Optical Properties Of Quantum Wires Research Articles

The effect of the magnetic field on optical properties of quantum wires

Interband and intersubband optical transitions in quantum wires are studied theoretically with allowance for scattering of carriers by both long-wavelength acoustic vibrations in a homogeneous magnetic field with the strength vector H of which is directed perpendicularly to the nanostructure axis. The features of the absorption spectra of a weak electromagnetic wave with an increase in the magnetic field are studied. In particular, it is shown that new channels of light absorption appear for interband optical transitions in the presence of a magnetic field. With an increase in the magnitude of H, absorption maximums are shifted to the region of higher frequencies; in this process, distances between the absorption bands increase. For intersubband optical transitions, frequency dependences of light absorption coefficients are calculated and analyzed with allowance for light polarization (linearly and circularly polarized light). Conditions under which processes of carrier scattering on a rough surface have a significant effect on optical characteristics of nanowires are discussed.

Spin–orbit interaction effects on the optical properties of quantum wires under the influence of in-plane magnetic fields

In this work, we studied the effects of in-plane magnetic field and Rashba spin–orbit interaction on the intersubband optical absorption coefficients and changes in refractive index for transitions between the first two lower-lying electronic levels of quantum wire represented by a parabolic confining potential. The analytical expressions of the linear and third-order nonlinear optical absorption coefficients and refractive index changes are obtained by using the compact-density matrix formalism and iterative scheme. Optical properties are investigated as a function of quantum wire radius, strength and orientation of magnetic field, strength of Rashba spin–orbit interaction and photon energies. Numerical results reveal that the optical absorption coefficients and refractive index changes are strongly affected and thus can be controlled by these parameters.

Optical properties of quantum wires: Rashba effect and external magnetic field

In the present paper, the optical properties of a InAs quasi-one-dimensional (1D) quantum wire are studied under an external magnetic field parallel to the z-direction and the Rashba spin–orbit interaction (SOI). For this purpose, we have used analytical expressions for optical properties obtained by the compact-density matrix formalism. In this work, we have investigated the intersubband optical absorption coefficients and refractive index changes as a function of applied magnetic field (B) and strength of the SOI. According to the obtained results, it is found that (i) The total refractive index changes decrease and shift towards lower energies when the magnetic field and strength of the SOI increases. (ii) The total absorption coefficient decreases and shift towards lower energies as the magnetic field and strength of the SOI decreases.

Epitaxial Semiconductor Quantum Wires

The investigation on the direct epitaxial quantum wires (QWR) using MBE or MOCVD has been persuited for more than two decades, more lengthy in history as compared with its quantum dot counterpart. Up to now, QWRs with various structural configurations have been produced with different growth methods. This is a reviewing article consisting mainly of two parts. The first part discusses QWRs of various configurations, together with laser devices based on them, in terms of the two growth mechanisms, self-ordering and self-assembling. The second part gives a brief review of the electrical and optical properties of QWRs.

Optical Properties of Quantum-Wires Grown Using Lateral Composition Modulation Induced by (InP)1/(GaP)1 Short-Period Superlattices

ABSTRACTThe optical properties of quantum wires (QWRs) grown using lateral composition modulation (LCM) were studied by photoluminescence (PL) measurement as a function cryostat temperature (Tcr). 3 stacked arrays of QWRs were formed by sequential growth of ∼ 180 Å-thick LCM layers (lateral period: ∼ 90 Å) induced by (InP)1/(GaP)1 short-period superlattices, and 200 Å-thick InGaP spacers at the growth temperature of 490 °C. The formation of QWRs was confirmed by a transmission electron microscopy measurement. By the analysis of the dependence of PL intensity and peak energy of the QWRs on Tcr, the origin of higher energy peak (H) and lower energy peak (L) were investigated. While behavior of the H peak is similar to that of an ordered InGaP, the L peak shows the insensitivity of PL peak energy to Tcr. This is attributed to compensation of the bandgap by competition of strain in the QWR region and indicates the L peak is related to the QWRs. Strong dependence of the L peak on the position of polarizer also supports this. Additionally, the PL peak intensity of the L peak has the maximum value not at the lowest Tcr (10 K) but at 50 K, while the H peak decrease continuously as T increases. We introduced the idea of compensation of the thermal expansion coefficient to explain this phenomenon.

Optical properties of quantum wires: Disorder scattering in the Lloyd model

The Lloyd model is extended to the exciton problem in quasi one-dimensional structures to study the interplay between the Coulomb attraction and disorder scattering. Within this model the averaging and resummation of the locator series can be performed analytically. As an application, the optical absorption in quantum box wires is investigated. Without electron-hole interaction, fluctuations in the well-width lead to an asymmetric broadening of the minibands with respect to the lower and upper band-edges.

Consequences of Low Symmetry in Quantum Wires: Exciton Degeneracies and Quasi-Equilibrium High-Density Regime

We analyze the role of the confinement potential symmetry on the optical properties of quantum wires in the high-density regime. Within the Hartree-Fock approximation we go beyond a global group-theoretical analysis and analyse the structural details of the symmetrized set of microscopic polarization equations including band-mixing. We show that neglecting conduction band mixing leads to further degeneracies of the quantum wire excitonic fine structure. We also find that the decoupling of the symmetrized polarization equations is persistent in the quasi-equilibrium high-density regime.

Spatially resolved luminescence investigation of AlGaAs/GaAs single quantum wires modified by selective implantation and annealing

Single Al0.5Ga0.5As/GaAs V-groove quantum wires (QWR) modified by selective implantation and rapid thermally annealing were investigated by spatially resolved microphotoluminescence (micro-PL). The PL from the necking region was clearly observed at room temperature. Optical properties of QWR and the adjacent quantum well structures were strongly degraded by the implantation. The recovery properties of the PL signals from all the structures were dependent on the implantation dose. A critical dose of 1×1013 cm−2 was found for the selective implantation, over which the PL from the necking region could not be recovered. Also the blueshifts of QWR and the necking-region PL peaks were observed for all the annealed samples. This blueshift is caused by the interface intermixing, which is very useful to increase the confinement of carriers in QWR region for optoelectronic device applications.

Theoretical studies of the effects of a magnetic field on excitonic nonlinear optical properties of quantum wires.

The magnetic-field dependence of the third-order excitonic nonlinear susceptibility ${\mathrm{\ensuremath{\chi}}}^{(3)}$ in a quantum wire is explored within the rotating wave approximation. Both the real and imaginary parts of ${\mathrm{\ensuremath{\chi}}}^{(3)}$, arising from population saturation of the excitonic state under optical pumping, are calculated for a GaAs wire as a function of magnetic field and pump-probe detuning frequencies. The imaginary part of ${\mathrm{\ensuremath{\chi}}}^{(3)}$ exhibits a negative peak associated with the bleaching of the excitonic resonance and a positive, broad, off-resonance absorption peak associated with biexciton formation. The amplitude, line shape, and spectral frequency of both these peaks can be modulated by a magnetic field which indicates the possibility of using such a field to probe the mechanism underlying optical nonlinearity in a quantum wire. Furthermore, the field can also be used to tune the optical nonlinearity over a range of frequencies which has device applications. \textcopyright{} 1996 The American Physical Society.

Optical properties of quantum wires: Fermi-edge singularity exponents and the low-density limit.

A simple many-body treatment of the Fermi-edge singularities in absorption and photoemission in quasi-one-dimensional quantum wires is presented. The problem of calculating the transition probabilities is reduced to numerically evaluating a sufficient number of determinants describing the overlap of the appropriate many-body wave functions. It is found that the edge singularity exponents can be determined from the size dependence of these determinants. The well-known connection between these exponents and the phase shifts at the Fermi surface are explicitly checked for one-dimensional quantum wires. The singular edge behavior is interpreted in terms of Friedel's replacement transitions and is found to be due to a replacement transition to the bound state, a result confirmed by considering the evolution of the spectra in the low-density limit. \textcopyright{} 1996 The American Physical Society.

INTERBAND OPTICAL PROPERTIES OF QUANTUM WIRES: THEORY AND APPLICATION

The electronic states near the fundamental gap largely determine the most commonly investigated linear and nonlinear optical properties of quantum wires. We first discuss heavy-hole-light-hole band-mixing effects and illustrate the consequences with an application in opto-electronics, namely a quantum-wire-array based polarization modulator. Next, an overview of polariton effects which determine the time scale for excitonic radiative decay is given and comparison with recent experiments made.

ELECTRONIC AND OPTICAL PROPERTIES OF QUASI-ONE-DIMENSIONAL CARRIERS IN QUANTUM WIRES

We review structural and optical properties of quantum wires (QWRs), grown on nonplanar substrates. Our approach of in situ wire formation on patterned substrates allows us to fabricate defect free QWRs of high optical quality, suitable for laser and other optoelectronic applications. Several types of wires and wire arrangements are investigated in detail: single QWRs, vertical QWR stacks, lateral sub-μm pitch QWR arrays and pseudomorphic QWRs. Theoretical calculations are performed for electronic eigenstates (with inclusion of strain effects) as well as the lineshape of spontaneous radiative recombination. The lateral bandgap modulation, carrier capture into the QWRs, subsequent intersubband relaxation, cooling, interband recombination, and bandgap renormalization are systematically investigated and compared to current theories and previously obtained results for quantum wells or bulk material.

Fermi edge singularities in doped quantum wires and quantum wells

The optical properties of quantum wires and quantum wells are investigated. We are concerned with the effects produced by the localization of electrons and holes either in the same (direct transitions) or different space regions (indirect transitions). The emission and absorption spectra are obtained by solving an effective Bethe-Salpeter equation including a static screened Coulomb interaction. Fermi Edge Singularities (FES) are obtained and their dependence on temperature and density of carriers are analyzed. In order to obtain strong FES, it is necessary that two special points are fulfilled: (i) the hole mass goes to infinity; and (ii) a spatial symmetry breaking of the system (spatially indirect transitions). When the latter condition is satisfied and the bottom of the second conduction subband approaches the first one, a new channel of scattering is favoured and the spectra show a large enhancement at the Fermi level. We compare our results with the experimental situations.

Optical processes in quantum confined semiconductors

A short overview of the optical properties of quantum wires and quantum dots is given. The results of recent spectroscopic experiments are discussed to elucidate the correlations between the structural and the optical properties of low-dimensional systems.

Theory of optical properties of quantum wires in porous silicon.

We present theoretical studies of the electronic and optical properties of free-standing Si quantum wires which exist in porous Si. We use a second-neighbor empirical tight-binding model which includes d orbitals and spin-orbit interaction. The excitonic effects are included within the effective-mass approximation. We found that for narrow quantum wires with widths around 8 A\r{}, the averaged exciton oscillator strength is comparable to that of bulk GaAs. However, the average exciton oscillator strength decreases dramatically (faster than 1/${\mathit{L}}^{5}$) as the quantum-wire width L increases. The radiative lifetimes of excitons in quantum wires are estimated and we find that the lifetime of the shortest-lived exciton ranges from 57 ns to 170 \ensuremath{\mu}s for wire widths from 7.7 to 31 A\r{}. We have also calculated the absorption spectra and found strong polarization dependence.

Fabrication and optical properties of quantum wires by selective metal organic chemical vapor deposition

Fabrication and optical properties of quantum wires by selective metal organic chemical vapor deposition

Polarization dependence of optical absorption and emission in quantum wires

The polarization dependence of absorption and emission spectra in cylindrical quantum wires is studied using a formulation of multiband envelope-function theory in a representation of cylindrical Bloch waves. The work presented is an analysis of the optical properties of quantum wires, which includes band-coupling effects. Contrary to the assumptions employed in previous studies, the valence states involved in these transitions are a strong admixture of light- and heavy-hole character. Band-coupling effects are therefore essential to understanding valence-subband dispersion and density of states, the energy dependence of interband optical-transition matrix elements, and polarization anisotropies. Analytical expressions are derived for the polarization dependence of optical matrix elements, and hence, optical-absorption and emission spectra. Applicability of the results derived for cylindrical quantum wires to the case of wires with lower symmetry is discussed.

Optical properties of quantum wires produced by strain patterning of GaAsAlGaAs quantum wells

We have confined excitons to wires within continuous GaAsAlGaAs quantum wells. The confinement is produced by inhomogeneous strain created by patterning and etching a compressively stressed overlayer of amorphous carbon. Potential wells for excitons beneath 400 nm wide wires are 31 meV, as measured by the red-shift of the exciton emission. We compare our results to expectations based upon finite-element calculations of the strain tensor, and discuss the complicated effect of the inhomogeneous strain on valence-band structure.