Yongjin Wang

Suspended p–n Junction InGaN/GaN Multiple-Quantum-Well Device With Selectable Functionality

We demonstrate optoelectronic devices implemented on suspended p-n junction InGaN/GaN multiple quantum wells (MQWs) for the further monolithic integration of an optical source, a waveguide, and a photodetector on the same GaN-on-silicon wafer. The fabricated suspended membrane device exhibits selectable functionalities either for efficient light-emitting diodes (LEDs) or sensitive photodetectors. Typical current-voltage (I-V) characteristics are obtained for the device operated under the LED mode, and the emitted light intensity is effectively modulated by the applied voltage. Lateral in-plane propagation of emitted light in a suspended membrane is experimentally presented. The simulation results show that the thickness-dependent optical performance can be tuned by back wafer thinning for epitaxial films. The device operated under the photodetector mode exhibits a static photocurrent on-off ratio ηs of 2.25 × 105 at a 1-V bias voltage with the illumination power of 690 μW and the wavelength of 450 nm. The photocurrent also shows a rectangular pulse response of the same duration as a 1-s rectangular illumination pulse at a 0-V bias voltage with the illumination power of 1 mW. The temporal photocurrent on-off ratio ηt is around 1.01 × 105. This paper opens a promising way to realize the monolithic integration of a LED, a waveguide, and a photodetector on a GaN-on-silicon platform.

Suspended membrane GaN gratings for refractive index sensing

In this paper, we describe the fabrication and the novel procedure for back thinning the GaN layer, which leads to improved optical performance. Angular-resolved reflectance measurements are then conducted to characterize the GaN gratings and show the dependence of resonant wavelength on grating period and membrane thickness. The measured results compare well with electromagnetic modeling based on rigorous coupled wave analysis (RCWA). Altering the dielectric environment has been shown as a novel sensing mechanism, and this work opens the way to the fabrication of novel GaN resonant photonic devices for refractive index sensing applications in the visible wavelength range.

Guided-Mode Resonant

Subwavelength HfO2 gratings are realized on a freestanding 200-nm-thick HfO2 membrane with air as the low refractive index materials on top and bottom. Strong coupling between the incident light and HfO2 grating is characterized by angular-resolved reflectivity measurement, and guided-mode resonances are experimentally demonstrated with the sensitivities to the parameters and shapes of grating and the polarization of incident beam. The experimental results are consistent with numerical simulation. This work opens the way to fabricate guided-mode resonant HfO2 photonic devices in the visible wavelength range.

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This Letter describes a double-sided process to fabricate freestanding membrane devices on a GaN-on-silicon platform. The photoluminescence measurement is taken to characterize the optical performance. A large portion of the excited light from InGaN/GaN multiple quantum wells is trapped as waveguide modes and propagates in different directions. Experimental results show that the propagation direction of the waveguide mode can be converted into the direction normal to the surface at the edge of a freestanding membrane, and the emitted light is attenuated due to light propagation loss before it gets out from the edge. Subwavelength grating can also convert waveguide modes into air modes on a freestanding membrane. These results suggest that the emission efficiency can be greatly improved by employing more efficient light extraction methods and that GaN-based photonic waveguides are promising in the visible range.

Grating at Visible Wavelength Range

We proposed and demonstrated pulsed fiber lasers Q-switched and mode-locked by using a large-angle tilted fiber grating, for the first time to our best knowledge. Owing to the unique polarization properties of the large-angle tilted fiber grating (LA-TFG), i.e. polarization-dependent loss and polarization-mode splitting, switchable dual-wavelength Q-switched and mode-locked pulses have been achieved with short and long cavities, respectively. For the mode-locking case, the laser was under the operation of nanosecond rectangular pulses, due to the peak-power clamping effect. With the increasing pump power, the durations of both single- and dual-wavelength rectangular pulses increase. It was also found that each filtered wavelength of the dual-wavelength rectangular pulse corresponds to an individual nanosecond rectangular pulse by employing a tunable bandpass filter.

Experimental observation of lateral emission in freestanding GaN-based membrane devices

This letter presents the fabrication and characterization of freestanding circular GaN gratings on a GaN-on-silicon platform. Optical modes propagate within the freestanding GaN membrane and their number decreases as the thickness of GaN membrane is reduced. Backside thinning of freestanding GaN membranes is used to obtain thinner GaN membranes that are helpful to reduce resonance modes and to broaden the reflectance interference fringes in the visible range. Strong coupling between the incident light and the circular grating is confirmed by angular-resolved reflectance measurement. The influences of the grating parameters on the reflectance spectra are investigated. This letter opens the way to fabricate single-layer GaN resonant gratings in the visible wavelength range.

Switchable dual-wavelength Q-switched and mode-locked fiber lasers using a large-angle tilted fiber grating

We demonstrate here guided mode resonance (GMR) concentric circular grating filters (CCGFs) on an HfO2-on-silicon platform. The fabricated devices are realized with a membrane structure by completely removing the silicon substrate beneath the HfO2 device layer and, thus, can operate across the entire visible wavelength band. Polarization-insensitive properties under normal incidence are experimentally achieved resulting from their rotational symmetry. Substantially differing from 1-D linear grating filters, CCGFs typically feature a couple of resonant peaks for normal incident arbitrary linear polarization, which, from a local point of view, can be physically attributed to the coexistence of azimuthal and radial components of the incident light electric field. The CCGFs here have potential applications, such as polarization-insensitive two-color mixing devices and polarization-insensitive color filtering devices.

Circular GaN Membrane Gratings

In this paper, we propose an advanced hyperspectral video imaging system (AHVIS), which consists of an objective lens, an occlusion mask, a relay lens, an Amici prism and two cameras. An RGB camera is used for spatial reading and a gray scale camera is used for measuring the scene with spectral information. The objective lens collects more light energy from the observed scene and images the scene on an occlusion mask, which subsamples the image of the observed scene. Then, the subsampled image is sent to the gray scale camera through the relay lens and the Amici prism. The Amici prism that is used to realize spectral dispersion along the optical path reduces optical distortions and offers direct view of the scene. The main advantages of the proposed system are improved light throughput and less optical distortion. Furthermore, the presented configuration is more compact, robust and practicable.

Polarization-Insensitive Concentric Circular Grating Filters Featuring a Couple of Resonant Peaks

In this paper, we design and theoretically investigate an ultracompact Fabry-Pérot filter (FPF) consisting of two silicon-based high-contrast gratings (HCGs). The filter is proposed for implemention on a double silicon-on-insulator (double-SOI) platform. By using the rigorous coupled wave analysis method, we examine and discuss the influences of various factors on the transmission properties of the filter. The location of the transmission peak is capable of being tuned efficiently by changing the grating period, thickness, duty cycle, or filter cavity length. An ultrahigh quality factor (e.g., as high as 625960) can be readily achieved by merely choosing the appropriate grating period or thickness. Moreover, with increasing silicon substrate thickness, the modulation effect of the substrate is capable of decomposing the transmission peak into a bundle of subpeaks. In addition, structural differences between the two HCGs may degrade the peak transmittivity severely. In a word, this paper is devoted to a systematic report on the particular transmission properties of the filter and, thus, provides a clear picture of its overall performance, which is highly important for its practical design and applications.

Advanced hyperspectral video imaging system using Amici prism

We propose a free-standing membrane light-emitting diode (LED) on a GaN-on-silicon platform. Finite-difference time-domain (FDTD) simulation is used to investigate thickness-dependent performance of a free-standing membrane LED. The simulation results show that the light extraction efficiency is increased, and a directed emission pattern is obtained in free-standing membrane LEDs, which are experimentally implemented by removing the silicon substrate and back wafer thinning of the epitaxial layer. The light extraction efficiency and current-voltage (I-V) performance of membrane LEDs is increased in comparison with LEDs with silicon substrate.