Intersubband Plasmons Research Articles

Role of geometry for strong coupling in active terahertz metamaterials

We discuss the influence of geometry and type of meta-atom resonance on important parameters for strong and ultrastrong light-matter coupling experiments based on localized plasmons in planar metamaterials and intersubband plasmons in quantum wells. An analytic model based on the density matrix formalism is used to extract the radiative linewidth, the achievable coupling rate, and its dependence on distance between a single quantum well and a metasurface for five different types of meta-atoms. Depending on these values, the optical response of the coupled system gives rise to different regimes ranging from the weak-coupling regime over the hybridization-induced transparency regime to the strong-coupling regime. As a result of our investigation, we show that the achievable coupling rates exhibit only a weak dependence on the actual geometry and thus the physical nature of the metamaterial resonance. In all cases, the coupling rate can be made large enough to reach the strong-coupling regime by using multiple quantum wells.

Gate controlled coupling of intersubband plasmons

The optical response of a heavily doped quantum well, with two occupied subbands, has been investigated as a function of the electronic density. It is shown that the two optically active transitions are mutually coupled by dipole-dipole Coulomb interaction, which strongly renormalizes their absorption amplitude. In order to demonstrate this effect, we have measured a set of optical spectra on a device in which the electronic density can be tuned by the application of a gate voltage. Our results show that the absorption spectra can be correctly described only by taking into account the Coulomb coupling between the two transitions. As a consequence, the optical dipoles originating from intersubband transitions are not independent, but rather coupled oscillators with an adjustable strength.

Charge-Induced Coherence between Intersubband Plasmons in a Quantum Structure

In this Letter we investigate a low dimensional semiconductor system, in which the light-matter interaction is enhanced by the cooperative behavior of a large number of dipolar oscillators, at different frequencies, mutually phase locked by Coulomb interaction. We experimentally and theoretically demonstrate that, owing to this phenomenon, the optical response of a semiconductor quantum well with several occupied subbands is a single sharp resonance, associated with the excitation of a bright multisubband plasmon. This effect illustrates how the whole oscillator strength of a two-dimensional system can be concentrated into a single resonance independently from the shape of the confining potential. When this cooperative excitation is tuned in resonance with a cavity mode, their coupling strength can be increased monotonically with the electronic density, allowing the achievement of the ultrastrong coupling regime up to room temperature.

Collective excitations of electron gas on the nanotube surface in a magnetic field

The spectra of magnetoplasma waves, zero sound and spin waves in an electron gas on a surface of a nonferromagnetic nanotube in longitudinal magnetic field are considered. The initial spectrum of the electron energy is assumed to be parabolic, and the interelectron interaction is taken into account within the random-phase approximation. It is shown that in a magnetic field there is a shift of the frequency of an intersubband plasmon proportional to a magnetic flux through a cross-section of a tube. The frequency and the damping constant of magnetoplasma waves in a nondegenerate electron gas are calculated. For a great number of filled subbands the frequencies of the zero sound and spin waves in a degenerate electron gas experience oscillations of the type of de Haas-van Alphen and Aharonov-Bohm with changing the density of electrons and magnetic flux.

Magnetoplasma oscillations of a two-dimensional electron system with spin-orbit interaction in a lateral superlattice

Low-frequency magnetoplasmon modes in a one-dimensional lateral superlattice with Rashba spin-orbit splitting are considered. Such modes correspond to oscillations related to virtual transitions within a Landau level that become possible due to the broadening of each Landau level into a band produced by the superlattice potential. The specificity of the one-dimensional intersubband plasmons emerging in such a system has been revealed.

Intersubband plasmons in a quasi-one-dimensional system

It has been shown that plasma oscillations in quasi-one-dimensional systems feature the presence of two branches of intersubband plasmons for each pair of subbands. The second (low-frequency) branch lies in the region of the frequency-momentum plane, where a plasma-wave-induced single-particle transition is allowed but there is no Landau damping.

The electrostatic coupling of longitudinal optical phonon and plasmon in wurtzite InN thin films

We utilize coherent longitudinal optical phonon as an optical nanoprobe to investigate the plasmonic behavior of wurtzite c-plane InN thin films. The transition from the three-dimensional electron gas in InN bulk toward the two-dimensional electron gas in InN thin film is revealed via measuring the coupling strength of coherent A1(LO) phonon and plasmon coupling mode. The coupling strength diminishes as the film thickness reduces and finally vanishes at 3±1 nm. This phenomenon is ascribed to the detuning of the intersubband plasmon frequency above the A1(LO) frequency, which is originated from the electronic quantum confinement in InN thin films.

Controlled generation of resonant electron-electron scattering induced current in quantum well structures

Current voltage measurements on samples designed for the resonant excitation of intersubband plasmons are reported. These resonances which represent collective electron-electron scattering processes contribute a significant amount to the total current under controlled bias conditions. Measurements with applied magnetic field give additional evidence for the resonant mechanism when the cyclotron energy (or multiples of it) matches the subband splitting.

Model of plasmon excitations in a bundle and a two-dimensional array of nanotubes

We calculate the plasma excitations in a bundle as well as a two-dimensional (2D) periodic array of aligned parallel multishell nanotubes on a substrate. The carbon nanotubes are oriented perpendicular to the substrate. The model we use for the system is an electron gas confined to the surface of an infinitely long cylinder embedded in a background dielectric medium. Electron tunneling between individual tubules is neglected. We include the Coulomb interaction between electrons on the same tubule and on different tubules for the same nanotube and neighboring nanotubes. We present a self-consistent field theory for the dispersion equation for intrasubband and intersubband plasmon excitations. For both the bundle and 2D array of aligned parallel nanotubes, the dispersion relation of the collective modes is determined by a three-dimensional wave vector with components in the direction of the nanotube axes and in the transverse directions. The dispersion equation is solved numerically for a single-wall nanotube 2D array as well as a bundle, and the plasmon excitation energies are obtained as a function of wave vector. The intertube Coulomb interaction couples plasmons with different angular momenta $m$ in individual nanotubes, lifting the $\ifmmode\pm\else\textpm\fi{}m$ degeneracy of the single-nanotube modes. This effect is analyzed numerically as a function of the separation between the tubules. We show that the translational symmetry of the lattice is maintained in the plasmon spectrum for the periodic array, and the plasmon energies have a periodic dependence on the transverse wave vector ${\mathbf{q}}_{\ensuremath{\perp}}$. For the bundle, the Coulomb interaction between nanotubes gives rise to optical plasmon excitations.

Theory of the Pseudospin Resonance in Semiconductor Bilayers

The pseudospin degree of freedom in a semiconductor bilayer gives rise to a collective mode analogous to the ferromagnetic-resonance mode of a ferromagnet. We present a many-body theory of the dependence of the energy and the damping of this mode on layer separation d. Based on these results, we discuss the possibilities of realizing transport-current driven pseudospin-transfer oscillators in semiconductors, and of using the pseudospin-transfer effect as an experimental probe of intersubband plasmons.

Intersubband excitations at finite temperatures and their roles in different quasi-one-dimensional systems

We theoretically study many-body excitations in three different quasi-one-dimensional (Q1D) electron systems: (i) those formed on the surface of liquid Helium; (ii) in two coupled semiconductor quantum wires; and (iii) Q1D electrons embedded in polar semiconductor-based quantum wires. Our results show intersubband coupling between higher subbands and the two lowest subbands affecting even the lower energy intersubband plasmons on the liquid Helium surface. Concerning the second system, we show a pronounced extra peak appearing in the intersubband impurity spectral function for temperatures as high as 20 K. We finally show coupled intersubband plasmon-phonon modes surviving for temperatures up to 300 K.

Confined acoustic and optical plasmons in double-layered quantum-wire arrays with strong tunneling

We investigate electronic excitations in $\mathrm{Ga}\mathrm{As}\text{\ensuremath{-}}{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ double-layered quantum wire arrays with strong tunneling coupling by resonant inelastic light scattering. By applying an external electric field, we can change the one-dimensional (1D) electron density and the symmetry of the double quantum-well (DQW) structure at the same time. We identify confined optical 1D intersubband plasmons (COP) and confined acoustic 1D intersubband plasmons (CAP). Due to the tunneling coupling, the energies of the CAP exhibit a minimum for a symmetric DQW potential, whereas the energies of the COP are dominated by the total carrier density, and are nearly insensitive to the symmetry of the potential.

Two-Electron Linear Intersubband Light Absorption in a Biased Quantum Well

We point out a novel manifestation of many-body correlations in the linear optical response of electrons confined in a quantum well. Namely, we demonstrate that along with the conventional absorption peak at a frequency omega close to the intersubband energy delta, there exists an additional peak at frequency h omega approximately = 2delta. This new peak is solely due to electron-electron interactions, and can be understood as excitation of two electrons by a single photon. The actual peak line shape is comprised of a sharp feature, due to excitation of pairs of intersubband plasmons, on top of a broader band due to absorption by two single-particle excitations. The two-plasmon contribution allows us to infer intersubband plasmon dispersion from linear absorption experiments.

Effects of collective excitations on the quantum well intersubband absorption

The dependence of the intersubband absorption spectra on the Coulomb interaction and quantum well (QW) width is studied. Rather than following the Fermi–Dirac distribution, we have solved the intersubband equations of motion to determine the subband population self-consistently. We have gone beyond the linear absorption theory to show the effect of various many-body interactions on the absorption spectra. It is found that the redistribution of electrons in excited states reduces the absorption. Our results indicate that the line shape and peak position are determined by the interplay of different collective excitations, such as the Fermi edge singularity and the intersubband plasmon. The dependence of the absorption spectrum on the QW width and the subband effective masses is also discussed.

New Branch of Intersubband Plasmons in a Nonequilibrium Two-Layer System

The plasma oscillation spectrum for 2D electrons in a double quantum well is calculated. It is shown that an additional branch of intersubband plasmons can exist without experiencing Landau damping in the case of nonequilibrium population of subbands. In an asymmetric structure, this branch is responsible for both the emergence of instabilities and the possibility of amplification of plasma waves.

Propagation characteristics of surface-plasmon waveguides operating in the mid- and far infrared: Nonperturbative approach

The modal propagation characteristics of metal surface-plasmon waveguides (MSPWs) containing n-doped multiple quantum wells with normal and inverted subband occupation are studied theoretically. “Ordinary” (“enhanced”) waveguides modeled by simple three (four) media structures are considered. The appropriate dispersion equations are derived employing the transfer-matrix approach and solved numerically. Analytical solutions are also obtained employing the thin-layer approximation. Special attention is paid to the role of the coupling between the modes guided by passive MSPWs and the intersubband plasmon. The obtained results indicate that modification of the propagation characteristics induced by the above-mentioned coupling plays a very important role. We show that the commonly used perturbed approach based on the concept of the confinement factor has a restricted range of applicability, particularly in the case of the ordinary MSPWs.

Theory of Intersubband Raman Laser in Modulation-doped Asymmetric Coupled Double Quantum Wells

A microscopic theory is developed for the laser gain due to stimulated intersubband electronic Raman scattering pumped by CO 2 laser in modulation-doped GaAs/AlGaAs asymmetric coupled double quantum wells (ACDQWs). Based on the charge-density-excitation (CDE) mechanism, the formula for electronic Raman scattering cross-section is given, taking into account the coupling between intersubband plasmon and optical phonons including GaAs confined LO phonons and interface phonons. Stimulated Raman gain factor is then derived from the cross-section. The optimization of Raman gain, the gain saturation and threshold condition are also discussed. Numerical analysis for temporal variation of stimulated Stokes photon density, subband populations and output Raman laser power is carried out by using the self-consistent conventional rate equations. The theory can predict the presence or lack of coupled modes in lasing in agreement with the recent experimental results.

Induced Transparency by Intersubband Plasmon Coupling in a Quantum Well

We study coupling of two intersubband plasmons associated with dipole-allowed cascading transitions in a quantum well. We show that the coupling can lead to the disappearance of the lower-energy resonance accompanied by an anticrossing behavior. Such coupling induced anomalies are of collective and resonant nature and provide the first example of Coulomb interaction induced transparency. Our numerical results from a microscopic theory are confirmed by an analytical model.

Grating-coupler assisted infrared spectroscopy on anisotropic multilayer systems: A comparative study

We examine two different methods to calculate the electromagnetic fields and the optical properties of multilayer structures of anisotropic media with gratings: (i) A modal-expansion method and (ii) a coupled-wave method. We show that both methods, which have been developed independently for different fields of applications and implemented on different platforms, give the same results to a very high precision. This is true even for such very crucial cases like for the higher-order excitation maxima of the (0-0) intrasubband plasmon ω 00 p or of the lineshape concerning any details in the spectrum of the (1-0) intersubband plasmon ω 10 p of a quasi-two-dimensional electron gas of GaAs–Ga 1− x Al x As multilayer structures with a finite height silver grating on top of the sample.

Symmetry driven plasmon transformations in a bilayer electron system

The effect of space symmetry on plasma excitations in a tunnel coupled bilayer electron system has been examined by means of inelastic light scattering spectroscopy. In the symmetrical state there exists a tunnel plasma mode with a long wavelength gap, a positive linear dispersion, and a large spectral weight. The mode of the acoustical plasmon softens and shifts to the single particle excitation continuum. When the system is driven to the asymmetric state the tunnel plasmon converts to an intersubband plasmon with different dispersion properties and small spectral weight, whereas the acoustical plasmon mode gains energy and spectral weight.