Femtosecond pump-probe spectroscopy is a powerful technique to study the primary photoinduced dynamical processes in molecules. In its standard implementation it requires macroscopic samples, so that the experimental results are averaged over many mesoscopic domains. For many applications, however, it is required to study single nano/micrometer-sized objects or to map the inhomogenous distribution of structured samples in order to correlate the physical processes with the sample morphology. For this purpose we have developed a new instrument combining broadband femtosecond pump-probe spectroscopy and confocal microscopy, with high temporal (150fs) and diffraction-limited spatial (300nm) resolution, acquiring four-dimensional differential-transmission maps as a function of x-y sample position, probe delay and wavelength in the visible to near-IR region.
Experimental setup of the ultrafast confocal microscope. BS1, BS2, BS3: beam splitters; SHG: second-harmonic generation crystal; IF1, IF2: interference fi lters; OBJ: microscope objective; PZT: piezotranslator; PMT: photomultiplier.
We have successfully applied this instrument to study polymer blends for organic solar-cell devices. One of the major technological challenges in organics optoelectronics is the control of the physical processes occurring at the molecular interface between materials that form the bulk heterojunction, which determine the ultimate device performance. Such phenomena are poorly understood due to the variety of possible electronic states and processes and to their complicated dynamics, requiring a combination of high spatial and temporal resolution to be observed. Our recent results:
- Thin films of a polyfluorene dispersed in PMMA, where pump-probe dynamical imaging reveal different photorelaxation pathways, highlighting PFO domains with aggregated/isolated chains in the center/border of the region. Reference: Polli, D., Grancini, G., Clark, J., Celebrano, M., Virgili, T., Cerullo, G., Lanzani, G. “Nanoscale imaging of the interface dynamics in polymer blends by femtosecond pump-probe confocal microscopy” (2010) Advanced Materials, 22 (28), pp. 3048-3051.
Pump-probe maps at λ = 510 nm (a) and λ = 640 nm (b) probe wavelengths at τ = 200 fs delay; scalebar: 2 μm. (c,d) pump-probe dynamics obtained at sample positions “A” and “B”, representative of the isolated and aggregated PFO regions, respectively.
- A donor/acceptor bulk heterojunction sample, where a peculiar charge-transfer state is pinpointed at the interface between crystals. Reference: Grancini, G., Polli, D., Fazzi, D., Cabanillas-Gonzalez, J., Cerullo, G., Lanzani, G., “Transient absorption imaging of P3HT:PCBM photovoltaic blend: Evidence for interfacial charge transfer state” (2011) Journal of Physical Chemistry Letters, 2 (9), pp. 1099-1105.
Pump-probe image on a 5μm x 5μm region of a P3HT:PCBM blend, showing the charge-transfer state identified at the interface between the domains.