Lamb-Dip Spectroscopy in Rotational Levels of Acetonitrile Molecules using a Narrow-Linewidth UTC-PD Terahertz Light Source

Transitions between the rotational levels characteristic of gas molecules appear in the terahertz (THz) frequency region. Since the rotational levels are arranged at non-equidistant intervals in terms of frequency, this system can be considered as a two-level system. In addition, it is expected to be a new quantum operation platform due to its extremely long phase relaxation time [1]. However, it should be noticed that molecules also exhibit translational motion at finite temperatures, resulting in a Doppler width of several hundred kHz at room temperature. While precise high-resolution spectroscopic techniques have been developed to overcome the Doppler width in the visible and ultraviolet light regions, the development of such techniques for the THz region is limited, with only a few practical applications currently available [2]–[4].