Since the first demonstration of optical frequency combs in the infrared frequency regime, the study of frequency combs in various wavelengths attracts more and more interests. Because of high frequency stability and short pulse generation (if mode-locking can be applicable), the frequency comb can significantly improve the spectral and time accuracies, which can then be used in fundamental scientific research and high-resolution applications. In the terahertz frequency regime, the semiconductor-based electrically-pumped terahertz quantum cascade laser (THz QCL) is an ideal candidate for achieving terahertz frequency comb operations due to its characteristics of high output power, narrow far-field divergence, and wide frequency coverage. Although the active mode-locking of THz QCLs has been successfully demonstrated for generating THz pulses, the setup is very complex due to the microwave electronics and bulky femtosecond laser system. Therefore, scientists have been working on more efficient and simpler passive techniques to lock the THz QCLs. However, due to the insufficient nonlinearities of conventional saturable absorbers and fast gain recovery time, the passive mode-locking of THz QCL combs has been never demonstrated.

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 and Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, demonstrate the enhanced passive frequency comb operation of THz QCLs by coupling a multi-layer graphene sample into the laser cavity. By exploiting the saturable absorption and dispersion compensation of the graphene sample, the frequency stability of the laser comb is significantly improved. The z-scan measurements prove the saturable absorption behavior of the graphene sample. The measured linewidth of the inter-mode beat note signal of the graphene-coupled THz QCLs shows a record of 700 Hz. Furthermore, a THz pump-probe technique is employed to directly measure the pulse width of the passive frequency comb. The measured 16 ps pulses separated by the round trip time of ~160 ps is a sign that the THz QCL is mode-locked.  

For more details, please read the paper “Graphene-Coupled Terahertz Semiconductor Lasers for Enhanced Passive Frequency Comb Operation” published in Advanced Science (https://doi.org/10.1002/advs.201900460).