High-speed, room-temperature, quantum well infrared photodetectors (QWIPs) at λ ∼ 4.9 μm have been realized in a strain compensated In 0.1 Ga 0.9 As/Al 0.4 Ga 0.6 As heterostructure grown on a GaAs substrate. The high-speed properties at room temperature have been optimized by using a specifically designed air-bridge structure, which greatly reduces the time constant of the effective RC circuit, thus, allowing transmission and detection of high-frequency signals. By modulating a high-speed quantum cascade laser (QCL) centered at λ ∼ 4.7 μm, we were able to record a modulation of the photocurrent up to ∼26 GHz, which is limited by our setup. At 300 K and under a bias voltage of −5 V our device shows high responsivity and detectivity of 100 mA/W and 1 × 10 7 Jones, respectively. The developed high-performance QWIPs at this wavelength are highly promising for optical heterodyne measurement, high-speed free space communications in microwave optical links and frequency comb QCLs characterisations.
The free‐spectral range (FSR) of quantum cascade lasers (QCLs) emitting at 4.7 µm can be tuned through direct microwave modulation. The intrinsic short carrier lifetime of inter‐subband transitions, along with impedance matched electrical packaging, allows a high‐speed modulation up to more than 30 GHz. A significant broadening and flattening of the lasing spectrum is observed under radio frequency (RF) injection with frequencies close to the round‐trip cavity. An accurate analysis of the high resolution spectra of the laser shows a comb‐like regime for both free‐running and RF modulated QCLs, if the modulation frequency is within the locking‐range of the device. One of the main advantages of collecting high‐resolution mid‐infrared spectra, over the plain investigation of the beatnote in the microwave region, is the access to all the longitudinal modes and thus the accurate measure of the FSR over the whole optical spectrum. The use of high‐resolution spectroscopy provides an in‐depth and comprehensive analysis of lasing spectra under microwave modulations.
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