THz Time-Domain Spectroscopy

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Terahertz spectroscopy is a tool for collecting spectral measurements in the far infrared (approximately 3 – 100 wavenumbers). Data collection in this frequency range has many advantages; biological samples do not ionize from terahertz radiation, and many compounds, like explosives and pharmaceuticals, have unique fingerprints in this region.  Terahertz time-domain spectroscopy, THz-TDS, is a spectroscopic technique for determining the properties of a sample probed by short pulses of terahertz radiation.

In order to create THz pulses, a diode-pumped CW laser at 532 nm pumps a mode-locked Ti:sapphire crystal to create ultra short (100 fs or shorter) optical pulses at 800 nm at a repetition rate of 80 MHz.  The optical pulses are sent through a prism pair to compensate for dispersion1 and then split into two parts: one used for detection and the other for generation.  Photoconductive antennas are used for both generation and detection.2 A series of parabolic mirrors are used to guide the terahertz pulses from the emitter to the sample and then to the detector.  A delay stage is used in the generation arm in order to vary the arrival time of the signal with respect to the optical pulse used for detection.  By scanning the delay line, the electric field amplitude and phase of the THz waveform can be mapped out as a function of time.  A LabVIEW program collects the electric field amplitude as a function of time from a digital lock-in amplifier that measures the signal from the detector.

Figure 1. Representation of the THz-TDS experimental setup.

References

  1. Fork, R. L.; Martinez, O. E.; Gordon, J. P., Negative Dispersion Using Pairs of Prisms. Optics Letters 1984, 9 (5), 150-152.
  2. Beard, M. C.; Turner, G. M.; Schmuttenmaer, C. A., Terahertz spectroscopy. Journal of Physical Chemistry B 2002, 106 (29), 7146-7159.

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Aschaffenburg, D. J., M. R. C. Williams, D. Talbayev, D. F. Santavicca, D. E. Prober, and C. A. Schmuttenmaer , Efficient measurement of broadband terahertz optical activity , Appl. Phys. Lett. , 2012 , 100

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