Femtosecond optical spectroscopy is a unique tool for the study of the dynamics of ultrafast light-triggered processes in physics, chemistry, and biology, such as translation or rotation of parts of a molecule, transport of energy or charge, formation or breaking of a chemical bond. Such elementary reactions typically take place on timescales ranging from 10 to 100 fs, which are the times characteristic for the motion of nuclei over interatomic distances, i.e. for molecular vibrations.

The basic time-resolved optical experiment is the so-called “pump–probe”. In this technique a first light pulse, the “pump” pulse, triggers a photoinduced process; the subsequent system evolution is monitored by a delayed “probe” pulse. The temporal resolution of the experiment is determined by the duration of pump and probe pulses, so that very short pulses are necessary in order to observe fast processes. In addition, the need to excite a system on resonance and probe optical transitions occurring at different frequencies requires tunability of both pump and probe pulses. In particular, the bandwidth of the pump pulse should not be too broad in order to enable selective excitation of an optical transition, thus calling for a compromise between temporal and spectral resolution; the probe pulse, on the other hand, should be as short as possible (ideally a transform-limited, d- function white light pulse enables simultaneous measurement of transmission changes at many different wavelengths). Therefore an ideal system for pump–probe spectroscopy should provide pulses that are as short as possible and broadly tunable; in addition, the frequencies of pump and probe pulses should be independently selectable.

Pump-probe schemeWant to know more? Have a look at these publications:

Spectroscopy of biomolecules:

  • Polli, D., Altoè, P., Weingart, O., Spillane, K.M., Manzoni, C., Brida, D., Tomasello, G., Orlandi, G., Kukura, P., Mathies, R.A., Garavelli, M., Cerullo, G. “Conical intersection dynamics of the primary photoisomerization event in vision” (2010) Nature, 467 (7314), pp. 440-443.
  • Polli, D., Antognazza, M.R., Brida, D., Lanzani, G., Cerullo, G., De Silvestri, S. “Broadband pump-probe spectroscopy with sub-10-fs resolution for probing ultrafast internal conversion and coherent phonons in carotenoids” (2008) Chemical Physics, 350 (1-3), pp. 45-55.
  • Cerullo, G., Manzoni, C., Lüer, L., Polli, D. “Time-resolved methods in biophysics. 4. Broadband pump-probe spectroscopy system with sub-20 fs temporal resolution for the study of energy transfer processes in photosynthesis” (2007) Photochemical and Photobiological Sciences, 6 (2), pp. 135-144.
  • Polli, D., Cerullo, G., Lanzani, G., De Silvestri, S., Yanagi, K., Hashimoto, H., Cogdell, R.J. “Conjugation length dependence of internal conversion in carotenoids: Role of the intermediate state” (2004) Physical Review Letters, 93 (16), pp. 163002-1-163002-4.
  • Cerullo, G., Polli, D., Lanzani, G., De Silvestri, S., Hashimoto, H., Cogdell, R.J. “Photosynthetic light harvesting by carotenoids: Detection of an intermediate excited state” (2002) Science, 298 (5602), pp. 2395-2398.

Spectroscopy of carbon nanotubes, nanorods and magnetoresistive manganites:

  • Lüer, L., Hoseinkhani, S., Polli, D., Crochet, J., Hertel, T., Lanzani, G. “Size and mobility of excitons in (6, 5) carbon nanotubes” (2009) Nature Physics, 5 (1), pp. 54-58.
  • Lupo, M.G., Sala, F.D., Carbone, L., Zavelani-Rossi, M., Fiore, A., Lüer, L., Polli, D., Cingolani, R., Manna, L., Lanzani, G. “Ultrafast electron-hole dynamics in core/shell CdSe/CdS Dot/Rod nanocrystals” (2008) Nano Letters, 8 (12), pp. 4582-4587.
  • Polli, D., Rini, M., Wall, S., Schoenlein, R.W., Tomioka, Y., Tokura, Y., Cerullo, G., Cavalleri, A. “Coherent orbital waves in the photo-induced insulator-metal dynamics of a magnetoresistive manganite” (2007) Nature Materials, 6 (9), pp. 643-647

Spectroscopic techniques: