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Low-temperature site-selective fluorescence (SSF) spectroscopy is employed to study morphological effects on the conformation of poly(p-phenylene vinylene) (PPV) and its phenyl-substituted, soluble derivative poly(phenylphenylenevinylene) (PPPV). Samples of PPV prepared as spin-coated thin films and stretch-aligned free-standing films, and samples of PPPV prepared as cast films and as blends with poly(methylmethacrylate) and polycarbonate have been studied. The results that the authors present are considered with the notion that each polymer sample consists of an array of ordered chain segments whose average length reflects the perfection of the local structure. The statistical distribution of the segment lengths is responsible for inhomogeneous broadening of the optical spectra (absorption and emission). The dominant electronic excitation created by photoexcitation across the pi - pi * energy gap is a singlet exciton that can execute a random walk among the chain segments. SSF spectroscopy allows the authors to distinguish the contributions to the apparent fluorescence Stokes shift that arise from energy relaxation through excitation migration (spectral diffusion) and from structural relaxation of the polymer chain (self-localization). The structural contribution to the Stokes shift approaches zero in well aligned PPV and reaches values of up to 500 cm-1 in highly disordered PPPV films. The SSF method also provides a means of assessing the extent of phase separation that occurs in PPPV blends.

We have investigated the stimulated emission of an evaporated thin Blm of alpha -sexithienyl at 20 g. We have performed femtosecond differential transmission measurements with excitation energies around 2.35 eV and probing the spectral region from 2 to 2.65 eV. The results are compared to time-resolved photoluminescence spectra. We observe amplification of the probe light at the spectral position of a sharp peak measured in time-resolved photoluminescence. The stimulated emission decays with a time constant of 2.5 ps

Polycrystalline powders and thin films of a novel rotaxane, methyl-exopyridine-anthracene rotaxane (EPAR-Me), and of the related thread and stoppers 10-[3,5-di (ter butyl)phenoxy]decyl-2-({2-[(9-anthrylcarbonyl) amino] acetyl}amino) acetate (ANTPEP), have been characterised by photoluminescence, absorption and photoluminescence excitation spectroscopy. A rather unusual, i.e. unstructured and largely red-shifted, photoluminescence spectral behaviour of the rotaxane has been found. Preliminary time resolved measurements indicate a fast energy transfer from the anthracene unit to different species the nature of which is still not assigned.

We demonstrate an all-optical switch based on a waveguide-embedded 1D photonic crystal cavity fabricated in silicon-on-insulator technology. Light at the telecom wavelength is modulated at high-speed by control pulses in the near infrared, harnessing the plasma dispersion effect. The actual absorbed switching power required for a 3 dB modulation depth is measured to be as low as 6 fJ. While the switch-on time is on the order of a few picoseconds, the relaxation time is almost 500 ps and limited by the lifetime of the charge carriers.

A planar nanophotonic Fabry-Perot-like resonator that can defy strong absorption of about 20 000 cm(-1) in the cavity material is demonstrated. Visible laser emission is observed from two silicon subwavelength-sized high index contrast gratings with embedded polymer gain material. The size of the laser is reduced by an order of magnitude compared to established designs based on photonic bandgap structures. As silicon constitutes the most common carrier for electronics, the cost-efficient integration of compact laser sources for visible wavelengths comes within reach.

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.

Arguments are presented in favour of an exciton description of the optical absorption of poly(phenylenevinylenes) (PPVs) and polythiophenes (PTs). (i) Absorption spectra can be deconvoluted into a series of inhomogeneously broadened vibronic transitions characteristic of chromophores no longer than about 10 repeat units. (ii) Site-selective fluorescence studies are in accord with the concept of spectral diffusion among chromophores differing in excitation energy. (iii) Electro-absorption measurements bear out a quadratic Stark effect acting on an exciton the spatial extent of which is 2–3 repeat units only; no evidence for transitions among delocalized states is found. (iv) Action spectra of intrinsic photoconductivity can be interpreted in terms of disorder-promoted expansion of singlet excitons to form geminate e-h pairs that can subsequently dissociate. (v) By comparison with the absorption spectra of chemically prepared anions of oligo-and poly(phenylenevinylenes) the photoinduced absorption of PPV is assigned to the absorption of monovalent ordered polymer segments of finite length.

Site-selective fluorescence spectroscopy has been employed to study spectral diffusion and site relaxation in molecularly doped polymers (TAPC and DEASP in polycarbonate binder). The width of the inhomogeneously broadened manifold of singlet states turns out to be 6–10 times larger than the binding energy of excitonic polarons. Adopting the same ratio for charge carriers suggests that the contribution of polaron effects on the activation energy of charge carrier transport does not exceed a few percent and is negligible compared to the contribution arising from disorder.

We have investigated the nature of primary excitations in ?-sexithienyl thin film by comparing excitations above and below the charge transfer exciton. We performed femtosecond differential transmission spectroscopy at low temperature. In the probed spectral region from 2 to 2.8 eV, we found a bleaching of the first excited singlet with a superimposed absorption into higher excited gerade states. During the first few picoseconds, the dynamics of the photoexcitations in the range of the first excited singlet manifold are independent of the excitation photon energy. When exciting below the origin of the first excited singlet, we observe an optical Stark effect. Thermal effects dominate the change in transmission for longer delay times for all excitation energies.

We study circular grating resonators (CGRs) which are formed by a central defect surrounded by concentric rings composing a grating and which display perfect azimuthal modal-symmetry. Because of their radial symmetry they exhibit a complete band gap for a minimal index contrast. However, as is the case for all 2D resonators their quality factors are limited by vertical losses. To reduce the vertical losses we introduce a chirp of the grating period by reducing it towards the central defect. The chirped CGRs exhibit drastically improved quality factors of up to tens of millions with a modal volume of a few cubic wavelengths.

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.

Low-temperature site-selective fluorescence spectroscopy was performed on PDBG present either as solid film or dissolved in a MTHF glass. The spectra are discussed in terms of spectral diffusion of singlet excitations within two different density-of-states (DOS) distributions. The higher-energy DOS derives from disordered chain segments that are subject to structural relaxation after excitation, while the lower-energy DOS is related to rigid ordered chain segments differing in length. The analogy with charge transport is pointed out.

Low temperature site-selective fluorescence spectroscopy was performed on poly (di- n -hexylsilylene) and poly (methyl- phenylsilylene), both present as solid film and matrix-isolated in a 2-MTHF glass. The experiments confirm that absorption and emission are exciton processes controlled by disorder arising from the length distribution of ordered chain segments that depends on the morphology of the sample. Upon exciting into the center portion of the density of states (DOS), excitations relax to a common occupational density of states mapped by the inhomogeneously broadened fluorescence spectrum. Upon tail state excitation, resonant emission shifting linearly with the excitation energy becomes progressively important. Coupling to both phonons and vibrations of the chain is very weak. The experiments are in quantitative accord with the results of Monte Carlo computer simulations for the random walk of excitations in an array of hopping sites featuring a Gaussian distribution of site energies. The existing analogy between exciton and charge transport suggests that site-selective fluorescence studies are a useful tool also for delineating the key features of charge carrier motion in polymeric systems.

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 Förster radii of about 50-70 A. Here, we report on resonant energy transfer of nonequilibrium 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 couple to the quantum dot. Hence, the coupling strength is much weaker than that anticipated in previous work and requires treatment beyond Förster 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.

L'etude spectroscopique demontre la generation d'etats excitoniques delocalises sur jusqu'a 10 unites repetitives et faiblement couples aux distorsions du reseau (phonons)

By comparing the widths of spectral holes burnt into either the electronic origin band or the first vibronic band of anthracene, tetracene and pentacene doped into MTHF or alcoholic glasses with the width of their non-resonant replica holes, the inhomogeneous broadening of the vibrational energies has been measured. It is site-dependent. Evidence is presented for spectral diffusion caused by vibrational excitation.

A detailed time resolved investigation of the photoluminescence of a thin tetracene film deposited on highly oriented pyrolytic graphite is presented. In agreement with Lim et al. [Phys. Rev. Lett. 92, 107402 (2004)], we find strong evidence for superradiance: an increase of the relative intensity of the pure electronic transition with respect to the vibronic sideband and a concomitant decrease of the radiative lifetime from 10 to 1.83 ns upon cooling from 300 to 4 K. For lower temperatures, a redshift (approximately 200 cm(-1)) of the free exciton is observed. Previously, this shift was attributed to a structural phase transition. Our time resolved spectra reveal that the spectral shift is related to a dynamical relaxation process which occurs within the first 50 ps.

We report on stimulated emission from a solid conjugated methyl-substituted ladder-type poly(paraphenylene). The threshold for stimulated emission as well as the spectral narrowing is measured. Upon exciting within the low energy tail of the S1-S0 (0–0) absorption band narrow emission lines offset from the excitation by the energies of the dominant vibrational modes are observed. From these measurements we conclude that vibronic sublevels play a major role in the optical gain mechanism. Hence, the gain mechanism can be explained in terms of molecularly excited states.

This paper addresses the issue of in-plane coupling into and out of circular-grating resonators with optical waveguides for all-optical switching applications. Two different designs are proposed: the first design consists in directly accessing the cavity with the waveguides. The second design is based on optimization with an evolutionary algorithm that randomly reshapes the gratings along the waveguides. Passive linear simulations for both approaches are performed with the finite-elements method, and the results will be presented.

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.

We report on the optical properties of tris-(stilbene)amine (TSA) and its application as a light-emitting layer in a microcavity. Time-resolved photoluminescence spectroscopy on a blend of TSA in polycarbonate (PC) demonstrates that the dynamics of the optical excitations significantly depend on the detection energy and temperature. This behaviour is characteristic for hopping relaxation within an inhomogeneously broadened density of states. We have realized a microcavity based on TSA where the emission shows spectral narrowing, intensity enhancement and a change of the radiative lifetime at the wavelength resonant to the cavity mode.

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.

Optically pumped organic lasers are fabricated onto a 2-D photonic band gap structure. Lasing occurs at the band-edge or in the band gap in excellent agreement with our modeling. Furthermore, frequency up-converted lasing is demonstrated.

The dynamics of photoexcitations in conjugated polymers and polymer blends is studied by means of time resolved luminescence spectroscopy. After excitation into the vibronic progression, the luminescence rises quasi-instantaneously. The luminescence decay depends strongly on the detection energy and the polymer concentration. The results are explained within a hopping model.

We investigated a phenyl-substituted poly(p-phenylenevinylene) (PPPV) blended into a polymethylmetacrylat (PMMA) matrix in a ratio of 0.1% by weight. The absorption and photoluminescence spectra show a large spectral shift between absorption and emission that efficiently reduces reabsorption of the emission. A block of the polymer blend with polished end faces was placed into a resonator, consisting of a Littrow grating and an output coupler. The polymer laser was transversely pumped with a dye laser.

Microcavities containing a J-aggregate forming organic dye were prepared in the strong-coupling regime. Changing the position of the neat organic films within the microcavity allows a tuning of the giant Rabi splitting observed. We set the Rabi splitting to 25 meV and study the thermal activation of the higher-energy polariton emission under excitation off resonance. This is found to exhibit an Arrhenius activation with an energy corresponding to the polariton branch splitting. A constant offset to the upper polariton is observed at low temperature. Microsecond gated spectroscopy reveals that thermalization of the system occurs on the time scale of milliseconds, which contrasts with the polariton lifetime in the range of picoseconds. Bulk sample heating cannot fully account for the slow polariton thermalization observed in steady-state experiments, suggesting that long-lived macroscopic polariton coherence influences the polariton relaxation.

Picosecond luminescence studies on poly(p‐phenylene vinylene) (PPV) and related compounds in the temperature range of 10–300 K are reported. We identify a fast nonradiative and considerably slower radiative recombination channel. The fastest decay is observed in PPV. In the phenyl substituted derivative poly(phenyl‐p‐phenylenevinylene) a significant contribution from a slow component (∼1.3 ns) is found. Dilution of the polymer in a blend leads to an increase of the contribution of the slow component. We attribute the fast decay to luminescence quenching due to energy dependent trapping of mobile excitations whereas the slow component reflects the radiative recombination.

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.

Rotaxanes are a new class of functional materials. They consist of mechanically interlocked supermolecular systems where a central linear thread passes through the cavity of a macrocyclic ring. The ring is constrained within the two extremities of the thread by two bulky stoppers. In this work, we report on spectroscopic characterisation of an anthracene-based novel rotaxane (EPAR-Me) and on the electroemission (EL) from a basic rotaxane component (ANTPEP). An OLED has been prepared using the anthracene-based thread (ANTPEP) of the EPAR-Me rotaxane in polycarbonate matrices. Spectroscopic measurements are reported, supporting excimer-like EL and field dependent electroexcimer for the ANTPEP thin film. Tentative assignment of emitting species of EPAR-Me are also discussed.

Upon photo-excitation an organic molecule is promoted to a non-equilibrium state from which it relaxes back to the ground state. Time-resolved photo- luminescence spectroscopy is the method of choice for obtaining information on the relaxation pathway(s). If the primary excitation event were to create a vibrationally hot singlet (S1) state or a higher lying Sn state, then relaxation can, in principle, involve vibrational cooling, internal conversion, intersystem crossing, readjustment of the molecular selection, and, finally, radiative or non-radiative decay to the ground state. In dense media solvation effects, as well as energy transfer and dissociation into a pair of charge carriers, can also occur. Which of these processes can be monitored by time-resolved spectroscopy depends on the time resolution of the experiment. Conventional methods with ns-time resolution yield the lifetime of the emitting states, the preceding history of that state remaining concealed, however. The advent of picosecond (ps) and femtosecond (fs) laser systems as excitation sources and of Streak camera and up-conversion techniques on the detection side allow the extension of the time resolution of fluorescence spectroscopy into the sub-picosecond regime. This has opened the possibility of studying ultra-fast photophysical relaxation processes such as the solvation of an excited chromophore in liquid solution, dephasing of optical excitations, and energy transfer in condensed systems. This article will focus on the latter aspect drawing heavily on the application of time and spectrally resolved fluorescence to elucidate the fate of optically excited states in conjugated polymers and their oligomeric counterpart structures.

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.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report on the realization of a solid state Fabry-Perot-like microcavity that uses a small Gaussian-shaped deformation inside the cavity to achieve strong lateral photon confinement on the order of the wavelength. Cavities with a mode volume V 1000 are fabricated by means of focused ion beam milling, removing the necessity for etched sidewalls as required for micropillar cavities. Perylene-diimide dye doped polystyrene was embedded in the microcavity and probed by time-resolved microphotoluminescence. A Purcell enhancement of the spontaneous emission rate by a factor of 3.5 has been observed at room temperature.

We present a Monte Carlo study of the relaxation and dissociation dynamics of optical excitations in poly(p-phenylenevinylene). The quantitative comparison with experimental data gives a complete picture of the excitation dynamics, the relevant parameters of the spectral relaxation, and the trapping processes involved. Our simulations are extended to include the primary step of the dissociation of a neutral photoexcitation in an electric field. The comparison with the results of field-induced photoluminescence quenching experiments enables us to understand the dissociation dynamics and to determine the exciton binding energy to 0.3\ifmmode\pm\else\textpm\fi{}0.1 eV. \textcopyright{} 1996 The American Physical Society.

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.

Polythiophene is an interesting system for its remarkable charge transport and optical properties as well as for the investigation of the electronic excitations and their dynamics in one-dimensional structures. However, conformation and configuration defects in the real material result in physical properties depending on sample preparation and processing1–3. On the contrary, thiophene oligomers are well-defined chemical systems with a precise structural definition of the condensed phase and provide considerable insight into the physical properties of polythiophene4–6. Heteromultilayer organic structures represent one further step for the understanding of basic theoretical aspects such as the electronic level ordering and the dynamics of optical excitations. Considerable theoretical attention has been recently devoted to the linear and non-linear optical properties of organic multilayer systems7–10. Besides the electronic properties of a single molecule the disorder present in the thin films leads to strong implications. It is, in fact, well kown that the optical and electrical properties of organic heteromultilayer structures are highly sensitive to the degree of structural order. Recently, it has been shown that highly oriented oligo-thiophene polycrystalline thin films can be grown by vacuum sublimation11.

We present a design concept for an optimized surface-emitting laser with two-dimensional feedback structure and organic gain material. The basic laser structure consists of an array of holes within a thin film of Ta2O5. The optical properties of such feedback structures are investigated theoretically and experimentally. Combining first-order with second-order photonic structures leads to a higher quality factor of the feedback structure, resulting in a lower laser threshold and/or a much smaller footprint of the laser.

We demonstrate the suitability of microcavities based on circular grating resonators (CGRs) as fast switches. This type of optical resonator is characterized by a high quality factor and very small mode volume. The waveguide-coupled CGRs are fabricated with silicon-on-insulator technology compatible with standard complementary metal-oxide semiconductor (CMOS) processing. The linear optical properties of the CGRs are investigated by transmission spectroscopy. From 3D finite-difference time-domain simulations of isolated CGRs, we identify the measured resonances. We probe the spatial distribution and the parasitic losses of a resonant optical mode with scanning near-field optical microscopy. We observe fast all-optical switching within a few picoseconds by optically generating free charge carriers within the cavity.

The electroactive and photoactive properties of conjugated systems have attracted the interest of many laboratories, both for a basic understanding of their photophysics as well as for possible applications in organic-based optoelectronic devices, α-sexithienyl has been recognized as a model compound for a basic understanding of the electronic properties of conjugated materials. We report on ultrafast pump-probe measurements on ultra-high vacuum deposited thin films of α-sexithienyl, Femtosecond transient spectroscopy experiments were performed at low temperatures and over a wide wavelength range of the probe beam (from 420 nm up to 680 nm). The dynamics of the photoexcited species are discussed in the frame of the excitonic model.

The lasing properties of an optimized two-dimensional photonic crystal structure with an organic gain material are investigated. The feedback structure which is fabricated in a thin film of Ta2O5 increases both the index contrast from the gain material as well as the optical confinement. By combining first-order and second-order photonic crystal structures, losses occuring 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.

The emission of a two-dimensional organic photonic crystal structure is investigated. A photon emitted by a molecule in this structure can take two different coupled pathways. It can either be emitted directly into the surroundings or first be transferred to a discrete state of the photonic crystal structure from where it is then emitted. This unique coupling of the pathways results in a particular asymmetric spectral line shape referred to as Fano resonance. By studying the Fano line shape as a function of the quality factor, the authors can gain insights into the coupling between the pathways within the photonic crystal structure.