In TA spectroscopy, a tuneable ca.100 fs ‘pump’ pulse is used to initially populate higher energy excited states. A second ‘probe’ white light pulse is then used to measure the differential absorption of these species. Detection wavelengths currently available span the Visible (420-810 nm) and Near Infra-Red (850-1600 nm) regions. Using a continuum based white light source, we are able to extend the detection time window from ~200 fs (FHWM IRF) to ~1 msec.

Types ofpolarized waves

The UQ-PULSE laboratories were commissioned in May 2014, facilitating the study of molecular excited state dynamics on very short timescales.

Light can be described as an electromagnetic wave traveling through space. For purposes of ellipsometry, it is adequate to discuss the waves’s electric field behavior in space and time, also known as polarization. The electric field of a wave is always orthogonal to the propagation direction. Therefore, a wave traveling along the z-direction can be described by its x- and y- components. When the light has completely random orientation and phase, it is considered unpolarized. For ellipsometry, however, we are interested in the kind of electric field that follows a specific path and traces out a distinct shape at any point. This is known as polarized light. When two orthogonal light waves are in-phase, the resulting light will be linearly polarized. The relative amplitudes determine the resulting orientation. If the orthogonal waves are 90° out-of-phase and equal in amplitude, the resultant light is circularly polarized. The most common polarization is “elliptical”, one that combines orthogonal waves of arbitrary amplitude and phase. This is where ellipsometry gets its name.

Our system is based on a 1 kHz 4W amplified Ti:Sapphire laser (Spectra Physics - Spitfire ACE) and Optical Parametric Amplifier (Light Conversion – Topas Prime) providing gap free sample photoexcitation from 240-2600nm.Various detection systems are available allowing transient absorption (Ultrafast Systems – HELIOS) and time resolved fluorescence (Ultrafast Systems – HALCYONE) measurements.

Polarized wavesin physics

Alternatively, fluorescent samples can be analysed using Femtosecond Optically Gated (FOG) lifetime methods. A tuneable ca. 100 fs ‘pump’ pulse is used to excite the sample, and the resulting emission is mixed with a second ‘gate’ pulse in a non-linear crystal, yielding an upconverted signal and providing the best available time resolution. Using this technique, fluorescence lifetimes with femtosecond time resolution can be obtained. Longer lived samples (> 2 ns) can be analysed using more traditional Time Correlated Single Photon Counting (TCSPC) approaches.

Ultrafast spectroscopy is used to measure the kinetics for a variety of important photophysical processes in chemical and biological research, including electronic structure, photoisomerizations, energy and/or electron transfer, charge transport, optical non-linear effects, and many other processes.

The primary techniques available include ‘pump-probe’ Transient Absorption (TA) and Femtosecond Optically Gated (FOG) fluorescence lifetimes.