Intraband Optical Transitions Research Articles

Intraband optical transition in InGaAs/GaAs pyramidal quantum dots

We present the results of our numerical analysis of intraband optical transitions within the valence band of InxGa1−xAs/GaAs pyramidal quantum dots. We study the possibility of tuning of the corresponding absorption spectra by varying the size and composition of the dots. The composition of the dot is varied by changing the value of ‘x’ between x = 0.1 and x = 1.0. The size of the quantum dot base is varied from 5 nm to 21 nm. The absorption spectra are calculated within the eight-band k.p model, which includes the effects of strain. The absorption spectra of the quantum dots are more sensitive to the in-plane, i.e., perpendicular to the growth direction and polarization of the incident light.

Intraband absorption in finite, inhomogeneous quantum dot stacks for intermediate band solar cells: Limitations and optimization

We present a theoretical analysis of intraband optical transitions from the intermediate pseudo-band of confined states to the conduction band in a finite, inhomogeneous stack of self-assembled semiconductor quantum dots (QDs). The chain is modeled with an effective Hamiltonian including nearest-neighbor tunnel couplings and the absorption under illumination with both coherent (laser) and thermal radiation is discussed. We show that the absorption spectrum already for a few coupled dots differs from that of a single dot and develops a structure with additional maxima at higher energies. We find out that this leads to an enhancement of the overall transition rate under solar illumination by up to several percent which grows with the number of QDs but saturates already for a few QDs in the chain. The decisive role of the strength of inter-dot coupling for the stability of this enhancement against QD stack inhomogeneity and temperature is revealed.

Linear and nonlinear optical absorption coefficients in GaAs/Ga1−xAlxAs concentric double quantum rings: Effects of hydrostatic pressure and aluminum concentration

The linear and nonlinear intra-band optical absorption coefficients in GaAs/Ga1−xAlxAs two-dimensional concentric double quantum rings are investigated. Taking into account the combined effects of hydrostatic pressure and aluminum concentration the energies of the ground (n=1,l=0) and the first excited state (n=2,l=1) have been found using the effective mass approximation and the transfer matrix formalism. The energies of these states and the corresponding threshold energy of the intra-band optical transitions are examined as a function of hydrostatic pressure and aluminum concentration for different sizes of the structure. We also investigated the dependencies of the linear, nonlinear, and total optical absorption coefficients as functions of the incident photon energy for different values of hydrostatic pressure, aluminum concentration, sizes of the structure, and incident optical intensity. Its is found that the effects of the hydrostatic pressure and the aluminum concentration lead to a shifting of the resonant peaks of the intra-band optical spectrum.

Optical self-energy in graphene due to correlations

In highly correlated systems one can define an optical self-energy in analogy to its quasiparticle (QP) self-energy counterpart. This quantity provides useful information on the nature of the excitations involved in inelastic scattering processes. Here we calculate the self-energy of the intraband optical transitions in graphene originating in the electron–electron interaction (EEI) as well as electron–phonon interaction (EPI). Although optics involves an average over all momenta (k) of the charge carriers, the structure in the optical self-energy is nevertheless found to mirror mainly that of the corresponding quasiparticles for k equal to or near the Fermi momentum kF. Consequently, plasmaronic structures which are associated with momenta near the Dirac point at k = 0 are not important in the intraband optical response. While the structure of the electron–phonon interaction (EPI) reflects the sharp peaks of the phonon density of states, the excitation spectrum associated with the electron–electron interaction is in comparison structureless and flat and extends over an energy range which scales linearly with the value of the chemical potential. We introduce a method whereby detailed quantitative information on such excitation spectra can be extracted from optical data. Modulations seen on the edge of the interband optical conductivity as it rises towards its universal background value are traced to structure in the quasiparticle self-energies around kF of the lower Dirac cone associated with the occupied states.

Electronic states and intraband terahertz optical transitions in InGaAs quantum rods

Strain-dependent eight-band k·p method is used to analyze the electronic structure and intraband optical transitions in self-assembled InGaAs quantum rods in the terahertz range. The calculation of absorption spectra for the growth- and in-plane-polarized radiation shows some similarities to those of quantum well and single quantum dot structures, augmented with contribution from transitions between the dot and quantum well states. The influence of rod height on the electronic structure and the intraband absorption spectra is also investigated. It is found that the energy of maximal terahertz absorption can be tailored by the rod height for both in-plane and in-growth polarized radiation.

Intraband optical transitions in semiconductor quantum dots: Radiative electronic-excitation lifetime

Intraband optical transitions in semiconductor quantum dots (QDs) in the forms of a parallelepiped, sphere, and cylinder have been considered. It is shown that the size dependence of the matrix elements of electron-photon interaction, which includes the intraband transitions, differs in the E · r and A · p representations of electron-photon coupling, which are widely used to describe various optical processes. The radiative intraband relaxation rates of QD electron excitations have been calculated, depending on the QD size and shape and the parameters of the QD material. It is shown that the radiative intraband transition rate may reach 109 s−1.

Probing Interband Coulomb Interactions in Semiconductor Nanostructures with 2D Double-Quantum Coherence Spectroscopy

Employing the interband exciton scattering model, we have derived a closed set of equations determining the 2D double-quantum coherence signal sensitive to the interband Coulomb interactions (i.e., many-body Coulomb interactions leading to the couplings between exciton and biexciton bands) in semiconductor nanostructures such as nanocrystals, quantum wires, wells, and carbon nanotubes. Our general analysis of 2D double-quantum coherence resonances has demonstrated that the interband Coulomb interactions lead to new cross-peaks whose appearance can be interpreted as a result of exciton and biexciton state mixing. The presence of the strongly coupled resonant states and weakly coupled background of off-resonant states can significantly simplify cross-peak analysis by eliminating the congested background spectrum. Our simulations of the 2D double-quantum coherence signal in PbSe NCs have validated this approach.

Spin dephasing and pumping in graphene due to random spin-orbit interaction

We consider spin effects related to the random spin-orbit interaction in graphene. Such a random interaction can result from the presence of ripples and/or other inhomogeneities at the graphene surface. We show that the random spin-orbit interaction generally reduces the spin dephasing (relaxation) time, even if the interaction vanishes on average. Moreover, the random spin-orbit coupling also allows for spin manipulation with an external electric field. Due to the spin-flip interband as well as intraband optical transitions, the spin density can be effectively generated by periodic electric field in a relatively broad range of frequencies.

Decoherence in quantum dots due to real and virtual transitions: A nonperturbative calculation

We investigate theoretically acoustic phonon induced decoherence in quantum dots. We calculate the dephasing of fundamental (interband or intraband) optical transitions due to real and virtual transitions with higher energy levels. Up to two acoustic phonon processes (absorption and/or emission) are taken into account simultaneously in a non-perturbative manner. An analytic expression of acoustic phonon induced broadening is given as a function of the electron-phonon matrix elements and is physically interpreted. The theory is applied to the dephasing of intersublevel transitions in self-assembled quantum dots.

Decoherence effects in the intraband and interband optical transitions in InAs/GaAs quantum dots

We present a review of coherence properties of interband and intraband optical transitions in self assembled InAs/GaAs quantun dots. Indeed, recent experimental and theoretical investigations of the optical transitions in both spectral domains have allowed a better understanding of the different phenomena that affects the interaction of confined carriers with light. These studies point out the many different ways the electron-phonon interactions play a role on the optical response of quantum dots. They also stress the primary role of the close environment on the coherence characteristics of quantum dots.

Interband and intraband optical transitions in InAs nanocrystal quantum dots: A pseudopotential approach

An atomistic pseudopotential method is used to investigate the electronic and optical properties of spherical InAs nanocrystals. Our calculated interband (valence-to-conduction) absorption spectra reproduce the features observed experimentally both qualitatively and quantitatively. The results relative to intraband (valence-to-valence and conduction-to-conduction) absorption successfully reproduce the recently measured photoinduced absorption spectra, which had so far been addressed only qualitatively. They exclude the hypothesis of a thermal activation process between dot-interior-delocalized hole states to explain the temperature dependence observed experimentally. Furthermore, based on the agreement of our data with the experimental valence intersublevel transitions and the almost complete overlap of the latter with scanning tunneling microscopic (STM) measurements, we question the simplistic attribution of the observed STM peaks obtained for negative bias.

The effect of plastic strain on interband and intraband optical transitions in germanium crystals

This paper shows that the absorption spectrum in the 1.6–15‐μm region in germanium crystals, which results from plastic strain, is mainly determined by two parameters: the initial carrier concentration and the degree of plastic strain. The free-carrier absorption is unambiguously associated with the electric parameters only when the strain is not too great (<6%).

Pauli blocking versus electrostatic attenuation of optical transition intensities in chargedPbSequantum dots

Quantum dots can be charged selectively by electrons or holes. This leads to changes in the intensity of interband and intraband optical transitions. Using atomistic pseudopotential calculations, we show that (i) when carriers are injected into dot-interior quantum-confined states, the intensity of interband transitions that have those states as their initial or final states is attenuated (``Pauli blocking'') and (ii) when carriers are injected into localized states near the surface of the dots, the electrostatic field set up by these charges attenuates all optically allowed interband transitions. We describe and explain these two mechanisms of intensity attenuation in the case of charged $\mathrm{PbSe}$ quantum dots. In addition, this study reveals a new assignment of the peaks in the absorption spectrum. The absorption spectrum of charged $\mathrm{PbSe}$ dots was previously interpreted assuming that all injected electrons reside in dot-interior states. This assumption has led to the suggestion that the second absorption peak originates from ${S}_{h}\text{\ensuremath{-}}{P}_{e}$ and ${P}_{h}\text{\ensuremath{-}}{S}_{e}$ optical transitions, despite the fact that such transitions are expected to be dipole forbidden. Our results show that the observed bleaching of absorption peaks upon electron or hole charging does not imply that the ${S}_{h}\text{\ensuremath{-}}{P}_{e}$ or ${P}_{h}\text{\ensuremath{-}}{S}_{e}$ transitions are allowed. Instead, the observed bleaching sequence is consistent with charging of both dot-interior and surface-localized states and with the assignment of the second absorption peak to the allowed ${P}_{h}\text{\ensuremath{-}}{P}_{e}$ transition.

Carrier multiplication in semiconductor nanocrystals via intraband optical transitions involving virtual biexciton states

We propose and analyze a physical mechanism for photogeneration of multiexcitons by single photons (carrier multiplication) in semiconductor nanocrystals, which involves intraband optical transitions within the manifold of biexciton states. In this mechanism, a virtual biexciton is generated from nanocrystal vacuum by the Coulomb interaction between two valence-band electrons, which results in their transfer to the conduction band. The virtual biexciton is then converted into a real, energy-conserving biexciton by photon absorption on an intraband optical transition. The proposed mechanism is inactive in bulk semiconductors as momentum conservation suppresses intraband transitions. However, it becomes highly efficient in the case of zero-dimensional nanocrystals, where quantum confinement results in relaxation of momentum conservation, which is accompanied by the development of strong intraband absorption. Our calculations show that the efficiency of the carrier multiplication channel mediated by intraband optical transitions can be comparable to or even greater than that for impact-ionization-like processes mediated by interband transitions.

Sum rules for the optical and Hall conductivity in graphene

Graphene has two atoms per unit cell with quasiparticles exhibiting the Dirac-like behavior. These properties lead to interband in addition to intraband optical transitions and modify the $f$-sum rule on the longitudinal conductivity. The expected dependence of the corresponding spectral weight on the applied gate voltage $V_g$ in a field effect graphene transistor is $\sim {const}- |V_g|^{3/2}$. For $V_g =0$, its temperature dependence is $T^3$ rather than the usual $T^2$. For the Hall conductivity, the corresponding spectral weight is determined by the Hall frequency $\omega_H$ which is linear in the carrier imbalance density $\rho$, and hence proportional to $V_g$, and is different from the cyclotron frequency for Dirac quasiparticles.

Magneto-optical effect induced by spin chirality of the itinerant ferromagnetNd2Mo2O7

It is demonstrated both theoretically and experimentally that the spin chirality associated with a noncoplanar spin configuration produces a magneto-optical effect. Numerical study of the two-band Hubbard model on a triangle cluster shows that the optical Hall conductivity $\sigma_{xy}(\omega)$ is proportional to the spin chirality. The detailed comparative experiments on pyrochlore-type molybdates $R_2$Mo$_2$O$_7$ with $R=$Nd (Ising-like moments) and $R=$Gd (Heisenberg-like ones) clearly distinguishes the two mechanisms, i.e., spin chirality and spin-orbit interactions. It is concluded that for $R$=Nd, $\sigma_{xy}(\omega)$ is dominated by the spin chirality for the dc ($\omega=0$) and the $d \to d$ incoherent intraband optical transitions between Mo atoms.

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.

Electronic propertiesof InAs/GaAs self‐assembled quantum dots: beyond the effective mass approximation

AbstractThe electronic properties of InAs/GaAs self‐assembled quantum dots (SAD) are investigated as a function of their shapes, sizes and stoichiometric compositions with an eight‐band k � p method. Emphasis is placed on the three‐dimensional strain distribution, which is obtained by minimizing the strain energy around the dot region, and the piezoelectric potential, which is calculated by solving the corresponding Poisson equation. Novel features of intraband and interband optical transitions are presented and discussed in relation with the 3D SAD k � p band structure, and their response to external electric fields. Finally, particular attention is paid to the quantum Stark effect in vertically coupled lens‐shaped SADs.

Efficient intraband optical transitions in Si nanocrystals

Efficient intraband optical transitions in the infrared range are predicted in n-type Si nanocrystals using tight-binding calculations of no-phonon and one-phonon-assisted processes taking into account each vibrational mode. New types of transitions are reported, with no equivalence in direct gap semiconductors, between the confined states in the six valleys of the conduction band. They involve phonons with wave vectors either at the center or at the edge of the Brillouin zone. The efficiency of the main no-phonon and phonon-assisted transitions is comparable to the one in III-V semiconductor quantum dots.

Electromodulation of the interband and intraband absorption of Ge/Si self-assembled islands

We have investigated the interband and the intraband absorption properties of Ge/Si self-assembled islands. The investigated structure consists of a p–i–n junction containing Ge/Si self-assembled islands embedded in a Si 0.98Ge 0.02 waveguiding layer. The variation of transmission associated with carrier injection under forward bias is monitored both in the near-infrared and in the mid-infrared spectral ranges. We show that the carrier injection leads to an absorption resonant at 185 meV which is polarized along the growth axis of the islands. This transition corresponds to an intraband optical transition from the island ground states to the two-dimensional wetting layer states. This assignment is supported by a two-dimensional band structure calculation performed in a 14 band k · p formalism. Meanwhile, the carrier injection leads to a bleaching of the interband absorption. We show that this electroabsorption spectroscopy is a useful tool for the study of self-assembled islands that is complementary of standard photoluminescence, electroluminescence or absorption spectroscopies.