Optimizing two-photon absorption for volumetric optical data storage

We study the requirements imposed on organic photochromes for two-photon absorption (2PA) terabyte volumetric optical storage. We present a quantitative model of signal-to-noise ratio (SNR) and signal-to-background ratio (SBR) when 2PA-induced photochromic switching is used for writing, and 2PA-induced fluorescence is used for readout. We show that single-channel data access rate > 100 MHz at minimum SNR > 4 implies minimum intrinsic 2PA cross section, sigma(2) > 10(3) GM. Resonance enhancement allows sigma(2) similar to 10(5) GM, however, it also lowers SBR due to thermally-activated one-photon absorption. We model the critical trade-off between SNR and SBR as a function of laser frequency, intensity, and temperature. Acceptable parameter space may be achieved by careful choice of the above variables. We perform experiments with nonsymmetrical free-base phthalocyanines, which show efficient 2PA-induced photochromic switching between two tautomer forms and large sigma(2) similar to 10(4) GM, and show good potential for high-capacity data storage. (c) 2007 Optical Society of America.