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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 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.

BaTiO 3 (BTO) single crystals exhibit one of the largest Pockels coefficients (r 42 > 1000 pm/V) among oxides. This makes BTO an excellent active material for electro-optical (EO) devices such as switches, modulators or tuning elements. However, in order to harness these properties in silicon photonics circuits, the challenge is to integrate BTO as high quality thin films onto Si substrates. The effective Pockels coefficients can be enhanced in epitaxial films due to their tight relationship with the crystallographic symmetry and microstructure. We report on the EO properties of epitaxial BTO thin films on Si. The growth of BTO layers on Si(001) is performed by molecular beam epitaxy (MBE). A thin single-crystalline strontium titanate seed layer is grown on Si, followed by a 130 nm thick BTO layer. Electrodes to provide an electrical field parallel to the surface are patterned on the films using photolithography. Throughout this process, the BTO keeps an epitaxial relationship to the Si-substrate. Considering the tensor nature of the Pockels effect, the optical behavior of the BTO layers upon applying an electrical field is simulated, taking into account the films' crystalline multi-domain structure. An experimental way to access these EO properties is discussed, which utilizes polarization changes of a transmitted laser beam upon applying an electrical field to the film. Simulations of the measurement signals demonstrate the capability of resolving the expected EO response of the samples, which serves as a promising base for future experiments.

We present a design concept for an optimized surface-emitting two-dimensional second-order feedback structure consisting of an array of holes within a dielectric material surrounded by a mirror rim. The mirror rim consists of a first order photonic crystal structure. The lasing properties of such feedback structures with organic gain material are investigated theoretically and experimentally.

Chemically synthesized colloidal quantum dots can easily be incorporated into conjugated polymer host systems allowing for novel organic/inorganic hybrid materials combining the natural advantages from both organic as well as inorganic components into one system. In order to obtain tailored optoelectronic properties a profound knowledge of the fundamental electronic energy transfer processes between the inorganic and organic parts is necessary. Previous studies have attributed the observed efficient energy transfer to a dipole-dipole coupling with Foerster-radii of about 50-70A. Here, we report on resonant energy transfer of non-equilibrium excitons in an amorphous polyfluorene donor CdSe/ZnS core-shell nanocrystal acceptor system. By time-resolved photoluminescence (PL) spectroscopy we have investigated the PL decay behavior of the primarily excited polyfluorene as a function of temperature. We show that the transfer efficiency drops from about 30% at room temperature to around 5% at low temperature. These results shed light on the importance of temperature-activated exciton diffusion in the energy transfer process. As a consequence the exciton has to migrate very close to the surface of the quantum dot in order to accomplish the coupling. Hence, the coupling strength is much weaker than anticipated in previous work and requires treatment beyond Forster theory.

We investigate circular grating resonators (CGR) with a very small footprint. Photonic devices based on cir-cular grating resonators are computationally designed, optimized and studied in their functionality using “nite-dierence time-domain (FDTD) method. A wide variety of critical quantities such as transmission and re”ection,resonant modes, resonant frequencies, and “eld patterns are calculated. Due to their computational size someof these calculations have to be performed on a super computer (e.g. parallel B lue Gene machine). The de-vices are fabricated in SOI using the computational design parameters. First they are de“ned by electron-beamlithography. Then the pattern transfer is achieved by an inductively coupled reactive-ion etch process. Finally,the devices are characterized by co upling light from a tunable laser with a tapered lensed “ber. As predictedfrom the simulations the measured transmission spectra exhibit a wide range of dierent type of resonances withquality factors exceeding 1000.

We present the design of an optimized mixed-order photonic crystal laser structure. The lasing properties of this two-dimensional photonic crystal structure with an organic gain material are investigated theoretically and experimentally. A feedback structure fabricated in a thin film of Ta2O5 increases both the index contrast from the gain material as well as the optical confinement. Furthermore, by combining first order photonic crystal structures with second order ones losses occurring at the edge of the second order structure are dramatically reduced leading to a lower laser threshold and / or to a much smaller footprint of the laser.

Circular grating resonators could lead to the development of very advanced silicon-on-insulator (SOI) based nano-photonic devices clearly beyond state of the art in terms of functionality, size, speed, cost, and integration density. The photonic devices based on the circular grating resonators are computationally designed and studied in their functionality using finite-difference time-domain (FDTD) method. A wide variety of critical quantities such as transmission and field patterns are calculated. Due to their computational size some of these calculations have to be performed on a supercomputer like a massive parallel Blue Gene machine. Using the computational design parameters the devices are fabricated on SOI substrates consisting of a buried oxide layer and a 340-nm-thick device layer. The devices are defined by electron-beam lithography and the pattern transfer is achieved in a inductively coupled reactive-ion etch process. Then the devices are characterized by coupling light in from a tunable laser with a lensed fiber. As predicted the measured transmission spectra exhibit a wide range of different type of resonances with Q-factors over 1000 which compares very well with the computations.