We present a study of free carrier photogeneration and multicarrier bound states, such as biexcitons and trions (charged excitons), in semiconducting single-walled carbon nanotubes. Pump-and-probe measurements performed with fs pulses reveal the effects of strong Coulomb interactions between carriers on their dynamics. Biexciton formation by optical transition from exciton population results in an induced absorption line (binding energy 130 meV). Exciton-exciton annihilation process is shown to evolve at high densities towards an Auger process that can expel carriers from nanotubes. The remaining carriers give rise to an induced absorption due to trion formation (binding energy 190 meV). These features show the dynamics of exciton and free carriers populations.

The three-dimensional photonic crystals used in this study were synthetic opals, composed of submicron silica spheres, close-packed in a face-centered cubic structure with a period of 200 nm, that exhibit photonic stopbands around 600 nm. We present measurements of the optical gain of CdS quantum dots (QDs) embedded inside the interstitials between the silica spheres. Unlike the usual gain spectra of CdS QDs in glass matrices, which display maximum gain at energies of the first quantum-confined transitions, for QDs embedded in photonic crystals the gain maximum is shifted toward the high-frequency edge of the photonic stopband (2.2 eV) far below the absorption edge of the semiconductor (2.5 eV). Studies of temperature, intensity, and orientation dependencies of the gain spectra allow one to ascribe the observed effect to gain enhancement caused by multiple coherent Bragg scattering of light in the periodic photonic crystal.

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Silicon nanocrystals (SiNCs) smaller than 5 nm are a material with strong visible photoluminescence (PL). However, the physical origin of the PL, which, in the case of oxide-passivated SiNCs, is typically composed of a slow-decaying red-orange band (S-band) and of a fast-decaying blue-green band (F-band), is still not fully understood. Here we present a physical interpretation of the F-band origin based on the results of an experimental study, in which we combine temperature (4-296 K), temporally (picosecond resolution) and spectrally resolved luminescence spectroscopy of free-standing oxide-passivated SiNCs. Our complex study shows that the F-band red-shifts only by 35 meV with increasing temperature, which is almost 6 times less than the red-shift of the S-band in a similar temperature range. In addition, the F-band characteristic decay time obtained from a stretched-exponential fit decreases only slightly with increasing temperature. These data strongly suggest that the F-band arises from the core-related quasi-direct radiative recombination governed by slowly thermalizing photoholes.

On the method of photoluminescence spectral intensity ratio imaging of silicon bricks: Advances and limitations J. Appl. Phys. 112, 063116 (2012) Measurement of net dopant concentration via dynamic photoluminescence J. Appl. Phys. 112, 063704 (2012) Luminescence and deep-level transient spectroscopy of grown dislocation-rich Si layers AIP Advances 2, 032152 (2012) The role of excess minority carriers in light induced degradation examined by photoluminescence imaging J. Appl. Phys. 112, 033703 (2012) Effect of hydrofluoric acid concentration on the evolution of photoluminescence characteristics in porous silicon nanowires prepared by Ag-assisted electroless etching method

To investigate the properties of a two-dimensional carrier gas at intermediate densities, we perform picosecond transient four-wave mixing experiments on trions ͑charged excitons͒ and neutral excitons in modulation p-doped CdTe/Cd 1ϪxϪy Mg x Zn y Te quantum wells. The determination of trion and exciton dephasing rates reveals a localization of both trions and holes in potential fluctuations induced by the ionized remote acceptors. We demonstrate that trions can be efficiently used as a charged optical probe sensitive to electrostatic potential fluctuations which are imperceptible for neutral excitons. ͓S0163-1829͑99͒15731-X͔ Modifications of optical spectra due to the introduction of carriers into a semiconductor quantum well ͑QW͒ recently have been of considerable interest, in particular as they provide information on the electronic properties in the vicinity of the metal-insulator transition, and on the role of disorder. 1 It has been known for a long time that excitons characterize the optical spectra of undoped semiconductors while bandto-band transitions are observed on strongly doped samples. It has been recognized more recently that spectra of moderately doped QW's feature also charged excitons, either positively or negatively according to the type of doping-the so-called trions. 1-5 Extensive investigations have been reported on the binding energy of trions, 2-5 on their polarization in a magnetic field, 2,3 and on their behavior in an electric field 6 and at different carrier densities. 1,3,4,7 Up to now, however, only very few papers have been devoted to the dynamics of trions, including transport and relaxation. [7][8][9] Actually, there has been some controversy in the literature about the possible localization of trions in modulation doped QW's ͑MDQW͒, in which carrier gases are formed in the QW's by introducing doping layers in the barriers. It was argued that free carriers would screen the Coulomb interaction thus forbidding the formation of excitons and trions. 1,2 Hence the charge carriers should be localized by electrostatic potential fluctuations caused by the ionized remote donors or acceptors. Recent near-field photoluminescence experiments 10 directly evidence the localization of trions in n-doped GaAs QW's and conclude indirectly to the localization of the carriers themselves. Other authors 3,11 have reported the observation of free trions in high-quality MDQW's with relatively thick spacer layers, which decreases the fluctuation depth.In this paper we demonstrate that the study of the coherence properties of trions and excitons by degenerate fourwave mixing ͑FWM͒ is a very efficient method to investigate a possible localization of both trions and carriers in a MDQW. The idea is as follows: First, localized quasiparticles have a strongly reduced scattering efficiency as compared to free ones. Second, this scattering plays an important role in the dephasing of the quasiparticles. Consequently, the determination of exciton and trion dephasing rates under different conditions ͑exciton, trion a...

We propose an analysis of the emission properties of anatase and rutile titanium dioxide (TiO 2) that emphasizes the role of the strong electron-phonon interaction. We performed measurements of photoluminescence (PL) spectra of bulk monocrystals under continuous wave-laser excitation and of their temperature dependence. We show that in both anatase and rutile, weakly bound self-trapped excitons are actually made out from carrier polarons and give rise to a broad emission band in the visible spectral range. The thermal activation of carrier motion allows their hopping to distant sites that leads to the observed quenching of luminescence. In the specific case of rutile TiO 2 , the PL spectral shape and its intensity-quenching scenario reveal the presence of dark trap states. Moreover, an additional narrow line structure shows up at low temperatures. The latter is due to localized impurity states that can be attributed to oxygen vacancies and can be fitted with a large Huang-Rhys parameter S = 2.5 within a Franck-Condon model. Both phases show thus a very strong interaction between the photogenerated carriers and the lattice.