THz Time-Domain Spectroscopy

[IMAGE: Optical table with two racks of electronics, a motorised delay stage, fibre-optic cables, and a sealed sample chamber with gas lines]

Figure 1: Fibre-coupled THz-TDS system on the optical table in Building 3. The femtosecond fibre laser (left rack) delivers <100 fs pulses at 80 MHz to the photoconductive antenna emitter and detector modules via dispersion-compensated fibre. The optical delay stage is visible at centre. The sample compartment (purged with dry N\u2082) is at right.

Overview

The THz Spectroscopy programme operates a fibre-coupled time-domain spectroscopy (TDS) system for broadband characterisation of materials in the 0.1–4 THz frequency range. The system is used for both fundamental studies of carrier dynamics in semiconductors and for applied measurements in support of other Institute programmes and external collaborators.

The THz-TDS system was developed in-house and serves as one of three nodes in the Nordic THz Network.

System Description

THz Generation & Detection

Broadband THz pulses are generated by optical rectification of femtosecond laser pulses (<100 fs, 80 MHz, 800 nm centre wavelength) in a 1 mm-thick ZnTe crystal. Alternatively, photoconductive antenna (PCA) emitters fabricated on low-temperature-grown GaAs provide higher bandwidth at the expense of lower THz field strength.

Detection uses either free-space electro-optic sampling in a second ZnTe crystal (for the highest bandwidth) or a fibre-coupled PCA detector (for flexibility and alignment stability). The optical delay stage provides a 200 ps scan range with 1 μm step resolution, corresponding to a frequency resolution better than 5 GHz after apodisation.

[IMAGE: Dual-panel plot: top shows a sub-picosecond THz pulse in time domain; bottom shows the corresponding amplitude spectrum on a log scale spanning 0-4 THz]

Figure 2: Representative THz time-domain waveform and corresponding Fourier-transform amplitude spectrum for a high-resistivity float-zone silicon reference sample. Useful bandwidth extends from ~100 GHz to ~3.5 THz with >60 dB dynamic range at the peak.

Current Research Applications

Semiconductor Carrier Dynamics

Optical-pump/THz-probe measurements of photoexcited carrier relaxation in bulk and quantum-well semiconductors. Recent work has focused on high-resistivity silicon for photoconductive switch applications and on InGaAs/InAlAs quantum-cascade structures. [Chen, IEEE Trans. THz Sci. Technol. 2002]

Pharmaceutical Polymorph Identification

Many pharmaceutical compounds exhibit characteristic THz absorption features corresponding to intermolecular vibrational modes. The TDS system has been used to distinguish polymorphic forms of common excipients and active pharmaceutical ingredients, in collaboration with an industrial partner. [Project summary]

Composite Material Inspection

Non-destructive THz imaging of glass-fibre and carbon-fibre composite panels for detection of delaminations, voids, and impact damage. Time-of-flight analysis of reflected THz pulses permits depth-resolved defect mapping. [Project summary]

Related Facilities

The THz-TDS system is housed in Building 3, adjacent to the shielded anechoic chamber used for free-space THz beam pattern measurements. The femtosecond laser source is shared with the Photonics Division ultrafast spectroscopy programme.

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