Observation of a cascaded process in intracavity terahertz optical parametric oscillators based on lithium niobate

Abstract

Cascaded difference frequency generation has been observed in intracavity optical parametric oscillators based on bulk lithium niobate and producing nanosecond pulses of terahertz radiation. Two idler waves are generated, namely: the primary idler wave associated with the parametric down conversion process itself; and a secondary idler wave, due to difference frequency generation. Experimental investigations of the frequency, temporal evolution, propagation direction, intensity, phase matching and oscillation threshold of the generated down-converted waves are reported. The overall generation efficiency for the terahertz radiation is enhanced, thereby overcoming the Manley-Rowe limit. Advantages of the present approach over schemes based on periodically poled lithium niobate are identified.

Cascaded Stimulated Polariton Scattering in a Single-Pass KTP Crystal under Picosecond Pumping

“…In the majority of these works, a pump beam of 1064 nm wavelength was used. Tunable THz polariton lasers based on polariton scattering in crystals and pumped by Q-switched lasers were realized in a number of papers [1][2][3][4][5][6][7][8][9][10]. Nonlinear crystals, LiNbO 3 [1][2][3][4], KTP [5][6][7][8], RTP [9] and KTA [10], were fixed on a rotating stage together with polariton cavity mirrors.…”

“…Tunable THz polariton lasers based on polariton scattering in crystals and pumped by Q-switched lasers were realized in a number of papers [1][2][3][4][5][6][7][8][9][10]. Nonlinear crystals, LiNbO 3 [1][2][3][4], KTP [5][6][7][8], RTP [9] and KTA [10], were fixed on a rotating stage together with polariton cavity mirrors. This stage allowed the authors to tune the small angle θ between the polariton cavity axis and pump laser beam and in this way tune the signal (Stokes) and idler (THz) frequencies intermittently.…”

“…The sufficiently short length of the THz polariton cavity (up to ~10 cm) allowed the authors to increase the intensity of the signal and THz waves during the nanosecond pumping pulses (7-30 ns). The polariton cavity is either inside the fundamental cavity [3][4][5][8][9][10] or outside it [1,2,6,7]. Intermittent tuning of the signal and THz waves in the process of the angle θ tuning was interpreted as the result of stimulated polariton scattering (SPS).…”

Phonon-assisted nonlinear optical processes in ultrashort-pulse pumped optical parametric amplifiers

“…Recently, the topic of coherent interactions between electromagnetic radiation and the vibrational modes in the NLO crystals has been receiving a revived interest in light of the ongoing developments in terahertz parametric oscillators (TPOs) 5 6 7 8 9 10 11 12 and Raman lasers 13 14 . In this regard, KTP and its isomorphs such as potassium titanyl arsenate (KTiOAsO 4 , KTA), RbTiOPO 4 etc., represent a versatile group of NLO materials that possess relatively high 2 nd -order nonlinear coefficients 15 , wide spectral transparency window (~400–4500 nm for KTP; ~350–5000 nm for KTA) 16 17 , and relatively high damage thresholds (reaching 100’s of GW/cm 2 in femtosecond regime 16 ).…”

“…Notably, the SRS of the parametrically amplified signal wave was shown to enhance when the idler wave was tuned into resonance with the second overtone band of the phosphate ion 18 and, moreover, when the pump pulse duration was shortened to a few-picosecond durations 19 thus approaching the dephasing times of the Raman-/IR-active phonons 31 . A distinct feature of these studies that utilize picosecond (relatively narrowband) pump pulses is that the Raman scattering occurs in cascading manner to the parametric processes 11 30 32 33 , i.e. the Stokes-shifted signal/pump spectra can be clearly distinguished from the major peaks corresponding to the OPO-generated signal and idler waves.…”