KFU scientists describe anomalous optical heating of non-light-absorbing materials
Researchers of the Quantum Photonics and Metamaterials Laboratory of the Institute of Physics of the Kazan Federal University, led by Professor Sergey Kharintsev, Chair of the Department of Optics and Nanophotonics, have established that in environments with strong spatial dispersion, the main mechanism of optical heating is light scattering, not absorption, as was previously believed.
This experimental result is of great importance for the development of nonlocal photonics, an emerging field of modern optics that studies the interaction of light with spatially limited environments.
The results of the work are presented in Nanophotonics. The work was supported by the industrial partner, Ostec-ArtTool LLC (Moscow).
"The interaction of light and matter plays an important role in the development of modern technologies in optoelectronics, biomedicine, and renewable energy. Optical radiation interacts well with homogeneous media only under resonance conditions, in which the photon energy is directly converted into the energy of an electron. This phenomenon is known as light absorption and obeys the Bouguer-Lambert-Beer law. It is absorption that leads to photo-heating of opaque homogeneous media. However, optical radiation interacts poorly with homogeneous media outside of resonance due to the mismatch of electron and photon momenta," Sergey Kharintsev explains.
According to the scientist, inhomogeneous media contain a large number of nanoscale optical inhomogeneities, such as defects, interphase boundaries, inclusions, twinning regions, etc. When illuminated by a laser, these inhomogeneities generate near-field photons with increased momenta, which participate in indirect (with a change in electron momentum) optical transitions. As a result, optically transparent inhomogeneous (spatially limited) media can heat up.
To explain this phenomenon, the team suggested that the general optical losses in inhomogeneous media arise not only from direct/indirect absorption, but also from direct/indirect scattering. In their work, the authors demonstrated anomalous optical heating of spatially confined solids due to electron scattering of light, which is the dominant mechanism of interaction of light with matter.
"Anomalous optical heating of spatially limited media is caused by indirect optical transitions due to the matching of electron and photon momentum. While indirect absorption is determined by the laws of conservation of energy and momentum, indirect scattering of light is limited only by the requirement of conservation of electron momentum," emphasizes Elina Battalova, co-author, master's student of the Institute of Physics.
This radiation, namely electron scattering of light, carries important information about the spatial structure of inhomogeneous media and does not depend on their chemical composition.
"It is important to note that this mechanism leads to an increase in the charge concentration in the conduction band and, therefore, provides an increase in the refractive index, optical heating, photoconductivity, and optical nonlinearity. Today, electron light scattering is widely used for the structural analysis of inhomogeneous and disordered solids and has the potential for further application in photovoltaic and thermo-optical technologies and devices," continues Dr. Kharintsev.
Electron light scattering has important practical applications in optoelectronics for the development of white LEDs and cavityless microlasers, silicon solar cells with an efficiency exceeding the Shockley-Queisser limit (32 percent). The high refractive index of spatially confined media opens the way to the creation of optically transparent conductive materials. Electron light scattering can serve as a spectroscopic tool for flaw detection in solids, which will find application in the express characterization of large-area silicon wafers. This noninvasive tool can also be used in geological exploration to determine the spatial structure of porous rocks, permeability and oil and gas saturation of reservoirs. The proposed physical principle allows increasing the flow rate of wells by reducing the viscosity of heavy oils.
Of greatest interest are applications in biomedicine. In particular, electron light scattering can be used for optical detection of peptide and protein conformations at room temperature. Anomalous photo-heating of spatially limited media underlies targeted thermo-optical diagnostics and therapy of neurodegenerative diseases and some types of cancer. Finally, this mechanism sheds light on the behavior of open chemical and biological systems with adjustable complexity to create 'conscious' artificial intelligence.
More information:
Heat generation in spatially confined solids through electronic light scattering
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Provided by Kazan Federal University
