Ultrafast Spectroscopy

One of the specialties of the Rosenthal group is ultrafast fluorescence upconversion spectroscopy. The Rosenthal group pioneered the use of this technique to study the carrier dynamics of CdSe nanocrystals on the ultrafast time scale. This allows us the ability to study the photogenerated charges virtually in real time.

Our ultrafast titanium:sapphire laser system is a commercial system from Coherent, Inc. It consists of a Verdi V-18 laser which pumps a mode-locked Ti:sapphire laser (Mira 900 Basic), Regenerative Amplifier (RegA 9000) and Optical Parametric Amplifier (OPA 9400). The OPA gives us the ability to produce wavelengths from 400 nm to ~2000 nm by combining the signal, idler, and residual second harmonic pump source.

The experiment itself is probably the most intuitive of the ultrafast laser spectroscopic techniques. The data comes in the form of fluorescence with respect to time, which is analogous to what most commercial fluorimeters would call a kinetics script for observing fluorescence intensity with time. The major difference between the two is that instead of time of observation being counted in minutes, seconds or even milliseconds, we observe fluorescence on the femtosecond (10-15 seconds) time scale.

Ultrafast fluorescence upconversion is a technique that requires two ultra-short laser pulses: an excitation pulse and a gate pulse. The wavelength of the excitation pulse is tuned to the sample's absorption spectrum. The gate pulse acts much like a camera shutter, in that only when the gate pulse is mixed temporally and spatially with the sample fluorescence is a signal observed, much like an image being collected only while the shutter exposes the film to light. Time resolution is achieved by delaying one pulse relative to the other by changing the distance the excitation pulse must travel.

Typical fluorescence upconversion data appears as a sum of decaying exponentials, with the caveat that the shape of the laser pulse must be taken into account. This is necessary because the laser pulse duration is nearly on the order of the time constants for the decay of the fluorescence intensity. The data fitting function is therefore a convolution of a function representing the laser pulse (in our case a Gaussian) and a sum of decaying exponentials.

 

References

Albert D. Dukes III, Michael A. Schreuder, Jessica A. Sammons, James R. McBride, Nathanael J. Smith, and Sandra J. Rosenthal. Journal of Chemical Physics, accepted

Maria Danielle Garrett, Albert D. Dukes III, James R. McBride, Nathanael J. Smith, Stephen J. Pennycook, and Sandra J. Rosenthal: J. Phys. Chem. C, 112, pp. 12736–12746 (2008)

Garrett, M. D., Bowers, M. J., McBride, J. R., Orndorff, R. L., Pennycook, S. J., Rosenthal, S. J. Journal of Physical Chemistry C 112 pp. 436-442 (2008)

Kippeny, T. C.; Bowers, M. J.; Ii; Dukes, A. D.; Iii; McBride, J. R.; Orndorff, R. L.; Garrett, M. D.; Rosenthal, S. J. The Journal of Chemical Physics 128 pp. 084713-7 (2008)