Magic photoheating of non-absorbing media
Researchers from Kazan Federal University discovered anomalous optical heating of materials that do not absorb light.
Researchers from the laboratory of Quantum Photonics and Metamaterials, led by Prof. Sergey Kharintsev, have found that the main mechanism of optical heating of media with strong spatial dispersion is light scattering rather than absorption, as previously thought.
These findings, just published in Nanophotonics, contribute to nonlocal photonics, an emerging field in optics that studies the interaction of light with spatially confined media.
“The interaction of light and matter plays a vital role in advancing technologies in optoelectronics, biomedicine and renewable energy. However, light interacts with homogeneous matter only at resonance, when the energy of a photon is converted into the energy of an electron. This phenomenon is well-known as light absorption and obeys the Beer-Lambert law. It is absorption that leads to photoheating of opaque, homogeneous media (Fig. 1a). Beyond resonance, light poorly interacts with homogeneous matter due to electron-photon momentum mismatching,” Sergey Kharintsev explained.
Heterogeneous media are enriched with nanoscale optical moieties, such as defects, phase boundaries, interstitials, twins etc. Upon illumination, these inhomogeneities generate near-field photons with expanded momenta allowing for indirect electronic transitions. As a result, optically transparent inhomogeneous (spatially confined) media can be heated (Fig. 1b).
Fig. 1 Photoheating of opaque homogeneous (a) and transparent heterogeneous (b) solids through absorption and scattering of light, respectively
To explain this phenomenon, Prof. Kharintsev’s group proposed that the total optical losses in heterogeneous media include not only direct/indirect absorption, but also direct/indirect scattering (Fig. 2). In their study, the authors demonstrated the anomalous optical heating of spatially confined solids due to electronic light scattering, which is the dominant mechanism of light-matter interaction.
Fig. 2 Optical melting of a silicon AFM tip apex (left) and electronic transitions in a direct bandgap semiconductor (right).
“Anomalous optical heating of spatially confined matter is caused by indirect optical transitions due to electron-photon momentum matching. While indirect absorption is governed by both energy and momentum matching conditions, indirect light scattering is constrained only by the electron-photon momentum matching,” emphasized Elina Battalova, co-author of this work.
Electronic light scattering carries important information about the spatial structure of inhomogeneous media and is independent of their chemical composition.
“Electronic light scattering increases electronic population of the conduction band, which can lead to an increase in the refractive index, photoheating, photocurrent and nonlinearity. Today, electronic light scattering is widely used for structural analysis of inhomogeneous and disordered solids and has potential for further applications in photovoltaic and thermo-optical technologies and devices,” Dr Kharintsev noted.
Electronic light scattering has significant practical applications in optoelectronics, including white light-emitting diodes and room-temperature mirrorless lasers, silicon solar cells with efficiency beyond the Shockley-Queisser limit. The high refractive index of spatially confined media paves the way for creating optically transparent conducting electrodes. Electronic light scattering serves as a spectroscopic probe for monitoring defects on large-scale crystalline wafers. This non-invasive tool can also be used in geological exploration to determine the spatial structure of porous rocks, as well as the permeability and hydrocarbon saturation of reservoirs. The proposed physical principle allows for increased flow rate of wells by reducing the viscosity of heavy oils. Of particular interest are applications in biomedicine. Specifically, electronic light scattering can be used for room-temperature optical recognition of peptide and protein conformations. Anomalous photoheating of spatially confined media forms the basis for targeted thermo-optical diagnostics and therapy of neurodegenerative diseases and specific types of cancers. Finally, this mechanism sheds light on the behavior of open chemical and biological systems with tunable complexity for the creation of conscious artificial intelligence
The research was supported by the industrial partner – Ostec-ArtTool LLC.