Conduction switching of photochromic molecules

We report a theoretical study of single molecule conduction switching of photochromic dithienylethene molecules. The light-induced intramolecular transformation drives a swapping of the highest occupied molecular orbital and lowest unoccupied molecular orbital between two distinct conjugated paths. The shuffling of single and double bonds produces a significant conductance change when the molecule is sandwiched between metal electrodes. We model the switching event using quantum molecular dynamics and the conductance changes using Green's function electronic transport theory. We find large on-off conductance ratios (between 10 and over 100) depending on the side group outside the switching core.     

Gap transit-time effects in fast optoelectronic semiconductor switching devices

The output shape of an electric pulse, which traverses the gap of a fast optoelectronic semiconductor switching device, is influenced by the transit time of the pulse. This dependence as well as the use of transit-time effects for pulse shortening are studied. To this end in the present paper optical and electrical time-dependent properties of the central unit, I.e. the laser-excited semiconductor, and the external passive electric elements and sources are considered and a time-variant telegraphic equation obeying appropriate boundary conditions is established. Numerical solutions for physically important cases are calculated and discussed.     

z-Scan characterization of zwitterionic chromophores for optoelectronic switching

Single beam z-scan measurements have been made on films containing amorphous polycarbonate and the zwitterionic chromophore, PYR-3, that has a very high second order nonlinear optical (NLO) figure of merit. The third order NLO figure of merit is ≈1.6 at 1030 nm and is comparable to that found in organic compounds optimized for high n ₂ values. The two-photon absorption coefficient is 2.1×10⁻¹² m/W, which is very low and advantageous for NLO device applications. The third order NLO refractive index is −1.4×10⁻¹⁸ m²/W.     

Two-dimensional optoelectronic devices for future optical switching and information processing

Recent progress is reviewed in surface-normal optoelectronic devices primarily for use in optical switching and information processing. A type of optoelectronic integrated circuit (OEIC), the surface-normal two-dimensional array, is fabricated using these devices. This improves on first-generation OEICs by featuring large-scale integration in a small area, which results in a higher production yield. It also has structures which can easily be integrated with electronic circuits and can meet multichannel requirements. This approach supports optoelectronic progress towards optical information processing.     

Magnetic properties of single crystals based on photochromic molecules of spiropyrans and chromium oxalates

Physics of the Solid State - The influence of UV irradiation on the effective magnetic moment and the Weiss temperature of single crystals based on photochromic molecules of spiropyrans and...     

Enantiospecific adsorption of chiral molecules on chiral gold clusters

Enantioselectivity in gold clusters is investigated by studying the adsorption of a chiral amino acid (cysteine) on a chiral Au55 cluster using density functional calculations. The highest adsorption energies were found when the amino and thiolate functional groups of cysteine are bonded to the lowest coordinated edges of the chiral cluster. Enantiospecific adsorption is primarily obtained from the different bond location and strength, at the cluster edge, of the carboxyl groups forming the left- and right-handed enantiomers. These results provide theoretical support to convey enantioselectivity in asymmetric nanocatalysts using chiral gold clusters.     

Utilization of photoreversible optical nonlinearities intrans-cis photochromic molecules for spatial light modulation

In this paper we examine the nonlinear optical properties of molecular systems undergoing acis-trans photoisomerization process. Such a process gives rise to dynamic photochromism on a picosecond time scale significant for designing a molecular Spatial Light Modulator (SLM).A kinetic analysis of a generalcis-trans nonlinear organic absorber is performed. The analysis is used to derive an expression for the imaginary part of the complex nonlinear molecular index of refraction. The properties of a molecular SLM based on such an absorber are discussed by examining the azo-type molecular system. These calculation predict an optimum contrast ratio and fast on-off switching for the SLM performance, achieved at low read-write laser intensities.     

Picosecond optoelectronic switching in CdS0.5Se0.5

So far only few materials have been investigated for their application in fast opto-electronic switching. These materials are Si, GaAs and InP. The difficulty in obtaining a high resistivity specimen of group IV elemental semiconductors puts constraints on their high power handling capability. Most of the group III–V compounds have the possibility of showing up of Gunn effect at high fields, which has undesirable effect in high voltage switching. We have, therefore, investigated a mixed II–VI compound CdS_0.5Se_0.5 for its potential use as an opto-electronic switch.     

4×8 optoelectronic matrix switch equipment using InGaAsP/InP heterojunction switching photodiodes

A four input-port and an eight output-port optoelectronic matrix switch using InGaAsP/InP hetero-junction switching photodiodes was fabricated and its characteristics studied. The characteristics of the broadband switching equipment, as well as those of individual components, such as the switching photodiodes, InGaAsP/InP semiconductor lasers, eight-port optical dividers and receiver circuits, are discussed from the viewpoint of analogue signal switching. The signal-to-noise ratio attained is more than 40dB for a 30MHz bandwidth television signal. Differential gain and differential phase are less than 2% and 2°, respectively, isolation is higher than 80 dB. Crosstalk between adjacent channels is suppressed below-60 dB over the band between d.c. and 100 MHz by a simple metallic shield at the input and output electric circuits.     

C60分子瞬态全光开关

本文以YAG脉冲激光为控制光,He-Ne连续激光为信号光,在C_(60)甲苯溶液中首次实现瞬态全光开关.由光开关时间推断出C_(60)分子三重态的寿命.     

Reversible photomechanical switching of individual engineered molecules at a metallic surface

We have observed reversible light-induced mechanical switching for individual organic molecules bound to a metal surface. Scanning tunneling microscopy (STM) was used to image the features of individual azobenzene molecules on Au(111) before and after reversibly cycling their mechanical structure between trans and cis states using light. Azobenzene molecules were engineered to increase their surface photomechanical activity by attaching varying numbers of tert-butyl (TB) ligands ("legs") to the azobenzene phenyl rings. STM images show that increasing the number of TB legs "lifts" the azobenzene molecules from the substrate, thereby increasing molecular photomechanical activity by decreasing molecule-surface coupling.     

Reversible switching of single tin phthalocyanine molecules on the InAs(111)A surface

Individual tin phthalocyanine (SnPc) molecules adsorbed on the InAs(111)A surface were studied by low-temperature scanning tunnelling microscopy (STM) at 5?K. Consistently with the nonplanar molecular structure, SnPc adopts two in-plane adsorption geometries with the centre Sn atom either above (SnPc(up)) or below (SnPc(down)) the molecular plane. Depending on the current and bias applied to the tunnel junction, the molecule can be reversibly switched between the two conformations, implying a controlled transfer of the Sn atom through the molecular plane. The SnPc(down) conformer is characterized by an enhanced surface bonding as compared to the SnPc(up) conformer. SnPc(up) molecules can be repositioned by the STM tip by means of lateral manipulation, whereas this is not feasible for SnPc(down) molecules. The reversible switching process thus enables one to either laterally move the molecule or anchor it to the semiconductor surface.     

On the mechanism of the photochromic process in copper-halide photochromic glasses

It is demonstrated that copper oxides (specifically cuprous oxide) play an important role in the photochromism of standard FKhS-7 glasses. The absorption spectra of the FKhS glasses are compared with the spectrum of glasses with microscopic cuprous oxide particles and the spectra of cuprous oxide deposited on various substrates. Solid-phase photochemical reactions in heterosystems are shown to be responsible for photochromism. Zh. Tekh. Fiz. 69, 93–96 (May 1999)     

Breaking the far-field diffraction limit in optical nanopatterning via repeated photochemical and electrochemical transitions in photochromic molecules

By saturating a photochromic transition with a nodal illumination (wavelength, λ), one isomeric form of a small molecule is spatially localized to a region smaller than the far-field diffraction limit. A selective oxidation step effectively locks this pattern allowing repeated patterning. Using this approach and a two-beam interferometer, we demonstrate isolated lines as narrow as λ/8 (78 nm) and spacing between features as narrow as λ/4 (153 nm). This is considerably smaller than the minimum far-field diffraction limit of λ/2. Most significantly, nanopatterning is achieved via single-photon reactions and at low light levels, which in turn allow for high throughput.     

Pulling monatomic gold wires with single molecules: an Ab initio simulation

Car-Parrinello molecular dynamics simulations demonstrate that pulling a single thiolate molecule anchored on a stepped gold surface does not preferentially break the sulfur-gold chemical bond. Instead, it is found that this process leads to the formation of a monoatomic gold nanowire, followed by breaking a gold-gold bond with a rupture force of about 1.2 nN. The simulations also indicate that previous single-molecule thiolate-gold and gold-gold rupture experiments both probe the same phenomenon, namely, the breaking of a gold-gold bond within a gold nanowire.     

Fluorescence quenching of dye molecules near gold nanoparticles: radiative and nonradiative effects

The radiative and nonradiative decay rates of lissamine dye molecules, chemically attached to differently sized gold nanoparticles, are investigated by means of time-resolved fluorescence experiments. A pronounced fluorescence quenching is observed already for the smallest nanoparticles of 1 nm radius. The quenching is caused not only by an increased nonradiative rate but, equally important, by a drastic decrease in the dye's radiative rate. Assuming resonant energy transfer to be responsible for the nonradiative decay channel, we compare our experimental findings with theoretical results derived from the Gersten-Nitzan model.     

Optical characterization of photochromic effect

Photochromic powders are powders whose color is sensitive to UV irradiation. When such a powder is exposed to the sunlight or any other UV rays, it becomes darker and then looks back progressively to its initial color. An optical method has been set up in order to quantify these colors variations. We can then quantify the variations in the colorimetric space, and the time response of the coming back phenomenon.