Nov 17 (Tue): "High Performance Semipolar InGaN Distributed Feedback Laser Diodes with Surface Gratings," Haojun Zhang, ECE PhD Defense
The impact of III-Nitride material and solid-state lighting on the world has been far-reaching. The last decade has seen the III-Nitride LEDs and laser didoes (LDs) steadily becoming essential components in a broad range of applications. Compared with traditional lighting systems, their great improvement in efficiency and portability for lighting systems, the reduced environmental impact as well as the potentials for the future visible light communication systems is profound.
Properly engineered distributed feedback (DFB) laser diodes (LDs) are well known for their excellent portability, easy integration, and ability to offer robust single longitudinal mode stability. Among them, III-nitride material based visible DFB-LDs are gaining special attention due to many emerging applications such as atomic clocks, medical sensing, laser cooling and visible light communication. Previous GaN DFB-LDs have been conventionally fabricated with epitaxially buried gratings, which is often difficult and costly. In this dissertation, the design of DFB-LDs with a novel surface grating structure and their enhanced performance will be presented. Starting with the design and optical simulation of the devices, continuing through nanofabrication and testing, major parts and analysis of these novel III-nitride devices with surface gratings will be discussed. We show that under continuous-wave operation, the 445 nm laser diode with a uniform first-order etched surface grating emits with a FWHM (full width half maximum) of 5 pm, SMSR (side mode suppression ratio) of more than 25 dB and output power of more than 80 mW from a single facet. Moreover, an enhanced performance has been also achieved in laser diodes with phase-shifted third-order non-etched hydrogen silsesquioxane (HSQ) gratings. These improved single-mode blue lasers achieve a high output power, SMSR of more than 35 dB and FWHM of less than 2 pm even under high current injection.
In addition to the DFB-LDs, surface gratings have also been historically employed in many other optical and photonic devices as an approach to realize novel functions. Efficient polarized light source, as a key component in mainstream LCD displays, plays an important role in energy saving. Specifically, in this dissertation, we also demonstrate that, by combining an ITO surface grating and Al surface coating, a highly polarized dual-wavelength semipolar InGaN micro-LED can be achieved. Such devices exhibit a 1.7-fold enhancement of the polarization ratio as compared to the conventional semipolar micro-LEDs.
Haojun Zhang obtained B.S. degree from Tsinghua University. He is now a Ph.D. candidate at the Solid State Lighting & Energy Electronics Center (SSLEEC), working on the design and development of GaN light emitting devices and their applications.
Hosted by: Professor Steven P. DenBaars
Submitted by: Haojun Zhang <email@example.com>