Persistent spectral hole burning by simultaneous two-photon absorption

We show for the first time persistent spectral hole burning by simultaneous absorption of two 1138 nm photons in an inhomogeneously broadened S1←S0 (Qy) 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/cm2) of near-infrared femtosecond pulses used for excitation. Two-photon absorption cross-section is measured for both stable and unstable photo-tautomers to be σ2=(0.5±0.2)×10−50 cm4 s photon−1.