Optical frequency combs (FCs) find numerous applications in time metrology, absolute distance measurements at long range, and spectroscopy. FC spectroscopy is particularly appealing in the terahertz (THz) spectral range where THz detectors based on direct power detection have low sensitivity, slow response time, and may require cryogenic cooling but inexpensive heterodyne detectors, such as Schottky diodes, are available and provide high sensitivity at room temperature.
Researchers at The University of Texas at Austin have developed an optical terahertz (THz) frequency comb (FC) using THz Cherenkov difference-frequency generation in a mid-infrared quantum cascade laser (QCL) frequency comb. The THz comb is expected to span from 0.5 THz to 10 THz and to operate at room temperature. This invention would be the first compact, completely monolithic, mass producible, electrically-pumped broadband THz frequency comb that operates at room temperature.
This invention generates a THz FC in the core of a mid-IR QCL. The QCL active core is engineered with broadband mid-IR gain and third order optical nonlinearity to generate a mid-IR FC. The mid-IR gain is designed to be evenly distributed amongst all frequencies in the gain bandwidth and results in a flat, dispersionless gain profile critical for mid-IR FC generation. Additional results are described here, here, and, finally, here.
Figure 1. (a) Schematic of Cerenkov THz DFG emission in a QGL. (b) Design layout for the QGL devices.
Figure 2. (c) Vertical refractive index profile in the devices form a mid-IR (lambda = 9um) and 4.5 THz light. Position x=0 corresponds to the middle of the active region.