Resonance enhancement of twophoton crosssection for optical storage in the presence of hot band abso

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.
Lukyanets (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
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Need for 2PA-sensitive photochromes
Access with 1 pulse 100fs, 100MHz gt 1TB
read/write in 24 hrs Each bit have to be written
and read by only 1 femtosecond pulse!
5
2PA resonance enhancement

• 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

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2PA vs. 1PA
Absorption spectra at different temperatures as
calculated from fluorescence spectrum
Power dependence of the fluorescence signal
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2PA-sensitive phtalocyanines
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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.

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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
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Summary

• Because of the requirement of fast speed writing
  and readout, the storage materials need to have
  high molecular 2PA cross section, ?2gt103-104 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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than that for sequential operation. Comparable
densities can be achieved by using a
three-dimensional waveguiding structure.