Silicon Ridge Research Articles

Dual-band and ultra-broadband photonic spin-orbit interaction for electromagnetic shaping based on single-layer silicon metasurfaces

Achieving electromagnetic wave scattering manipulation in the multispectral and broad operation band has been a long pursuit in stealth applications. Here, we present an approach by using single-layer metasurfaces composed of space-variant amorphous silicon ridges tiled on a metallic mirror to generate high-efficiency dual-band and ultra-wideband photonic spin-orbit interaction and geometric phase. Two scattering engineered metasurfaces have been designed to reduce specular reflection; the first one can suppress both specular reflectances at 1.05–1.08 μm and 5–12 μm below 10%. The second one is designed for an ultra-broadband of 4.6–14 μm, which is actually implemented by cleverly connecting two bands of 4.6–6.1 μm and 6.1–14 μm. Furthermore, the presented structures exhibit low thermal emission at the same time due to the low absorption loss of silicon in the infrared spectrum, which can be regarded as an achievement of laser–infrared compatible camouflage. We believe the proposed strategy may open a new route to implement multispectral electromagnetic modulation and multiphysical engineering applications.

Ultra-wideband multi-level optical modulation in a Ge2Sb2Te5-based waveguide with low power consumption and small footprint

A Ge2Sb2Te5 (GST)-based four-level pulse amplitude optical modulator is proposed and its optical and electrical properties are studied. The four-level pulse amplitude modulation (PAM4) scheme is used as a solution to overcome the inherent speed limitation of the phase-change based optical modulators. The modulator is formed by implementing two GST-based active waveguide sections in a silicon ridge waveguide where the propagation loss of each active waveguide section can be varied by changing the GST phase. Optical simulation results show that the four different output states of the modulator are separated by more than 2.5dB in the ultra-wide wavelength range of 1.4μm<λ<1.7μm, while the small overall footprint of 3.28μm2, results in low insertion loss of 1.19dB. Electrical and thermal properties of the structure are also investigated which reveal the very small energy consumption of 11.97pJ/bit and high modulation speed of 0.428Gbit/sec for the proposed PAM4 modulator. These calculations show substantial improvement in the performance parameters of the proposed structure compared to the previously reported phase-change material (PCM)-based optical modulators in terms of modulation speed and energy consumption.

Submicron gap reduction of micro-resonator based on porous silica ridge waveguides manufactured by standard photolithographic process

In this paper, we demonstrate a new method to obtain a low gap between the access waveguide and the racetrack cavity of a micro-resonator based on porous silica material and using standard photolithographic process. The photolithographic process is first carried out on porous silicon layers to obtain porous silicon ridge waveguides that constitutes the racetrack micro-resonator. Then the full oxidation of the micro-resonator is performed. Because of the volume expansion of silicon that occurs during its full oxidation, the gap is reduced. Lower gaps and then lower coupling distances can be obtained using a low cost photolithographic process compared to e-beam technology. This original method proposed in this paper allows to improve the miniaturization of such porous silica micro-resonators which is of great interest for integrated optical biosensor application.

High-performance thermo-optic tunable grating filters based on laterally supported suspended silicon ridge waveguide.

We demonstrate a novel thermally tunable grating optical filter based on suspended silicon ridge waveguide with low power consumption, fast response and relatively high mechanical stability. An enhanced power efficiency of ~300 pm/mW is achieved by employing isolation trench. Periodic lateral tethers and anchors are utilized to support the overall waveguide for maintaining mechanical robustness. 10%-90% rising time and falling time are measured to be 78 μs and 52 μs, respectively. Simulations and experiments both indicate that the demonstrated device has a relatively homogeneous temperature distribution, which is essential for practical integrated photonic devices.

Broadband spectra translation between mid-infrared band and telecom band in a nonlinear silicon-based symmetric hybrid plasmonic waveguide

A symmetric hybrid plasmonic waveguide is designed to realize high-efficient conversion between mid-infrared (MIR) and telecom wavelength bands. The proposed hybrid plasmonic waveguide consists of an insulator–metal–insulator structure buried into a silicon ridge on silica substrate. By using geometry optimization, the dispersion of waveguide is optimized, leading to broadband (resulting in a 3-dB bandwidth of 1,919[Formula: see text]nm from 1.496[Formula: see text][Formula: see text]m to 3.415[Formula: see text][Formula: see text]m) and efficient (resulting in a peak conversion efficiency of [Formula: see text]18.9[Formula: see text]dB) four-wave mixing (FWM) in a 200-[Formula: see text]m-long waveguide pumped by a 1.725-[Formula: see text]m source of 0.5 W. This waveguide configuration shows its great potential in all-optical switching between MIR band and telecom band.

Ultracompact Si-GST Hybrid Waveguides for Nonvolatile Light Wave Manipulation

Phase change materials combined with silicon photonics are emerging as a promising platform to realize miniature photonic devices. We study the basic optical properties of a subwavelength-dimension silicon ridge waveguide with a 20-nm-thick Ge2Sb2Te5 (GST) top-clad layer. Numerical simulations show that the effective index of the Si-GST hybrid waveguide varies significantly when the GST changes from the amorphous to the crystalline states. This change can be utilized to make micron-size photonic devices. To experimentally verify the effectiveness of the Si-GST hybrid waveguide on light wave manipulation, we fabricated a series of unbalanced Mach-Zehnder interferometers with one arm connected with a section of Si-GST hybrid waveguide in different lengths. The transmission spectra are measured and the complex effective indices are extracted for GST at crystalline, amorphous, and intermediate phases. The experimental results overall agree well with the simulation ones. The nonvolatile property of GST makes it attractive to reduce the static power consumption. This research represents a significant step toward the realization of ultracompact Si-GST hybrid devices that will play a key role in high-density photonic integrated circuits, opening the door to many potential applications, including optical switch, memory, and logic operation.

Mid-infrared self-similar compression of picosecond pulse in an inversely tapered silicon ridge waveguide

On chip high quality and high degree pulse compression is desirable in the realization of integrated ultrashort pulse sources, which are important for nonlinear photonics and spectroscopy. In this paper, we design a simple inversely tapered silicon ridge waveguide with exponentially decreasing dispersion profile along the propagation direction, and numerically investigate self-similar pulse compression of the fundamental soliton within the mid-infrared spectral region. When higher-order dispersion (HOD), higher-order nonlinearity (HON), losses (α), and variation of the Kerr nonlinear coefficient γ(z) are considered in the extended nonlinear Schrodinger equation, a 1 ps input pulse at the wavelength of 2490 nm is successfully compressed to 57.29 fs in only 5.1-cm of propagation, along with a compression factor Fc of 17.46. We demonstrated that the impacts of HOD and HON are minor on the pulse compression process, compared with that of α and variation of γ(z). Our research results provide a promising solution to realize integrated mid-infrared ultrashort pulse sources.

Design and fabrication of hybrid SPP waveguides for ultrahigh-bandwidth low-penalty terabit-scale data transmission

Here we design and fabricate a hybrid surface plasmon polarities (SPP) waveguide on the silicon-on-insulator (SOI) photonics platform. The designed hybrid SPP waveguide is composed of a metal ridge, an air gap, and a silicon ridge. We simulate the mode characteristics in the structure and design the waveguide with a wide air gap that can simplify the fabrication process and maintain the advantages of the hybrid SPP mode. The performance of ultrahigh-bandwidth data transmission through the proposed waveguide is then investigated using 161 wavelength-division multiplexing (WDM) channels, each carrying a 11.2-Gbit/s orthogonal frequency-division multiplexing (OFDM) 16-ary quadrature amplitude modulation (16-QAM) signal. The bit-error rates (BERs) of all 161 channels are less than 1e-3. The favorable results show the prospect of on-chip optical interconnection using the proposed hybrid SPP waveguide.

Porous silicon micro-resonator implemented by standard photolithography process for sensing application

A micro-resonator based on porous silicon ridge waveguides is implemented by a large scale standard photolithography process to obtain a low cost and sensitive sensor based on volume detection principle instead of the evanescent one usually used. The porous nature of the ridge waveguides allows the target molecules to be infiltrated in the core and to be detected by direct interaction with the propagated light. Racetrack resonator with radius of 100 μm and a coupling length of 70 μm is optically characterized for the volume detection of different concentrations of glucose. A high sensitivity of 560 nm/RIU is reached with only one micro-resonator and a limit of detection of 8.10−5 RIU, equivalent to a glucose concentration of 0.7 g/L, is obtained.

Electric field-induced second-order nonlinear optical effects in silicon waveguides

The demand for nonlinear effects within a silicon platform to support photonic circuits requiring phase-only modulation, frequency doubling, and/or difference frequency generation, is becoming increasingly clear. However, the symmetry of the silicon crystal inhibits second order optical nonlinear susceptibility, $\chi^{(2)}$. Here, we show that the crystalline symmetry is broken when a DC field is present, inducing a $\chi^{(2)}$ in a silicon waveguide that is proportional to the large $\chi^{(3)}$ of silicon. First, Mach-Zehnder interferometers using the DC Kerr effect optical phase shifters in silicon ridge waveguides with p-i-n junctions are demonstrated with a $V_{\pi}L$ of $2.4Vcm$ in telecom bands $({\lambda}_{\omega}=1.58{\mu}m)$ without requiring to dope the silicon core. Second, the pump and second harmonic modes in silicon ridge waveguides are quasi-phase matched when the magnitude, spatial distribution of the DC field and $\chi^{(2)}$ are controlled with p-i-n junctions. Using these waveguides, second harmonic generation at multiple pump wavelengths are observed with a maximum efficiency of $P_{2{\omega}}/P_{\omega}^2$=12%/W at ${\lambda}_{\omega}=2.29{\mu}m$ in a 1mm long waveguide. This corresponds to a field-induced $\chi^{(2)}=41pm/V$, comparable to non-centrosymmetric media (LiNbO3, GaAs, GaN). The field-induced nonlinear silicon photonics will lead to a new class of CMOS compatible integrated devices spanning from near to mid infrared spectrum.

Photon-assisted tunneling for sub-bandgap light detection in silicon PN-doped waveguides.

We demonstrate silicon ridge waveguide photo-detectors capable of sub-bandgap light absorption and avalanche multiplication. The proposed waveguide photo-detectors contain highly doped PN junction, where a strong electric field can generate the photon-assisted tunneling current for sub-bandgap light incidence and amplify the generated photo-current by the avalanche multiplication effect. The voltage-dependent sub-bandgap absorption coefficient and multiplication gain are experimentally evaluated for various doping configurations to find optimal photo-response with low dark currents. As a result, our representative silicon waveguide photo-detector gives sub-bandgap responsivities of ~10 and ~2 A/W under the applied reverse bias voltage of -8.3 V for near-infrared wavelengths of 1.31 and 1.52 μm, respectively. The voltage-dependent frequency photo-response is also demonstrated with theoretical verification.

Research progress of III–V laser bonding to Si

The vigorous development of silicon photonics makes a silicon-based light source essential for optoelectronics' integration. Bonding of III–V/Si hybrid laser has developed rapidly in the last ten years. In the tireless efforts of researchers, we are privileged to see these bonding methods, such as direct bonding, medium adhesive bonding and low temperature eutectic bonding. They have been developed and applied to the research and fabrication of III–V/Si hybrid lasers. Some research groups have made remarkable progress. Tanabe Katsuaki of Tokyo University successfully implemented a silicon-based InAs/GaAs quantum dot laser with direct bonding method in 2012. They have bonded the InAs/GaAs quantum dot laser to the silicon substrate and the silicon ridge waveguide, respectively. The threshold current of the device is as low as 200 A/cm2. Stevan Stanković and Sui Shaoshuai successfully produced a variety of hybrid III–V/Si laser with the method of BCB bonding, respectively. BCB has high light transmittance and it can provide high bonding strength. Researchers of Tokyo University and Peking University have realized III–V/Si hybrid lasers with metal bonding method. We describe the progress in the fabrication of III–V/Si hybrid lasers with bonding methods by various research groups in recent years. The advantages and disadvantages of these methods are presented. We also introduce the progress of the growth of III–V epitaxial layer on silicon substrate, which is also a promising method to realize silicon-based light source. I hope that readers can have a general understanding of this field from this article and we can attract more researchers to focus on the study in this field.

Optical bound states in slotted high-contrast gratings

This paper investigates the optical bound states in the continuum (BIC) supported by a slotted high-contrast grating (sHCG) structure. The sHCG structure consists of a periodic array of silicon ridges with a slot in each ridge. Given that the BICs are perfectly confined, their spectral locations are identified using a finite-element method formulated from a generalized eigenvalue problem. The real eigenvalues represent the wavelengths of BIC modes and the associated eigenvectors correspond to the electric field distributions. In the spectral and angular vicinity of the BICs, we studied the leaky waveguide modes using the rigorous coupled-wave analysis. The combination of the full-wave eigenvalue solver and the coupled-wave analysis provides an ideal setting to investigate the optical BICs of periodic structures for various applications. The simulation results show, for example, that the sHCG structures can support symmetry-protected bound states with a zero in-plane wave vector as well as high-quality-factor (high-Q) resonances for both TE and TM polarizations. By adjusting the slot, we can turn the BIC mode into high-Q modes and determine the linewidth of the mode by the degree of asymmetry.

Proposal of a broadband, polarization-insensitive and high-efficiency hot-carrier schottky photodetector integrated with a plasmonic silicon ridge waveguide

We propose a broadband, polarization-insensitive and high-efficiency plasmonic Schottky diode for detection of sub-bandgap photons in the optical communication wavelength range through internal photoemission (IPE). The distinctive features of this design are that it has a gold film covering both the top and the sidewalls of a dielectric silicon ridge waveguide with the Schottky contact formed at the gold–silicon interface and the sidewall coverage of gold can be easily tuned by an insulating layer. An extensive physical model on IPE of hot carriers is presented in detail and is applied to calculate and examine the performance of this detector. In comparison with a diode having only the top gold contact, the polarization sensitivity of the responsivity is greatly minimized in our photodetector with gold film covering both the top and the sidewall. Much higher responsivities for both polarizations are also achieved over a broad wavelength range of 1.2–1.6 μm. Moreover, the Schottky contact is only 4 μm long, leading to a very small dark current. Our design is very promising for practical applications in high-density silicon photonic integration.

Silicon nanoridge array waveguides for nonlinear and sensing applications.

We fabricate and characterize waveguides composed of closely spaced and longitudinally oriented silicon ridges etched into silicon-on-insulator wafers. Through both guided mode and bulk measurements, we demonstrate that the patterning of silicon waveguides on such a deeply subwavelength scale is desirable for nonlinear and sensing applications alike. The proposed waveguide geometry simultaneously exhibits comparable propagation losses to similar schemes proposed in literature, an enhanced effective third-order nonlinear susceptibility, and high sensitivity to perturbations in its environment.

Substrate topography guides pore morphology evolution in nanoporous gold thin films

This paper illustrates the effect of substrate topography on morphology evolution in nanoporous gold (np-Au) thin films. One micron-high silicon ridges with widths varying between 150nm and 50μm were fabricated and coated with 500nm-thick np-Au films obtained by dealloying sputtered gold–silver alloy films. Analysis of scanning electron micrographs of the np-Au films following dealloying and thermal annealing revealed two distinct regimes where the ratio of film thickness to ridge width determines the morphological evolution of np-Au films.

Enhanced performance of graphene-based electro-absorption waveguide modulators by engineered optical modes

Electro-absorption modulators based on electrically contacted double-layer graphene optimally incorporated in plasmonic and photonic waveguide configurations were simulated and analyzed in terms of the device performance at telecom wavelengths. It is shown that increasing the mode electric field strength on the graphene layers enhances absorption of graphene and, in consequence, improves the electro-optic performances. The ratio of the change in extinction ratio and the waveguide loss (Δα/α) is used as a figure of merit. A plasmonic waveguide configuration with a silicon ridge has a simulated 3 dB modulation depth for a device length of ~140 nm and Δα/α ~ 20. The calculated energy consumption per bit is as low as ~240 aJ bit−1 and ~1.8 aJ bit−1 for plasmonic modulators with polymer and silicon ridge waveguides respectively. Much higher figures of merit were obtained for modulators based on photonic waveguides with Δα/α exceeding 220 for a waveguide with a TM-supported mode. This comes at the cost of the modulator length, which increases to over 500 nm, and the calculated energy per bit of 1.93 fJ bit−1 for polymer and ~10.3 aJ bit−1 for silicon waveguides. The photonic waveguides were designed to support both TM and TE modes. The TE mode requires a much longer modulation length of ~10 µm to achieve a 3 dB modulation depth and shows a lower figure of merit of ~12 compared to the TM mode, but has a low energy per bit of ~44.0 aJ bit−1. The TE mode is in the OFF state at low applied voltage.

The Lens Descends. Optics, reimagined.

The article focuses on research into planar lenses, stating in 2012 physicist and engineer Federico Capasso and colleagues developed a rudimentary flat, ultrathin lens that focused light through microscopic silicon ridges, and mentions his team has moved beyond proof of concept to develop a lens that can yield multicolor images.

Optical biosensor based on a silicon nanowire ridge waveguide for lab on chip applications

We propose a novel sensor using a silicon nanowire ridge waveguide (SNRW). This waveguide is comprised of an array of silicon nanowires on an insulator substrate that has the envelope of a ridge waveguide. The SNRW inherently maximizes the overlap between the material-under-test and the incident light wave by introducing voids to the otherwise bulk structure. When a sensing sample is injected, the voids within the SNRW adopt the refractive index of the material-under-test. Hence, the strong contribution of the material-under-test to the overall modal effective index will greatly augment the sensitivity. Additionally, the ridge structure provides a fabrication convenience as it covers the entire substrate, ensuring that the etching process would not damage the substrate. Finite-difference time-domain simulations are conducted and showed that the percentage change in the effective index due to a 1% change in the surrounding environment is more than 170 times the change perceived in an evanescent-detection based bulk silicon ridge waveguide. Moreover, the SNRW proves to be more sensitive than recent other, non-evanescent sensors. In addition, the detection limit for this structure was revealed to be as small as 10−8. A compact bimodal waveguide based on SNRW is designed and tested. It delivers high sensitivity values that offer comparable performance to similar low-index light-guiding sensing configurations; however, our proposed structure has much smaller footprints and allows high dense integration for lab-on-chip applications.

Ultra-Compact Silicon Photonic 512 × 512 25 GHz Arrayed Waveguide Grating Router

This paper discusses design, fabrication, and characterization of a 512 × 512 arrayed waveguide grating router (AWGR) with a channel spacing of 25 GHz. The dimensions of the AWGR is 16 mm × 11 mm and is fabricated on a 250 nm silicon-on-insulator platform. The measured channel crosstalk is approximately -4 dB without any compensation for the phase errors in the arrayed waveguides. The AWGR spectrum in the arrayed waveguide grating arms were characterized by using an optical vector network analyzer. Fabrication details of obtaining low loss silicon ridge waveguides are also discussed.