Tunable narrowband THz pulses from a large-area photoconductive emitter

THz radiation in the range from 0.2 – 10 THz, generated by ultrafast lasers via optical rectification or highly excited dipole antennas typically consist of 0.5 to 2 cycles of light and therefore cover a broad frequency range. This makes these pulses ideal for broadband spectroscopic applications, where spectra can be acquired at the highest possible time-resolution. For applications such as free space imaging or intense driving of a specific resonance, such THz pulses are less useful due to water absorption and quantitative studies become difficult. In these cases narrowband sources such as Free-Electron Lasers proved to be the ideal light source, as well as cw-sources (e.g. quantum cascade lasers) and photomixers for imaging. However, utilizing the broad spectra of ultrafast lasers, one technique to generate narrowband THz radiation is based on mixing two time- delayed linearly chirped pulses in a nonlinear media: Narrowband, tunable THz pulses up to 780 GHz were successfully generated in a dipole antenna [1] via a fast oscillating current induced by the beat frequency of the mixed laser pulses. Higher frequency pulses were recently demonstrated in ZnTe, where a second order process is utilized [2]. In this work we show that an Auston-switch based emitter concept can serve as a high frequency emitter for narrowband THz pulses. We use our recently introduced concept of a large area emitter with interdigitated electrodes, based on low-temperature grown and semi-insulating substrates. The two pulses for THz generation as well as the probe pulses for electro-optic sampling were taken from a 250 kHz regenerative Ti:sapphire amplifier, whose output was positively chirped to a pulse duration of 3.3 ps. These pulses were split into three beams, where one, undergoing compression in a grating compressor, serves as the probe pulse for field-resolved detection. The other two pulses are sent – with an adjustable optical delay in one of the arms – to a combining beamsplitter and to the THz emitter. The beat frequency – set by the delay in the interferometer – now controls the emission frequency of the emitter, while the length of the input pulse gives the spectral width. The normalized spectra of the pulses for different delays are presented in Fig. 1 and show a frequency coverage from 0.35 to 2.3 THz. All spectra have a spectral width (FWHM) of about 200 GHz which could be further reduced by employing longer driving pulses. Fig. 2 demonstrates the excellent tuning behavior of the emitter, which is perfectly linear with the given time-delay (black dots). The maximum power is obtained around 1 THz and steeply drops towards higher frequencies (red dots) and resembles the power distribution of the emitter excited with a single 50 fs pulse.
[1] A. S. Weling and D. H. Auston, J. Opt. Soc. Am. B 13, 2783 (1996). [2] J. R. Danielson et al., J. Appl. Phys. 104, 033111 (2008).