G. C. La Rocca

Excitons and optical nonlinearities in hybrid organic-inorganic nanostructures

We present a theoretical review of the properties of electronic excitations in nanostructures based on combinations of organic materials with inorganic semiconductors, having respectively Frenkel excitons and Wannier-Mott excitons with nearly equal energies. We show that in this case the resonant coupling between organic and inorganic quantum wells (or wires or dots) may lead to several interesting effects, such as splitting of the excitonic spectrum and enhancement of the resonant optical nonlinearities. First, we discuss the properties of hybrid Frenkel-Wannier-Mott excitons, which appear when the energy splitting of the excitonic spectrum is large compared to the width of the exciton resonances (the case of strong resonant coupling). Such peculiar excitations share at the same time both the properties of the Wannier excitons (e.g., the large radius) and those of the Frenkel excitons (e.g., the large oscillator strength). We discuss mainly two-dimensional configurations (interfaces or coupled quantum wells) which are the most extensively studied. In particular, we show that hybrid excitons are expected to have resonant optical nonlinearities significantly enhanced with respect to those of traditional inorganic or organic systems. We also consider analogous phenomena in microcavities where the exciton resonances are close to the cavity photon mode resonance. Next, we consider the case of weak resonant coupling and show the relevance of the Forster mechanism of energy transfer from an inorganic quantum well to an organic overlayer. Such an effect may be especially interesting for applications: the electrical pumping of excitons in the semiconductor quantum well can be used to efficiently turn on the organic material luminescence.

Efficient electronic energy transfer from a semiconductor quantum well to an organic material

We predict an efficient electronic energy transfer from the two-dimensional Wannier-Mott excitons confined in a semiconductor quantum well to the optically active organic molecules of the nearby medium (substrate and/or overlayer). The energy transfer mechanism is of the Förster type and, at semiconductor-organic distances of several nanometers, can easily be as fast as 100 ps, which is about an order of magnitude shorter than the average exciton lifetime in an isolated quantum well. Under such conditions, the Wannier-Mott exciton luminescence is quenched and the organic luminescence is efficiently turned on. Our calculations, combining a microscopic quantum mechanical exciton model with a macroscopic electrodynamic description of the organic medium, take into account the dielectric constant discontinuities and can be applied to any organic-inorganic multilayer structure.

Controlling the photonic band structure of optically driven cold atoms

An analytical method based on a two-mode approximation is here developed to study the optical response of a periodically modulated medium of ultracold atoms driven into a regime of standing-wave electromagnetically induced transparency. A systematic comparison with the usual approach based on the coupled Maxwell-Liouville equations shows that our method is very accurate in the frequency region of interest. Our method, in particular, explains in a straightforward manner the formation of a well-developed photonic bandgap in the optical Bloch wave vector dispersion. For ultracold 87Rb atoms nearly perfect reflectivity may be attained and a light pulse whose frequency components are contained within the gap is seen to be reflected with little loss and deformation.

Optical nonreciprocity of cold atom Bragg mirrors in motion.

Reciprocity is fundamental to light transport and is a concept that holds also in rather complex systems. Yet, reciprocity can be switched off even in linear, isotropic, and passive media by setting the material structure into motion. In highly dispersive multilayers this leads to a fairly large forward-backward asymmetry in the pulse transmission. Moreover, in multilevel systems, this transport phenomenon can be all-optically enhanced. For atomic multilayer structures made of three-level cold 87Rb atoms, for instance, forward-backward transmission contrast around 95% can be obtained already at atomic speeds in the meter per second range. The scheme we illustrate may open up avenues for optical isolation that were not previously accessible.

Microscopic theory of polariton lasing via vibronically assisted scattering

Polariton lasing has recently been observed in strongly coupled crystalline anthracene microcavities. A simple model is developed describing the onset of the nonlinear threshold based on a master equation including the relevant relaxation processes and employing realistic material parameters. The mechanism governing the buildup of the polariton population, namely, bosonic stimulated scattering from the exciton reservoir via a vibronically assisted process, is characterized and its efficiency calculated on the basis of a microscopic theory. The role of polariton-polariton bimolecular quenching is identified and temperature-dependent effects are discussed.

New concept for organic LEDs: non-radiative electronic energy transfer from semiconductor quantum well to organic overlayer

Abstract The resonant electronic energy transfer from an excited semiconductor quantum well to a nearby strongly absorbing organic medium may occur on time scales of several tens of picoseconds for II–VI semiconductors and of several hundreds picoseconds for III–V semiconductors, which is in both cases significantly less than the semiconductor excitation life-time in the absence of such a transfer. Thus, a large fraction of the semiconductor excitation energy can be transfered to the organic medium. Due to energy transfer the non-radiative processes in the semiconductor quantum well with characteristic times larger than that of the energy transfer, will be suppressed. The advantage of the hybrid structures would be also the combination of comparatively good transport properties of semiconductors (allowing for electrical pumping) and good light-emitting properties of organic substances (high quantum yield, colour tuning). Such a favorable combination may be exploited in constructing new type of LEDs.

Slow group velocity and Cherenkov radiation.

We study theoretically the effect of ultraslow group velocities on the emission of Vavilov-Cherenkov radiation in a coherently driven medium. We show that in this case the aperture of the group cone on which the intensity of the radiation peaks is much smaller than that of the usual wave cone associated with the Cherenkov coherence condition. As a specific example, we consider a coherently driven ultracold atomic gas where such singular behavior may be observed.

Electromagnetically induced one-photon and two-photon transparency in rubidium atoms

Abstract We investigate electromagnetically induced transparency (EIT) for one-photon and resonant two-photon transitions in rubidium atomic vapors. One-photon transparency is obtained in the 5S–5P–5D three level ladder system. At the same time, we study electromagnetically induced transparency in the two-photon transition (EITT) from the 5S to the 7S state resonant with the 5P intermediate state, when the latter is coupled to the 5D state by a control beam. The dependence of the transparency profile on the controlling laser intensity, linewidth, and detuning is investigated. Experimental results are obtained for isotopes 85 Rb and 87 Rb characterized by different hyperfine structures. Our findings for both the one-photon and the resonant two-photon transparency agree well with the prediction from three and four-level theory, respectively, with independently determined spectroscopic parameters.

Transverse Fresnel-Fizeau drag effects in strongly dispersive media

A light beam normally incident upon an uniformly moving dielectric medium is, in general, subject to bendings due to a transverse Fresnel-Fizeau light drag effect. In most familiar dielectrics, the magnitude of this bending effect is very small and hard to detect. Yet, the effect can be dramatically enhanced in strongly dispersive media where slow group velocities in the m/s range have been recently observed taking advantage of the electromagnetically induced transparency effect. In addition to the usual downstream drag that takes place for positive group velocities, we discuss a significant anomalous upstream drag which is expected to occur for negative group velocities. Furthermore, for sufficiently fast speeds of the medium, higher-order dispersion terms are found to play an important role and to be responsible for light propagation along curved paths or the restoration of the time and space coherence of an incident noisy beam. The physics underlying this class of slow-light effects is thoroughly discussed.

Stationary light pulses in cold thermal atomic clouds

Fourier-expanded Maxwell-Liouville equations are employed to study the light pulse dynamics in atomic samples coherently driven by a standing-wave light field. Solutions are obtained by a suitable truncation of the Maxwell-Liouville equations that contain the number of spin and optical Fourier coherence components appropriate to the sample temperature. This approach is examined here for cold but thermal atoms where the Doppler broadening is still not negligible and familiar secular approximations no longer hold. In this temperature regime higher-order momentum Fourier coherence components are shown to be important for achieving excellent agreement with a recent experiment done in cold {sup 87}Rb clouds at several hundred microkelvins.