Terahertz pulse induced femtosecond optical second harmonic generation in transparent media with cubic nonlinearity

Abstract

Second harmonic (SH) generation of femtosecond laser radiation induced by a short terahertz pulse in a layer of isotropic medium with cubic nonlinearity was investigated. A comprehensive 1D theory of SH field dynamics with allowance for different propagation velocities of interacting terahertz, fundamental, and SH pulses was developed. In particular, it was shown that for media with a coherent length of the fundamental and second optical harmonics smaller than the walk-off length of the fundamental optical and THz pulses, a SH radiation signal is generated only when the fundamental optical and terahertz pulses overlap near the entrance and exit surfaces of the nonlinear layer rather than when the pulses are superimposed in the bulk. The prediction was verified experimentally by using a 3-mm-thick fused quartz. Good agreement between the theory and the experiment was demonstrated.

Cubic Nonlinearity of Tellurite and Chalcogenide Glasses: Terahertz-Field-Induced Second Harmonic Generation vs. Optical Kerr Effect

“…(3) (τ) (in our case this field is propagated only in SiO 2 ), in contrast, is rather sensitive to the mutual propagation dynamics of the terahertz and optical pulses [31]. In fused quartz, the coherent length of the fundamental and SH radiation (~10 µm) is significantly less than the walk-off length of the optical and terahertz pulses: ~200 µm.…”

“…A possible advancement of the EFISHG technique is to use terahertz electromagnetic pulses instead of a DC electric field. This so-called terahertz-field-induced SHG (TFISHG) technique was used for the investigation (primarily the transmission geometry) of the nonlinear response of different materials, including ferroelectric layers [27,28], electrooptic crystals [29,30], SiO 2 , and optical adhesives [31,32]. Superstrong (above the DC-field breakdown threshold) terahertz fields (~20 MV/cm) were employed for studying SHG in p-doped Si under below-band-gap optical excitation [33].…”

“…The specific feature of the TFISHG measurements, shown in Refs. [31,34], is the influence of ambient gas (air) on the detected second harmonic (SH) signal (this influence was also discussed in regards to EFISHG [35]). In the previous studies, due to a rather strong SH signal from the sample or/and some overlapping geometry of the fundamental optical and terahertz pulses, the SHG from ambient gas could be neglected or strongly suppressed by using helium gas instead of air [31,34].…”

“…[31,34], is the influence of ambient gas (air) on the detected second harmonic (SH) signal (this influence was also discussed in regards to EFISHG [35]). In the previous studies, due to a rather strong SH signal from the sample or/and some overlapping geometry of the fundamental optical and terahertz pulses, the SHG from ambient gas could be neglected or strongly suppressed by using helium gas instead of air [31,34]. However, in the case of a small SH signal from the sample (due to its small nonlinearity), even rather weak generation from the ambient gas may be significant.…”

Local measurement of terahertz field-induced second harmonic generation in plasma filaments

“…Using a 3 mm thick fused quartz plate, Refs. [ 139 , 140 ] show that time delay data can be used to identify materials and hidden layers within materials. Here, the mechanism of TFISH is used to estimate the electron number density.…”