Formaldehyde laser-induced fluorescence imaging with a multi-band transmission filter.

A method for laser-induced fluorescence (LIF) imaging of formaldehyde (CH2O) that discriminates against the interfering signal from polycyclic aromatic hydrocarbons (PAHs) is presented. This technique uses an interference filter with 11 transmission bands that closely match the most prominent fluorescence features of CH2O upon excitation at 355 nm. The signal increases by a factor of 12 with the multi-band filter compared to a single-band filter. Slight angle-tuning of the filter shifts the transmission maxima to the minima in between the CH2O-LIF features. The pixel-by-pixel difference between the images collected at the two filter angles thus provides CH2O images free of PAH interference, providing the capability for selective single-pulse CH2O-LIF imaging in engine combustion even under fuel-rich conditions.

Variable-focus terahertz lens.

We present a variable focus lens for the THz range. The focal length can be changed by pumping a medical white oil in and out of the lens body. Due to the optical transparency of the liquid and a similar refractive index in the visible frequency range, the THz beam path can be aligned using conventional optical light sources. This type of lens might find applications in terahertz based quality control, stand-off detection and wireless communication systems.

Terahertz form birefringence.

We report on one-dimensional photonic crystals designed to exhibit a pronounced form birefringence at terahertz frequencies. The crystals can be used as volumetric quasioptical elements for a broad frequency range. Theoretical simulations of the dielectric parameters of these structures are presented as well as measurement results of a polymeric crystal that exhibit a birefringence of 0.25 at 300 GHz. As a potential application, the device is exemplarily used as terahertz wave plate.

Terahertz imaging: applications and perspectives.

Terahertz (THz) spectroscopy, and especially THz imaging, holds large potential in the field of nondestructive, contact-free testing. The ongoing advances in the development of THz systems, as well as the appearance of the first related commercial products, indicate that large-scale market introduction of THz systems is rapidly approaching. We review selected industrial applications for THz systems, comprising inline monitoring of compounding processes, plastic weld joint inspection, birefringence analysis of fiber-reinforced components, water distribution monitoring in polymers and plants, as well as quality inspection of food products employing both continuous wave and pulsed THz systems.

Terahertz birefringence for orientation analysis.

A terahertz time-domain spectrometer is employed to study different birefringent samples. We develop a method based on the temporal waveform and the impulse response of a sample to map the anisotropy of their inner structure. To validate our algorithm, we study the polarization-affecting structure of various classes of materials such as crystals, plastics, and natural products. Among all samples we observe the largest birefringence for a rutile crystal with Deltan=3.3 at 1 THz.