Resonant Optical Excitation Research Articles

Dynamic exciton-polariton macroscopic coherent phases in a tunable dot lattice

We report on the enhancement of the nonlinear properties of exciton-polariton macroscopic coherent phases created by resonant optical excitation under the spatial modulation of a square acoustic lattice. The confinement increases the local particle density by preventing particle drift due to repulsive interactions and modifies the coupling to the exciting laser field. Both effects lead to the enhancement of the nonlinear properties of the system, which is reflected in a reduction of the pump laser intensity threshold of formation of the coherent phase, pronounced blueshift of the emission energy and screening of the acoustic potential.

Progress of ultra trace determination of technetium using laser resonance ionization mass spectrometry

Laser resonance ionization mass spectrometry (RIMS) represents one of the most sensitive and selective techniques for ultra trace determination of long-lived radioisotopes. The isotope (99g)Tc constitutes a specific candidate of high relevance concerning its environmental behavior as well as fundamental research applications. Based on the recent precision determination of the ionization potential of technetium by laser resonance ionization, refined resonant optical excitation pathways have been derived for analytical determination of ultra trace amounts of (99g)Tc by laser mass spectrometric approaches. The state of the art and the specifications of RIMS-based ultra trace determination for (99g)Tc, leading to a level of detection of ε ≈ 3 × 10(-4) atoms (3 μBq), are reported.

Dynamic Nuclear Spin Polarization in the Resonant Laser Excitation of an InGaAs Quantum Dot

Resonant optical excitation of lowest-energy excitonic transitions in self-assembled quantum dots leads to nuclear spin polarization that is qualitatively different from the well-known optical orientation phenomena. By carrying out a comprehensive set of experiments, we demonstrate that nuclear spin polarization manifests itself in quantum dots subjected to finite external magnetic field as locking of the higher energy Zeeman transition to the driving laser field, as well as the avoidance of the resonance condition for the lower energy Zeeman branch. We interpret our findings on the basis of dynamic nuclear spin polarization originating from noncollinear hyperfine interaction and find excellent agreement between experiment and theory. Our results provide evidence for the significance of noncollinear hyperfine processes not only for nuclear spin diffusion and decay, but also for buildup dynamics of nuclear spin polarization in a coupled electron-nuclear spin system.

Optically erasing disorder in semiconductor microcavities using polariton–polariton interactions

AbstractThrough resonant coherent optical excitation of upper branch polaritons in semiconductor microcavities, an effective screening of the disorder potential experienced by lower branch polaritons is predicted. The effect is due to polariton–polariton interactions, which generate bistability for the upper‐polariton states and introduce an effective potential for lower‐polaritons. The effect represents a simple tool to improve the characteristics of devices operating in the low density regime.

Resonant optical excitations in complementary plasmonic nanostructures

We compare the plasmonic response of two complementary structures to a scanning electron probe; a silver nanowire and a nanoslot in a silver film of comparable dimensions, desirable for their localized electromagnetic enhancement and enhanced optical transmission respectively. Through electron energy loss spectroscopy, multiple plasmonic resonant harmonics setup in both structures are resolved with inverted phase, in agreement with Babinet's principle, and of consequence in the design and fabrication of nanostructures.

Bistability and nonequilibrium transitions in the system of cavity polaritons under nanosecond-long resonant excitation

The polarization dependence of nonequilibrium transitions in a multistable cavity-polariton system is studied under a nanosecond long resonant optical excitation at the normal and magic angle incidences with various polarizations of the pump beam. The temporal correlations between the frequency, intensity, and optical polarization of the intra-cavity field, which all undergo sharp threshold-like changes due to the spin dependent interaction of cavity polaritons, are visualized. The observed dynamics cannot be reproduced within the conventional semi-classical model based on the Gross-Pitaevskii equations. To explain the observed phenomena, it is necessary to take into account the unpolarized exciton reservoir which brings on additional blueshift of bright excitons, equal in the $\sigma^+$ and $\sigma^-$ polarization components. This model explains the effect of polarization instability under both pulsed and continuous wave resonant excitation conditions, consistently with the spin ring pattern formation that has recently been observed under Gaussian shaped excitation.

High-fidelity projective read-out of a solid-state spin quantum register

Initialization and read-out of coupled quantum systems are essential ingredients for the implementation of quantum algorithms. Single-shot read-out of the state of a multi-quantum-bit (multi-qubit) register would allow direct investigation of quantum correlations (entanglement), and would give access to further key resources such as quantum error correction and deterministic quantum teleportation. Although spins in solids are attractive candidates for scalable quantum information processing, their single-shot detection has been achieved only for isolated qubits. Here we demonstrate the preparation and measurement of a multi-spin quantum register in a low-temperature solid-state system by implementing resonant optical excitation techniques originally developed in atomic physics. We achieve high-fidelity read-out of the electronic spin associated with a single nitrogen-vacancy centre in diamond, and use this read-out to project up to three nearby nuclear spin qubits onto a well-defined state. Conversely, we can distinguish the state of the nuclear spins in a single shot by mapping it onto, and subsequently measuring, the electronic spin. Finally, we show compatibility with qubit control: we demonstrate initialization, coherent manipulation and single-shot read-out in a single experiment on a two-qubit register, using techniques suitable for extension to larger registers. These results pave the way for a test of Bell's inequalities on solid-state spins and the implementation of measurement-based quantum information protocols.

Resonant Photoconductance of Molecular Junctions Formed in Gold Nanoparticle Arrays

We investigate the photoconductance properties of oligo(phenylene vinylene) (OPV) molecules in metal-molecule-metal junctions. The molecules are electrically contacted in a two-dimensional array of gold nanoparticles. The nanoparticles in such an array are separated by only few nanometers. This allows to bridge the distance between the nanoparticles with molecules considered as molecular wires such as OPV. We report on the photoconductance of electrically contacted OPV upon resonant optical excitation of the molecules. This resonant photoconductance is sublinear in laser intensity, which suggests that trap state dynamics of the optically excited charge carriers dominate the optoelectronic response.

Abnormal anti-Stokes Raman emission as a coherent anti-Stokes Raman scattering-like process in disordered media

In this paper, we demonstrate that, by continuous single beam excitation, one can generate an abnormal anti-Stokes Raman emission (AASRE) whose properties are similar to a coherent anti-Stokes Raman scattering (CARS). The effect has been observed in materials which possess intrinsically nonlinear properties (LiNbO3 and CdS), which have the electric susceptibility of third order different from zero, χ(3) ≠ 0, as well as in materials that become nonlinear under resonant optical excitation. In the latter case, we used poly-3,4-ethylendioxythiophene (PEDOT) in its undoped state deposited electrochemically on Au support. Raman studies corroborated with images of optical microscopy demonstrate that the production of AASRE is conditioned by the existence of a particular morphology of the sample able to ensure efficient transport of the light inside the sample through a multiple light scattering mechanism. In this context, it was found that LiNbO3 and CdS in powder form as well as the PEDOT films layered on a rough Au substrate are suitable morphological forms. We explain AASRE as resulting from a wave-mixing mechanism of the incident laser light ωl with a Stokes-shifted Raman light ωS produced by a spontaneous Raman light scattering process, both strongly scattered inside the sample. As a CARS process, AASRE is conditioned by the achievement of phase-matching requirements, which makes the difference between the wave vectors of mixing light close to zero, Δk =/2kl − kS − kCARS/≈ 0. In condensed media, the small dispersion of the refractive index makes Δk ≈ 0 so that the formation of a favourable phase-matching geometry may be accomplished even at a crossing angle θ of travelling scattered light ωl and ωS. For tightly focused beams, the requirement of phase matching relaxes; it is no longer sensitive to the Raman shift, so that a wide intense anti-Stokes Raman spectrum is observed at an angle larger than the Stokes Raman spectrum.

Electrochemical functionalisation of SWNTs with poly(3,4‐ethylenedioxythiophene) evidenced by anti‐Stokes/Stokes Raman spectroscopy

AbstractThe capability of anti‐Stokes/Stokes Raman spectroscopy to evaluate chemical interactions at the interface of a conducting polymer/carbon nanotubes is demonstrated. Electrochemical polymerisation of the monomer 3,4‐ethylenedioxythiophene (EDOT) on a Au support covered with a single‐walled carbon nanotube (SWNT) film immersed in a LiClO4/CH3CN solution was carried out. At the resonant optical excitation, which occurs when the energy of the exciting light coincides with the energy of an electronic transition, poly(3,4‐ethylenedioxythiophene) (PEDOT) deposited electrochemically as a thin film of nanometric thickness on a rough Au support presents an abnormally intense anti‐Stokes Raman spectrum. The additional increase in Raman intensity in the anti‐Stokes branch observed when PEDOT is deposited on SWNTs is interpreted as resulting from the excitation of plasmons in the metallic nanotubes. A covalent functionalisation of SWNTs with PEDOT both in un‐doped and doped states takes place when the electropolymerisation of EDOT, with stopping at +1.6 V versus Ag/Ag+, is performed on a SWNT film deposited on a Au plate. The presence of PEDOT covalently functionalised SWNTs is rationalised by (1) a downshift by a few wavenumbers of the polymer Raman line associated with the symmetric CC stretching mode and (2) an upshift of the radial breathing modes of SWNTs, both variations revealing an interaction between SWNTs and the conjugated polymer. Raman studies performed at different excitation wavelengths indicate that the resonant optical excitation is the key condition to observe the abnormal anti‐Stokes Raman effect. Copyright © 2010 John Wiley & Sons, Ltd.

Subharmonic Resonant Optical Excitation of Confined Acoustic Modes in a Free-Standing Semiconductor Membrane at GHz Frequencies with a High-Repetition-Rate Femtosecond Laser

We propose subharmonic resonant optical excitation with femtosecond lasers as a new method for the characterization of phononic and nanomechanical systems in the gigahertz to terahertz frequency range. This method is applied for the investigation of confined acoustic modes in a free-standing semiconductor membrane. By tuning the repetition rate of a femtosecond laser through a subharmonic of a mechanical resonance we amplify the mechanical amplitude, directly measure the linewidth with megahertz resolution, infer the lifetime of the coherently excited vibrational states, accurately determine the system's quality factor, and determine the amplitude of the mechanical motion with femtometer resolution.

External field control of charge transmission through single molecules: Switching effects and transient currents

Net electron transfer through a single molecule attached to nano electrodes can undergo a pronounced change by resonant optical excitation. Caused by the population of excited electronic states of the molecule new transmission channels are opened which alter the current–voltage characteristics of the junction. This recent suggestion is further investigated here by studying the transient behavior of the current if the external laser pulse excitation is switched-on and -off within a certain time-interval. The computations concentrate on the case of weak and intermediate molecule–lead coupling. They are carried out in using rate equations which govern the populations of the different electron-vibrational states of the molecule being in its neutral or charged state. The interrelation of the switching-time and the time of charging and discharge of the molecule as well as of the time of vibrational relaxation is demonstrated by working in a switching-time range of some hundreds fs up to about 50ps.

Controlling the Interaction of Electron and Nuclear Spins in a Tunnel-Coupled Quantum Dot

We present a technique for manipulating the nuclear spins and the emission polarization from a single optically active quantum dot. When the quantum dot is tunnel coupled to a Fermi sea, we have discovered a natural cycle in which an electron spin is repeatedly created with resonant optical excitation. The spontaneous emission polarization and the nuclear spin polarization exhibit a bistability. For a σ(+) pump, the emission switches from σ(+) to σ(-) at a particular detuning of the laser. Simultaneously, the nuclear spin polarization switches from positive to negative. Away from the bistability, the nuclear spin polarization can be changed continuously from negative to positive, allowing precise control via the laser wavelength.

Low power resonant optical excitation of an optomechanical cavity

We demonstrate the actuation of a double beam opto-mechanical cavity with a sinusoidally varying optical input power. We observe the driven mechanical motion with only 200 nW coupled to the optical cavity mode. We also investigate the pump power dependence of the radio-frequency response for both the driving power and the probe power. Finally, we investigate the dependence of the amplitude of the mechanical motion on mechanical cavity quality factor.

Optically monitoring electron spin relaxation in a single quantum dot using a spin memory device

We demonstrate optical single-electron spin initialization, storage, and readout in a single self-assembled InGaAs quantum dot using a spin memory device. Single-electron spin relaxation is monitored over time scales exceeding $\ensuremath{\ge}30\text{ }\ensuremath{\mu}\text{s}$, defined only by extrinsic experimental parameters such as the optical detection efficiency. The selective generation of a single electron in the dot is performed by resonant optical excitation and subsequent partial exciton ionization; the hole is removed from the dot while the electron remains stored. When subject to a magnetic field applied in Faraday geometry, we show how the spin of the electron can be prepared with a well-defined spin projection relative to the light propagation direction simply by controlling the voltage applied to the gate electrode. The spin is stored then in the dot before being read out using an optical implementation of spin to charge conversion, whereby the spin projection of the electron is mapped onto a more robust variable, the charge state of the dot. After spin to charge conversion, we show how the charge occupancy can be repeatedly and nonperturbatively measured by pumping a luminescence recycling transition. The approach is shown to provide a readout signal ${10}^{4}$ times stronger per spin when compared to previous methods. In combination with spin manipulation using the optical Stark effect or microwaves, our approach provides an ideal basis for probing spin coherence in single self-assembled quantum dots over long-time scales and the development of methods for coherent spin control.

Design of optomechanical cavities and waveguides on a simultaneous bandgap phononic-photonic crystal slab

In this paper we study and design quasi-2D optomechanical crystals, waveguides, and resonant cavities formed from patterned slabs. Two-dimensional periodicity allows for in-plane pseudo-bandgaps in frequency where resonant optical and mechanical excitations localized to the slab are forbidden. By tailoring the unit cell geometry, we show that it is possible to have a slab crystal with simultaneous optical and mechanical pseudo-bandgaps, and for which optical waveguiding is not compromised. We then use these crystals to design optomechanical cavities in which strongly interacting, co-localized photonic-phononic resonances occur. A resonant cavity structure formed by perturbing a ;;linear defect' waveguide of optical and acoustic waves in a silicon optomechanical crystal slab is shown to support an optical resonance at wavelength lambda(0) approximately 1.5 mum and a mechanical resonance of frequency omega(m)/2pi approximately 9.5 GHz. These resonances, due to the simultaneous pseudo-bandgap of the waveguide structure, are simulated to have optical and mechanical radiation-limited Q-factors greater than 10(7). The optomechanical coupling of the optical and acousticresonances in this cavity due to radiation pressure is also studied, with a quantum conversion rate, corresponding to the scattering rate of a single cavity photon via a single cavity phonon, calculated to be g/2pi = 292 kHz.

Optical coupling of two distant InAs/GaAs quantum dots by a photonic-crystal microcavity

Long distance $(1.4\text{ }\ensuremath{\mu}\text{m})$ interaction of two different InAs/GaAs quantum dots in a photonic crystal microcavity is observed. Simultaneous coupling of both quantum dots to the cavity is demonstrated by Purcell effect measurements. Resonant optical excitation in the $p$ state of any of the quantum dots, results in an increase in the $s$-state emission of the other one. The cavity-mediated coupling can be controlled by varying the excitation intensity. These results represent an experimental step toward the realization of quantum logic operations using distant solid-state qubits.

Whispering-gallery mode excitation in a microdroplet illuminated by a train of chirped ultrashort laser pulses

The peculiarities of resonant optical field excitation inside a water microdroplet under illumination by a spatially bounded Gaussian beam with a temporal regime of a single chirped ultrashort laser pulse and a chirped pulse train are considered. It is established that the coupling efficiency of incident radiation to a selected high-Q whispering-gallery mode significantly depends on the interpulse interval in the train and chirping parameter of pumped laser radiation. The influence of the geometry of particle illumination by a laser beam and of the number of pulses in the train on the whispering-gallery mode buildup and its peak intensity is investigated.

Coherent optical control of the wave function of zero-dimensional exciton polaritons

Control of the wave function of confined microcavity polaritons is demonstrated experimentally and theoretically by means of tailored resonant optical excitation. Three dimensional confinement is achieved by etching mesas on top of the microcavity spacer layer. Resonant excitation with a continuous-wave laser locks the phase of the discrete polariton states to the phase of the laser. By tuning the energy and momentum of the laser, we achieve precise control of the momentum pattern of the polariton wave function. This is an efficient and direct way for quantum control of electronic excitations in a solid.

On resonant optical excitation and carrier escape in GaInN/GaN quantum wells

Recently, photoluminescence studies using resonant optical excitation in GaInN layers have been used to investigate the physical origin of efficiency droop in GaInN/GaN light-emitting diodes. In these studies, it has been assumed that in the case of resonant excitation, where electron-hole pairs are generated in the GaInN layers only, carrier transport effects play no role. We report that in contrast to this assumption, carrier escape from quantum wells does take place and shows strong dependence upon the duration of excitation and bias conditions. We also discuss the time scales required to reach steady-state conditions under pulsed optical excitation.