Tunable backward terahertz-wave parametric oscillator centered at a high frequency of 0.87 THz with injection seeding

We report the first demonstration of a frequency tunable backward THz-wave parametric oscillator (BW-TPO) centered at a high frequency of 0.87 THz using a slant-stripe-type magnesium oxide-doped periodically poled lithium niobate (PPLN) crystal as the nonlinear medium. Down-converted THz and idler beams generate upon excitation of the PPLN with a sub-nanosecond pulsed source of λ = 1064.44 nm. The resulting first idler has a wavelength of 1067.75 nm, equivalent to an oscillation frequency of 0.872 THz as per the spectral line separation from the pump. We also present angle tuning of the BW-TPO frequency ranging from 0.836-0.905 THz through PPLN rotation. The threshold pump intensity for BW-TPO is determined to be 5.6 GW/cm² while obtaining a conversion efficiency as high as 12.3% at a pump energy (intensity) of 15.25 mJ (8.90 GW/cm²). A reduction of the BW-TPO threshold energy and improved pump-to-idler energy conversion efficiency resulted from injection seeding with a CW laser at the same wavelength as the first idler. The THz output is also directly proportional to seed power.