Terahertz Applications – Spectroscopy

: Absorption line of α-lactose monohydrate, recorded with TOPTICA’s cw terahertz.

Numerous gases and organic solids show terahertz “fingerprints”
Time-domain and frequency-domain spectroscopy feasible
Complex refractive index of a sample can be determined
TOPTICA supplies lasers for pulsed and cw terahertz applications

Applications of terahertz spectroscopy are manifold. Numerous gases and organic solids, including chemical agents and explosives, show distinct absorption features – so-called spectral “fingerprints” – in the terahertz range. Compared to infrared sensing, advantages of terahertz spectroscopy are a great flexibility (a terahertz spectrometer can detect and identify a multitude of gaseous compounds, which would require a different IR laser each), and a high sensitivity, due to the oscillator strength of the resonant rotational / librational transitions.

Terahertz transmission spectroscopy can be performed using either pulsed or cw terahertz radiation. In a time domain (pulsed) setup, an absorption signature manifests itself in a decrease of the amplitude and a delay of the detected terahertz pulse. In a frequency domain (cw) experiment, one likewise observes an attenuated signal, and a phase shift of the terahertz wave. In both pulsed and cw measurements, a comparison of spectra acquired with and without the sample in place enables a determination of the complex refractive index of the material under test.

The figure above shows an absorption spectrum α-lactose monohydrate, recorded with TOPTICA's cw laser packages + spectroscopy kit. Normalizing the sample data to the reference scan yields the transmittance of the lactose pellet, and additionally eliminates any signatures of water vapor, present in the raw data. The transmittance shows a pronounced absorption feature at 0.53 THz. The data are in excellent agreement with a fit based on the Lorentz model for the dielectric function. The spectrum even shows effects of multiple reflections in the sample, which appear as Fabry-Perot fringes in the ranges of high transmittance [1].

TOPTICA provides systems for both frequency domain and time domain applications: The FemtoFiber pro series offers femtosecond laser sources in a turn-key assembly, whilst a modular set of product packages is available for cw terahertz applications. This includes state-of-the-art photomixers for terahertz generation and coherent signal detection.

Reference: [1] A. Roggenbuck et al., New J. Phys. 12 (2010) 043017.