Quantum cascade lasers, or QCLs, are tiny – smaller than a grain of rice – but they pack a punch. Compared to traditional lasers, QCLs offer higher power output and can be tuned to a wide range of infrared wavelengths. They can also be used at room temperature without the need for bulky cooling systems.
But because they’re difficult and costly to produce, QCLs aren’t used much outside the Department of Defense.
They have also developed a simpler process for creating such lasers, with comparable performance and better efficiency.
Optical power vs current and voltage vs current characteristics for an uncoated 3.15 mm × 9 μm laser mounted on an AlN submount and tested in the pulsed mode (200 ns; 10 kHz) at 300 K. Measured slope efficiency − 4.9 W/A and threshold current density − 1.7 kA/cm2. Inset: emission spectrum is centered at 5.65 μm. Citation: Appl. Phys. Lett. 109, 121109 (2016); http://dx.doi.org/10.1063/1.4963233
“The previous record was achieved using a design that’s a little exotic, that’s somewhat difficult to reproduce in real life,” Lyakh said. “We improved on that record, but what’s really important is that we did it in such a way that it’s easier to transition this technology to production. From a practical standpoint, it’s an important result.”
That could lead to greater usage in spectroscopy, such as using the infrared lasers as remote sensors to detect gases and toxins in the atmosphere. Lyakh, who has joint appointments with UCF’s NanoScience Technology Center and the College of Optics and Photonics, envisions portable health devices. For instance, a small QCL-embedded device could be plugged into a smartphone and used to diagnose health problems by simply analyzing one’s exhaled breath.
“But for a handheld device, it has to be as efficient as possible so it doesn’t drain your battery and it won’t generate a lot of heat,” Lyakh said.
The method that previously produced the highest efficiency called for the QCL atop a substrate made up of more than 1,000 layers, each one barely thicker than a single atom. Each layer was composed of one of five different materials, making production challenging.
The new method developed at UCF uses only two different materials – a simpler design from a production standpoint.
5.6 μm quantum cascade lasers based on the Al0.78In0.22As/In0.69Ga0.31As active region composition with the measured pulsed room temperature wall plug efficiency of 28.3% are reported. Injection efficiency for the upper laser level of 75% was measured for the design by testing devices with variable cavity lengths. A threshold current density of 1.7 kA/cm2 and a slope efficiency of 4.9 W/A were measured for uncoated 3.15 mm × 9 μm lasers. Threshold current density and slope efficiency dependence on temperature in the range from 288 K to 348 K for the structure can be described by characteristic temperatures T0 ∼ 140 K and T1 ∼ 710 K, respectively.
SOURCES – University of Central Florida, Applied Physics Letters