Silicon Photonic Crystal Waveguide Research Articles

Microcavity induced by a few-layer GaSe crystal on a silicon photonic crystal waveguide for efficient optical frequency conversion.

We demonstrate the post-induction of high-quality microcavities on a silicon photonic crystal (PC) waveguide by integrating a few-layer GaSe crystal, which promises efficient on-chip optical frequency conversions. The integration of GaSe shifts the dispersion bands of the PC waveguide mode into the bandgap, resulting in localized modes confined by the bare PC waveguides. Thanks to the small contrast of refractive index at the boundaries of the microcavity, it is reliable to obtain quality factors exceeding 104. With the enhanced light-GaSe interaction by the microcavity modes and GaSe's high second-order nonlinearity, remarkable second-harmonic generation (SHG) and sum-frequency generation (SFG) are achieved with continuous-wave (CW) lasers.

Application of slow-light engineered chalcogenide and silicon photonic crystal waveguides for double-dimensional microwave frequency identification

Four-wave mixing effect was utilized within short dispersion-engineered slow-light silicon and AMTIR1 chalcogenide photonic crystal waveguides with group velocities of c/33 and c/23, respectively, for simultaneous measurement of frequency and power level of an RF tone over a frequency range of 0.01–80 GHz. In our numerical simulations, we considered the impact of various nonlinear phenomena that affected the system outputs. An FWM conversion efficiency of approximately − 45 dB was achieved for a 10 mW continuous-wave pump and probe in an 80 µm AMTIR1 PhC waveguide. In our system, two orthogonal and independent outputs were generated simultaneously. These independent outputs were achieved using a microwave photonics Hilbert transformer implemented using the transversal filtering approach. Since the process was inherently incoherent, the nonlinearity of the four-wave mixing did not affect the resulting signals orthogonality. The ability to simultaneously identify both the RF frequency and the power level of a microwave signal would make this system a perfect candidate for various applications, such as instantaneous frequency measurement receivers.

Design of multifunctional all-optical logic gates based on photonic crystal waveguides.

The explosive development of the big data era has driven the rapid growth of silicon photonics, and logic operators based on photonic circuits have also been intensively investigated. Photonic integrated logic operators possess a high degree of design freedom and novel prospects, and they are regarded as promising platforms for future signaling and data processing. In this work, considering all-optical logic operation with lower power consumption and a smaller device footprint, multifunctional all-optical logic gates based on silicon photonic crystal (PhC) waveguides and phase-encoded light beams are proposed and applied to realize several logic operators, including XNOR, XOR, NOR, AND gates as well as a half adder and half subtractor. The initial phases (π and 0) of incident light represent the input digital states (1 and 0), and the logic operation results are determined by the output light intensity. Also, simulations are carried out to verify the proposed concept and to investigate the rise time, fall time, and cross talk of the devices. Theoretically, the bit rate for the proposed device can reach 1.25Tb/s, and the proposed structures have the potential to be extremely compact due to PhC waveguides.

Waveguide-Integrated MoTe2 p-i-n Homojunction Photodetector.

Two-dimensional (2D) materials, featuring distinctive electronic and optical properties and dangling-bond-free surfaces, are promising for developing high-performance on-chip photodetectors in photonic integrated circuits. However, most of the previously reported devices operating in the photoconductive mode suffer from a high dark current or a low responsivity. Here, we demonstrate a MoTe2 p-i-n homojunction fabricated directly on a silicon photonic crystal (PC) waveguide, which enables on-chip photodetection with ultralow dark current, high responsivity, and fast response speed. The adopted silicon PC waveguide is electrically split into two individual back gates to selectively dope the top regions of the MoTe2 channel in p- or n-types. High-quality reconfigurable MoTe2 (p-i-n, n-i-p, n-i-n, p-i-p) homojunctions are realized successfully, presenting rectification behaviors with ideality factors approaching 1.0 and ultralow dark currents less than 90 pA. Waveguide-assisted MoTe2 absorption promises a sensitive photodetection in the telecommunication O-band from 1260 to 1340 nm, though it is close to MoTe2's absorption band-edge. A competitive photoresponsivity of 0.4 A/W is realized with a light on/off current ratio exceeding 104 and a record-high normalized photocurrent-to-dark-current ratio of 106 mW-1. The ultrasmall capacitance of p-i-n homojunction and high carrier mobility of MoTe2 promise a high dynamic response bandwidth close to 34.0 GHz. The proposed device geometry has the advantages of employing a silicon PC waveguide as the back gates to build a 2D material p-i-n homojunction directly and simultaneously to enhance light-2D material interaction. It provides a potential pathway to develop 2D material-based photodetectors, laser diodes, and electro-optic modulators on silicon photonic chips.

Impact of thermal and refractive index tuning on the bandgap and band-edges of a silicon photonic crystal waveguide with sensing applications

This paper proposes the design of a silicon photonic crystal waveguide with a hexagonal lattice structure. In the proposed structure, the effect of temperature variation on the photonic band edges has been studied. Structural design parameters have been optimized for TM and TE modes to obtain bandgap in the optical communication window of 1550 nm at room temperature. The variation in the refractive index of the silicon slab with temperature ranging from 300 K–400 K has been considered and the tuning of photonic band-edge wavelength with thermo-optic effect is presented. The band edges are found to shift linearly with changes in temperature for both TE and TM modes. A sensitivity of 0.0595 nm/K for TM mode and 0.0587 nm/K for TE mode has been observed and thus the proposed structure can be used as a temperature sensor. This paper also presents the tuning of the bandgap and band-edges by changing the refractive index of the holes in the proposed structure. The wavelength of the band-edges is found to increase whereas the bandgap is found to decrease on increasing the refractive index of the holes for both TM and TE modes. It is observed that the proposed structure offers a refractive index sensitivity of 10.105 nm/RIU for TM mode and 34.953 nm/RIU for TE mode and thus can be used as a refractive index sensor.

Observation of slow light in glide-symmetric photonic-crystal waveguides.

We report optical transmission measurements on suspended silicon photonic-crystal waveguides, where one side of the photonic lattice is shifted by half a period along the waveguide axis. The combination of this glide symmetry and slow light leads to a strongly enhanced chiral light-matter interaction but the interplay between slow light and backscattering has not been investigated experimentally in such waveguides. We build photonic-crystal resonators consisting of glide-symmetric waveguides terminated by reflectors and use transmission measurements as well as evanescent coupling to map out the dispersion relation. We find excellent agreement with theory and measure group indices exceeding 90, implying significant potential for applications in slow-light devices and chiral quantum optics. By measuring resonators of different length, we assess the role of backscattering induced by fabrication imperfections and its intimate connection to the group index.

Recent Progress in Silicon-Based Slow-Light Electro-Optic Modulators.

As an important optoelectronic integration platform, silicon photonics has achieved significant progress in recent years, demonstrating the advantages on low power consumption, low cost, and complementary metal–oxide–semiconductor (CMOS) compatibility. Among the different silicon photonics devices, the silicon electro-optic modulator is a key active component to implement the conversion of electric signal to optical signal. However, conventional silicon Mach–Zehnder modulators and silicon micro-ring modulators both have their own limitations, which will limit their use in future systems. For example, the conventional silicon Mach–Zehnder modulators are hindered by large footprint, while the silicon micro-ring modulators have narrow optical bandwidth and high temperature sensitivity. Therefore, developing a new structure for silicon modulators to improve the performance is a crucial research direction in silicon photonics. Meanwhile, slow-light effect is an important physical phenomenon that can reduce the group velocity of light. Applying slow-light effect on silicon modulators through photonics crystal and waveguide grating structures is an attractive research point, especially in the aspect of reducing the device footprint. In this paper, we review the recent progress of silicon-based slow-light electro-optic modulators towards future communication requirements. Beginning from the principle of slow-light effect, we summarize the research of silicon photonic crystal modulators and silicon waveguide grating modulators in detail. Simultaneously, the experimental results of representative silicon slow-light modulators are compared and analyzed. Finally, we discuss the existing challenges and development directions of silicon-based slow-light electro-optic modulators for the practical applications.

Single Virus Detection on Silicon Photonic Crystal Random Cavities.

On-chip silicon microcavity sensors are advantageous for the detection of virus and biomolecules due to their compactness and the enhanced light-matter interaction with the analyte. While their theoretical sensitivity is at the single-molecule level, the fabrication of high quality (Q) factor silicon cavities and their integration with optical couplers remain as major hurdles in applications such as single virus detection. Here, label-free single virus detection using silicon photonic crystal random cavities is proposed and demonstrated. The sensor chips consist of free-standing silicon photonic crystal waveguides and do not require pre-fabricated defect cavities or optical couplers. Residual fabrication disorder results in Anderson-localized cavity modes which are excited by a free space beam. The Q≈105 is sufficient for observing discrete step-changes in resonance wavelength for the binding of single adenoviruses (≈50nm radius). The authors' findings point to future applications of CMOS-compatible silicon sensor chips supporting Anderson-localized modes that have detection capabilities at the level of single nanoparticles and molecules.

Pure-quartic solitons and their generalizations—Theory and experiments

Solitons are wave packets that can propagate without changing shape by balancing nonlinear effects with the effects of dispersion. In photonics, they have underpinned numerous applications, ranging from telecommunications and spectroscopy to ultrashort pulse generation. Although traditionally the dominant dispersion type has been quadratic dispersion, experimental and theoretical research in recent years has shown that high-order, even dispersion enriches the phenomenon and may lead to novel applications. In this Tutorial, which is aimed both at soliton novices and at experienced researchers, we review the exciting developments in this burgeoning area, which includes pure-quartic solitons and their generalizations. We include theory, numerics, and experimental results, covering both fundamental aspects and applications. The theory covers the relevant equations and the intuition to make sense of the results. We discuss experiments in silicon photonic crystal waveguides and in a fiber laser and assess the promises in additional platforms. We hope that this Tutorial will encourage our colleagues to join in the investigation of this exciting and promising field.

Utilization of slow light enhancement of four-wave mixing within a silicon photonic crystal for microwave frequency measurement purposes

We report the demonstration of an instantaneous frequency measurement system based on the four-wave mixing (FWM) effect in short dispersion engineered slow-light silicon photonic crystal waveguides for RF frequency measurement purposes within a range of 10 MHz to 80 GHz. Three nonlinear media were investigated including 3 mm ridge waveguide, 80 µm nanowire, and 80 µm photonic crystal (PhC). The system size could thus be decreased, and as a result system integration would become possible. We have shown that the optical power required to excite FWM is low enough to remove any need for optical amplification, and hence the system noise floor will be kept low. Issues due to the amplifier saturation will also be resolved this way. As a result, no noise reduction system, like lock-in amplification, would be required. A better system latency will also be achieved accordingly. The system dynamic range would also be improved in two ways. First, due to the low noise floor, and second, because of removing any optical amplifier that possibly could become saturated at higher power levels. All three media behaviors were simulated by the split step Fourier method, and the results showed that the best medium to be used is PhC.

Giant anomalous self-steepening and temporal soliton compression in silicon photonic crystal waveguides

Self-steepening (SS) is enhanced by slow-light effects in photonic crystal waveguides (PhCWs), with coefficients as large as hundreds of femtoseconds, and it plays an important role in temporally compressed solitons with narrow widths. Here, we investigate the soliton evolution in silicon PhCWs through experiments and numerical simulations; the simulated results agree well with the experimental measurements and help in revealing the physical mechanism of high-order soliton evolution. The dual opposite effects of giant anomalous SS on temporal soliton compression are demonstrated for the first time, i.e., the SS weakens or improves the compression competing with the effects of third-order dispersion (TOD) through two different physical mechanisms. It is also found that SS flattens or steepens the pulse leading edge depending on the strength of the positive TOD perturbation. These results promote the understanding of high-order solitons and can help with the design of suitable dispersion engineered silicon waveguides for superior on-chip temporal pulse compression for optical interconnects, data processing, and microwave photonics.

Particle swarm optimization of silicon photonic crystal waveguide transition.

Slow light generated through silicon (Si) photonic crystal waveguides (PCWs) is useful for improving the performance of Si photonic devices. However, the accumulation of coupling loss between a PCW and Si optical wiring waveguides is a problem when slow-light devices are connected in a series in a photonic integrated circuit. Previously, we reported a tapered transition structure between these waveguides and observed a coupling loss of 0.46 dB per transition. This Letter employed particle swarm optimization to engineer the arrangement of photonic crystal holes to reduce loss and succeeded in demonstrating theoretical loss value of 0.12 dB on average in the wavelength range of 1540-1560 nm and an experimental one of 0.21 dB. Crucially, this structure enhances the versatility of slow light.

Improving Low-Dispersion Bandwidth of the Silicon Photonic Crystal Waveguides for Ultrafast Integrated Photonics

We design a novel slow-light silicon photonic crystal waveguide which can operate over an extremely wide flat band for ultrafast integrated nonlinear photonics. By conveniently adjusting the radii and positions of the second air-holes rows, a flat slow-light low-dispersion band of 50 nm is achieved numerically. Such a slow-light photonic crystal waveguide with large flat low-dispersion wideband will pave the way for governing the femtosecond pulses in integrated nonlinear photonic platforms based on CMOS technology.

Silicon Photonic Crystal Modulators for High-Speed Transmission and Wavelength Division Multiplexing

For next-era optical interconnects in data centers, development of compact, energy-efficient, low-cost, and high-speed optical transceivers are required, for which high-performance external modulators in silicon photonics will be key components. We present a silicon photonic crystal waveguide slow light Mach-Zehnder modulator suitable for this purpose. The enhancement in the modulation efficiency via the slow light effect reduces the halfwave voltage-length product VπL, maintaining a wide working spectrum over 15 nm. The frequency response of the slow light modulator is constricted by an electrooptic phase mismatch between slow light and RF signals. In this study, this was dramatically improved by matching the phase using meanderline electrodes that delay RF signals. The cutoff frequency was experimentally evaluated to be 32-38 GHz. Using this device, we demonstrated high-speed modulation, including 64-Gbps on-off keying, 100-Gbps pulse amplitude modulation, and 50-Gbps/ch wavelength division multiplexing in 170-200-μm- long devices.

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.

Effects of third-order dispersion on temporal soliton compression in dispersion-engineered silicon photonic crystal waveguides

High-order temporal soliton compression in dispersion-engineered silicon photonic crystal waveguides will play an important role in future integrated photonic circuits compatible with complementary metal–oxide–semiconductors. Here, we report the physical mechanisms of high-order temporal soliton compression affected by third-order dispersion (TOD) combined with free carrier dispersion (FCD) in a dispersion engineered silicon photonic crystal waveguide with wideband low anomalous dispersion. Through numerical temporal soliton evolution analysis, we report what we believe is the first demonstration of the dual opposite effects of TOD on temporal soliton compression, which are strengthening or weakening through two different physical mechanisms, not only depending on the sign of TOD but also the relative magnitude of TOD-induced equivalent group velocity dispersion (GVD) β 2 , equ to the original GVD β 2 . We further find that FCD counteracts the effects of negative TOD on the soliton compression, while it reinforces the effects of positive TOD on the soliton compression. These results will help to design suitable dispersion-engineered silicon waveguides for superior on-chip temporal pulse compression in optical communications and processing application fields.

Optimization of silicon-photonic crystal (PhC) waveguide for a compact and high extinction ratio TM-pass polarization filter

A silicon photonic crystal waveguide based design of a highly-compact transverse-magnetic pass polarization filter has been proposed in this paper. The device utilizes both the index guiding and bandgap property simultaneously to realize its operation as a polarizer. Optimizations of different device-parameters, such as the radius of the holes, width, thickness, and length of the waveguide, have been performed for attaining its paramount performance. A small waveguide length, in the order of 5 μm, has shown a high extinction ratio, i.e., 45 dB, at the wavelength of 1550 nm. A uniform bandwidth of ≈120 nm is observed, beyond the extinction ratio of 40 dB, along with a remarkably low insertion loss, i.e., ≈0.6 dB. Investigations are also performed to evaluate performances of the polarizer under possible fabrication disorders, which depicts a sustained performance up to at least a random fabrication disorder of 30 nm. These merits make the polarizer suitable for applications in densely integrable photonic integrated circuits.

Slow-Light Frequency Combs and Dissipative Kerr Solitons in Coupled-Cavity Waveguides

We study Kerr frequency combs and dissipative Kerr solitons in silicon photonic crystal coupled-cavity waveguides (CCW) with globally optimized dispersion at telecom wavelengths. The corresponding threshold for comb generation is found to explicitly depend on the main CCW figures of merit, namely, mode volume, normal mode quality factor and slow-light group index. Our analysis is carried out by solving the non-linear dynamics of the CCW Bloch modes in presence of Kerr non-linearity and two-photon absorption. Our results open the way to CCW comb generation via dispersion engineering and slow-light enhancement.

Coupling of a whispering gallery mode to a silicon chip with photonic crystal.

We demonstrate the efficient coupling (99.5%) of a silica whispering gallery mode microresonator directly with a silicon chip by using a silicon photonic crystal waveguide as a coupler. The efficient coupling is attributed to the small effective refractive index difference between the two devices. The large group index of the photonic crystal waveguide mode also contributes to the efficient coupling. A coupling Q of 2.68×106 is obtained, which allows us to achieve the critical coupling of a silica whispering gallery mode with an intrinsic Q of close to 107 with a Si chip.

Double-layer graphene on photonic crystal waveguide electro-absorption modulator with 12 GHz bandwidth

Abstract Graphene has been widely used in silicon-based optical modulators for its ultra-broadband light absorption and ultrafast optoelectronic response. By incorporating graphene and slow-light silicon photonic crystal waveguide (PhCW), here we propose and experimentally demonstrate a unique double-layer graphene electro-absorption modulator in telecommunication applications. The modulator exhibits a modulation depth of 0.5 dB/μm with a bandwidth of 13.6 GHz, while graphene coverage length is only 1.2 μm in simulations. We also fabricated the graphene modulator on silicon platform, and the device achieved a modulation bandwidth at 12 GHz. The proposed graphene-PhCW modulator may have potentials in the applications of on-chip interconnections.