Terahertz field enhancement via coherent superposition of the pulse sequences after a single optical-rectification crystal

Abstract

Terahertz electromagnetic pulses are frequently generated by optical rectification of femtosecond laser pulses. In many cases, the efficiency of this process is known to saturate with increasing intensity of the generation beam because of two-photon absorption. Here, we demonstrate two routes to reduce this effect in ZnTe(110) crystals and enhance efficiency, namely by (i) recycling the generation pulses and by (ii) splitting each generation pulse into two pulses before pumping the crystal. In both methods, the second pulse arrives ~1 ns after the first one, sufficiently long for optically generated carriers to relax. Enhancement is achieved by coherently superimposing the two resulting terahertz fields.Over the past two decades, electromagnetic radiation from the terahertz (THz) frequency window has proven to be a highly useful probe of numerous fundamental excitations such as crystal lattice vibrations (in particular in molecular crystals), 1 free or weakly bound electrons, 2,3 spin waves, 4 and spin currents. 5 In addition to such linear THz probing, the availability of intense THz pulses has recently opened the door to applications like nonlinear THz spectroscopy, 6 THz control over matter and light 7,8 and large-area imaging. 9 Table-top sources of strong THz fields generally rely on frequency conversion of intense femtosecond laser pulses in various media, for example, optical rectification (OR) in nonlinear-optical crystals, large-area photoconductive antennas or, in gases, the mixing of the fundamental optical beam and its second harmonic.10 A frequently employed material for OR is the nonlinear-optical crystal ZnTe, because of its relatively large second-order optical nonlinearity, low THz absorption (absorption coefficient 1.3 cm -1 at 1THz) and a relatively large phase-matching coherence length ( >1 mm for 1 to 3 THz and 800 nm). 11One may expect higher OR efficiency in ZnTe by just rising the pump intensity or increasing the thickness of the ZnTe crystal. However, this strategy is severely limited because additional nonlinear optical processes become operative. In particular, two-photon absorption (2PA) of the pump beam results in the loss of pump energy and the generation of free charge carriers that dissipate the THz radiation through Drude-type or free-carrier absorption (FCA).12 Consequently, the generated THz power saturates when the pump power is increased further. 12,13,14,15 The use of thicker crystals is also problematic as it increases the 2PA-related FCA of THz radiation. (For example, if the thickness of the ZnTe crystal is doubled, THz waves generated in the first half slice of the crystal will undergo additional absorption in the second half slice.)To keep the impact of 2PA and, thus, FCA on a tolerable level, ZnTe(110) crystals with larger aperture have been used, resulting in reduced pump intensity (power per area) and increased opticalto-THz conversion efficiency. 16,17 For example, using a ZnTe wafer with a diameter as large as 75 mm and an 800-nm pump pulse with an energy of 48 mJ,...

Ultrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm−1 from a metallic spintronic emitter

“…Finally, in terms of the emitter itself, a number of degrees of freedom can be tuned, including the emitter temperature, the choice of materials with large spin Hall angle 36 , the layer sequence and the arrangement of cascaded emitters. 37 See supplementary material for details on the sample preparation and on the calculation of the pulse energy.…”

Quarter-cycle engineering of terahertz field waveforms

“…It is worth noting that two pump beams yielded a higher THz output power than a single beam with the same total power, the power enhancement factor being around 1.3 (THz field enhancement of 1.1). This could be explained by the reduced free carrier absorption of the THz wave, as was the main motivation in [24].…”

“…This energy scaling-up concept based on coherent synthesis has been proposed for the OR technique [23]. More recently, Sajadi et al [24] reported THz field enhancement by generating and then combing two THz pulses through two temporally separated collinear pump beams in ZnTe, a technique that could reduce the absorption of THz radiation by free carriers induced by the two photon absorption of a strong pump.…”

Optically Gated Terahertz-Field-Driven Switching of Antiferromagnetic CuMnAs

“…For GaP, the time constants of the decay of Δ𝑅 vary from 140 ps to 800 ps, depending on excitation density, and are of the same order of magnitude as time scales found for other large-gap semiconductors such as 6H-SiC (ref. 37) and ZnTe 38 . We emphasize that comparison of carrier lifetimes is useful only in a qualitative sense.…”