Delay-resolved gain measurements of intersubband devices

Engineered intersubband structures like quantum cascade lasers are ideal for mid-infrared and terahertz applications. While the theoretical intersubband gain and absorption predictions are well-developed, performance often deviates due to reasons such as variations in molecular beam epitaxy (MBE) machines, requiring experimental investigation. Existing methods either produce inaccurate gain profiles or are limited in their ability to assess dependence on frequency, bias, and temperature. In this work, we present a delay-resolved absolute-referencing method for accurate gain evaluation without these limitations. Our findings show that gain profiles match theoretical predictions at low-temperature biases below the threshold, with gain and dispersion clamping after lasing. A faster-than-expected degradation in gain performance was also observed at higher temperatures. These results provide a precise experimental evaluation of temperature-dependent gain performance, including high-temperature conditions that differ from theoretical predictions, and offer insights for optimizing high-temperature performance and future quantum cascade laser-based frequency comb design.