Photothermal spectroscopy is a group of high sensitivity spectroscopy techniques used to measure optical absorption and thermal characteristics of a sample. The basis of photothermal spectroscopy is the change in thermal state of the sample resulting from the absorption of radiation. Light absorbed and not lost by emission results in heating. The heat raises temperature thereby influencing the thermodynamic properties of the sample or of a suitable material adjacent to it. Measurement of the temperature, pressure, or density changes that occur due to optical absorption are ultimately the basis for the photothermal spectroscopic measurements.
As with photoacoustic spectroscopy, photothermal spectroscopy is an indirect method for measuring optical absorption, because it is not based on the direct measure of the light which is involved in the absorption. In another sense, however, photothermal (and photoacoustic) methods measure directly the absorption, rather than e.g. calculate it from the transmission, as is the case of more usual (transmission) spectroscopic techniques. And it is this fact that gives the technique its high sensitivity, because in transmission techniques the absorbance is calculated as the difference between total light impinging on the sample and the transmitted (plus reflected, plus scattered) light, with the usual problems of accuracy when one deals with small differences between large numbers, if the absorption is small. In photothermal spectroscopies, instead, the signal is essentially proportional to the absorption, and is zero when there is zero true absorption, even in the presence of reflection or scattering.
There are several methods and techniques used in photothermal spectroscopy. Each of these has a name indicating the specific physical effect measured.
* Photothermal lens spectroscopy (PTS or TLS) measures the thermal blooming that occurs when a beam of light heats a transparent sample. It is typically applied for measuring minute quantities of substances in homogeneous gas and liquid solutions.
* Photothermal deflection spectroscopy (PDS), also called the mirage effect, measures the bending of light due to optical absorption. This technique is particularly useful for measuring surface absorption and for profiling thermal properties in layered materials.
* Photothermal diffraction, a type of four wave mixing, monitors the effect of transient diffraction gratings "written" into the sample with coherent lasers. It is a form of real-time holography.
* Photothermal emission measures an increase in sample infrared radiance occurring as a consequence of absorption. Sample emission follows Stefan's law of thermal emission. This methods is used to measure the thermal properties of solids and layered materials.
* Photothermal single particle microscopy. This technique allows the detection of single absorbing nanoparticles via the creation of a spherically symmetric thermal lens for imaging and correlation spectroscopy.