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Microcavities with embedded optically active materials allow to create exciton–polariton condensates in the strong light–matter interaction regime. These condensates exhibit quantum fluid properties up to room temperature, and, when crystal-like lattices are imprinted in the cavity, they can be used to emulate and study solid-state physics toy models. Here, we demonstrate room temperature polariton condensation in a nano-fabricated two-dimensional Lieb lattice with an organic polymer. We exploit the tunability of our open cavity to selectively condense into the s-, p- and d-lattice band manifolds. Furthermore, we interferometrically measure long-range first-order coherence across the lattice and assess the influence of the disorder in the system. These are key first steps to investigate extended topological polariton systems at ambient conditions. Artificial periodic lattices of microcavities filled with optically active materials can serve as a platform to explore a range of physical phenomena. Here, the authors prepare and analyse 2D Lieb lattices made from a tunable cavity array with an organic polymer which operates in the strong exciton–photon coupling regime and exhibits polariton condensation at room temperature.

Ligand-capped nanocrystals (NCs) of lead halide perovskites, foremost fully inorganic CsPbX3 NCs, are the latest generation of colloidal semiconductor quantum dots. They offer a set of compelling characteristics—large absorption cross section, as well as narrow, fast, and efficient photoluminescence with long exciton coherence times—rendering them attractive for applications in light-emitting devices and quantum optics. Monodisperse and shape-uniform, broadly size-tunable, scalable, and robust NC samples are paramount for unveiling their basic photophysics, as well as for putting them into use. Thus far, no synthesis method fulfilling all these requirements has been reported. For instance, long-chain zwitterionic ligands impart the most durable surface coating, but at the expense of reduced size uniformity of the as-synthesized colloid. In this work, we demonstrate that size-selective precipitation of CsPbBr3 NCs coated with a long-chain sulfobetaine ligand, namely, 3-(N,N-dimethyloctadecylammonio)-propanesulfonate, yields monodisperse and sizable fractions (>100 mg inorganic mass) with the mean NC size adjustable in the range between 3.5 and 16 nm and emission peak wavelength between 479 and 518 nm. We find that all NCs exhibit an oblate cuboidal shape with the aspect ratio of 1.2 × 1.2 × 1. We present a theoretical model (effective mass/k·p) that accounts for the anisotropic NC shape and describes the size dependence of the first and second excitonic transition in absorption spectra and explains room-temperature exciton lifetimes. We also show that uniform zwitterion-capped NCs readily form long-range ordered superlattices upon solvent evaporation. In comparison to more conventional ligand systems (oleic acid and oleylamine), supercrystals of zwitterion-capped NCs exhibit larger domain sizes and lower mosaicity. Both kinds of supercrystals exhibit superfluorescence at cryogenic temperatures—accelerated collective emission arising from the coherent coupling of the emitting dipoles. ISSN:2374-7951

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. ISSN:1936-0851 ISSN:1936-086X

Nanocrystal (NC) self-assembly is a versatile platform for materials engineering at the mesoscale. The NC shape anisotropy leads to structures not observed with spherical NCs. This work presents a broad structural diversity in multicomponent, long-range ordered superlattices (SLs) comprising highly luminescent cubic CsPbBr3 NCs (and FAPbBr3 NCs) coassembled with the spherical, truncated cuboid, and disk-shaped NC building blocks. CsPbBr3 nanocubes combined with Fe3O4 or NaGdF4 spheres and truncated cuboid PbS NCs form binary SLs of six structure types with high packing density; namely, AB2, quasi-ternary ABO3, and ABO6 types as well as previously known NaCl, AlB2, and CuAu types. In these structures, nanocubes preserve orientational coherence. Combining nanocubes with large and thick NaGdF4 nanodisks results in the orthorhombic SL resembling CaC2 structure with pairs of CsPbBr3 NCs on one lattice site. Also, we implement two substrate-free methods of SL formation. Oil-in-oil templated assembly results in the formation of binary supraparticles. Self-assembly at the liquid–air interface from the drying solution cast over the glyceryl triacetate as subphase yields extended thin films of SLs. Collective electronic states arise at low temperatures from the dense, periodic packing of NCs, observed as sharp red-shifted bands at 6 K in the photoluminescence and absorption spectra and persisting up to 200 K., ACS Nano, 16 (5), ISSN:1936-0851, ISSN:1936-086X

We use silicon sub-wavelength high index contrast gratings to create on-chip integrated multimode waveguides. We succeeded in guiding visible light at 600 nm wavelength with losses down to 6 dB/cm despite strong material absorption.

Self-assembly of colloidal nanocrystals (NCs) holds great promise in the multiscale engineering of solid-state materials, whereby atomically engineered NC building blocks are arranged into long-range ordered structures - superlattices (SLs) - with synergistic physical and chemical properties. Thus far, the reports have by far focused on single-component and binary systems of spherical NCs, yielding SLs isostructural with the known atomic lattices. Far greater structural space, beyond the realm of known lattices, is anticipated from combining NCs of various shapes. Here, we report on the co-assembly of steric-stabilized CsPbBr3 nanocubes (5.3 nm) with disk-shaped LaF3 NCs (9.2-28.4 nm in diameter, 1.6 nm in thickness) into binary SLs, yielding six columnar structures with AB, AB2, AB4, and AB6 stoichiometry, not observed before and in our reference experiments with NC systems comprising spheres and disks. This striking effect of the cubic shape is rationalized herein using packing-density calculations. Furthermore, in the systems with comparable dimensions of nanocubes (8.6 nm) and nanodisks (6.5 nm, 9.0 nm, 12.5 nm), other, noncolumnar structures are observed, such as ReO3-type SL, featuring intimate intermixing and face-to-face alignment of disks and cubes, face-centered cubic or simple cubic sublattice of nanocubes, and two or three disks per one lattice site. Lamellar and ReO3-type SLs, employing large 8.6 nm CsPbBr3 NCs, exhibit characteristic features of the collective ultrafast light emission - superfluorescence - originating from the coherent coupling of emission dipoles in the excited state. ISSN:1936-0851 ISSN:1936-086X

The recent progress in nanotechnology1,2 and single-molecule spectroscopy3–5 paves the way for emergent cost-effective organic quantum optical technologies with potential applications in useful devices operating at ambient conditions. We harness a π-conjugated ladder-type polymer strongly coupled to a microcavity forming hybrid light–matter states, so-called exciton-polaritons, to create exciton-polariton condensates with quantum fluid properties. Obeying Bose statistics, exciton-polaritons exhibit an extreme nonlinearity when undergoing bosonic stimulation6, which we have managed to trigger at the single-photon level, thereby providing an efficient way for all-optical ultrafast control over the macroscopic condensate wavefunction. Here, we utilize stable excitons dressed with high-energy molecular vibrations, allowing for single-photon nonlinear operation at ambient conditions. This opens new horizons for practical implementations like sub-picosecond switching, amplification and all-optical logic at the fundamental quantum limit. Nonlinearity induced by a single photon is desirable because it can drive power consumption of optical devices to their fundamental quantum limit, and is demonstrated here at room temperature.

Atomically defined assemblies of dye molecules (such as H and J aggregates) have been of interest for more than 80 years because of the emergence of collective phenomena in their optical spectra1,2,3, their coherent long-range energy transport, their conceptual similarity to natural light-harvesting complexes4,5, and their potential use as light sources and in photovoltaics. Another way of creating versatile and controlled aggregates that exhibit collective phenomena involves the organization of colloidal semiconductor nanocrystals into long-range-ordered superlattices6. Caesium lead halide perovskite nanocrystals7,8,9 are promising building blocks for such superlattices, owing to the high oscillator strength of bright triplet excitons10, slow dephasing (coherence times of up to 80 picoseconds) and minimal inhomogeneous broadening of emission lines11,12. So far, only single-component superlattices with simple cubic packing have been devised from these nanocrystals13. Here we present perovskite-type (ABO3) binary and ternary nanocrystal superlattices, created via the shape-directed co-assembly of steric-stabilized, highly luminescent cubic CsPbBr3 nanocrystals (which occupy the B and/or O lattice sites), spherical Fe3O4 or NaGdF4 nanocrystals (A sites) and truncated-cuboid PbS nanocrystals (B sites). These ABO3 superlattices, as well as the binary NaCl and AlB2 superlattice structures that we demonstrate, exhibit a high degree of orientational ordering of the CsPbBr3 nanocubes. They also exhibit superfluorescence—a collective emission that results in a burst of photons with ultrafast radiative decay (22 picoseconds) that could be tailored for use in ultrabright (quantum) light sources. Our work paves the way for further exploration of complex, ordered and functionally useful perovskite mesostructures., Nature, 593 (7860), ISSN:0028-0836, ISSN:1476-4687

The recent progress in nanotechnology

An ensemble of emitters can behave differently from its individual constituents when it interacts coherently via common vacuum light modes. One example of a many-body collective coupling is so-called superfluorescent coupling, where the excited emitters are initially fully uncorrelated and coherence is established through spontaneously triggered correlations from quantum fluctuations. Subsequently, the coupled emitters emit a strong superfluorescent pulse. Since this phenomenon requires low inhomogeneity and a fine balance of interactions between the resonant emitters and their decoupling from the environment, superfluorescence has only been observed in a limited number of systems, such as certain atomic and molecular gases and a few solid-state systems. Here, we investigate densely packed arrays of fully inorganic cesium lead halide perovskite quantum dots[1], known as superlattices. These quantum dots obtain exceptional optical properties such as an lowest bright triplet state with an ultrafast radiative decay that is 1000x faster compared to other conventional nanocrystals at cryogenic temperatures[2]. The resulting high oscillator strength and a long exciton dephasing time[3] are key ingredients for strong light-matter interactions. In a solvent-drying-induced assembly process, perovskite quantum dots form densely packed cuboidal superlattices that show key signatures of superfluorescence[4]. We observe a more than twenty-fold accelerated radiative decay with dynamically red-shifted emission, extension of the first-order coherence time by more than a factor of four, photon bunching and an intensity-dependent time delay after which the photon burst is emitted. Also, at high excitation density, the superfluorescent decay exhibits a Burnham-Chiao ringing behavior, reflecting the coherent Rabi-type interaction.

We create exciton-polariton (strongly coupled light-matter quasi particles) Bose-Einstein condensates at room temperature [1] by optically exciting a ladder-type conjugated polymer [2] placed inside a tuneable Fabry-Perot microcavity [3]. By Focussed Ion Beam (FIB) milling we fabricate different structure for inplane confinement, ranging from 0D (single Gaussian-shaped defect) to 2D ones (arrays of coupled Gaussian defects). In the first part of this work we focus on the basic building blocks of our system, the strong light-matter coupling and the exciton-polariton condensation localized in a 0D Gaussian-shaped defect structure. The strong coupling between the excitonic resonance of the conjugated polymer and the individual cavity modes is obtained by exciting the system while detuning the cavity length till the anti-crossing behaviour of the modes appears. By increasing the excitation power above threshold, we reach room temperature condensation in a single Gaussian defect. The condensation features are demonstrated by showing non-linear emission intensity, spectral line-narrowing and a characteristic blue shift as function of excitation power. To better characterize the condensation in such a system, we studied the angular emission (Fig. 1 (a) and (b)) and the first order coherence (Fig. 1 (c) and (d)) of the exciton-polaritons below and above threshold.

Using a ladder-type polymer in an optical microcavity in strong coupling, we demonstrate an all-optical transistor with record net gain of ~10 dB/μm, sub-ps switching time, cascaded amplification and all-optical logic operation at ambient conditions.

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.

We demonstrate strong light-matter interaction at ambient conditions between a ladder-type conjugated polymer and the individual modes of a vertical microcavity with tunable resonance frequencies. Zero-dimensional wavelength-scale confinement for the polaritons is achieved through a sub-micron sized Gaussian defect, resulting in a vacuum Rabi splitting of the polariton branches of 2g = 166 meV. By placing a second Gaussian defect nearby, we create a polaritonic molecule with tunnel coupling strength of up to 2J ~ 50 meV. This platform enables the creation of tailor-made potential landscapes with wavelength-scale dimensions and tunable coupling strengths beyond the thermal energy, opening a route towards room-temperature polariton-based quantum simulators.

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 CsPbBr

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_3, 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_3 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.

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., Nano Letters, 18 (12), ISSN:1530-6984, ISSN:1530-6992

We create exciton-polaritons in a zero-dimensional (0D) microcavity filled with organic ladder-type conjugated polymer in the strong light-matter interaction regime. Photonic confinement at wavelength scale is realized in the longitudinal direction by two dielectric Bragg mirrors and laterally by a sub-micron Gaussian-shaped defect. The cavity is separated into two parts allowing nanometer position control and enabling tuning of exciton and photon fractions of the polariton wavefunction. Polariton condensation is achieved with non-resonant, picosecond optical excitation at ambient conditions and evidenced by a threshold behavior with non-linear increase of the emission intensity, line narrowing and blue-shift of the emission peak. Furthermore, angular emission spectra show that condensation occurs in the ground state of the 0D-cavity, and first order coherence measurements reveal the coherent nature. These experiments open the door for polariton quantum fluids in complex external potentials at room temperature.

We demonstrate control and tunability of the exciton fine-structure splitting by properly engineering a nanojunction consisting of a CdSe nanocrystal core and an asymmetric rod-like CdS shell. Samples with small core and/or thick rod diameters exhibit a strongly reduced fine-structure splitting resulting from a reduced electron-hole exchange interaction. These results shed light onto the electronic configuration of such nanosystems and, apart from being of fundamental interest, could enable the use of colloidal nanocrystals as a source of entangled photons.

The exciton fine structure of small (2-3.5 nm) wurtzite (WZ) and zincblende (ZB) CdSe quantum dots (Qdots) has been investigated by means of nanosecond and picosecond time-resolved photoluminescence spectroscopy, at temperatures ranging from 5 K to room temperature. For both crystal structures, we observe a similar dark-bright energy level splitting of 2.4-5 meV, with a larger splitting corresponding to smaller Qdots. In addition, spectrally resolved streak camera images collected at 5 K reveal the presence of a third state, split from the lower dark-bright manifold by 30-70 meV, again independently of the crystal structure of the Qdots. The data thus reveal that small WZ and ZB CdSe Qdots are optically indistinguishable. This contrasts with theoretical calculations within the effective-mass approximation, which, in the limit of spherical Qdots, yield a different fine structure for both. However, experimental and theoretical results converge when taking the Qdot shape into account. With transmission electron microscopy, we determined that our Qdots are prolate, with an aspect ratio of 1.15:1. Incorporating this value into our calculations, we obtain a similar fine structure for both WZ and ZB Qdots. Moreover, the opposite sign of the crystal field and shape anisotropy in CdSe suggests that the lowest energy level in small CdSe Qdots has an angular momentum projection F = 0, in contrast with (perfectly) spherical Qdots, where the lowest level corresponds to the dark ±2 state. From the experimental and theoretical data we conclude that shape anisotropy and exchange interactions dominate over the crystal field anisotropy-induced splitting in this size range.

We investigate a hybrid nanocomposite combining fluorescent dyes and ultrasmall (

Colloidal semiconductor quantum structures allow controlling the strong confinement of charge carriers through material composition and geometry. Besides being a unique platform to study fundamental effects, these materials attracted considerable interest due to their potential in opto-electronic and quantum communication applications. Heteronanostructures like CdSe/CdS offer new prospects to tailor their optical properties as they take advantage of a small conduction band offset allowing tunability of the electron delocalization from type-I toward quasi-type-II. Here, we report on a detailed study of the exciton recombination dynamics in CdSe/CdS heterorods. We observed a clear size-dependent radiative lifetime, which can be linked to the different degree of electron wave function (de)localization. Moreover, by increasing the temperature from 70 to 300 K, we observed a considerable increase of the radiative lifetime, clearly demonstrating a reduction of the conduction band offset at higher temperatures. Understanding and controlling electron delocalization in such heterostructures will be pivotal for realizing efficient and low-cost photonic devices.

A polarization-independent photodetector device is demonstrated that can be combined with electronic integrated circuits on a single chip. The photodetector device is fully compatible with the standard silicon-on-insulator (SOI) complementary metal-oxide-semiconductor (CMOS) process without requiring process modification or postprocessing.

The integration of a thin layer of the high refractive index material Ta"2O"5 in organic laser structures is studied. A complete fabrication process of Ta"2O"5 based photonic crystals is described including deposition, electron beam lithography and reactive ion etching. Functional devices using methyl-substituted ladder-type poly(para-phenylene) (MeLPPP) as gain material are characterized with regard to the effectivity of the fabricated feedback structures.

We create exciton-polariton quasi-particles by exciting optically a microcavity filled with a ladder-type conjugated polymer in the strong coupling regime. At room temperature thermalization of these quasi-particles occurs while it is suppressed at low temperature due to a relaxation bottleneck. Above a certain excitation threshold with incoherent offresonant picosecond laser pulses, we observe the emergence of non-equilibrium Bose-Einstein condensation in the lower polariton branch. This is evidenced by several distinct features such as a blue-shifted emission peak at zero in-plane momentum, accompanied by a nonlinear increase in the emission intensity and a sudden drop of the line width. In contrast to conventional lasing, we find a strong increase in threshold when decreasing the temperature, which can be explained by the peculiar thermalization properties. Single-shot measurements of the emission spectrum allow studying single realizations of the condensate, giving access to non-averaged properties from each individual condensation process. Our approach demonstrates a radically simplified route to investigate Bose-Einstein condensation physics at ambient conditions with easy-to-process non-crystalline materials.

Photonic-crystal lasers whose feedback is enhanced by a thin layer of titanium dioxide (TiO2) have been experimentally investigated. The use of TiO2 increases the index contrast in the photonic crystal as well as the confinement in the waveguide, resulting in larger feedback of the lasing modes. This allows lower thresholds and much smaller devices. Vertically emitting laser devices, using the methyl-substituted ladder-type poly(p-phenylene) (MeLPPP) as gain material, have been fabricated according to optimized parameters, and the measured dispersion relation of the structures matches excellently with simulations. The devices feature a three to five times lower threshold than devices whose feedback structure is etched directly into the fused silica substrate.

The mode coupling of organic photonic crystal lasers is enhanced by using a photonic crystal that consists of a thin layer of a high-index material. The high-index material increases the index contrast, the confinement in the waveguide and thus the mode coupling. Using such a photonic crystal gives rise to new design criteria. We investigate these criteria, and employ them in the design of an organic photonic crystal laser. Calculations of the coupling constant of the organic laser show that using high-index materials results in much higher coupling constants and thus smaller devices.

Optically pumped organic polymer lasers are fabricated by spin coating a thin polymer film onto a nanopatterned SiO2 circular-grating surface-emitting distributed Bragg reflector. For certain grating parameters, we observe a peak inside the stop band that leads to lasing with a reduced threshold. An analytical model, based on the transfer-matrix method, has been developed to investigate the origin of this peak. The theoretical results are in good agreement with the experimental findings.

Strain in colloidal heteronanocrystals with non-centrosymmetric lattices presents a unique opportunity for controlling optoelectronic properties and adds a new degree of freedom to existing wavefunction engineering and doping paradigms. We synthesized wurtzite CdSe nanorods embedded in a thick CdS shell, hereby exploiting the large lattice mismatch between the two domains to generate a compressive strain of the CdSe core and a strong piezoelectric potential along its c-axis. Efficient charge separation results in an indirect ground-state transition with a lifetime of several microseconds, almost one order of magnitude longer than any other CdSe/CdS nanocrystal. Higher excited states recombine radiatively in the nanosecond time range, due to increasingly overlapping excited-state orbitals. k̇p calculations confirm the importance of the anisotropic shape and crystal structure in the buildup of the piezoelectric potential. Strain engineering thus presents an efficient approach to highly tunable single- and multiexciton interactions, driven by a dedicated core/shell nanocrystal design., Quantum dots confine electrons to a nanometre length scale, and this gives rise to numerous quantum effects. Here, the authors directly control the excitonic structure of nanocrystal quantum dots by manipulating intra-particle piezoelectric fields.

Optical organic ‘‘semiconductor’’ microcavities exhibiting strong coupling were prepared. Thin layers of the dye 1,1 0 diethyl-2,2 0 -cyanine (PIC), which is known to form J-aggregates, were used as the optically active material. Spin coating of a specific dye salt made it possible to prepare thin J-aggregate layers within a k=2-cavity without the need of any polymer-matrix. SiOx buffer layers were used to place the optically active J-aggregate layer at different positions inside the cavity. Transmission measurements revealed different vacuum Rabi-splitting energies between 26 and 52 meV. A dipole approximation was used to explain the dependence of the vacuum Rabi-splitting energy on the spatial position inside the cavity. 2002 Elsevier Science B.V. All rights reserved.