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 fil
m at low temperature. Spectrally selective hole burning is achieved due to
high (10%) quantum efficiency of photo-transformation in that system and hi
gh peak intensity (tens of GW/cm(2)) of near-infrared femtosecond pulses us
ed 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
.