|
A novel ultra-broadband transient spectrometer with millisecond
measurement range
We use a Ti:sapphire CPA femtosecond laser system for the generation of intense, tunable pump pulses (e.g. by noncollinear optical parametric amplification: NOPA) and ultrabroadband probe pulses from a supercontinuum fiber laser (SCFL) to probe transient dynamics on the time scale of nanoseconds to seconds. Since we use two independent laser sources for pump and probe pulses, the measurement time window is not limited by mechanical delay lines. The advantage over a classical laser flash setup is the largely reduced thermal load on the sample and the improved time resolution. The supercontinuum probe pulse allows us to record transient spectra from 450 to 2000 nm (presently limited by the photodetector) without changes to the setup.
Due to the different repetition rates of CPA (1 kHz) and SCFL (24 kHz) each excitation pulse is followed by 24 probe pulses. Since the probe pulses are low in energy, they do not affect the photodynamics of the sample and not just the first, but all 24 pulses can be used to probe the excitation induced dynamics. For each probe pulse the time delay with respect to the pump pulse is measured. To allow for data averaging, the time range is divided into time bins. All transmission measurements with time delays that fall into the same bin are averaged. Due to the asynchronous relation between the CPA and the SCFL the pulses populate the bins in an arbitrary order. This can be compared to a fast scan procedure [J. A. Moon., Rev. Sci. Instrum. 64, 1775 (1993)]. The single shot transmission measurements that are averaged in each time bin are thus not correlated, and averaging yields a true 1/sqrt(N) noise reduction.
The temporal width of the bins gives the time resolution of the measurement. To complete the transient measurement in a reasonable amount of time (5 min.), this width cannot be chosen arbitrarily small as a certain number of probe pulses (N ~ 2000) are required to fall in each bin for sufficient noise reduction. For a measurement time window of 800 µs, a bin width of 0.5 – 1 µs proved reasonable.
 
In our setup we presently use single channel detection to record single time traces at different wavelengths. The spectral selection is achieved through a motorized monochromator. Transient spectra can be reconstructed from time traces by choosing a suitably narrow wavelength step. Time traces at 56 equally spaced wavelengths in the VIS and NIR spectral range from 450 to 1000 nm have been measured to reconstruct the time evolution of the transient spectrum of Nd:YAG.
| 
