Terahertz Time-Domain Spectroscopy

THz Time-Domain Spectroscopy (THz-TDS) is a technique that directly accesses the complex THz electric field, i.e. amplitude and phase. By measuring the change in electric field amplitude and phase shift, one can extract the complex refractive (or conductivity) without the need of using Kramers-Kronig transformations. This is currently a widely spread technique as it can access materials properties without any electrical contacts. To achieve this, the beam from a femtosecond laser is split in two parts:

(1) a generation beam used to produce THz pulses via a photoconductive antenna or optical rectification in a nonlinear crystal, and

(2) a gating beam used to detect the THz electric field via a photoconductive antenna or Pockels effect in a nonlinear crystal.

  • Generation via photoconductive antennas

A photoconductive antenna consists of two metal electrodes deposited on a semiconductor (e.g. GaAs). By exciting the gap between the DC biased electrodes, the semiconductor becomes conductive on a very short time scale generating a current pulse that radiates a picosecond long quasi-monocycle electric-field transient in the THz range.

  • THz optics for transmission measurements

To collect the generated THz radiation, a set of 90° off-axis parabolic mirrors are used. The studied sample is placed at the focal point after the second parabolic mirror. The transmitted THz pulses are then collected and focused on the detection system and overlaps with the gating beam.

  • Electro-optic sampling

In the presence of THz electric field, the electro-optic crystal (e.g. ZnTe) becomes birefringent inducing a change in the polarization of the gating beam (measured by an optical balance composed of quarter-wave plate, a Wollaston prism and photodiode balance). This change is directly proportional to the amplitude of the THz electric field (Pockels effect).  By moving a delay stage on the gating beam path, it is possible to sample the THz electric field amplitude at different time delays and map the THz Time-Domain waveform. The spectrum is obtained via a Fourier transform.

  • Optical pump – THz probe (OPTP)

A powerful approach is the combined use of the pump-probe spectroscopy with THz-TDS spectroscopy. A high fluence pump photoexcites a material which is then probed with a phase resolved THz probe. By doing so, one can access spectrally resolved pump-induced change in complex conductivity or refractive index. This provides direct access to the dynamical evolution of photoexcited charged and neutral particles. Used in combination with Drude-like model fittings, on can retrieve, for example, the interplay between free carriers and excitons in a material.

Example of THz-TDS at FSU. We use interdigitated photoconductive antennas excited by sub-15fs NIR pulses in a reflective configuration and electro-optic detection to achieve 0.1 – 20 THz (3 mm – 16 µm) bandwidth.
THz temporal waveform and FFT spectrum

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