High-power photon sources at mid-infrared wavelengths have commercial potential in areas having large societal impact. The proposed work will create a new device that can be used in ultra-sensitive systems for environmental (greenhouse gas detection, ground water and wastewater monitoring), industrial (chemical process sensing and automotive emission sensors), and homeland security (explosive detection) applications. The device could also be used as an enabling component in new systems being developed for biological imaging and in areas such as cancer cell detection. In all of these applications, the higher output powers expected with the "transistor-injected quantum-cascade laser" should allow systems with greater sensitivity than systems where mid-infrared photons are generated with conventional options. The anticipated ability to modulate at high speed (> 20 GHz) also opens the possibility of producing free-space optical links in non-absorbing atmospheric windows. Such links could be used either in place of or to augment microwave-frequency point-to-point communication channels. This work will also provide other societal benefits. Basic knowledge gained will be incorporated into courses at the University of Illinois on compound semiconductor devices, and will be disseminated through peer-reviewed literature and at conferences. The funding provided will allow undergraduate and graduate research projects to proceed. The PI is also committed to advancing STEM education, and has provided seminars to high school students and teachers during various REU/RET programs at Illinois. The PI is also committed to the education of women and underrepresented groups, and a portion of the project funds will be used to support a female graduate student.
It is the objective of this research program to design, fabricate, and test a novel device architecture for the generation of coherent radiation at mid-infrared and longer wavelengths. The transistor-injected quantum cascade laser is proposed as a 3-terminal device that allows independent control of the field across a quantum cascade region located in the reverse-biased junction of a heterojunction bipolar transistor and the amplitude of the injected current, controlled using the forward biased emitter-base junction. Independent control of cascade structure field stabilizes parameters such as state energy and lifetime that fundamentally impact laser operation. This approach provides several advantages over the 2-terminal quantum cascade laser, which is the incumbent solution for the generation of mid-IR coherent radiation. First, free carrier absorption is lower in the proposed device because both the cascade region and surrounding portions of the p-base and n-collector are within the depletion region of the base-collector junction. Because free-carrier absorption is a key contributor to internal loss at long wavelength, threshold current densities are projected to be lower and both slope efficiency and wall-plug efficiency higher. Additionally, because the field in the cascade region can be fixed at a value that provides optimal resonant coupling between the energy levels in adjacent wells and stable energy state lifetimes, the gain as a function injected current is expected to be more stable and have better linearity. Higher drive currents will not cause gain reduction due to field-related misalignment of quantum states and modification of state lifetimes. Finally, because a small base current is used to control the operation of the device, modulation in the multi-gigahertz frequency range is expected to be possible. Successful demonstration of this device has the potential to create new areas of research in both mid-infrared wavelength through terahertz frequency devices and in the applications for those devices.
