement technique. In our photonic crystal, the quality factors measured at 1314, 1328, 1377, 1599, and 1630 nm are high and can be utilized for designing low-threshold micro-lasers and narrowband optical filters in the silicon slab platform.

Spectral-selective high-efficient light absorption of large-area monolayer transition-metal dichalcogenides via critical coupling

Through the consideration of light trapping theory, a hybrid structure with silicon (Si) photonic crystal slabs and silica (SiO2) spacer layer on gold (Au) substrate has been proposed to enhance the light absorption of monolayer transition-metal dichalcogenides (TMDCs). Complete monolayer TMDCs is utilized without any demand of manufacture process to form periodic patterns, and multiple absorption peaks with near unity absorbance at visible wavelengths can be achieved. Further analysis indicates that the strong confinement of electromagnetic (EM) field through the principle of critical coupling gives rise to the high absorption. The method is a general way to realize the total absorption in any lossy atomically thin two-dimensional (2D) materials. This favorable field enhancement and tunability of absorption not only open up new approaches to accelerate the light-matter interaction in monolayer TMDCs, but also may be of great significance in wavelength-selective photoluminescence and photodetection.

Microwatts continuous-wave pumped second harmonic generation in few- and mono-layer GaSe

We demonstrate the first achievement of continuous-wave (CW) pumped second harmonic generation (SHG) in few- and mono-layer gallium selenide (GaSe) flakes, which are coated on silicon photonic crystal (PC) cavities. Because of ultrahigh second order nonlinearity of the two-dimensional (2D) GaSe and localized resonant mode in the PC cavity, SHG’s pump power is greatly reduced to microwatts. In a nine-layer GaSe coated PC cavity, while the optical power inside the GaSe flake is only 1.5% of that in the silicon PC slab, the SHG in GaSe is more than 650 times stronger than the third harmonic generation in silicon slab, indicating 2D GaSe’s great potentials to strengthen nonlinear processes in silicon photonics. Our study opens up a new view to expand 2D materials’ optoelectronic applications in nonlinear regime and chip-integrated active devices.

Extraordinary wavelength reduction in terahertz graphene-cladded photonic crystal slabs.

Photonic crystal slabs have been widely used in nanophotonics for light confinement, dispersion engineering, nonlinearity enhancement, and other unusual effects arising from their structural periodicity. Sub-micron device sizes and mode volumes are routine for silicon-based photonic crystal slabs, however spectrally they are limited to operate in the near infrared. Here, we show that two single-layer graphene sheets allow silicon photonic crystal slabs with submicron periodicity to operate in the terahertz regime, with an extreme 100× wavelength reduction from graphene’s large kinetic inductance. The atomically thin graphene further leads to excellent out-of-plane confinement, and consequently photonic-crystal-slab band structures that closely resemble those of ideal two-dimensional photonic crystals, with broad band gaps even when the slab thickness approaches zero. The overall photonic band structure not only scales with the graphene Fermi level, but more importantly scales to lower frequencies with reduced slab thickness. Just like ideal 2D photonic crystals, graphene-cladded photonic crystal slabs confine light along line defects, forming waveguides with the propagation lengths on the order of tens of lattice constants. The proposed structure opens up the possibility to dramatically reduce the size of terahertz photonic systems by orders of magnitude.

Theory of pulsed four-wave mixing in one-dimensional silicon photonic crystal slab waveguides

We present a comprehensive theoretical analysis and computational study of four-wave mixing (FWM) of optical pulses co-propagating in one-dimensional silicon photonic crystal waveguides (Si-PhCWGs). Our theoretical analysis describes a very general set-up of the interacting optical pulses, namely we consider nondegenerate FWM in a configuration in which at each frequency there exists a superposition of guiding modes. We incorporate in our theoretical model all relevant linear optical effects, including waveguide loss, free-carrier (FC) dispersion and FC absorption, nonlinear optical effects such as self- and cross-phase modulation (SPM, XPM), two-photon absorption (TPA), and cross-absorption modulation (XAM), as well as the coupled dynamics of FCs and optical field. In particular, our theoretical analysis based on the coupled-mode theory provides rigorously derived formulae for linear dispersion coefficients of the guiding modes, linear coupling coefficients between these modes, as well as the nonlinear waveguide coefficients describing SPM, XPM, TPA, XAM, and FWM. In addition, our theoretical analysis and numerical simulations reveal key differences between the characteristics of FWM in the slow- and fast-light regimes, which could potentially have important implications to the design of ultra-compact active photonic devices.

Extremely low-loss terahertz waveguide based on silicon photonic-crystal slab.

We pursued the extremely low loss of photonic-crystal waveguides composed of a silicon slab with high resistivity (20 kΩ-cm) in the terahertz region. Propagation and bending losses as small as <0.1 dB/cm (0.326-0.331 THz) and 0.2 dB/bend (0.323-0.331 THz), respectively, were achieved in the 0.3-THz band. We also developed 1.5-Gbit/s terahertz links and demonstrated an error-free uncompressed high-definition video transmission by using a photonic-crystal waveguide with a length of as long as 50 cm and up to 28 bends thanks to the low-loss properties. Our results show the potential of photonic crystals for application as terahertz integration platforms.

Dielectric measurements of nanoliter liquids with a photonic crystal resonator at terahertz frequencies

We present a highly sensitive technique for determining the complex permittivity of nanoliter liquid samples in the terahertz band based on a photonic crystal resonator and microcapillary. Liquids are characterized by using a capillary tube to introduce a ∼4 nl liquid sample into the electromagnetic field of a resonant mode confined by an L3 resonant cavity in a high-resistivity silicon photonic crystal slab. Monitoring the perturbation of the resonant frequency and unloaded Q-factor of the resonant mode at 100 GHz and ∼5800, respectively, allows a sample's permittivity to be calculated. An analytical model describing the system response based on perturbation theory and quasi-static analysis of the electric field within the capillary is also presented and found to agree well with FEM simulations and experimental measurements of ethanol-water mixtures of various concentrations for low to moderate loss tangents of the liquid samples. We demonstrate the utility of this approach by measuring the complex permittivity of several bioliquids, including suspensions of red and white blood cells. These results represent a step towards a lab-on-a-chip device for the analysis of extremely small quantities of biological, toxic, explosive, and other liquid types at terahertz frequencies.

Photonic surfaces for designable nonlinear power shaping

We propose a method for designing nonlinear input-output power response based on absorptive resonances of nanostructured surfaces. We show that various power transmission trends can be obtained by placing a photonic resonance mode at the appropriate detuning from the laser wavelength. We demonstrate our results in a silicon photonic crystal slab at a laser wavelength of 808 nm. We quantify the overall spectral red shift as a function of laser power. The shift results from absorptive heating and the thermo-optic effect. We then demonstrate devices with increasing, decreasing, and non-monotonic transmission as a function of laser power. The transmission changes are up to 7.5 times larger than in unpatterned silicon. The strong nonlinear transmission is due to a combination of resonantly enhanced absorption, reduced thermal conductivity, and the resonant transmission lineshape. Our results illustrate the possibility of designing different nonlinear power trends within a single materials platform at a given wavelength of interest.

5 × 5 cm² silicon photonic crystal slabs on glass and plastic foil exhibiting broadband absorption and high-intensity near-fields.

Crystalline silicon photonic crystal slabs are widely used in various photonics applications. So far, the commercial success of such structures is still limited owing to the lack of cost-effective fabrication processes enabling large nanopatterned areas (≫ 1 cm2). We present a simple method for producing crystalline silicon nanohole arrays of up to 5 × 5 cm2 size with lattice pitches between 600 and 1000 nm on glass and flexible plastic substrates. Exclusively up-scalable, fast fabrication processes are applied such as nanoimprint-lithography and silicon evaporation. The broadband light trapping efficiency of the arrays is among the best values reported for large-area experimental crystalline silicon nanostructures. Further, measured photonic crystal resonance modes are in good accordance with light scattering simulations predicting strong near-field intensity enhancements greater than 500. Hence, the large-area silicon nanohole arrays might become a promising platform for ultrathin solar cells on lightweight substrates, high-sensitive optical biosensors, and nonlinear optics.

All-optical analog to electromagnetically induced transparency effects for multiple wavelengths in a silicon photonic crystal coupled cavity system

We design and theoretically study the all-optical analog to electromagnetically induced transparency (EIT) effect in a system consisting of two heterojunction cavities and one waveguide in a silicon photonic crystal slab with a triangular lattice. Multiple modes with a high Qint/Qc ratio are found in both cavities. By tuning the resonance difference and phase difference between cavities, EIT-like phenomena for two modes at different wavelengths are found. Our analyses show that optical delays of 0.34 and 0.20ns are realized for each optical EIT peak, thereby benefiting further research and applications, such as multiple-wavelength switches, optical buffer devices, and signal processing. The relationship between the characteristics of the optical EIT-like peak and several parameters is investigated, and the results can provide a reference for future experiments. The 3D finite-difference time-domain method is used to analyze the field distribution in the process.

Design of an air-slot mode-gap nanocavity in a two dimensional photonic crystal slab

We present a novel and simple design of an air-slot mode-gap photonic crystal (PC) nanocavity by introducing a linear air slot to the center of a line-defect waveguide in a two dimensional triangular-lattice silicon PC slab. A high quality factor (Q factor) of 8.42×105 and an ultrasmall mode volume of 0.998 cubic wavelength are achieved in an optimized air-slot nanocavity which is a suitable choice for the strong matter-field interaction in free space. The high Q cavities with ultrasmall mode volume are important for applications such as quantum computation and nonlinear optics.

Femtogram dispersive L3-nanobeam optomechanical cavities: design and experimental comparison

We present the design and experimental comparison of femtogram L3-nanobeam photonic crystal cavities for optomechanical studies. Two symmetric nanobeams are created by placing three air slots in a silicon photonic crystal slab where three holes are removed. The nanobeams' mechanical frequencies are higher than 600 MHz with ultrasmall effective modal masses at approximately 20 femtograms. The optical quality factor (Q) is optimized up to 53,000. The optical and mechanical modes are dispersively coupled with a vacuum optomechanical coupling rate g(0)/2π exceeding 200 kHz. The anchor-loss-limited mechanical Q of the differential beam mode is evaluated to be greater than 10,000 for structures with ideally symmetric beams. The influence of variations on the air slot width and position is also investigated. The devices can be used as ultrasensitive sensors of mass, force, and displacement.

Linear and passive silicon optical isolator

On-chip optical isolation plays a key role in optical communications and computing based on silicon integrated photonic structures and has attracted great attentions for long years. Recently there have appeared hot controversies upon whether isolation of light can be realized via linear and passive photonic structures. Here we demonstrate optical isolation of infrared light in purely linear and passive silicon photonic structures. Both numerical simulations and experimental measurements show that the round-trip transmissivity of in-plane infrared light across a silicon photonic crystal slab heterojunction diode could be two orders of magnitudes smaller than the forward transmissivity at around 1,550 nm with a bandwidth of about 50 nm, indicating good performance of optical isolation. The occurrence of in-plane light isolation is attributed to the information dissipation due to off-plane and side-way scattering and selective modal conversion in the multiple-channel structure and has no conflict with the reciprocal principle.

Fano Resonance Filtering Characteristic of High-Resistivity Silicon Photonic Crystal Slab in Terahertz Region

We investigate experimentally and numerically the Fano resonances of terahertz waves through a high resistivity silicon photonic crystal slab with a hexagonal array of circular air holes. Finite-difference time-domain simulation and terahertz time-domain spectroscopy transmission measurement show good agreement with the data, in support of the Fano resonance mechanism. The frequency dependent transmission spectra show there is a Fano resonance bandpass filtering characteristic at the center frequency of 2.10 THz with a Q factor of 191, which is consistent with that of Fano resonance theory calculation.