Organic Microcavity Research Articles

Anomalous reflection properties in high density limit fibril shaped PIC‐J aggregates in microcavity structure

AbstractWe report our recent observations on anomalous reflection properties of organic microcavity containing PIC J‐aggregates in fibril‐shaped structures as one of their highest‐density limiting case. Even in ordinary low density PIC‐Js dispersed in polymer matrices, they easily go into the exciton‐photon strong coupling regime and new quantum states, i.e. exciton‐polaritons, are formed. Vacuum Rabi‐splitting, an indicator of the coupling strength, can be controlled by the density of PIC‐Js in the microcavities in low density regime. Fibril‐shaped PIC‐J aggregates are bundle‐like, self‐assembled higher‐order structure of Js and are considered as a high‐density limiting case. Angular dependence of the local reflection spectra are observed by our improved microscopy. While in the low density cases, clear anti‐crossing behaviours of the two branches appear, which is the evidence of the polariton formations, the highest‐density limiting cases show one or two anomalous reflection dip structures with almost no angular dependence. The result is inconsistent with neither ordinary polariton reflection property nor bare exciton nor cavity photon properties. We tentatively consider that the anomaly is inherent to the fibril formation and is caused by possible breakdown of polariton coherence due to the inter‐fiber energy transfer. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polariton-polariton scattering in organic microcavities at high excitation densities

Polariton-polariton scattering phenomena in organic strongly coupled microcavities have been theoretically assessed on the basis of the kinematic interaction of Frenkel excitons. Experiments have been focused on resonant pumping at magic angle, as well established in inorganic quantum-well microcavities, but insofar no experimental evidence of parametric amplification has been found. Here, we show that in J-aggregate-based microcavities a peculiar polariton-polariton scattering process applies, i.e., the scattering of two resonantly pumped polaritons toward a polariton of lower energy and a localized excitation of the exciton reservoir. This process, because of the localized nature of the final state, is not limited by momentum conservation, and we show that in a high-quality factor microcavity it could be exploited to setup resonant pumping experiments not requiring a magic angle configuration.

Coupled multiple organic microcavities

Recently, organic based microcavities became a focal point in the boundary photonic research due to the special features of the organic active molecules whose properties may be easily modulated. By using the thermal evaporation we realized double and triple coupled-microcavity structures containing one or more active layers of light emitting organic molecules. The resonators are tuned at the same wavelength and coupled by low reflectivity mirrors. In particular we grew an asymmetric double system with only one active organic layer embedded in one microcavity. The reflectivity measurements show that the coupling between the two resonator results in a splitting of the optical modes. The photoluminescence spectra display a double wavelength emission. The comparison of the diagonalized effective Hamiltonian with the observed resonances further confirms the strong coupling between the two cavities. A quantum statistical approach for interacting quantum systems in the strong coupling regime reproduces with very good agreement the experimental results. The triple structure has been designed to improve the coupling among the resonators. The device shows three emission peaks at different wavelength and represents a very interesting start up to realize white organic based LEDs and ultrafast optical amplifiers.

Polariton lasing

The experimental demonstration of polariton lasing from an organic optical microcavity is an important step towards the development of practical devices that exploit coupled states of light and matter and operate at room temperature.

Organic polariton laser

The present invention relates to organic lasers. More specifically, the present invention is directed to an organic laser that provides a self-stimulated source of coherent radiation originating from organic microcavity polaritons. The organic polariton laser of the present invention comprises a substrate, a resonant microcavity comprising an organic polariton emission layer; and an optical pump. In preferred embodiments the optical pump is a microcavity OLED allowing for the fabrication of a self-contained or integrated device.

Room-temperature polariton lasing in an organic single-crystal microcavity

The optical properties of organic semiconductors are almost exclusively described using the Frenkel exciton picture1. In this description, the strong Coulombic interaction between an excited electron and the charged vacancy it leaves behind (a hole) is automatically taken into account. If, in an optical microcavity, the exciton–photon interaction is strong compared to the excitonic and photonic decay rates, a second quasiparticle, the microcavity polariton, must be introduced to properly account for this coupling2. Coherent, laser-like emission from polaritons has been predicted to occur when the ground-state occupancy of polaritons 〈ngs〉, reaches 1 (ref. 3). This process, known as polariton lasing, can occur at thresholds much lower than required for conventional lasing. Polaritons in organic semiconductors are highly stable at room temperature, but to our knowledge, there has as yet been no report of nonlinear emission from these structures. Here, we demonstrate polariton lasing at room temperature in an organic microcavity composed of a melt-grown anthracene single crystal sandwiched between two dielectric mirrors. Polaritons in organic semiconductors are highly stable at room temperature, but so far nonlinear emission from these structures has not been demonstrated. Here, polariton lasing at room temperature in an organic microcavity composed of a melt-grown anthracene single crystal sandwiched between two dielectric mirrors is reported.

Strong coupled organic microcavities

Strong coupled organic-inorganic microcavities device has been realized and studied. One of the two cavities contains an organic thin film of tetrakis(4-methoxyphenyl)porphyrin, whereas the other microcavity is a dielectric structure coupled to the organic one by means of a LiF/ZnS Bragg mirror. Reflectivity spectra show the presence of two well defined cavity dips. We observe an energy splitting of the two cavity-modes. Despite only one cavity contains the active layer, the photoluminescence spectra display two peaks at the same energy of the reflectivity dips. These observations indicate the strong coupling of the two cavities. The comparison of the diagonalized effective Hamiltonian with the observed resonances further confirms the strong coupling.

Dependence of polarization splitting on mode tuning in microcavities

We present an in-depth analysis of the angle and mode tuning dependence of the polarization splitting in detuned organic microcavities. The system consists of highly reflecting dielectric mirrors comprising a continuous wedge-shaped cavity layer of 4,4′-bis(N-carbazole)-biphenyl, doped with 2 wt % of 4,4′-bis[(N-carbazole)styryl]-biphenyl as the active material. By varying the measurement position across the sample, we shift the resonant mode with respect to the center of the cavity stop band. Starting from small detunings, the splitting value increases non-linearly up to 58 meV, when the resonant modes reach the stop band edge. We compare this behavior with results from transfer matrix calculations and theoretical predictions.

Polarization splitting of discrete states in square shaped organic photonic dots

We analyze the polarization dependence of the discrete photonic states in an organic microcavity system that is laterally confined on the micron scale. Via microscopic photoluminescence measurements on square shaped photonic dots, we demonstrate a polarization splitting that increases up to 10 meV for higher order modes. These experimental results are evaluated using a vectorial waveguide approach and the splitting is attributed to phase shifts, which occur as a result of reflection at the side walls of the mesa structures.

Selective excitation of laser modes in an organic photonic dot microcavity

We experimentally investigate variable laser mode excitation in an organic photonic dot microcavity by shifting the excitation beam position. The sample comprises two highly reflective dielectric mirrors (R>99.9%) and a square-shaped organic dye mesa of a DCM doped (2 wt %) Alq3-matrix. Its wavelength-size (≈5×5 μm2) transforms the cavity mode dispersion to a set of discrete states, each with a different intensity distribution of the electromagnetic field in space. Numerical simulations, including absorption and gain, confirm the experimentally observed relation between mode distribution and progression on the excitation condition.

Vertical coupled double organic microcavities

A light emitting structure consisting of two coupled microcavities has been realized and studied. One of the two cavities contains a luminescent organic thin film of tetrakis(4-methoxyphenyl)porphyrin, whereas the other microcavity is a dielectric structure coupled to the organic one by means of a LiF/ZnS Bragg mirror. Reflectivity spectra show the presence of two well defined cavity dips. We observe an energy splitting of the two cavity modes. Despite the fact that only one cavity contains the active layer, the photoluminescence spectra display two peaks with comparable intensities at the same energy of the reflectivity dips. These observations indicate the strong coupling of the two cavities. The comparison of the diagonalized effective Hamiltonian with the observed resonances further confirms the strong coupling.

Origin of giant polarization splitting in high quality organic microcavities

We study the emission properties of a high quality monolithic microcavity with an embedded ultrathin organic tetrakis(4-methoxyphenyl)porphyrin layer. Spectral and angle-resolved photoluminescence measurements show a well defined polarized doublet at detection angles larger than 15°. The splitting energy ranges up to 70 meV. The usual employed mechanism, based on the mismatch between the center of the mirror stop band and the wavelength of the microcavity, accounts only for a small fraction of the observed splitting. A transfer matrix model taking into account the optical anisotropy of the organic layer, has been employed to reproduce the observed experimental results. The anisotropy of the organic layer, confirmed by optical absorption spectroscopy, origins from a local molecular order as inferred by atomic force microscopy measurements.

Multimode laser emission from laterally confined organic microcavities

Room-temperature multimode laser emission is observed in a microcavity consisting of dielectric mirrors and small-molecular-weight organic photonic dots as a cavity layer. The structure shows simultaneous lasing of a wide variety of transverse modes. A comparison of the laser operating characteristics with those of unpatterned structures shows an enhancement in the spontaneous emission coupling factor by more than two orders of magnitude due to the lateral confinement. The spectral features are in quantitative agreement with calculations of quantized photonic states in three-dimensional optical cavities.

Organic photonic bandgap microcavities doped with semiconductor nanocrystals for room-temperature on-demand single-photon sources

We report the first experimental observation of fluorescence from single semiconductor nanocrystals (colloidal quantum dots) in microcavities. In these room-temperature experiments we observed photon antibunching from single CdSe nanocrystals doped into a chiral one-dimensional photonic bandgap liquid-crystal microcavity. The chirality resulted in high-purity, circular polarization of definite handedness of the emitted single photons. We also report the fabrication of chiral microcavities for telecom wavelengths doped with PbSe nanocrystals as well as a solution-processed-polymer microcavity with a defect layer doped with CdSe nanocrystals between two distributed Bragg reflectors. These systems with their low host fluorescence background are attractive for on-demand single-photon sources for quantum information and communication.

Cavity polariton dispersion of a single‐crystalline anthracene film embedded in a microcavity

AbstractWe study anisotropic behavior of cavity polaritons in an organic microcavity composed of a single crystalline anthracene film grown into the gap of the Fabry‐Perot cavity by a method through capillary action. The anisotropic response of the cavity polariton appeared on the spectra depending on the linear polarized directions; two kinds of the polarization correspond to the principal axes of anthracene crystal. Transmission spectra were measured with various thickness of the anthracene film. The peak‐shift of the lower polariton mode depending on the thickness was obviously observed, whereas that of the upper polariton mode was not clearly observed be cause of the complicated shape of the spectra. This behavior for the upper polariton mode was analyzed on the basis of the convolution between the spectra of the cavity mode and the absorption due to the vibronic transition in anthracene. Cavity‐polariton dispersion as a function of thickness was obtained from peak‐shift on the spectra. Each thickness was determined by spectral fitting through calculation using the transfer matrix theory. Large vacuum Rabi splitting energies of this system were obtained from the dispersion as 0.20 and 0.34 eV for two kinds of the polarizations. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Longitudinal coherence of organic-based microcavity lasers

We report on the measurement of the longitudinal coherence of organic microcavity lasers based on a conjugated polymer. By using a modified Michelson interferometer configuration enabling single-shot measurements of the coherence length, the transition from spontaneous emission to lasing is investigated. The measured coherence length grows upon increasing the pumping fluence, saturating around 45 microm above threshold. At large fluences, possible thermal and photo-oxidation processes occurring in the gain medium limit the further increase of the coherence length. Our results are important for understanding lasing emission in organic microcavities and optimizing the device design and performances.

Photophysics of J‐aggregate microcavities

AbstractWe have developed a model in order to account for the photo‐excitation dynamics of J‐aggregate films and strongly coupled J‐aggregate microcavities. The J‐aggregates are described as a disordered Frenkel exciton system in which relaxation occurs due to the presence of a thermal bath of molecular vibrations. The link between the J‐aggregate film and the microcavities is obtained introducing a model polariton wavefunction mixing cavity photon modes and J‐aggregate superradiant excitons. With the same approach we have calculated both absorption and luminescence spectra for the J‐aggregate film and the photoluminescence of strongly coupled organic microcavities. The model is able to explain the organic microcavities fast relaxation dynamics and polariton luminescence following non resonant pumping starting from the excitonic properties of the optically active layer. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Strongly coupled organic microcavities with vibronic progressions

AbstractThe optical properties of an organic polycrystalline film of NTCDA with one vibronic replica, embedded in a microcavity, were measured by Holmes and Forrest in 2004. They observed strong exciton‐photon coupling and hybridization between the cavity mode and both the 0‐0 and 0‐1 vibronic transitions. The purpose of this work is to build a model to describe optical spectra of an organic microcavity with vibronic replicas in both weak and strong coupling regimes between photon and exciton modes. The problem has been handled with two approaches, a microscopic model and a macroscopic one. The active material vibronic spectra have been calculated in the Franck‐Condon framework.With these models we have reproduced the experimental spectra and we have also analyzed more complicated systems such as anisotropic monocrystalline materials. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulation of J-aggregate microcavity photoluminescence

We have developed a model in order to account for the photoexcitation dynamics of J-aggregate films and strongly coupled J-aggregate microcavities. The J aggregates are described as a disordered Frenkel exciton system in which relaxation occurs due to the presence of a thermal bath of molecular vibrations. The correspondence between the photophysics in J-aggregate films and that in J-aggregate microcavities is obtained by introducing a model polariton wave function mixing cavity photon modes and J-aggregate super-radiant excitons. With the same description of the material properties, we have calculated both absorption and luminescence spectra for the J-aggregate film and the photoluminescence of strongly coupled organic microcavities. The model is able to account for the fast relaxation dynamics in organic microcavities following nonresonant pumping and explains the temperature dependence of the ratio between the upper polariton and the lower polariton luminescence.