Volume 85, Issue 3, 01 February 1999
Index of content:
- GENERAL PHYSICS: NUCLEAR, ATOMIC, AND MOLECULAR (PACS 01-39)
85(1999); http://dx.doi.org/10.1063/1.369258View Description Hide Description
In this article, the propagation of digital and analog signals through media which, in general, are both dissipative and dispersive is modeled using the one-dimensional telegraph equation. Input signals are represented using impulsive, Heaviside unit step, Gaussian, rectangular pulse, and both unmodulated and modulated sinusoidal pulse type boundary data. Applications to coaxial transmission lines and freshwater signal propagation, for both digital and analog signals, are included. The analysis presented here supports the finding that digital transmission in dispersive media is generally superior to that of analog. The boundary data (input signals) give rise to solutions of the telegraph equation which contain propagating discontinuities. It is shown that the magnitudes of these discontinuities, as a function of distance, can be found without the need of solving the governing equation. Thus, for digital signals in particular, signal strength at a given distance from the input source can be easily determined. Furthermore, the magnitudes of these discontinuities are found to be independent of both the dispersion coefficient k and the elastic coefficient b. In addition, it is shown that, depending on the algebraic sign of k, one of two distinct forms of dispersion is possible and that for small-time intervals, solutions are approximately independent of k.
Femtosecond transient absorption and luminescence decay studies of spectrally sensitized photographic emulsions85(1999); http://dx.doi.org/10.1063/1.369259View Description Hide Description
Fast electronic processes taking place immediately after excitation of sensitized silver halide emulsions were studied with picosecond time-correlated single-photon counting (SPC) and femtosecond transient absorption (TA). The fluorescence decays of the aggregate obtained by SPC measurements were also analyzed as a sum of three exponentials. For the aggregates of a benchmark dye, a dimethyl-μ-ethyl-thiacarbocyanine, a component with a 20 ps decay time, which was absent in samples containing no AgBr crystals, was attributed to aggregates of the sensitizer adsorbed on the AgBr microcrystals. The fluorescence decay results were compared with photographic quantum yields in an attempt to show the relevance of the spectroscopicmeasurements. The second component (150 ps) is a combination of fluorescence decays of the unaggregated sensitizer molecules not adsorbed on the AgBr microcrystals and monomers of the dye on AgBr surface. The fluorescence decay was found to be independent of the crystal coverage. Prolonged illumination bleached the fluorescence of the fast component, which is consistent with depletion of the aggregates by electron injection. The decay time of this fast component shows strong dependence on both the structure of the dye and the composition of the silver halide. Excitation in the narrow absorption band of the aggregates with a femtosecond pump pulse resulted in ground state bleaching followed by a recovery that was analyzed as a sum of three exponentials. The fast (170 fs and 2 ps) components of the recovery are related to the relaxation of bi- or multiexciton states and to singlet-singlet annihilation processes. The remaining component (1 ns) can be attributed to the radiative relaxation of dye molecules not adsorbed on the silver halide crystals. The observed bleaching recovery demonstrated a complex dependence on both the excitation flux and the total illumination dose.
Feasibility of molecular-resolution fluorescence near-field microscopy using multi-photon absorption and field enhancement near a sharp tip85(1999); http://dx.doi.org/10.1063/1.369260View Description Hide Description
Aperture-based near-field microscopy suffers from fundamental limitations imposed by the electromagnetic skin depth of the aperture material and a rapidly decreasing throughput as the aperture is made smaller. Apertureless approaches without these limitations have been demonstrated for coherent imaging but are not easily applicable to incoherent processes such as fluorescence or Raman scattering and to photochemicalsurface modification. Using multi-photon processes in conjunction with the field enhancement that occurs at a sharp tip in close apposition to a substrate should permit substantial localization of absorption and excitation to a nm sized volume. The expected enhancement of the optical field at the tip edge is calculated here for various combinations of metallic and nonmetallic tip and substrate materials. It is estimated that when using 100 fs pulses repeating at 100 MHz average laser powers of about 10 mW should be sufficient to reach saturating field strengths for three-photon absorption. Steady state and instantaneous temperature rises at the tip are estimated and found likely not to be a limiting factor. Fluorescence quenching is expected to limit the resolution achievable with metallic tips to about 5 nm, but tips made from highly refracting insulators or semiconductors should allow truly molecular resolution.
85(1999); http://dx.doi.org/10.1063/1.369261View Description Hide Description
We have measured the time dependence of the flux of non-thermal phononsgenerated by α particles in a single crystal of mass 1 g using a Series Array of Superconducting Tunnel Junctions. Taking advantage of the almost perfect elastic symmetry of we approximate the equations of quasidiffusive propagation to give a simple diffusion equation characterized by an effective diffusion coefficient where K is a material constant and L the distance between the phonon source and the point of detection. We use this model to perform pulse-shape analysis on our data and show that agreement is excellent, and highly preferred over a ballistic pulse shape. The distributions of values of K determined for different lengths L are found to overlap strongly, in support of the law We suggest that pulse-shape analysis may provide an alternative to time-difference measurements to achieve position sensitivity in a phonon-mediated detector based on a absorber.