N. Makarov, A. Rebane, M. Drobizhev, D. Peone

1. Resonance enhancement of two-photon cross-section for optical storage in the presence of hot band absorption N. Makarov, A. Rebane, M. Drobizhev, D. Peone (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) E. Makarova, E. Luk’yanets (Organic Intermediates and Dyes Institute, Moscow, Russia)

2. Outline • Principles of 3D 2PA optical memory • 2PA-sensitive photochromes • Resonance enhancement • 2PA vs. 1PA • 2PA in phthalocyanines • Summary

3. Principles of 3D 2PA optical memory PD DM dv hv M dh hh write read nL nL nL nL form A form B form A form B

4. Need for 2PA-sensitive photochromes Access with 1 pulse: 100fs, 100MHz => 1TB read/write in 24 hrs Each bit have to be written and read by only 1 femtosecond pulse!

5. 2PA resonance enhancement Wavelength, nm 700 800 900 1000 1100 Qx(A) 18000 Qy(A) 16000 14000 12000 s2, GM 10000 8000 6000 4000 long wavelength tail region 2000 0 15000 14000 13000 12000 11000 10000 9000 Frequency, cm-1 • A fundamental trade-off between 2PA and 1PA: • tune laser frequency as close as possible to the resonance • tune as far as possible to decrease 1PA background

6. 2PA vs. 1PA 1 Fluorescence 240K 10-1 4.0 900 nm, a=2.14 ±0.16 10-2 I(P)= Pa 890 nm, a=1.99 ±0.05 3.5 880 nm, a=1.68 ±0.04 Absorbance, a.u. 870 nm, a=1.54 ±0.09 3.0 10-3 860 nm, a=1.36 ±0.10 850 nm, a=1.24 ±0.17 2.5 300K Fluorescence intensity, a.u. 10-4 2.0 240K 1.5 850-900 nm 10-5 1.0 0 500 1000 1500 2000 2500 Frequency detuning n1PA-nL, cm-1 0.5 850 nm 860 nm 870 nm 880 nm 890 nm 900 nm 0.0 1 2 3 4 5 6 7 8 9 Laser pulse energy, a.u. Absorption spectra at different temperatures as calculated from fluorescence spectrum Power dependence of the fluorescence signal

7. 2PA-sensitive phtalocyanines Qy Qy Qx Qx Qy Qx But N N N H N N H Qy Qx+Qy Qx Qy But N N N Qx But 2 6 1 3 4 5

8. 2PA-sensitive phtalocyanines • The change of substituents from butyl groups at -positions to alkoxy groups at -positions (molecule 1 vs. 2) increases 2PA cross-sections by a factor of nearly 2. This also results in the red shift of entire 1PA spectrum by 30 nm (500 cm-1). The 2PA spectrum also experiences the red shift. This shows that addition of oxygen atoms increases -conjugation. • Addition of extra CHO group (molecule 1 vs. 6) results in a slight decrease of 2PA cross-section as compared to better purified compound 1 and in slight increase of the cross-sections compared to 1a and 1b. The 1PA spectrum practically does not change. • Substituting an external benzene ring with another alkoxy group (molecule 4 vs. 6) produces a nearly symmetrical molecule. This shifts both Qx and Qy peaks closer to each other so that they overlap. A similar shift appears in 2PA spectrum. The value of 2PA cross-section reduces by a factor of nearly 2, which is probably because of reduce of the difference in dipole moments in more symmetrical molecule. • Addition of extra hydrogen atoms (molecule 3 vs. 4) reduces degree of symmetry. This slightly increases the 2PA cross-section for molecule 3. However, its cross-section is smaller than for molecules 1 and 6. The reason is more symmetry and thus less difference in dipole moments in the molecule 3 • Change of substituent from molecule 3 to 5 makes the molecule less symmetrical, and thus increase 2PA cross-section. However, molecule 6, and especially 1 have the highest 2PA cross-sections among all studied samples.

9. 2PA-sensitive phtalocyanines: comparison for 3D memory *For molecules 3 and 5 absorption spectra of tautomer forms T1 and T2 significantly overlaps that makes them not practical as photochromes for 3D optical memory

10. SNR-SBR comparison

11. Summary • Because of the requirement of fast speed writing and readout, the storage materials need to have high molecular 2PA cross section, 2>103-104 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 • A brief analysis of changes in 2PA spectra and cross-sections due to different substituent groups is provided and allow to deduce structure-to-properties relations • We conclude that from the set of studied molecules compound 1 is the most promising for rewritable 3D optical memory.

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