Femtosecond resolution of the fluorescence is obtained using the upconversion technique. The sample is excited and its fluorescence is mixed with that of a femtosecond laser pulse (called the gate beam) in a sum-frequency generating crystal. The resultant signal is detected using a PMT. By varying the delay between the gate beam and the fluorescence, the dynamics of the fluorescence can be observed.

Recently this technique has been used to study both plant and bacterial photosynthetic complexes as well as single-walled nanotubes.

References:

• Carotenoid to Chlorophyll Energy Transfer in Light Harvesting Complex II from Arabidopsis thalaina Probed by Femtosecond Fluorescence Upconversion.  N. E. Holt, J. T. M. Kennis, L. dall’Osto, R. Bassi and G. R. Fleming.  Chem. Phys. Lett. , 379, 305-319, 2003.
  
• Evidence for Direct Carotenoid Involvement in the Regulation of Photosynthetic Light Harvesting, Y.-Z Ma, N. Holt, X.-P. Li, K.K. Niyogi, and G. R. Fleming. Proc National Acad Sci USA. 100, 4377-4382 (2003).
  
• Light Harvesting by Chlorophlls and Carotenoids in the Photosystem I Core Complex of Synechococcus Elongatus : A Fluorescence Upconversion Study , J. T. M. Kennis, B.Gobets, I. H. M. van Stokkum, J. P. Dekker, R. van Grondelle, and G. R. Fleming, J. Phys. Chem., 105, 4485-4494 (2001).
• Excitation Energy Transfer in Dimeric Light-Harvesting Complex I: A Combined Streak-Camera/Fluorescence Upconversion Study , B. Gobets, J. T. M. Kennis, J. A.Ihalainen, M. Brazzoli, R.Croce, I H.M. van Stokkum, R. Bassi, J. P. Dekker, H. van Amerongen, G. R. Fleming, and R. van Grondelle, J Phys Chem B., 105, 10132-10139 (2001).
• Fs-transient Absorption and Fluorescence Upconversion after Two-Photon Excitation of Carotenoids in Solution and in LHC II, P. J. Walla, P. A. Linden and G. R. Fleming, In Ultrafast Phenomena XII (Springer), 671-673 (2001).
• Two Photon Excitation Spectrum of LHC II and Fluorescence Upconversion After One-and Two-Photon Excitation of the Carotenoids , P. J. Walla, J. Yom, B. P. Krueger, and G. R. Fleming. J. Phys. Chem. B, 104, 19, 4799-4806 (2000).
• Fluorescence Upconversion and ab initio Studies of the Light Harvesting Function of Carotenoids in Bacterial Light Harvesting Antenna, B.P. Krueger, G.D. Scholes, I.R. Gould, and G.R. Fleming. Ultrafast Phenomena XI, Springer, 666-668 (1998).
• Ultrafast Energy Transfer in the Core Light Harvesting Complex of the Photosynthetic Bacterium Rhodobacter Sphaeroides Observed by Flourescence Upconversion. S. Bradforth, R. Jimenez, V. Fidler, G. Fleming, S. Nagarajan, J. Norris, F. van Mourik and R. van Grondelle. In: Ultrafast Phenomena IX, Vol. 60, Springer, New York (1994) pp. 441-442.
• Fluorescence Upconversion Study of Cis-Stilbene Isomerization , D.C. Todd, J.M. Jean, S.J. Rosenthal, A.J. Ruggiero, D. Yang and G.R. Fleming, J. Chem. Phys. 93, 8658 (1990).