Porous silicon bandgap broadening at natural oxidation

Emission and excitation photoluminescence spectra of porous silicon thin layers have been investigated at natural oxidation. The shift of both types of spectra to high-energy region with time has been shown. Analysis of excitation spectra points out the indirect behavior of electron transitions responsible for visible photoluminescence, which remains unaltered at natural oxidation. The value of optical bandgap is estimated in each case. It is shown that the optical bandgap broadens during oxidation due to size reduction of silicon nanocrystallites.     

Optical properties of InN—the bandgap question

The recent controversy on the bandgap of InN is addressed, with reference to optical data on single crystalline thin film samples grown on sapphire. The optical absorption spectra deduced from transmission data or spectroscopic ellipsometry are consistent with a lowest bandgap around 0.7 eV in the low doping limit. Further, these data from a number of different independent authors and samples give values for the absorption coefficient within a factor 2 well above the absorption edge, supporting an intrinsic direct bandgap process. The presence of Mie resonances due to In inclusions in the InN matrix affects the shape of the absorption above the edge, but is less relevant for the discussion of the bandgap for pure InN. The alternative model of a deep level to conduction band transition requires the presence of a deep donor at a concentration close to 10²⁰ cm⁻³; in addition this concentration has to be the same within a factor 2 in all samples studied so far. This appears implausible, and no such deep donor could so far be identified from SIMS data in the highest quality samples studied. The line shape of the photoluminescence spectra can be quite well reproduced in a model for the optical transitions from the conduction band states to localized states above the valence band, including the Coulomb effects of the impurity potentials. A value of 0.69 eV for the bandgap of pure InN is deduced at 2 K. For samples that appear to be only weakly degenerate n-type two narrow peaks are observed in the photoluminescence at low temperature, assigned to conduction band—acceptor transitions. These peaks can hardly be explained in the deep level model. Recent cathodoluminescence data on highly n-doped InN films showing that the emission appears to be concentrated around In inclusions can also be explained as near bandgap recombination, considering the plausible enhancement due to interface plasmons. Finally, recent photoluminescence data on quantum structures based on InN and InGaN with a high In content appear to be consistent with moderate upshifts of the emission from a 0.7 eV value due to electron confinement.     

Possible generation of electric multipole radiation from solid state quantum rings

The self-organized crystal growth of semiconductor quantum rings has opened a new possibility to study and exploit optical transitions between ring-shaped quantum states. In such states, orbital angular momenta of particle envelope functions are well-defined. We investigate theoretically the intraband interlevel transitions between such states and examine the possibility of electrical multipole radiations (EMRs). Selections rules due to envelope function quantum numbers are deduced. To enhance the EMR efficiency, we propose a novel coupled dot–ring structure, by which the lowest allowed EMR can be selected and manipulated, allowing the efficient radiation of multipole photons.     

Optical properties of GaN/AlN multiple quantum wells

Optical properties of GaN/AlN multiple quantum wells (MQW) have been investigated by Raman scattering, photoluminescence and photoluminescence excitation measurements. A careful examination of the Raman spectrum reveals the fact that the constituent layers of GaN/AlN MQWs are well strained. The experimental results of emission and absorption in MQWs were compared with the calculated solutions of the finite quantum well and the bound states involved in the optical transitions were identified. It is found that the interband transitions up to n=3 bound state can be observed in the strained GaN/AlN MQWs sample. The temperature dependence of the heavy-hole transitions shows an interesting phenomenon, in which the peak energy first increases with increasing temperature and then decreases with the temperature rapidly. The observation can be explained in a consistent way by the strain effects of lattice mismatch due to the interplay between the thermal expansion of GaN and AlN layers. Our results indicate that pseudomorphic GaN/AlN MQWs with good quality can be readily grown, and their applications in optoelectronics can be expected in the near future.     

Magnetic polaron-related optical properties in Ni(Ⅱ)-doped Cd S nanobelts: Implication for spin nanophotonic devices

Emissions by magnetic polarons and spin-coupled d-d transitions in diluted magnetic semiconductors(DMSs)have become a popular research field due to their unusual optical behaviors.In this work,high-quality NiI₂(Ⅱ)-doped CdS nanobelts are synthesized via chemical vapor deposition(CVD),and then characterized by scanning electron microscopy(SEM),x-ray diffraction,x-ray photoelectron spectroscopy(XPS),and Raman scattering.At low temperatures,the photoluminescence(PL)spectra of the Ni-doped nanobelts demonstrate three peaks near the band edge:the free exciton(FX)peak,the exciton magnetic polaron(EMP)peak out of ferromagnetically coupled spins coupled with FXs,and a small higher-energy peak from the interaction of antiferromagnetic coupled Ni pairs and FXs,called antiferromagnetic magnetic polarons(AMPs).With a higher Ni doping concentration,in addition to the d-d transitions of single Ni ions at 620 nm and 760 nm,two other PL peaks appear at 530 nm and 685 nm,attributed to another EMP emission and the d-d transitions of the antiferromagnetic coupled Ni²⁺-Ni²⁺pair,respectively.Furthermore,single-mode lasing at the first EMP is excited by a femtosecond laser pulse,proving a coherent bosonic lasing of the EMP condensate out of complicated states.These results show that the coupled spins play an important role in forming magnetic polaron and implementing related optical responses.     

Ab-initio study of doped nanostructures—Mn-doped ultrathin ZnS films

An ab-initio study of the effects of the quantum confinement has been performed for the first time in the ultrathin ZnS films: unpassivated, passivated and the Mn-doped ones. A self-consistent full potential linear muffin tin orbital (FP-LMTO) method has been employed. The studied films have comparatively a large thickness range of 2.7–29.7 Å. The fundamental band gap increases exponentially with decrease in the size of the quantum confinement. The Mn-doped films reveal the localized impurity-induced states within the band gap and also in the conduction band region. The intense optical transitions between the Mn-induced states will appear at about 2.1 eV which is in excellent agreement with the observed peak in the photoluminescence experiments.     

Preparation and electronic structure of phase pure K3C60

Phase pure K₃C₆₀ films have been grown using vacuum distillation. The structure of such films could be shown to be face centered cubic consistent with X-ray diffraction studies. The electronic structure of the films has been studied using electron energy-loss spectroscopy in transmission. From C1s core excitation measurements the unoccupied density of states has been determined. Performing the dielectric function has been derived in a wide energy range (0–45 eV). It is shown that the low energy part of the optical conductivity cannot be understood within a simple free electron model but that interband transitions between the three conduction bands have to be taken into account. The spectral weight of interband transitions between valence and conduction bands shows strong momentum dependence due to optical selection rules demonstrating the molecular-like nature of the electronic states.     

GaN激子跃迁的时间分辨光谱学研究

用时间分辨光谱学方法研究低压有机金属化学汽相沉积生长的ＧａＮ中自由，束缚激子（ＢＸ）的跃迁，讨论了这些跃迁的光致发光谱，复合寿命及其与温度的关系，给出了中性施主束缚激子和自由激子（ＦＸ）的辐射复合寿命分别为０．１２ｎｓ和０．４ｎｓ。     

A tight-binding study of LUMO states in ellipsoidal silicon nanocrystals

In this paper we report on tight-binding calculations of lowest unoccupied molecular orbitals states for silicon ellipsoidal nanocrystals. The electronic structure has been calculated for different nanocrystal shapes either keeping constant or varying the number of silicon atoms. We have found that changing the ellipsoid aspect ratio a non-obvious energy level structure is obtained. The implications for the infrared optical transitions and their relationship with the polarization of the radiation involved are discussed.     

Some optical properties of amorphous and crystalline antimony trisulphide thin films

The change in optical properties accompanying the amorphous crystalline transition has been studied for antimony trisulphide thin films. The real and imaginary parts of the dielectric constant are found to be much lower for amorphous films at lower photon energies, possibly because of a large number of defect states which would mostly disturb the top of the valence band and the conduction band comprised, respectively, of chalcogen lone pair and antibonding states. The crystalline material shows some structure in the imaginary part of the dielectric constant that corresponds to interband transitions, and apparently the direct band edge is at 1.88 eV. In the edge region the power law absorption has been observed in the amorphous material from which the extrapolated optical gap has been found to be 1.7 eV.     

Contactless electroreflectance spectroscopy of optical transitions in low dimensional semiconductor structures

The authors present the application of contactless electroreflectance (CER) spectroscopy to study optical transitions in low dimensional semiconductor structures including quantum wells (QWs), step-like QWs, quantum dots (QDs), quantum dashes (QDashes), QDs and QDashes embedded in a QW, and QDashes coupled with a QW. For QWs optical transitions between the ground and excited states as well as optical transitions in QW barriers and step-like barriers have been clearly observed in CER spectra. Energies of these transitions have been compared with theoretical calculations and in this way the band structure has been determined for the investigated QWs. For QD and QDash structures optical transitions in QDs and QDashes as well as optical transitions in the wetting layer have been identified. For QDs and QDashes surrounded by a QW, in addition to energies of QD and QDash transitions, energies of optical transitions in the surrounded QW have been measured and the band structure has been determined for the surrounded QW. Finally some differences, which can be observed in CER and photo-reflectance spectra, have been presented and discussed for selected QW and QD structures.     

Oscillatory behavior of magneto-optical interband emissions in asymmetric quantum well structures

We report on the oscillatory behavior of the photoluminescence intensity from asymmetric AlGaAs/InGaAs/GaAs quantum well structures in the presence of a perpendicular magnetic field. Two distinct photoluminescence peaks originating from transitions from the ground (e₁) and the first excited (e₂) electronic states to the heavy hole state (hh₁) are observed. The opposite phase of the oscillations shows clearly the competitive process between the transitions from the ground and first excited states. Electron transfer mechanisms cannot explain the origin of these oscillations. The optical oscillations emerge from changes in the effective electron–hole interaction.     

Temperature dependence of photoluminescence of semiconductor quantum dots upon indirect excitation in a SiO2 dielectric matrix

The processes of excitation and relaxation of confined excitons in semiconductor quantum dots upon indirect high-energy excitation have been considered. The temperature behavior of photoluminescence of quantum dots in a SiO₂ dielectric matrix has been described using a model accounting for the process of population of quantum-dot triplet states upon excitation transfer through mobile excitons of the matrix. Analytical expressions that take into account the two-stage and three-stage schemes of relaxation transitions have been obtained. The applicability of these expressions for analyzing fluorescence properties of semiconductor quantum dots has been demonstrated using the example of silicon and carbon nanoparticles in the thin-film SiO₂ matrix. It has been shown that the complex character of the temperature dependences of the photoluminescence upon indirect excitation can be an indication of a multistage relaxation of excited states of the matrix and quantum dots. The model concepts developed in this study allow one to predict the form of temperature dependences of the photoluminescence for different schemes of indirect excitation of quantum dots.

     

Exclusive (e,e′pp) knockout reactions

Exclusive cross sections of the ¹⁶O(e,e′pp) ¹⁴C knockout reaction are calculated for transitions to the low-lying discrete final states of the residual nucleus. Short-range correlations and two-body currents, due to the excitation of a Δ resonance in the intermediate state, are included in the calculations. Final-state interactions are taken into account by means of phenomenological spin-dependent optical potentials. Recoil-momentum distributions for transitions to states with different angular momentum exhibit different shapes, which are basically determined by the cm orbital angular momentum of the initial proton pair.     

多壁纳米碳管的频率上转换效应研究

实验上测量了多壁纳米碳管的吸收光谱和光致发光谱,观察到了多壁纳米碳管的光频率上转换效应,激发波长为1064 nm,发射光谱为带状光谱,峰值波长为780 nm.由吸收光谱上观察到了纳米碳管的态密度分布的范霍夫奇点,这些奇点对应的吸收峰位置为685nm,719nm和894nm.上转换过程是纳米碳管的电子经双光子吸收,再经无辐射跃迁布居在范霍夫奇点,最后经辐射跃迁而产生荧光.     

Influence of two-pair continuum correlations following resonant excitation of excitons

The memory structure induced by coherent transitions to the exciton-exciton scattering continuum is shown to have significant influence on spectrally resolved four-wave-mixing signals even under selective excitation of 1s excitons. Comparisons between experiments and calculations that account nonperturbatively for these quantum kinetic Coulomb correlations demonstrate large compensations between mean-field contributions and transitions to the two-pair continuum. Experiments with different polarizations of the laser pulses show that two-pair continuum correlations are responsible for delay-time dependent shifts of the excitonic emission as well as for substantial deformations of the line shape.     

Radiated recombination in CdGa2Se4

The photoluminescence of single crystals of CdGa₂Se₄ is studied as a function of temperature, of the excitation intensity, and of the polarization of the exciting light. A possible level scheme for optical transitions in the perfect crystal is given and evidence is also found for donor-acceptor pair recombination luminescence.     

Resonant Raman scattering of optical phonons in self-assembled quantum dots

We have investigated the carrier relaxation mechanism in InGaAs/GaAs quantum dots by photoluminescence excitation (PLE) spectroscopy. Near-field scanning optical microscope successfully shows that a PLE resonance at a relaxation energy of 36 meV can be seen in all single-dot luminescence spectra, and thus can be attributed to resonant Raman scattering by a GaAs LO phonon to the excitonic ground state. In addition, a number of sharp resonances observed in single-dot PLE spectra can be identified as resonant Raman features due to localized phonons, which are observed in the conventional Raman spectrum. The results reveal the mechanism for the efficient relaxation of carriers observed in self-assembled quantum dots: the carriers can relax within the continuum states, and make transitions to the excitonic ground state by phonon emission.     

Controlled emission from dye saturated single and coupled microcavities

Modified photoluminescence is demonstrated from the dye saturated porous silicon based single and coupled microcavities. When photonic cavity mode is weakly coupled to the emission states of the dye, photoluminescence line narrowing and intensity enhancement have been observed. Our experimental work and transfer matrix simulations and cavity modelling convincingly explain the tunability and optical field confinement within the microcavity. We also show that the photoluminescence enhancement is due to one-dimensional microcavity effect. These optically active hybrid materials from inexpensive fabrication may become an important consideration for many photonic applications.     

Ultrafast intrachain photoexcitation of polymeric semiconductors

We report on excited state dynamics in isolated poly(9,9-dioctylfluorene) chains obtained by embedding the polymer in an inert plastic matrix. Early events (<300 fs) of intrachain photophysics are detected by pump-probe spectroscopy using tunable UV 25-fs pump pulses and sub-10-fs visible probe pulses. We show that higher-lying optical states, reached by multiphoton transitions, give rise to on-chain charge separation on the ultrafast time scale. The intrachain charge pair decays geminately within 500 fs to the lowest singlet state. Characteristic time scales for internal conversion and intramolecular vibrational redistribution are also determined.