Christine M. Steenkamp

A portable fibre-probe ultraviolet light emitting diode (LED)-induced fluorescence detection system

A portable fibre-probe fluorescence detection system comprising a continuous-wave high-power ultraviolet light emitting diode (UV LED) emitting at 365 nm as excitation source, a bifurcated fibre probe with a six-around-one fibre configuration to illuminate and read from a large target area (~3.6 mm2) and an integrated PC-coupled spectrometer has been developed. The construction, calibration and operation of the fluorescence detection system are described. Demonstrative test measurements with the system for possible inspection of different ripening stages on some batches of horticultural and agricultural products (lemon, mandarin, banana leaf and ivy leaf) have been performed and results presented. The system is portable, comparatively low cost, easily operated and relative immune to ambient light, thus being suitable for field measurements.

Faster learning algorithm convergence utilizing a combined time-frequency representation as basis

Light is capable of directly manipulating and probing molecular dynamics at its most fundamental level. One versatile approach to influencing such dynamics exploits temporally shaped femtosecond laser pulses. Oftentimes the control mechanisms necessary to induce a desired reaction cannot be determined theoretically a priori. However under certain circumstances these mechanisms can be extracted experimentally through trial and error. This can be implemented systematically by using an evolutionary learning algorithm (LA) with closed loop feedback. Most frequently, pulse shaping algorithms operate within either the time or frequency domain, however seldom both. This may influence the physical insight gained due to dependence on the search basis, as well as influence the speed the algorithm takes to converge. As an alternative to the Fourier domain basis, we make use of a combined time-frequency representation known as the von Neumann basis where we observe temporal and spectral effects at the same time. We report on the numerical and experimental results obtained using the Fourier, as well as the von Neumann basis to maximize the second harmonic generation (SHG) output in a non-linear crystal. We show that the von Neumann representation converges faster than the Fourier domain when compared to searches in the Fourier domain. We also show a reduced parameter space is required for the Fourier domain to converge efficiently, but not for von Neumann domain. Finally we show the highest SHG signal is not only a consequence of the shortest pulse, but that the pulse central frequency also plays a key role. Taken together these results suggest that the von Neumann basis can be used as a viable alternative to the Fourier domain with improved convergence time and potentially deeper physical insight.