Circumventing limitations of tilted-pulse-front terahertz generation using a stair-step echelon

Abstract

Abstract-Ultrafast terahertz (THz) transients are of great interest for linear and nonlinear THz spectroscopy and compact particle acceleration. Tilted-pulse-front (TPF) [1] THz generation in lithium niobate (LN) has become ubiquitous due to its compatibility with easily accessible highpower 800 nm/1 µm laser technology. However, the use of diffraction grating (DG) based TPFs (DG-TPFs) present formidable imaging challenges, particularly for large pump bandwidths and beam sizes. Furthermore, as the optical pump spectrum red-shifts and broadens due to repeated down-conversion to THz frequencies (referred to as cascading), a spatio-temporal break-up of the pump pulse results [2]. This limits the energy conversion efficiency (η) and produces THz pulses with spatio-temporal distortions.We describe new mechanisms to eliminate these limitations by engineering the transverse-momentum distribution of the pump pulse via a stair-step echelon [3]. The structure ( Fig. 1(a)) produces a discrete optical pulse front composed of many beamlets. Each beamlet propagating in LN at the optical group velocity c/n g produces a THz wavelet that propagates at phase velocity c/n THz in

Analysis of terahertz generation by beamlet superposition

“…In effect, cascading is just an increase in the bandwidth of the pulse and hence the improvement in performance at larger bandwidths signals alleviation of cascading limitations. While initial simultations appear to attest to this hypothesis [30], future work will address the ultimate limits of beamlet superposition.…”

“…(29), notice that the distribution in transverse momentum k x is centered about k(Ω)βv g and has a much narrower spread in k x space as w is much larger than σ. Additionally, there is a reduction in the amplitude of E T HZ (Ω, k x , z) by a factor of proportional to (1+k T Ω 2 w 2 /τ 2 ) −1/2 as captured by the effective beam radius term w 1 in Eq. (30). This shall be proven in a subsequent section to most negatively affect terahertz generation produced by DG-TPF's.…”

“…This is because the effect of GVD-AD is more adverse for larger bandwidths (see Eqs. (29)- (30) ). The average frequency also reduces (blue dashed, Fig.10(b)) compared to the w = 2 mm case (black dashed, Fig.9(d)).…”

“…Most importantly, as the angular spread is uniform for all pump spectral components, phase matching may be maintained across the entire pump pulse bandwidth. This allows the approach to potentially circumvent limitations due to bandwidth and cascading effects [30]. However, a detailed theoretical investigation has not yet been performed to explore this approach comprehensively.…”

Simultaneous generation and compression of broadband terahertz pulses in aperiodically poled crystals

“…This is because, at high conversion efficiencies, cascading effects produce large spectral and angular broadening of the pump. This issue may be alleviated using a superposition of beamlets [31][32][33] and variants thereof [34,35]. Alternatively, attempts to eliminate the drawbacks of tilted pulse fronts in lithium niobate by utilizing smaller pulse-front tilt angles are being pursued [36,37].…”