Electrons intraband transitions

Reports on several selected systems have produced various dynamical models for ultrafast excitations in nanoparticles [62, 107, 112, 113]. Many have used a widely accepted model to account for both the excitation and decay processes in metals. This involves the optical excitation of the electrons by interband and intraband transitions. These transitions are followed by a loss of coherence,... 

Analyses of the electronic and electron spin resonance (ESR) spectra of the radical cation and anion of polysilanes make it possible to elucidate the structure of HOMO and LUMO, because an unpaired electron in the radical anion or cation occupies HOMO or LUMO, respectively. As schematically depicted in Fig. 10, the radical ions of polysilanes show absorption bands in UV and near-IR regions [29 31]. The former band corresponds to intraband transitions between valence and conduction bands. The latter band corresponds to transitions within the valence or the conduction band [32,33]. Because the near-... 

To describe the optical properties of the crystals over a wide temperature range and to derive the basic phenomenological parameters of the electronic transition, we fitted the R(co) and a(co) spectra using the Drude-Lorentz model. The Drude part describes the intraband transition of free carriers,... 

The electronic structure is similar within each group as far as optical properties are concerned. For example, at the photon energy used in our experiments the optical response of the noble metals is entirely due to intraband optical transitions [22, while that of Ti, Zr, and Hf is dominated by interband transitions [23,24], The optical structures of Ni and Pd are more complicated, and both interband and intraband transitions can contribute [24,25. This interpretation is supported by the observation that the signals of largest and smallest magnitude occur in Zr and Ag, respectively. 

The action of a circularly polarised pulse leads to a repopulation of electrons and holes between states with a different angular momentum component in the direction of the pump pulse wave vector. The corresponding SIFE relaxation time describes the recovery of the symmetry of the initial angular momentum distribution. Again it is preferable that the probe response is dominated by intraband transitions or else inelastic scattering may obscure the relaxation of hot electron (hole) angular momentum. 

Carrier relaxation due to both optical and nonradiative intraband transitions in silicon quantum dots (QDs) in SiOa matrix is considered. Interaction of confined holes with optical phonons is studied. The Huang-Rhys factor governing intraband multiphonon transitions induced by this interaction is calculated. The new mechanism of nonradiative relaxation based on the interaction with local vibrations in polar glass is studied for electrons confined in Si QDs. 

Radiative spontaneous transition rates for intraband transitions have been calculated within dipole approximation both for electrons and holes. Fig. 3 demonstrates, that for a QD with the diameter of 3.1 nm the transition time varies in the range of 0.1-100 ps. The fastest transitions correspond to the levels separated by the energies of about 1 eV. Intraband transitions between a number of hole levels are forbidden due to the selection rule. 

Our calculations demonstrate that the fastest radiative intraband transitions correspond to the levels separated by the energies around 1 eV and are characterized by the rates of the order of 10 ns for both holes and electrons. 

Figure 1 illustrates how a pump laser radiation resonance to the transition 1 4 (Bh Pe) transfers an electron from state Ph in the valence band to state Pe in the conduction band. The electron transfer creates the inversion population for both intraband transition 4 3 in the conduction band and intraband transition 2 1 in the valence band. Although the number of electrons on Ph level is bigger then that on Sii level, we use the term inversion population because absorption of photons on the transition 1 2 is absent since all states on the level Ph are occupied. 

The wavefunctions for three-dimensional confinement have three quantum numbers (n, I, m) plus spin. The selection rules for dipole-allowed absorption and emission were given above for interband transitions. For intraband transitions between the ladder of electron or hole states, the selection rules for the simplest case of non-interacting electrons and holes are An 0 AL = 0, + 1 Am = 0, + 1. 

Noble metals - copper, silver and gold - are monovalent elements with a /cc-like crystallographic structure in the bulk phase under normal conditions. Their dielectric function has been the subject of various experimental investigations in the past [1-6]. A compilation and an analyse of the main results can be found in [7]. The response of noble metals to an electromagnetic excitation in the UV-visible range cannot be described, contrarily to the case of alkalis, by the only behaviour of the quasi-free conduction electrons (sp band), but must include the Influence of the bound electrons of the so-called d bands [8]. Hence, the total dielectric function of noble metals can be written as the sum of two contributions, one due to electronic transitions within the conduction band (intraband transitions) and the other stemming from transitions from the d bands to the conduction one (Interband... 

Various other electronic transitions are possible upon light excitation. Besides the band-band transitions, an excitation of an electron from a donor state or an impurity level into the conduction band is feasible (transition 2 in Fig. 1.9). However, since the impurity concentration is very small, the absorption cross-section and therefore the corresponding absorption coefficient will be smaller by many orders of magnitude than that for a band-band transition. At lower photon energies, i.e. at ph < g, an absorption increase with decreasing ph has frequently been observed for heavily doped semiconductors. This absorption has been related to an intraband transition (transition 4 in Fig. 1.9), and is approximately described by the Drude theory [4]. This free carrier absorption increases with the carrier density. It is negligible for carrier densities below about 10 cm ... 

For an intraband transition between the states s) and s ) of the electron-hole pair one may simple average t(r) c(r) and ipl(r)ipv(r) over the unit cell using the Bloch functions at the band extremes, since the principal term does not already vanish. As a result, the corresponding matrix element of the charge density is given by the sum of the electron and hole contributions ... 

Electron energy-loss spectroscopy at low excitation energies is a surface sensitive technique to study the electronic structure by exciting collective oscillations or electrons from occupied into unoccupied states. In metals with a high density of states arising from d electrons, the excitation of plasmon losses has a relatively low probability. Therefore, the spectra are dominated by interband or intraband transitions. In rare earth metals, excitations of the partially filled/shell are observed that are assigned to be dipole-forbidden 4/4/transitions. These transitions are enhanced near the 4ii-4/threshold [56]. 

It has to be kept in mind, however, that the plasma frequency A>p as given in eq. (4) is modified due to limited validity of the nearly free electron model. Interference of plasmon excitation with interband and intraband transitions may shift the plasma frequency in a complicated way, as discussed by Raether (1965). 

Intraband relaxation in most systems occurs on timescales that are short relative to the spontaneous emission lifetimes of the intraband transition, and this has hindered potential applications of intraband transitions for infrared detection and emission. Quantum dots with large number of surface traps have however been shown to rapidly extract electrons from the P levels of the nanocrystal. Mid infrared photons incident on such a material induce a PL quench at room temperatures, allowing for the visualization of mid-infrared light. 

In ordinary optical absorption there are two components associated with intraband transitions (Drude component) and interband transitions, respectively. A similar situation is encountered in magneto-optical spectroscopy. Of special interest is the interband component which is related to the joint density of states. The intensity of the magneto-optical transitions is proportional to the product of spin-orbit coupling strength and net electron-spin polarization of states excited by the incident light (Erskine and Stern, 1973). 

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