Organic Semiconductor Microcavities Research Articles

Room temperature, cascadable, all-optical polariton universal gates

Today, almost all information processing is performed using electronic logic circuits operating at several gigahertz frequency. All-optical logic holds the promise to allow for up to three orders of magnitude higher speed. Whereas essential all-optical transistor functionalities were demonstrated across a range of platforms, utilising them to implement a complete Boolean logic gate set and in particular negation, i.e. switching off an optical signal with another, weaker, optical signal, poses a major challenge. Here, we realize a cascadable NOT gate by introducing the concept of non-ground-state polariton amplification in organic semiconductor microcavities under non-resonant optical excitation. We unravel the importance of vibron-mediated stimulated scattering in room temperature operation of the inverter. Moreover, we extend the concept to a multi-input universal NOR logic gate, where in the presence of any of the input signals non-ground-state amplification supersedes spontaneous ground-state condensation, resulting in a NOR gate with ~1 ps switching time. The realisation of an ultrafast universal logic gate constitutes an essential step for more complex optical circuitry that could boost information processing applications.

Ultrafast Optical Control of Polariton Energy in an Organic Semiconductor Microcavity

AbstractThe manipulation of exciton–polaritons and their condensates is of great interest due to their applications in polariton simulators and high‐speed, all‐optical logic devices. Until now, methods of trapping and manipulating such condensates are not dynamically reconfigurable or resulted in an undesirable reduction in the exciton–photon coupling strength. Here, a new strategy for the ultrafast control of polariton resonances via transient modification of an optical cavity mode is presented. Multilayer organic semiconductor microcavities that contain two absorbers are constructed: one strongly‐coupled to the cavity photon mode and one that is out‐of‐resonance. By selectively exciting the out‐of‐resonance absorber using ultrashort laser pulses, the cavity refractive index is modulated, and fully‐reversible blueshifts of the lower polariton branch by up to 8 meV in sub‐ps timescales with no corresponding reduction in the exciton–photon coupling strength are generated. This work demonstrates the ability to manipulate polariton energy landscapes over ultrafast timescales with important applications in emerging computing technologies.

Molecular polariton electroabsorption

We investigate electroabsorption (EA) in organic semiconductor microcavities to understand whether strong light-matter coupling non-trivially alters their nonlinear optical [{chi }^{(3)}left(omega,{{{{mathrm{0,0}}}}}right)] response. Focusing on strongly-absorbing squaraine (SQ) molecules dispersed in a wide-gap host matrix, we find that classical transfer matrix modeling accurately captures the EA response of low concentration SQ microcavities with a vacuum Rabi splitting of hslash Omega approx 200 meV, but fails for high concentration cavities with hslash Omega approx 420 meV. Rather than new physics in the ultrastrong coupling regime, however, we attribute the discrepancy at high SQ concentration to a nearly dark H-aggregate state below the SQ exciton transition, which goes undetected in the optical constant dispersion on which the transfer matrix model is based, but nonetheless interacts with and enhances the EA response of the lower polariton mode. These results indicate that strong coupling can be used to manipulate EA (and presumably other optical nonlinearities) from organic microcavities by controlling the energy of polariton modes relative to other states in the system, but it does not alter the intrinsic optical nonlinearity of the organic semiconductor inside the cavity.

Exciton-Polaritons and Their Bose-Einstein Condensates in Organic Semiconductor Microcavities.

Exciton-polaritons are half-light, half-matter bosonic quasiparticles formed by strong exciton-photon coupling in semiconductor microcavities. These hybrid particles possess the strong nonlinear interactions of excitons and keep most of the characteristics of the underlying photons. As bosons, above a threshold density they can undergo Bose-Einstein condensation to a polariton condensate phase and exhibit a rich variety of exotic macroscopic quantum phenomena in solids. Recently, organic semiconductors have been considered as a promising material platform for these studies due to their room-temperature stability, good processability, and abundant photophysics and photochemistry. Herein, recent advances of exciton-polaritons and their Bose-Einstein condensates in organic semiconductor microcavities are summarized. First, the basic physics is introduced, and then their emerging applications are highlighted. The remaining questions are also discussed and a personal viewpoint about the potential directions for future research is given.

Polariton condensation in an organic microcavity utilising a hybrid metal-DBR mirror

We have developed a simplified approach to fabricate high-reflectivity mirrors suitable for applications in a strongly-coupled organic-semiconductor microcavity. Such mirrors are based on a small number of quarter-wave dielectric pairs deposited on top of a thick silver film that combine high reflectivity and broad reflectivity bandwidth. Using this approach, we construct a microcavity containing the molecular dye BODIPY-Br in which the bottom cavity mirror is composed of a silver layer coated by a SiO2 and a Nb2O5 film, and show that this cavity undergoes polariton condensation at a similar threshold to that of a control cavity whose bottom mirror consists of ten quarter-wave dielectric pairs. We observe, however, that the roughness of the hybrid mirror—caused by limited adhesion between the silver and the dielectric pair—apparently prevents complete collapse of the population to the ground polariton state above the condensation threshold.

Observation of photon-mode decoupling in a strongly coupled multimode microcavity.

We have fabricated organic semiconductor microcavities having an extended optical path-length (up to 2 µm) that contain J-aggregates of a cyanine dye. These structures are studied using optical-reflectivity and are found to be characterized by a series of polaritonic modes. By changing the effective oscillator strength of the dye within the cavity, we evidence a transition from "normal" strong coupling in which the photon modes are coupled to one another via the excitonic transition of the molecular dye to a state in which photon-modes become decoupled. We use an eight-level modified Hamiltonian to describe the optical properties of the system and compare the distribution of the confined optical field in coupled and decoupled structures.

Incident angle and photon energy dependence of polariton lasing in an organic microcavity

Organic semiconductor microcavities forming strongly coupled exciton-polaritons are interesting systems due to their nonlinear coherent emission observed even at room temperature. Here, we investigate the incident angle and photon-energy-dependent photoemission from a planar organic microcavity to evaluate the relaxation efficiency towards the bottom of the lower polariton branch. We observe nonlinear increase in the vertical emission from the microcavity when the sample is pumped resonantly to the lower polariton branch with an incident angle of 30 to 55°. The observed results support the fast and efficient relaxation of organic microcavity polaritons within the lower polariton branch.

Charged Polariton Luminescence from an Organic Semiconductor Microcavity

Strong coupling light to polaron optical transitions in an organic semiconductor microcavity leads to an unusual class of polariton that possesses a net charge. This species may offer a range of technologically useful optoelectronic properties but has thus far only been observed in passive reflectivity measurements. Here, we report room-temperature photoluminescence from both upper and lower branch polaron polariton states that originate from hole excitations in a p-doped 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) microcavity. Our observations under nonresonant excitation are in good agreement with classical dipole emission modeling, which suggests that polariton states in this system are populated radiatively via fluorescence from the polaron excited state reservoir. These results constitute one of the first observations of polaron luminescence from an organic semiconductor thin film and are an important prerequisite for realizing a charged polariton condensate.

Generation of Anti-Stokes Fluorescence in a Strongly Coupled Organic Semiconductor Microcavity

We explore the generation of anti-Stokes fluorescence from strongly coupled organic dye microcavities following resonant ground-state excitation. We observe polariton emission along the lower polariton branch, with our results indicating that this process involves a return to the exciton reservoir and the absorption of thermal energy from molecules in a vibrationally excited ground-state. We speculate that the generation of a population of "hot" polaritons is enhanced by the fact that the cavity supresses the emission of Stokes-shifted fluorescence, as it is located energetically below the cut-off frequency of the cavity.

Charged Polaron Polaritons in an Organic Semiconductor Microcavity.

We report strong coupling between light and polaron optical excitations in a doped organic semiconductor microcavity at room temperature. Codepositing MoO_{3} and the hole transport material 4, 4^{'}-cyclohexylidenebis[N, N-bis(4-methylphenyl)benzenamine] introduces a large hole density with a narrow linewidth optical transition centered at 1.8eV and an absorption coefficient exceeding 10^{4} cm^{-1}. Coupling this transition to a Fabry-Pérot cavity mode yields upper and lower polaron polariton branches that are clearly resolved in angle-dependent reflectivity with a vacuum Rabi splitting ℏΩ_{R}>0.3 eV. This result establishes a path to electrically control polaritons in organic semiconductors and may lead to increased polariton-polariton Coulombic interactions that lower the threshold for nonlinear phenomena such as polariton condensation and lasing.

Polariton Condensates: A Yellow Polariton Condensate in a Dye Filled Microcavity (Advanced Optical Materials 18/2017)

David G. Lidzey, Pavlos G. Lagoudakis, and co-workers demonstrate room temperature polariton condensation in the yellow part of the visible spectrum from a planar organic semiconductor microcavity containing the molecular dye BODIPY-Br in article number 1700203. Single-shot polariton dispersion imaging is exploited to observe the collapse of the energy distribution and a blueshift of the emission wavelength upon reaching condensation threshold.

A Yellow Polariton Condensate in a Dye Filled Microcavity

Polariton condensation in the yellow part of the visible spectrum from a planar organic semiconductor microcavity containing the molecular dye bromine‐substituted boron‐dipyrromethene is observed. This study provides experimental fingerprint of polariton condensation under nonresonant optical excitation, including the nonlinear dependence of the emission intensity and wavelength blueshift with increasing excitation density, single excitation pulse dispersion imaging, and real space interferometry. The latter two allow to visualize the collapse of the energy distribution and the long‐range coherence of the polariton condensate.

Exploring Light–Matter Interaction Phenomena under Ultrastrong Coupling Regime

Exciton-polaritons are bosonic quasiparticles that arise from the normal mode splitting of photons in a microcavity and excitons in a semiconductor material. One of the most intriguing extensions of such a light–matter interaction is the so-called ultrastrong coupling regime. It is achieved when the Rabi frequency (ΩR, the energy exchange rate between the emitter and the resonant photonic mode) reaches a considerable fraction of the emitter transition frequency, ω0. Here, we report a Rabi energy splitting (2ℏΩR) of 1.12 eV and record values of the coupling ratio (2ΩR/ω0) up to 0.6-fold the material band gap in organic semiconductor microcavities and up to 0.5-fold in monolithic heterostructure organic light-emitting diodes working at room temperature. Furthermore, we show that with such a large coupling strength it is possible to undress the exciton homogeneous linewidth from its inhomogeneous broadening, which allows for an unprecedented narrow emission line (below the cavity finesse) for such organic LE...

Imaging the polariton relaxation bottleneck in strongly coupled organic semiconductor microcavities

We use angular-resolved photoluminescence excitation spectroscopy to probe energy relaxation towards the lowest lying states (${k}_{\ensuremath{\parallel}}=0$) on the lower polariton branch (LPB) of two different strong-coupled $J$-aggregate organic semiconductor microcavities. We find that states along the upper polariton branch (UPB) are able to undergo efficient relaxation to populate states around ${k}_{\ensuremath{\parallel}}=0$ on the LPB; a process that occurs through the rapid relaxation of UPB polaritons to the exciton reservoir. In contrast, LPB states having a photon fraction $g$0.5 are not apparently able to undergo further relaxation towards ${k}_{\ensuremath{\parallel}}=0$, constituting an effective relaxation bottleneck.

Ultrastrongly Coupled Exciton–Polaritons in Metal‐Clad Organic Semiconductor Microcavities

Ultrastrong exciton–photon coupling of Frenkel molecular excitons is demonstrated at room temperature in a metal‐clad microcavity containing a thin film of 2,7‐bis[9,9‐di(4‐methylphenyl)‐fluoren‐2‐yl]‐9,9‐di(4‐methylphenyl)fluorene. A giant Rabi splitting of Ω ∼ 1 eV is measured using angle‐resolved reflectivity and bright photoluminescence is observed from the lower polariton branch. To obtain the virtual photon and exciton content of the polariton ground state, the results are interpreted in terms of the full Hopfield Hamiltonian, including anti‐resonant terms. Also included is an analytical treatment of the often ignored and sometimes misinterpreted TM‐polarized metal–insulator–metal plasmon–polariton.

Characterizing the Electroluminescence Emission from a Strongly Coupled Organic Semiconductor Microcavity LED

The fabrication of a strongly coupled organic semiconductor light emitting diode (OLED) is described. The device is characterised through angle‐resolved photoluminescence and electroluminescence spectroscopy, with the efficiency of the polariton OLED being compared to that of an equivalent weakly coupled device. The external emission efficiency of the polariton OLED is approximately 6 times smaller than that of the OLED; an effect that is attributed to the relatively slow scattering of excitons towards states at the bottom of the lower polariton branch. The temporal responses of the polariton and standard OLED devices are measured under pulsed voltage excitation, and device dynamics are found to be limited by charge transport and recombination.

Ultra-fast polariton dynamics in an organic microcavity

We study an organic semiconductor microcavity operating in the strong- coupling regime using femtosecond pump-probe spectroscopy. By probing the photo- induced absorption bands, we characterize the time-dependent population densities of states in the two polariton branches. We found evidence of a scattering process from the upper-branch cavity polaritons to the exciton reservoir having a rate of (150 fs)−1. A slower process similarly populates lower-branch polaritons with a rate of around (3ps)−1

Polarization splitting in polariton electroluminescence from an organic semiconductor microcavity with metallic reflectors

Organic semiconductors have received considerable attention as the active medium in microcavity devices that exploit the regime of strong exciton–photon coupling. The eigenstates of these systems are microcavity polaritons, whose properties are an admixture of the uncoupled exciton and photon. Organic microcavities are particularly interesting due to their large exciton binding energy which permits the electrical excitation of polaritons at room temperature. Measurements of electroluminescence are often facilitated through the use of metallic reflectors that form the optical microcavity and also serve as device electrodes. Here, we demonstrate that such structures exhibit a significant polarization splitting under both optical and electrical excitation. The size of the polarization splitting rivals those observed in strongly coupled microcavities based on distributed Bragg reflectors having a long optical penetration depth.

Controlling the interactions between polaritons and molecular vibrations in strongly coupled organic semiconductor microcavities

We study the microscopic processes that determine the dynamics of polaritons in strongly-coupled organic-semiconductor microcavities. Using a quantum kinetic theory, we show that resonant couplings and interactions between cavity photons, excitons, and localized molecular vibrations strongly affect the polariton resonance line shapes in $J$-aggregate microcavities. We use our many-body calculation to reproduce the measured polariton emission at exciton-photon resonance which is determined as a function of the Rabi-splitting energy.

Polariton dynamics in disordered microcavities

The behaviour of cavity polaritons is strongly affected by disorder scattering in traditional inorganic semiconductor microcavities and, much more, in organic semiconductor microcavities. In the presence of disorder coherent cavity polaritons having a well defined in-plane wavevector coexist with incoherent states having a localized character. We here study through a numerical simulation of a one-dimensional disordered microcavity in the strong coupling regime, the polariton wavefunction character and the dynamical behaviour of excited wave packets. A detailed microscopic picture is obtained which substantiates the coexistence of both propagating and localized states over time and length scales of experimental relevance.