communications Figure 6. a) SEM image of the free-standing silver pillar on the cover slip, made by using a laser power of 1.14 mW and a linear scanning speed of 3 mm s À1 , taken at an observation angle of 45 8. The inset is a close-up view of the silver pillar parallel to the substrate, which demonstrates the linewidth of the smallest portion of the silver pillar as 180 nm. b) SEM image of silver pyramids, fabricated with a laser power of 1.3 mW and scanning speed of 2.5 mm s À1 , taken at an observation angle of 45 8. The inset on the left is a top view of the silver-pyramid array. The inset on the right is a close-up view of the silver pyramid.

The lateral spatial resolution (LSR) in two-photon induced polymerization was improved to 80nm by using an anthracene derivative (9,10-bis-pentyloxy-2,7-bis[2-(4-dimethylamino-phenyl)-vinyl]anthracene (BPDPA)) as a highly sensitive and efficient photoinitiator. Photocurable resin containing 0.18mol% BPDPA exhibited a low polymerization threshold of 0.64mW at 800nm. Theoretical calculations showed that the LSR can be increased by reducing the laser power, indicating that the LSR could be improved using more sensitive initiators in the future.

A series of five fluorescent and ionic dimethylamino stilbazolium homologues with increasing conjugation length (from ethenyl to decapentaenyl) is investigated by high-frequency, amplitude-modulated femtosecond hyper-Rayleigh scattering at 1300 nm. A hyperpolarizability value that is not overestimated by the presence of a multiphoton fluorescence contribution is obtained from the Fourier analysis of the hyper-Rayleigh scattering signal. The demodulation curve (decrease of Fourier amplitude versus modulation frequency) is characterized by both the hyperpolarizability value and the fluorescence decay parameters. The fluorescence decay parameters are accurately determined independently by single-photon counting. A detailed analysis of the fluorescence decay parameters from the hyper-Rayleigh scattering demodulation curve and of their relation to the fluorescence decay parameters obtained from single-photon counting experiments is presented. The inherent hyperpolarizability value for these chromophores shows a maximum of (2045 Ϯ 35) ϫ 10 Ϫ30 esu or (760 Ϯ 13) ϫ 10 Ϫ50 C 3 m 3 J Ϫ2 for the hexatrienyl conjugation length. A comparison with theoretical calculations suggests the importance of trans-cis isomerization in the excited state.

Second-order hyperpolarizabilities (β), electron distribution, dipole moment, and excitation energy together with oscillator strength of stilbazolium cations have been investigated by a semiempirical calculation with PM3 parametrization of the MNDO Hamiltonian. Since stilbazoliums were found to generally have several times larger β value than nitrostilbene derivatives with similar experimental absorption maximum wavelength, such ionic species are concluded to be advantageous to create new second-order nonlinear optical materials having large β and short cutoff. Among the stilbazolium derivatives investigated, the β values increase with increasing electron-donating ability of the substituent at the para position of benzene ring, and exceptionally chlorinated one was found to have comparatively large β value withstanding its short absorption maximum wavelength. The main reason for the large β values of stilbazolium cations is the strong acceptor property of the pyridinium ring resulting in large difference in dipole moment between the ground and excited states, which could be shown using the two-level model. The tendency of calculated β values of stilbazolium cations shows good agreement with that obtained from the hyper Rayleigh scattering (HRS) measurement.

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The second-order hyperpolarizabilities (0) of two categories of organic ionic species were evaluated by the hyper Rayleigh scattering method. The 101 of (dimethy1amino)stilbazolium analogues in methanol with long cut-off wavelength more than 600 nm, which can be used for electro-optic modulation, were evaluated to be in the order of esu using 1064 nm light as a fundamental beam. Aromatic sulfonates in the second category were found to have comparatively large 101 values irrespective of short cut-off wavelength: In the case of sodiump-toluenesulfonate with cut-off of 280 nm, 101in methanol was 2 . 2~1 0 '~~ esu and was about two thirds ofp-nitroaniline with cut-off of 445 nm.