arXiv subjects

Patanjali Kambhampati

Publications and source records attributed to Patanjali Kambhampati.

7 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 Fröhlich 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↗

arXiv2026

Optical gain in colloidal quantum dots is limited by biexciton absorption, not biexciton recombination

Despite three decades of experimental study, optical gain in colloidal quantum dots still lacks a microscopic theory capable of explaining gain thresholds approaching one exciton per dot, their size dependence, or the anomalously small effective stimulated-emission cross sections observed across materials. Existing descriptions treat quantum dots as effective two-level systems comprised of an exciton and a biexciton, attributing gain thresholds to biexciton Auger recombination. This assumption is inconsistent with state-resolved optical pumping experiments and basic spectroscopic constraints. Here we present a microscopic theory of optical gain explicitly anchored in the Einstein relations governing absorption and stimulated emission. Within this framework, gain is determined by a spectral balance between stimulated emission from single excitons and excited-state absorption into biexcitonic manifolds, rather than by biexciton lifetimes. Using a spin-boson description of excitons coupled to a lattice bath, we show that gain thresholds and effective gain cross sections are controlled by the interplay of biexciton stabilization and exciton-lattice dressing. The theory unifies disparate materials by quantitatively explaining all longstanding gain phenomenology in CdSe quantum dots and predicts a continuous crossover to effective four-level, near-thresholdless gain in dynamically disordered lattices such as perovskite quantum dots.

cond-mat.mes-hall↗

arXiv2025

Time-Reversed Superfluorescence in a Polaronic Quantum Material

Superfluorescence, the cooperative burst of spontaneous emission from an ensemble of dipoles, arises when microscopic oscillators spontaneously synchronize their phases. Here we show that this process can be reversed in time within quantum materials. Coherent multidimensional spectroscopy of halide perovskite quantum dots reveals a delayed cooperative absorption burst, the mirror image of superfluorescent emission, driven by transient polaron fields that phase-lock unit-cell dipoles within 100 fs. The effect scales systematically with quantum-dot size and halide composition, reaching near-unity coherence fidelity even at 300 K. A microscopic exciton-polaron model reproduces the buildup and decay of the coherent state, identifying lattice polarons as the mediators of synchronization. These results demonstrate that many-body temporal coherence can self-organize and persist at room temperature, opening routes toward engineered collective optical states and superabsorbing quantum devices.

cond-mat.mtrl-sci↗

arXiv2025

Landau Polarons as Generators of Quantum-Coherent States

Since Landau's theory, polarons have been understood as quasiparticles in which charges are dressed by the lattice field, yet decades of transport and spectroscopic studies have yielded only static indirect renormalizations. Whether such dressing can dynamically reorganize electronic spectra to generate new quantum-coherent states has remained unresolved. Here we use femtosecond coherent multidimensional spectroscopy on size and composition controlled perovskite quantum dots to track polaronic field-induced dynamics in real time, revealing their consequences. We observe a delayed condensation into a confined spectrum of coherent states on 50-150 fs timescales, with couplings between these states evolving dynamically on the same timescale. The splittings are robust, exhibit anomalous linear size dependence, exceed single-particle splittings and manifest at 300 K. A Raman-constrained spin-boson Hamiltonian captures both the anomalous scaling and dynamical onset, establishing polarons as generators of coherent manifolds that enable collective quantum phenomena including superradiance, superfluorescence and superabsorption.

cond-mat.mes-hall↗

arXiv2025

A Path towards Thresholdless Colloidal Quantum Dot Lasers by Solving Decades of Mythology on Optical Gain

The semiconductor quantum dot (CQD) was first conceived in the 1980s as offering potential for future lasers. Following high quality solution phase synthesis of colloidal CQD (CCQD) in 1993, optical gain was first demonstrated in 2000 via stimulated emission (SE) measurements. Decades of phenomenology have given rise to a Standard Model of optical gain in CCQD based upon two assumptions: a biexciton is required to achieve optical gain, and short biexciton lifetimes limit the efficient development of amplified spontaneous emission (ASE). The Standard Model predicts the solution to efficient ASE is slowing Auger recombination, a task which now consumes the CCQD field. Here, we show that the Standard Model is physically incorrect, leading to misdirected materials development. Inspection of the phenomenology reveals a Simple Model which uniquely reproduces all observed phenomena. The Simple Model provides the physical foundation for developing CCQD lasers with quantum leaps in performance, including thresholdless gain.

physics.optics↗