Ultrafast tristable spin memory of a coherent polariton gas

Unraveling the Ultrafast Coherent Dynamics of Exciton Polariton Propagation at Room Temperature

Exciton polaritons are widely considered as promising platforms for developing room-temperature polaritonic devices, owing to the high-speed propagation and nonlinear interactions. However, it remains challenging to explore the dynamics of exciton polaritons specifically at room temperature, where the lifetime could be as small as a few picoseconds and the prevailing time-averaged measurement cannot give access to the true nature of it. Herein, by using the time-resolved photoluminescence, we have successfully traced the ultrafast coherent dynamics of a moving exciton polariton condensate in a one-dimensional perovskite microcavity. The propagation speed is directly measured to be ∼12.2 ± 0.8 μm/ps. Moreover, we have developed a time-resolved Michelson interferometry to quantify the time-dependent phase coherence, which reveals that the actual coherence time of exciton polaritons could be much longer (nearly 100%) than what was believed before. Our work sheds new light on the ultrafast coherent propagation of exciton polaritons at room temperature.

Room-Temperature Electrical Field-Enhanced Ultrafast Switch in Organic Microcavity Polariton Condensates

Integrated electro-optical switches are essential as one of the fundamental elements in the development of modern optoelectronics. As an architecture for photonic systems exciton polaritons, hybrid bosonic quasiparticles that possess unique properties derived from both excitons and photons, have shown much promise. For this system, we demonstrate a significant improvement of emitted intensity and condensation threshold by applying an electric field to a microcavity filled with an organic microbelt. Our theoretical investigations indicate that the electric field makes the excitons dipolar and induces an enhancement of the exciton-polariton interaction and of the polariton lifetime. Based on these electric field-induced changes, a sub-nanosecond electrical field-enhanced polariton condensate switch is realized at room temperature, providing the basis for developing an on-chip integrated photonic device in the strong light-matter coupling regime.

Nonlinear polariton parametric emission in an atomically thin semiconductor based microcavity

Parametric nonlinear optical processes are at the heart of nonlinear optics underpinning the central role in the generation of entangled photons as well as the realization of coherent optical sources. Exciton-polaritons are capable to sustain parametric scattering at extremely low threshold, offering a readily accessible platform to study bosonic fluids. Recently, two-dimensional transition-metal dichalcogenides (TMDs) have attracted great attention in strong light-matter interactions due to robust excitonic transitions and unique spin-valley degrees of freedom. However, further progress is hindered by the lack of realizations of strong nonlinear effects in TMD polaritons. Here, we demonstrate a realization of nonlinear optical parametric polaritons in a WS₂ monolayer microcavity pumped at the inflection point and triggered in the ground state. We observed the formation of a phase-matched idler state and nonlinear amplification that preserves the valley population and survives up to room temperature. Our results open a new door towards the realization of the future for all-optical valley polariton nonlinear devices.

Engineering Photon Statistics in a Spinor Polariton Condensate

We implement full polarization tomography on photon correlations in a spinor exciton-polariton condensate. Our measurements reveal condensate pseudospin mean-field dynamics spanning from stochastic switching between linear polarization components, limit cycles, and stable fixed points, and their intrinsic relation to the condensate photon statistics. We optically harness the cavity birefringence, polariton interactions, and the optical orientation of an incoherent exciton reservoir to engineer photon statistics with precise control. Our results demonstrate a smooth transition from a highly coherent to a super-thermal state of the condensate polarization components.

All-optical switching based on interacting exciton polaritons in self-assembled perovskite microwires

Ultrafast all-optical switches and integrated circuits call for giant optical nonlinearity to minimize energy consumption and footprint. Exciton polaritons underpin intrinsic strong nonlinear interactions and high-speed propagation in solids, thus affording an intriguing platform for all-optical devices. However, semiconductors sustaining stable exciton polaritons at room temperature usually exhibit restricted nonlinearity and/or propagation properties. Delocalized and strongly interacting Wannier-Mott excitons in metal halide perovskites highlight their advantages in integrated nonlinear optical devices. Here, we report all-optical switching by using propagating and strongly interacting exciton-polariton fluids in self-assembled CsPbBr₃ microwires. Strong polariton-polariton interactions and extended polariton fluids with a propagation length of around 25 μm have been reached. All-optical switching on/off of polariton propagation can be realized in picosecond time scale by locally blue-shifting the dispersion with interacting polaritons. The all-optical switching, together with the scalable self-assembly method, highlights promising applications of solution-processed perovskites toward integrated photonics operating in strong coupling regime.

Long-Time Memory and Ternary Logic Gate Using a Multistable Cavity Magnonic System

Multistability is an extraordinary nonlinear property of dynamical systems and can be explored to implement memory and switches. Here we experimentally realize the tristability in a three-mode cavity magnonic system with Kerr nonlinearity. The three stable states in the tristable region correspond to the stable solutions of the frequency shift of the cavity magnon polariton under specific driving conditions. We find that the system staying in which stable state depends on the history experienced by the system, and this state can be harnessed to store the history information. In our experiment, the memory time can reach as long as 5.11 s. Moreover, we demonstrate the ternary logic gate with good on-off characteristics using this multistable hybrid system. Our new findings pave a way towards cavity magnonics-based information storage and processing.

Role of non-thermal electrons in ultrafast spin dynamics of ferromagnetic multilayer

Understanding of ultrafast spin dynamics is crucial for future spintronic applications. In particular, the role of non-thermal electrons needs further investigation in order to gain a fundamental understanding of photoinduced demagnetization and remagnetization on a femtosecond time scale. We experimentally demonstrate that non-thermal electrons existing in the very early phase of the photoinduced demagnetization process play a key role in governing the overall ultrafast spin dynamics behavior. We simultaneously measured the time-resolved reflectivity (TR-R) and the magneto-optical Kerr effect (TR-MOKE) for a Co/Pt multilayer film. By using an extended three-temperature model (E3TM), the quantitative analysis, including non-thermal electron energy transfer into the subsystem (thermal electron, lattice, and spin), reveals that energy flow from non-thermal electrons plays a decisive role in determining the type I and II photoinduced spin dynamics behavior. Our finding proposes a new mechanism for understanding ultrafast remagnetization dynamics.

Polaritonic Cross Feshbach Resonance

We demonstrate the existence of a cross Feshbach resonance by strongly driving a lower polariton mode and by monitoring in time the transmission of a short optical pulse at the energy of the upper polariton mode in a semiconductor microcavity. From the signatures of the optical resonance, strength, and sign of the energy shift, we attribute the origin of the scattering process between polariton modes with opposite circular polarization to a biexciton bound state. From this study, we infer the conditions required for a strong enhancement of the generation of entangled photon pairs.

Tracking Dark Excitons with Exciton Polaritons in Semiconductor Microcavities

Dark excitons are of fundamental importance for a wide variety of processes in semiconductors but are difficult to investigate using optical techniques due to their weak interaction with light fields. We reveal and characterize dark excitons nonresonantly injected into a semiconductor microcavity structure containing InGaAs/GaAs quantum wells by a gated train of eight 100 fs pulses separated by 13 ns by monitoring their interactions with the bright lower polariton mode. We find a surprisingly long dark exciton lifetime of more than 20 ns, which is longer than the time delay between two consecutive pulses. This creates a memory effect that we clearly observe through the variation of the time-resolved transmission signal. We propose a rate equation model that provides a quantitative agreement with the experimental data.

Optical Bistability under Nonresonant Excitation in Spinor Polariton Condensates

We realize bistability in the spinor of polariton condensates under nonresonant optical excitation and in the absence of biasing external fields. Numerical modeling of the system using the Ginzburg-Landau equation with an internal Josephson coupling between the two spin components of the condensate qualitatively describes the experimental observations. We demonstrate that polariton spin bistability strongly depends on the condensate's overlap with the exciton reservoir by tuning the excitation geometry and sample temperature. We obtain noncollapsing bistability hysteresis loops for a record range of sweep times, [10  μs, 1 s], offering a promising route to spin switches and spin memory elements.

Spin Order and Phase Transitions in Chains of Polariton Condensates

We demonstrate that multiply coupled spinor polariton condensates can be optically tuned through a sequence of spin-ordered phases by changing the coupling strength between nearest neighbors. For closed four-condensate chains these phases span from ferromagnetic (FM) to antiferromagnetic (AFM), separated by an unexpected crossover phase. This crossover phase is composed of alternating FM-AFM bonds. For larger eight-condensate chains, we show the critical role of spatial inhomogeneities and demonstrate a scheme to overcome them and prepare any desired spin state. Our observations thus demonstrate a fully controllable nonequilibrium spin lattice.

High-speed flow of interacting organic polaritons

The strong coupling of an excitonic transition with an electromagnetic mode results in composite quasi-particles called exciton polaritons, which have been shown to combine the best properties of their individual components in semiconductor microcavities. However, the physics and applications of polariton flows in organic materials and at room temperature are still unexplored because of the poor photon confinement in such structures. Here, we demonstrate that polaritons formed by the hybridization of organic excitons with a Bloch surface wave are able to propagate for hundreds of microns showing remarkable third-order nonlinear interactions upon high injection density. These findings pave the way for the study of organic nonlinear light-matter fluxes and for a technologically promising route of the realization of dissipation-less on-chip polariton devices operating at room temperature.

Cellular automata in photonic cavity arrays

We propose theoretically a photonic Turing machine based on cellular automata in arrays of nonlinear cavities coupled with artificial gauge fields. The state of the system is recorded making use of the bistability of driven cavities, in which losses are fully compensated by an external continuous drive. The sequential update of the automaton layers is achieved automatically, by the local switching of bistable states, without requiring any additional synchronization or temporal control.

A sub-femtojoule electrical spin-switch based on optically trapped polariton condensates

Practical challenges to extrapolating Moore's law favour alternatives to electrons as information carriers. Two promising candidates are spin-based and all-optical architectures, the former offering lower energy consumption, the latter superior signal transfer down to the level of chip-interconnects. Polaritons-spinor quasi-particles composed of semiconductor excitons and microcavity photons-directly couple exciton spins and photon polarizations, combining the advantages of both approaches. However, their implementation for spintronics has been hindered because polariton spins can be manipulated only optically or by strong magnetic fields. Here we use an external electric field to directly control the spin of a polariton condensate, bias-tuning the emission polarization. The nonlinear spin dynamics offers an alternative route to switching, allowing us to realize an electrical spin-switch exhibiting ultralow switching energies below 0.5 fJ. Our results lay the foundation for development of devices based on the electro-optical control of coherent spin ensembles on a chip.

Photonic architectures for equilibrium high-temperature Bose-Einstein condensation in dichalcogenide monolayers

Semiconductor-microcavity polaritons are composite quasiparticles of excitons and photons, emerging in the strong coupling regime. As quantum superpositions of matter and light, polaritons have much stronger interparticle interactions compared with photons, enabling rapid equilibration and Bose-Einstein condensation (BEC). Current realizations based on 1D photonic structures, such as Fabry-Pérot microcavities, have limited light-trapping ability resulting in picosecond polariton lifetime. We demonstrate, theoretically, above-room-temperature (up to 590 K) BEC of long-lived polaritons in MoSe₂ monolayers sandwiched by simple TiO₂ based 3D photonic band gap (PBG) materials. The 3D PBG induces very strong coupling of 40 meV (Rabi splitting of 62 meV) for as few as three dichalcogenide monolayers. Strong light-trapping in the 3D PBG enables the long-lived polariton superfluid to be robust against fabrication-induced disorder and exciton line-broadening.

Stochastic resonance in collective exciton-polariton excitations inside a GaAs microcavity

We report the first observation of stochastic resonance in confined exciton polaritons. We evidence this phenomena by tracking the polaritons behavior through two stochastic resonance quantifiers namely the spectral magnification factor and the signal-to-noise ratio. The evolution of the stochastic resonance in the function of the modulation amplitude of the periodic excitation signal is studied. Our experimental observations are well reproduced by numerical simulations performed in the framework of the Gross-Pitaevskii equation under stochastic perturbation.

Optomechanics with cavity polaritons: dissipative coupling and unconventional bistability

We study a hybrid system formed from an optomechanical resonator and a cavity mode strongly coupled to an excitonic transition inside a quantum well. We show that due to the mixing of cavity photon and exciton states, the emergent quasiparticles-polaritons-possess coupling to the mechanical mode of both a dispersive and dissipative nature. We calculate the occupancies of polariton modes and reveal bistable behavior, which deviates from conventional Kerr nonlinearity or dispersive coupling cases due to the dissipative coupling. The described system serves as a good candidate for future polaritonic devices.