arXiv subjects

Libai Huang

Publications and source records attributed to Libai Huang.

14 recordsLinked to original sources

arXiv2026

Few-Body Decay Dynamics in Colloidal CsPbI3 Quantum-Dot Clusters

Colloidal perovskite quantum dots combine bright emission with the ability to self- assemble into closely spaced structures, enabling radiative dynamics to be studied from isolated emitters to few-dot clusters. Here, we characterize CsPbI3 perovskite quan- tum dots from isolated emitters to self-assembled clusters containing up to ten dots. We estimate the number of emitters in each cluster using a combination of photon autocorrelation, blinking statistics, and emission brightness. Compared with isolated dots, clusters containing two or more emitters exhibit biexponential decay, with a short lifetime that decreases with emitter number and an additional long-lived component. Photon-correlation Fourier spectroscopy shows that the single-dot emission remains far from the lifetime-limited regime. Nevertheless, we observe emitter-number-dependent decay dynamics even in this low-coherence regime. Together, these measurements es- tablish self-assembled perovskite quantum-dot clusters as a platform for studying how collective optical behavior emerges between the single-emitter and ensemble limits

quant-ph↗

arXiv2026

Long-Range Fermi-Polaron Blockade in Monolayer MoSe$_2$

Strong optical nonlinearities at the few-photon level are a central goal for quantum photonics, yet they remain difficult to realize in solid-state systems. In doped two-dimensional semiconductors, coupling between excitons and a degenerate Fermi sea gives rise to exciton-Fermi polarons, many-body quasiparticles whose optical response is governed by fermionic correlations. Here, using femtosecond pump-probe transient absorption microscopy, we directly image the spatially resolved nonlinear optical response of exciton-Fermi polarons in monolayer MoSe$_2$. We observe a pronounced spatial suppression of resonant absorption associated with the attractive Fermi polaron, from which we extract an optical blockade radius that is more than ten times larger than that of the neutral exciton. Microscopic analysis indicates that this extended nonlinearity arises primarily from fermion-mediated interactions between exciton-Fermi polarons. Our results establish exciton-Fermi polarons in two-dimensional semiconductors as electrically tunable, strongly interacting optical quasiparticles, and identify them as a promising platform for ultralow-power nonlinear optical devices.

cond-mat.mes-hall↗

arXiv2026

Ballistic Exciton Flow Driven by Intertwined Exciton-Electron Orders in a Moir\'e Superlattice

Moir\'e superlattices of transition-metal dichalcogenides (TMDs) host strongly interacting Bose-Fermi mixtures in which bosonic excitons coexist with correlated electron lattices. Using ultrafast, time- and energy-resolved photoluminescence (PL) and reflectance microscopy, we show that strong exciton-electron and exciton-exciton repulsion can enable collective ballistic exciton transport in a WSe$_2$/WS$_2$ heterobilayer. The ballistic transport is energy-selective: repulsive interactions drive excitons into a higher moir\'e exciton band, where enhanced intersite hopping enables rapid spatial expansion. Correspondingly, the exciton mean-squared displacement (MSD) exhibits a quadratic time dependence ($\propto t^2$). This ballistic expansion is enhanced at fractional electron fillings where the electrons form generalized Wigner-crystal (GWC) orders. Afterwards, the system transitions into a mixed electron-exciton Mott state as Auger recombination and density depletion conclude the ballistic expansion. A one-dimensional Bose-Fermi Hubbard model solved using density-matrix renormalization group (DMRG) qualitatively reproduces the measured exciton transport and time-dependent response. It further confirms that strong cross-species interactions allow the electron crystal to perforate the exciton Mott background, accelerating its melting and enhancing exciton motion. Our results establish moir\'e TMDs as highly tunable platforms for realizing strongly interacting Bose-Fermi mixtures, which we employ here to demonstrate real-time control of intertwined bosonic and electronic order and to establish a route to the exciton insulator-fluid transition.

cond-mat.str-el↗

arXiv2025

Many-Body Entanglement in Solid-State Emitters

The preparation and control of quantum states lie at the heart of quantum information science (QIS). Recent advances in solid-state quantum emitters (QEs) and nanophotonics have transformed the landscape of quantum photonic technologies, enabling scalable generation of quantum states of light and matter. A new frontier in solid-state quantum photonics is the engineering of many-body interactions between QEs and photons to achieve robust coherence and controllable many-body entanglement. These entangled states, including photonic graph and cluster states, superradiant emission, and emergent quantum phases, are promising for quantum computation, sensing, and simulation. However, intrinsic inhomogeneities and decoherence in solid-state platforms pose significant challenges to realize such complex entangled states. This review provides an overview of the fundamental many-body interactions and dynamics at the light-matter interfaces of solid-state QEs, and discusses recent advances in mitigating decoherence and harnessing robust many-body coherence.

quant-ph↗

arXiv2025

Cavity QED with molecular defects coupled to a photonic crystal cavity

We implement permanent spectral tuning to bring lifetime-limited emitters into collective resonance within an integrated photonic cavity. This addresses a fundamental challenge in solid-state cavity QED: combining multiple coherent quantum emitters with scalable nanophotonics. Our hybrid approach decouples emitter synthesis from nanophotonic fabrication using straightforward techniques that make cavity QED broadly accessible. High doping densities allow us to couple several coherent emitters to a single cavity mode, while optically-induced frequency shifting provides long-lived spectral control. By tuning two molecules into resonance, we demonstrate controlled formation of collective quantum states, establishing a scalable platform for many-body cavity QED. This opens pathways toward chemically-designed quantum systems where optical properties are engineered through synthetic chemistry.

quant-ph↗

arXiv2024

Slow Dephasing of Coherent Optical Phonons in Two-dimensional Lead Organic Chalcogenides

Hybrid organic-inorganic semiconductors with strong electron-phonon interactions provide a programmable platform for developing a variety of electronic, optoelectronic, and quantum materials by controlling these interactions. However, in current hybrid semiconductors, such as halide perovskites, anharmonic vibrations with rapid dephasing hinder the ability to coherently manipulate phonons. Here, we report the observation of long-lived coherent phonons in lead organic chalcogenides (LOCs), a new family of hybrid two-dimensional semiconductors. These materials feature harmonic phonon dynamics despite distorted lattices, combining long phonon dephasing times with tunable semiconducting properties. Dephasing time as long as 75 ps at 10 K, with up to 500 cycles of phonon oscillation between scattering events, was observed, corresponding to a dimensionless harmonicity parameter more than an order of magnitude larger than that of halide perovskites. The phonon dephasing time is significantly influenced by anharmonicity and centrosymmetry, both of which can be tuned through the design of the organic ligands thanks to the direct bonding between the organic and inorganic motifs. This research opens new opportunities for the manipulation of electronic properties with coherent phonons in hybrid semiconductors.

cond-mat.mtrl-sci↗

arXiv2023

Square Moir\'e Superlattices in Twisted Two-Dimensional Halide Perovskites

Moir\'e superlattices have emerged as a new platform for studying strongly correlated quantum phenomena, but these systems have been largely limited to van der Waals layer two-dimensional (2D) materials. Here we introduce moir\'e superlattices leveraging ultra-thin, ligand-free halide perovskites, facilitated by ionic interactions. Square moir\'e superlattices with varying periodic lengths are clearly visualized through high-resolution transmission electron microscopy. Twist-angle-dependent transient photoluminescence microscopy and electrical characterizations indicate the emergence of localized bright excitons and trapped charge carriers near a twist angle of ~10{\deg}. The localized excitons are accompanied by enhanced exciton emission, attributed to an increased oscillator strength by a theoretically forecasted flat band. This work illustrates the potential of extended ionic interaction in realizing moir\'e physics at room temperature, broadening the horizon for future investigations.

cond-mat.mtrl-sci↗

arXiv2023

Superradiant and subradiant states in lifetime-limited organic molecules through laser-induced tuning

An array of radiatively coupled emitters is an exciting new platform for generating, storing, and manipulating quantum light. However, the simultaneous positioning and tuning of multiple lifetime-limited emitters into resonance remains a significant challenge. Here we report the creation of superradiant and subradiant entangled states in pairs of lifetime-limited and sub-wavelength spaced organic molecules by permanently shifting them into resonance with laser-induced tuning. The molecules are embedded as defects in an organic nanocrystal. The pump light redistributes charges in the nanocrystal and dramatically increases the likelihood of resonant molecules. The frequency spectra, lifetimes, and second-order correlation agree with a simple quantum model. This scalable tuning approach with organic molecules provides a pathway for observing collective quantum phenomena in sub-wavelength arrays of quantum emitters.

quant-ph↗

arXiv2023

Interface engineering of charge-transfer excitons in 2D lateral heterostructures

The existence of bound charge transfer (CT) excitons at the interface of monolayer lateral heterojunctions has been debated in literature, but contrary to the case of interlayer excitons in vertical heterostructure their observation still has to be confirmed. Here, we present a microscopic study investigating signatures of bound CT excitons in photoluminescence spectra at the interface of hBN-encapsulated lateral MoSe$_2$-WSe$_2$ heterostructures. Based on a fully microscopic and material-specific theory, we reveal the many-particle processes behind the formation of CT excitons and how they can be tuned via interface- and dielectric engineering. For junction widths smaller than the Coulomb-induced Bohr radius we predict the appearance of a low-energy CT exciton. The theoretical prediction is compared with experimental low-temperature photoluminescence measurements showing emission in the bound CT excitons energy range. Our joint theory-experiment study presents a significant step towards a microscopic understanding of optical properties of technologically promising 2D lateral heterostructures.

cond-mat.mes-hall↗

arXiv2022

Hyperspectral imaging of excitons within a moir\'e unit-cell with a sub-nanometer electron probe

Electronic and optical excitations in two-dimensional moir\'e systems are uniquely sensitive to local atomic registries, leading to materials- and twist-angle specific correlated electronic ground states with varied degree of localization. However, there has been no direct experimental correlation between the sub-nanometer structure and emergent excitonic transitions, comprising tightly-bound pairs of photoexcited electrons and holes. Here, we use cryogenic transmission electron microscopy and spectroscopy to simultaneously image the structural reconstruction and associated localization of the lowest-energy intralayer exciton in a rotationally aligned heterostructure of WS2 and WSe2 monolayers. In conjunction with optical spectroscopy and ab initio calculations, we determine that the exciton center-of-mass wavefunction is strongly modulated in space, confined to a radius of ~ 2 nm around the highest-energy stacking site in the moir\'e unit-cell, forming a triangular lattice. Our results provide direct evidence that atomic reconstructions lead to the strongly confining moir\'e potentials and that engineering strain at the nanoscale will enable new types of excitonic lattices.

cond-mat.mtrl-sci↗

arXiv2021

Interlayer exciton landscape in WS$_2$/tetracene heterostructures

The vertical stacking of two-dimensional materials into heterostructures gives rise to a plethora of intriguing optoelectronic properties and presents an unprecedented potential for technological development. While much progress has been made combining different monolayers of transition metal dichalgonenides (TMDs), little is known about TMD-based heterostructures including organic layers of molecules. Here, we present a joint theory-experiment study on a TMD/tetracene heterostructure demonstrating clear signatures of spatially separated interlayer excitons in low temperature photoluminescence spectra. Here, the Coulomb-bound electrons and holes are localized either in the TMD or in the molecule layer, respectively. In particular, we reveal both in theory and experiment signatures of the entire intra- and interlayer exciton landscape in the photoluminescence spectra. In particular, we find both in theory and experiment a pronounced transfer of intensity from the intralayer TMD exciton to a series of energetically lower interlayer excitons with decreasing temperature. In addition, we find signatures phonon-sidebands stemming from these interlayer exciton states. Our findings shed light on the microscopic nature of interlayer excitons in TMD/molecule heterostructures and could have important implications for technological applications of these materials.

cond-mat.mes-hall↗

arXiv2021

Non-Equilibrium First-Order Exciton Mott Transition at Monolayer Lateral Heterojunctions Visualized by Ultrafast Microscopy

Atomically precise lateral heterojunctions based on transition metal dichalcogenides provide a new platform for exploring exciton Mott transition in one-dimension. To investigate the intrinsically non-equilibrium Mott transition, we employed ultrafast microscopy with ~ 200 fs temporal resolution to image the transport of different exciton phases in a type II WSe2-WS1.16Se0.84 lateral heterostructure. These measurements visualized the extremely rapid expansion of a highly non-equilibrium electron-hole (e-h) plasma phase with a Fermi velocity up to 3.2*10^6 cm*s-1. An abrupt first-order exciton Mott transition at a density of ~ 5*10^12 cm-2 at room temperature was revealed by ultrafast microscopy, which could be disguised as a continuous transition in conventional steady-state measurements. These results point to exciting new opportunities for designing atomically thin lateral heterojunctions as novel highways of excitons and collective e-h plasma for high-speed electronic applications.

cond-mat.mes-hall↗

arXiv2019

Anomalous Interlayer Exciton Diffusion in Twist-Angle-Dependent Moiré Potentials of WS$_2$-WSe$_2$ Heterobilayers

The nanoscale periodic potentials introduced by moiré patterns in semiconducting van der Waals (vdW) heterostructures provide a new platform for designing exciton superlattices. To realize these applications, a thorough understanding of the localization and delocalization of interlayer excitons in the moiré potentials is necessary. Here, we investigated interlayer exciton dynamics and transport modulated by the moiré potentials in WS$_2$-WSe$_2$ heterobilayers in time, space, and momentum domains using transient absorption microscopy combined with first-principles calculations. Experimental results verified the theoretical prediction of energetically favorable K-Q interlayer excitons and unraveled exciton-population dynamics that was controlled by the twist-angle-dependent energy difference between the K-Q and K-K excitons. Spatially- and temporally-resolved exciton-population imaging directly visualizes exciton localization by twist-angle-dependent moiré potentials of ~100 meV. Exciton transport deviates significantly from normal diffusion due to the interplay between the moiré potentials and strong many-body interactions, leading to exciton-density- and twist-angle-dependent diffusion length. These results have important implications for designing vdW heterostructures for exciton and spin transport as well as for quantum communication applications.

cond-mat.mtrl-sci↗

arXiv2018

Homogenization of Halide Distribution and Carrier Dynamics in Alloyed Organic-Inorganic Perovskites

Perovskite solar cells have shown remarkable efficiencies beyond 22%, through organic and inorganic cation alloying. However, the role of alkali-metal cations is not well-understood. By using synchrotron-based nano-X-ray fluorescence and complementary measurements, we show that when adding RbI and/or CsI the halide distribution becomes homogenous. This homogenization translates into long-lived charge carrier decays, spatially homogenous carrier dynamics visualized by ultrafast microscopy, as well as improved photovoltaic device performance. We find that Rb and K phase-segregate in highly concentrated aggregates. Synchrotron-based X-ray-beam-induced current and electron-beam-induced current of solar cells show that Rb clusters do not contribute to the current and are recombination active. Our findings bring light to the beneficial effects of alkali metal halides in perovskites, and point at areas of weakness in the elemental composition of these complex perovskites, paving the way to improved performance in this rapidly growing family of materials for solar cell applications.

cond-mat.mtrl-sci↗