Carrier-diffusion-limited remote optical addressing of single quantum dots

We present a scheme for remotely addressing single quantum dots (QDs) by means of near-field optical microscopy that simply makes use of the polarization of light. A structure containing self-assembled CdTe QDs is covered with a thin metal film presenting sub-wavelength holes. When the optical tip is positioned some distance away from a hole, surface plasmons in the metal coating are generated which, by turning the polarization plane of the excitation light, transfer the excitation towards a chosen hole and induce emission from the underlying dots. In addition, our procedure gives valuable insight into the diffusion of photo-excited carriers in the QD plane that can put limits to the addressing scheme.     

Robust quantum dot exciton generation via adiabatic passage with frequency-swept optical pulses

The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can be coherently controlled with resonant laser pulses. Long coherence times allow the observation of Rabi flopping of a single dipole transition in a solid state device, for which occupancy of the upper state depends sensitively on the dipole moment and the excitation laser power. We report on the robust population inversion in a single quantum dot using an optical technique that exploits rapid adiabatic passage from the ground to an excited state through excitation with laser pulses whose frequency is swept through the resonance. This observation in photoluminescence experiments is made possible by introducing a novel optical detection scheme for the resonant electron hole pair (exciton) generation.     

半导体自组织量子点量子发光机理与器件

介绍了自组织量子点单光子发光机理及器件研究进展.主要内容包括：半导体液滴自催化外延GaAs纳米线中InAs量子点和GaAs量子点的单光子发光效应、自组织InAs/GaAs量子点与分布布拉格平面微腔耦合结构的单光子发光效应和器件制备,单量子点发光的共振荧光测量方法、量子点单光子参量下转换实现的纠缠光子发射、单光子的量子存储效应以及量子点单光子发光的光纤耦合输出芯片制备等.     

Growth and time-resolved photoluminescence study of self-organized CdSe quantum dots in ZnSe

Employing two different growth methods: standard molecular beam epitaxy (MBE) and low-temperature atomic layer epitaxy (ALE) with subsequent annealing, we have obtained high-quality quantum dot structures consisting of CdSe embedded in ZnSe. Single dot emission lines are observed in micro-luminescence. The samples have been investigated by further optical methods including time-resolved photoluminescence under resonant excitation at 4.2 K. Distinct properties of systems with three-dimensional confinement are observed such as the suppression of the interaction between isolated quantum dots (QDs). In standard quantum wells tunneling/hopping processes generally lead to a pronounced red shift of the luminescence over time due to a lateral localization of excitons in potential fluctuations. A much less pronounced red shift is observed for the QDs reflecting only the different lifetimes of single dots and higher excited states. The red shift completely vanishes under resonant excitation that selectively excites only a few QDs of the ensemble in the layer. Typical behaviour is also observed from the halfwidth of the quantum dot emission.     

Scalable fabrication of optical resonators with embedded site-controlled quantum dots

We present a scalable method for the fabrication of site-controlled quantum dots (QDs) embedded in optical resonators. The position of the quantum dots is determined by nucleating their growth in an array of nano-holes that is aligned to a set of markers, allowing the precise overlay of the resonator geometry over the QD array. Coupling of the QDs to the resonators is demonstrated by the resonant enhancement of the spontaneous emission observed in photoluminescence experiments.     

Effects of p-type doping on the optical properties of InAs/GaAs quantum dots

The effects of p-type doping on the optical properties of self-organized InAs/GaAs quantum dots (QDs) were investigated by both micro-photoluminescence and degenerated pump–probe reflection measurements. As compared to undoped InAs/GaAs QDs, it was observed that the transitions between the ground and the first excited states of electrons and holes levels appeared at higher energies for p-doped InAs/GaAs QDs. In addition, the PL intensities for both undoped and p-doped QDs were found to decrease when the excitation power exceeded a critical value. The critical excitation power for p-doped QDs appeared to be much lower than that for undoped ones. In the pump–probe experiments, it was revealed that the value and sign of the differential reflectivity depends strongly on excitation wavelength. P-doped QDs exhibited a response behavior that is different from that of undoped ones. It is believed that the large build-in population of holes plays a crucial role in determining the transient reflection spectrum.     

Spectral Optical Properties of Polymer Composite Nanomaterials Based on Carbon Nanotubes in a High-Density Polyethylene Matrix

Spectral, kinetic, and nonlinear optical regularities that demonstrate the exchange of electronic excitations between the components of hybrid associates of Ag₂S colloid quantum dots (1.7–1.8 nm) in gelatin with molecules of thiazine dyes (Ds) are found. When the IR luminescence of Ag₂S quantum dots (QDs) is excited by radiation from the thionine absorption region, its enhancement due to nonradiative resonant energy transfer is observed. The association with methylene-blue molecules blocked the IR luminescence of Ag₂S QDs upon its excitation by radiation from the absorption region of the dye due to the transfer of charge carriers. It is demonstrated that the hybrid association of thionine molecules and Ag₂S QDs adversely affects the nonlinear optical properties of the latter, which manifests itself in inverse saturated absorption by the action of 10-ns second-harmonic pulses (532 nm) of a Nd³⁺:YAG laser. For the associates of Ag₂S QDs with methylene-blue molecules, the radiation focusing caused by the transfer of charge carriers from the dye and the change in the population of small traps in nanocrystals is found. It is concluded that the direction of the transfer of electronic excitations and the photophysical processes in these objects are determined by the mutual arrangement of the HOMO–LUMO levels of the dye with respect to the levels of dimensional quantization of the Ag₂S QDs.     

Electronic structure and optical gain of InAsPN/GaP(N) quantum dots

The electronic structure and optical gain of InAsPN/GaP(N) quantum dots (QDs) are investigated in the framework of the effective-mass envelope function theory. The strain distribution is calculated using the valence force field (VFF) method. With GaP barrier, for smaller InAsPN QDs, the minimum transition energy may occur at a lower phosphorous (P) composition, but for larger QDs, the transition energy increases as P composition increases due to the increased bandgap of alloy QDs. When the nitrogen (N) composition increases, the transition energy decreases due to the stronger repulsion between the conduction band (CB) and the N resonant band, and the transition matrix element (TME) is more affected by the transition energy rather than N–CB mixing. To obtain laser materials with a lattice constant comparable to Si, we incorporated 2% of N into the GaP barrier. With this GaP_0.98N_0.02 barrier, the conduction band offset is reduced, so the quantum confinement is lower, resulting in a smaller transition energy and longer wavelength. At the same time, the TME is reduced and the optical gain is less than those without N in the barrier at a low carrier density, but the peak gain increases faster when the carrier density increases. Finally it can surpass and reach a greater saturation optical gain than those without N in the barrier. This shows that incorporating N into GaP barriers is an effective way to achieve desirable wavelength and optical gain.     

Excitation transfer in novel self-organized quantum dot structures

We report on studies of excitation transfer processes in vertically self-organized pairs of unequal-sized quantum dots (QDs), created in InAs/GaAs bilayers having differing InAs deposition amounts in the first (seed) and subsequent layer. The former and latter enable independent control, respectively, of the density and the size distribution of the second layer QDs. This approach allows us to enhance the average volume and improve the uniformity of InAs QDs, resulting in low-temperature photoluminescence at 1.028 eV with a linewidth of 25 meV for 1.74 ML (seed)/3.00 ML InAs stacking. The optical properties of the formed pairs of unequal-sized QDs with clearly discernible ground-state transition energy depend on the spacer thickness and composition. Photoluminescence results provide evidence for nonresonant energy transfer from the smaller QDs in the seed layer to the larger QDs in the second layer in such asymmetric QD pairs. Transfer times down to 20 ps (36 ML GaAs spacer) are estimated, depending exponentially on the GaAs spacer thickness.     

Selective excitation of self-assembled quantum dots by using shaped pulse

We carried out optical selective excitation of individual self-assembled quantum dots by using phase-modulated pulses. Based on scattered photoluminescence excitation resonances in individual QDs, the excitation pulses modulated in the spectral region allows for addressing individual ground states emission. The photoluminescence spectra including several QDs showed intensity changes according to both the modulation energies and phases. The results also suggested the individual control of selective QDs even in collective excitation.     

Synthesis,COSMO-RS analysis and optical properties of surface modified ZnS quantum dots using ionic liquids

Zinc sulfide (ZnS) quantum dots (QDs) were synthesized using the microwave assisted ionic liquid (MAIL) route. Three ionic liquids (ILs), namely, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF₄]), trihexyl(tetradecyl) phosphonium bis(trifluoromethanesulfonyl) amide ([P_6,6,6,14][TSFA]) and trihexyl(tetradecyl) phosphonium chloride ([P_6,6,6,14][Cl]) were used in this study. The size and structure of the QDs were characterized by high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) pattern, respectively. The synthesized QDs were of wurtzite crystalline structure with size less than 5 nm. The QDs were more uniformly distributed while using the phosponium based ILs as a reaction medium during synthesis. The optical properties were investigated by UV–vis absorption and photoluminescence (PL) emission spectroscopy. The optical properties of QDs showed the quantum confinement effect in their absorption and the effect of cation and anion structural moiety was observed on their bandedge emission. The QDs emission intensity was measured higher for [P_6,6,6,14][Cl] due to their better dispersion as well as high charge density of Cl anion. The capability of the ILs in stabilizing the QDs was interpreted by density functional theory (DFT) computations. The obtained results are in good agreement with the theoretical prediction.     

Characterization of an optical frequency comb using modified direct frequency comb spectroscopy

We introduce a method for determination of the absolute frequencies of comb lines within an optical frequency comb spectrum. The method utilizes the experimental and theoretical approach of the velocity-selective optical pumping of the atomic ground state hyperfine levels induced by resonant pulse-train excitation. The information on the laser pulse repetition frequency and carrier–envelope offset are physically mapped onto the ⁸⁷Rb ground state hyperfine level population velocity distributions. Theoretical spectra are calculated using an iterative analytic solution of the optical Bloch equations describing the resonant pulse-train excitation of four-level ⁸⁷Rb atoms. They are employed to fit the measured spectra and obtain the parameters of the frequency comb, thus providing a practical algorithm which can be used in real-time measurements.     

Manufacturing of Volumetric Glass–Based Composites with Single‐ and Double‐QD Doping

Quantum dot (QD)‐based light‐emitting materials are gaining increased attention because of their easily tunable optical properties desired for various applications in biology, optoelectronics, and photonics. However, few methods can be used to manufacture volumetric materials doped with more than one type of QD other than QD‐polymer hybrids, and they often require complicated preparation processes and are prone to luminescence quenching by QD aggregation and separation from the matrix. Here, simultaneous doping of a volumetric glass‐based nanocomposite with two types of QDs is demonstrated for the first time in a single‐step process using the nanoparticle direct doping method. Glass rods doped with CdTe, CdSe/ZnS, or co‐doped with both QDs, are obtained. Photoluminescence and lifetime experiments confirm temperature‐dependent double emission with maxima at 596 and 720 nm with mean lifetimes up to 16 ns, as well as radiative energy transfer from the short wavelength–emitting QDs to the long wavelength–emitting QDs. This approach may enable the simple and cost‐efficient manufacturing of bulk materials that produce multicolor luminescence with cascade excitation pumping. Applications that could benefit from this include broadband optical fiber amplifiers, backlight systems in LCD screens, high‐power LEDs, or down‐converting solar concentrators used to increase the efficiency of solar panels.     

Optical control of excitons in a pair of quantum dots coupled by the dipole-dipole interaction

We demonstrate coherent nonlinear-optical control of excitons in a pair of quantum dots (QDs) coupled via dipolar interaction. The single-exciton population in the first QD is controlled by resonant picosecond excitation, giving rise to Rabi oscillations. As a result, the exciton transition in the second QD is spectrally shifted and concomitant Rabi oscillations are observed. We identify coupling between permanent excitonic dipole moments as the dominant interaction mechanism, whereas quasiresonant (F?rster) energy transfer is weak. Such control schemes based on dipolar interaction are a prerequisite for realizing scalable quantum logic gates.     

The influence of the quantum dot/polymethylmethacrylate composite preparation method on the stability of its optical properties under laser radiation

Photoluminescent semiconductor nanocrystals, quantum dots (QDs), are nowadays one of the most promising materials for developing a new generation of fluorescent labels, new types of light-emitting devices and displays, flexible electronic components, and solar panels. In many areas the use of QDs is associated with an intense optical excitation, which, in the case of a prolonged exposure, often leads to changes in their optical characteristics. In the present work we examined how the method of preparation of quantum dot/polymethylmethacrylate (QD/PMMA) composite influenced the stability of the optical properties of QD inside the polymer matrix under irradiation by different laser harmonics in the UV (355 nm) and visible (532 nm) spectral regions. The composites were synthesized by spin-coating and radical polymerization methods. Experiments with the samples obtained by spin-coating showed that the properties of the QD/PMMA films remain almost constant at values of the radiation dose below ~10 fJ per particle. Irradiating the composites prepared by the radical polymerization method, we observed a monotonic increase in the luminescence quantum yield (QY) accompanied by an increase in the luminescence decay time regardless of the wavelength of the incident radiation. We assume that the observed difference in the optical properties of the samples under exposure to laser radiation is associated with the processes occurring during radical polymerization, in particular, with charge transfer from the radical particles inside QDs. The results of this study are important for understanding photophysical properties of composites on the basis of QDs, as well as for selection of the type of polymer and the composite synthesis method with quantum dots that would allow one to avoid the degradation of their luminescence.     

Ultraslow optical solitons in a cold four-state medium

We show the formation of ultraslow optical solitons in a lifetime broadened four-state atomic medium under Raman excitation. With appropriate conditions we demonstrate, both analytically and numerically, that both bright and dark ultraslow optical solitons can occur in such a highly resonant medium with remarkable propagation characteristics. This work may open other research opportunities in condensed matter and may result in a substantial impact on technology.     

Tamper resistance in optical excitation transfer based on optical near-field interactions

We present tamper resistance in optical excitation transfer via optical near-field interactions based on the energy dissipation process occurring locally in nanometric devices such as quantum dots. A theoretical comparison with electrical systems is also shown, focusing on the required environmental conditions. Numerical simulations based on virtual photon models demonstrate high tamper resistance.     

Formation of QD-porphyrin molecule complexes in aqueous solutions

The spectral and luminescent manifestations of the electrostatic formation of quantum dot (QD)-porphyrin complexes are studied. The QD luminescence in these complexes is found to be efficiently quenched. The luminescence of molecules complexed with QDs is also partially quenched. The luminescence excitation spectra of porphyrin molecules associated with QDs exhibit a contribution of the QD absorption spectrum, which indicates that energy is transferred from QDs to porphyrin. The efficiency of the nonradiative resonant energy transfer from a QD to a porphyrin molecule is estimated. The observed experimental data agree well with a proposed model of formation of complexes of the QD-organic molecule type.     

ZnO/Ag纳米复合物的制备及光学性质

采用ZnO纳米晶表面还原Ag+的方法合成了ZnO/Ag纳米复合物,并研究了其光学性质.透射电镜和XRD谱表征了ZnO/Ag纳米复合物的晶体形貌和结构,吸收光谱和荧光光谱证明ZnO和Ag之间存在电子传递.在325 nm激光激发下观察到了ZnO/Ag纳米复合物的表面增强共振拉曼散射.     

Experimental measurements on transverse vibration characteristics of piezoceramic rectangular plates by optical methods

This study provides two non-contact optical techniques to investigate the transverse vibration characteristics of piezoceramic rectangular plates in resonance. These methods, including the amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) and laser Doppler vibrometer (LDV), are full-field measurement for AF-ESPI and point-wise displacement measurement for LDV, respectively. The edges of these piezoceramic rectangular plates may either be fixed or free. Both resonant frequencies and mode shapes of vibrating piezoceramic plates can be obtained simultaneously by AF-ESPI. Excellent quality of the interferometric fringe patterns for the mode shapes is obtained. In the LDV system, a built-in dynamic signal analyzer (DSA) composed of DSA software and a plug-in waveform generator board can provide the piezoceramic plates with the swept-sine excitation signal, whose gain at corresponding frequencies is analyzed by the DSA software. The peaks appeared in the frequency response curve are resonant frequencies. In addition to these optical methods, the numerical computation based on the finite element analysis is used to verify the experimental results. Good agreements of the mode shapes and resonant frequencies are obtained for experimental and numerical results.