Abstract
We experimentally investigate the low-frequency (below 1 THz) spectral content of broadband terahertz (THz) emission from two-color femtosecond filament formed by the 2.7-mJ, 40-fs, 800+400-nm pulse focused into air. For incoherent detection, we screened the Golay cell by the bandpass filters and measured the THz angular distributions at the selected frequencies ν=0.5, 1, 2 and 3 THz. The measured distributions of THz fluence were integrated over the forward hemisphere taking into account the transmittance of the filters, thus providing the estimation of spectral power at the frequencies studied. The spectral power decreases monotonically with the frequency increasing from 0.5 to 3 THz, thus showing that the maximum of THz spectrum is attained at ν≤0.5 THz. The THz waveform measured by electro-optical sampling (EOS) based on ZnTe crystal and transformed into the spectral domain shows that there exists the local maximum of the THz spectral power at ν≈1 THz. This disagrees with monotonic decrease of THz spectral power obtained from the filter-based measurements. We have introduced the correction to the spectral power reconstructed from EOS measurements. This correction takes into account different focal spot size for different THz frequencies contained in the broadband electromagnetic pulse. The corrected EOS spectral power is in semi-quantitative agreement with the one measured by a set of filters.
Highlights
Two-color femtosecond filament [1] with the intensity of ∼100 TW/cm2 [2,3,4], the beam diameter of 50–100 μm [5,6,7], and the plasma density of 1015 –1019 cm−3 [8,9] is a terahertz (THz) source with the most broadband spectrum up to 50–100 THz [10,11,12], depending on the pump pulse duration [13]
The incoherent measurements of the frequency-angular distribution of THz emission from two-color femtosecond filament indicate the monotonic decrease in spectral power from 0.5 to 3 THz and, the absence of spectral maximum in this range
The measured by the rotation of the Golay cell screened by the bandpass filters THz angular distributions were integrated over the forward hemisphere taking into account the filters’ transmittance
Summary
Two-color femtosecond filament [1] with the intensity of ∼100 TW/cm2 [2,3,4], the beam diameter of 50–100 μm [5,6,7], and the plasma density of 1015 –1019 cm−3 [8,9] is a terahertz (THz) source with the most broadband spectrum up to 50–100 THz [10,11,12], depending on the pump pulse duration [13]. The experiments [21,22,23] show the unimodal on-axis angular distribution of THz emission from two-color filament for ν . The spectral distribution of the low-frequency THz emission from a two-color filament is still unknown. Even the position of the low-frequency maximum νmax in the THz spectra measured in the close conditions (pulse energy and duration, geometry of the experiment etc.) can vary by an order of magnitude from one experiment to another. We collected the corresponding data, which shows that as the pump pulse duration varies in the range of 30–50 fs, the position of the spectral maximum travels in a wide frequency range from 0.4 THz [27] to 13 THz [28]. Self-consistent numerical simulations based on Unidirectional pulse propagation Equation (UPPE [29]) predict the maximum of THz spectrum at the frequency of about a few hundreds of gigahertz [16,17]
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