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Although gas phase rotational spectroscopy is a mature field for which millions of rotational spectral lines have been measured in hundreds of molecules with sub-MHz accuracy, it remains a challenge to measure these rotational spectra in excited vibrational modes with the same accuracy. Recently, it was demonstrated that virtually any rotational transition in excited vibrational modes of most molecules may be made to lase when pumped by a continuously tunable quantum cascade laser (QCL). Here, we demonstrate how an infrared QCL may be used to enhance absorption strength or induce lasing of terahertz rotational transitions in highly excited vibrational modes in order to measure their frequencies more accurately. To illustrate the concepts, we used a tunable QCL to excite v₃R-branch transitions in N₂O and either enhanced absorption or induced lasing on 20 v₃rotational transitions, whose frequencies between 299 and 772 GHz were then measured using either heterodyne or modulation spectroscopy. The spectra were fitted to obtain the rotational constants B₃and D₃, which reproduce the measured spectra to within the experimental uncertainty of ± 5 kHz. We then show how this technique may be generalized by estimating the threshold power to make any rotational transition lase in any N₂O vibrational mode.

The terahertz region of the electromagnetic spectrum has been the least utilized owing to inadequacies of available sources. We introduce a compact, widely frequency-tunable, extremely bright source of terahertz radiation: a gas-phase molecular laser based on rotational population inversions optically pumped by a quantum cascade laser. By identifying the essential parameters that determine the suitability of a molecule for a terahertz laser, almost any rotational transition of almost any molecular gas can be made to lase. Nitrous oxide is used to illustrate the broad tunability over 37 lines spanning 0.251 to 0.955 terahertz, each with kilohertz linewidths. Our analysis shows that laser lines spanning more than 1 terahertz with powers greater than 1 milliwatt are possible from many molecular gases pumped by quantum cascade lasers.

Since the days of Hertz, radio transmitters have evolved from rudimentary circuits emitting around 50 MHz to modern ubiquitous Wi-Fi devices operating at gigahertz radio bands. As wireless data traffic continues to increase, there is a need for new communication technologies capable of high-frequency operation for high-speed data transfer. Here, we give a proof of concept of a compact radio frequency transmitter based on a semiconductor laser frequency comb. In this laser, the beating among the coherent modes oscillating inside the cavity generates a radio frequency current, which couples to the electrodes of the device. We show that redesigning the top contact of the laser allows one to exploit the internal oscillatory current to drive a dipole antenna, which radiates into free space. In addition, direct modulation of the laser current permits encoding a signal in the radiated radio frequency carrier. Working in the opposite direction, the antenna can receive an external radio frequency signal, couple it to the active region, and injection lock the laser. These results pave the way for applications and functionality in optical frequency combs, such as wireless radio communication and wireless synchronization to a reference source.