Coupled Plasmon Modes Research Articles

Silica encapsulation of quantum dots and metal clusters

The use of nanometre thick silica shells as a means to stabilize metal clusters and semiconductor particles is discussed, and its potential advantages over conventional organic capping agents are presented. Shell deposition depends on control of the double layer potential, and requires priming of the core particle surface. Chemical reactions are possible within the core, via diffusion of reactants through the shell layer. Quantum dots can be stabilized against photochemical degradation through silica deposition, whilst retaining strong fluorescence quantum yields and their size dependent optical properties. Ordered 3D and 2D arrays of a macroscopic size with uniform particle spacing can be created. Thin colloid films can also be created with well-defined interparticle spacing, allowing controlled coupling of exciton and surface plasmon modes to be investigated. A number of future core–shell nanocomposite structures are postulated, including quantum bubbles and single electron capacitors based on Au@SiO2.

MANY-BODY EFFECTS IN FRICTIONAL DRAG BETWEEN COUPLED TWO-DIMENSIONAL ELECTRON SYSTEMS

Independently contacted coupled quantum wells separated by barriers which allow significant interlayer interactions but no tunneling have been fabricated. When current is passed through one layer, the interlayer interactions drag carriers in the second layer, resulting in a voltage response (for open circuits). The magnitude of the response gives a quantitative measure of the effective interlayer interactions and response functions of the system, and hence this is an excellent laboratory for the study of many-body phenomena in two-dimensional electron gases. We review the Boltzmann and Kubo formalisms for the theory of drag effects in coupled quantum wells and discuss three specific cases where many-body effects significantly affect the drag: (1) acoustic phonon-mediated drag, (2) large enhancements due to coupled plasmon modes, and (3) interplay of screening and Landau levels in large magnetic fields.

Plasmonics: Electromagnetic Energy Transfer and Switching in Nanoparticle Chain-Arrays Below the Diffraction Limit

ABSTRACTA model is given for the transport of electromagnetic energy through structures that consist of chains of closely spaced metal nanoparticles. This transport relies on the near-field electrodynamic interaction between metal particles that sets up coupled plasmon modes. The model predicts that the propagation is coherent and the group velocities can exceed typical saturation velocities of electrons in semiconductors (≈ 105 m/s). Furthermore, it is shown that in these structures propagation of energy around sharp corners (bending radius << the wavelength of visible light) is possible at high efficiency. This efficiency is a strong function of the frequency and polarization direction of the plasmon mode. The operation of a plasmon switch is modeled, in which plasmon waves can be switched or modulated. Finally, suggestions are given for the choice of metal particle and host material.

The investigation of 1D and 2D phenomena using double-layer electron systems

The physics of low-dimensional electron systems continues to be an active field of research; using two examples we show how double-layer electron gases can extend these investigations. First, after reviewing the properties of 2D–2D tunnelling, we present equilibrium tunnelling measurements between a two-dimensional electron gas (2DEG) and an array of independent and identical 1D wires. Using a parallel magnetic field full information about the wires can be obtained, their 1D energies and wave functions, and hence the functional form of the confining potential. In the second case, the Coulomb drag (where a voltage is induced in one layer when a current is passed through the other) is measured between two separately contacted and electrically isolated 2DEGs. At temperatures comparable to the Fermi temperature, the measured transresistivity shows a plasmon enhancement. A comparison between theory and experiment shows the importance of many-body corrections to the coupled plasmon modes.

Electron–electron scattering between closely spaced two-dimensional electron gases

Electron–electron interactions between two closely spaced electron gases can be directly measured by the drag voltage induced in one layer when a current is driven through the other. We have used such drag measurements of GaAs/AlGaAs double quantum wells to probe both the electronic excitations within each layer and the interlayer Coulomb interaction. Two contrasting regimes have been explored. First, at high temperatures we measure an enhanced drag due to the “antiscreening” of the coupled plasmon modes – a result which is in good agreement with theoretical predictions. Moreover, we show the importance of many-body corrections to the plasmon modes, enabling us to test a local-field description of the correlations. Second, drag measurements in the quantum Hall regime show more pronounced structure than the Shubnikov–de Haas oscillations; the drag is more sensitive to spin splitting at high filling factors and enhances the features seen at low filling factors. We show that the localised states away from the Landu level centre influence the drag.

Raman investigations of GaN films grown by pulsed laser deposition

GaN epitaxial films grown by pulsed laser deposition have been investigated by Raman scattering measurements. Room temperature electron concentrations and mobilities of films grown on 6H-SiC were estimated by A1(LO) mode analyses according to the theory of plasmon-coupled modes yielding values of about n=5×1016cm−3 and μ=550cm2/V·s, respectively. The Raman microprobe has served to explore the homogeneity of the films with respect to distributions of the residual strain and carrier concentration.

Correlation Effects on the Coupled Plasmon Modes of a Double Quantum Well

At temperatures comparable to the Fermi temperature, we have measured a plasmon enhanced Coulomb drag in a GaAs/AlGaAs double quantum well electron system. This measurement provides a probe of the many-body corrections to the coupled plasmon modes, and we present a detailed comparison between experiment and theory testing the validity of local field theories. Using a perpendicular magnetic field to raise the magnetoplasmon energy we can induce a crossover to single-particle Coulomb scattering.

Microraman Characterization Of Microdefects In Bulk Sic

AbstractMicroRaman spectroscopy is used for characterizing defects in large SiC crystals with micrometer spatial resolution. The ability to identify microscopic inclusions of polytypes different than the crystal matrix is demonstrated; silicon and carbon inclusions and disorder effects are found in micropipes. A study of the Longitudinal Optic Phonon Plasmon Coupled (LOPC) Raman modes allowed to obtain local fluctuations of the net donor concentration, ND-NA, and the electron mobility around defects, which allowed impurity gettering effects to be observed.

Plasmon enhancement of Coulomb drag in double-quantum-well systems.

We derive an expression for the drag rate (i.e., interlayer momentum transfer rate) for carriers in two coupled two-dimensional gases to lowest nonvanishing order in the screened interlayer electron-electron interaction, valid for arbitrary intralayer scattering mechanisms, using the Boltzmann transport equation. We calculate the drag rate for experimentally relevant parameters, and show that for moderately high temperatures (T\ensuremath{\gtrsim}0.2${\mathit{T}}_{\mathit{F}}$, where ${\mathit{T}}_{\mathit{F}}$ is the Fermi temperature) the dynamical screening of the interlayer results in a large enhancement of the drag rate due to the presence of coupled plasmon modes. This plasmon enhancement causes the scaled drag rate to have a peak (i) as a function of temperature at T\ensuremath{\approxeq}0.5${\mathit{T}}_{\mathit{F}}$, and (ii) as a function of the ratio of densities of the carriers in the two layers when their Fermi velocities are equal. We also show that the drag rate can be significantly affected by the intralayer scattering mechanisms; in particular, the drag rate changes approximately by a factor of 2 when the dopant-layer modulation-doped structures are moved in from 400 to 100 \AA{}.

Coulomb Drag as a Probe of Coupled Plasmon Modes in Parallel Quantum Wells.

We show theoretically that the Coulomb drag rate between two parallel quasi-two-dimensional electron gases with equal Fermi velocities is substantially enhanced by the coupled acoustic and optic plasmon modes at temperatures $T\ensuremath{\gtrsim}0.2{T}_{F}$ (where ${T}_{F}$ is the Fermi temperature) for experimentally relevant parameters. The acoustic mode causes a sharp upturn in the scaled drag rate with increasing temperature at $T\ensuremath{\approx}0.2{T}_{F}$. Other experimental signatures of the plasmon-dominated drag rate are a ${d}^{\ensuremath{-}3}$ dependence on the well separation $d$ and a peak as a function of relative densities at matched Fermi velocities.

Metal/ n-InP interfaces studied by photoreflectance and Raman spectroscopies

We report electrical and optical studies of metal/ n-InP interfaces formed at room temperature (RT = 300 K) and low temperature (LT = 77 K). From current-voltage ( I- V) and capacitance-voltage ( C- V) data, it was found that the leakage current density, J 0 of the LT diode was reduced by more than six orders of magnitude with respect to the RT diode. The Schottky barrier height, φ B was increased from 0.44 to 0.83 eV with Pd metal. This suggested Fermi-level unpinning in the LT diodes. Better surface preservation was revealed by photoreflectance spectroscopy (PR) measurement. The surface electric field, ξ built-in potential, V bi and broadening parameter, Γ were also determined from PR spectra. These results were further related to Raman spectra, in which both the LO phonon mode from the depletion layer, and the coupled plasmon mode, L + from the bulk were observed. The intensity of the LO mode increased with the width of the depletion layer. The Raman shift of the L + was sensitive to the carrier density in the bulk region. The relative intensity of LO and L + was used to compare the width of the depletion layer for RT and LT samples. Very good agreement was obtained between the electrical and optical results.

Monte Carlo simulations of field and carrier density dependent hole transport in an InGaAs/GaAs strained layer quantum well

The authors have carried out Monte Carlo simulations of hole transport in a range of InGaAs/GaAs strained quantum well structures. The simulations include phonon, impurity and alloy scattering, with rates obtained using k.p band structure. The simulations give good agreement with the experimentally determined magnitude and carrier density dependence of the low field hole mobility, for samples with carrier densities ps<or=4.0*1011 cm-2. The results suggest that alloy scattering is primarily responsible for the low mobilities observed in these structures. At ps=4.0*1011 cm-2 strong coupling of optical phonon and plasmon modes leads to enhanced scattering and consequent reduction of the hole drift velocity, but in the current model this effect still does not fully account for the strong saturation of the drift velocity observed experimentally.

THE OPTICAL PHONONS,,PLASMON AND LO PHONON-PLASMON COUPLING MODE IN MIXED CRYSTAL GaAs_（1-x）_P_x_ ROWN BY LIQUID PHASE EPITAXY

在40--7 0 0 (1/cm)波数和4.2--300 K 温度范围内研究了不同组份液相外延n 型GaAs_（1-x）_P_x__样品的红外反射谱. 对反射谱进行了鹰谐振子拟合与K-K 关系计算.从而获得了有关描述GaAs_（1-x）_P_x__样品的光学声子模、等离子体激元、LO 声子-等离子体激元藕合模的重要物理参量及红外光学常数信息. 基于这些计算结果, 提出了双导带谷并计及与x 能谷相联系的施主能级模型, 用它解释等离子体激元频率和实验观察的三支主要的祸合模的温度变化规律。

Raman scattering characterization of Ga1−xAlxAs/GaAs heterojunctions: Epilayer and interface

We have used Raman spectroscopy to characterize Ga1−xAlxAs/GaAs heterojunctions (x≊0.9) grown by liquid phase epitaxy. The GaAs substrate had a (100) surface and the Ga1−xAlxAs epilayer was about 5000 Å thick. Signals were observed from both the Ga1−xAlxAs epilayer and the GaAs substrate at the interface. Information about the quality of the epilayer can be gained from several aspects of the spectrum including linewidths, violations of polarization selection rules (e.g., observation of symmetry forbidden TO mode), and the amplitude of the disorder-activated LA mode (DALA). These results are correlated with the properties of the substrate and growth condition. In addition the signal observed from the GaAs substrate revealed interesting features such as coupled plasmon modes and hence yielded information about band- bending and dopant diffusion at the interface.

Band‐to‐Band Recombination via Spontaneous Emission of Phonon–Plasmon Coupled Modes in Lead Chalcogenides

AbstractIn lead chalcogenides with degenerate statistics the small‐signal lifetime for spontaneous emission of phonon–plasmon coupled modes is calculated in dependence on temperature. The results are compared with corresponding data concerning Auger recombination. Within the approximation of undamped collective modes plasmon emission dominates as compared to Auger recombination for doping levels ND &gt; 1019 cm−3 in PbSnSe at sufficiently low temperatures.

Reflectivity of Tm monochalcogenides: No evidence of valence fluctuations

The reflectivity of metallic TmS, TmSe and semiconducting TmTe has been measured between 0.03 eV and 12 eV and has been interpreted as 4f n -4f n−1 5d and p-d interband transitions and coupled plasmon modes.

Study of the Dispersion Relations of the Coupled Phonon-Damped Plasmon Mode inn-GaAs by Light Scattering

Recent developments of the study on the coupled LO phonon-electron system are described. A special attention is paid to the behavior of a coupled mode in the Landau damping region in n·GaAs. Capability of the light scattering is also discussed for getting information about the semiconductor surfaces as well as the dispersion and damping of the mode. The study of electron and phonon interactions is of fundamental im­ portance in polar semiconductors. The longitudinal optical (LO) phonons interact with carriers through longitudinal polarization field, causing the well known polaron effects!) as well as a shift of the LO phonon frequency. 2>•3> The existence of the coupled LO phonon-plasmon modes was first predicted by Yokota in the long wavelength limit. 4> Gurevich et al. independently discussed the dispersion of the coupled modes in connection with the possibility of superconductivity in semiconductors. 5) The LO phonon-plasmon system was further studied theoretically for the degenerate polar semiconductors, G),7) where the usual self consistent field method is reliable. 8) Such a treatment is closely related to the theory of the coupled electron-ion system in simple metals. 9>• 10> Fallowing the invention of the laser sources, the opening of the Renaissance was notified in the light scattering spectroscopy which had been a valuable tool in the investigation of vibrational and rotational energy levels of molecules as well as the determination of phonon frequencies in crystalline solids.11> Accordingly theoretical aspects of light scattering also entered upon a new phase. Many exciting effects have been discovered in the field of light scatter­ ing in this decade. Elementary excitations such as phonons, plasmons, magnons and polaritons as well as single particle excitations have been studied extensively_l2)~ 15) Experimental researches on LO phonon and high density