Jonathon David White

Real‐time data acquisition incorporating high‐speed software correlator for single‐molecule spectroscopy

Single‐molecule spectroscopy and detection are powerful techniques for the study of single fluorescent particles and their interaction with their environment. We present a low‐cost system for simultaneous real‐time acquisition, storage of inter‐photon arrival times and the calculation and display of the fluorescence time trace, autocorrelation function and distribution of delays histogram for single‐molecule experiments. From a hardware perspective, in addition to a multi‐core computer, only a standard low‐cost counting board is required as processing is software‐based. Software is written in a parallel programming environment with time crucial operations coded in ANSI‐C. Crucial to system performance is a simple and efficient real‐time autocorrelation algorithm (acf) optimized for the count rates (approximately 104 cps) encountered in single‐molecule experiments. The algorithms time complexity is independent of temporal resolution, which is maintained at all time delays. The system and algorithms performance was validated by duplicating the signal from the photon detector and sending it to both the ordinary counter board and a commercial correlator simultaneously. The data acquisition systems robustness under typical single‐molecule experimental conditions was tested by observing the diffusion of Rhodamine 6G molecules in deionized water.

Graphene reduction dynamics unveiled

The reduction dynamics of micron-sized defects created on chemical vapor deposition- (CVD) grown graphene through scanning probe lithography (SPL) is reported here. CVD-grown graphene was locally oxidized using SPL and subsequently reduced, making use of a focused beam of soft x-rays. During this whole process, the reduction dynamics was monitored using a combination of micro-Raman spectroscopy (μ-RS) and micro-x-ray photoelectron spectroscopy (μ-XPS). After x-ray reduction, the graphene film was found to be chemically identical (μ-XPS) but structurally different (μ-RS) from the original graphene. During reduction the population of C–C bonds was found to first increase dramatically and then decrease exponentially. By modeling the dynamics of the C=O → C–O → C–C → C=C reduction process with four coupled-rate equations and three rate constants, the conversion from C–C to C=C bonds was found to be the limiting rate.

Observing and modeling light propagation in polymer films

Light propagation within a thin luminescent polymer film was quantified experimentally and examined using ray tracing simulation. Spot excitation provided a localized source of photoluminescence. Parallel grooves extracted the light far away from the source. By varying the separation between excitation and extraction sites, the propagation of photoluminescence within the film was analyzed. Simulation was then applied to determine the contribution of scattering to the observed propagation distance. Relying only on the relative light intensities, it provides a robust method to quantify and understand light propagation within polymer films.

On the potential of solid state LED strips utilizing an organic color converter for non-line of sight visible light communication

LED strip lighting can provide high quality uniform shadow-free diffuse lighting at low cost as numerous emission sources are controlled by a single transformer. Organic LEDs offer the additional advantages of UV free emission and, for visible light communication, picosecond fluorescent lifetimes allowing the whole visible spectrum to be used without filters. Using parameters determined experimentally for solid-state LED strip lighting and fluorescent lifetimes typical of organic phosphors as the input for a Monte Carlo based ray-tracing simulation, we evaluate the potential bandwidths obtainable for indoor communication. Our work suggests that raw data transfer rates of 4 to 10 Mbps are obtainable in a standard 5m by 5m by 3m room compatible with Internet of Things (IoT) applications.

Controlling photoluminescent emission from polymer membranes through the creation of local deformation zones.

Understanding and optimizing light propagation and extraction in light-emitting systems, such as fluorescent chemical sensors, is important for the production of more efficient sensors. We apply Monte Carlo ray tracing to model the effects of one-dimensional perturbations of film thickness on the luminescent emission (spatial, directional, spectral) of a freestanding transparent polymer film embedded with luminescent chromophores. Such modification not only enhances light extraction but also allows its location and direction to be controlled. Optimization of the deformation geometry allows for a 3.6-fold increase in intensity for a far-field detector.

Observing the propagation of fluorescent light in doped polymer films

By making use of spot excitation to generate photoluminescence, and parallel grooves to extract light, light propagation in a polymer film was quantified. Scatterings contribution in limiting the propagation distance was determined by simulation.

High-speed low-cost correlator for single molecule fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) has been extensively applied to study the kinetics and photophysics of molecules as well as interactions between molecules by extracting information from the fluctuation of signals. In particular, single molecule applications of FCS promise the greatest amounts of information. Ideally, one would like to carry out FCS in real-time; however, due to the time-consuming nature of the correlation process, performing the correlation in real-time is totally nontrivial. Generally an expensive hardware correlator or a TCSPC board is required for this purpose. Recently highly-efficient algorithms based on multi-tau method have been proposed to build up a software correlator. In this work, we set forth an innovative algorithm capable of realizing the real-time correlation, without turning to the multi-tau method. This algorithm takes advantage of the low count rate generally existing in the FCS experiments, directly using the time interval between each photon its adjacent photon to efficiently update the correlation function. Based on this efficiency, it is possible to build a low-cost software correlator with just an ordinary counter board. We practically demonstrate the feasibility by setting up this correlator to measure the diffusion motion of rhodamine 6G in water using FCS. The algorithm was validated by duplicating the signal from the photon detector and sending it to both the ordinary counter board with our software correlator and a commercial correlator simultaneously. The perfect coincidence of the correlation curves from these two correlators and the real-time display of the correlation function indicate the validity and practicability of our approach.

Single molecule fluorescence spectroscopy of luminescent conjugated polymers with different chain length

Fluorescence intensity traces of different length luminescent conjugated polymers are studied by single molecule spectroscopy. The fundamental difference between one and three-dimensional system is used to explain the discrepancy in the photophysics of different chain length.