Optical Rectification In Nonlinear Crystals Research Articles

Time-frequency analysis of two-photon absorption effect during optical rectification in a ZnTe crystal pumped at 1.024 µm.

Optical rectification in nonlinear crystals is a well-established method for generating terahertz (THz) waves from ultra-short optical pulses. To achieve high conversion efficiency, the phase-matching conditions between the pump pulse and the generated THz wave within the nonlinear medium must be satisfied. For a ytterbium laser operating at 1.024 µm, a severe phase mismatch occurs in the zinc telluride (ZnTe) crystal, preventing the efficient generation of broadband THz pulses. Using time-frequency analysis, we show that the ultrafast charge carrier dynamic, mainly induced by two-photon absorption, generated in the nonlinear medium during optical rectification processes in ZnTe, plays a crucial role in the filtering of the out-of-phase components of the THz signal, thus enabling the recovery of broadband THz pulse generations.

High-intensity THz pulse generation by TW laser radiation in ionized gas and nonlinear crystals

Intense electromagnetic fields can be obtained now in THz frequency range. THz pulses with electric field intensity above 1 MV/cm are now requested for strong-field applications like particle acceleration, short electron bunches measurements, controlling the state of the matter. The advantage of a THz pulse in comparison with the visible frequency range is the almost non-oscillating field that interacts with charged particles more efficiently. The advantage over a direct field is the ability to remotely focus this radiation in space and in time. Except for the huge free-electron lasers facilities, the only way to get THz pulses of MV/cm intensity is the conversion of high-power femtosecond laser pulses. We compare two ways of generating intense THz pulses using the radiation of TW and GW laser systems operating at 800 nm. They are two-color filamentation in gas and optical rectification in nonlinear crystals. The prospects for scaling of THz generation methods for a petawatt laser are discussed.

Optimized Spintronic Terahertz Emitters Based on Epitaxial Grown Fe/Pt Layer Structures

We report on generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The experimentally determined optimum layer thicknesses were in qualitative agreement with simulations of the spin current induced in the ferromagnetic layer. Our model takes into account generation of spin polarization, spin diffusion and accumulation in Fe and Pt and electrical as well as optical properties of the bilayer samples. Using the device in a counterintuitive orientation a Si lens was attached to increase the collection efficiency of the emitter. The optimized emitter provided a bandwidth of up to 8 THz which was mainly limited by the low-temperature-grown GaAs (LT-GaAS) photoconductive antenna used as detector and the pulse length of the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at a repetition rate of 75 MHz) have been used for terahertz generation. This and the general performance make the spintronic terahertz emitter compatible with established emitters based on optical rectification in nonlinear crystals.

Generating terahertz radiation via optical rectification in nonlinear crystals: Theory and experimental results

Key principles of terahertz radiation generation via laser pulse optical rectification are reviewed. The development of theoretical concepts is considered in connection with recent experimental results. The usability of resonant effects and metamaterials for the further development of this technique of generation is analyzed.

Generation of terahertz pulses with arbitrary elliptical polarization

We employ two different methods to generate controllable elliptical polarization of teraherz (THz) pulses. First, THz pulses are generated via optical rectification in nonlinear crystals using a pair of temporally separated and perpendicularly polarized optical pulses. The THz ellipticity is controlled by adjusting the relative time delay and polarization of the two optical pulses. We generate mixed polarization states of single-cycle THz pulses using ZnTe, and elliptically polarized multicycle THz pulses in periodically poled lithium niobate crystals. Second, we generate elliptically polarized THz pulses by making a THz “wave plate” using a combination of a wire-grid THz polarizer and a mirror to transform linearly polarized multicycle THz pulses into elliptical polarization.