The dual-comb is a novel technique that can be used in many practical applications in spectroscopy, imaging, communications, and so on. In the spectroscopic field, the traditional techniques, e.g., Fourier transform infrared spectroscopy, time-domain spectroscopy, etc., require a moving part to obtain the spectra of samples, which makes it hard to perform the real-time spectral measurement. However, the dual-comb spectroscopy that employs two frequency combs with slightly different repetition rates can directly map the sample spectrum (transmission or reflection) in the optical frequency to the microwave frequency. Because no moving part is needed, the dual-comb technique can perform the spectroscopy in real-time. In the terahertz regime, the electrically-pumped quantum cascade laser, which is characterized by high power, broad frequency coverage, narrow beam divergence, is suitable for frequency comb and dual-comb operations.
In this work, the researchers at the Key Laboratory of Terahertz Solid State Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, in collaboration with East China Normal University, demonstrate the compact and real-time terahertz spectroscopy employing THz QCL frequency combs. In our experiment, two free-running QCLs generate approximately 120 GHz wide combs centered at 4.2 THz, with slightly different repetition frequencies. We observe that ～490 nW terahertz power coupling of one laser into the other suffices for laser-self-detecting the dual-comb spectrum that is registered by a microwave spectrum analyzer. Furthermore, we demonstrate practical terahertz transmission dual-comb spectroscopy with our device, by implementing a short air path at room temperature. Spectra are shown of semiconductor samples and of moist air, the latter allowing rapid monitoring of the relative humidity. Our devices should be readily extendable to perform imaging, microscopy, and near-field microscopy in the terahertz regime.
The work has been published in ACS Photonics 7, 49 (2020) and selected to be featured on the front cover of the February issue of 2020. The details can be found in the paper and Supporting Information at DOI: 10.1021/acsphotonics.9b01427.
Left: Schematic illustration of the compact terahertz dual-comb system based on terahertz quantum cascade lasers. The inset is the photo of the dual-comb lasers mounted on a specifically designed cold finger. Right: Front cover image of the paper published in ACS Photonics