Search

Many of the scientific applications for X-ray free-electron lasers seek to exploit the ultrashort pulse durations of intense X-rays to obtain femtosecond time resolution of various processes in a "pump-probe" scheme. One of the limiting factors for such experiments is the timing jitter between the X-rays and ultrashort pulses from more conventional lasers operating at near-optical wavelengths. In this work, we investigate the potential of using X-ray-induced changes in the optical second harmonic generation efficiency of a nonlinear crystal to retrieve single-shot arrival times of X-ray pulses with respect to optical laser pulses. Our experimental results and simulations show changes to the efficiency of the second harmonic generation of 12%, approximately three times larger than the measured changes in the transmission of the 800 nm center-wavelength fundamental pulse. Further experiments showing even larger changes in the transmission of 400 nm center-wavelength pulses show that the mechanism of the second harmonic generation efficiency modulation is mainly the result of X-ray-induced changes in the linear absorption coefficients near 400 nm. We demonstrate and characterize a cross-correlation tool based on this effect in reference to a previously demonstrated method of X-ray/optical cross-correlation.

We show that it is possible to phase multidimensional infrared spectra generated by a boxcars geometry four-wave mixing spectrometer directly from the signal generated by the molecular vibration of interest, without the need for auxiliary phasing measurements. For isolated vibrations, the phase profile of the 2D response smoothly varies between fixed phase limits, allowing for a general target for phasing independent of the degree of anharmonicity exhibited between the ground and excited state. As a proof of principle, the 2D response of the ∼2155 cm -1 thiocyanate stretch vibration of MeSCN, a system exhibiting anharmonicity such that the 0-1 and 1-2 transitions are spectrally isolated, is successfuly phased directly from the experimental spectra. The methodology is also applied to correctly phase extremely weak signals of the unnatural amino acid azidohomoalanine following background subtraction.

The supercontinuum generated with a linearly polarized near-IR (775 nm) pump in rotated calcium fluoride is shown to have intrinsic intensity and polarization modulations. To mask the rotation of the crystal plate, we circularly polarize the pump and find greatly improved output parameters for the generated white light: intensity fluctuations of 0.5% limited only by pump laser stability, and a circular polarization state-matching that of the pump-over the entire visible spectrum. This polarization conservation allows the return of the supercontinuum to a linear polarization state or to a pair of linearly polarized beams with correlated intensity fluctuations. We were also able to extend the supercontinuum source deep into the ultraviolet with a frequency doubled (387 nm) pump, to serve as an new source to probe the region where most molecular photochemistry occurs.