Advances in photothermal and photoacoustic metrology

The absorption of electromagnetic radiation by fluids or solids results in a temperature change, which, in turn, causes a change in volume resulting in the generation of a mechanical wave. When the emitted acoustic waves are recorded, the effect is referred to as a photoacoustic or optoacoustic effect. If the thermal radiation resulting from the light absorption is recorded the effect is known as a photothermal effect. Keen interest and intense investigation into the basic physics and applications of these two effects followed as a consequence of the invention of the laser, which provided a light source of sufficiently high intensity to make photothermal, and photoacoustic effects strong enough to be detected. Since the generation of temperature fields and sound waves is almost universally one of the consequences of light absorption, photothermal and photoacoustic methods, over the years, have found application to numerous areas of research from physical sciences to medicine.

A complete list of all of the fields that have incorporated photoacoustic methods would be difficult to compile. However, the main applications include infrared and visible spectroscopy, trace gas detection, non-destructive testing, medical imaging, photochemistry, chemical reactions dynamics, generation of bulk, surface, and interface ultrasonic waves, impulsive fracture, studies of nonequilibrium transport of heated electrons in metals, generation of acoustic solitons in crystals, soft matter research, and the investigation of acousto-electronic properties of semiconductors. Moreover, photothermal techniques have found applications in studies of coherent phonon transport, characterization of superlattices, quasiballistic thermal transport, ferroelectric resonance, semiconductors properties, mechanical characterization of thin films, plates and membranes, non-destructive testing and materials evaluation, food characterization, as well as the determination of thermal properties of grains, thin films, viscoelastic fluids, and solids. The “Photothermics” Special Topic in the Journal of Applied Physics aims to highlight recent advances and modifications of photothermal and photoacoustic methods for non-contact characterization, as well as experimental and theoretical research based on these methods.

Many of the modern thermal measurement methods rely on photothermal and photoacoustic effects, including thermoreflectance, Raman scattering, Brillouin light scattering, and thermal radiation. A Tutorial by Sandell et al.¹ summarizes the state-of-art non-contact methods for thermal characterization based on thermoreflectance and Raman scattering. They show how modifications of traditional time-domain thermoreflectance methods² enable in-plane thermal measurements on nanostructures as well as making possible the probing of phonon mean free path spectra. They also describe two-laser Raman thermometry, which enables thermal mapping at the microscale. Contributions by Zenji et al.³ and Zhang et al.⁴ focus on the role of metal transducers in thermoreflectance measurements and their properties at different frequencies. Flizikowski et al.⁵ discuss the effects on the photothermal-induced phase shift measured by a homodyne quadrature laser interferometer and compare the experiments with a theoretical description of thermoelastic surface displacement in metals. Their proposed method enables measurements of anisotropic thermophysical properties of bulk and thin membranes without the need for an optical transducer. Using two different photothermal techniques, micro-Raman thermometry and scanning thermal microscopy, Massoud et al.⁶ measured the thermal conductivity of porous silicon, showing that both methods yielded consistent results. Cancellier et al.⁷ used time-domain thermoreflectance to measure the thermal conductivity of multilayered structures.

As for the photoacoustic methods, Pérez-Cota and co-workers⁸ review the field of picosecond ultrasonics and, in particular, methods based on Brillouin scattering for imaging and characterization of biological cells. Macias et al.⁹ used a photoacoustic technique to measure the thermal conductivity of epitaxial heterostructures.

Photoacoustic and photothermal methods also have important applications in imaging—each with their own contrast mechanism and response to materials properties. In thermal imaging, the limited spatial resolution remains one of its main restrictions. In this regard, Chien and Schmid¹⁰ present a scheme for optimization of photothermal microscopy, improving the precision of localization down to 3 Å, while Kovács et al.¹¹ propose deep learning approaches. Thompson and co-workers¹² describe an imaging technique that uses laser-induced ultrasound generated by the photoacoustic effect. A paper by Thummerer et al.¹³ tackles an interesting virtual wave concept for photothermal parameter estimation and image reconstruction. Koskelo et al.¹⁴ report the use of noise to enhance stochastically the resolution of a charge-coupled device (CCD) in thermoreflectance imaging. Hamaoui et al.¹⁵ propose a photothermal radiometry system with a spatial resolution of 33 $μ$m using a mixed scan in both the frequency and spatial domains.

Several contributions focus on the theoretical modeling of photothermal signals in particular circumstances. Chirtoc et al.¹⁶ propose a theory to account for non-linearities in photothermal radiometry. A work by Somer et al.¹⁷ discusses anomalous thermal diffusion. Liu et al.¹⁸ report on modeling photothermal lock-in thermographic images obtained on curved surfaces. Isidro-Ojeda et al.¹⁹ exploit ring-shaped laser beams to investigate disk-like geometries. A paper by Barbalinardo et al.²⁰ contributes with lattice dynamics simulations of heat transport in crystalline and disordered solids. Xu et al.²¹ tackled the modeling of infrared radiation coming off a rotating missile.

Some papers of this Special Topic are not specifically categorized as photothermal or photoacoustic but are nevertheless related. James et al.²² discuss the effect of surface roughness on the efficiency of solar energy conversion. As far as semiconductor science is concerned, Lofti et al.²³ present an intriguing two-temperature model using generalized thermoelasticity theory with implications for photothermal transport process. Kumar et al.²⁴ report on the nanoparticle-mediated photoporation, which results in various bio-effects including intracellular delivery of molecules. In addition, Paoloni et al.²⁵ highlight successful explorations of the use of photothermal methods in the domain of cultural heritage.

Some groups demonstrate the performance of photothermal methods for the characterization of particular devices and materials: quantum cascade lasers,²⁶ Tb$3+$-doped tungsten–zirconium–tellurite glasses,²⁷ nearly perfect absorbers,²⁸ and omnithermal metamaterials.²⁹ 

In addition to research papers, this Special Topic includes several Perspectives. The article by Li Voti³⁰ discusses photothermal lensing and beam deflection techniques and their applications in various fields and of the relative advantages of using these techniques. Usoltseva et al.³¹ focus on the interesting features of nanofluids in general, and the use of the (photo)thermal lens technique for their characterization in particular. Pérez-Cota et al.⁸ review the state of the art of picosecond ultrasonics focusing on superoptical resolution and application to cell imaging and characterization. A Perspective by Shen et al.³² reviews how photothermal methods are applied to characterization of biodiesel and petroleum diesel fuels. Finally, Xiong et al.³³ summarize recent advances in near-field radiation and thermal transport in nanogaps and two-dimensional materials.

The fields of photoacoustic and photothermal science have witnessed amazing growth resulting in a spectacular increase in the number of publications and patents, scientific applications to a range of research fields, and the formation of small businesses worldwide. This Special Topic shows how the photothermal and photoacoustic methods have been improved and expanded over the past decade, enabling contact-free measurements with a higher resolution and greater precision than previously possible. Photothermal phenomena have found applications in photovoltaics,²² thermoelectrics,²³ optomechanics,³⁴ plasmonics,³⁴ phase-change materials,³⁵ photovoltaics,²² and other areas. We hope that the photothermics community will benefit from the collection of articles in this Special Topic.