Frequency-domain gratings by simultaneous absorption of two photons

Abstract

We study frequency-domain coherence gratings produced by interference in process of simultaneous absorption of two photons in an inhomogeneously broadened medium. In our experiment, we create a grating at a visible wavelength by illuminating an organic dye-doped polymer at low temperature with phase-locked pairs of 100-fs laser pulses at a near-infrared wavelength equal to two times the transition wavelength. We show that two-photon excited coherence can be detected either by measuring the spectrum of fluorescence or, after prolonged illumination, by observing spectrally modulated persistent spectral hole. We analyze the phase and contrast of the grating at different temperatures and obtain, for the first time to our knowledge, temperature dependence of Debye–Waller factor and phonon spectrum of two-photon transition of organic molecule in presence of large inhomogeneous broadening. We also study wavelength dependence of absolute two-photon absorption cross-section in a series of porphyrins and show that at some wavelengths in porphyrin B-band region the cross-section is resonantly enhanced by nearby one-photon allowed Q-transition. These experiments indicate that large organic molecules such as porphyrins are promising candidates for further investigation of multi-photon excitation of coherence.