Supersymmetric nitride-based semiconductor lasers

Figure 1: Configuration of SUSY-laser with two ridges

Figure 2: Schematic illustration of the refractive index profile of two separate (left) and optically coupled (right) waveguides corresponding distribution of the modes indicated by red lines

This project is a joined work between the Lodz University of Technology, Institute of High Pressure Physics Polish Academy of Science (IHPP PAS), and our group at Chemnitz University of Technology.
The visible to UV range of the spectrum is dominated by nitride-based lasers. However, high-power laser diodes made of this semiconductor have a wide laser stripe and only multimode emission. To achieve fundamental lateral mode operation, common laser diode structures use a sufficiently narrow etched laser ridge that suppresses or cuts off the higher order modes and includes only the fundamental mode. This results in a small volume of the active region and low emission power. With this new project we want to achieve high power devices with fundamental mode operation. The supersymmetry (SUSY) theory is used to boost fundamental mode emission by implementing absorption for all supermodes. In genral, SUSY enables a widening of the ridge with sustaining the fundametal mode, which leads to a high quality of the laser beam and enhances the optical power.
We propose SUSY-lasers production by fabricating two adjacent ridges (see fig. 1). Only the so-called main ridge has an electrical contact and is significantly broader than the typical single-mode lasers, which will lead to a higher emission power. The second ridge (partner) has no contact and therefore remains passive. The active area of both ridges consists of a multiple quantum well (MQW) structure. Furthermore, due to the optically coupling of both waveguides every mode that has a partner will split into supermode pairs that are propagating in both waveguides (see fig. 2 right side). However, the fundamental mode of the main and thus broader waveguide is localized in its own ridge structure and therefore not propagating in both ridges. The partner ridge was developed to implement SUSY, where the intensity distribution of all supermode pairs overlaps with the lossy regions of the MQW. As a result, all supermodes that experience absorption under the partner ridge will be suppressed and none of them will be present in the laser spectrum. Only the fundamental mode from the main ridge will be confined in the positive gain region of the main ridge. Therefore, the light intensity distribution of the fundamental mode of the main ridge is almost unaffected by the absorptions and thus enables high optical power emission in fundamental mode operation.

The first two main aims of the project are mainly pursued by our collaborators and relate to the design and realization of SUSY lasers. Initially, SUSY lasers in pulse mode and later also lasers in cw mode will be developed. It is important to mention that the operating modes require different structures. Only electrically operated devices will be manufactured. The third main topic is implemented by our group and focuses on the experimental characterization of SUSY lasers. The aim is to discover the physical fundamentals for stable single-mode operation. A thorough understanding of the fundamental physical phenomena that occur in SUSY lasers with respect to the ambiguous role of exception points (EP) and parity-time symmetry (PT) is important to be able to implement the concept in other structures. It is assumed that EP and PT are the most important physical phenomena limiting single-mode emission.
start: June 2024