Frequency-resolved characterization of broadband two-color air-plasma terahertz beam profiles

Abstract

The frequency-resolved terahertz (THz) beam profile characteristics of a two-color air-plasma THz source were investigated in the broadband frequency range (1-15 THz). The frequency resolution is achieved by combining THz waveform measurements and the knife-edge technique. Our results show that the THz focal spot size is strongly frequency dependent. This has important implications on nonlinear THz spectroscopy applications where accurate knowledge of the applied THz electrical field strength onto the sample is important. In addition, the transition between the solid and hollow beam profile of the air-plasma THz beam was carefully identified. Far from the focus, the features across the 1-15 THz range have also been carefully examined, revealing the characteristic conical emission patterns at all frequencies.

Non-Linear Conductivity Response of Graphene on Thin-Film PET Characterized by Transmission and Reflection Air-Plasma THz-TDS

“…A small portion of the output directly from the laser amplifier was employed as the optical probe beam for the THz waveform detection. An 8 mm diameter hole in the center of the first off-axis parabolic mirror (OPM1, 4” effective focal length) eliminated most of the residual laser beam while reflecting most of the THz beam at the same time due to the conical emission pattern of the air-plasma THz wave emission [ 22 , 23 ]. Three off-axis parabolic mirrors (OPM2-4, all with 3” effective focal length) were employed to generate an intermediate focus on the sample, as well as the final focus at the high voltage (HV) electrodes (OPM3 was skipped for the R-mode configuration).…”

“…Multiple (up to 10) high-resistivity silicon wafers with minimal dispersion and absorption in broadband THz frequency range were used to adjust the incident THz pulse field strength onto the graphene sample [ 25 ] according to the attenuation formula , where N is the number of wafers inserted in the beam. With zero attenuation wafers, the THz pulse energy was 0.51 µJ at the sample location, and the calculated peak THz electrical field strength was 1050 kV/cm, based on measurement of the beam profile and the time trace detected by ABCD [ 23 ]. The incident THz field can be reduced with the insertion of wafers to a minimum of 30 kV/cm with 10 wafers inserted.…”

Nonlinear Conductivity Response of Graphene on Thin-Film PET Characterized by Transmission and Reflection Air-Plasma THz-TDS

“…Multiple (up to 10) high-resistivity silicon wafers with minimal dispersion and absorption in broadband THz frequency range were used to adjust the incident THz pulse field strength onto the graphene sample [25] according to the attenuation formula E N = (0.7) N , where N is the number of wafers inserted in the beam. With zero attenuation wafers, the THz pulse energy was 0.51 µJ at the sample location, and the calculated peak THz electrical field strength was 1050 kV/cm, based on measurement of the beam profile and the time trace detected by ABCD [23]. The incident THz field can be reduced with the insertion of wafers to a minimum of 30 kV/cm with 10 wafers inserted.…”

Remote terahertz spectroscopy from extended two-color plasma filaments: The ALTESSE 2 project

“…4(c), (d). Although carrying lower energy, THz frequencies with small conical emission < 1 • [59,60] are more sensitive to the hole of the parabolic mirror P1 (see fig. 1), which may cause fluctuations in the output THz profile.…”