Ultrafast metrology

3D Space-time Ultrafast Holography

Optical pulse in 3D: Optical pulse cross-section with complex space-time structure - the pulse shape and arrival time depends non-trivially on transverse position.

The most important aspect of any experiment is the ability to measure a signal that depends on the feature that one is trying to study. In ultrafast science, one is attempting to observe dynamical systems on a sub-picosecond or even sub-femtosecond timescale using optical pulses of a similar or shorter brevity. These timescales are too short to be observed by conventional electronic means, thus one has to resort to advanced metrological methods. Typically these methods are based around measuring the pulse spectrum (the range of frequencies or colours present in the pulse) and the spectral phase (the arrival time of each colour with respect to each other). The shortest pulse is obtained when all the colours arrive simultaneously, and therefore the spectral phase is equal for all frequencies. Spatial holography recods the spatial phase, i.e. distance. In ultrfast optics, we desire the temporal information for all spatial positions.

SEA-SPIDER concept: Concept of SEA-SPIDER measurements that can be used to measure the spatially resolved pulse shape, thus recovering space-time couping.

Another consequence of the short pulse duration is that the electric field strength at the peak of the pulse can become very strong, even for modest pulse energies. These pulses can exhibit nonlinear effects in the medium as the pulse propagates, and can result in the pulses generating more frequencies, solitonic behaviour, self-focusing or even self-confinement, giving rise to a wide-range of applications including long-distance pulse propagation and pulse compression (i.e. further reducing the pulse duration). However, the consequence of this nonlinear interaction between the optical field and the propagating medium is that the pulses become much more complex and the spatial and temporal degrees of freedom become coupled, such that the temporal profile of the pulse can depend in a non-trivial way on the transverse position within the pulse (figure 1). This places much more stringent demands on the metrology required to extract useful information about the pulse.

One aspect of our research is to develop advances in complete ultrashort pulse metrology that enable the measurement of these complex pulses in full space and time, requiring the additional measurement of the spectrally dependent spatial wavefront. Using this information one should be able to gain further understanding into the complex nonlinear dynamics of particular interactions and ultimately lead to control of the process towards desired goals. Our technique is based on shearing interferometry[1] – the relative arrival times of adjacent frequencies are measured by interfering two spectrally shifted replicas of the pulse being measured, thus providing the spectral dispersion of the pulse (i.e. spread in arrival times). The spatial wavefront is simultaneously measured in a similar manner – adjacent positions of the wavefront of two identical replicas of the pulse are interfered to provide the local wavefront curvature. Spectrally and spatially resolving both measurements provides a complete measurement of the field. Initial two dimensional (time plus one transverse spatial coordinate) measurements are shown in figure 2.

[1] I. A. Walmsley and C. Dorrer. Characterization of ultrashort electromagnetic pulses. Adv. Opt. Photon., 1:308–437, 2009.