Non-resonant Excitation Conditions Research Articles

A - Si nanolayer induced enhancement of the 1.53μm photoluminescence in Er3+ doped a-Al2O3 thin films

A structured film formed by an active Er3+-doped amorphous Al2O3 layer located between two amorphous silicon nanolayers (NLs) in as-grown conditions shows an enhancement of the photoluminescence (PL) intensity and lifetime at 1.53μm of one order of magnitude when compared to a similar Er3+-doped film without silicon NLs. The film can be pumped even under nonresonant excitation conditions as a result of a long range energy transfer from the a-Si NLs to the Er3+ ions. In addition, the PL shows a single exponential decay with a lifetime value as high as 2.4ms. The lifetime enhancement is associated with an improvement of the emission efficiency of the Er3+ ions.

Resonant enhancement of third-order optical nonlinearities in single-walled carbon nanotubes

Third-order nonlinear optical response in single-walled carbon nanotubes (SWNTs) has been investigated by means of femtosecond pump–probe spectroscopy. The imaginary part of nonlinear susceptibility Im χ (3) measured under the nonresonant excitation conditions shows the resonant enhancement at each optical transition of the specific ( n, m) tube in miscelle-suspended SWNTs. With the selective excitation of the E 22 exciton transition for the (7,6) tube, the value of Im χ (3) is increased by an order of magnitude compared with the nonresonant excitation case. The nonlinear response signal at the E 22 transition energy for the (8,7) tube has been also enhanced for the excitation at ∼0.22 eV above the E 22 transition energy. This result is interpreted in terms of the phonon-assisted E 22 transition for the (8,7) tube, indicating strong exciton–phonon coupling.

Theoretical Design of an Aromatic Hydrocarbon Rotor Driven by a Circularly Polarized Electric Field

An aromatic hydrocarbon rotor without functional groups is theoretically designed. Such a molecular rotor is free from long-range electrostatic interactions. Induced dipole interactions are the rotor-driving forces under a nonresonant excitation condition. As an example, a molecular rotor with a condensed aromatic ring, a pentacene moiety mounted on a phenyl-acetylene axle that is driven by a circularly polarized electric field is considered. Results of simulations of the quantum dynamics of a rotor that take into account short-range rotor-bath interactions are presented by numerically solving the density matrix equations of the rotational motions.

Interaction between a high-density polariton phase and the exciton environment in semiconductor microcavities

Spontaneous polariton coherence is observed following creation of a cool exciton distribution in a GaAs-based semiconductor microcavity. Interactions between the high density polariton state and high-$k$ excitons result in a blueshift in energy and are shown to limit the temporal coherence of the polariton phase to $10--20\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. The effect of exciton-polariton interaction is also revealed in two beam experiments where the coherence of the polariton optical parametric oscillator is strongly suppressed by the creation of a reservoir population. These interactions are likely to be important in any polariton high density phase created under conditions of nonresonant excitation.

Polariton and spin dynamics in semiconductor microcavities under non-resonantexcitation

Semiconductor microcavities offer an ideal scenario to study strong radiation–matterinteractions. In this paper we review the temporal dynamics of polaritons in II–VI andIII–V based microcavities under non-resonant excitation conditions. We present evidence offinal-state stimulated scattering and discuss the spin-dependent emission, which exhibits aremarkably rich behaviour.

Surface-Enhanced Raman Scattering in Local Optical Fields of Silver and Gold NanoaggregatesFrom Single-Molecule Raman Spectroscopy to Ultrasensitive Probing in Live Cells

This Account discusses surface-enhanced Raman scattering at extremely high enhancement levels that can occur for molecules attached to silver and gold nanoclusters. Strongly enhanced and highly confined local optical fields enable surface-enhanced Stokes and anti-Stokes Raman spectroscopy of single molecules even under nonresonant excitation conditions as well as extremely large effective cross sections in two-photon excited Raman spectroscopy. The ability for very sensitive and spatially confined molecular structural probing makes gold and silver nanoclusters very promising tools for studies of small structures in biological materials, such as cellular compartments.

New organic materials and microcavity structures for strong exciton‐photon coupling

We discuss the properties of a selection of cyanine dyes that form J-aggregates with narrow absorption bands, suitable for studying strong exciton-photon coupling in optical microcavities at room temperature. In particular, we show how the optical properties of J-aggregate films can be tailored for microcavity applications. Microcavities are fabricated by lamination of the organic material between 2 dielectric mirrors. Such cavities contain an optical wedge, so the relative tuning between the organic exciton and the cavity mode can be realized by changing the sample position with regard to the light beam. Transmission measurements performed at room temperature show well resolved anticrossing behaviour with Rabi splitting of around 80 meV. Moreover, photoluminescence measurements allow us to study polariton emission under conditions of non-resonant excitation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Coherent spin dynamics in semiconductor quantum dots

The anisotropic exchange interaction (AEI) between electrons and holes is shown to play a central role in quantum dots (QDs) spin dynamics. In neutral QDs, AEI is at the origin of spin quantum beats observed under resonant excitation between the lowest energy doublet of linearly dipole-active eigenstates. In negatively charged QDs, AEI is at the origin of QD emission with opposite helicity to the optic al excitation, under non-resonant excitation conditions. Finally, the possibility of leaving a spin information in the system after recombination of the photo-injected electron-hole pair is discussed with respect to the type and the level of the doping. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Oscillatory behaviour in the nonlinear emission of semiconductor microcavities

We have observed marked oscillations in the time-resolved photoluminescence of a semiconductor microcavity under non-resonant excitation conditions. Hot excitons, created with an ultrashort light pulse, rapidly relax into polaritons in the cavity with a large in-plane momentum K. Shortly after illumination, above a certain excitation power, the polaritons accumulate into an energy trap at the bottom of the dispersion and the light emission, governed by final-state stimulated scattering, starts at K ∼ 0 (0° with respect to the normal of the sample). The angular dependence of the photoluminescence at negative detunings reveals that the emission is rapidly transferred to K ∼ 2 × 104 cm−1 (∼15°), close to the point of inflection of the lower polariton branch, giving rise to an annular emission. The oscillations arise from a macroscopic coherent population in the lower polariton branch.

Continuous wave stimulation in semiconductor microcavities in the strong coupling limit

We describe recent continuous wave studies of stimulated polariton scattering in semiconductor microcavities, with emphasis on the variety of new physics which has been observed in the last few years. The ability to manipulate both exciton and photon properties, and how they interact together to form strongly coupled exciton–photon coupled modes, exciton polaritons, leads to a number of novel phenomena, previously unreported in bulk semiconductors or quantum wells. The unusual dispersion of the lower polariton branch, which allows energy and momentum conserving polariton–polariton parametric scattering under resonant excitation, is shown to be a key factor enabling much of the new physics. A classical theoretical treatment of the phenomena which draws parallels between the equivalent pictures of stimulated polariton scattering and those of a triply resonant optical parametric oscillator is discussed. Higher order parametric phenomena and the factors which control the threshold for stimulation are also described. The results obtained under resonant excitation are contrasted with phenomena which occur under conditions of non-resonant excitation. Prospects to achieve ‘polariton lasing’ under non-resonant excitation are discussed.

Giant enhancement of polariton relaxation in semiconductor microcavities by polariton-free carrier interaction: Experimental evidence and theory

Very large changes in the population distributions of microcavity polaritons are reported under conditions of simultaneous resonant and nonresonant excitation. The strong relaxation bottleneck, which results in weak emission from the lower polariton branch for resonant excitation alone, is fully suppressed if additional nonresonant excitation is employed. By comparison with theory, the massive enhancement of the polariton relaxation is shown to arise from scattering of polaritons by free carriers, present in quantum wells due to residual doping, which are then heated by the nonresonant excitation.

High-occupancy effects and stimulation phenomena in semiconductor microcavities

This paper describes recent work on high-occupancy effects in semiconductor microcavities, with emphasis on the variety of new physics and the potential for applications that has been demonstrated recently. It is shown that the ability to manipulate both exciton and photon properties, and how they interact together to form strongly coupled exciton-photon coupled modes, exciton polaritons, leads to a number of very interesting phenomena, which are either difficult or impossible to achieve in bulk semiconductors or quantum wells. The very low polariton density of states enables state occupancies greater than one to be easily achieved, and hence stimulation phenomena to be realized under conditions of resonant excitation. The particular form of the lower polariton dispersion curve in microcavities allows energy and momentum conserving polariton-polariton scattering under resonant excitation. Stimulated scattering of the bosonic quasi-particles occurs to the emitting state at the center of the Brillouin zone, and to a companion state at high wave vector. The stimulation phenomena lead to the formation of highly occupied states with macroscopic coherence in two specific regions of k space. The results are contrasted with phenomena that occur under conditions of nonresonant excitation. Prospects to achieve "polariton lasing" under nonresonant excitation, and high-gain, room-temperature ultrafast amplifiers and low-threshold optical parametric oscillator under resonant excitation conditions are discussed.

Transition from strong to weak coupling and the onset of lasing in semiconductor microcavities

Angular-dependent emission spectra are investigated in a strongly coupled InGaAs-GaAs-AlAs-based semiconductor microcavity as a function of excitation intensity and of detuning between the uncoupled exciton and photon modes. Under conditions of nonresonant excitation, it is shown that the onset of stimulated emission always occurs in the weak coupling regime. Angular-dependent studies show that the transition to weak coupling occurs when the linewidth of high k excitons becomes of the order of the normal mode splitting of the exciton-polariton coupled modes. We conclude that, under nonresonant excitation, ``polariton lasing,'' where stimulated polariton scattering followed by photon emission occurs in the strong coupling regime, is only likely to be achieved in systems with larger exciton binding energy than in GaAs-based structures or possibly also in GaAs microcavities containing excess concentrations of free electrons. The experimental results below threshold are found to be in good agreement with numerical solution of Boltzmann kinetic equations for the photoexcited polariton distribution.

The Role of Spin in Interacting Excitonic Gases

The spin-dependent exciton-exciton interactions in GaAs quantum well (QW) systems have been studied, under resonant and non-resonant excitation conditions at liquid helium temperatures. In the former case, a pure exciton gas is created, whereas in the latter the excitons are surrounded by a charged electron-hole plasma. In a double QW structure, the distance between electrons and holes comprising the excitons could be externally tuned by applying an electric field perpendicular to the well planes. The interactions were studied in terms of their dependence on excitation intensity, excess energy, spin polarization and electron-hole separation using time- and polarization-resolved photoluminescence up-conversion spectroscopy.

Polariton–polariton interactions and stimulated scattering in semiconductor microcavities

Recent work on polariton–polariton scattering in semiconductor microcavities under continuous wave excitation conditions is reviewed. For weak non-resonant laser excitation, a marked bottleneck in the polariton distribution is observed, but which is suppressed by polariton–polariton scattering as the laser intensity is increased. However, the high excitation conditions necessary to observe stimulated emission lead to loss of strong coupling and conventional lasing in the weak coupling regime. By contrast for resonant excitation, polaritons are created directly in the polariton trap formed by the microcavity dispersion curve. Stimulated scattering of the bosonic quasi-particles occurs to the emitting state at the centre of the Brillouin zone, and to a companion state at high wavevector. The stimulation phenomena lead to condensation of the bosonic quasi-particles to two specific regions of k-space, and to the formation of a new state with macroscopic coherence. The prospects to achieve a polariton laser under conditions of non-resonant excitation are discussed.

Stimulated scattering and its dynamics in semiconductor microcavities at 80 K under nonresonant excitation conditions

We have observed stimulated scattering in a II-VI microcavity at 80 K under nonresonant excitation conditions. This stimulated scattering manifests itself through probe amplification in a pump-probe reflectivity experiment and through a highly nonlinear emission at an energy corresponding to a k? ? 0 state on the lower polariton branch. We were able to follow the onset and decline of stimulated scattering by measuring the temporal evolution of the probe amplification.

Excitonic absorption of GaN epilayers on sapphire: Dynamics, intensity, and temperature dependence

Using test-pump measurements, we studied the excitonic absorption of GaN epilayers on sapphire substrate under nanosecond and picosecond pulsed excitation conditions. We measured a short exciton lifetime of 25 ps, which is consistent with results obtained from degenerate four-wave mixing measurements. Since the radiative lifetime should be much longer, this short characteristic time shows the dominance of nonradiative processes. Under nonresonant excitation condition, we observed an intensity dependent shift and damping of the exciton resonance, attributed to an inhomogeneous temperature distribution. This thermal effect was not observed under resonant excitation conditions. This behavior is tentatively explained by the presence of shallow centers that have a rather long lifetime and recombine nonradiatively.

Coherent spin dynamics of excitons in quantum wells

Coherent spin dynamics of excitons in quantum wells is investigated theoretically. Resonant and nonresonant excitation conditions are considered. The single-particle hole spin-flip mechanism within the exciton is always the dominant cause for breaking the exchange coupling. The two-dimensional confinement and low temperature are the most important factors which stabilize the hole spin orientation within the exciton. The experimental observations reported previously are interpreted.

Measurements of frequency upconversion and picosecond excitation-correlation luminescence spectra in GaAs quantum wells and determination of time constants describing exciton dynamics

We study the picosecond dynamics of free carriers and excitons in high-quality GaAs quantum wells at 8 K using frequency upconversion (UC) and nonlinear picosecond excitation-correlation (PEC) luminescence spectroscopy under nonresonant excitation conditions with carrier densities of a few 1010 cm−2. We analyze the measurements using a theoretical model of the coupled free electron–hole pair and exciton dynamics, incorporating important density-dependent nonlinear effects, caused by exciton collisions and fermion exclusion. The time-resolved UC photoluminescence (PL) spectra are used to deduce time constants related to free electron–hole (e–h)-pair transformation into excitons and exciton transfer from large K to small K (k is the exciton wave vector). It is shown that unambiguous determination of exciton radiative and nonradiative decay times are possible when both UC and PEC PL spectra are considered simultaneously. We carry out a detailed line-shape analysis of the picosecond UC and PEC PL spectra. We find that while collision broadening is mainly responsible for the observed nonlinear excitonic PEC signal, the PEC PL spectra can be fully explained only when additional nonlinear effects, such as density-dependent radiative lifetime and fermion exclusion, are included in the analysis. From a comparison of the time evolution of both the PEC and UC PL spectra with theory, the time constants introduced in the model to describe free carrier density decay, transfer of excitons with K≠0 to K≊0 states, and their radiative and nonradiative decay are estimated to be about 50, 15, 60, and 1800 ps respectively.

Thermal lensing effects in -doped luminescent glass

Optical absorption, photoluminescence and fluorescence decay time of in a glass were measured in the 400 - 800 nm range. Under continuous (CW) laser excitation the characteristics of thermal lensing (TL) were measured at room temperature, under resonant and non-resonant conditions. Our results are analysed based on a theoretical model for CW laser induced thermal lensing under a gaussian-beam approximation. The effects of the power input and the response time for TL are explained in terms of the thermal properties of the glass host, for non-resonant excitation, and in terms of the coupling of rare earth ions to the local vibrations for resonant excitations. We also report, for the first time, the observation of laser induced permanent changes in the refractive index of bulk rare earth doped oxide glasses, under non-resonant excitation conditions.