ABSTRACT
The crucial point in all laser spectroscopic/imaging modalities is how the interaction response between laser photons and probed samples is ascertained. In this chapter, the main emphasis is on direct measurement methodologies, based on photon detection (classified as “nondestructive” sample probing) and particle detection (in the form of charged particles, constituting “destructive” probing). In both cases, it is discussed how spectral information can be extracted from the measurement data, using filters, spectrometers, and interferometers in the case of photon detection; and time-of-flight (mass) spectrometers for electron or ion detection. In addition, the most common detection devices are briefly introduced, including single-element photodiodes, avalanche devices, and photomultipliers as well as array sensors (e.g., CCDs). Their basic properties and operating parameters are summarized, highlighting their pros and cons. The list of measurement modalities concludes with a brief outline of the “indirect” methodologies of photothermal and photoacoustic spectroscopy. In a final section, the fundamental issues of “signal” and “noise” are addressed, and the most common methodologies of signal recovery from noisy response data are discussed, centering on lock-in techniques in the case of continuous signals and gated (boxcar) techniques for signals of short or random duration (“pulsed” measurement modalities).
