Professor Paul Ewart

Abstract:

A technique is reported for simultaneous, time- and space-resolved measurements of temperature using laser-induced thermal grating scattering, LITGS, from four points on a 1-D line. Signals from four separate points on the line, separated by 1 mm, in toluene-seeded nitrogen flows, were imaged onto a fibre-optic array and delivered to separate photo-diode detectors to record their temporal evolution from which the temperatures at each point were derived with a spatial resolution of 1 mm and a precision of 0.7% at atmospheric pressure. Effects of variation of composition on the accuracy of the measurements were compensated by a calibration method providing good agreement with values inferred from thermocouple measurements. Temperature gradients at the boundary between parallel gas flows and at the surface of hot and cold surfaces were measured with a resolution of 5 Kmm-1 . Extension of the technique to more measurement points and improvements in spatial resolution are briefly discussed.

Abstract:

An interband cascade laser (ICL) operating at 3.7 μm has been used to perform multimode absorption spectroscopy, MUMAS, at scan rates up to 10 kHz. Line widths of individual modes in the range 10-80 MHz were derived from isolated lines in the MUMAS signatures of HCl. MUMAS data for methane covering a spectral range of 30 nm yielded a detection level of 30 μbar·m for 1 s measurement time at 100 Hz. Simultaneous detection of methane, acetylene, and formaldehyde in a gas mixture containing all three species is reported.

Abstract:

We demonstrate that optical activity in amorphous isotropic thin films of pure Ge2Sb2Te5 and N-doped Ge2Sb2Te5N phase-change memory materials can be induced using rapid photo crystallisation with circularly polarised laser light. The new anisotropic phase transition has been confirmed by circular dichroism measurements. This opens up the possibility of controlled induction of optical activity at the nanosecond time scale for exploitation in a new generation of high-density optical memory, fast chiroptical switches and chiral metamaterials.