Chien-Yao Lu

High speed, low noise ultraviolet photodetectors based on GaN p-i-n and AlGaN(p)-GaN(i)-GaN(n)structures

We have investigated the spectral response of front-surface-illuminated GaN and AlGaN/GaN p-i-n ultraviolet photodetectors prepared by reactive molecular beam epitaxy on sapphire substrates. GaN homojunction p-i-n photodiodes exhibited a peaked response near the band edge. This enhanced response was absent in the AlGaN/GaN heterojunction p-i-n detectors. We analyzed the effect of p-layer thickness of the GaN p-i-n diodes on the magnitude of the peak photoresponse. The AlGaN/GaN photodiodes had a maximum zero-bias responsivity of 0.12 A/W at 364 nm, which decreased by more than 3 orders of magnitude for wavelengths longer than 390 nm. A reverse bias of −10 V raised the responsivity to 0.15 A/W without any significant increase in noise. The root-mean-square noise current in a 1 Hz bandwidth is ∼1.0 pA, corresponding to a noise-equivalent-power of ∼8.3 pW. We measured extremely fast decay times of 12 ns for the AlGaN/GaN and 29 ns for the GaN photodiodes.

Metal-cavity surface-emitting microlaser at room temperature

We propose and realize a substrate-free metal-cavity surface-emitting microlaser with both top and sidewall metal and a bottom distributed Bragg reflector as the cavity structure. The transfer-matrix method is used to design the laser structure based on the round-trip resonance condition inside the cavity. The laser is 2.0 μm in diameter and 2.5 μm in height, and operates at room temperature with continuous-wave mode. Flip-bonding the device to a silicon substrate with a conductive metal provides efficient heat removal. A high characteristic temperature about 425 K is observed from 10 to 27 °C.

Metal-cavity surface-emitting microlaser with hybrid metal-DBR reflectors

We demonstrate a metal-cavity surface-emitting microlaser at room temperature using hybrid metal/distributed Bragg reflectors as well as substrate removal. Our devices operate under continuous-wave current injection at room temperature. The smallest laser is 1.0 μm in radius and ~4.0 μm in height with a circular beam shape and an output over 8 μW. The device lases at 995 nm wavelength with a threshold current of about 2.6 mA.

A surface-emitting 3D metal-nanocavity laser: proposal and theory

A novel three-dimensional (3D) metal-nanocavity (or nano-coin) semiconductor laser suitable for electrical injection is proposed and analyzed. Our design uses metals as both the cavity sidewall and the top/bottom reflectors (i. e., a fully metal encapsulated nanolaser) and maintains the surface-emitting nature. As a result of the large permittivity contrast between the dielectric and metal, the optical energy can be well-confined inside the metal nanocavity. With a proper design and the choice of the HE111 mode, which has the best top surface radiation pattern, a laser with a physical size smaller than 0.01λ(0)(3) is achievable at 1.55 μm wavelength with a reasonable semiconductor gain at room temperature. We provide a detailed theoretical model starting from the waveguide analysis to full 3D structure simulations by taking into account both geometry and metal dispersion. We show a systematic procedure for analyzing this class of 3D metal-cavity (or nano-coin) lasers with discussions on the optimization of the performance such as light output power, threshold reduction, and output beam shaping.

CW substrate-free metal-cavity surface microemitters at 300 K

In this paper substrate-free metal-cavity surface microemitters are demonstrated. The optical cavity is formed by a metal reflector, metal-surrounded sidewall and n-doped distributed-Bragg reflector, which provides optical feedback and carrier injection. We describe a simple design principle with the modal properties modified by geometry and metal-insulator cladding. Both resonant cavity light-emitting diodes (1.85 µm diameter and 0.6 µm height) and lasers (2.0 µm diameter and 2.5 µm height) are successfully fabricated and characterized. These two types of devices operate at room temperature under continuous-wave (CW) operation. Since the devices are substrate-free, they can be bonded to any substrates. From the threshold currents of the lasers, we obtain a high characteristic temperature of 425 K in the range of 10–27 °C. We also discuss a general approach to improve the diffraction from small-aperture devices.

Quantum-dot laser with a metal-coated waveguide under continuous-wave operation at room temperature

We fabricated and characterized Fabry–Perot quantum-dot lasers with metal-coated waveguides. Lasing action at room temperature under continuous-wave operation was observed, contrary to the belief that metal is too lossy to serve as the waveguide material at optical frequencies. We extracted the optical gain and group index from the amplified spontaneous emission spectra. A high group index of about 4.2 has been observed from these metal-coated devices, which is much larger than a value of 3.2 measured from an uncoated quantum-dot laser. It is believed that the metal coating contributes to the high group index in these devices.

Low Thermal Impedance of Substrate-Free Metal Cavity Surface-Emitting Microlasers

The thermal impedance of a substrate-free metal cavity surface-emitting microlaser is measured. The lasing wavelength shifts with temperature at a ratio of 4.367 × 10^-2 nm/°C. An extremely low thermal impedance below 1.730°C/mW is extracted for this microlaser with a 1.0-μm radius and 3.5-μm height in the temperature window of 20°C -80°C. The full metal coverage, substrate-free configuration, and metal bonding lead to efficient heat removal.

Cavity-Volume Scaling Law of Quantum-Dot Metal-Cavity Surface-Emitting Microlasers

Quantum-dot (QD) metal-cavity surface-emitting microlasers are designed, fabricated, and characterized for various sizes of cavity volume for both lateral and vertical confinements. Microlasers using submonolayer QDs in the active region are fabricated according to our design model optimized for a resonant metal cavity. The cavity-volume scaling law is studied by our theoretical modeling and experimental demonstration. The smallest laser has a diameter of 1

Theory of Metal-Cavity Surface-Emitting Microlasers and Comparison With Experiment

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Metal-Cavity Surface-Emitting Microlasers With Size Reduction: Theory and Experiment

with silver metal cladding operating at room temperature with electrical injection in pulsed mode. Our experimental results show significant self-heating effect in the smaller devices with a diameter of a few micrometers due to high series resistance and high threshold gain. With the use of hybrid metal-DBR mirrors, the number of DBR pairs in the top hybrid mirror can be reduced from 19.5 to 5.5 without sacrificing threshold current density.