arXiv subjects

Priya Nagpal

Publications and source records attributed to Priya Nagpal.

4 recordsLinked to original sources

arXiv2026

Quantum Back-Action Expands the Excitonic Hilbert Space in a Soft Polar Semiconductor

Electronic excitations in solids are commonly described within a hierarchy in which the excitonic Hamiltonian is defined first and the lattice acts later through renormalization, relaxation, and dephasing. This picture assumes that the optically accessible excitonic manifold is already present at the moment of photoexcitation. Here we show that this assumption fails in a soft polar semiconductor. Using femtosecond coherent multidimensional spectroscopy on lead-halide perovskite nanocrystals, we observe quantum back-action between an electronic excitation and a collective lattice-polarization field that expands the excitonic Hilbert space in real time. The optical pulse first prepares an excitonic polarization, X1. A second configuration, X2, emerges only after the polaron field develops, while coherent X1-X2 coupling appears at later times. State formation and coherence formation are therefore resolved as distinct stages of quasiparticle formation. In contrast, CdSe quantum dots exhibit the conventional limit in which excitonic states and couplings are present at time zero and are only weakly perturbed by phonons. The observed diagonal and anti-diagonal splittings increase with nanocrystal size and correlate with radiative oscillator strength, opposite to expectations from simple quantum confinement. A dynamical polaron-field model describes the lattice polarization as an order parameter that expands the optically accessible manifold and generates time-dependent coherent coupling. These results show that strong system-bath coupling can actively create excitonic states and the coherent manifold in which they evolve.

cond-mat.mes-hall↗

arXiv2026

Real-Time Formation of a Landau Polaron

Polarons are electronic excitations dressed by a self-consistent lattice distortion, yet their formation has not been directly resolved in real time. We develop a microscopic lineshape framework that connects the growth of a collective lattice polarization to the population-time evolution of the anti-diagonal linewidth in coherent multidimensional spectroscopy. Within this formalism, the anti-diagonal linewidth directly tracks the decay of lattice frequency-frequency correlations. Underdamped phonon environments produce oscillatory linewidth modulation, whereas overdamped collective polarization dynamics generate monotonic exponential broadening. Applying this framework to multidimensional measurements on perovskite quantum dots, we show that the observed approximately 150 femtosecond exponential anti-diagonal broadening reflects the decay of a collective polarization order parameter. These results establish anti-diagonal linewidth dynamics as a direct real-time signature of Landau polaron formation.

cond-mat.other↗

arXiv2026

Dynamic Lattice Disorder and Collective Dipole Coupling Give Rise to Dicke Physics in Perovskite Quantum Dots

Halide perovskite quantum dots exhibit cooperative optical phenomena that are absent in conventional semiconductor nanocrystals, including exciton superradiance, superabsorption, and biexciton superradiance within individual dots. Here we develop a microscopic theory that identifies the physical origin of these Dicke effects and establishes how they can be controlled by materials parameters. The central result is that cooperative emission emerges from a competition between collective coupling of optical transition dipoles and lattice-induced disorder, with the balance governed by the Raman-derived phonon spectral density and the excitonic oscillator strength. At elevated temperature, strong Fr\"ohlich coupling and glassy lattice dynamics produce dynamic disorder that suppresses dipole synchronization and yields incoherent emission. Upon cooling, lattice fluctuations freeze and cooperative coherence emerges when the collective coupling exceeds residual static disorder, defining size- and composition-dependent crossover temperatures that we map as phase diagrams. Extending the framework to biexcitons, we show that a confined biexciton constitutes a single correlated charge distribution dressed by a shared lattice configuration, enabling pathway-indistinguishable decay and cooperative radiative enhancement. The theory quantitatively accounts for observed size, composition, and temperature trends in radiative-rate constant ratios and biexciton binding energies, while explaining why full Dicke saturation is not universal. More broadly, the results establish Raman spectral weight and oscillator strength as design parameters for engineering cooperative quantum-optical behavior in quantum materials.

physics.optics↗

arXiv2024

Highly Sensitive Label-free Biomolecular Detection Using Au-WS2 Nanohybrid Based SERS Substrates

Recent advancements in nanotechnology have led to the development of surface-enhanced Raman spectroscopy (SERS) based rapid and low-cost technologies for ultra-sensitive label-free detection and identification of molecular analytes. Herein, we utilized the synergistic plasmonic and chemical enhancement effects of Au-WS2 nanohybrids to attain the high-intensity Raman signals of targeted analytes. To develop these nanohybrids, a series of monodispersed Au nanoparticles (NPs) of varying diameters from 20 to 80 nm was chemically synthesized and successively blended with liquid-phase exfoliated WS2 nano-flakes of average lateral size 90 nm. They provided a maximum enhancement factor (EF) of ~1.80 109 corresponding to the characteristic peaks at 1364 cm-1 and 1512 cm-1 for R6G analyte molecules. Theoretical studies based on the finite-difference time-domain simulations on Au-WS2 nanohybrid systems revealed a huge field-intensity enhancement with an EF of more than 1000 at the plasmonic hotspots, which was induced by the strong coupling of individual plasmon oscillations of the adjacent Au NPs upon light interactions. These electromagnetic effects along with the chemical enhancement effects of WS2 nanoflakes were found to be mainly responsible for such huge enhancement in Raman signals. Furthermore, these hybrids were successfully employed for achieving highly sensitive detection of the E. coli ATCC 35218 bacterial strain with a concentration of 104 CFU/mL in phosphate-buffered saline media, indicating their real capabilities for practical scenarios. The findings of the present study will indeed provide vital information in the development of innovative nanomaterial-based biosensors, that will offer new possibilities for addressing critical public health concerns.

physics.app-ph↗