Laser pulse duration (35-50 fs vs 100-200 fs) effect on probe light generation in femtosecond transient absorption spectroscopy: white light stability, temporal resolution, optics damage.

Have you ever compared the white light stability and the Helios performance with 100 fs and 35 fs pulses from an amplified laser? I am wondering whether there are any drawbacks in choosing a 35 fs amplifier instead of a 100 fs one. Does a 35 fs pulse produce more optics damage or reduces supercontinuum stability? Is the temporal resolution really better with 35 fs pulses?

While we have not noticed any more than usual damage to a non-linear crystal due to a shorter pulse, 35 fs pulses are more difficult to handle. Due to their broad-spectrum 35 fs pulses need to be negatively pre-chirped and their duration is very sensitive to the amount of refractive material in the beam path. This makes working with them more challenging. If a user is inexperienced they may not be able to match the amount of refractive material in the 800 nm beam path before the white light generation in an OPA and the Helios. This will result in a less stable white light in either an OPA or the Helios, or both.

Since ultrashort pulses are much easier to stretch, a few extra lenses or windows will easily push you beyond 200 fs. As a result, the IRF with a 35 fs pulse can be not shorter (and often is longer) than with a 100 fs pulse.

35 fs pulses have a much wider spectrum, which makes the regen stretcher and compressor much more sensitive to the room temperature and the air currents in the lab which translated to the white light instability and higher noise level of the deltaA signal.

Additionally, the spectrally broad OPA output reduces the excitation wavelength selectivity in your experiment.

So in general, we would recommend using 35-50 fs for transient absorption only if you really need better resolution and know how to work with it, while 100 fs amplified systems provide more stability, especially at a multiuser facility.

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