Jau Tang

Structure of Rhodopseudomonas sphaeroides R-26 reaction center

The molecular replacement method has been succesfully used to provide a structure for the photosynthetic reaction center of Rhodopseudomonas sphaeroides at 3.7 Å resolution. Atomic coordinates derived from the R. viridis reaction center were used in the search structure. The crystallographic R‐factor is 0.39 for reflections between 8 and 3.7 Å. Validity of the resulting model is further suggested by the visualization of amino acid side chains not included in the R. viridis search structure, and by the arrangements of the reaction centers in the unit cell. In the initial calculations quinones or pigments were not included; nevertheless, in the resulting electron density map, electron density for both quinones qa and qb appears along with the bacteriochlorophylls and bacteriopheophytins. Kinetic analysis of the charge recombination shows that the secondary quinone is fully functional in the R. sphaeroides crystal.

A general model of electron spin polarization arising from the interactions within radical pairs

We present a general description of electron spin polarization observed for interacting radical pairs. Unlike previous treatments, both chemically induced dynamic electron polarization and correlated radical pair polarization are included for ST0 mixing. A key feature of this model is that the members of the pair can remain interacting throughout the time sequence of electron spin polarization. Explanations of the time‐dependent evolution of the electron spin polarization are presented using the density matrix and its associated vector diagram. A new vector method is formulated for calculating and visualizing chemically induced electron spin polarization. The approach is based on the vector, Δρ(t) which represents the displacement of the density vector ρ(t) from its direction at the time of its ‘‘birth.’’ In addition, these electron spin polarization phenomena are described in spectroscopic terms as well as in the usual mathematical manner.

The Shannon channel capacity of dispersion-free nonlinear optical fiber transmission

We extend the Shannon information theory to a nonlinear system and present a model for calculating the channel capacity of an optical-fiber transmission system using dispersion-free fiber. For this particular fiber, a closed-form solution for the nonlinear Schroedinger equation exists. This allows us to derive an analytical result for the channel capacity that is exact and valid for arbitrary input power. We will study the single-span case and examine the dependence of the capacity on operating input power, the number of channels (N/sub c/), the noise power (P/sub W/), etc. The maximum capacity is shown to follow a simple scaling law with log/sub 2/(1+CN/sub c//sup -2/3/P/sub W//sup -2/3/) dependence, multiplexing (WDM) systems.

A highly efficient graphene oxide absorber for Q-switched Nd:GdVO4 lasers

We demonstrated that graphene oxide material could be used as a highly efficient saturable absorber for the Q-switched Nd:GdVO4 laser. A novel and low-cost graphene oxide (GO) absorber was fabricated by a vertical evaporation technique and high viscosity of polyvinyl alcohol (PVA) aqueous solution. A piece of GO/PVA absorber, a piece of round quartz, and an output coupler mirror were combined to be a sandwich structure passive component. Using such a structure, 104 ns pulses and 1.22 W average output power were obtained with the maximum pulse energy at 2 µJ and a slope efficiency of 17%.

Effects of the Duschinsky mode-mixing mechanism on temperature dependence of electron transfer processes

Electron transfer processes involving a multimode mixing mechanism (the Duschinsky rotation) are systematically examined. Such processes can be analyzed with a very general “spin–boson” model with N displaced and quadratically coupled harmonic potentials. The very general Franck–Condon factor obtained here is applicable to the studies of electron transfer as well as energy transfer processes, where frequency shifts and the Duschinsky rotation are involved. Although there are several numerical studies of such a mechanism, the derivation of an analytical expression for the rate constant is presented here and the temperature dependence is examined. As, in general, at very low temperatures where the thermal energy is smaller, the electron transfer rate becomes temperature independent due to nuclear quantum tunneling. However, in the presence of Duschinsky rotation, the pre-exponential factor in the rate constant can deviate from the characteristic 1/√T dependence of the Marcus theory. For processes with no or...

Single particle versus ensemble average: From power-law intermittency of a single quantum dot to quasistretched exponential fluorescence decay of an ensemble

Light-induced diffusion-controlled electron transfer is proposed as an underlying mechanism for the intermittency (power law and breakdown) of a single quantum dot and ensemble-averaged fluorescence decay. The intensity decay can be approximated to a stretched exponential expression. The physical links to the free energy gap, reorganization energy, electronic coupling, and diffusion correlation times are discussed. A procedure is described for extracting these molecular-based parameters from experiments and is demonstrated with examples using existing data.

Antibunching Single-Photon Emission and Blinking Suppression of CdSe/ZnS Quantum Dots

We demonstrated that by properly coupling to silver nanoprisms, single CdSe/ZnS semiconductor quantum dots (QDs) exhibited suppressed blinking behavior, an enhanced fluorescence intensity ( approximately 2.5 fold), increased radiative decay rates ( approximately 12.5 fold), and antibunching single-photon emission. All these modifications significantly promote the overall performance of the proposed single-photon sources based on colloidal semiconductor QDs.

The multispan effects of Kerr nonlinearity and amplifier noises on Shannon channel capacity of a dispersion-free nonlinear optical fiber

We present a general treatment of multispan effects on Shannon channel capacity for dispersion-free nonlinear optical fiber transmission. We have derived a closed-form formula that is exact and applicable for arbitrary input signal power and noise power. The nonlinear interference among the accumulated Kerr nonlinear noises and the amplifier noises along each span will be investigated. The dependence of the channel capacity on various operating renditions and system parameters will be studied. We have derived a simple scaling law for the Shannon capacity as given by log, [1+CN/sub s//sup -4/3/N/sub c//sup -2/3//spl gamma//sup -2/3/P/sub W//sup -2/3/] for Nb spans and N/sub c/ channels, where /spl gamma/ is the Kerr nonlinearity coefficient and P/sub W/ is the average noise power density for each amplifier.

Effects of a fluctuating electronic coupling matrix element on electron transfer rate

The effects of a fluctuating electronic coupling matrix element on an electron transfer process are examined theoretically. An analytical expression for the electron transfer rate constant was derived using the spin‐boson model. In the limit of long correlation time (τv) for the fluctuation, our results are identical to the Marcus expression for the static coupling. The electron transfer rate in the intermediate range of τv, however, can be either enhanced or reduced, depending on the free energy gap and the reorganization energy. In the limit of very short τv, the electron transfer rate decreases with τv, and depends only on τv and the coupling matrix element.

Efficient electron transfer in carbon nanodot–graphene oxide nanocomposites

Carbon nanodots (CNDs) have emerged as fascinating materials with exceptional electronic and optical properties, and thus they offer many promising applications in photovoltaics and photocatalysis. In this paper we investigate electron transfer in nanocomposites of CNDs–graphene oxide (GO), –multi-walled carbon nanotubes (MWNTs) and –TiO2 nanoparticles without linker molecules, using steady state and time-resolved spectroscopy. Significant fluorescence quenching was observed in the CND–GO system, and it is attributed to the ultrafast electron transfer from CNDs to GO with a time constant of 400 fs. In comparison, carbon nanotubes result in static quenching of fluorescence in CNDs. No charge transfer was observed in both CND–MWNT and CND–TiO2 nanocomposites. This finding suggests that the CND–GO nanocomposite can be an excellent candidate for hot carrier solar cells due to the effective carrier extraction, broad spectral absorption, weak electron–phonon scattering, and thus a slow cooling rate for hot carriers.