Over the last decade there have been spectacular developments in ultrafast laser technology, due to the introduction of solid state active materials and of new mode-locking and amplification techniques. These advances, together with the discovery of new nonlinear optical crystals, have fostered the introduction of ultrafast optical parametric amplifiers as a practical source of femtosecond pulses tunable across the visible and infrared spectral ranges. This article summarizes the recent progress in the development of ultrafast optical parametric amplifiers, giving the basic design principles for different frequency ranges and in addition presenting some advanced designs for the generation of ultrabroadband, few-optical-cycle pulses. Finally, we also briefly discuss the possibility of applying parametric amplification schemes to large-scale, petawatt-level systems.

Time-resolved absorption spectroscopy on the femtosecond time scale has been used to monitor the earliest events associated with excited-state relaxation in tris-(2,2′-bipyridine)ruthenium(II). The data reveal dynamics associated with the temporal evolution of the Franck-Condon state to the lowest energy excited state of this molecule. The process is essentially complete in ∼300 femtoseconds after the initial excitation. This result is discussed with regard to reformulating long-held notions about excited-state relaxation, as well as its implication for the importance of non-equilibrium excited-state processes in understanding and designing molecular-based electron transfer, artificial photosynthetic, and photovoltaic assemblies in which compounds of this class are currently playing a key role.

Graphene is emerging as a viable alternative to conventional optoelectronic, plasmonic and nanophotonic materials. The interaction of light with charge carriers creates an out-of-equilibrium distribution, which relaxes on an ultrafast timescale to a hot Fermi-Dirac distribution, that subsequently cools emitting phonons. Although the slower relaxation mechanisms have been extensively investigated, the initial stages still pose a challenge. Experimentally, they defy the resolution of most pump-probe setups, due to the extremely fast sub-100 fs carrier dynamics. Theoretically, massless Dirac fermions represent a novel many-body problem, fundamentally different from Schrödinger fermions. Here we combine pump-probe spectroscopy with a microscopic theory to investigate electron–electron interactions during the early stages of relaxation. We identify the mechanisms controlling the ultrafast dynamics, in particular the role of collinear scattering. This gives rise to Auger processes, including charge multiplication, which is key in photovoltage generation and photodetectors.

Pull, pull, pulling electrons along: Organic photovoltaics operate by transferring charge from a light-absorbing donor material to a nearby acceptor. Falke et al. show that molecular vibrations smooth the way for this charge transfer to proceed. A combination of ultrafast spectroscopy and theoretical simulations revealed an oscillatory signal in a model donor/acceptor blend that implicates carbon-carbon bond stretching in concert with the electronic transition. This vibrational/electronic, or vibronic, process maintains a quantum-mechanical phase relationship that guides the charge more rapidly and directly than an incoherent migration from donor to acceptor. Science , this issue p. 1001