The results of high-field terahertz transmission experiments on n-doped silicon (carrier concentration of 8.7×10 cm) are presented. We use terahertz pulses with electric field strengths up to 3.1 MV cm and a pulse duration of 700 fs. A huge transmittance enhancement of ∼90 times is observed with increasing of the terahertz electric field strengths within the range of 1.5-3.1 MV cm.
We demonstrate a simple approach to retrieve the original peak electric field (E-field) strength of high-intensity THz pulses using an electro-optic sampling (EOS) technique and the Poynting flux approach. The latter supposes assessment of THz pulse intensity by measurement of pulse energy, duration and spot size, but its applicability to a few-cycle THz pulse needs detailed consideration. We applied a deconvolution procedure to the raw EOS data to retrieve the THz field waveform. We describe a two-step procedure that allows us to assess the field strength of an extreme THz field. First, the EOS measurements of the THz field should be performed at low pulse energies to retrieve the THz waveform and estimate pulse duration and amplitudes of each particular oscillation. Next, the field strength of an extreme THz pulse can be assessed from the Poynting flux approach with correction to the abovementioned data obtained from the EOS measurements. We show good experimental coincidence between peak strength estimation from the EOS directly and from the combined approach at 'low' field strength. Hence, an extreme THz E-field strength can also be assessed from preliminary EOS measurements and full energy measurements based on the Poynting flux approach. We also show that the Poynting flux approach for extreme few-cycle THz pulses gives prominent, at least two-fold, underestimation without preliminary EOS measurements.
We report on a new technique of silicon surface nanostructuring in liquid with a pair of Gaussian-shaped femtosecond laser pulses. The bubble, generated in liquid near the molten silicon surface by the first pulse, serves as a dynamic microscale obstacle for spatial modulation of the intensity profile of the second pulse following at a certain delay via scattering processes. As a result, the circular ripple patterns with anomalously high surface-relief modulation, undersurface annular nanocavities, and interfacial smoothness are produced at the surface. The possibility of the control over the specific pattern through the laser intensity variation is shown.
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