1. Materials for 3D optical storage: two-photon access vs. one-photon background N.S. Makarov, A. Rebane, M. Drobizhev (Department of Physics, Montana State University, Bozeman, MT 59717, USA) H. Wolleb, H. Spahni (Ciba Specialty Chemicals Inc, P.O. Box Ch-4002 Basle, Switzerland)

2. Outline Principles of 3D 2PA optical memory Lack of 2PA-sensitive photochromes 2PA resonance enhancement 2PA vs. 1PA 2PA-sensitive phtalocyanines Summary References

3. Principles of 3D 2PA optical memory hvhv dvdv h dhdh write form Bform A M L L read form Bform A PD DM L L

4. Lack of 2PA-sensitive photochromes Access with 1 pulse: 100fs, 100MHz => 1TB read/write in 22.2 hrs Each bit have to be written and read by only 1 femtosecond pulse! Compound  1, cm 2 (, nm)  2,GM (, nm) FF ABAB Fulgide-based 3.38  10 -16 (650) 2 (780)0.160.045 Spiropyrans-based 8.27  10 -18 (352) 100 (694)0.050.01 Diarylethene-based 1.33  10 -16 (530-600) 70 (750)0.50.4

5. 2PA resonance enhancement A fundamental trade-off between 2PA and 1PA may be formulated as follows: On the one hand, one would like to tune laser frequency as close as possible to the resonance in order to increase useful signal, but on the other hand, one would like to tune as far as possible to decrease detrimental background.

6. 2PA vs. 1PA 10 -4 1 Absorbance, a.u. 05001000150020002500 10 -3 10 -1 10 -5 10 -2 Frequency detuning 1PA - L, cm -1 240K 300K Fluorescence 240K 850-900 nm

7. 2PA-sensitive phtalocyanines Compound  1, cm 2 (, nm)  2,GM (, nm) FF ABAB Pc 3 Nc 4.8  10 -16 (752) 816 (865)0.320.0080 Pc 3 An 5.7  10 -16 (778) 1300 (920)0.260.0138

8. Summary Because of the requirement of fast speed writing and readout, the storage materials need to have high molecular 2PA cross section,  2 >10 3 - 10 4 GM It is evident that the crucial points in this approach are the two-photon sensitivity of a molecule and the possibility of its photochemical transformation from one form to another Careful choice of excitation frequency, along with suitable combination of 1PA and 2PA properties allow minimizing the negative impact of underlying near resonance hot band absorption Our model allows to predict the appropriateness of chromophores for the 2PA-based optical storage

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10. M.E. Marhic, “Storage limit of two-photon-based three-dimensional memories with parallel access”, Opt. Lett., 16, 1272-1273 (1991). “For systems that use parallel access by simultaneous writing or reading of bits located in an entire common plane, diffraction sets a limit to the storage density that is far smaller than that for sequential operation. Comparable densities can be achieved by using a three-dimensional waveguiding structure.”