Reaction functions for weakly bound systems

We propose a new technique to analyze total reaction cross sections. In this technique, which has been previously applied to fusion reactions, the experimental data are used to build a dimensionless reaction function, which does not depend on the system size or details of the optical potential. In this way, total reaction cross sections for different systems can be directly compared. We employ this technique to perform a systematic study of reaction cross sections of weakly bound systems in different mass ranges, and compare their reaction functions with the ones of tightly bound systems with similar masses. We show that breakup reactions and neutron transfers in halo systems lead to large reaction functions, well above the ones of typical tightly or weakly bound stable systems.     

"Two-Photon" coincidence imaging with a classical source

Coincidence imaging is a technique that extracts an image of a test system from the statistics of photons transmitted by a reference system when the two systems are illuminated by a source possessing appropriate correlations. It has recently been argued that quantum entangled sources are necessary for the implementation of this technique. We show that this technique does not require entanglement, and we provide an experimental demonstration of coincidence imaging using a classical source. We further find that any kind of coincidence imaging technique which uses a "bucket" detector in the test arm is incapable of imaging phase-only objects, whether a classical or quantum source is employed.     

Bayesian approach to network modularity

We present an efficient, principled, and interpretable technique for inferring module assignments and for identifying the optimal number of modules in a given network. We show how several existing methods for finding modules can be described as variant, special, or limiting cases of our work, and how the method overcomes the resolution limit problem, accurately recovering the true number of modules. Our approach is based on Bayesian methods for model selection which have been used with success for almost a century, implemented using a variational technique developed only in the past decade. We apply the technique to synthetic and real networks and outline how the method naturally allows selection among competing models.     

Direct streak mesurement of frequency sweeping and self focusing in single picosecond pulse

We describe a direct time resolved technique of frequency sweeping or fringe pattern shift measurements in a single picosecond laser pulse, based on the detection of a Fraunhofer diffraction pattern. The technique will obviously help to increase the information reliability of laser fusion experiments.     

Selective opening of airholes in photonic crystal fiber

We introduce a femtosecond-laser-based technique for selective opening of airholes in photonic crystal fibers (PCFs). With this technique, selective filling and inflation of the airholes in the PCF cladding are demonstrated. The technique may find important applications in tailoring or altering PCF characteristics and make it possible to seamlessly integrate various components/functions into PCFs.     

Direct measurement of transverse coherence length of hard X rays from interference fringes

We propose a simple interferometric technique for hard x-ray spatial coherence characterization, recording a Fresnel interference pattern produced by a round fiber or a slit. We have derived analytical formulas that give a direct relation between a visibility of interference fringes and either the source size or the transverse coherence length. The technique is well suited to third-generation synchrotron radiation sources and was experimentally applied to determine the spatial coherence length and the source size at the European Synchrotron Radiation Facility.     

Phase-dispersion optical tomography

We report on phase-dispersion optical tomography, a new imaging technique based on phase measurements using low-coherence interferometry. The technique simultaneously probes the target with fundamental and second-harmonic light and interferometrically measures the relative phase shift of the backscattered light fields. This phase change can arise either from reflection at an interface within a sample or from bulk refraction. We show that this highly sensitive (~5 degrees ) phase technique can complement optical coherence tomography, which measures electric field amplitude, by revealing otherwise undetectable dispersive variations in the sample.     

Dimension-Splitting for Simplifying Diffusion in Lattice-Gas Models

We introduce a simplified technique for incorporating diffusive phenomena into lattice-gas molecular dynamics models. In this method, spatial interactions take place one dimension at a time, with a separate fractional timestep devoted to each dimension, and with all dimensions treated identically. We show that the model resulting from this technique is equivalent to the macroscopic diffusion equation in the appropriate limit. This technique saves computational resources and reduces the complexity of model design, programming, debugging, simulation and analysis. For example, a reaction-diffusion simulation can be designed and tested as a one-dimensional system, and then directly extended to two or more dimensions. We illustrate the use of this approach in constructing a microscopically reversible model of diffusion-limited aggregation as well as in a model of growth of biological films.     

Determination of Resting Membrane Potential of Individual Neuroblastoma Cells (IMR-32) Using a Potentiometric Dye (TMRM) and Confocal Microscopy

The potentiometric dye, Tetramethylrhodamine methyl ester (TMRM) has been extensively used with fluorometry or optical microscopy to evaluate the electric potential across plasma or mitochondrial membranes. We present here a TMRM confocal microscopy-based potential measurement technique. Corrections are introduced to minimize nonspecific dye binding and insensitivity to low background levels. We have used this technique to compare the resting membrane potential of proliferating and differentiated human neuroblastoma cells (IMR-32).     

Control of chaos in excitable physiological systems: A geometric analysis

Model-independent chaos control techniques are inherently well-suited for the control of physiological systems for which quantitative system models are unavailable. The proportional perturbation feedback (PPF) control paradigm, which uses electrical stimulation to perturb directly the controlled system variable (e.g., the interbeat or interspike interval), was developed for excitable physiological systems that do not have an easily accessible system parameter. We develop the stable manifold placement (SMP) technique, a PPF-type technique which is simpler and more robust than the original PPF control algorithm. We use the SMP technique to control a simple geometric model of a chaotic system in the neighborhood of an unstable periodic orbit (UPO). We show that while the SMP technique can control a chaotic system that has UPO dynamics which are characterized by one stable manifold and one unstable manifold, the success of the SMP technique is sensitive to UPO parameter estimation errors. (c) 1997 American Institute of Physics.     

Variable resolution with pupil masks

There are many applications in which pupil-plane masks are useful for point-spread-function (PSF) apodization or superresolution. A limitation of this technique is that once a mask is fabricated, the corresponding PSF characteristics are fixed. To overcome this drawback we introduce a technique for easily varying the performance of pupil-plane masks. This technique is based on the modification of the transmittance of each of the mask zones and, thus, can be implemented using a spatial light modulator or linear polarizers, e.g., we apply the technique to binary phase-only masks and we check that the figures of merit that characterize the PSF can be easily controlled. We study different configurations that allow us to modify resolution or peak intensity in a continuous way and we derive analytical expressions for these figures of merit.     

Acousto-optic laser optical feedback imaging

We present a photon noise and diffraction-limited imaging method combining an imaging laser and ultrasonic waves. The laser optical feedback imaging (LOFI) technique is an ultrasensitive imaging method for imaging objects through or embedded within a scattering medium. However, LOFI performances are dramatically limited by parasitic optical feedback occurring in the experimental setup. In this Letter, we have tagged the ballistic photons by an acousto-optic effect in order to filter the parasitic feedback effect and to reach the theoretical and ultimate sensitivity of the LOFI technique. We present the principle and the experimental setup of the acousto-optic laser optical feedback imaging technique, and we demonstrate the suppression of the parasitic feedback.     

Terahertz time-domain attenuated total reflection spectroscopy in water and biological solution

We demonstrate time-domain attenuated total reflection spectroscopy in terahertz frequency region. Geometry of the reflection measurement is well optimized to obtain accurate optical constants of water or aqueous biomolecular system. We determine the dielectric constants of distilled water and sucrose solutions with this technique. This technique will open new aspects in the research field of biological systems in water.     

“Triple Observation Method (TOM)” to Discriminate Optically Autofluorescence from Porphyrins Versus that from Copper-Metallothioneins

We propose a conclusive difference observed between the excitation conditions required to observe porphyrins and copper-metallothioneins in cells and/or tissues using an ordinary fluorescence microscope. We have emphasized the importance of examining the spectral properties of the emissions to avoid any serious mistakes such as confusing porphyrins with copper-metallothioneins in the liver and kidneys. However, microspectrophotometry is not a conventional method for either histochemical, cytochemical, or pathological studies because microspectrophotometers are both expensive and difficult to operate. Therefore, we demonstrate a simple comparative method using ordinary excitation filter arrangements. When using our technique, it becomes possible to optically discriminate more accurately between the autofluorescence properties arising from porphyrins and those arising from copper-metallothioneins. We would like to name our simple technique “Triple Observation Method (TOM)”.     

Nonlinear coupling of nanomechanical resonators to josephson quantum circuits

We propose a technique to couple the position operator of a nanomechanical resonator to a SQUID device by modulating its magnetic flux bias. By tuning the magnetic field properly, either linear or quadratic couplings can be realized, with a discretely adjustable coupling strength. This provides a way to realize coherent nonlinear effects in a nanomechanical resonator by coupling it to a Josephson quantum circuit. As an example, we show how squeezing of the nanomechanical resonator state can be realized with this technique. We also propose a simple method to measure the uncertainty in the position of the nanomechanical resonator without quantum state tomography.     

Pulsed ultrasound-modulated light tomography

Ultrasound-modulated light tomography is a new technique that combines laser light and ultrasound to provide a representation of the light density inside turbid media. We present a method that can produce two- or three-dimensional light density representations with standard ultrasonic pulses. This technique should allow simple, direct fusion of ultrasonic images with optical tomography.     

Three-dimensional photoacoustic imaging of blood vessels in tissue

We applied photoacoustics as a tissue tomography technique for the detection of blood concentrations, e.g., angiogenesis around tumors. We imaged blood vessels in highly scattering samples, using 532-nm light, to depths of ~1 cm . The samples were real tissue (chicken breast) or 10% dilutions of Intralipid-10%. The blood flowed through nylon capillaries. Polyvinylidene difluoride (PVdF) piezoelectric detectors were used in a surface-scanning mode. We demonstrate the sensitivity of the technique by photoacoustic detection of single red blood cells upon a glass plate. Lateral resolution is limited by the detector diameter (200 microm). The depth resolution is ~10 microm.     

An automatic technique for the measurement of thermoelectric power at high pressures

We describe an automatic technique for direct recording of the themoelectric power of metals either as a function of temperature or pressure. This technique facilitates the process of measurement especially when one has to scan a wide temperature and pressure range. Typical experimental recordings of the γ-α electronic phase transition in cerium are also presented.     

Nonlinear optical technique for precise retardation measurements

We present a highly sensitive nonlinear optical technique to measure optical retardation. The technique is based on second-harmonic generation from thin films using two beams at the fundamental frequency. The sensitive polarization dependence of the process allows measuring optical retardation very precisely. The technique relies on fundamental symmetry principles and does therefore not require complicated experimental arrangement or data analysis. The technique was demonstrated by determining the retardation of a nominal half-wave plate to a precision and repeatability better than lambda/10(4).     

Simulating biochemical networks at the particle level and in time and space: Green's function reaction dynamics

We present a technique, called Green's function reaction dynamics (GFRD), for particle-based simulations of reaction-diffusion systems. GFRD uses a maximum time step such that only single particles or pairs of particles have to be considered. For these particles, the Smoluchowski equations are solved analytically using Green's functions, which are used to set up an event-driven algorithm. We apply the technique to a model of gene expression. Under biologically relevant conditions, GFRD is up to 5 orders of magnitude faster than conventional particle-based schemes.