Post-Process Frequency Tuning of Single-Mode Quantum Cascade Laser at 4.7 THz

We present a systematic study of the optical design, fabrication, and characterization of quantum cascade laser devices with a frequency around 4.7 THz, intended for use as local oscillators in the GREAT heterodyne receiver aboard SOFIA (Heyminck, et al., 2012), (Risacher et al., 2018). The measured devices exhibit consistent spectral performance, with approximately 75% of them having their emission frequency within a 6 GHz band relative to their nominal value. We present surface-emitting lasers capable of covering the required 4743-4748 GHz frequency interval, with powers up to 2.2mW at 40K in continuous wave. Their emission frequency can be tuned up to +2 GHz with current over 80mA and -5 GHz over the 20-60K range with temperature. Additionally, we explain how processing variability is exploited to shift the emission frequency post-process and post-measurement: occurring during an etching step, the undesired height difference between different sample areas can be minimized using custom thicknesses for depositing various materials. This alters the effective refractive index of the optical mode, thus changing the laser's emission frequency.