Quantum-cascade local oscillators beyond 5 THz

Many molecular species that comprise the interstellar medium have strong spectral features in the terahertz range, and the high spectral resolution provided by heterodyne spectroscopy is required to obtain ~km/s velocity resolution to resolve their complicated lineshapes and disentangle them from the background. Understanding the kinetics and energetics within the gas clouds of the interstellar medium is critical to understanding star formation processes and validating theories of galactic evolution. THz quantum-cascade (QC) lasers are candidates for local oscillators that deliver high-powers sufficient to pump large arrays of heterodyne mixers, particularly above 2 THz where electronic techniques provide diminishing output power. To date, THz QC-lasers have been deployed as local oscillators on two heterodyne instruments (upGREAT/SOFIA and GUSTO) to map the [OI] line at 4.74 THz. However, there exists no viable local oscillator above 5 THz, which is needed to access important spectral lines including [OIII] at 5.79 THz and [NIII] at 5.23 THz. Previous observations have shown that oxygen and nitrogen lines often dominate the emission from galactic HII regions, and originate from regions rich with activity: early-type stars in the process of formation, shocks and supernova remnants. The proposed work would develop the underlying technology for THz QC-laser local oscillators in the 5-6 THz spectral range. Such heterodyne receivers may find future use in a future far-IR probe mission, or balloon platform, SmallSat, or Explorer-class mission. To date, operation of THz QC-lasers at frequencies above 5 THz has proven especially difficult because of increased losses and reduced gain associated with to the proximity of the laser frequency to the polar-optical-phonon resonances within the III-V semiconductor gain medium. This project will address these issues through fundamental research to develop advanced active regions that mitigate this degradation above 5 THz. In addition, the QC-laser will be implemented using a new architecture previously developed by our team: the QC vertical-external-cavity-surface-emitting-laser (QC-VECSEL). QC-VECSELs have excellent beam quality, and are capable of single-frequency output tunable over as much as 20% of their center frequency – an asset for a frequency agile local oscillator. Furthermore, the output power from a QC-VECSEL is scalable from sub-milliwatt levels (suitable for few-pixel instruments on power constrained platforms) to over 10 milliwatts of output power (suitable for pumping future large format heterodyne array instruments of 100 pixels (or more) for rapid mapping of the interstellar medium). Our research goal in this proposal is to develop quantum-cascade laser local oscillator sources that operate above 5-6 THz for the first time. We present a multi-themed research plan that spans from (a) fundamental development of the QC laser material, (b) its implementation in a frequency-agile metasurface QC-VECSEL, and (c) frequency stabilization of the QC-VECSEL and its insertion in a practical heterodyne receiver testbed as a proof-of-concept demonstration. We choose 5.23 THz and 5.79 THz as specific demonstration frequencies, so as to correspond with the aforementioned [NIII] and [OIII] lines. At the outcome, we expect to demonstrate QCL local oscillators in the region, which exhibit fine tuning to enable spectral alignment with the lines of interest. Particular attention will be paid to exploring the trade-offs between output power, wall-plug efficiency, operating temperature, and power consumption, with a mind to meeting power budget requirements of future SmallSat and balloon missions.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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