Intraband Optical Transitions Research Articles

Ultrafast switch-off of an electrically pumped quantum-dot laser

We report on the subpicosecond switch-off of the emission of an electrically pumped quantum-dot (QD) laser with perturbation by a 100 fs optical pulse. The observed effect is in contrast to known ones for quantum-well laser diodes. This is explained by unique properties of the QDs, namely by the fast modal gain saturation and localization-enhanced intraband absorption. The QD laser response can be described in terms of hot carrier absorption in the barrier states via intraband optical transitions leading to the transient suppression of the laser mode and, hence, to the switch-off.

Linear Interaction (Transformation) of de Broglie Waves of the Γ- and X-Valleys and Intraband Optical Transitions in GaAs/AlAs Quantum-Well Heterostructures in a Lateral Electric Field

We discuss the linear interaction between the de Broglie waves of different electron valleys in heterostructures where a constant electric field is applied along the heterointerfaces (lateral field). The probabilities of phononless and dipole optical transitions of electrons between the Γ- and X-subbands in GaAs/AlAs quantum-well heterosructures with a lateral electric field are calculated. It is shown that the electric field has a strong effect on the probabilities of phononless and direct optical dipole Γ-X electron transitions. Moreover, a lateral electric field changes the spectrum of the intersubband Γ-X light absorption, i.e., there is an intersubband analogue of the Franz-Keldysh effect. The considered linear interaction of de Broglie waves is compared with the well-known cases of linear wave interaction in a magnetized plasma, the Zener breakdown, and the Franz-Keldysh effect in solids.

The Influence of the Electron Effective Mass Mismatch on the Intraband Optical Transition in a Semiconductor Superlattice

The effective mass mismatch of a conduction band electron in a semiconductor.superlattice renormalizes the electron confining potential. Taking this effect into account we calculate the absorption coefficient of the intraband optical transition in an InGaAs/InAlAs superlattice. We find that the electron effective mass mismatch causes a notable broadening of spectrum linewidth, which can well account for the experimental result.

Analysis of absorption and amplification in a unipolar semiconductor structure with quantum dots

The allowed intraband optical transitions in a quantum dot are considered in the context of their application in quantum-well IR detectors and unipolar semiconductor lasers. The atom-like quantum dot is described by using the harmonic oscillator model. An expression is obtained for the absorption cross section which is independent of the wavelength and the quantum-dot size. Optical absorption and amplification are estimated.

Ultrafast dynamics of inter- and intraband transitions in semiconductor nanocrystals: Implications for quantum-dot lasers

Application of femtosecond transient absorption in the visible and near-IR spectral ranges and time-resolved photoluminescence allows us to separate electron and hole relaxation paths and to map the structure of interband and intraband optical transitions in CdSe and CdS nanocrystals (NC's) with a wide range of surface properties. In contrast to electron relaxation, which is controlled by NC surface passivation, depopulation of hole quantized states is extremely fast (sub-ps-to-ps time scales) in all types samples, independent of NC surface treatment (including NC's overcoated with a ZnS layer). Our results suggest that ultrafast hole dynamics are not due to trapping at localized surface defects such as a vacancy, but rather arise from relaxation into intrinsic NC states or intrinsically unpassivated interface states.

Intraband population inversion and amplification of IR radiation through charge-carrier injection into quantum wells and quantum dots

A mechanism is proposed for obtaining intraband population inversion of electrons in size-quantization levels through the injection of electron-hole pairs into the i region of a heterostructure with quantum wells or quantum dots. Key elements of the mechanism are the simultaneous generation of interband (hv≈Eg) near-IR radiation and the presence of a “metastable” level. In quantum wells such a level can be produced by making use of the weak overlap of the wave functions of electrons in the levels of a quantum well of complicated configuration and exploiting the characteristic features of the interaction of electrons with optical phonons in polar semiconductors. In quantum dots such a level forms as a result of the phonon bottleneck effect. Estimates are made of the gain for mid-IR radiation in intraband optical transitions of electrons.

Intraband optical absorption in semiconductor coupled quantum dots

In the framework of effective mass envelope function theory, absorption coefficients are calculated for intraband (intersubband in the conduction band) optical transition in InAs/GaAs coupled quantum dots. In our calculation the microscpic distributon of the strain is taken into account. The absorption in coupled quantum dots is quite different from that of superlattices. In superlattices, the absorption does not exist when the electric vector of light is parallel to the superlattice plane (perpendicular incident). This introduces somewhat of a difficulty in fabricating the infrared detector. In quantum dots, the absorption exists when light incident along any direction, which may be good for fabricating infrared detectors.

Oscillator Strengths of the Intraband Transitions in a Semiconductor Superlattice in an Electric Field

AbstractThe oscillator strengths of the intraband optical transitions in a semiconductor superlattice in an electric field parallel to the growth axis is calculated using a perturbation model with Bloch envelope functions. The applied electric field induces strength asymmetry between the oblique +p and — p transitions of the Wannier‐Stark ladder.

Luminescence of Bi 4Ge 3O 12 (BGO) crystals under KrF and XeF laser excitation

The results of a study of the luminescence of Bi 4Ge 3O 12 (BGO) crystals at low excitation intensity by a KrF laser ( hv = 4.98 eV) and a two-photon excitation by an intense XeF laser (2 hv = 7 eV) confirm that the main emission band (2.5 eV) is due to optical transitions in local Bi 3+ ions. Under intense KrF laser excitation a broad structureless emission is observed. This emission may be assigned to the intraband optical transitions of highly excited electrons in the conduction band.

Normal incidence parallel intraband photoconductivity in GaAs/AlGaAs multiquantum wells

For the first time, resonant parallel photoconductivity due to intraband optical transitions is observed in GaAs/AlGaAs multiquantum wells. The angle of incidence dependence of the detected signals indicates that the photoconductivity process is not directly related to intersubband transitions. Indeed, contrary to usual detectors based on intersubband transitions, infrared radiations are detected at normal incidence with responsivity as high as (0.05 A/W). Photoconductivity spectra reproduce roughly the absorption spectra with, however, significant differences in peak positions, indicating that the intersubband transitions are indirectly involved in the detection process. Elements of an explanatory model based on hot-electron bolometric effects are discussed.

Intraband optical absorption in semiconductor superlattices

Absorption coefficients are calculated for intraband (intersubband in the conduction band) optical transition in semiconductor superlattices. The absorption line broadening is introduced by taking the fluctuation of well thickness into account. The selection rule is quite different for two kinds of transitions: transitions from the ground state to states below and above the conduction-band offset. The former follows the usual selection rule, whereas the latter does not. The calculated results well explain the experiments for ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/${\mathrm{In}}_{0.53}$${\mathrm{Al}}_{0.47}$As superlattices.

On the linewidth of Γ- and X-valley intersubband optical transitions in semiconductor quantum wells

Intraband optical transitions between bound Γ-valley levels in GaAs quantum wells, and between bound X-valley levels in AlAs quantum wells grown in [100] direction are investigated. Considerably lower values of linewidths are found for X-valley transitions, making the indirect semiconductor quantum wells preferable in applications where narrow linewidth is important.