Stress-induced stimulated emission from excitonic molecule in CdS

Stimulated emission has been observed at the M line (2.543 eV) in optically excited CdS crystals under an applied uniaxial stress perpendicular to the C axis at 4.2 K. This emission is strongly polarized with an electric vector parallel to the applied stress. The observed polarization characteristics are successfully accounted for by assuming a radiative annihilation of an excitonic molecule, providing an experimental evidence for ascribing this emission to the excitonic molecule in CdS.     

Single molecule diffraction

For solving the atomic structure of organic molecules such as small proteins which are difficult to crystallize, the use of a jet of doped liquid helium droplets traversing a continuous high energy electron beam is proposed as a means of obtaining electron diffraction patterns (serial crystallography). Organic molecules (such as small proteins) within the droplet (and within a vitreous ice jacket) may be aligned by use of a polarized laser beam. Iterative methods for solving the phase problem are indicated. Comparisons with a related plan for pulsed x-ray diffraction from single proteins in a molecular beam are provided.     

Plasma induced optical nonlinearities and optical bistability in CdS

The transition from a low density gas of excitons to an electron-hole plasma is connected with strong variations of the optical properties. In this contribution we investigate both the optical nonlinearities produced in CdS by an intense laser beam in the spectral region close to the absorption edge and at the position of the laser itself. In a second part it is shown that the nonlinearities at the photon energy of the incident laser may lead to absorptive and dispersive optical bistability.     

单分子操纵与单分子生物物理

文章介绍了近年来发展起来的一些单分子操纵实验技术如光镊、磁镊、微针、斯托克斯拖曳技术，以及应用这些技术拉伸、旋转、解链DNA分子，从而研究其力学性质所取得的研究进展．各种蛋白质如T7 DNA聚合酶、拓扑异构酶，SWI／SNF染色质重建复合体、RNA聚合酶与DNA的作用在生化过程中十分重要，因此，文章也介绍了这些蛋白质与DNA在单分子的水平上相互作用所取得的研究进展．     

单分子操纵与单分子生物物理

文章介绍了近年来发展起来的一些单分子操纵实验技术如光镊、磁镊、微针、斯托克斯拖曳技术,以及应用这些技术拉伸、旋转、解链DNA分子,从而研究其力学性质所取得的研究进展.各种蛋白质如T7 DNA聚合酶、拓扑异构酶,SWI/SNF染色质重建复合体、RNA 聚合酶与DNA的作用在生化过程中十分重要,因此,文章也介绍了这些蛋白质与DNA在单分子的水平上相互作用所取得的研究进展.     

Polar optical phonons in semiconducting CdS nanocrystals

We have experimentally and theoretically studied IR-active optical phonons, which are spatially confined in the volume of semiconducting CdS nanocrystals of various shapes synthesized in a dielectric matrix (porous aluminum oxide). Within an approach admitting the mixing of all expected types of vibrations, the complete sets of phonon modes are determined for a spherical quantum dot (QD) and a cylindrical quantum wire (QW) in this matrix. Based on these results, the polarizability spectra of QDs and QWs, as well as the effective dielectric function of a composite material containing such nanoparticles, are calculated for the far-IR wavelength range. It is established that the spectrum of the dielectric function exhibits specific features in the region between the transverse and longitudinal optical phonon frequencies of the massive semiconductor material. These features explain the rather unusual structure of the IR spectra of the composite samples studied.     

Recombination waves in CdS induced by optical quenching

Low-frequency current oscillations induced by optical quenching were observed in highly photosensitive CdS crystals. No evidence of a negative differential resistivity was found. The current oscillations were observed at room temperature at electric field strengths of 100–800 V/cm. As shown by probe measurements, the instabilities resulted from a bulk phenomenon. The oscillations are related to the excitation of recombination waves, predicted by Konstantinov and Perel.     

Single molecule vibrationally mediated chemistry

Tunnelling electrons may scatter inelastically with an adsorbate, releasing part of their energy through the excitation of molecular vibrations. The resolution of inelastic processes with a low temperature scanning tunnelling microscope (STM) provides a valuable tool to chemically characterize single adsorbates and their adsorption mechanisms. Here, we present a molecular scale picture of single molecule vibrational chemistry, as resolved by STM. To understand the way a reaction proceed it is needed knowledge about both the excitation and damping of a molecular vibration. The excitation is mediated by the specific coupling between electronic molecular resonances present at the Fermi level and vibrational states of the adsorbate. Thus, the two-dimensional mapping of the inelastic signal with an STM provides the spatial distribution of the adsorbate electronic states (near the Fermi level) which are predominantly coupled to the particular vibrational mode observed. The damping of the vibration follows a competition between different mechanisms, mediated via the creation of electron-hole pairs or via anharmonic coupling between vibrational states. This latter case give rise to effective energy transfer mechanisms which eventually may focus vibrational energy in a specific reaction coordinate. In this single-molecule work-bench, STM provides alternative tools to understand reactivity in the limit of low excitation rate, which demonstrate the existence of state-specific excitation strategies which may lead to selectivity in the product of a reaction.     

Quantum optical waveform conversion

Proposals for long-distance quantum communication rely on the entanglement of matter-based quantum nodes through optical communications channels, but the entangling light pulses have poor temporal behavior in current experiments. Here we show that nonlinear mixing of a quantum light pulse with a spectrally tailored classical field can compress the quantum pulse by more than a factor of 100 and flexibly reshape its temporal waveform while preserving all quantum properties, including entanglement. Our scheme paves the way for quantum communication at the full data rate of optical telecommunications.     

Wavelength conversion in optical transport networks

The introduction of optical technology in the path layer of a transport network is analyzed; in particular, the impact of all-optical wavelength conversion is examined. Two basic optical cross-connect schemes based on space switching and on wave-length switching, respectively, and two types of wavelength converters are considered. The wavelength converters are, respectively, based on four-wave miring (FWM) in semiconductor optical amplifiers (SOAs) and on cross-phase modulation (XPM) obtained by two SOAs put in an interferometric configuration. An analysis of transmission performances of optical transport networks employing both types of wavelength conversion and both the node architectures is reported. The performances are evaluated by means of a system modeling that facilitates considering physical phenomena occurring during transmission and all the important transmission impairments. The analysis reveals that both the types of converters can be employed with different features and limitations, according to the considered network and switching architecture.     

光参量过程中的逆转换问题

逆转换是影响光参量变换转换效率、参量光光束质量以及参量光输出稳定性的关键因素,随光斑分布和时间变化,逆转换现象很难消除.文中分析了光参量变换过程中的逆转换问题,研究了影响逆转换的关键因素.分析得出,适当的晶体长度、优化的抽运光斑截面类型、合适的谐振腔参数(对于振荡器)有利于降低逆转换,提高参量转换效率,改善参量光光束质量.根据理论分析结果,设计了脉冲砷酸钛氧钾(KTA)光参量振荡器,实验获得了270 mJ信号光和150 mJ闲频光输出,有效地抑制了逆转换的影响,参量转化效率达到了43%. 关键词： 逆转换 光参量放大 光参量振荡     

Single-wavelength pulsed optical logic based on dichroism in CdS

A new type of pulsed (picosecond) nonlinear optical logic element, that exploits polarisation dichroism, is proposed and investigated experimentally. The device is based on a nonlinear Fabry-Pérot cavity and excite/probe operation. Unlike previous similar devices, it operates at a single wavelength and uses band-edge dichroism to distinguish the excite and probe pulses according to their polarisation. The concept has been tested using CdS as the dichroic nonlinear material and differential gain levels greater than unity have been observed. Experimental data are also included regarding the magnitudes of the nonlinear refractive cross-section and other relevant parameters in CdS.     

Single molecule acid-base kinetics and thermodynamics

We report a method in which temperature dependent single-molecule fluorescence measurements are used to study the kinetics and thermodynamics of the acid-base interaction in films of photoresist polymer. We use the two distinct fluorescent prototropic forms of Coumarin 6 (C6-->C6+) to indicate the state of the acid-base system. Data are analyzed using a statistical model of the intensity probability distributions, yielding temperature dependent proton exchange rates, which is confirmed through agreement with a simple two-state Monte Carlo model. The temperature dependent rates are used to calculate the activation enthalpy for proton exchange.     

Efficient microwave-induced optical frequency conversion

Frequency conversion process is studied in a medium of atoms with a configuration of levels, where transition between two lower states is driven by a microwave field. In this system, conversion efficiency can be very high by virtue of the effect of electromagnetically induced transparency (EIT). Depending on intensity of the microwave field, two regimes of EIT are realized: “dark-state” EIT for the weak field, and Autler-Townes-type EIT for the strong one. We study both cases via analytical and numerical solution and find optimum conditions for the conversion. Received 13 December 1999 and Received in final form 6 March 2000     

Single molecule electron transfer dynamics in complex environments

We propose a new theoretical approach to study the kinetics of the electron transfer (ET) under the dynamical influence of the complex environments with the first passage times (FPT) of the reaction events. By measuring the mean and high order moments of FPT and their ratios, the full kinetics of ET, especially the dynamical transitions across different temperature zones, is revealed. The potential applications of the current results to single molecule electron transfer are discussed.     

Structural and optical properties of CdS thin films

CdS thin films are deposited onto glass substrates by vacuum evaporation at 373 K and the films are annealed at different temperatures. Rutherford backscattering spectrometry (RBS) and X-ray diffraction techniques are used to determine the thickness, composition, crystalline structure and grain size of the films. The films show a predominant hexagonal phase with small crystallites. The optical band gap of the films are estimated using the optical transmittance measurements. A decrease in the band gap is observed for the annealed films. The Raman peak position of the CdS A₁ (LO) mode did not change much whereas, the full width at half maximum (FWHM) is found to decrease with annealing.     

Prospects for the optical cooling of CdS nanocomposites

The possibility of using CdS nanocomposites for optical cooling is analyzed on the basis of their spectroscopic properties. The physical principles of laser cooling in semiconductor structures are discussed.