
- Journal of Semiconductors
- Vol. 40, Issue 5, 050402 (2019)
Abstract
Frequency combs[
Figure 1.(Color online) Frequency comb and its applications in spectroscopy and nonlinear dynamics of materials.
Although the first demonstrations of frequency combs were developed in infrared wavelengths, the coherent comb operations in other wavelengths have been much in demand for various spectral applications. Here, we pay more attention to the terahertz frequency range (between 0.1 and 10 THz) which is also called “terahertz gap” in the entire electromagnetic spectrum due to the lack of efficient radiation sources in this frequency band. However, the terahertz frequency range is of great importance for practical applications since diverse characteristic absorption lines of gases and chemicals are located in this range. The electrically-pumped semiconductor-based terahertz quantum cascade laser (QCL)[
Terahertz combs have been realized in QCLs by employing an active mode-locking or phase seeding technique assisted by femto-second lasers[
Currently, the application of terahertz QCL frequency combs is still in the preliminary stage. Although terahertz QCL combs have been successfully demonstrated in actively mode-locked or free-running lasers, the practical application of such combs for spectroscopy normally requires an implementation of dual-comb technique. The on-chip dual-comb terahertz source has been demonstrated using double-metal waveguide QCLs by employing a self-detection scheme[
Since the dual-comb spectroscopy shows advantages of high-resolution and fast data acquisition without a need of moving parts in the system over the traditional Fourier transform infrared (FTIR) spectroscopy, it will renovate the terahertz spectroscopy in the future. The spectral resolution is expected to be increased significantly by at least three orders from GHz (a typical value of a commercial FTIR) to MHz by employing the terahertz QCL dual-comb technique. The following two difficulties should be overcome for pushing the dual-comb technique for commercialization. First of all, the optical bandwidth of a single QCL-based frequency comb should be broadened for detecting absorption lines of various molecules. This is the main difficulty because the QCL is specially designed for narrow emission frequency. Moreover, as the frequency becomes wider, the dispersion compensation would becomes harder and harder. Secondly, the fine frequency tuning of QCL combs is critical for accurately measuring narrow absorption lines with high spectral resolutions. Therefore, external perturbations, for instance, by radio frequency modulation, is necessary to be employed.
It is worth mentioning that although optical pulses generated from terahertz QCL combs have been observed, it is still a way to go to use the terahertz pulses for nonlinear study in materials. Terahertz QCL combs are characterized by high output power. However, due to the high repetition rate of GHz level that is more than 1000 times larger than the femto-second laser pulses and much wider pulse duration of dozens of picoseconds resulted from the limited optical bandwidth, the peak power of a terahertz QCL comb is much weaker than that of a femto-second laser combs. Therefore, in the near future, to develop ultra-broadband terahertz QCL frequency combs with ultrashort pulse widths and low repetition rates is of great interest for spectroscopic and nonlinear studies.
Acknowledgements
This work was supported by the "Hundred-Talent" Program of Chinese Academy of Sciences, the National Natural Science Foundation of China (61875220, 61575214, 61404150, 61405233, and 61704181), the National Key R&D Program of China (2017YFF0106302 and 2017YFA0701005), and Shanghai Municipal Commission of Science and Technology (17YF1430000).
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