Persistent spectral hole burning by simultaneous two-photon absorption

We show for the first time persistent spectral hole burning by simultaneous absorption of two 1 138 nm photons in an inhomogeneously broadened S-1 <-- S-0 (Q(y)) transition of unstable photo-tautomer of chlorin in polymer film at low temperature. Spectrally selective hole burning is achieved due to high (10%) quantum efficiency of photo-transformation in that system and high peak intensity (tens of GW/cm(2)) of near-infrared femtosecond pulses used for excitation. Two-photon absorption cross-section is measured for both stable and unstable photo-tautomers to be <sigma>(2) = (0.5 +/- 0.2) x 10(-50) cm(4) s photon(-1). (C) 2001 Elsevier Science B.V. All rights reserved.