Field correlation effects on the resonant light scattering

Effects of incident photon field coherence on resonant light scattering have been investigated. In order to obtain the scattering intensity and the photon counting rate, an expression for the reduced density matrix for the scattered field has been derived. The expression involves the first-order correlation function of the incident field. The relation between the line shape of the scattered light and the bandwidth of the incident field has been clarified. Model calculations of the photon counting rate have been performed in the case of an incident field without first-order coherence. In our treatment, the transverse and longitudinal relaxation constants have been taken into account by using the impact approximation.     

Incoherent subharmonic light scattering in isotropic media

Incoherent subharmonic light scattering in isotropic media is a new kind of nonlinear light scattering, which involves single input photon and multiple output photons of equal frequency. We investigate theoretically the dependence of the subharmonic scattering intensity on the hyperpolarizability of molecules and the incident intensity using nonlinear optics theory similar to that used for Hyper-Rayleigh scattering and degenerate optical parametric oscillators. It is derived that the subharmonic scattering intensities grow exponentially or superexponentially with the hyperpolarizability of molecules and the incident intensity.     

Theory of correlation measurements of resonance light scattering

The boson nature of radiation is shown to give rise to a purely quantum mechanical exchange contribution to the intensity-intensity correlation function for resonant light scattering by an atomic or molecular system. The exchange contribution can be decomposed into three components, one involving the intensity correlation for a pair of coherently scattered photons (“resonant Raman” processes), another for a pair of incoherently scattered ones (“resonance fluorescence”), and the last involving the exchange correlation one of each. The intensity correlation measurements of Kimble et al., on optically pumped atomic beams of sodium atoms are interpreted with the theory, producing values of the decay rate of the excited sodium atoms and of the coherence time of the exciting radiation in good agreement with expectations.     

Depolarization of atomic two-photon radiation

A technique of depolarization is used to investigate the coherence and spectral properties of the radiation emitted in the two-photon decay of metastable atomic deuterium. The results agree with the quantum mechanical predictions but may also be interpreted to show that a single photon of a two-photon pair can be considered to have a very short coherence length, a very short coherence time and a broad bandwidth, all limited, in practice, by the transmission properties of the detection system.     

Angular-ratiometric plasmon-resonance based light scattering for bioaffinity sensing

We describe an exciting opportunity for affinity biosensing using a ratiometric approach to the angular-dependent light scattering from bioactivated and subsequently aggregated noble metal colloids. This new model sensing platform utilizes the changes in particle scattering from very small colloids, which scatter light according to traditional Rayleigh theory, as compared to the changes in scattering observed by much larger colloidal aggregates, formed due to a bioaffinity reaction. These larger aggregates no longer scatter incident light in a Cos(2) theta dependence, as is the case for Rayleigh scattering, but instead scatter light in an increased forward direction as compared to the incident geometry. By subsequently taking the ratio of the scattered intensity at two angles, namely 90 degrees and 140 degrees , relative to the incident light, we can follow the association of biotinylated bovine serum albumin-coated 20 nm gold colloids, cross-linked by additions of streptavidin. This new model system can be potentially applied to many other nanoparticle assays and has many advantages over traditional fluorescence sensing and indeed light-scattering approaches. For example, a single nanoparticle can have the equivalent scattered intensity as 10(5) fluorescing fluorescein molecules substantially increasing detection; the angular distribution of scattered light from noble metal colloids is substantially easier to predict as compared to fluorescence; the scattered light is not quenched by biospecies; the ratiometric measurements described here are not dependent on colloid concentration as are other scattering techniques; and finally, the noble metal colloids are not prone to photodestruction, as is the case with organic fluorophores.     

State dependence of rayleigh scattering from an=2 state of hydrogenlike atoms

We consider Rayleigh scattering from a hydrogenlike atom in an arbitrary excitedn=2 state, and we investigate theoretically the dependence of the scattered radiation intensity and Stokes parameters on the state multipoles for the case of unpolarized incident radiation. Because in then=2 case Rayleigh scattering can not be accompanied by the change of the electron angular momentum, only 10 out of the 16 state multipoles influence the scattered radiation attributes. Our study reveals the existence of a measurable quantity which is determined only by Rayleigh scattering from 2p states. For the particular case of excitation by electron impact, some quantitative predictions are made, at the photon scattering angle ?=π/2, based on values for the state multipoles extracted from the literature (Blum and Kleinpoppen, Band). The vicinity of Balmer α and Lyman α resonances are studied in detail.     

Single molecule photon counting statistics for quantum mechanical chromophore dynamics

We extend the generating function technique for calculation of single molecule photon emission statistics (Zheng, Y.; Brown, F. L. H. Phys. Rev. Lett. 2003, 90, 238305) to systems governed by multi-level quantum dynamics. This opens up the possibility to study phenomena that are outside the realm of purely stochastic and mixed quantum-stochastic models. In particular, the present methodology allows for calculation of photon statistics that are spectrally resolved and subject to quantum coherence. Several model calculations illustrate the generality of the technique and highlight quantitative and qualitative differences between quantum mechanical models and related stochastic approximations when they arise. Calculations suggest that studying photon statistics as a function of photon frequency has the potential to reveal more about system dynamics than the usual broadband detection schemes.     

Quantum trajectories in atom-surface scattering with single adsorbates: the role of quantum vortices

In this work, a full quantum study of the scattering of He atoms off single CO molecules, adsorbed onto the Pt(111) surface, is presented within the formalism of quantum trajectories provided by Bohmian mechanics. By means of this theory, it is shown that the underlying dynamics is strongly dominated by the existence of a transient vortitial trapping with measurable effects on the whole diffraction pattern. This kind of trapping emphasizes the key role played by quantum vortices in this scattering. Moreover, an analysis of the surface rainbow effect caused by the local corrugation that the CO molecule induces on the surface, and its manifestation in the corresponding intensity pattern, is also presented and discussed.     

Proton dynamics in an extended array of hydrogen bonds: normal coordinates, proton transfer and macroscopic quantum entanglement in the ground state

After a brief introduction to neutron scattering techniques, illustrated with the scattering function for harmonic oscillators, some new aspects of proton dynamics in the KHCO₃ crystal are presented. The full scattering function for the proton modes measured on single crystals provides a graphic view of proton dynamics. Vibrational states are fully characterized with three quantum numbers. The effective oscillator mass of 1 amu confirms the decoupling of protons from the lattice. Combining infrared, Raman and inelastic neutron scattering techniques, the double minimum potential for the transfer of a single proton along hydrogen bonds is totally determined. Elastic neutron scattering techniques probe dynamics in the fully degenerate ground state. Quantum entanglement arising from normal coordinates gives rise to quantum interference. With diffraction techniques, the dynamical structure arising from large-scale quantum coherence is observed as ridges of intensity, well separated from Bragg's peaks. The vibrational wave function in the ground state must be regarded as a superposition of non-factorable macroscopic wave function.     

Electronic coherence effects in photosynthetic light harvesting

Photosynthetic light harvesting is a paradigmatic example for quantum effects in biology. In this work, we review studies on quantum coherence effects in the LH2 antenna complex from purple bacteria to demonstrate how quantum mechanical rules play important roles in the speedup of excitation energy transfer, the stabilization of electronic excitations, and the robustness of light harvesting in photosynthesis. Subsequently, we present our recent theoretical studies on exciton dynamical localization and excitonic coherence generation in photosynthetic systems. We apply a variational-polaron approach to investigate decoherence of exciton states induced by dynamical fluctuations due to system-environment interactions. The results indicate that the dynamical localization of photoexcitations in photosynthetic complexes is significant and imperative for a complete understanding of coherence and excitation dynamics in photosynthesis. Moreover, we use a simple model to investigate quantum coherence effects in intercomplex excitation energy transfer in natural photosynthesis, with a focus on the likelihoods of generating excitonic coherences during the process. Our model simulations reveal that excitonic coherence between acceptor exciton states and transient nonlocal quantum correlation between distant pairs of chromophores can be generated through intercomplex energy transfer. Finally, we discuss the implications of these theoretical works and important open questions that remain to be answered.     

Light scattering from tilted two-dimensional spherulites

The theory of small-angle light scattering was developed for oblique incidence of the light beam on the surface of a two-dimensional spherulite. Results of the theory were compared with previously reported results of light scattering from two-dimensional and three-dimensional spherulites for normal incidence, and with some experimental patterns. The comparisons suggest that the scattering intensity distributions of two-dimensional spherulites deviate from those of three-dimensional spherulites when the sample surface is tilted with respect to the propagation direction of the incident beam, although they are almost identical when the sample surface is normal to the incident beam. Observation of the change of scattered intensity distributions upon tilting the samples thus provides a method of distinguishing between two-dimensional and three-dimensional spherulites. Moreover, this observation makes it possible to determine the degree of planar orientation of the optic axes of optically anisotropic scattering elements within two-dimensional spherulites. The calculations were carried out for special cases of two-dimensional spherulites with the optic axis orientation confined to the two-dimensional plane and randomly or helicoidally rotated around the spherulite radii.     

Light absorption at high intensities. Comparison of quantum theory and semi-classical results

An error in the author's previous treatment of the interaction of an absorber with high intensity light is noted. The correct development of Mower is applied to determine the amplitude of the initial state of the radiation-matter system. Comparison of the quantum theory solution for a damped absorber exposed to a square light pulse with the results of semiclassical theories based on the undamped optical Bloch equations is effected by determining the effective in- and out-of-phase components of the transition dipole response. In general, the out-of-phase (or absorptive) part of the transition dipole is zero at the outset and strongly time-dependent at short times, evolving to a steady-state value at longer times. For resonance radiation the system exhibits either overdamped irreversible decay or underdamped oscillations, depending upon the relative magnitudes of the incident light intensity and the radiative damping rate. Specific results are presented for a variety of limiting values of the physical parameters, including light intensity, damping rate, and amount of off-resonance.     

Photosensitized Damage Inflicted on Plasma Membranes of Live Cells by An Extracellular Generator of Singlet Oxygen—A Linear Dependence of A Lethal Dose on Light Intensity

We describe a study of the influence of a dose rate, i.e. light intensity or photon flux, on the efficiency of induction of a loss of integrity of plasma membranes of live cells in culture. The influence of a photon flux on the size of the light dose, which was capable of causing lethal effects, was measured in an experimental system where singlet oxygen was generated exclusively outside of live cells by ruthenium(II) phenantroline complex. Instantaneous, sensitive detection of a loss of integrity of a plasma membrane was achieved by fluorescence confocal imaging of the entry of this complex into a cell interior. We demonstrate that the size of the lethal dose of light is directly proportional to the intensity of the exciting light. Thus, the probability of a photon of the exciting light inflicting photosensitized damage on plasma membranes diminishes with increasing density of the incident photons.     

How Quantum Coherence Assists Photosynthetic Light Harvesting

This perspective examines how hundreds of pigment molecules in purple bacteria cooperate through quantum coherence to achieve remarkable light harvesting efficiency. Quantum coherent sharing of excitation, which modifies excited state energy levels and combines transition dipole moments, enables rapid transfer of excitation over large distances. Purple bacteria exploit the resulting excitation transfer to engage many antenna proteins in light harvesting, thereby increasing the rate of photon absorption and energy conversion. We highlight here how quantum coherence comes about and plays a key role in the photosynthetic apparatus of purple bacteria.     

Langevin-Bloch equations for a spin bath

We derive the Bloch equations for a two-level system coupled to a spin bath of infinitely many two-level atoms to examine phase and energy relaxation of an optically excited system. We show that increasing temperature assists coherence. This is reflected in a number of anomalous features of relaxation of the system, e.g., decrease of integrated absorption coefficient with temperature, nonlinear variation of linewidth with incident power. We also predict that thermally induced coherence may result in anomalous narrowing of linewidth, reminiscent (but distinct) of "motional narrowing" of spectral line. The theoretical results are discussed in the light of absorption-emission experiments on single quantum dots.     

Raman Scattering at Resonant or Near-Resonant Conditions: A Generalized Short-Time Approximation

We investigate the dynamics of resonant Raman scattering in the course of the frequency de-tuning. The dephasing in the time domain makes the scattering fast when the photon energy is tuned from the absorption resonance. This makes frequency detuning to act as a cam-era shutter with a regulated scattering duration and provides a practical tool of controlling the scattering time in ordinary stationary measurements. The theory is applied to resonant Raman spectra of a couple of few-mode model systems and to trans-1,3,5-hexatriene and guanine-cytosine (G-C) Watson-Crick base pairs (DNA) molecules. Besides some particular physical effects, the regime of fast scattering leads to a simplification of the spectrum as well as to the scattering theory itself. Strong overtones appear in the Raman spectra when the photon frequency is tuned in the resonant region, while in the mode of fast scattering, the overtones are gradually quenched when the photon frequency is tuned more than one vibra-tional quantum below the first absorption resonance. The detuning from the resonant region thus leads to a strong purification of the Raman spectrum from the contamination by higher overtones and soft modes and purifies the spectrum also in terms of avoidance of dissociationand interfering fluorescence decay of the resonant state. This makes frequency detuning a very useful practical tool in the analysis of the resonant Raman spectra of complex systems and considerably improves the prospects for using the Raman effect for detection of foreign substances at ultra-low concentrations.     

Fluorescence kinetics of emission from a small finite volume of a biological system

The fluorescence decay, apparent quantum yield and transmission from chromophores constrained to a microscopic volume using a single picosecond laser excitation were measured as a function of incident intensity. The β subunit of phycoeryhthrin aggregate isolated from the photosynthetic antenna system of Nostoc sp. was selected since it contains only four chromophores in a volume of less than 5.6×10⁴ ų. The non-exponential fluorescence decay profiles were intensity independent for the intensity range studied (5 × 10¹³ - 2 × 10¹⁵ photon cm^?2 per pulse). The apparent decrease in the relative fluorescence quantum yield and increase of the relative transmission with increasing excitation intensity is attributed to the combined effects of ground state depletion and upper excited state absorption. Evidence suggests that exciton annihilation is absent within isolated β subunits.     

THE NON-RADIATIVE DISSIPATION OF EXCITATION ENERGY IN SOLID CINNAMIC ACID BY DIMER FORMATION

Abstract— The quantum efficiency for the photodimerization of trans -cinnamic acid in the solid state is independent of intensity and is found to have a value approaching two. Thus, the reaction involves one excited and one un-excited molecule.
During the exposure, a dimer film developing on the surface of the cinnamic acid layer attenuates the intensity of the radiation incident on the unreacted cinnamic acid. This gives the appearance of a decrease in the quantum yield with increasing number of photons incident.