We report measurements of exciton dynamics in GaAs multiple-quantum-well structures at low temperature using three-pulse picosecond four-wave mixing. The time-resolved measurements of the emission show both a delayed signal (stimulated photon echo) and a prompt signal (free polarization decay) with different decay times. The results suggest that, in addition to the inhomogeneously broadened resonance, a homogeneously broadened resonance contributes to the optical response of the exciton.PACS numbers: 78.47.+p, 42.50.Md, 42.65.Re, 71.35,+z Transient nonlinear laser spectroscopy methods provide a powerful experimental tool for the study of electronic excitation in solids. In the simplest experiment, optical excitation by a short pump pulse, with wave vector ki, creates a nonequilibrium state of the system and modifies the absorption, which can be measured as a function of time and frequency with a second beam with wave vector k2. The decay rates reflect the return to equilibrium and arise from several mechanisms such as radiative decay and scattering due to interactions with other perturbers. The time evolution may be very complex. Using the language developed for simpler systems, this decay can be referred to as longitudinal relaxation. Electronic excitation by coherent optical radiation also creates a macroscopic polarization reflecting the fact that the system is in a superposition of states that give rise to the dipole. This polarization is characterized by a decay that is usually distinct from the longitudinal relaxation and can be designated transverse or dephasing relaxation, corresponding to the homogeneous linewidth in the frequency domain. In the time domain, the dephasing rate can be measured by transient four-wave mixing * (FWM) where two beams separated in time (with wave vectors kj and k2) create a spatial modulation that results in scattering of the incident beams in direction 2ki -k2 or 2k2 _ kj. For a homogeneously broadened system, the signal is prompt [called a free polarization decay (FPD)] with respect to the second pulse, while for an inhomogeneously broadened system, the signal emission is delayed [called a two-pulse photon echo (PE)] and the delay time is the interval between the input pulses. In this paper we examine both the longitudinal and transverse relaxation of resonantly excited excitons in GaAs/AlGaAs multiple-quantum-well (MQW) structures using a powerful extension of the above techniques based on time-separated, three-pulse excitation 2 in a backward FWM geometry.In an ideal quantum well at low temperature, the quasi-two-dimensional excitons are delocalized. For the HH1 exciton (HH denotes heavy hole), phonon and defect scattering result in rapid dephasing [measured using transient FWM in a single high-quality QW, width = 13.5 nm, to be of order 2 ps corresponding to the homogeneous linewidth of order 0.6 meV (Ref.3)]. This scattering also gives rise to an initial fast decay in the longitudinal relaxation rate measurement due to scattering to exciton k values away from the init...
A phylogenetic analysis is conducted for all 15 of the Oriental genera of Orthopagini (OGO), and representatives in Arjunini, Dictyopharini, and Hastini, so as to investigate the monophyly and phylogenetic relationships of the OGO. Although the length, thickness, and curvature of the cephalic process vary dramatically in different taxa of Orthopagini, most OGO make up a good monophylum if Emeljanovina Xing & Chen, 2013, is excluded. The revised OGO include 14 genera and can be divided into four unambiguous clades: Medeusa, Dictyopharina, Orthopagus, and Dictyotenguna. On the basis of the results of the above analysis, the phylogenetic analysis for all species of Centromeria Stå l, 1870 is presented based on 48 adult characters. The revised Centromeria is monophyletic and can be separated into two unambiguous lineages: the speilinea clade and the longipennis clade. With the combination of the phylogeny and the updated geographical distribution, a Sundaland origin and a historically widespread distribution in Southeast Asia are suggested for Centromeria. Eight new Centromeria species, one new genus, and four new combinations are presented. Two synonymized Centromeria species are resurrected and two new specific synonyms are recognized. Diagnoses and descriptions, along with structural illustrations of male genitalia, are provided for all new species and four previously described species, excepting Centromeria inspinata Haupt, 1917. Keys to the OGO and to the species of Centromeria are provided.
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