Intersubband Excitations Research Articles

Hot Electron Cooling in n-Doped Colloidal Nanoplatelets Following Near-Infrared Intersubband Excitation.

Intersubband transition was recently discovered in colloidal nanoplatelets, but the associated intersubband carrier relaxation dynamics remains poorly understood. In particular, it is crucial to selectively excite the intersubband transition and to follow the hot electron dynamics in the absence of valence-band holes. This is achieved herein by exciting the predoped electrons in CdSe/ZnS nanoplatelets using near-infrared femtosecond pulses and monitoring nonequilibrium electron dynamics using broad-band visible pulses. We find that the n = 2 electrons relax to the n = 1 subband and establish a Fermi-Dirac distribution within 200 fs, and finally reach an equilibrium with the lattice within a few ps. The cooling dynamics depend mainly on the excitation fluence but weakly on the doping density and the lattice temperature. These characteristics are well captured by our numerical simulation that explicitly accounts for the state occupation effect and optical phonon scattering.

Raman photogalvanic effect: Photocurrent at inelastic light scattering

We show theoretically that electromagnetic waves propagating in the transparency region of a noncentrosymmetric medium can induce a DC electric current. The origin of the effect is the Raman scattering of light by free carriers in the system. Due to the photon scattering, electrons undergo real quantum transitions resulting in the formation of their anisotropic momentum distribution and in shifts of electronic wave packets giving rise to a steady-state photocurrent. We present the microscopic theory of the Raman photogalvanic effect (RPGE) focusing on two specific situations: (i) generic case of a bulk gyrotropic semiconductor and (ii) a quantum well structure where the light is scattered by intersubband excitations. We uncover the relation of the predicted RPGE and the traditional photogalvanic effect at the light absorption.

Intersubband excitations in ultrathin core-shell nanowires in the one-dimensional quantum limit probed by resonant inelastic light scattering

The observation of electronic Raman scattering in one-dimensional semiconductors has so far been limited to arrays of laterally structured quantum wells. Recent progress in the growth of core-shell nanowires has opened the possibility to reach the quantum confinement regime for single nanowires. Here, the authors present measurements of the one-dimensional subband structure of single wurtzite-phase GaAs nanowires by resonant Raman spectroscopy. The obtained energetic distances between consecutive electron or hole subbands are nicely reproduced by realistic k∙p band structure calculations.

Mono-cycle terahertz pulses from intersubband shift currents in asymmetric semiconductor quantum wells

Both second-order frequency mixing in nonlinear optical media and photoconductive antennas have provided terahertz (THz) transients in a wide parameter range. Here, we demonstrate a novel type of ultrafast nonlinear optical response in asymmetric semiconductor quantum wells, originating from electron shift currents. Resonant intersubband excitation by a femtosecond mid-infrared pulse induces a transient spatial shift of electronic charge, which leads to the emission of a mono-cycle THz pulse. This mechanism is characterized and separated from conventional difference frequency mixing by nonlinear two-dimensional THz spectroscopy. The amplitude of the THz electric field emitted by the shift current reaches several percent of the mid-infrared driving field, significantly higher than in difference frequency mixing. Changes in time structure of the THz pulse upon nonlinear saturation of intersubband absorption allow for generating tailored THz transients. The present concept can be implemented in highly compact devices driven by mid-infrared pulses at megahertz repetition rates to provide versatile THz pulses for spectroscopy and optoelectronics.

Intersubband Absorption in a Microwave-Excited Inhomogeneous 2D Electron Gas under Quantizing Magnetic Fields

We present the first theoretical description of microwave-absorption magnetooscillations in a 2D electron gas on liquid helium occupying two subbands with the inhomogeneous inter-subband excitation...

Magnetoconductivity oscillations induced by intersubband excitation in a degenerate 2D electron gas

Magnetoconductivity oscillations and absolute negative conductivity induced by nonequilibrium populations of excited subbands in a degenerate multisubband two-dimensional electron system are studied theoretically. The displacement from equilibrium, which can be caused by resonant microwave excitation or by any other reason, is assumed to be such that electron distributions can no longer be described by a single Fermi level. In this case, in addition to the well-known conductivity peaks occurring at the Shubnikov-de Haas conditions and small peaks of normal intersubband scattering, sign-changing oscillations with a different shape are shown to be possible. We found also that even a small fraction of electrons transferred to the excited subband can lead to negative conductivity effects.

Magneto-Oscillations and Anomalous Current States in a Photoexcited Electron Gas on Liquid Helium

The paper reviews a novel class of phenomena observed recently in the two-dimensional (2D) electron system formed on the free surface of liquid helium in the presence of a magnetic field directed normally and exposed to microwave radiation. The distinctive feature of these nonequilibrium phenomena is magnetoconductivity oscillations induced by inter-subband (out-of-plane) and intra-subband (in-plane) microwave excitations. The conductivity magneto-oscillations induced by intra-subband excitation are similar to remarkable microwave-induced resistance oscillations (MIRO) reported for semiconductor heterostructures. Investigations of microwave-induced conductivity oscillations on liquid helium helped with understanding of the origin of MIRO. Much stronger microwave-induced conductivity oscillations were observed and well described theoretically for resonant inter-subband microwave excitation. At strong powers, such excitation leads to zero-resistance states, the in-plane redistribution of electrons, self-generated audio-frequency oscillations, and incompressible states. These phenomena are caused by unusual current states of the 2D electron system formed under resonant microwave excitation.

The fundamentals of harnessing the magneto-optics of quantum wires for designing optical amplifiers: Formalism

Quantum wires occupy a unique status among the semiconducting nanostructures with reduced dimensionality, no other system seems to have engaged researchers with as many appealing features to pursue. This paper aims at a core issue related with the magnetoplasmon excitations in the quantum wires characterized by the confining harmonic potential and subjected to a longitudinal electric field and a perpendicular magnetic field in the symmetric gauge. Despite the substantive complexity, we obtain the exact analytical expressions for the eigenfunction and eigenenergy, using the scheme of ladder operators, which fundamentally characterize the quantal system. Crucial to this inquiry is an intersubband collective excitation that evolves into a magnetoroton — above a threshold value of magnetic field — which observes a negative group velocity between the maxon and the roton. The evidence of negative group velocity implies anomalous dispersion in a gain medium with the population inversion that forms the basis for the lasing action of lasers. Thus, the technological pathway that unfolds is the route to devices exploiting the magnetoroton features for designing the novel optical amplifiers at nanoscale and hence paving the way to a new generation of lasers.

Intersubband plasmon excitations in doped carbon nanotubes

We theoretically investigate intersubband plasmon excitations in doped single wall carbon nanotubes (SWNTs) by examining the dependence of plasmon frequency on the nanotube diameter, chirality, and Fermi energy. The intersubband plasmons can be excited by light with polarization perpendicular to the nanotube axis and thus the plasmon excitations corresponds to optical transitions between the two different subbands, which are sensitive to the Fermi energy. In every SWNT, this mechanism leads to the emergence of the optical absorption peak at the plasmon frequency for a given Fermi energy, $E_F$. The plasmon frequencies calculated for many SWNTs with diameter $d_t < 2$ nm exhibit a dependence on $(1/d_t)^{0.7}$ and the frequencies are further affected by Fermi energy as $E_F^{0.25}$. With this knowledge, it is possible to develop a map of intersubband plasmon excitations in doped SWNTs that could be useful to quickly estimate the doping level and also be an alternative way to characterize nanotube chirality.

Intersubband plasmons induced negative refraction at mid-IR frequency in heterostructured semiconductor metamaterials

In this contribution, we report on negative refraction effect occurring in layered semiconductors. The origin of this effect is attributed to the presence of intersubband plasmon resonance induced by the electronic confinement of the electrons in thick quantum wells. Relying on the effective medium theory, we analyse both the in plane and the out of plane effective optical properties of highly doped ZnO/ZnMgO semiconductor material. We theoretically show and experimentally demonstrate that thicknesses and doping levels of each layer can be carefully chosen to feature strong intersubband transitions, leading to type 1 and type 2 hyperbolic response. Understanding the optical properties of intersubband materials in the frame of hyperbolic materials would shed new light on which physical mechanisms are controlling the radiative decay of intersubband plasmon excitations. This approach could be further utilized to designing efficient mid-IR sources.

Harnessing the magneto-optics of quantum wires for designing optical amplifiers

Quantum wires occupy a unique status among the semiconducting nanostructures with reduced dimensionality – no other system seems to have engaged researchers with as many appealing features to pursue. This letter aims at a core issue related with the magnetoplasmon excitations in the quantum wires characterized by the confining harmonic potential and subjected to a longitudinal electric field and a perpendicular magnetic field in the symmetric gauge. Despite the substantive complexity, we obtain the exact analytical expressions for the eigenfunction and eigenenergy, using the scheme of ladder operators, which fundamentally characterize the quantal system. Crucial to this inquiry is an intersubband collective excitation that evolves into a magnetoroton – above a threshold value of magnetic field – which observes a negative group velocity between the maxon and the roton. The evidence of negative group velocity implies anomalous dispersion in a gain medium with the population inversion that forms the basis for the lasing action of lasers. Thus, the technological pathway that unfolds is the route to devices exploiting the magnetoroton features for designing the novel optical amplifiers at nanoscale and hence paving the way to a new generation of lasers.

Hybrid k·p tight-binding model for subbands and infrared intersubband optics in few-layer films of transition-metal dichalcogenides: MoS2 , MoSe2 , WS2 , and WSe2

We present a density functional theory parametrized hybrid $\mathrm{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{p}$ tight-binding model for electronic properties of atomically thin films of transition-metal dichalcogenides, $2\mathrm{H}\ensuremath{-}M{X}_{2}\phantom{\rule{0.16em}{0ex}}(M=\mathrm{Mo},\phantom{\rule{0.16em}{0ex}}\mathrm{W};\phantom{\rule{0.16em}{0ex}}X=\mathrm{S}$, Se). We use this model to analyze intersubband transitions in $p$- and $n$-doped $2\mathrm{H}\text{\ensuremath{-}}M{X}_{2}$ films and predict the line shapes of the intersubband excitations, determined by the subband-dependent two-dimensional electron and hole masses, as well as excitation lifetimes due to emission and absorption of optical phonons. We find that the intersubband spectra of atomically thin films of the 2H-$M{X}_{2}$ family with thicknesses of $N=2$ to 7 layers densely cover the infrared spectral range of wavelengths between 2 and $30\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$. The detailed analysis presented in this paper shows that for thin $n$-doped films, the electronic dispersion and spin-valley degeneracy of the lowest-energy subbands oscillate between odd and even number of layers, which may also offer interesting opportunities for quantum Hall effect studies in these systems.

5a-J-6 磁場中量子細線のサブバンド間励起

5a-J-6 磁場中量子細線のサブバンド間励起

Anisotropic excitons and their contributions to shift current transients in bulk GaAs

Shift current transient are obtained for near band gap excitation of bulk GaAs by numerical solutions of the semiconductor Bloch equations in a basis obtained from a 14 band k.p model of the band structure. This approach provides a transparent description of the optically induced excitations in terms of interband, intersubband, and intraband excitations which enables a clear distinction between different contributions to the shift current transients and fully includes resonant as well as off-resonant processes. Using a geodesic grid in reciprocal space in our numerical solutions, we are able to include the electron-hole Coulomb attraction in combination with our anisotropic three-dimensional band structure. We obtain an excitonic absorption peak and an enhancement of the continuum absorption and demonstrate that the excitonic wave function contains a significant amount of anisotropy. Optical excitation at the excitonic resonance generates shift current transients of significant strength, however, due to the electron-hole attraction the shift distance is smaller than for above band gap excitation. We thus demonstrate that our approach is able to provide important information on the ultrafast electron dynamics on the atomic scale.

Temperature dependence of photogalvanic effect in GaAs/AlGaAs two-dimensional electron gas at interband and intersubband excitation

The linear (LPGE) and circular photogalvanic effects (CPGE), induced by interband (532 nm) and intersubband (1064 nm) excitation, have been investigated in a temperature range from 77 to 300 K in GaAs/AlGaAs two-dimensional electron gas. The temperature dependences of the CPGE current induced by Rashba and Dresselhaus spin orbit coupling (SOC) under the interband and intersubband excitation are obtained, respectively. It is revealed that the CPGE and LPGE current induced by the intersubband excitation almost increases with increasing temperature, while that induced by interband excitation nearly decreases with increasing temperatures. These phenomena may be attributed to the different variation trends of photo-induced carrier density and SOC with increasing temperatures between interband and intersubband excitation. The evolutions of the ratio of the Rashba and Dresselhaus SOC (RD ratios) and the anisotropic ratio of linear photogalvanic tensors, corresponding to the interband and intersubband excitations, with temperatures are determined. The power dependence of CPGE and LPGE current induced by intersubband excitation is also investigated. It is found that for the power range from 0 to 275 mW, the CPGE current nearly increases linearly with increasing power, while the LPGE current shows a decreasing generation rate at higher power. This study also provides a method to manipulate the photogalvanic effects by temperatures and by using excitation light of different wavelengths.

Observation of collective excitations in MgZnO/ZnO two-dimensional electron systems by resonant Raman scattering

The neutral excitations in two-dimensional electron systems (2DES) confined at ${\mathrm{Mg}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{O}/\text{ZnO}$ heterojunctions were studied by a resonant inelastic light scattering (ILS) technique. At zero magnetic field the ILS spectra of intra- and intersubband collective excitations are detected in the wavelength range close to the ZnO direct band gap. The intersubband spectrum is represented by collective modes of charge- and spin-density excitations along with single-particle excitation continuum. The energies of these excitations were characterized on the series of samples and shown to grow sublinearly with the two-dimensional electron density. The intrasubband 2D plasmon was observed at plasma frequency and its square-root momentum dispersion was ascertained. Strong electron-electron correlations are manifested in the energy structure of intersubband excitations. Depolarization and excitonic contributions to the charge- and spin-density excitation energies are extracted in the wide range of 2D-electron densities. The negative excitonic contribution dominates among the many-particle energy terms and results in an unusual hierarchy of 2DES intersubband collective excitation energies.

Ultrafast shift and rectification photocurrents in GaAs quantum wells: Excitation intensity dependence and the importance of band mixing

A microscopic approach that is based on the multisubband semiconductor Bloch equations formulated in the basis of a 14-band ${\mathbf k} \cdot {\mathbf p}$ model is employed to compute the temporal dynamics of photocurrents in GaAs quantum wells following the excitation with femtosecond laser pulses. This approach provides a transparent description of the interband, intersubband, and intraband excitations, fully includes all resonant as well as off-resonant excitations, and treats the light-matter interaction non-perturbatively. For linearly polarized excitations the photocurrents contain contributions from shift and rectification currents. We numerically compute and analyze these currents generated by the excitation with femtosecond laser pulses for [110]- and [111]-oriented GaAs quantum wells. It is shown that the often employed perturbative $\chi^{(2)}$-approach breaks down for peak fields larger than about 10~kV/cm and that non-perturbative effects lead to a reduction of the peak values of the shift and rectification currents and to temporal oscillations which originate from Rabi flopping. In particular, we find a complex oscillatory photon energy dependence of the magnitudes of the shift and rectification currents. Our simulations demonstrate that this dependence is the result of mixing between the heavy- and light-hole valence bands. This is a surprising finding since the bandmixing has an even larger influence on the strength of the photocurrents than the absorption coefficient. For [110]-oriented GaAs quantum wells the calculated photon energy dependence is compared to experimental results and a good agreement is obtained which validates our theoretical approach.

Resonant electronic Raman scattering: A BCS-like system

In this paper we investigate the resonant intersubband Raman scattering of two-dimensional electron systems in GaAs-AlGaAs single quantum wells. Self-consistent calculations of the polarized and depolarized Raman cross sections show that the appearance of excitations at the unrenormalized single-particle energy are related to three factors: the extreme resonance regime, the existence of degeneracy in intersubband excitations of the electron gas, and, finally, degeneracy in the interactions between pairs of excitations. It is demonstrated that the physics that governs the problem is similar to the one that gives rise to the formation of the superconducting state in the BCS theory of normal metals. Comparison between experiment and theory shows an excellent agreement.

Saturable Two-Step Photocurrent Generation in Intermediate-Band Solar Cells Including InAs Quantum Dots Embedded in Al0.3Ga0.7/GaAs Quantum Wells

We studied in detail the photocurrent generation process in two-step photon absorption in intermediate-band solar cells, including InAs quantum dots embedded in Al $_{0.3}$ Ga $_{0.7}$ As/GaAs quantum wells at room temperature. The photocurrent generated by the two-step photon absorption exhibited saturation as the interband excitation intensity increased in strength. On the other hand, as the intersubband excitation intensity increased, the two-step photoexcitation current deviated from a power law. Furthermore, the two-step photoexcitation current exhibiting saturation and deviation strongly depended on both the interband and intersubband excitation intensities. To interpret these phenomena, we performed a theoretical simulation of the two-step photoexcitation current. The results suggest that the photocurrent saturation and deviation were caused by filling of the intermediate states with electrons. Furthermore, our calculated results indicate that the electron-recombination lifetime in the intermediate states is extremely long. The results of the temperature dependence of the two-step photoexcitation current and the excitation intensity dependence of photoluminescence suggest that efficient electron–hole separation extends electron lifetime.

Electrical excitation of superradiant intersubband plasmons

We present a detailed experimental analysis on mid-infrared superradiant emission from highly doped quantum wells. The emission originates from the radiative decay of intersubband plasmon excitations, which are electrically injected by directly contacting the two-dimensional electron gas. For the experiment, we processed an InGaAs/AlInAs quantum well into a three terminal field effect transistor with a leaky gate. Depending on the current pathway, we observe either a purely thermal excitation (source to drain injection) or a combination between thermal and resonant excitations (gate to source and drain injection). The scaling of the optical power with the density of electrons in the channel is in excellent agreement with the superradiant nature of the plasmon decay.