ABSTRACT
Quantum cascade detectors (QCD) are photovoltaic mid-infrared detectors based on intersubband transitions. Owing to the sub-picosecond carrier transport between subbands and the absence of a bias voltage, QCDs are ideally suited for high-speed and room temperature operation. Here, we demonstrate the design, fabrication, and characterization of 4.3â µm wavelength QCDs optimized for large electrical bandwidth. The detector signal is extracted via a tapered coplanar waveguide (CPW), which was impedance-matched to 50â Ω. Using femtosecond pulses generated by a mid-infrared optical parametric oscillator (OPO), we show that the impulse response of the fully packaged QCDs has a full-width at half-maximum of only 13.4â ps corresponding to a 3-dB bandwidth of more than 20â GHz. Considerable detection capability beyond the 3-dB bandwidth is reported up to at least 50â GHz, which allows us to measure more than 600 harmonics of the OPO repetition frequency reaching 38â dB signal-to-noise ratio without the need of electronic amplification.