Hybrid Frenkel-Wannier-Mott excitons at interfaces and in microcavities

We discuss the linear and nonlinear optical properties of organic-inorganic nanostructures (quantum wells and superlattices) brought about by resonance interactions between Frenkel excitons in organic QWs and Wannier-Mott excitons in semiconductor QWs. We show that such a coupling (Coulomb dipole-dipole at an interface and through the cavity photons in a microcavity) is responsible for the appearance of mixed Frenkel-Wannier excitations. We demonstrate that the new hybrid states and their dispersion curves can be tailored to engineer the enhancement of resonant optical nonlinearity, fluorescence efficiency and relaxation processes.     

Selective optical pumping of charged excitons in unintentionally doped InAs quantum dots

We have investigated the selective optical pumping of charged excitonic species in a sample containing quantum dots of different sizes and low areal density by photoluminescence and excitation of the photoluminescence microspectroscopy. We study the selective optical excitation of negatively charged excitons as an alternative to commonly used electrical methods. We demonstrate that under resonant excitation in impurity related bands, the selective pumping efficiency can be as high as 85% in small quantum dots having one electron shell and emitting at around 930?nm, and around 65% in big quantum dots having four electron shells and emitting at 1160?nm.     

多模光纤温度传感器系统

研制了钴盐溶液热色传感介质光学双波长差分吸收多模光纤温度传感器系统。自制了光纤耦合器件,采用了锁相电路,利用微机直接进行数字锁相探测,信号相除运算和温度校正。该系统实时显示温度,测温范围30℃—50℃,准确度±0.15℃,分辨率0.02℃,在40℃时6小时稳定性±0.05℃,12小时稳定性±0.18℃。主要用于抗电磁干扰的微波辐射治疗和谷物加热干燥等测温应用。     

Dark‐Exciton‐Mediated Fano Resonance from a Single Gold Nanostructure on Monolayer WS2 at Room Temperature

Strong spatial confinement and highly reduced dielectric screening provide monolayer transition metal dichalcogenides with strong many‐body effects, thereby possessing optically forbidden excitonic states (i.e., dark excitons) at room temperature. Herein, the interaction of surface plasmons with dark excitons in hybrid systems consisting of stacked gold nanotriangles and monolayer WS₂ is explored. A narrow Fano resonance is observed when the hybrid system is surrounded by water, and the narrowing of the spectral Fano linewidth is attributed to the plasmon‐enhanced decay of dark K‐K excitons. These results reveal that dark excitons in monolayer WS₂ can strongly modify Fano resonances in hybrid plasmon–exciton systems and can be harnessed for novel optical sensors and active nanophotonic devices.     

Origin of defect-insensitive emission probability in In-containing (Al,In,Ga)N alloy semiconductors

Group-III-nitride semiconductors have shown enormous potential as light sources for full-colour displays, optical storage and solid-state lighting. Remarkably, InGaN blue- and green-light-emitting diodes (LEDs) emit brilliant light although the threading dislocation density generated due to lattice mismatch is six orders of magnitude higher than that in conventional LEDs. Here we explain why In-containing (Al,In,Ga)N bulk films exhibit a defect-insensitive emission probability. From the extremely short positron diffusion lengths (<4 nm) and short radiative lifetimes of excitonic emissions, we conclude that localizing valence states associated with atomic condensates of In-N preferentially capture holes, which have a positive charge similar to positrons. The holes form localized excitons to emit the light, although some of the excitons recombine at non-radiative centres. The enterprising use of atomically inhomogeneous crystals is proposed for future innovation in light emitters even when using defective crystals.     

Polariton parametric amplification in semiconductor microcavities

Modifying the photonic environment of a semiconductor quantum well by embedding it in a high-reflectivity microcavity gives rise to new fundamental optical excitations, half-quantum well excitons, half-photons. These particles, called polaritons, have a light mass, as cavity photons, meaning that they have a large De Broglie wavelength. On the other hand, polaritons, like excitons, are subject to Coulomb interaction, a feature generating strong optical nonlinearities. Such properties favour quantum degeneracy and collective phenomena related to the bosonic statistics of polaritons. We review experiments on stimulated scattering of polaritons. In particular we concentrate on the resonant excitation of polaritons somewhere on the dispersion curve and the stimulation of their scattering into the fundamental state by means of an optical probe beam. The process is called polariton parametric amplification and results in very large and ultrafast optical amplification of the probe beam. The model, based on a Hamiltonian of interacting bosons, suggests that the amplification is related to the coherence between polaritons. We demonstrate that in clearly designed samples, this coherence can be preserved almost up to room temperature, so that intersting applications of this phenomenon can be conceived. At the same time we have been able to improve dramatically the efficiency of the parametric process, making the microcavity an unprecedented optical amplifier.     

Optical phase contrast measurement of ultrasonic fields

This report describes an optical phase contrast imaging technique for the measurement of wide bandwidth ultrasound fields in water. In this method, a collimated optical wavefront (λ_l = 810 nm) impinges on a wide bandwidth ultrasound pulse. The method requires that refractive index perturbations induced by the ultrasound field be sufficiently small. Specifically, on exit from the acoustic field, the phase of the optical wavefront must be proportional to the ray sum of local density taken in the direction of propagation of the incident optical wave. A similar restriction is placed on the dimensions of the ultrasound pulse. Repeated measurement of this phase as the ultrasound field is rotated through 180° about an axis normal to the direction of propagation of the incident optical wave generates the Radon transform of the ultrasonically induced refractive index perturbation. Standard tomographic reconstruction techniques are used to reconstruct the full three-dimensional refractive index perturbation. A simple two-lens imaging system and an optical signal processing element from phase contrast microscopy provide a method of directly measuring an affine function of the desired optical phase for small optical phase shifts. The piezo- and elasto-optic coefficients (the first partial derivatives of refractive index with respect to density and pressure) relate refractive index to density and pressure via a linear model. The optical measurement method described in this paper provides a direct, quantitative measurement of the piezo- and elasto-optic coefficients (from the density or pressure fields)     

Urbach tail of gadolinium trihydride films

The fundamental absorption edge of the gadolinium trihydride has been studied by performing optical transmission measurements at temperatures ranging from 298 K to 353 K. Urbach tails and the photon absorption spectra give information about the exciton dynamics, distinguishing between free and trapped excitons in the lattice. Our experimental results can be accounted for in terms of the theory of Toyozawa and co-workers, which ascribe the Urbach tail to the momentary localization of excitons due to phonon interaction.     

Lasing in submonolayer InAs/AlGaAs structures without external optical confinement

Structures having a set of planes with submonolayer InAs inclusions in an AlGaAs matrix were fabricated and studied. Lasing was observed as a result of optical excitation. It is shown that lasing takes place via the ground state of excitons localized at InAs islands and may be achieved without external optical confinement of the active region by wide-gap layers of lower refractive index. The low threshold excitation density shows that these structures may be used to develop low-threshold injection lasers in the visible range, exciton waveguides, and self-contained microcavities. Pis’ma Zh. Tekh. Fiz. 24, 58–66 (July 26, 1998)     

The problem of substance in chemistry — an approach by means of large deviation statistics

Different substances can phenomenologically be defined by use of Gibbs' phase rule. In quantum mechanics, on the other hand, the notion of a substance is not clearly understood: The thermal density operator D_β corresponding to some Hamiltonian and some chosen inverse temperature β is uniquely defined, which implies that different isomers exhibit the same thermal density operator. Coexistence of isomers, substances or phases at some temperature, on the other hand, would need different thermal density operators, one for each isomer, substance or phase. Here we try to understand this problem for the classical van der Waals gas using large deviation statistics. We show that the gaseous and liquid phase of the van der Waals gas emerge with increasing numbers of particles. Intermediate states — neither gaseous nor liquid — exist but die out with increasing number of particles. Extension of our method to quantum mechanics is not straightforward, but looks promising.     

Exciton recombination dynamics in CdSe nanowires: bimolecular to three-carrier Auger kinetics

Ultrafast relaxation dynamics of charge carriers in CdSe quantum wires with diameters between 6 and 8 nm are studied as a function of carrier density. At high electron-hole pair densities above 10(19) cm(-3) the dominant process for carrier cooling is the "bimolecular" Auger recombination of one-dimensional (1D) excitons. However, below this excitation level an unexpected transition from a bimolecular (exciton-exciton) to a three-carrier Auger relaxation mechanism occurs. Thus, depending on excitation intensity, electron-hole pair relaxation dynamics in the nanowires exhibit either 1D or 0D (quantum dot) character. This dual nature of the recovery kinetics defines an optimal intensity for achieving optical gain in solution-grown nanowires given the different carrier-density-dependent scaling of relaxation rates in either regime.     

Exciton Dynamics in Semiconductor Nanocrystals

This review article provides an overview of recent advances in the study and understanding of dynamics of excitons in semiconductor nanocrystals (NCs) or quantum dots (QDs). Emphasis is placed on the relationship between exciton dynamics and optical properties, both linear and nonlinear. We also focus on the unique aspects of exciton dynamics in semiconductor NCs as compared to those in bulk crystals. Various experimental techniques for probing exciton dynamics, particularly time‐resolved laser methods, are reviewed. Relevant models and computational studies are also briefly presented. By comparing different materials systems, a unifying picture is proposed to account for the major dynamic features of excitons in semiconductor QDs. While the specific dynamic processes involved are material‐dependent, key processes can be identified for all the materials that include electronic dephasing, intraband relaxation, trapping, and interband recombination of free and trapped charge carriers (electron and hole). Exciton dynamics play a critical role in the fundamental properties and functionalities of nanomaterials of interest for a variety of applications including optical detectors, solar energy conversion, lasers, and sensors. A better understanding of exciton dynamics in nanomaterials is thus important both fundamentally and technologically.     

The rich photonic world of plasmonic nanoparticle arrays

Metal nanoparticle arrays that support surface lattice resonances have emerged as an exciting platform for manipulating light–matter interactions at the nanoscale and enabling a diverse range of applications. Their recent prominence can be attributed to a combination of desirable photonic and plasmonic attributes: high electromagnetic field enhancements extended over large volumes with long-lived lifetimes. This Review will describe the design rules for achieving high-quality optical responses from metal nanoparticle arrays, nanofabrication advances that have enabled their production, and the theory that inspired their experimental realization. Rich fundamental insights will focus on weak and strong coupling with molecular excitons, as well as semiconductor excitons and the lattice resonances. Applications related to nanoscale lasing, solid-state lighting, and optical devices will be discussed. Finally, prospects and future open questions will be described.     

A new set-up for the observation of magnetic structures in uniaxial materials at temperatures above 4.2K

A polarization microscope, designed especially for the magneto-optical observation of domain structures in ferromagnetic or superconducting thin films as well as bulk samples at low temperatures, is described. With this system, temperature controlled domain observations in the temperature range 4.3 K < T < 40 K and magnetic fields up to 0.8 T can be performed with an optical resolution of 2 μm. The imaging of the intermediate state in type—I superconductors, of phase boundaries between the Meissner- and Shubnikov-phase in type—II superconductors and of magnetic domains structures in uniaxial ferromagnetic materials is demonstrated.     

Optical readout for optical storage with phase jump

A novel, to our knowledge, optical readout for optical storage with phase jump is presented. In the readout scheme two coherent laser beams are focused on an optical disk with one beam scanning along pits and the other along land. When the probe beam scans across a pit, two phase jumps will take place in the interference resultant of the two beams if the phase difference between two beams is prefixed at pi, resulting in a phase pulse of 180 deg. The slopes of rising and falling edges of the phase pulse are infinite, and they are not affected by the intensity variation of the light source, stray light, and the vibration of the disk. Therefore this phase pulse can be used to read out the information on an optical disk. The use of phase jump will improve the signal-to-noise ratio of the readout signal and enhance the density of optical storage. An optical readout with phase jump was constructed. Both the theoretical design and the experimental verification are conducted. Experimental results show that the proposed optical readout is feasible.     

Excitonic Aharonov–Bohm Oscillations in Core–Shell Nanowires

Phase coherence in nanostructures is at the heart of a wide range of quantum effects such as Josephson oscillations between exciton–polariton condensates in microcavities, conductance quantization in 1D ballistic transport, or the optical (excitonic) Aharonov–Bohm effect in semiconductor quantum rings. These effects only occur in structures of the highest perfection. The 2D semiconductor heterostructures required for the observation of Aharonov–Bohm oscillations have proved to be particularly demanding, since interface roughness or alloy fluctuations cause a loss of the spatial phase coherence of excitons, and ultimately induce exciton localization. Experimental work in this field has so far relied on either self‐assembled ring structures with very limited control of shape and dimension or on lithographically defined nanorings that suffer from the detrimental effects of free surfaces. Here, it is demonstrated that nanowires are an ideal platform for studies of the Aharonov–Bohm effect of neutral and charged excitons, as they facilitate the controlled fabrication of nearly ideal quantum rings by combining all‐binary radial heterostructures with axial crystal‐phase quantum structures. Thanks to the atomically flat interfaces and the absence of alloy disorder, excitonic phase coherence is preserved even in rings with circumferences as large as 200 nm.     

Superfluidity and Vortex Lattice Formation in Quasi Two-Dimensional Exciton System

Bose-Einstein condensation(BEC) and superfluidity of excitons in type-II quantum wells are studied theoretically. As a typical example, we consider GaAs/AlAs quantum wells with each layer thickness L less than 37Å. First, we investigate the interaction between excitons, thus finding it repulsive for L less than about 15Å, which shows the possibility of BEC. Secondly, we study the stationary pattern of the phase of the BEC order parameter under the external current J. The interaction between excitons and the weak laser light violates conservation of number of excitons and fixes that phase. There appears a vortex lattice with net supercurrent when J is larger than a critical value. The experimental observation of this critical current will give an evidence that BEC and superfluidity occur in exciton system.     

Random-dot pattern design of a light guide in an edge-lit backlight: integration of optical design and dot generation scheme by the molecular-dynamics method

We present the methodology of an integration of a dot generation scheme by a molecular-dynamics (MD) method and the subsequent software optical design phase by software. The MD dot generation scheme proposed has great advantages when integrated into the optical design phase. These advantages include the variable r-cut and reflective boundary condition techniques, both of which could achieve a high-density variation of dot distribution. In addition, we use a cell technique where the domain is divided into a number of smaller cells, allowing for flexibility in adjusting the dot density within each cell, as well as for using the add-on or remove-from technique of the dots in one cell to achieve an equal-luminance condition. In addition, a simple proportional rule of luminance to dot density is also proposed to perform dot optimization. Finally, an illustration is shown for the optimal dot distribution of a LED backlight. The result that a two-dimensional dot density distribution near the light source changes gradually to a one-dimensional dot density distribution with increasing distance from the light source shows the validity of the present integration of the MD dot generation scheme into the optical design phase.     

Polarized Light-Emitting Diodes Based on Anisotropic Excitons in Few-Layer ReS2

An on-chip polarized light source is desirable in signal processing, optical communication, and display applications. Layered semiconductors with reduced in-plane symmetry have inherent anisotropic excitons that are attractive candidates as polarized dipole emitters. Herein, the demonstration of polarized light-emitting diode based on anisotropic excitons in few-layer ReS₂, a 2D semiconductor with excitonic transition energy of 1.5–1.6 eV, is reported. The light-emitting device is based on minority carrier (hole) injection into n-type ReS₂ through a hexagonal boron nitride (hBN) tunnel barrier in a metal–insulator–semiconductor (MIS) van der Waals heterostack. Two distinct emission peaks from excitons are observed at near-infrared wavelength regime from few-layer ReS₂. The emissions exhibit a degree of polarization of 80% reflecting the nearly 1D nature of excitons in ReS₂.     

Coexistence of 1D and quasi-0D photoluminescence from single silicon nanowires

Single silicon nanowires (Si-NWs) prepared by electron-beam lithography and reactive-ion etching are investigated by imaging optical spectroscopy under variable temperatures and laser pumping intensities. Spectral images of individual Si-NWs reveal a large variability of photoluminescence (PL) along a single Si-NW. The weaker broad emission band asymmetrically extended to the high-energy side is interpreted to be due to recombination of quasi-free 1D excitons while the brighter localized emission features (with significantly variable peak position, width, and shape) are due to localization of electron-hole pairs in surface protrusions acting like quasi-0D centers or quantum dots (QDs). Correlated PL and scanning electron microscopy images indicate that the efficiently emitting QDs are located at the Si-NW interface with completely oxidized neck of the initial Si wall. Theoretical fitting of the delocalized PL emission band explains its broad asymmetrical band to be due to the Gaussian size distribution of the Si-NW diameter and reveals also the presence of recombination from the Si-NW excited state which can facilitate a fast capture of excitons into QD centers.