Subdiffraction far-field imaging of luminescent single-walled carbon nanotubes

Far-field near-infrared fluorescence microscopy of single-walled carbon nanotubes (SWNTs) has been hampered by the diffraction limit to resolution. A new analysis method is presented that allows subwavelength (相似文献   

Long-wavelength quantum-dot lasers

Quantum dots (QD) of (InGaAs/GaAs) on GaAs substrate with long-wavelength emission (1300 nm) have been fabricated using metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) for use in surface-emitting laser diodes. QDs are obtained by employing two different approaches, seeding and overgrowth with a quantum well, yielding similar recombination spectra. Despite the shift to long wavelengths, a large separation (80 meV) between excited states is maintained. The introduction of such QDs into a vertical cavity leads to a strong narrowing of the emission spectrum. Lasing from 1300-nm QD VCSEL is reported.     

Transferring the spatial state of an entangled photon pair to a single photon by photon detection

We show theoretically that the spatial state of entangled photons generated by parametric down-conversion can be transferred to the spatial state of an idler photon by signal photon detection. This study considered the general condition with an arbitrary pump field profile and the detection of a signal photon at an arbitrary distance from a nonlinear crystal where the entangled photons are generated. Upon the detection of a signal photon, the two-photon state function of the entangled state can be transferred to a single-photon state function of the idler field due to the EPR type correlation between the signal and idler fields. The spatial state of the idler field contains more information on the original two-photon state.     

Polarization-selective plasmon-enhanced silicon quantum-dot luminescence

The photoluminescence intensity of silicon quantum dots is enhanced in a polarization-selective way by coupling to elongated Ag nanoparticles. The observed polarization dependence provides direct proof that the PL enhancement is due to electromagnetic coupling of the silicon quantum-dot emission dipoles with dipolar plasmon modes of the Ag nanoparticles. The polarization selectivity demonstrates the potential of engineered plasmonic nanostructures to optimize and tune the performance of light sources in a way that goes beyond solely enhancing the emission and absorption rates.     

The engineering of quantum-dot devices

Insofar as quantum-dot structures represent the extrapolation of today's microstructures used for electronic and optoelectronic device applications, their low-cost manufacture presents many challenges which will take some time to overcome if tunnel and quantum-well devices are any guide.     

Fabrication of quantum-dot devices in graphene

Abstract

We describe our recent experimental results on the fabrication of quantum-dot devices in a graphene-based two-dimensional system. Graphene samples were prepared by micromechanical cleavage of graphite crystals on a SiO₂/Si substrate. We performed micro-Raman spectroscopy measurements to determine the number of layers of graphene flakes during the device fabrication process. By applying a nanofabrication process to the identified graphene flakes, we prepared a double-quantum-dot device structure comprising two lateral quantum dots coupled in series. Measurements of low-temperature electrical transport show the device to be a series-coupled double-dot system with varied interdot tunnel coupling, the strength of which changes continuously and non-monotonically as a function of gate voltage.     

Reappraisal of variable-range hopping in quantum-dot solids

The temperature dependence of the electrical conductivity of assemblies of ZnO nanocrystals, studied with an electrochemically gated transistor is very accurately described by the relation ln sigma=ln sigma0-(T0/T)(x) with x=2/3 over the entire temperature range from 7 to 200 K, independent of charge concentration and dielectric environment. These results cannot be explained by existing models but are supported by results on Au nanocrystals where an identical temperature dependence was observed (Zabet-Khosousi et al., Phys. Rev. Lett. 2006, 96 (15), 156403). We propose an adaptation of the Efros-Shklovskii variable-range hopping model by introducing an expression for nonresonant tunneling based on local energy fluctuations, which yields exactly the temperature dependence that is observed experimentally.     

Dynamic viscosity measurements by photon correlation spectroscopy

It is shown that absolute values for the dynamic viscosity of liquids can be obtained using photon correlation spectroscopy. The technique is based on the measurement of the particle diffusion coefficient of spheres dispersed in a liquid. In the experiments both PMMA and silica particles were used as a seed. This includes the first application of newly industrially produced silica samples for the goal of viscosity measurements. Experiment results are presented for various liquids at ambient conditions, including some alcohols, alkanes, and the refrigerant R123. Most of the data agree quite well with reference values. Requirements for suitable seed particles and possible reasons for deviations from reference data are discussed. Measurements in liquid mixtures indicate the general possibility for the simultaneous determination of dynamic viscosity and molecular diffusion coefficient in a single experiment.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994. Boulder, Colorado, U.S.A.     

Luminous and photon standards by trap detectors

This paper presents the realization of photometric and photon standards with the same detector. To this aim, a measurement system has been set up at the Istituto Elettrotecnico Nazionale (IEN) based on trap detectors able to measure both luminous and photon flux of a light source. To validate the experimental results, the Planck's constant is obtained from the measured data. This paper highlights some of the problems encountered and discusses the relationship among lumen and mole of photons for electromagnetic radiation in the visible range     

Split current quantum-dot cellular automata-modeling and simulation

We examine a novel quantum-dot cellular automata device concept using the interaction of resonant tunneling currents through a system of four quantum wells. The interaction of resonant tunneling currents forces the total current to flow predominantly in the wells along one of the two diagonals, effectively polarizing the cell. We refer to this device concept as split current quantum cellular automata (SCQCA). A free cell will settle to a random diagonal, whereas charge interactions between adjacent cells will cause the polarization to synchronize between cells. In contrast with the standard QCA cell, this device does not require tunneling between dots. Electron tunneling occurs along the vertical direction, where highly controllable deposition techniques are able to deposit very thin films and effectively tune the device parameters. Clocking of an SCQCA cell is performed by controlling the bias across the device, and none of the potential barriers between the dots need to be controlled. We believe this device concept lends itself to fabrication using currently available fabrication technologies.     

Serial bit-stream analysis using quantum-dot cellular automata

Quasi-adiabatically switched quantum-dot cellular automata (QCA) devices present the opportunity to extend our efforts from the implementation of combinational logic devices to more useful sequential logic devices. One very important application of sequential logic is in the recognition of patterns in serial bit streams. This is important, for example, in Internet applications, where particular bit patterns are designated as "sentinel" characters that indicate a particular action should be taken. The foundation of a serial bit-stream analyzer is a shift register, which can be implemented very easily using quasi-adiabatically switched QCA devices. In addition to the shift register, the device will require a multiple-bit comparator, which has not yet been demonstrated in a QCA architecture. We will present a multiple-bit serial stream analyzer that combines the functions of the shift register and the comparator. This device will be analyzed first using behavioral and structural models developed specifically for this project, then its correct quasi-adiabatic behavior will be demonstrated using well established quantum mechanical models. We will show that the proposed device can be used to detect any arbitrary bit pattern appearing in a serial stream of data applied to its serial input. The bit pattern to be detected can be changed using device inputs, and a successful match will be indicated by asserting an output.     

Ultrafast dynamics of quantum-dot semiconductor optical amplifiers

We report on a series of experiments on the dynamical properties of quantum-dot semiconductor optical amplifiers. We show how the amplifier responds to one or several ultrafast (170 fs) pulses in rapid succession and our results demonstrate applicability and ultimate limitations to application of quantum-dot amplifiers in e.g. amplification of signals in a telecommunications system. We also review experiments on pulse propagation control and show the possibility to slow down or speed up 170 fs pulses in a quantum-dot based device.     

Growth and characterization of InP ringlike quantum-dot molecules grown by solid-source molecular beam epitaxy

In this paper, we have studied the fabrication of InP ringlike quantum-dot molecules on GaAs(001) substrate grown by solid-source molecular beam epitaxy using droplet epitaxy technique and the effect of In deposition rate on the physical and optical properties of InP ringlike quantum-dot molecules. The In deposition rate is varied from 0.2 ML/s to 0.4, 0.8 and 1.6 ML/s. The surface morphology and cross-section were examined by ex-situ atomic force microscope and transmission electron microscope, respectively. The increasing of In deposition rate results in the decreasing of outer and inner diameters of InP ringlike quantum-dot molecules and height of InP quantum dots but increases the InP quantum dot and ringlike quantum-dot molecule densities. The photoluminescence peaks of InP ringlike quantum-dot molecules are blue-shifted and FWHM is narrower when In deposition rate is bigger.     

Multiexciton generation by a single photon in nanocrystals

We have theoretically shown that efficient generation of multi-electron-hole pairs by a single photon observed recently in semiconductor nanocrystals1-4 is caused by breaking the single electron approximation for carriers with kinetic energy above the effective energy gap. Due to strong Coulomb interaction, these states form a coherent superposition with charged excitons of the same energy. This concept allows us to define the conditions for dominant two-exciton generations by a single photon: the thermalization rate of a single exciton, initiated by light, should be lower than both the two-exciton state thermalization rate and the rate of Coulomb coupling between single and two exciton states. Possible experimental manifestations of our model are discussed.     

Collective cell guidance by cooperative intercellular forces

Cells comprising a tissue migrate as part of a collective. How collective processes are coordinated over large multi-cellular assemblies has remained unclear, however, because mechanical stresses exerted at cell-cell junctions have not been accessible experimentally. We report here maps of these stresses within and between cells comprising a monolayer. Within the cell sheet there arise unanticipated fluctuations of mechanical stress that are severe, emerge spontaneously, and ripple across the monolayer. Within that stress landscape, local cellular migrations follow local orientations of maximal principal stress. Migrations of both endothelial and epithelial monolayers conform to this behaviour, as do breast cancer cell lines before but not after the epithelial-mesenchymal transition. Collective migration in these diverse systems is seen to be governed by a simple but unifying physiological principle: neighbouring cells join forces to transmit appreciable normal stress across the cell-cell junction, but migrate along orientations of minimal intercellular shear stress.     

Spontaneous long-wavelength interlevel emission in quantum-dot laser structures

Spontaneous emission has been observed for the first time as a result of interband transitions of holes and electrons between size-quantization levels in vertically coupled quantum dots and also as a result of transitions from quantum-well states to a quantum-dot level. The spectral range of the emission was in the far-infrared (λ≅10–20 μm). The long-wavelength emission was only recorded simultaneously with short-wavelength interband emission (λ≅0.94 μm) in InGaAs/AlGaAs quantum-dot laser structures at above-threshold currents. Pis’ma Zh. Tekh. Fiz. 24, 20–26 (August 12, 1998)     

Investigations for InAs/GaAs multilayered quantum-dot structure treated by high energy proton irradiation

This paper focuses on the high energy proton irradiation effect of InAs/GaAs multilayers quantum-dot (QD) wafer and photodetector. With high energy proton path simulation, the releases of proton energy and trap distribution in QD multilayers are predicted well. Treated by 1 and 3 MeV protons, all protons almost penetrate the multilayers of QD structures and stop deeply in GaAs substrate. InAs QD multilayer structures/Infrared photodetector have been irradiated by protons with different energies (1 and 3 MeV) and doses (1 × 10⁹∼ 1 × 10¹³ protons/cm²). The photoluminescence (PL) and photoresponsivity (PR) spectrum of samples were measured and discussed with as grown and post irradiation.     

Optical properties of a carbon-zirconia quantum-dot photonic crystal

We report the optical properties of a new type of photonic crystal: a transparent fused silica matrix containing quantum dots—nanoparticles of another material. In this study, nanoparticles consist of graphite zones several nanometers in size, stabilized by zirconia. The photonic crystal is prepared by high-temperature annealing (1200°C) of synthetic opal infiltrated with zirconia and a small amount of carbon. We demonstrate selective reflection of visible light from the surface of the quantum-dot crystal under broadband illumination. Such crystals are potentially attractive as narrow-band selective filters that would reflect the exciting light in Raman measurements and might be used to convert short-wavelength broadband radiation to quasi-monochromatic light in the visible range.     

Fabrication of a self-aligned cross-wire quantum-dot chain light emitting diode by molecular beam epitaxial regrowth

The fabrication of a cross-wire p-i-n light emitting diode (LED) by molecular beam epitaxial overgrowth on mesa-patterned GaAs(100) substrates is presented. Micron-wide mesa stripes fabricated by standard photolithography are subsequently narrowed to sub-micron dimensions by GaAs overgrowth due to net migration towards the mesa top. Chains of InAs quantum dots (QDs) can then be grown in a self-aligned manner on top of the narrow GaAs ridge mesa, forming the active region of the QD-chain LED. The kinetics of the overgrowth is discussed and the electroluminescence operation of the LED is presented.