THz Generation by Optical Rectification and Competition with Other Nonlinear Processes

Abstract

We present a study of the competition between tera-hertz (THz) generation by optical rectification in <110> ZnTe crystals, two-photon absorption, second harmonic generation and free-carrier absorption. The two-photon nonlinear absorption coefficient, second harmonic generation efficiency and free-carrier absorption coefficient in the THz range are measured independently. The incident pump field is shown to be depleted by two-photon absorption and the THz radiation is shown to be reduced, upon focusing, by free-carrier absorption. The reduction of the generated THz radiation upon tight focusing is explained, provided that one also takes into account diffraction effects from the sub-wavelength THz source.

Influence of free-carrier absorption on terahertz generation from ZnTe(110)

“…TPA of the excitation beam and FCA of terahertz radiation have been identified as processes that compete with optical rectification. [8][9][10][11][12] We find that FCA attenuates the latter part of the terahertz pulse, while TPA depletes the power available for terahertz generation.…”

“…When a material undergoes TPA, the intensity I at a distance l within a sample illuminated with incident intensity I 0 is given by I = I 0 / ͑1+I 0 ␤l͒, where ␤ is the TPA coefficient ͑␤ = 4.6 cm/ GW for ZnTe͑110͒ and 800 nm light͒. 11,12,25 Fig. 5͑b͒ plots the relative terahertz amplitude as a function of pump fluence, and it is seen that the attenuation of the terahertz pulse is accounted for quantitatively by considering the TPA of the pump pulse.…”

“…More recently, studies have shown that processes such as second harmonic generation ͑SHG͒, twophoton absorption ͑TPA͒, and free-carrier absorption ͑FCA͒ can all compete with optical rectification. [8][9][10][11][12] SHG is the sum-frequency analog of optical rectification and removes photons from the excitation pulse that would normally produce terahertz radiation. TPA also depletes the excitation beam intended for optical rectification and more importantly, produces free carriers, which can then absorb the generated terahertz radiation through FCA.…”

Aspects of Terahertz Reflection Spectroscopy

“…Finally, we can estimate the TPA-induced THz absorption α Ω = 2Ωκ Ω /c. Using the TPA coefficient β ≈ 5 cm/GW[211], τ rc, ZnTe ≈ 25 ns[210], ϵ ∞ = n with n Ω = 3.2 as measured at f = 1 THz, Γ = 20 THz[212] and m eff = 0.15 × m e[213] together with the experiment parameters, we obtain α Ω ≈ 80 cm −1 . Likewise, we can fit the modified Eq.…”