Organic Microcavity Research Articles

Reduced angular dependence of the emission from tris(8-hydroxyquinoline) aluminum based microcavity

Organic microcavity light emitting diodes typically exhibit a significant blue shift of the emission wavelength with the viewing angle. In this work, we demonstrate significant reduction of the emission angular dependence of a microcavity device consisting of two organic layers, N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) as a hole transport layer and tris(8-hydroxyquinoline) (Alq) as emitting layer, between two metal mirrors. The reduction of the angular dependence of the emission was achieved by optimizing the thickness of the layers within the device. The reasons for the reduced emission wavelength shift with the viewing angle are discussed.

Tuning the exciton-photon coupling in a strongly coupled organic microcavity containing an optical wedge

We have fabricated strongly coupled organic microcavities by laminating an organic thin film between two dielectric mirrors. The organic film consisted of J aggregates of a cyanine dye suspended in a transparent matrix. By applying a nonuniform force across the substrate during lamination, the optical path length of the cavity changed linearly across the sample, thereby forming a wedge. This enabled cavity mode tuning by changing the position of the incident light beam with respect to the surface of the cavity. Transmission measurements are employed to investigate strong exciton-photon coupling in such microcavities. The results show a well-resolved anti-crossing behavior, which is confirmed by numerical analysis.

Fast polariton relaxation in strongly coupled organic microcavities

We consider the regime of strong light-matter coupling in an organic microcavity, where large Rabi splitting can be achieved. As has been shown, the excitation spectrum of such a structure, besides coherent polaritonic states, contains a number of strongly spatially localized incoherent excited states. These states form the majority of the excited states of the microcavity and are supposed to play the decisive role in the relaxation dynamics of the excitations in the microcavity. We consider the non-radiative transition from an incoherent excited state into one of the coherent states of the lower polaritonic branch accompanied by emission of a high-energy intramolecular phonon. It is shown that this process may determine the lifetime of incoherent excited states in the microcavity. This observation may be important in the discussion of pump–probe experiments with short pulses. This process may also play an important role for the population of the lowest energy states in organic microcavities, and hence in the problem of condensation of cavity polaritons.

Near-infrared emission from tris(8-hydroxyquinoline) aluminum based microcavity light emitting diodes

Organic microcavity devices emitting in the near-infrared spectral region were fabricated. The devices consisted of two metal mirrors and two organic layers, N, N′-di(naphthalene-1-yl)- N, N′-diphenylbenzidine (NPB) as a hole transport layer and tris (8-hydroxyquinoline) (Alq) as emitting layer. Two different metals, copper and silver, were used as a bottom mirror, while silver was used as a top mirror. The devices were studied using reflectance, photoluminescence, and electroluminescence measurements. Angular dependence of the emission was also investigated. The origin and properties of this infrared emission are discussed.

Time-resolved and cw photoluminescence from strongly coupled organic microcavities

The optical properties of microcavities (MCs) are strongly dependent on both polarization of incident and emitted light and its angle of observation. Here we report the measurements of cw- and time-resolved photoluminescence (PL) observed at negative detuning and at resonance for s- and p-polarization in the strong coupling regime of a planar MC containing J-aggregates of a cyanine dye. Following non-resonant excitation, the emission spectra consist of three types of features: direct J-aggregate exciton emission, polariton emission, and uncoupled monomer emission through the transmission maxima of the distributed Bragg reflector beyond the stop-band. We compare our experimental results with a transfer-matrix calculation of the transmission for s- and p-polarization and explain the different positions of the polariton branches, the stop-band width, and the high- and low energy transmission maxima of the MC. Time-resolved PL experiments show an increase in the decay lifetime of the exciton-like mode when it is positioned far from the cavity mode. Close to resonance, the lower polariton branch decays with the natural lifetime of the J-aggregates.

Exciton-polariton dynamics in a transparent organic semiconductor microcavity

Exciton-polariton dynamics are studied in organic semiconductor microcavities operating in the strongly coupled polariton regime, with thin films of cyanine J-aggregates sandwiched between low-loss dielectric mirrors. The delocalized excitons are subject to polariton normal mode splittings which are modulated by near resonant femtosecond excitation pulses, while orientational disorder effects on the formation of polariton states play an important role in polariton dynamics.

Optically pumped lasing from organic microcavity

An optically pumped organic laser has been fabricated using a DCM (4-dicyanmethylene-2-methyl-6-4H-pyran)-doped Alq (tris (8-hydroxyquinoline) aluminum) film as the gain medium sandwiched between a high-reflectance distributed Bragg reflector and a silver reflector. The stimulated emission properties were investigated by short-pulse laser pumping. The intensity ratio of the resonant peak at 604 nm to the DBR (distributed Bragg reflector)-modulated free space emission band changed greatly with increasing pump energy density. The dependence on the pump energy density of the output properties including the cavity mode output, the total output, and the ratio of the cavity output versus total output, indicates a clear threshold energy density of 400 ± 20 µJ/cm2. The effective gain coefficient at the threshold was about 250 ± 14 cm–1. PACS No.: 42.55.Sa

Asymmetric Bragg mirrors for the reduction of emission wavelength dependence on the viewing angle in organic microcavity light emitting diodes

Asymmetric Bragg mirrors for the reduction of emission wavelength dependence on the viewing angle in organic microcavity light emitting diodes

Asymmetric Bragg mirrors for the reduction of emission wavelength dependence on the viewing angle in organic microcavity light emitting diodes

Organic microcavity light emitting diodes typically exhibit a blue shift of the emitting wavelength with increasing viewing angle. While the wavelength shift can be reduced with the appropriate choice of organic materials and metal mirrors, for further reduction of the emission wavelength shift it is necessary to consider a mirror whose phase shift can partly compensate the effect of the change of optical path within the cavity. In this work, we used a genetic algorithm (GA) to design an asymmetric Bragg mirror in order to minimize the emission wavelength shift with viewing angle. Based on simulation results, the use of asymmetric Bragg mirrors represents a promising way to reduce the emission wavelength shift. Detailed comparison between GA optimized and conventional Bragg mirrors in terms of resonant wavelength dependence on the viewing angle, spectral narrowing, and brightness enhancement is given.

Exciton-polaritons in a crystalline anisotropic organic microcavity

Investigation of polaritons in organic microcavities with strong light-matter interaction is of fundamental and practical interest. In the paper we study for the first time the dispersion of exciton–polaritons in a microcavity utilizing anisotropic organic crystals with one and two molecules per unit cell as optically active material. It is known that in bulk anisotropic organic crystals (like anthracene, tetracene and similar ones) the energies of Coulomb excitons are non-analytical functions of their wave vector k at small k (i.e. the energy of the exciton is different for different directions of the wave vector in the limit k → 0). We show that this is not the case in a planar microcavity where Coulomb exciton states are two-dimensional and the energy of Coulomb excitons at small wave vectors is an analytical function. Also, in contrast to widely used inorganic microcavities, in an anisotropic organic microcavity the cavity photon modes with both possible polarizations mix with the exciton states in the formation of polariton states. We demonstrate that these two factors result for small wave vectors in an almost isotropic dispersion of polaritons in an anisotropic organic crystalline microcavity, which can be observed in reflection, transmission and photoluminescence spectra of organic microcavity. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Organic microcavities with anisotropic optically active materials

We study the eigenmodes and the linear optics of an organic bulk microcavity whose active layer is formed of oriented molecular aggregates. Such active medium can be considered microscopically as uniaxial with optical axis determined by the orientation of the dipole moment of the excitonic transition. Using the 4 × 4 transfer matrix formalism we calculate the cavity polariton dispersion and the reflection and transmission spectra as a function of the in-plane wavevector, with special attention to the birefringent effect and to the formation of mixed symmetry polariton branches. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Strong coupling in high-finesse organic semiconductor microcavities

We report the fabrication of high-finesse strongly coupled microcavities composed of a polystyrene film doped with the dye tetraphenyl–porphyrin zinc positioned between two high reflectivity dielectric mirrors. The bottom mirror was deposited by plasma enhanced chemical vapor deposition, and was composed of 11 λ/4 thick (silicon oxide/silicon nitride) pairs. The organic layer was deposited on to this by spin coating. Finally, the top mirror was deposited by thermal evaporation and consisted of 12 λ/4 thick (tellurium oxide/lithium fluoride) pairs. Such cavities are characterized by Q factors of between 440 and 620. Strong coupling was evidenced via white light reflectivity measurements. Due to the high cavity Q factor, a Rabi splitting of 135 meV at resonance was very clearly resolved.

Strong coupling in organic semiconductor microcavities

We discuss the similarities and differences between inorganic and organic semiconductor microcavities operating within the strong coupling regime and identify novel features of the organic systems that can be exploited in new structures. Experimental results for σ-conjugated poly(n-butylphenylsilane) [PBPS] based structures show a giant Rabi splitting energy of between 210 and 230 meV. PBPS represents the first of a new class of organic materials, namely the σ-conjugated polysilanes, that are well suited for strong coupling studies. The polysilanes possess large oscillator strengths, large exciton binding energies and can have relatively narrow exciton linewidths. These properties ensure large Rabi energies and allow observation at room temperature. The experimental results for both cavity reflection and emission spectra correspond well to our transfer matrix model calculations. We report the successful engineering of a flat dispersion for the lower polariton branch that results in insensitivity to emission angle for the photoluminescence emission peak. This invariance is promising for display devices that use microcavities to define colour since it ensures that there is a negligible blue-shift of the emission for off-axis viewing. As other examples of novel features that can be obtained with organic systems we describe the use of birefringent polymers to mediate multi-component polariton interactions and describe the opportunities for amplifiers based on stimulated scattering of polaritons. We also discuss prospects for inorganic/organic hybrid materials that may allow the rules that apply to matter and light to be further stretched, potentially seeding a new paradigm in optoelectronic devices.

Organic Microcavity Photodiodes

AbstractThe interaction of light and matter lies at the heart of the principle of optoelectronic devices. By tuning the strength of the electric field component of the light wave, one can gain control over this interaction. A simple way of achieving this is by employing microcavities, which are one‐dimensional photonic structures. These give rise to an effective quantization of the light field in one direction. The largest enhancements in the strength of light–matter coupling are achieved for cavities with dimensions on the order of the effective wavelength of light. As organic materials have the very large oscillator strengths required for light–matter coupling, as well as excellent thin film forming properties, they are ideal materials with which to exploit tunable electron–photon coupling. We demonstrate the influence of the optical field strength in organic microcavity photodiodes. Besides allowing tunability of the response spectrum by varying the effective resonator thickness, a large increase in the photocurrent sensitivity is observed below the absorption threshold of the optically active material. The microcavity induced field enhancement plays a particularly important role under two‐photon excitation. In this case we observe a 500‐fold increase in the photocurrent response with respect to a non‐cavity device. This opens up a range of applications for organic microcavity photodiodes as nonlinear detector elements.

Narrow-band polarized light emission from organic microcavity fabricated by sol-gel technique

We report the fabrication and optical properties of an organic light-emitting microcavity. The cavity is a laser dye-doped polycarbonate film sandwiched between two Bragg reflectors, fabricated by alternately depositing TiO2 and SiO2 sol-gel thin films. Reflectivity up to 98% was achieved using the Bragg reflectors. The cavity has a photonic band gap between 597 and 789 nm with a defect state inside the gap. Narrow-band optical pumped light emission from the cavity and its polarization dependence were observed. A cavity quality factor as high as 354 was achieved. Angular and polarization dependence of light emission were analyzed using the transfer matrix method. All experimental results agree well with theoretical calculations.

Photoluminescence emission and Raman scattering polarization in birefringent organic microcavities in the strong coupling regime

We have investigated the polarization properties of photoluminescence and Raman scattering in strongly coupled organic microcavities containing cyanine dye J aggregates. The birefringence of radially aligned J aggregates results in a large energy splitting of polariton modes when the electric field of the incoming laser light is perpendicular or parallel to the alignment direction. This splitting allows the degree of polarization of doubly resonant Raman processes involving the vibrational modes of J aggregates to be controlled, where the ingoing and outgoing channels are in resonance with the lower polariton branch. As well as providing insight into the properties of polaritons in organic microcavities, these experiments are a sensitive probe of alignment effects arising during the spin-coating deposition process.

One- and two-photon photocurrents from tunable organic microcavity photodiodes

We have constructed multilayer thin-film organic microcavity photodiodes with the photoactive layer comprised of a spin-coated conjugated polymer and an evaporated C60 layer. The electrodes are designed as semitransparent mirrors which form a resonant cavity structure. The photocurrent spectra show distinct maxima at the optical resonances of the cavities, which are located up to 200 nm below the fundamental optical transition of the polymer. The design allows a simple tuning of the spectral response by varying the layer thickness. Microcavity photodiodes are also shown to be highly sensitive two-photon detectors, which exhibit a factor 500 improvement in the two-photon response compared to devices without photonic confinement.

Polariton emission from polysilane-based organic microcavities

We report the observation of strong coupling between exciton and photon modes in a conjugated polymer-based semiconductor microcavity. Thin films of the σ-conjugated poly[bis(p-butylphenyl)silane] (PBPS) were inserted between metal and dielectric mirrors to form the microcavity structures. Variation of the PBPS film thickness between 80 and 140 nm allowed the cavity photon resonance to be tuned in the vicinity of the free exciton energy. The expected anticrossing behavior, with intensity and linewidth averaging, was observed at room temperature in the cavity reflection spectra and the vacuum Rabi splitting was found to be ⩽430 meV. This large value is consistent with the expectations of transfer matrix reflectivity calculations performed with optical constants data derived from a Kramers–Kronig analysis of the PBPS absorption spectrum. Angle-dependent photoluminescence measurements were performed for the microcavity with a 120 nm thickness PBPS layer. Unlike the emission from a standard, weakly coupled, cavity, the polariton emission shows almost no blueshift with angle, a desirable feature for potential display applications.

Low-threshold lasing in a microcavity of fluorene-based liquid-crystalline polymer blends

We report the lasing characteristics of a microcavity device made of fluorene-based luminescent polymer blends with liquid crystallinity. Poly(2,7-bis(p-stiryl)-9,9′-di-n-hexylfluorene sebacate) (PBSDHFS) and poly(9,9′-di-n-hexyl fluorenediylvinylene-alt-1,4-phenylenevinylene) (PDHFPPV) were used to comprise a binary blend system. We employed the Förster-type energy transfer from a liquid crystalline donor to a non-liquid-crystalline acceptor to obtain a low lasing threshold. The binary blend film of PBSDHFS/PDHFPPV (98/2 by wt) demonstrated a very low-threshold energy (∼3 nJ/cm2/pulse) for microcavity lasing, which is lower than any other values previously reported on the organic or polymeric microcavity devices with metal or dielectric mirrors. This result implies that the liquid crystalline polymer blends could be a good candidate for the gain material of photo and electrically pumped lasing devices.

Enhanced chromaticity of organic electroluminescence from siliconbased organic microcavity

Electroluminescence (EL) from Sibased organic microcavity has firstly been reported in the literature. The microcavity is made up of the central active multilayers sandwiched between a silver film and a porous silicon Bragg reflector (PSDBR), formed by electrochemical etching of p+Si substrate in the electrolyte of HF: C2H5OH:H2O. The central active multilayers consist of Al (1 nm) / LiF (05 nm) / Alq3 / Alq3: DCJTB / NPB / CuPc / ITO / SiO2． The reflectivity (relative to an Al mirror) of the PSDBR is up to 99%, and the stopband is about 160 nm wide. Resonant cavity mode appears as a tip in the reflectivity spectrum of the Sibased organic multilayer films, indicating that the Sibased organic multilayer structure is indeed a microcavity. The peak widths of the EL spetra are greatly reduced from 70 nm to 12 nm as compared with those measured from noncavity structures. Note that the EL emission from the cavity devices is singlemode, and the offresonant optical modes are highly suppressed. Moreover, an increase of a factor of about 4 of the resonant peak intensity is observed. In addition, the currentbrightnessvoltage characteristics and the effect of parameters on the lifetime of the cavity devices are also discussed. The present technique for obtaining enhanced EL emission from Sibased organic microcavity may also be another novel effective method for realizing allSibased photonic devices and optoelectronics device integration.