Abstract
We report on efficient THz generation in DAST by optical rectification of intense mid-IR pulses centered at (i) 3.9 μm and (ii) its second harmonic at 1.95 μm. Suppression of multi-photon absorption shifts the onset of saturation of the THz conversion efficiency to pump energy densities, which are almost an order of magnitude higher as compared to conventional pump schemes at 1.5 μm. Despite strong linear absorption at 3.9 μm, DAST exhibits a high optical-to-THz conversion efficiency, which we attribute to resonantly enhanced nonlinearity and advantageous phase matching of the THz phase velocity and group velocity of the driving pulse. At 1.95 μm, we find that low linear and multi-photon absorption in combination with cascaded optical rectification lead to record optical-to-THz conversion efficiencies approaching 6%. The observed high sensitivity of the THz generation to the parameters of the mid-IR driving pulses motivates an in-depth study of the underlying interplay of nonlinear wavelength- and intensity-dependent effects.
Highlights
Within the last few decades, terahertz spectroscopy,1–3 where the THz pulse mainly acts as a linear probe, has become a versatile tool to explore a multitude of physical phenomena by probing lowenergy transitions situated in the THz range, governed by phonon, plasmon, and spin-resonances; intersubband passages; excitons; or molecular rotations.4,5 Intense terahertz (THz) pulses with an electric field strength exceeding the intrinsic field of atoms have large potential to engineer new dynamic states in a wide range of materials
Because the absorption edge of DAST is situated at around 700 nm,20 the crystal suffers from multi-photon absorption (MPA),21 which acts on the saturation of the conversion efficiency, as well as on the crystal damage threshold, and restricts the applicable energy density of the driving pulses to a range of 20 mJ/cm2
To generate THz radiation by optical rectification (OR) in a ∼ 170 μm thick DAST crystal (Swiss Terahertz LLC), we use a high power mid-IR optical parametric chirped pulse amplifier (OPCPA),25,26 operating at a repetition rate of 20 Hz and generating 30 mJ, 100 fs pulses centered at 3.9 μm
Summary
Within the last few decades, terahertz spectroscopy, where the THz pulse mainly acts as a linear probe, has become a versatile tool to explore a multitude of physical phenomena by probing lowenergy transitions situated in the THz range, governed by phonon-, plasmon-, and spin-resonances; intersubband passages; excitons; or molecular rotations. Intense terahertz (THz) pulses with an electric field strength exceeding the intrinsic field of atoms have large potential to engineer new dynamic states in a wide range of materials. Trend-setting experiments, wherein the THz pulse acts as a non-linear (NL) pump, have been demanding intense, broadband THz sources.6–8 This exciting new research area provides an opportunity to control matter by coherent lattice excitation, to trigger transient phase transitions and engineer new dynamic states of materials, such as THz enhanced superconductivity, polarization switching in ferroelectric materials, or ultrafast switching and controlling of magnetic domains.. Considerable progress has been achieved with organic crystals, such as DAST (4-N,Ndimethylamino-4′-N′-methylstilbazolium tosylate) and its derivative DSTMS (4-N,N-dimethylamino-4′-N′-methyl-stilbazolium 2,4,6 trimethylbenzenesulfonate) Due to their extraordinarily high nonlinearities, small dielectric constants, and small dispersion from low to optical frequencies, the organic crystals are excellent materials for broadband THz generation.. MPA and subsequent free-carrier absorption of the THz radiation were identified to be responsible for the reduction of THz generated via OR. NL absorption can be a limiting factor for a low crystal damage threshold in EO crystals.
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