# A Review of Photothermal Detection Techniques for Gas Sensing Applications
## 1 Introduction  
Photothermal spectroscopy (PTS) is a group of spectroscopy techniques commonly used to measure thermal characteristics and optical absorption of samples (solids, liquids or gases) [1; 2; 3]. Photothermal techniques were investigated already in the 80s', mainly as a method for measuring the absorption of thin samples and liquids. Generally, in PTS, the analyte is illuminated with a radiation source (e.g., a laser beam) and the change in the thermal state of the sample (or the consequences of the change) is monitored. A portion of the light absorbed by the sample is lost to emission, the remaining part contributes to a temperature raise of the sample. This in turn influences the thermodynamic properties of the sample (or particles adjacent to it), which can be observed as temperature, density or pressure change. In PTS, those three parameters are most commonly monitored to determine the composition of the target analyte. PTS is in principle similar to photoacoustic spectroscopy (PAS) [4; 5; 6] and relies on an indirect method of measuring the absorption of the sample, which is the key distinction when compared to traditional absorption spectroscopy techniques. In absorption spectroscopy techniques e.g., tunable diode laser absorption spectroscopy (TDLAS), a basic setup consists of a tunable diode laser light source and a suitable detector [7; 8]. The laser source used in the experiment is tuned across the characteristic absorption lines of the targeted gas in the path-length of the laser beam. Absorption of the molecules causes a reduction of the intensity, which is detected by a photodiode and signal-processed to determine the gas concentration or other properties of the gas (e.g., pressure, temperature, velocity) [9]. As this method usually relies on calculating the difference between the input light intensity and the light absorbed by the sample, the performance and accuracy of the sensor is limited especially in the case of small absorption and by reflections and scattering effects usually present in such systems. Moreover, accessing strong absorption features of numerous gas molecules requires using mid-infrared laser sources. In TDLAS, this implies incorporating expensive detectors (e.g., based on mercury cadmium telluride material). Unlike traditional absorption spectroscopy techniques, in PTS and PAS, the sensor does not rely on quantifying the signal directly involved in the absorption,but rather on the influence of the absorbed energy on the gas particles. Therefore, the amplitude of the registered PTS signal is proportional to the absorption, but the measurement does not require monitoring the parameters of the laser radiation exciting the gas molecules. Moreover, performed in a properly designed sensor, PTS is a zero-background measurement--no absorption equals to zero registered signal, thus PTS sensors require minimal calibration. Photothermal spectroscopy has been successfully employed in commercial lab-grade apparatus commonly used for analyzing solid samples. In such devices, photothermal deflection (PDS) or photothermal lens spectroscopy (TLS) effects are used [10; 11; 12; 13]. PTS of gas samples is usually performed in a scheme as presented in Figure 1.  
To perform PTS, the gas molecules have to be excited by an energy source. This is commonly accomplished by illuminating the gas sample with a laser source--_pump_ laser. Similarly to traditional absorption spectroscopy, the wavelength of the light has to be appropriately chosen to target strong transitions of the molecules; thus, a significant portion of the light will be absorbed. Part of the absorbed energy will locally heat the sample (due to non-radiative relaxation of the molecules), causing a change in the density, which in turn modulates the refractive index (RI) of the gas. Because the variations of the RI are small (\(\Delta n\sim 10^{-9}\)), most PTS gas sensors rely on an interferometric signal retrieval sensor configuration (e.g., a Mach-Zehnder interferometer).