arXiv subjects

Sayantan Patra

Publications and source records attributed to Sayantan Patra.

4 recordsLinked to original sources

arXiv2026

Extreme Polarization of the Optical Gap and High-Energy Exciton Landscape in CrSBr

We reveal a strongly anisotropic excitonic landscape in monolayer and bulk-like CrSBr using optical absorption spectroscopy and $GW$-Bethe-Salpeter equation $\textit{ab initio}$ calculations. The direct absorptive determination of the lowest bright optical onsets i.e. $X_0^a$ and $X_0^b$ excitons for the two in-plane polarization eigenaxes yield an in-plane optical gap anisotropy of $470 \pm 15$ meV. This is the highest observed value for any material in the near-infrared-to-visible spectral region to the best of our knowledge. Energetically above, we identify multiple strongly polarized excitons spanning $1.25$ eV to $3.1$ eV selectively aligned along the two orthogonal axes. A resonance $X^-$, located $24$ meV below the $X_0^b$ progressively transfers oscillator strength to $X_b^0$, a behavior consistent with a coupled trion (Fermi-polarion)/exciton pair. Our experiments also provide polarization-resolved broadband dielectric functions of CrSBr. These results establish CrSBr as a strongly polarization-selective excitonic system and highlight its potential for polarization-selective optoelectronics enabled with its large optical-gap anisotropy.

cond-mat.mes-hall↗

arXiv2026

Phonon-modulated Kerr nonlinearity in ultrathin 2H-MoTe2

Controlling nonequilibrium responses in optically driven quantum materials is essential for advancing applications in energy conversion, ultrafast electronics, and quantum computation. Nonlinear optical spectroscopy serves as a powerful tool to investigate ultrafast electron and phonon dynamics in these systems; however, conventional nonlinear approaches often require intense laser pulses (> 10 GW/cm2) and typically encounter a strong background. Here, we introduce a phase-sensitive nonlinear spectroscopic technique that operates at low laser powers (~ 10 kW/cm2, pulse energies ~ 10 pJ) and enables real-time monitoring and active control of coherent phonons in a few-layer (three to five) thick 2H-MoTe2. Upon excitation with ultrashort (~ 10 fs) pump pulses, we achieve displacive excitation of coherent phonons, which periodically modulate the Kerr nonlinearity of the material, leading to cross-phase modulation (XPM) of a delayed probe pulse. This phase modulation induces spectral broadening and oscillations in the center of mass (COM) of the probe spectrum in time, enabling the detection of subtle nonlinear optical responses in a background-free manner. The nonlinear response can be selectively amplified or attenuated by adjusting the strength of the pump pulse, which controls the distribution of photoexcited carriers in the electronic bands. By combining two-color nondegenerate pump-probe measurements and time-dependent density-functional theory (TDDFT) calculations, we directly resolve the coupled nonequilibrium electronic and phonon dynamics. A dual-pump pulse scheme enables precise control of phonon oscillations, allowing selective activation or suppression of specific phonon modes and correspondingly the modulation of the Kerr nonlinearity.

physics.optics↗

arXiv2026

Exciton-Selective Phonon Coupling in a Lead Halide Perovskite

Exciton-phonon interactions govern the optical response of semiconductors, yet disentangling multiple coupling channels in lead halide perovskites remains challenging. We investigate CsPbBr3 microcrystals using photoluminescence, Raman and reflectance spectroscopy at low temperature, revealing the simultaneous presence of high-energy and Rashba excitons, each accompanied by distinct phonon replica series. High-energy exciton replicas are uniquely spaced by approximately 9 meV, whereas Rashba exciton replicas exhibit a characteristic approximately 6 meV spacing, indicating the specificity of the exciton-phonon coupling. Unsupervised machine learning applied to a large low-temperature photoluminescence dataset reveals these replica features are prevalent. With increasing temperature, replica features broaden and merge, evolving into a dominant longitudinal optical phonon coupling regime at room temperature. This work establishes direct spectroscopic evidence for concurrent, exciton-specific phonon coupling within a single material, offering new pathways to engineer light-matter interactions for optoelectronic and phonon-photon-based quantum device applications.

cond-mat.mes-hall↗

arXiv2025

Universal Thickness-Dependent Absorption in Solids at the Nanoscale: Anomalous Enhancement in the Ultrathin Limit

Through systematic experimental and theoretical studies of layer-thickness-dependent absorption in semiconducting MoSe$_2$ and WS$_2$ across the visible to near-infrared spectral range, we demonstrate a universal absorption behavior in solids at nanoscale thicknesses. With increasing thickness, a non-monotonic evolution of absorption integrated over the measured spectral region is revealed which is accompanied by pronounced oscillatory features. This shows a strong deviation from the expected Beer-Lambert law. Below 10 nm, we observe a sharp anomalous increase in absorption, with deviations from Beer's law exceeding 50% in layered semiconductors. Our conclusions hold irrespective of the presence of any optical resonances such as excitons or plasmons within the spectral window. The observed behavior has origins in the electromagnetic interference effects taking place between the two surfaces of the thin crystals. The present work on 2D semiconductors is extendable to all kinds of solids such as conventional semiconductors (e.g. Si, GaAs, GaN, InP), (semi) metals (e.g. Al, Ag, Au, c-HOPG) and 2D magnetic materials (e.g. CrSBr and NiPS$_3$). Our results provide fundamental insights into light-matter interactions in solids at the nanoscale and are vital for optimally designing the new generation of absorption-based flexible optoelectronic devices.

cond-mat.mes-hall↗