Fs photo-isomerization in bacteriorhodopsin by few-cycle pulses

We have build up non-collinear optical parametric amplifier (NOPA) laser system with adaptive pulse tailoring, which enables to generate sub-4-fs laser pulses. Using the ultrashort laser pulses generated by NOPA, we have performed pump-probe measurement of bacteriorhodopsin (bR) using ultrashort pulses, which can observe real-time frequencies of molecular vibrations. Both of the spectra of pump and probe pulses are nearly the same and cover a spectral range from 500 to 710 nm with a nearly constant phase. The time-gated Fourier transform of the measured absorbance change trace revealed that the frequency of the C=C stretching mode was modulated along with the torsion motion around C13=C14 during the photoisomerization of bR. It shows the bond length of C13=C14 is modulated on the deformation around C13=C14 during the photoisomerization. The hydrogen-out-of-plane (HOOP) mode and the in-plane C=C-H bending mode coupled with C-C stretching mode are found to be intermixed with each other in the same period of the torsion motion. It also shows us the real-time conformation change of the molecule, which makes both of the modes indistinguishable