Cyclotron resonance in bilayer graphene

We present the first measurements of cyclotron resonance of electrons and holes in bilayer graphene. In magnetic fields up to B=18 T, we observe four distinct intraband transitions in both the conduction and valence bands. The transition energies are roughly linear in B between the lowest Landau levels, whereas they follow square root[B] for the higher transitions. This highly unusual behavior represents a change from a parabolic to a linear energy dispersion. The density of states derived from our data generally agrees with the existing lowest order tight binding calculation for bilayer graphene. However, in comparing data to theory, a single set of fitting parameters fails to describe the experimental results.     

Alloy composition effects on the gain and the differential gain for CdZnTe based II–VI semiconductor lasers

We study the variation of the gain and the differential gain for a quantum well laser based on the CdZnTe alloys. We calculate theoretically the optical gain of CdZnTe based quantum well laser, as function of the alloys composition for various values of carrier’s densities. Our study is based on the parabolic model with the intraband relaxation taken into account. Finally, we investigate how the composition alloys affects the differential gain of quantum well lasers.     

Optische Bestimmung der Nichtparabolizität des dritten Valenzbandes von Germanium

Measurements of the optical absorption of free holes in germanium produced by direct intraband transitions in the valence band have been made at temperatures of 300–390 °K. The experimental method of the photoabsorption was used which enables the absorption spectra to be determined at intrinsic material without an influence of the absorption edge due to interband transitions between the valence band and the conduction band. An analysis of the spectra gives an experimental determination of the shape of the non parabolic split-off valence band. By comparison withKane's theory a marked difference is found but good agreement with calculations fromFawcett is achieved. The temperature dependence of the shape of the split-off band is discussed concerning the optical absorption measurements on “hot holes” in high electric fields with respect of the non-equilibrium energy distribution of the carriers.     

Impurity optical absorption in parabolic quantum well

Optical absorption in GaAs parabolic quantum well in the presence of hydrogenic impurity is considered. The absorption coefficient associated with the transitions between the upper valence subband and donor ground state is calculated. The impurity ground state wave function and energy are obtained using the variational method. Dependence of the absorption spectra on impurity position in quantum well was investigated. It is shown, that along with quantum well width decrease the absorption threshold shifts to higher frequencies. Results obtained within frames of parabolic approximation are compared with results for rectangular infinite-barrier quantum well case. The acceptor state → conduction band transitions considered as well.     

Influence of lateral electric field on intraband optical absorption in concentric double quantum rings

The influence of lateral electric field on one-electron states and intraband absorption in two-dimensional concentric double quantum rings is investigated. The confining potential of the rings is modeled as a double harmonic central potential. Using the exact diagonalization technique, we calculate the dependence of the electron energy spectrum as a function of the electric field strength as well as the inner ring radius. Also, different values of confinement strength are considered. Selection rule is obtained for intraband transitions, caused by the direction of incident light polarization. The intraband absorption coefficient is calculated for different values of electric field strength, inner ring radius, confinement strength and incident light polarization direction. The combined influence of electric field strength and change of confining strength show that while the increment of the first one leads only to blueshift of absorption spectrum, the augment of the second one makes the redshift. In addition, both blueshift and redshift of the spectrum have been obtained with the enlargement of inner ring radius. Finally, we show that the absorption spectrum undergoes redshift by changing the polarization of incident light from x- to y-axis.     

Optical Absorption Spectra and Intraband Dynamics in Terahertz-Driven Semiconductor Superlattice

We have theoretically investigated the optical absorption spectrum and intraband dynamics by subjecting a superlattice to both a terahertz (THz)-frequency driving field and an optical pulse by using an excitonic basis.In the presence of a THz dc field, the satellite structures in the absorption spectra are presented. The satellite structure is a result from the THz nonlinear dynamics of Wannier-Stark ladder excitons. On the other hand, the coherent intraband polarization is investigated. We find that the excitonic Bloch oscillation is driven by the THz field and yields an intraband polarization that continues to oscillate at times much longer than the intraband dephasing time. The temporal evolution of the slowly varying components of the intraband polarization is dependent on the THz frequency.     

Exciton optical transitions in quantum wells with spin-orbit coupling

The energy spectrum of light-hole and heavy-hole excitons and optical absorption in a quantum well have been analyzed taking into account Rashba spin-orbit coupling. Interband and intraband exciton transitions have been considered. It has been shown that, in the presence of spin-orbit coupling, the probabilities of the interband and intraband photoelectric effects diverge in the vicinity of the threshold if the electron-hole interaction is neglected. The threshold probabilities of the interband and intraband photoelectric effects become finite when Coulomb interaction is taken into account.     

Emission spectrum of intraband luminescence for single parabolic band under excitation of wide-band-gap insulators by ionizing radiation and particles

Emission spectrum and intensity of picosecond intraband luminescence for single parabolic band approximation of electron conduction band in insulating crystals are calculated. It is shown that the spectral intensity of this intraband emission increases approximately as (?ω)^?1/2 with decrease in photon energy, while the spectral density of photons increases as (?ω)^?3/2. The intensity of this emission is inversely proportional to the longitudinal optical phonon frequency. The total emission yield in this approximation is low, less than 10 photons in the spectral window of silicon photomultipliers per MeV of ionizing particle energy.     

Optical spectroscopy of graphene: From the far infrared to the ultraviolet

The unique electronic structure of graphene leads to several distinctive optical properties. In this brief review, we outline the current understanding of two general aspects of optical response of graphene: optical absorption and light emission. We show that optical absorption in graphene is dominated by intraband transitions at low photon energies (in the far-infrared spectral range) and by interband transitions at higher energies (from mid-infrared to ultraviolet). We discuss how the intraband and interband transitions in graphene can be modified through electrostatic gating. We describe plasmonic resonances arising from the free-carrier (intraband) response and excitonic effects that are manifested in the interband absorption. Light emission, the reverse process of absorption, is weak in graphene due to the absence of a band gap. We show that photoluminescence from hot electrons can, however, become observable either through femtosecond laser excitation or strong electrostatic gating.     

Magnetic field effect on the electronic states and the intraband optical absorption spectrum of a laser dressed double quantum dot molecule

The simultaneous effects of intense terahertz (THz) laser, a homogeneous magnetic fields, and the modification of the structural parameters on the electronic states, and the intraband optical absorption spectrum in a two-dimensional double quantum dot molecule are theoretically investigated. The crossing and anticrossing are observed in the energy dependence on the magnetic field induction between the third and the fourth energy levels. Additionally, it is shown that an intense THz laser field always shifts the energy spectrum to higher values. The variation of the structural parameters leads to the change of the positions of the energy levels and the anticrossing point. Finally, we have found that the intraband optical absorption spectrum, particularly the absorption intensity and the peak position, can be effectively regulated by an intense THz laser and a magnetic fields, as well as by the variation of the structural parameters of the double quantum dot molecule.     

Intraband absorption and emission of light in quantum wells and quantum dots

High-resolution spectroscopy in the mid-infrared spectral range is used to study electronic transitions between size-quantization subbands in stepped quantum wells under picosecond interband excitation. The contributions from intersubband and intrasubband absorption of light are separated by using the difference in time profiles of the absorption coefficient for these cases. For stepped quantum wells, spontaneous interband luminescence and superluminescence are studied for different excitation levels. For structures with quantum dots, the intraband absorption spectra for n-and p-type structures and the spectra of photoinduced intraband absorption and emission (for polarized radiation) for undoped structures are studied.     

Impurity-modulated Aharonov–Bohm oscillations and intraband optical absorption in quantum dot–ring nanostructures

In this work we study the electronic states in quantum dot–ring complex nanostructures with an on-center hydrogenic impurity. The influence of the impurity on Aharonov–Bohm energy spectra oscillations and intraband optical absorption is investigated. It is shown that in the presence of a hydrogenic donor impurity the Aharonov–Bohm oscillations in quantum dot–ring structures become highly tunable. Furthermore, the presence of the impurity drastically changes the intraband absorption spectra due to the strong controllability of the electron localization type.     

Pure spin photocurrents in low-dimensional structures

As is well known, the absorption of circularly polarized light in semiconductors results in optical orientation of electron spins and helicity-dependent electric photocurrent, and the absorption of linearly polarized light is accompanied by optical alignment of electron momenta. Here, we show that the absorption of unpolarized light leads to the generation of a pure spin current, although both the average electron spin and electric current vanish. We demonstrate this for direct interband and intersubband as well as indirect intraband (Drude-like) optical transitions in semiconductor quantum wells.     

Quantum effects of potential fluctuations in GaAs δ-doping superlattices

The electronic and optical properties of δ-doping n-i-p-i superlattices are strongly influenced by the random distribution of donors and acceptors within the doping layers. A Monte-Carlo method is applied to investigate the resulting potential fluctuations and local changes of energy levels and wavefunctions. We study disorder effects on the density of states, the capacitance and the intraband absorption coefficient as a function of excitation level. In addition, the luminescence spectra are calculated and compared to electroluminescence measurements. Excellent agreement is achieved without using any fitting parameters, if the local wavefunction shrinkage of the tail states is included. While contributions of different subbands cannot be resolved in the luminescence, the simulation of conduction band (CB) intraband absorption confirms that this is possible using resonant Raman scattering.     

A study of light-hole electro-absorption in AlGaAs parabolic quantum wells,using a novel method

We report on polarization-resolved absorption measurements of parabolic quantum well structures embedded in an optical waveguide. This allows us to investigate in detail not only the heavy hole band structure, but also the light hole subbands whose contribution is suppressed in surface-normal incidence experiments. We present a novel experimental technique utilizing a segmented waveguide device that enables us to directly determine the modal absorption caused by the parabolic quantum well interband transitions. Experimental results for electro-absorption are given and compared to theoretical predictions.     

Monopolar optical orientation of electron spins in bulk semiconductors and heterostructures

It is shown that intraband absorption of circularly polarized light leads to spin polarization of the electron gas. A theory of this monopolar optical spin orientation is developed for indirect intraband transitions in bulk semiconductors and for indirect intrasubband and direct intersubband transitions in quantum wells.     

Impurity-related intraband absorption in coupled quantum dot-ring structure under lateral electric field

The effects of a lateral electric field on intraband absorption in GaAs/GaAlAs two-dimensional coupled quantum dot-ring structure with an on-center hydrogenic donor impurity are investigated. The confining potential of the system consists of two parabolas with various confinement energies. The calculations are made using the exact diagonalization technique. A selection rule for intraband transitions was found for x-polarized incident light. The absorption spectrum mainly exhibits a redshift with the increment of electric field strength. On the other hand, the absorption spectrum can exhibit either a blue- or redshift depending on the values of confinement energies of dot and ring. Additionally, electric field changes the energetic shift direction influenced by the variation of barrier thickness of the structure.     

Intraband Dynamics and Terahertz Emission in Biased GaAs/In0.53Ga0.47As Semiconductor Superlattices

Using an excitonic basis, we investigate the intraband polarization, opticalabsorption spectra, and terahertz emission of semiconductor superlattice withthe density matrix theory. The excitonic Bloch oscillation is driven by the dcand ac electric fields. The slow variation in the intraband polarizationdepends on the ac electric field frequency. The intraband polarizationincreases when the ac electric field frequency is below the Bloch frequency.When the ac electric field frequency is above the Bloch frequency, theintraband polarization downwards and its intensity decreases. The satellitestructures in the optical absorption spectra are presented. Due to excitonicdynamic localization, the emission lines of terahertz shift in different acelectric field and dc electric field.     

Study of sequential two photon absorption and conduction band electrons absorption in ZnSe

Using the Z-scan technique and pump–probe technique with 130 fs laser pulses at 800 nm, we verify that an intraband one-photon absorption follows the interband two photon absorption. Particularly, we find that there is an intraband relaxation interspersed between these two absorption processes for some of the conduction band electrons but not all of them. In this study, we measure the interband two photon absorption coefficient and the absorptive cross sections associated with both excitation pathways within the conduction band. In addition, we estimate the time for relaxation of electrons within the conduction band to be 250 fs.     

Electrodynamic response of a quantum nanotube in a parallel magnetic field

This paper reports on a theoretical study of intraband resonances arising in the absorption of electromagnetic radiation by a quantum nanotube both with and without involvement of optical phonons. Explicit relations are derived for the absorption coefficients. The relative intensities of the resonance peaks are analyzed.