David R. Demmer

Excitation pulse‐shape mimic technique for improving picosecond‐laser‐excited time‐correlated single‐photon counting deconvolutions

The determination of accurate subnanosecond fluorescence lifetimes by the time‐correlated single‐photon counting technique is often limited by difficulties in obtaining the correct instrumental response function f(λ,t) to the excitation pulse. These difficulties are increased when a grating monochromator is used to disperse the emission due to the introduction of an additional temporal broadening of f(λ,t). A technique for determining the correct f(λ,t) at the wavelength of sample emission, λem, is described. The technique consists of using a very short‐lived fluorophore to ‘‘mimic’’ the excitation pulse shape at λem, yielding the convoluted mimic decay function Cm(λem,t), and then to computationally extract f (λem,t) from Cm(λem,t). The technique is experimentally and computationally simple and yields the desired instrumental response function at λem which eliminates problems due to the sensitivity of f(λ,t) to λ (color shift artifact). The photomultiplier tube receives spatially equal illumination when ...

Three exponential fluorescence decay of horse liver alcohol dehydrogenase revealed by iodide quenching.

Abstract— The time resolved fluorescence decay of horse liver alcohol dehydrogenase was measured using a frequency doubled picosecond dye laser and time‐correlated single‐photon counting detection. A flow‐cell technique is described which eliminates the photodegradation artifacts which commonly occur with laser excitation. A procedure is introduced which uses fluorescence quenching to reveal minor fluorescence lifetime components. The decay of the unquenched tryptophanyl fluorescence could be described by a double exponential decay law, but experiments conducted in the presence of iodide ion showed that the fluorescence decay must be more complex than this. A model is presented in which the fluorescence decay consists of three exponential components, only two of which are susceptible to quenching by iodide ion. Several possibilities are presented for the origin of this minor decay component, the most reasonable of which is that it arises from conformational heterogeneity in the solvent‐exposed tryptophanyl residue.

Tracking polymer diffusion in a wet latex film with fluorescence resonance energy transfer.

We describe an instrument to measure the polymer interdiffusion between donor-labeled and acceptor-labeled latex polymers in a partially wet latex film with fluorescence resonance energy transfer (FRET). It is possible to temporarily arrest the drying process of a wet latex film by sealing the film in an airtight chamber. In our approach, we measure donor fluorescence decays from 0.5 mm diameter spots at various positions across an arrested latex film with time-correlated single photon counting. We interpret the resulting decays with a Monte Carlo simulation of the FRET process and extract information about the extent of polymer diffusion as a function of position on the film. These results enable us to determine the extent of polymer interdiffusion as a function of distance from the wet-dry edge in the latex film. To highlight this devices ability to capture the rapid early stages of latex interdiffusion, we report results from an acrylate copolymer latex.

Picosecond dynamics of the S2 excited state of azulene and its van der Waals complexes with Ar and Xe

Abstract The dynamics of the second excited singlet (S 2 ) states of jet-cooled azulene (Az) and its van der Waals complexes with one and two atoms of Ar and Xe have been investigated. Quantum interference effects due to restricted intramolecular vibrational redistribution in azulene have been reinterpreted. Lifetime shortening in the 1:1 and 1:2 complexes with Xe is attributed to an enhancement of the rate of S 2 T 1 intersystem crossing by the external heavy atom effect. Excitation of the complexes to states with E vib > E b produces energy-resolved fluorescence decays which exhibit sequential intramolecular vibrational redistribution-vibrational predissociation.

Using Photodiodes in High Temperature Applications

Photodiodes are used for optical sensing in a large variety of operating environments. Problems occur using photodiodes in high temperature environments as optical and electrical performance parameters degrade. High temperature photodiode behavior will be discussed along with methods to mitigate degrading performance and stretch the practical operating temperature range.

Application of the light valve to holographic stereograms

Holography as a medium for exploration by artists faces several major limitations. Conventional holography is restricted to that which can be brought in to the lab and illuminated with coherent light. Hologrpahic stereograms have made possible the creation of holograms of both real objects in natural or studio lighting and virtual objects created using three-dimensional graphics. There are several approaches for creating holograms from digital graphics. This paper discusses the application of the light valve to the process along with other related developments. Artists can not produce high quality inexpensive, medium-format holograms using this direct-link to digital media. Future developments will lead to even higher resolution Digital Image-Light-Amplifier (D- ILA) systems.

Simple, inexpensive autocorrelator for use with cw mode‐locked lasers

An improved method of obtaining variable pulse spacing for use in a cw SHG autocorrelation interferometer is described.