Trap Characterization Techniques for GaN-Based HEMTs: A Critical Review.

Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) have been considered promising candidates for power devices due to their superior advantages of high current density, high breakdown voltage, high power density, and high-frequency operations. However, the development of GaN HEMTs has been constrained by stability and reliability issues related to traps. In this article, the locations and energy levels of traps in GaN HEMTs are summarized. Moreover, the characterization techniques for bulk traps and interface traps, whose characteristics and scopes are included as well, are reviewed and highlighted. Finally, the challenges in trap characterization techniques for GaN-based HEMTs are discussed to provide insights into the reliability assessment of GaN-based HEMTs.

Silicon Photonic Phase Shifters and Their Applications: A Review.

With the development of silicon photonics, dense photonic integrated circuits play a significant role in applications such as light detection and ranging systems, photonic computing accelerators, miniaturized spectrometers, and so on. Recently, extensive research work has been carried out on the phase shifter, which acts as the fundamental building block in the photonic integrated circuit. In this review, we overview different types of silicon photonic phase shifters, including micro-electro-mechanical systems (MEMS), thermo-optics, and free-carrier depletion types, highlighting the MEMS-based ones. The major working principles of these phase shifters are introduced and analyzed. Additionally, the related works are summarized and compared. Moreover, some emerging applications utilizing phase shifters are introduced, such as neuromorphic computing systems, photonic accelerators, multi-purpose processing cores, etc. Finally, a discussion on each kind of phase shifter is given based on the figures of merit.

Mid-infrared silicon photonic phase shifter based on microelectromechanical system.

Mid-infrared (MIR) photonic integrated circuits have generated considerable interest, owing to their potential applications, such as thermal imaging and biochemical sensing. A challenging area in the field is the development of reconfigurable approaches for the enhancement of on-chip functions, where a phase shifter plays an important role. Here, we demonstrate a MIR microelectromechanical system (MEMS) phase shifter by utilizing an asymmetric slot waveguide with subwavelength grating (SWG) claddings. The MEMS-enabled device can be easily integrated into a fully suspended waveguide with SWG cladding, built on a silicon-on-insulator (SOI) platform. Through engineering of the SWG design, the device achieves a maximum phase shift of 6π, with an insertion loss of 4 dB and a half-wave-voltage-length product (VπLπ) of 2.6 V·cm. Moreover, the time response of the device is measured as 13 µs (rise time) and 5 µs (fall time).

Suspended Silicon Waveguide with Sub-Wavelength Grating Cladding for Optical MEMS in Mid-Infrared.

Mid-infrared (MIR) photonics are generating considerable interest because of the potential applications in spectroscopic sensing, thermal imaging, and remote sensing. Silicon photonics is believed to be a promising solution to realize MIR photonic integrated circuits (PICs). The past decade has seen a huge growth in MIR PIC building blocks. However, there is still a need for the development of MIR reconfigurable photonics to enable powerful on-chip optical systems and new functionalities. In this paper, we present an MIR (3.7~4.1 µm wavelength range) MEMS reconfiguration approach using the suspended silicon waveguide platform on the silicon-on-insulator. With the sub-wavelength grating claddings, the photonic waveguide can be well integrated with the MEMS actuator, thus offering low-loss, energy-efficient, and effective reconfiguration. We present a simulation study on the waveguide design and depict the MEMS-integration approach. Moreover, we experimentally report the suspended waveguide with propagation loss (-2.9 dB/cm) and bending loss (-0.076 dB each). The suspended waveguide coupler is experimentally investigated. In addition, we validate the proposed optical MEMS approach using a reconfigurable ring resonator design. In conclusion, we experimentally demonstrate the proposed waveguide platform's capability for MIR MEMS-reconfigurable photonics, which empowers the MIR on-chip optical systems for various applications.

Bi-layered composite gratings with high diffraction efficiency enabled by near-field coupling.

In this paper, we present a design method for bi-layered composite gratings to achieve high diffraction efficiency. These composite gratings feature strong near-field coupling between their constituent dielectric subwavelength gratings, thus enabling high-efficiency first-order diffraction in the far-field. An intuitive explanation based on a wavevector matching condition for such high diffraction efficiency composite gratings is provided. According to theoretical analysis, a design strategy for the proposed composite gratings is developed and verified by numerical simulations with gratings working in both TE and TM modes. The proposed strategy could open door to develop bi-layered composite gratings for manipulating diffracted waves with high efficiency, thus may potentially enable new applications in photonic systems.

Multifunctional mid-infrared photonic switch using a MEMS-based tunable waveguide coupler.

We demonstrate a multifunctional photonic switch on silicon-on-insulator platform operating at the mid-infrared wavelength range (3.85-4.05 µm) using suspended waveguides with sub-wavelength cladding and a micro-electro-mechanical systems (MEMS) tunable waveguide coupler. Leveraging the flip-chip bonding technology, a top wafer acting as the electrode is assembled above the silicon-on-insular wafer to enable the electrostatic actuation. Experimental characterizations for the functions of the proposed device include (1) an optical attenuator with 25 dB depth using DC voltage actuation, (2) a 1×2 optical switch with response time of 8.9 µs and -3dB bandwidth up to 127 kHz using AC voltage actuation, and (3) an on-chip integrated light chopper with the comparable performance of a commercial rotating disc light chopper.

Ultra-small photonic crystal (PhC)-based test tool for gas permeability of polymers.

We present an ultra-small photonic crystal-based test tool for gas permeability of polymers. It features a fully-etched photonic crystal (PhC) structure occupying an area of 20 µm × 800 µm on silicon-on-insulator wafer. The light-matter interaction in the PhC cavity with deformed Polydimethylsiloxane (PDMS) under pressure difference was investigated with finite element method and finite-difference time-domain method numerically. Next, three PDMS membranes of different mixing ratios were utilized for the characterization of gas permeation flux. The feasibility and effectiveness of the proposed working mechanism are verified through clearly distinguishing the gas permeability of these three testing samples. Compared with conventional test tools, this proposed test tool has fast response while it consumes less testing gas volume in a testing system with reduced footprint. Potentially, it can be integrated into lab-on-a-chip devices to measure gas permeation in nano scale.

Design of an on-chip Fourier transform spectrometer using waveguide directional couplers and NEMS.

A novel concept of on-chip Fourier transform spectrometer is proposed. It consists of semiconductor waveguide directional couplers and NEMS actuators. The optical path difference can be tuned by controlling the NEMS actuators to couple or decouple the directional couplers. With 9 stages of directional couplers, we demonstrate numerically that the spectral resolution can reach up to 4 nm in 1.5 µm to 1.8 µm wavelength range. Further enhancement can be achieved by increasing the number of integrated NEMS driven directional couplers. This design meets the requirement of small size, weight and power and may be useful in future on-chip spectroscopic sensors.

Applications of Photonic Crystal Nanobeam Cavities for Sensing.

In recent years, there has been growing interest in optical sensors based on microcavities due to their advantages of size reduction and enhanced sensing capability. In this paper, we aim to give a comprehensive review of the field of photonic crystal nanobeam cavity-based sensors. The sensing principles and development of applications, such as refractive index sensing, nanoparticle sensing, optomechanical sensing, and temperature sensing, are summarized and highlighted. From the studies reported, it is demonstrated that photonic crystal nanobeam cavities, which provide excellent light confinement capability, ultra-small size, flexible on-chip design, and easy integration, offer promising platforms for a range of sensing applications.