Monochromatic Cherenkov THz Source Pumped by a Singly Resonant Optical Parametric Oscillator

We report an improvement of terahertz (THz) output energy via Cherenkov-type difference frequency generation by developing the dual-wavelength pump source, which is based on a singly resonant near-degenerated optical parametric oscillator. Total utilization efficiency of pump energy is considerably increased by recycling the residual fundamental wave. The highest energy of Cherenkov-type monochromatic THz generation is achieved to be 1.58 nJ/pulse. A tuning range of 0.1-3.2 THz is demonstrated.

Terahertz (THz) technology has attracted considerable interest due to its potential for various applications, such as chemical and biomedical testing [1]. In these fields, monochromatic and tunable coherent THz source plays an important role, owing to its narrow linewidth, high spectral power density, and wide tuning range. Difference frequency generation (DFG) from laser pulses with two close wavelengths is an established way for monochromatic THz generation [2], due to such advantages: availability of both low cost laser pump source and nonlinear crystal, continuous and wide tunability, operating at room temperature, and also compactness of system. An amount of investigation has been made to improve the efficiency of THz-DFG process, including on the nonlinear materials (eg. GaSe [3]–[5], GaP [6], [7], GaAs [8], PPLN [9] and DAST [10]) and also on the phase-matching (PM) configurations (eg. birefringence [3]–[5], noncollinear [6], [8] and quasi PM [7], [9], as well as Cherenkov-type [11] and front-tilting [12] PM). Cherenkov PM in , first demonstrated by Kawase et al. [13], is a promising configuration, which can provide a wide tuning range without inherent dip [14], [15], as the PM condition is automatically satisfied. Its merits over the other geometries have been discussed in detail in [11]. However, the output pulse energy via Cherenkov-type THz-DFG is only around sub-nJ-level [15].