Laser frequency combs (LFC) provide a direct link between the radio frequency (RF) and the optical frequency
regime. The comb-like spectrum of an LFC is formed by exact equidistant laser modes, whose absolute optical
frequencies are controlled by RF-references such as atomic clocks or GPS receivers. While nowadays LFCs
are routinely used in metrological and spectroscopic fields, their application in astronomy was delayed until
recently when systems became available with a mode spacing and wavelength coverage suitable for calibration
of astronomical spectrographs. We developed a LFC based calibration system for the high-resolution echelle
spectrograph at the German Vacuum Tower Telescope (VTT), located at the Teide observatory, Tenerife, Canary
Islands. To characterize the calibration performance of the instrument, we use an all-fiber setup where sunlight
and calibration light are fed to the spectrograph by the same single-mode fiber, eliminating systematic effects
related to variable grating illumination.
The terahertz (or T-ray) spectra of many small molecules of biological relevance show very characteristic, specific
features that are sensitive to small changes of the molecular structure and even isomerization. On the other hand,
most packaging materials like plastics, paper or even clothing are transparent for T-rays. Therefore, it is possible
to differentiate and identify different substances by their spectral fingerprints, even through their packaging. This
supports the potential of this technique in a wide range of applications from safety and security applications, via
biosensing, through to pharmaceutical quality control. However, most of the molecular vibrations that give rise to the characteristic features in the T-ray spectra are
phonon-like intermolecular vibrations of weakly bound crystalline compounds. This can be easily demonstrated
by comparing the spectra of different crystals of the same molecule. Whereas this sensitivity on the intermolecular
structure can be used to probe the crystalline structure and detect phase transitions, it is a hurdle when it comes
to identify samples that lack such a well defined intermolecular structure. Yet, we have recently shown that
a comparison of the absolute absorption values can still be used to differentiate between complex biomolecules
such as RNA.
In this paper we will demonstrate, based on a wide range of spectra, the potential of T-ray spectroscopy for
biosensing and will show examples where this technique can be used to probe the crystalline configuration and
probe phase transitions and will discuss the feasibility of using this technique for biosensing.
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