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.
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.
Combining organic films with high Kerr-nonlinearities and highly
optimized photonic nanostructures could lead to new fast switching
elements. Fabry-Perot cavities are fabricated by incorporating an
organic material between two dielectric mirrors. Using femto-second
pump and probe measurements we characterize these hybrid 1-D
photonic band gap structures for various organic materials. By
varying the pump beam wavelength across the cavity resonance we are
able to delineate between the various underlying nonlinear
processes. Comparing these measurements with computations we are
able to quantify both the refractive and absorptive nonlinear
coefficients of various organic materials.
The mode coupling of organic distributed feedback lasers is enhanced by using a distributed feedback grating that is etched into a thin layer of titanium dioxide (TiO2). The use of TiO2 increases the index contrast in the grating and the confinement in the waveguide. The enhanced mode coupling results in larger feedback given to the lasing modes, which lowers the laser threshold and allows smaller devices to be built. The lasing threshold of the TiO2-enhanced devices is twice as low as that of conventional devices whose grating is etched directly into the quartz.
Organic two-dimensional photonic bandgap structures (2D PBG) have been fabricated by spin-coating a thin polymer film onto a nano-patterned SiO2 circular-grating surface-emitting distributed Bragg reflectors (CG-SE-DBR). When optically pumped and for certain grating parameters, these structures exhibit 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 excellent agreement with the experimental findings.
The phoenix project aims to develop all-optical switches based on
the combination of inorganic and organic materials in hybrid
devices. We present first results in developing low-loss ring
resonators fabricated in silicon-on-insulator (SOI) technology,
with Q-factors as high as 125.000, and losses of α≈3.5dB/cm in the ring.
We report on an optically pumped polymer laser based on a circular grating resonator. The circular gratings were fabricated by electron beam lithography into a fused silica substrate. Due to the importance of a precise circularity of the grating we used a Leica LION LV1 electron beam writer allowing for smooth curved grating lines. A thin film of a methyl-substituted ladder-type poly(p-phenylene), spin-coated onto the gratings, acts as active material. Since the grating period satisfies the second-order Bragg condition it provides a true 2D feedback for the emitted photons of the polymer. At the same time surface emission is achieved via first order diffraction normal to the sample surface. Using short pulse laser systems for optical pumping we observe a clear lazing threshold, highly directed emission and a narrow spectral linewidth. By changing the grating period one can tune the emission wavelength over the entire gain region of the polymer.
We report on a tunable laser based on a conjugated polymer blend system and investigate the underlying gain mechanism. A solid polymer blend consisting of the conjugated polymer poly(phenyl-p-phenylenevinyene) dispersed into an inert matrix of polymethylmetacrylat is examined. Emission line narrowing which can be attributed to amplified spontaneous emission (ASE) is observed at high excitation densities. Placing a block of the blend system into an external resonator yields true laser emission. Emission linewidth and peak intensity show a clear threshold behavior. The laser emission is highly collimated, coherent, and highly polarized. It can be tuned over a range of 300 meV. Gain spectra indicate that the gain mechanism can be explained within the molecular model for conjugated polymers in close analogy to the gain mechanism well known from dye lasers. Thin films of a methyl-substituted ladder-type poly(p- phenylene) are examined to check if the result obtained from a diluted system also hold for neat films. ASE is observed upon increasing the pump intensity. Additionally, quasi- resonant, spectrally very narrow emission lines can be observed. These emission lines are energetically offset from the excitation laser by energies corresponding to well known molecular vibrations. This confirms the previous assumption that the gain mechanism in conjugated polymers is linked to molecularly excited states.
High resolution cw photoluminescence spectroscopy, energy transfer studies, time and spectrally resolved fluorescence decay as well as quenching of photoluminescence by strong electric fields support the notion that photon absorption in PPV-type conjugated polymers creates neutral excitations. They undergo a random walk among segments of the polymer chain thereby relaxing energetically. In the presence of an electric field they can form off- chain geminate pairs acting as precursors for free charge carriers.