Quartz-enhanced photoacoustic spectroscopy is a photoacoustic spectroscopy technique which substitutes the conventional microphone by a quartz tuning fork which enables very sensitive detection in small volumes. The piezo-electric properties are used to transfer the oscillation amplitude into an electric signal. However, the opposite is also possible and the tuning fork can be driven electrically. By alternate electric and photoacoustic driving, it is possible to extract information about the background gas and to determine decay times rather than amplitudes which result presently in a comparable limit of detection as conventional QEPAS but still shows potential for improvement

In this paper, we report on the progress of the auto-triggered quartz-enhanced photoacoustic spectroscopy (QEPAS) technique which operates without external frequency generators and ensures permanent locking to the current resonance frequency of the tuning fork. This is obtained by incorporating the tuning fork in an oscillator circuit that autonomously oscillates at the present resonance frequency that shifts with changing environmental conditions, e.g., density and viscosity of the surrounding gas, temperature, and pressure. Both, the oscillation amplitude as well as the frequency can be read from the oscillator circuit. The photoacoustic signal appears as an offset of the electrically induced signal amplitude. Since the sum amplitude depends on the phase relation between the electrical and photoacoustic driving forces, the phase is permanently modulated, enabling the extraction of the photoacoustic component by use of a second lock-in amplifier stage which is being referenced with the phase modulation frequency. The functionality of this method is demonstrated for methane detection in a carbon dioxide atmosphere in a concentration range from 0 to 100% and ammonia in synthetic air employing a pulsed mid infrared QCL around 1280 cm−1. The gas mixtures are motivated by the demands in biogas-analysis.

Trace detection of energetic materials is a method to screen personnel, packages and other items for concealed explosives. It is likely, that a person who carries explosives will contaminate himself and the package with the explosive. We present first results on the development of a compact detector combining Pulsed Laser Fragmentation (PLF) with fragments detection. A passively q-switched UV microchip laser photofragments surface contaminations. The concentration ratio of the fragments NO and NO2 is a sufficient indicator for the presence of nitrogen-based explosives. For TNT, the results suggest, that a surface density as small as a few ng/cm2 can be reliably detected

Within the past years, an increasing number of optical techniques for the detection of harmful substances has been investigated on account of the threat of terroristic attacks. Optical technologies have the capability to overcome the problems of extensive sample preparation and long measurement cycles that are known from widely used detection techniques such as mass spectrometry and gas chromatography. The threat of bombings has also produced interest in standoff techniques — in addition to checkpoint applications for screening of persons. Even in this field, optical technologies are well suited: to date, Raman spectroscopy and laser‐induced breakdown spectroscopy (LIBS) seem to be the most promising technologies, but other optical methods are permanently developed. Particularly detection in the midinfrared (MIR) spectral region, providing selective fingerprints of molecules in the range from 3 to 11 µm, has experienced a renaissance with development of new laser sources, detectors, and spectroscopic techniques. In this paper, some comparatively new techniques for the detection of explosives in the MIR spectral range, relying on quantum cascade lasers (QCLs) and photoacoustic spectroscopy (PAS), are presented. Another emphasis is put on the indirect detection of explosives via characteristic decomposition products and their active generation with laser‐induced photodissociation techniques.