Active optical elements with ever smaller footprint and lower energy consumption are central to modern photonics. The drive for miniaturization, speed and efficiency, with the concomitant volume ...reduction of the optically active area, has led to the development of devices that harness strong light–matter interactions. By managing the strength of light–matter coupling to exceed losses, quasiparticles, called exciton-polaritons, are formed that combine the properties of the optical fields with the electronic excitations of the active material. By making use of polaritons in inorganic semiconductor microcavities, all-optical transistor functionality was observed, albeit at cryogenic temperatures1. Here, we replace inorganic semiconductors with a ladder-type polymer in an optical microcavity and realize room-temperature operation of a polariton transistor through vibron-mediated stimulated polariton relaxation. We demonstrate net gain of ~10 dB μm−1, sub-picosecond switching time, cascaded amplification and all-optical logic operation at ambient conditions.Net gain of ~10 dB µm–1 and sub-picosecond switching time are shown at room temperature for optical transistors using polymers in a microcavity.
A Bose-Einstein condensate (BEC) is a state of matter in which extensive collective coherence leads to intriguing macroscopic quantum phenomena. In crystalline semiconductor microcavities, bosonic ...quasiparticles, known as exciton-polaritons, can be created through strong coupling between bound electron-hole pairs and the photon field. Recently, a non-equilibrium BEC (ref. ) and superfluidity have been demonstrated in such structures. With organic crystals grown inside dielectric microcavities, signatures of polariton lasing have been observed. However, owing to the deleterious effects of disorder and material imperfection on the condensed phase, only crystalline materials of the highest quality have been used until now. Here we demonstrate non-equilibrium BEC of exciton-polaritons in a polymer-filled microcavity at room temperature. We observe thermalization of polaritons and, above a critical excitation density, clear evidence of condensation at zero in-plane momentum, namely nonlinear behaviour, blueshifted emission and long-range coherence. The key signatures distinguishing the behaviour from conventional photon lasing are presented. As no crystal growth is involved, our approach radically reduces the complexity of experiments to investigate BEC physics and paves the way for a new generation of opto-electronic devices, taking advantage of the processability and flexibility of polymers.
Metal-halide semiconductors with perovskite crystal structure are attractive due to their facile solution processability, and have recently been harnessed very successfully for high-efficiency ...photovoltaics and bright light sources. Here, we show that at low temperature single colloidal cesium lead halide (CsPbX3, where X = Cl/Br) nanocrystals exhibit stable, narrow-band emission with suppressed blinking and small spectral diffusion. Photon antibunching demonstrates unambiguously nonclassical single-photon emission with radiative decay on the order of 250 ps, representing a significant acceleration compared to other common quantum emitters. High-resolution spectroscopy provides insight into the complex nature of the emission process such as the fine structure and charged exciton dynamics.
Fully inorganic cesium lead halide perovskite nanocrystals (NCs) have shown to exhibit outstanding optical properties such as wide spectral tunability, high quantum yield, high oscillator strength as ...well as blinking-free single photon emission, and low spectral diffusion. Here, we report measurements of the coherent and incoherent exciton dynamics on the 100 fs to 10 ns time scale, determining dephasing and density decay rates in these NCs. The experiments are performed on CsPbBr2Cl NCs using transient resonant three-pulse four-wave mixing (FWM) in heterodyne detection at temperatures ranging from 5 to 50 K. We found a low-temperature exciton dephasing time of 24.5 ± 1.0 ps, inferred from the decay of the photon-echo amplitude at 5 K, corresponding to a homogeneous line width (fwhm) of 54 ± 5 μeV. Furthermore, oscillations in the photon-echo signal on a picosecond time scale are observed and attributed to coherent coupling of the exciton to a quantized phonon mode with 3.45 meV energy.
Nanostructured semiconductors emit light from electronic states known as excitons. For organic materials, Hund's rules state that the lowest-energy exciton is a poorly emitting triplet state. For ...inorganic semiconductors, similar rules predict an analogue of this triplet state known as the 'dark exciton'. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX
, with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin-orbit coupling in the conduction band of a perovskite is combined with the Rashba effect. We then apply our model to CsPbX
nanocrystals, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting, lasers and displays, these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.
An ensemble of emitters can behave very differently from its individual constituents when they interact coherently via a common light field. After excitation of such an ensemble, collective coupling ...can give rise to a many-body quantum phenomenon that results in short, intense bursts of light-so-called superfluorescence
. Because this phenomenon requires a fine balance of interactions between the emitters and their decoupling from the environment, together with close identity of the individual emitters, superfluorescence has thus far been observed only in a limited number of systems, such as certain atomic and molecular gases and a few solid-state systems
. The generation of superfluorescent light in colloidal nanocrystals (which are bright photonic sources practically suited for optoelectronics
) has been precluded by inhomogeneous emission broadening, low oscillator strength, and fast exciton dephasing. Here we show that caesium lead halide (CsPbX
, X = Cl, Br) perovskite nanocrystals
that are self-organized into highly ordered three-dimensional superlattices exhibit key signatures of superfluorescence. These are dynamically red-shifted emission with more than 20-fold accelerated radiative decay, extension of the first-order coherence time by more than a factor of four, photon bunching, and delayed emission pulses with Burnham-Chiao ringing behaviour
at high excitation density. These mesoscopically extended coherent states could be used to boost the performance of opto-electronic devices
and enable entangled multi-photon quantum light sources
.
When this happens, the electron can undergo a process called spontaneous emission: it returns to its initial state and releases the energy it has gained as a photon, the wavelength of which is ...defined by the energy difference between the two states. Amplified spontaneous emission has already been achieved with nanocrystals through optical excitation, and been used extensively in laser devices in the past two decades. In any case, whether they are made from colloidal quantum dots or organic materials, such unconventional integrated lasers driven exclusively by electricity would indeed be a game-changing technology.
A common signature of nearly all nanoscale emitters is fluorescence intermittency, which is a rapid switching between “on”-states exhibiting a high photon emission rate and “off”-states with a much ...lower rate. One consequence of fluorescence intermittency occurring on time scales longer than the exciton decay time is the so-called delayed photon emission, manifested by a long radiative decay component. Besides their dominant fast radiative decay, fully inorganic cesium lead halide perovskite quantum dots exhibit a long fluorescence decay component at cryogenic temperatures that is often attributed to the decay of the dark exciton. Here, we show that its origin is delayed photon emission by investigating temporal variations in fluorescence intensity and concomitant decay times found in single CsPbBr3 perovskite quantum dots. We attribute the different intensity levels of the intensity trace to a rapid switching between a high-intensity exciton state and an Auger-reduced low-intensity trion state that occurs when the excitation is sufficiently strong. Surprisingly, we observe that the exponent of this power-law-dependent delayed emission is correlated with the emission intensity, which cannot be explained with existing charge carrier trapping models. Our analysis reveals that the long decay component is mainly governed by delayed emission, which is present in both the exciton and trion state. The absence of a fine structure in trions clarifies the vanishing role of the dark exciton state for the long decay component. Our findings are essential for the development of a complete photophysical model that captures all observed features of fluorescence variations in colloidal nanocrystals.
One of the most attractive commercial applications of semiconductor nanocrystals (NCs) is their use in lasers. Thanks to their high quantum yield, tunable optical properties, photostability, and ...wet-chemical processability, NCs have arisen as promising gain materials. Most of these applications, however, rely on incorporation of NCs in lasing cavities separately produced using sophisticated fabrication methods and often difficult to manipulate. Here, we present whispering gallery mode lasing in supraparticles (SPs) of self-assembled NCs. The SPs composed of NCs act as both lasing medium and cavity. Moreover, the synthesis of the SPs, based on an in-flow microfluidic device, allows precise control of the dimensions of the SPs, i.e. the size of the cavity, in the micrometer range with polydispersity as low as several percent. The SPs presented here show whispering gallery mode resonances with quality factors up to 320. Whispering gallery mode lasing is evidenced by a clear threshold behavior, coherent emission, and emission lifetime shortening due to the stimulation process.
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