Optical fiber-based single-shot picosecond transient absorption spectroscopy.

A new type of single-shot transient absorption apparatus is described based on a bundle of optical fibers. The bundle contains 100 fibers of different lengths, each successively giving approximately 15 ps longer optical delay. Data are collected by imaging light from the exit of the bundle into a sample where it is overlapped with an electron pulse or laser excitation pulse, followed by imaging onto a charge coupled device (CCD) detector where the intensity of light from each fiber is measured simultaneously. Application to both ultrafast pump-probe spectroscopy and pulse radiolysis is demonstrated. For pulse radiolysis, the prototype bundle provides the ability to collect data with a time resolution limited only by the electron pulse width of 7-10 ps, over a total single-shot time window of approximately 1.5 ns. Tunable probe light is obtained from a titanium-sapphire laser and an optical parametric amplifier. Corrections are made to remove the fiber-to-fiber variations in signal magnitude due to the spatial overlap of the electron beam and probe image. High quality data can be collected over most of the sensitivity range of the CCD camera detectors. The single-shot instrument is valuable for measurement of samples that are only available in very limited quantities, are too viscous to flow, or are rigid. It is therefore excellent in applications, such as picosecond pulse radiolysis, where the thousands of pulses per kinetic trace typical in classical pump-probe experiments can damage the sample before useful results could be obtained.