Femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) is used as a probe for monitoring the vibrational dynamics of room temperature ionic liquids (ILs). The experiments are performed on a series of 1,3-dialkylimidazolium ILs containing the bis(trifluoromethylsulfonyl)imide [NTf2] anion. The effect of methylation of the cationic C2 position on the dephasing time is studied analyzing [NTf2]-ILs of 1-ethyl-3-methylimidazolium [EMIM], 1-ethyl-2,3-dimethylimidazolium [EMMIM], 1-butyl-3-methylimidazolium [BMIM], and 1-butyl-2,3-dimethylimidazolium [BMMIM]. Raman coherences are excited around ∼1400 cm(-1), and the vibrational dephasing of the modes in the fingerprint region is monitored as a function of time. The results indicate that vibrational energy transfer occurs governed by the interionic interactions. This is suggested by mode beating involving vibrations beyond the excitation spectrum as well as systematic differences in the temporal dephasing behavior. In contrast, the length of the cationic alkyl side chain has a negligible impact on the vibrational dynamics.

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Accessing ultrafast photoinduced molecular dynamics on a femtosecond time-scale with vibrational selectivity and at the same time sub-diffraction limited spatial resolution would help to gain important information about ultrafast processes in nanostructures. While nonlinear Raman techniques have been used to obtain highly resolved images in combination with near-field microscopy, the use of femtosecond laser pulses in electronic resonance still constitutes a big challenge. Here, we present our first results on coherent anti-Stokes Raman scattering (fs-CARS) with femtosecond laser pulses detected in the near-field using scanning near-field optical microscopy (SNOM). We demonstrate that highly spatially resolved images can be obtained from poly(3-hexylthiophene) (P3HT) nano-structures where the fs-CARS process was in resonance with the P3HT absorption and with characteristic P3HT vibrational modes without destruction of the samples. Sub-diffraction limited lateral resolution is achieved. Especially the height resolution clearly surpasses that obtained with standard microCARS. These results will be the basis for future investigations of mode-selective dynamics in the near field.

Molecules are known to change properties when in contact with metal surfaces. Therefore, dynamics of photoinduced molecular excitons in a semiconductor also are expected to be influenced by a metal contact. This effect, which is of considerable interest also for applications, is limited to interface excitons generated within just a few nanometer proximity to a metal layer. Up to now, however, a highly localized access to such excitonic events has not been presented, and diffraction-limited microspectroscopy did not yield any pattern in exciton dynamics other than that of bulk excitons, irrespective of an existing metal interface. In our work, we have combined femtosecond time-resolved spectroscopy with scanning near-field optical microscopy (SNOM) to study the interfacial dynamics of a gold-poly­(3-hexylthiophene) system (Au–P3HT) making use of tip-enhancement of the light fields of the ultrashort laser pulses by a gold rim surrounding the SNOM fiber tip, which collects the signal light. Next to annihilation of free excitons in P3HT, which is an efficient loss mechanism at the laser powers employed, a direct exciton decay highly confined within the near-field range right at the Au–P3HT interface has been observed. We show that the occurrence of the gold-coated SNOM-tip-induced near-field enhancement of the optical fields permits selective access to the highly confined interfacial exciton decay. The experiments reveal that the early ultrafast loss of charge pairs in P3HT becomes significantly faster at the Au–P3HT interface because of an additional pathway.

A Toffoli logic gate (CCNOT gate) is a universal reversible logic gate from which all other reversible gates can be constructed. It has a three-bit input and output. The goal of our work was to realize a Toffoli gate where all inputs and outputs are realized optically, which allows for ultrafast switching processes. We demonstrate experimentally that a Toffoli logic gate can be created based on nonlinear multi-wave interactions of light with matter. Using femtosecond laser pulses, the all-optical Toffoli gate is based on the coherence of the optical signals produced via the nonlinear optical processes. Sum frequency (SF) and second harmonic (SH) generations are combined in such a way so as to yield the complete truth table of the universal reversible logic gate.