Journal of Luminescence 98 (2002) 341–353 Frequency-domain gratings by simultaneous absorption of two photons Aleksander Rebane*, Mikhail Drobizhev 1 , Aliaksandr Karotki Department of Physics, Montana State University, Bozeman, MT 59717-3840, USA 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. r 2002 Elsevier Science B.V. All rights reserved. PACS: 42.65.k; 33.80.Wz Keywords: Two-photon absorption; Nonlinear spectroscopy of molecules 1. Introduction Study of coherent nonlinear optical effects such as time-and-space-domain holography, photon echo, spectral hole burning, multi-photon absorp- tion, electromagnetically induced transparency, stopping (and accelerating) of light pulses in a nonlinear medium etc. gives valuable insights into the physics of the interaction between atoms (or molecules) and light. This research is also paving a road for future advances in technology, especially in high-speed high-capacity data storage, ultra stable frequency- and time standards, as well as new ways of performing computations. The principle of time-and-space-domain holo- graphy is based on frequency-domain interference, which occurs due to coherence excited in an absorbing medium, by time-delayed object- and reference pulses [1–8]. Depending on the relative phase between different components of the field *Corresponding author. Tel.: +1-406-994-7831; fax: +1- 406-994-4452. E-mail address: rebane@physics.montana.edu (A. Rebane). 1 Permanent address: Lebedev Physics Institute, Leninsky pr., 53, 119991 Moscow, Russia. 0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0022-2313(02)00289-2