A type of optical analytic technique called photothermal spectroscopy is a powerful tool to study heat that is produced as a result of light absorption in a sample that has been exposed to radiation.
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Information is collected in traditional spectrometric methods by measuring the amounts of light that a sample transmits, reflects, or emits. Spectroscopic data is gathered in photothermal spectrometry by detecting the heat that is generated by nonradiative relaxation.
Photothermal spectroscopy has a variety of uses in chemistry, physics, biology, and engineering because of the photothermal effect's universality, which is similar to how heat evolution follows nearly all optical absorption.
A thermal wave is produced when a sample is heated by a laser beam, and it moves away from the heated area. A temperature oscillation that is dampened and exponentially decays with distance from the heated surface in a material is called a thermal wave. Studying the propagation dynamics and the other parameters of the thermal wave supplies useful information about the source and structure of the sample. The thermal analysis can be carried out using several different spectroscopic methods. Some of the widely used techniques are summarized below.
Photoacoustic Spectroscopy
The photoacoustic spectroscopy technique is based on the fundamental principles of acoustic waves. In a dedicated cell, a sample is placed for analysis and introduced to modulated laser exposure, which heats up the material. The pressure inside the acoustic cell fluctuates periodically as a result of the heat that is released which gets transferred to the gas medium in the cell. A very sensitive acoustic detector that is integrated inside the cell records the acoustic waves.
Open photoacoustic systems, that can detect pressure changes in air, have recently been developed. These systems are promising tools for remotely detecting chemical substances on solid surface areas. The sensitive element in these systems is a quartz tuning fork.
Photothermal Deflection Spectroscopy
In Photothermal Deflection Spectroscopy, a sample's surface is heated by a modulated laser beam. The surface layer of the medium that touches the sample is periodically heated by the beam. The probe beam's deviation from a plane parallel to the sample surface, which is detected by a position-sensitive photodetector, reveals the development of a temperature gradient in the medium at the surface and the corresponding change in its refractive index.
The optical absorption coefficient, thermal conductivity, temperature, and electrical characteristics of materials are all commonly measured using this method.
Thermal Lens Spectroscopy
In thermal lens spectroscopy, a temperature gradient is activated by nonuniform heating when an absorbing media is exposed to radiation with a Gaussian intensity distribution. According to the intensity distribution of the laser beam, a change in temperature affects changes in the medium's refractive index. The creation of an optical element that functions as a scattering lens as a result of the changing refractive index is known as a thermal lens.
The thermal lens is often scattered because most materials with transparencies lose refractive index as the temperature rises; as a result, the size of the transverse laser beam expands due to the heating of the medium.
The thermal lens approach is frequently employed in nonlinear spectroscopy, the analysis of trace quantities of materials, the kinetics of chemical reactions, and the measurement of weak absorption in solids and liquids.
Photothermal Radiometric Spectroscopy
An approach that actively reduces the impact of background radiation from external objects surrounding the sample being investigated is photothermal radiometry. In Photothermal radiometry, a sample is subjected to continuous wave laser radiation during measurements. The sample surface temperature and the measured heat flux are modulated at the pulse repetition frequency as a result of the partial absorption of laser energy and the resulting heat release. After measuring the dynamic component of the recorded thermal signal's time dependency and its phase, information on various physical characteristics of the object is acquired.
Photothermal radiometry is employed to assess low absorption in thin films, surface layers, and bulk materials. This technique is employed in thermal-wave microscopy, thermography, and remote spectral analysis to study the surfaces of materials and coatings, and investigate the electronic properties and structure of semiconductor materials.
Photothermal Interferometric Spectroscopy
To measure weak absorption in transparent mediums, photothermal interferometry has gained a lot of popularity due to its high sensitivity.
As mentioned in the thermal lens section, changes in the refractive index of an irradiated medium cause a radiation wave traveling through it to alter in phase. The interferometric approach, in which a medium is inserted in one of the interferometer arms, is the most sensitive way to detect phase change. The phase detection method offers more sensitivity when compared to the deflection and thermal lens procedures.
Outlook
The photothermal spectroscopy techniques summarized above that are used to investigate the spectral dependence of photoinduced absorption demonstrate their effectiveness for analyzing the defect structure of thin films, surface layers, and bulk materials. This is crucial for advancing manufacturing processes and maintaining the quality of the materials produced.
The development of photothermal techniques for nondestructive material diagnostics and the hardware required for their implementation are continuously being advanced. The successful application of photothermal technologies in the semiconductor industry and the creation of PCI-based photothermal measurement systems are extending the applications of photothermal spectroscopy in the industry.
More from AZoOptics: Using the Laser Flash Method for Thermal Analysis
References and Further Reading
Ilia M. Pavlovetc, Eduard A. Podshivaylov, Rusha Chatterjee, Gregory V. Hartland, Pavel A. Frantsuzov, and Masaru Kuno , "Infrared photothermal heterodyne imaging: Contrast mechanism and detection limits", Journal of Applied Physics 127, 165101 (2020)https://doi.org/10.1063/1.5142277
L A Skvortso. Laser photothermal spectroscopy of light-induced absorption. 2013. Quantum Electron. 43 1 DOI 10.1070/QE2013v043n01ABEH014912
S. Bialkowski : “Photothermal Spectroscopy Methods for Chemical Analysis” Volume 134 Chemical Analysis: A Series of Monographs on Analytical Chemistry and Its Applications, , J. D. Winefordner, Series Editor, John Wiley & Sons, Inc. 1996
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