Over the last decades, scientists have paid growing attention towards the terahertz properties of liquid crystals. On the
one hand, the dielectric properties of liquid crystals are relatively unexplored at terahertz frequencies, and the observed
low-energy phenomena are not yet well understood. On the other hand, terahertz technology requires switchable devices,
in which liquid crystals could potentially serve as a base material. This paper gives an overview of the research done so
far on the properties and applications of liquid crystals in the terahertz frequency range. The path from first liquid-crystal
terahertz experiments to comprehensive studies of their structure-property relation is outlined. Furthermore, the
evolution from basic concepts to first liquid-crystal terahertz devices is sketched, and prospects as well as future
challenges are discussed. Due to the development of compact and cost-efficient components, terahertz spectrometers
matured from room-filling laboratory instruments to compact, reliable scientific tools. Modern terahertz systems are thus
also covered in this report. Liquid-crystal devices could help terahertz technology continue this trend, and pave the way
to a wider range of application.
We utilize the reflectivity of one-dimensional metal gratings with sub-wavelength slits to realize mirrors for THz
frequencies. Two of them are combined to a Fabry-Perot filter, which features the corresponding transmission
bands. By appropriate choice of dimensions, the extraordinary transmission resonance of the sub-wavelength
gratings can be superimposed with the Fabry-Perot peak. By varying the resonator length between the grating
mirrors, the overlap of both transmission peaks can be controlled. This enables the tuning of the filter bandwidth.
The theoretical analysis shows that continuous tuning of the filter bandwidth up to 30% is possible for a two
mirror stage. For the performance of comparative measurements, an all-fiber continuous-wave THz system is
used. The experimental results are in fairly well agreement with the theoretically predicted tuning properties.
Distributed feedback (DFB) laser chips have recently become available at wavelengths that match the D1 and D2 resonance transitions of alkaline atoms. We investigated the spectral properties, tuning characteristics and modulation behavior of continuous wave, single-mode DFB diodes at 778-780 nm and performed high-resolution spectroscopy of Rubidium vapor. The mode-hop free tuning range of the DFB diodes was as large as 2.4 nm (1186 GHz). The line width of the laser diodes was examined both with a heterodyne beat experiment and with high-resolution Doppler-free two photon spectroscopy, yielding a half width of 2-2.5 MHz. The saturation spectra of the D2-line of 85Rb and 87Rb were recorded with a resolution close to the natural line width. The emission frequency was actively stabilized to Doppler-free transitions with a relative accuracy of better than 4 parts in 109 using commercially available servo devices only. The output power of 80 mW was sufficient to allow for two photon spectroscopy of the 5S-5D-transition of 87Rb.
We conclude that the performance of the DFB laser equals that of grating-stabilized external-cavity diode lasers (ECDLs), without the mechanical complexity of the latter systems. The DFB diode is thus well-suited to high-resolution applications in alkaline spectroscopy, including laser cooling and optical manipulation of ultra-cold atoms.
Frequency conversion of near-infrared diode lasers provides an efficient method to generate tunable laser radiation in the near-UV, violet and blue-green spectral range. High-power, coherent fundamental laser sources such as master oscillator-power amplifier (MOPA) configurations are now state of the art and commercially available.
A new, highly efficient material for second-harmonic generation (SHG) is Bismuth Triborate ("BiBO", stoichiometry BiB3O6). The material has a high effective non-linearity deff, is non-hygroscopic and transparent for wavelengths between 286 nm and 2.5 μm. Compared to other non-linear crystals, "walk-off" effects between fundamental laser radiation and frequency-doubled beam are considerably lower. We used a BiBO crystal in a resonant doubling cavity to convert the output of a 780 nm, 900 mW tapered amplifier system. A maximum UV power of 400 mW (conversion efficiency 44%) was attained. This value is 3-4 times higher than previous results obtained with LBO or BBO crystals and, to the best of our knowledge, represents the highest tunable cw power of a frequency-converted diode laser.