Pump-probe phase spectroscopy with submilliradian sensitivity and nanosecond time delay using Michelson interferometers.

Using two Michelson interferometers, we describe an experimental scheme for sensitive pump-probe spectral interferometry measurements at long time delays. It has practical advantages over the Sagnac interferometer method typically used when long-time delays are required. First, with the Sagnac interferometer, achieving many nanosecond delays requires expanding the size of the interferometer so that the reference pulse arrives before the probe pulse. Because the two pulses still pass through the same region of the sample, long-lived effects can still affect the measurement. In our scheme, the probe and reference pulses are spatially separated at the sample, alleviating the need for a large interferometer. Second, in our scheme, a fixed delay between probe and reference pulses is straightforward to produce and is continuously adjustable while maintaining alignment. Two applications are demonstrated. First, transient phase spectra are presented in a thin tetracene film with up to 5 ns probe delay. Second, impulsive stimulated Raman measurements are presented in Bi4Ge3O12. The signal-to-noise using the double Michelson technique is comparable to previously described methods with the added advantage of arbitrarily long pump-probe time delays.

Characterization of Interfacial Structure in Polymer-Fullerene Bulk Heterojunctions via

We introduce a new application of solid state NMR measurements towards characterizing the donor-acceptor interfaces within bulk heterojunction (BHJ) films. Rotational echo double resonance (REDOR) is used to measure dipolar couplings between ^{13}C nuclei on the acceptor phenyl-C_{61}-butyric acid methyl ester (PCBM) fullerene cage, which is ≈18% isotopically enriched with ^{13}C, and beta hydrogens on the donor poly(3-hexyl thiophene) (P3HT) main chain, which are >95% isotopically enriched with ^{2}H. The ^{13}C-^{2}H dipolar couplings are used for constraining possible models of molecular packing in the amorphous mixed phase of a P3HT/PCBM BHJ. The films studied are highly mixed (>80%) and have a maximum length scale of composition nonuniformity of ≈6 nm in the mixed phase, as demonstrated by ^{1}H spin diffusion NMR and supported by TEM. The REDOR results show that despite the lack of phase separation at length scales greater than ≈6 nm, neat P3HT and PCBM clusters exist on ≈3 nm size scales, and, for the average PCBM molecule, the number of nearest neighbors P3HTs is two.