Efficient light emission from inorganic and organic semiconductor hybrid structures by energy-level tuning.

The fundamental limits of inorganic semiconductors for light emitting applications, such as holographic displays, biomedical imaging and ultrafast data processing and communication, might be overcome by hybridization with their organic counterparts, which feature enhanced frequency response and colour range. Innovative hybrid inorganic/organic structures exploit efficient electrical injection and high excitation density of inorganic semiconductors and subsequent energy transfer to the organic semiconductor, provided that the radiative emission yield is high. An inherent obstacle to that end is the unfavourable energy level offset at hybrid inorganic/organic structures, which rather facilitates charge transfer that quenches light emission. Here, we introduce a technologically relevant method to optimize the hybrid structure's energy levels, here comprising ZnO and a tailored ladder-type oligophenylene. The ZnO work function is substantially lowered with an organometallic donor monolayer, aligning the frontier levels of the inorganic and organic semiconductors. This increases the hybrid structure's radiative emission yield sevenfold, validating the relevance of our approach.

Controlling the growth mode of para-sexiphenyl (6P) on ZnO by partial fluorination.

We report on the impact of partial fluorination of para-sexiphenyl (6P) on the growth mode when deposited on the non-polar ZnO(101̄0) surface. The evolution of the thin film structure and morphology is monitored by in situ atomic force microscopy and in situ real-time X-ray scattering. Both 6P and its symmetrical, terminally fluorinated derivative (6P-F4) grow in a highly crystalline mode, however, with a distinctly different morphology. While 6P films are characterised by the formation of two different phases with three-dimensional nanocrystallites and consequently a rather rough surface morphology, layer-by-layer growth and phase purity in case of 6P-F4 prevails leading to smooth terraced thin films. We relate the different growth behaviour to specifics of the thin film structure.

ZnO as a tunable metal: new types of surface plasmon polaritons.

The use of the free-electron gas in a heavily doped semiconductor (ZnO:Ga) enables the realization of almost arbitrarily shaped surface-plasmon-polariton dispersion curves in planar geometries. In particular, by preparing metal-metal-type interfaces, we demonstrate surface-plasmon polaritons exhibiting finite frequencies in the long-wavelength limit. Moreover, coupling of surface plasmon polaritons at adjacent interfaces allows for the controlled formation of frequency gaps or, alternatively, the opening of otherwise forbidden regions by an appropriate layer design. Our findings reveal a considerable plasmonic potential of this semiconductor-based approach, e.g., for achieving propagation control or phase matching for nonlinear optical processes as well as novel many-particle phenomena.

Electrostatic-field-driven alignment of organic oligomers on ZnO surfaces.

We study the physisorption of organic oligomers on the strongly ionic ZnO(1010) surface by using first-principles density-functional theory and nonempirical embedding methods. It turns out that the in-plane variation of the molecule-substrate interaction energy and the bonding dipole in the vertical direction are linked up by a linear relationship originating from the electrostatic coupling of the molecule with the periodic dipolar electric field generated by the Zn-O surface dimers. Long oligomers with a highly axial π-electron system such as sexiphenyl become well oriented with alignment energies of several 100 meV along rows of a positive electric field, in full agreement with recent experiments. These findings define a new route towards the realization of highly ordered self-assembled arrays of oligomers or polymers on ZnO(1010) and similar surfaces.

Odd-number theorem: optical feedback control at a subcritical Hopf bifurcation in a semiconductor laser.

A subcritical Hopf bifurcation is prepared in a multisection semiconductor laser. In the free-running state, hysteresis is absent due to noise-induced escape processes. The missing branches are recovered by stabilizing them against noise through application of phase-sensitive noninvasive delayed optical feedback control. The same type of control is successfully used to stabilize the unstable pulsations born in the Hopf bifurcation. This experimental finding represents an optical counterexample to the so-called odd-number limitation of delayed feedback control. However, as a leftover of the limitation, the domains of control are extremely small.

Random semiconductor lasers: scattered versus Fabry-Perot feedback.

As a result of growth imperfections, (Zn,Cd)O/ZnO quantum well structures exhibit random laser action. Fabrication of microresonators allows us to study and to compare directly cavity and scattered feedback. Our experimental and theoretical analysis shows that (i) pure random lasing generally requires a larger gain than in the standard Fabry-Perot regime, (ii) the presence of Mie scatterers in the semiconductor-based cavity does not substantially increase the lasing threshold, and (iii) the random feedback creates a subtle modal gain distribution that might be of particular importance for the dynamical properties, both with and without Fabry-Perot cavity.

Synchronization of quasiperiodic oscillations to a periodic force studied with semiconductor lasers.

We experimentally study synchronization processes in a system of two different multisection semiconductor lasers. Periodic self-pulsations of laser 1 are injected into laser 2, which is operating in a regime with two-frequency quasiperiodic self-pulsations. The experimental system demonstrates the new type of transitions to synchrony between three frequencies which has been recently revealed using generic coupled phase and van der Pol oscillator models. In particular, resonances of quasiperiodic oscillations at integer winding numbers three and five are shown to break up before locking to the injected periodic signal. Moreover, carefully determining the coherence of the noisy oscillations, we reveal so far unexplored processes of coherence transfer to nonsynchronized oscillations.

Tristability of a semiconductor laser due to time-delayed optical feedback.

We present an experimental and theoretical study of multistability of a single-mode laser subject to feedback through phase tuning and amplifier sections integrated on the same chip. Closely above threshold, a regime of tristability of continuous-wave (CW) states is found for multiple ranges of amplifier and phase currents. The separation between the tristable wavelengths agrees with the channel spacing of dense wavelength multiplexing in the C band of optical communication making the device interesting for ternary logic applications. Complementary theoretical investigations in the framework of the paradigmatic Lang-Kobayashi model provide a consistent understanding of the experimental findings and additionally yield an analytic formula expressing the maximum number of coexisting stable CW states by the linewidth-enhancement factor alpha . Tristability belongs to the alpha range from 5 to 8 in good agreement with experiment.

All-optical noninvasive chaos control of a semiconductor laser.

We demonstrate experimentally control of a chaotic system on time scales much shorter than in any previous study. Combining a multisection laser with an external Fabry-Perot etalon, the chaotic output transforms into a regular intensity self-pulsation with a frequency in the 10-GHz range. The control is noninvasive as the feedback from the etalon is minimum when the target state is reached. The optical phase is identified as a crucial control parameter. Numerical simulations agree well with the experimental data and uncover global control properties.

Nonequilibrium nuclear-electron spin dynamics in semiconductor quantum dots.

We study the spin dynamics in charged quantum dots in the situation where the resident electron is coupled to only about 200 nuclear spins and where the electron spin splitting induced by the Overhauser field does not exceed markedly the spectral broadening. The formation of a dynamical nuclear polarization as well as its subsequent decay by the dipole-dipole interaction is directly resolved in time. Because not limited by intrinsic nonlinearities, almost complete nuclear polarization is achieved, even at elevated temperatures. The data suggest a nonequilibrium mode of nuclear polarization, distinctly different from the spin temperature concept exploited on bulk semiconductors.

All-optical noninvasive control of unstable steady states in a semiconductor laser.

All-optical noninvasive control of a multisection semiconductor laser by means of time-delayed feedback from an external Fabry-Perot cavity is realized experimentally. A theoretical analysis, in both a generic model as well as a device-specific simulation, points out the role of the optical phase. Using phase-dependent feedback we demonstrate stabilization of the continuous-wave laser output and noninvasive suppression of intensity pulsations.

Electron spin dynamics in a self-assembled semiconductor quantum dot: the limit of low magnetic fields.

Using the trion as an optical probe, we uncover novel electron spin dynamics in CdSe/ZnSe Stranski-Krastanov quantum dots. The longitudinal spin lifetime obeys an inverse power law associated with recharging processes in the dot ensemble. No hint at spin-orbit mediated spin relaxation is found. At very weak magnetic fields (< 50 mT), electron spin dynamics related to the hyperfine interaction with the lattice nuclei is uncovered. A strong Knight field gives rise to nuclear ordering and formation of dynamical polarization on a 100-micros time scale under continuous electron spin pumping. The associated spin transients are temperature robust and can be observed up to 100 K.

Stimulated emission from the biexciton in a single self-assembled II-VI quantum dot.

Using two-photon excitation, stimulated emission from the biexciton state in a single CdSe/ZnSe quantum dot is observed in a two-pulse configuration. We directly time resolve the emission-absorption characteristics and verify the potential for laser action. By setting the polarization of the stimulation pulse, the recombination path of the biexciton and, by this, the state of the photons emitted in the decay cascade is controlled. We elaborate also the coherent response and address entanglement and disentanglement of the exciton-biexciton system.

Converting Wannier into Frenkel excitons in an inorganic/organic hybrid semiconductor nanostructure.

Electronic coupling between Wannier and Frenkel excitons in an inorganic/organic semiconductor hybrid structure is experimentally observed. Time-resolved photoluminescence and excitation spectroscopy directly demonstrate that electronic excitation energy can be transferred with an efficiency of up to 50% from an inorganic ZnO quantum well to an organic [2,2-p-phenylenebis-(5-phenyloxazol), alpha-sexithiophene] overlayer. The coupling is mediated via dipole-dipole-interaction analog to the Förster transfer in donor-acceptor systems.

Coherence resonance near a Hopf bifurcation.

We report on the observation of coherence resonance for a semiconductor laser with short optical feedback close to Hopf bifurcations. Noise-induced self-pulsations are documented by distinct Lorentzian-like features in the power spectrum. The character of coherence is critically related to the type of the bifurcation. In the supercritical case, spectral width and height of the peak are monotonic functions of the noise level. In contrast, for the subcritical bifurcation, the width exhibits a minimum, translating into resonance behavior of the correlation time in the pulsation transients. A theoretical analysis based on the generic model of a self-sustained oscillator demonstrates that these observations are of general nature and are related to the fact that the damping depends qualitatively different on the noise intensity for the subcritical and supercritical case.

Synchronization of delay-coupled oscillators: a study of semiconductor lasers.

Two delay-coupled semiconductor lasers are studied in the regime where the coupling delay is comparable to the time scales of the internal laser oscillations. Detuning the optical frequency between the two lasers, novel delay-induced scenarios leading from optical frequency locking to successive states of periodic intensity pulsations are observed. We demonstrate and analyze these dynamical phenomena experimentally using two distinct laser configurations. A theoretical treatment reveals the universal character of our findings for delay-coupled systems.

Two-photon coherent control of a single quantum dot.

We report on two-photon coherent control of the biexciton state in single Stranski-Krastanov CdSe quantum dots. Clear interference patterns are observed at twice the optical frequency. The decay of the interference contrast is nonexponential and caused by a dynamical inhomogeneous broadening of the energy levels due to long-term fluctuations in the dot environment.

Self-organization in semiconductor lasers with ultrashort optical feedback.

The dynamical behavior of a single-mode laser subject to optical feedback is investigated in the limit, when the delay time is much shorter than the period of the relaxation oscillations. Use of an integrated distributed feedback device allows us to control the feedback phase. We observe two kinds of Hopf bifurcations associated with regular self-pulsations of different frequencies.

Nonlinear dynamics of semiconductor lasers with active optical feedback.

An in-depth theoretical as well as experimental analysis of the nonlinear dynamics in semiconductor lasers with active optical feedback is presented. Use of a monolithically integrated multisection device of submillimeter total length provides access to the short-cavity regime. By introducing an amplifier section as a special feature, phase and strength of the feedback can be separately tuned. In this way, the number of modes involved in the laser action can be adjusted. We predict and observe specific dynamical scenarios. Bifurcations mediate various transitions in the device output, from single-mode steadystate to self-pulsation and between different kinds of self-pulsations, reaching eventually chaotic behavior in the multimode limit.