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Atomic Physics

Faculty of Engineering, LTH

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Biophotonics

Combining disciplines

The development and application of optical diagnostic tools and therapies.

Gas in scattering media absorption spectroscopy (GASMAS)

Free gases exhibit sharp and highly structured absorption spectra. Since spectra of solid materials are typically very dull, high-resolution spectroscopy allows selective detection of gases even when these are dispersed within porous and highly scattering solids such as wood, polystyrene foam, biological tissue and pharmaceutical tablets. This concept was developed at our division, and introduced to the scientific community via Optics Letters in 2001. We base our gas sensing on tunable diode laser absorption spectroscopy (TDLAS or TDLS). While conventional absorption spectroscopy is carried out with well-defined beamlines and known interaction pathlengths, this task involves dealing with diffuse light, unknown and uncontrollable pathlengths, severe backscattering, and devastating optical interference. These characteristics are further discussed below and motivate that the technique, since its introduction, is designated as gas in scattering media absorption spectroscopy (GASMAS).

Diffuse Reflectance Spectroscopy (DRS)

Photon Time-of-Flight Spectroscopy (pToFS)

Time-of-flight spectroscopy (TOFS) is a tool for characterisation and analysis of highly scattering (turbid) materials, such biological tissue, powders, and pharmaceutical samples. The main principle is to inject a very short laser pulse into the material, and to analyse the resulting pulse at some distance from the injection point. Due to the multiple scattering of photons in a turbid material, the detected pulse will be much broader than the original pulse. Some photons will reach the detector rather fast (short time-of-flight), while others will arrive much later (long time-of-flight). In combination with models of light propagation in scattering materials (i.e. photon migration), the distribution of photon time-of-flights can used to determine the absorption and scattering properties of the sample. The derived information about absorption can be used to determine the chemical composition of the material, while the scattering properties gives information on the physical/structural properties of the material. Since the idea is to resolve a transmitted or reflected light-pulse in time, the technique is sometimes referred to as time-resolved spectroscopy.

Since light travels very fast, the detector has to be extremely fast to be able to resolve the distribution of photon time-of-flights. In many applications, the time-resolution must be in the order of tens of picoseconds.