Nyan L. Aung

Importance of interface roughness induced intersubband scattering in mid-infrared quantum cascade lasers

The electron transit time of many different quantum cascade lasers has been measured and compared to the calculated upper laser level lifetimes with and without taking into account interface roughness induced intersubband scattering. A significantly better correlation is found between the experimental results and the calculation when including the contribution from the interface roughness (corr. coeff.: 0.79 vs. 0.43 with and without the consideration of interface roughness, respectively). This suggests that in addition to longitudinal optical phonons, interface roughness is also crucial in determining the intersubband lifetimes in mid-infrared quantum cascade laser and should routinely be included in design.

High power spiral cavity quantum cascade superluminescent emitter.

Quantum cascade devices have been shaped into compact, yet long spiral cavities to increase mid-infrared superluminescence power. A peak power of ~57 mW at 250 K is obtained with a coherence length of ~107 μm.

HIGH PEAK POWER QUANTUM CASCADE SUPERLUMINESCENT EMITTER

A high power quantum cascade superluminescent emitter employs low reflectivity facets including a tilted cleaved facet, a rounded shaped wet-etched sloped facet and a loop facet in either a linear or spiral configuration to increase ASE.

Threshold current reduction and directional emission of deformed microdisk lasers via spatially selective electrical pumping

We report on laser threshold current reduction and directional emission from quadrupole-shaped AlGaInAs microdisk diode lasers by selective electrical pumping. The directional emission results from breaking the 2-fold rotation symmetry of the system by the introduction of a triangle-shaped contact geometry, and the laser threshold reduction results from a small current injection area. Room temperature laser operation is achieved in both pulsed and continuous-wave operation for a microdisk radius of 50 μm and deformation constant of e = 0.09, with optical output power of more than 8 mW and 3 mW, respectively. Under pulsed operation, the minimum measured threshold current for selectively pumped microlasers is 42 mA, significantly lower than the minimum measured threshold current for uniformly pumped microlasers (58 mA) and standard ridge lasers (80 mA) of the same device size and material.

Suppression of pointing instability in quantum cascade lasers by transverse mode control

We report on a technique that serves to eliminate pointing instability in Quantum Cascade lasers by suppressing the propagation of higher order transverse modes within the laser cavity. This process, achieved via a short and lossy lateral constriction in the waveguide, only allows the fundamental mode to propagate. The beam pointing fluctuations of the modified devices are greatly reduced, with a 10-fold improvement in standard deviation, indicating a stable emitted beam. The far-field pattern has a Gaussian shape, and the slope efficiency of the received power on a 0.87 numerical aperture detector increases by ∼65%, without significant increase in threshold.

Transverse mode control in quantum cascade lasers via lossy lateral constrictions

Quantum Cascade (QC) lasers are semiconductor devices operating in the mid-infrared and terahertz regions of the electromagnetic spectrum. Since their first demonstration in 1994, they have evolved rapidly into high power devices. However, they also have intrinsic challenges, such as beam steering at high power. Such phenomenon has been observed in QC lasers and attributed to the interaction between the two lowest transverse modes in the laser cavity. In this project, we have used COMSOL Multiphysics simulations to first investigate how transverse mode propagation can be controlled with cavity spoilers. We have modeled this effect by creating short and lossy lateral constrictions from the top of the laser ridge to perturb the modes distributed more toward the sides of the laser ridge, while leaving the fundamental mode intact. After obtaining optimized dimensions for the constrictions, we have utilized focused ion beam (FIB) milling to etch two small trenches from the top of several laser ridges to create the simulated effect on our devices. We, then, filled them with platinum in an effort to completely suppress the propagation of higher order transverse modes in the cavity. The results obtained show minimal effect on threshold and a Gaussian far-field distribution at various current levels, indicating a complete suppression of the higher order transverse modes.

Hybrid mid-IR OCT system for imaging and spectroscopy using new high power low-coherence quantum cascade superluminescent emitters

We have successfully constructed and tested a new mid-infrared optical coherence tomography imaging system capable of simultaneous imaging and spatial spectroscopy using new quantum cascade superluminescent emitters.

Comparison study of the influence of taller electron exit barriers on the device performance for ultra-strong coupling Quantum Cascade laser designs

Two different ultra-strong coupling Quantum Cascade laser designs and their modified versions with two taller electron exit barriers are compared experimentally to study the influence of such taller electron exit barriers on the device performance.