Kazan University researchers discover new mechanism by which light interacts with structural defects

A research team led by Prof. Sergey Kharintsev has answered a long-standing question: why the refractive index of spatially confined media, such as quantum dots, nanocrystals and thin films, can exceed the fundamental limit.

The study, featured as the cover story of the December issue of Nano Letters, explains the origin of anomalous refractive index through quantum confinement of a medium in which optically driven electronic polarization in regions with broken spatial symmetry—such as point defects, phase boundaries and twins, arises.

"Perfection lies within imperfection," remarked Sergey Kharintsev. "Quantum confinement fundamentally reshapes light-matter interactions by enabling indirect optical transitions. In homogeneous bulk materials, such transitions are forbidden due to electron-photon momentum mismatching, by two to three orders of magnitude. As a result, only direct optical transitions are accessible, but light absorption dominates over scattering because these processes are governed by first- and second-order terms of perturbation theory, respectively."

He continued, "Quantum confinement imparts additional momentum to the electronic system, much like a phonon transfers momentum to an electron in indirect bandgap transitions. This extra momentum unlocks indirect optical transitions, significantly enhancing the oscillator strength. Since the electronic density of states peaks near the conduction band edge, indirect light scattering becomes the dominant process in spatially confined systems. This conclusion is supported by numerous observations of a broadband background emission in Raman spectra, known as electronic light scattering (ELS), which differs fundamentally from vibrational/electronic Raman scattering and fluorescence, since its spectral profile reflects the material's band structure and typically exhibits a linewidth ranging from 0.1 to 1 eV."

Importantly, ELS encodes vital information about the spatial structure of inhomogeneous media, independent of their chemical composition. It boosts the concentration of free carriers in the conduction band, leading to an anomalous increase in the refractive index.

"We studied this effect using isolated and clustered gold nanoparticles smaller than 5 nm," noted Elina Battalova, the paper's first author. "Under resonant conditions, their refractive index increased by more than an order of magnitude. Remarkably, this enhancement persists across a broad spectral range, even far from resonance. Moreover, aggregation of gold nanoparticles induces a redshift and further amplifies the intensity of electronic light scattering, resulting in additional refractive index enhancement over a wide bandwidth. Our findings indicate that spectral study of the RRI in spatially confined structures such as quantum dots, nanocrystals and ultrathin films using spectrophotometry and ellipsometry, should be interpreted with caution, as the total optical losses arise not only from direct/indirect absorption but also from direct/indirect scattering."

This breakthrough is allowed to revolutionize photovoltaic and thermo-optic technologies. The research team aims to harness this effect to develop single-junction, spectrum-splitting-free solar cells—commonly referred to as fourth-generation photovoltaics—with power conversion efficiencies exceeding the Shockley–Queisser limit (32% for silicon p-n junction). Moreover, the exceptionally high refractive index of spatially confined media opens the door to creating optically transparent yet electrically conductive materials, which could serve as high-performance electrodes in next-generation solar cells.

In addition, the high refractive index enables unprecedented optical control of electrons in silicon transistors at sub-nanometer scales. Electronic light scattering forms the basis of a novel optical defect inspection technique for rapid, non-destructive characterization of silicon wafers used in micro- and nano-electronics. In sensing applications, it offers a powerful method remotely to probe temperature and the electronic band structure of spatially confined systems.

The research was supported by industrial partners Ostec-ArtTool and NT-MDT BV.

More information:
Off-Resonance Enhancement of the Refractive Index via Indirect Optical TransitionsClick to copy article link
pubs.acs.org/doi/10.1021/acs.nanolett.5c04414

Provided by Kazan Federal University