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High-resolution rapid-scan THz spectrometer using a large-area photoconductive emitter in combination with asynchronous optical sampling

Dreyhaupt, A.; Winnerl, S.; Schneider, H.; Helm, M.; Bartels, A.; Thoma, A.; Janke, C.; Dekorsy, T.
We present an approach for photoconductive THz generation providing a broad bandwidth and exceptionally high electric-field amplitude. A large-area interdigitated two-electrode structure on a GaAs substrate offers high electric fields at moderate bias voltages. In order to avoid destructive interference of the THz waves in the far field, every second electrode gap is masked by an additional metallization [1]. Thus all semiconductor regions exposed to incident radiation exhibit parallel electric fields and photocarriers excited by a mode locked Ti:Sapphire laser with MHz or GHz repetition rates are accelerated in the same direction. Areas with anti-parallel fields do not contribute to the THz radiation, thus the resulting constructive interference gives rise to an intense THz output. Using electro-optic sampling, we detect a THz field amplitude of 1.7 kV/cm, which is almost one order of magnitude higher as compared to previous photoconductive emitters excited with pulses from an unamplified oscillator. This field value corresponds to an average THz power of 145 µW and yields a NIR-to-THz power-conversion efficiency as high as 2 × 10-4. We have employed this emitter concept in a compact THz spectrometer based on asynchronous optical sampling (ASOPS) [2]. ASOPS allows us to scan the THz electric field over a nanosecond time delay at a kilohertz scan rate without using a mechanical delay stage. To this end, two mode-locked femtosecond lasers with approximately 1 GHz repetition rate are combined at a fixed, stabilized kHz difference frequency ∆f. One laser delivers the THz excitation pulse, the other provides the probe pulses for electro-optic detection. In this way, the relative time delay between the THz pulses and the probe pulses is linearly ramped, thus enabling high-speed scanning over a 1 ns time delay with the scan rate ∆f. At a scan rate of 9 kHz a time resolution of 230 fs is accomplished. High-resolution spectra from 50 GHz up to 3 THz are obtained and water absorption lines with a width of 11 GHz are observed. A dynamic range larger than 3 orders of magnitude is achieved in a few 10 seconds averaging time.
[1] A. Dreyhaupt, S. Winnerl, T. Dekorsy, M. Helm, Appl. Phys. Lett. 86, 121114 (2005).
[2] A. Bartels, A. Thoma, C. Janke, T. Dekorsy, A. Dreyhaupt, S. Winnerl, M. Helm, Opt. Express 14, 430-437 (2006).
  • Poster
    2nd Workshop on Terahertz Technology, 01.-02.03.2006, Kaiserslautern, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-9080
Publ.-Id: 9080