Terahertz generation in lithium niobate driven by Ti:sapphire laser pulses and its limitations

Abstract

We experimentally investigate the limits to 800 nm-to-terahertz (THz) energy conversion in lithium niobate at room temperature driven by amplified Ti:Sapphire laser pulses with tilted-pulse-front. The influence of the pump central wavelength, pulse duration, and fluence on THz generation is studied. We achieved a high peak efficiency of 0.12% using transform limited 150 fs pulses and observed saturation of the optical to THz conversion efficiency at a fluence of 15 mJ/cm 2 . We experimentally identify two main limitations for the scaling of optical-to-THz conversion efficiencies: (i) the large spectral broadening of the optical pump spectrum in combination with large angular dispersion of the tilted-pulse-front and (ii) free-carrier absorption of THz radiation due to multi-photon absorption of the 800 nm radiation. Strong-field terahertz (THz) sources hold promise for enabling myriads of novel applications. They possess intense electric and magnetic fields at frequencies which are particularly amenable to studies of condensed matter dynamics [1][2][3], manipulation of molecules [4], highharmonic generation (HHG) [5], and compact chargedparticle acceleration [6][7], among others. Therefore, there is a great need for the development of robust and efficient strong-field THz sources.These sources are predominantly accelerator-based facilities (delivering up to 100 µJ THz energy) [8,9] or ultrafast laser-based table-top systems [10]. Laboratory scale systems are of particular interest due to accessibility and relatively low cost. In this category, laser-induced air/gas plasmas (delivering up to 5 µJ THz energy) [11] and optical rectification (OR) of infrared (IR) pulses in nonlinear optical crystals (delivering up to 0.4 mJ THz energy) [12] have emerged as the most common methods of all THz generation modalities. The highest optical-toTHz conversion efficiencies (henceforth referred to as conversion efficiency) in excess of 1% at room temperature have been achieved by OR employing angularly dispersed femtosecond IR pump pulses in lithium niobate [13,14]. As a result, these systems are especially relevant to generating mJ-level THz pulses [12]. In this approach, angular dispersion is introduced to compensate for the large difference in refractive indices between IR and THz frequencies by forming a tilted-pulse-front. The generated THz then propagates perpendicular to this tilted-pulsefront and phase matching is achieved in a non-collinear configuration. OR systems based on lithium niobate are powered predominantly by sources in the 1 µm and 800 nm wavelength regions. While the former yields much higher conversion efficiencies, today, 800 nm sources based on Ti:Sapphire systems prevail in ultrafast laser technology as the most accessible and widely employed sources. Consequently, exploring the limits to achievable conversion efficiency is a great value in the pursuit of accessible high-energy THz sources.In this paper we extensively investigate the limits of conversion efficiency with 800 nm systems through a systemati...

Theory of terahertz generation by optical rectification using tilted-pulse-fronts

“…On one hand cascading is responsible for conversion efficiencies which exceed the Manley-Rowe limit. On the other hand, in the presence of group velocity dispersion due to angular dispersion (GVD-AD) and material dispersion (GVD-MD), this spectral broadening inhibits further THz generation [18][19][20]. A comprehensive theoretical model should therefore be able to account for all of the above effects.…”

Temperature dependent refractive index and absorption coefficient of congruent lithium niobate crystals in the terahertz range

“…Among various THz generation methods, optical rectification with intra-pulse difference frequency generation is still the most efficient technique [9][10][11][12][13]. Percent level optical-to-THz energy conversion efficiencies have been demonstrated in organic crystals [14] and MgO-doped lithium niobate crystals [15].…”

“…Furthermore, with MgO doping in the congruent lithium niobate, the damage threshold can be increased to 100 GW/cm 2 . 2-inch congruent lithium niobate crystals with 6.0 mol% MgO doping level have been employed to generate THz radiation at 800 nm [11].…”

Longitudinally polarized single-cycle terahertz pulses generated with high electric field strengths

“…In recent years, magnesium-oxide doped stoichiometric lithium niobate (MgO:SLN) has become a popular non-linear material for use in the generation of linearly polarized terahertz radiation with a high peak electric field strength. [5][6][7][8][9] Electric field amplitudes in excess of 1 MV/cm have been demonstrated by employing a pulse-front tilt pumping scheme to enable the coherent transverse addition of the Cherenkov terahertz emission. 10,11 Whilst other non-linear crystal sources have been shown to produce terahertz radiation with similar electric field amplitudes, 12 the wide availability of 800 nm Ti:Sapphire lasers makes MgO:SLN more attractive as a terahertz source.…”

“…Whilst this method is able to maintain phase matching between the terahertz radiation and particle bunch, the presence of the dielectric material, in this case MgO:SLN, causes a lowering of the terahertz electric field through strong terahertz absorption. 5 Generating a large longitudinal electric field component whilst still maintaining the single-cycle nature of the pulsed terahertz radiation would present a distinct advantage for particle acceleration as it would enable a higher accelerating peak electric field and a higher acceleration efficiency for short (sub-ps) electron bunches.…”