Gigahertz Mid-Infrared Interband Cascade Detectors: Photo-Response Saturation by a Femtosecond Oscillator

Mid-infrared (mid-IR) interband cascade detectors (ICDs) combine interband optical transitions with fast intraband transport to achieve high-frequency and broad-wavelength operation at room temperature. To understand the photo-response and saturation characteristics of these detectors, we measured the bias-dependent impulse response to femtosecond pulses for the first time. To probe the ICD, we use an optical parametric oscillator emitting 104 fs pulses. The ICD, which is embedded in a ridge waveguide, contains four active stages consisting of a W -shaped quantum well, an electron and a hole extractor. We measured the photo-response as a function of reverse bias and average incident power. These measurements show that a reverse bias increases the peak voltage for a given input power as well as the saturation power. Viewing the dynamic response on a log scale reveals a faster exponential decay (around 40 ps, likely associated with electrons) followed by a slower second exponential decay (around 150 ps, likely associated with holes). We observed that both of these characteristic decay times reduce with increasing reverse bias. To determine the frequency response, we performed a direct measurement on a microwave spectrum analyzer and calculated the Fourier transform of the temporal response. We see that the 3-dB power bandwidth of the ICD increases from 1.24 GHz at zero bias to 1.67 GHz at -3.0 V of reverse bias. In conclusion, we have measured the saturation behavior and impulse response of an ICD for the first time. Our results provide insights into the ultrafast dynamics of ICDs, and will help enable the development and characterization of infrared light sources, especially mode-locked lasers.