Broadband Time-resolved terahertz spectroscopy

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The terahertz (THz) frequency range is usually defined as spanning two decades from 0.1 to 10 THz or 3 to 300 cm-1. Also known as the far-infrared, this part of the electromagnetic spectrum contains spectroscopic signatures for a multitude of physical phenomena, like optical phonons in crystals, superconductor gaps, Josephson plasma resonances, and other collective excitations.

An important class of THz sources relies on the frequency conversion of femtosecond lasers to deliver coherent few-cycle pulses, enabling time-domain THz spectroscopy with the unique ability to characterize the complete electric field of a THz pulse to yield full phase and amplitude information, an advantage over typical detectors that measure only intensity.

Among the THz probe generation processes used in our experiments, optical rectification in non-linear crystals like ZnTe, GaP or GaSe is widely used and it allows to cover the spectral range between 500 GHz and a few THz.

Figure 1: Optical rectification process.

Optical rectification is a second order nonlinear process in which a nonlinear polarization is generated by the incoming laser field, mediated by the second order nonlinear susceptibility. In the case of very short laser pulses, a polarization with the duration of the pulse itself and the shape of the pulse envelope is produced. If the duration of the laser pulse is less than 1 ps, the result will be a very short electromagnetic pulse with frequency content in the THz range (see Figure 1). The THz field transient generated in this way can then be detected with electro-optic sampling in the same non-linear crystal used for generation.

Figure 2: Scheme of a photoconductive switch.

In cases when the probed region needs to be extended far below 500 GHz, different kind of THz sources are preferable:  the so-called photoconductive switches. Here, a biased semiconductor device, known as the Auston switch,is electrically shortened by a femtosecond laser pulse. Photoconductive switches consist of a semiconductor material with a short carrier lifetime and an electrode structure (see Figure 2). The electrode is typically biased with a voltage of 10–50 V and when the laser pulse hits the biased gap of the semiconductor, free carriers are created, and are immediately accelerated by the bias field. The rapid change in polarization induced by the ultrafast acceleration of the carriers generates a sub-picosecond, single-cycle coherent electromagnetic pulse, with a spectral content extending into the sub-THz region.

Figure 3: Spectrum emitted by gas plasma compared with that generated via optical rectification in ZnTe.

A special mention among the THz sources used in our labs is deserved by theultra-broadband THz generation and detection by laser-ionized plasma. This scheme provides a much broader bandwidth with respect to all other sources introduced so far, extending well above 10 THz (see figure 3) and thus allowing for transient infrared spectroscopy of complex materials in a similar way as Fourier-transform spectroscopy is used at equilibrium.

Figure 4: Typical THz time-domain spectroscopy layout.

The layout of a typical time-domain THz experiment is shown in figure 4. The sample is first excited with intense pump pulses at near-infrared, mid-infrared or THz frequencies. Its transient response is then probed (either in reflection or in transmission geometry) with a second, delayed THz pulse generated with one of the techniques introduced above. The transient, pump-induced changes in the THz probe electric field reflected (or transmitted) by the sample are then measured via electro-optic sampling. In this way, the full optical response of the perturbed material (optical conductivity, dielectric function, refractive index) can be retrieved.

Related Experiment

THz reflection setup for mid-infrared pump / THz probe spectroscopy. THz probe pulses are generated with a photoconductive antenna.

Daniele fine tuning the mid-infrared pump / THz probe setup.

References

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