Synthesis of terahertz frequencies by optical frequency mixing
In the context of THz spectroscopy, investigation of narrow-line radiation-matter resonances requires tunable monochromatic radiation. A key feature, highly desirable for a variety of experiments, is the knowledge of the emitted THz frequency with a high accuracy. The current panorama of CW tunable THz generation includes: i) extreme-order microwave harmonic multiplication; ii) the only current compact high-power (10s mW) THz laser sources, the quantum cascade lasers (QCLs); iii) frequency mixing of two IR laser beams. Whereas the first approach is limited to the 100s of GHz range, due to the low power available at higher frequencies, and QCLs still work only at cryogenic temperature (< 200 K), frequency mixing appears at present as the most flexible generation scheme, suitable for high-resolution spectroscopy on the whole THz range (0.1-10 THz, 3 mm-30 μm).
To date, frequency mixing has been mostly performed in optoelectronic emitters (photo-mixers) where a THz frequency is generated in a semiconductor substrate from the beating of two incident CW IR beams. As concerns the knowledge of the generated THz frequency, photo-mixing has taken advantage from the introduction of optical frequency combs, which allow the realization of ultra-stable phase-coherent links between the optical frequency domain and the radiofrequency range. By locking the near-IR lasers to two teeth of an optical frequency comb with standard techniques, the difference frequency THz radiation can be readily referenced to a microwave frequency standard, thus giving access to THz synthesis. The THz power generated by photo-mixing can be as high as 100 μW at frequencies below 1 THz, but quickly decreases at higher frequency, due to the intrinsically limited response bandwidth of the photo-mixer.
Our strategy for the development of a highly efficient and coherent tunable source is based on optical frequency mixing of two near IR lasers in a non-linear optical crystal (second order non-linearity, chi2), via a phase-matched difference frequency generation process. As already demonstrated in the case of photo-mixing, the THz frequency can be referenced to a microwave standard by means of a frequency comb, enabling THz frequency synthesis. Owing to the broad-band of optical frequency combs and to the ultra-fast response of the optical non-linearity, the synthesizer holds the potential to cover any portion of the THz spectrum up to 10 THz, being limited only by the transparency of the non-linear medium employed.