Reduction of Droop for Antimonide-based Mid-Infrared Lasers

In the past 60-years since the demonstration of the first laser, great strides have been made in improving the efficiency of semiconductor lasers, including the integration of numerous stages where light is generated at repeated steps rather than at a single interface, multiplying the output power possible through a cascade configuration. Even with these advances, these devices do not yet operate at their theoretical ideal.

The goal of this project is to identify loss mechanisms that result in current not being converted to laser light and to apply that knowledge to improving the design of future cascade semiconductor lasers. This would increase output power and efficiency, particularly for devices emitting in the mid-infrared portion of the spectrum, a range of wavelengths important for applications in environmental monitoring, medicine, and homeland security, including chemical sensing and infrared countermeasures.

The approach will include not only current injection, optical excitation, and spectroscopy, but also the integration of two-dimensional sheets of graphene with three-dimensional semiconductor alloys containing elements from the third and fifth columns of the periodic table to create a top contact that is simultaneously optically transparent and both electrically and thermally conductive. Improving efficiency has the potential to substantially reduce input power requirements and operating costs while increasing portability.

Classroom and laboratory activities proposed in this work, including creation of videos and engaging lessons, will attract K-12 students to careers in electrical and computer engineering and will contribute to retention of students by exposing them to cutting edge research at the undergraduate and graduate level.

Interband cascade lasers employing type-II band alignment in antimonide-based heterostructures have demonstrated recent success at 3-6 micrometer wavelengths, an important spectral range for applications such as chemical sensing, infrared countermeasures, and free-space optical communications. However, there is a “droop” in efficiency when these lasers are driven above threshold that reduces the maximum power obtainable in continuous wave (cw) or single mode operation.

The physical cause for the limitations on output power at high temperatures is not understood. Identifying the fundamental mechanisms that prevent carrier pinning will permit creation of new wave function engineering approaches to increase the maximum output power of these lasers at or above room temperature. This will increase the efficiency of devices so that injected carriers above threshold will contribute to laser output and not be lost to spontaneous emission or non-radiative recombination mechanisms, and consequently substantially reduce input power requirements and operating costs.

Contacts will be optimized to collect spontaneous emission, and the physical mechanisms of the limiting behavior will be quantified through light-current, light-light, spectral, current-voltage, and pump-probe measurements, all enhanced by the integration of split-ridge fabrication and/or transparent graphene contacts. The high optical, electrical, and thermal conductivity of graphene will not only aid in the collection of data but will advance optoelectronic device development more broadly through the study of the graphene-semiconductor interface.

The results of this project will provide a new understanding of the mechanism required to achieve pinning of the carrier densities and ultimately to increase the high-temperature output power in these lasers. This contribution is significant because it will enable the redesign of the active and cladding regions of laser devices in order to increase the maximum cw output power and result in more efficient high temperature operation.

Thus, antimonide-based semiconductor lasers would have the potential to be an enabling technology for mid-infrared applications in homeland security, environmental monitoring, and medical applications such as breath analysis for early detection of asthma in children. Outreach activities include video and curriculum development for K-12 students and teachers as well as for freshman engineers.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.