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, 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. The resulting high oscillator strength and a long exciton dephasing time 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. 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.
BaTiO3 (BTO) single crystals exhibit one of the largest Pockels coefficients (r42 > 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 50-70Å.
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 Förster theory.
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.
We investigate circular grating resonators (CGR) with a very small footprint. Photonic devices based on circular
grating resonators are computationally designed, optimized and studied in their functionality using finite
difference time-domain (FDTD) method. A wide variety of critical quantities such as transmission and reflection,
resonant modes, resonant frequencies, and field patterns are calculated. Due to their computational size some
of these calculations have to be performed on a supercomputer (e.g. parallel Blue Gene machine). The devices
are fabricated in SOI using the computational design parameters. First they are defined by electron-beam
lithography. Then the pattern transfer is achieved by an inductively coupled reactive-ion etch process. Finally,
the devices are characterized by coupling light from a tunable laser with a tapered lensed fiber. As predicted
from the simulations the measured transmission spectra exhibit a wide range of different type of resonances with
quality factors exceeding 1000.
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.