This project is to research and develop a new type of thermophotonic device. The intellectual merit includes studies in the physics of heat and charge transport in charge controlled quantum wells combined with microcavity physics to achieve waste heat absorption. Thermophotonics has long been proposed as a potential advance to achieve solar energy conversion of close to 100%, due to its ability to utilize waste heat from above band gap absorption. However, nonoptimized device structures and internal efficiency problems have limited the overall efficiency. These problems are addressed through charge control in high quality quantum wells, and microcavity physics to reach high extraction efficiency. The proposed structures provide a new device route to overcoming past technology hurdles in the efficiency of waste heat absorption.
Broader Impact: The research project is gender diverse, and the broader impacts include the advancement of women in science and engineering, as well as the impact that thermophotonic device technologies can make on society. Devices that can produce high efficiency absorption of waste heat can be used in a wide range of applications, and thermophotonic devices are well suited for these since they can in principle produce heat absorption even for small temperature differences. Besides high efficiency photovoltaic cells, thermophotonic devices could find applications in consumer electronics such as computers, cell-phones, and various sensors. The thermophotonic devices can greatly increase the efficiency of these electronics and extend their battery lifetimes and operating lifetimes by reducing waste heat and temperature rises due to inefficient electronics.
This project made fundamental investigations into semiconductor light emitting diode heat pumps that could be integrated into various photonic devices including lasers and solar cells. The heat pump action has long been predicted for a forward biased high efficiency p-n junction light emitting diode (LED), under certain bias conditions. However the effect has been difficult to observe in the laboratory due to the difficulty in extracting the LED’s light. Our experimental approach has been to integrate the LED heat pump directly into photonic devices for which cooling is desired. This project has focused heavily on integrating the LED heat pump with a laser. In this case the LED heat pump provides optical pump light to the monolithically integrated semiconductor laser so that the LED is electrically pumped while the laser is optically pumped. The integrated chip overcomes the problems with Snell’s law, instead using it to advantage by absorbing the spontaneously radiated photons within the laser waveguide internal to the chip. The benefit to the laser can be reduced internal loss through optical pumping. In ideal operation the LED optical pump can also act as a heat pump, absorbing heat from the semiconductor laser and its electrical contacts. The integrated heat pump could make a dramatic change in semiconductor laser efficiency and brightness. The LED heat pump has long been known to be theoretically capable of delivering more optical power than the electrical power it draws, producing greater than unity optical to electrical power conversion efficiency. Our calculations based on material parameters indicate that with proper design, the integrated laser chip can also exceed unity efficiency with respect to the chip’s electrical input power. Combined with the potential for increased efficiency is the separate potential for high brightness. By removing dopants from overlapping the laser mode, lower internal optical loss can be achieved to increase the laser’s cavity length. Our experimental component of the project has made the first demonstration of the integrated laser chip. Electrical pumping is only to the LED, which optically pumps the integrated semiconductor laser. The results are compared with electrically injected semiconductor lasers with similar waveguide design. Threshold current densities in these electrically injected LED, optically pumped laser chips are only a factor of two increased in this initial demonstration. Power levels of about 1 W are obtained under room temperature operation. These first demonstrations are expected to pave the way for additional experiments and developments of integrated chips that can benefit from integrated LED heat pumps.
