Harald Röckel

Two-photon absorption and broadband optical limiting with bis-donor stilbenes

Large two-photon absorptivities are reported for symmetrical bis-donor stilbene derivatives with dialkylamino or diphenylamino groups. These molecules exhibit strong optical limiting of nanosecond pulses over a broad spectral range in the visible. Relative to bis(di-n-butylamino)stilbene, bis(diphenylamino)stilbene exhibits a 90-nm red shift of its optical limiting band but only a minimal shift of ~13 nm of its lowest one-photon electronic absorption band. Mixtures of these compounds offer an unprecedented combination of broad optical limiting bandwidth and high linear transparency.

New photopolymers based on two-photon absorbing chromophores and application to three-dimensional microfabrication and optical storage

Molecules exhibiting strong two-photon absorption hold great potential for a wide range of applications including two-photon fluorescence imaging, three-dimensional (3D) optical data storage, and 3D microfabrication. The authors have observed two-photon absorptivities as high as 1,500 x 10{sup {minus}50} cm{sup 4} s/photon in bis-donor diphenylpolyene derivatives that are correlated to simultaneous charge transfer from the end groups to the polyene bridge in the molecule. Many of these molecules are also excellent photoexcitable electron donors that can initiate charge-transfer reactions with acrylate monomers. Marcus theory is used to describe the efficiency of these charge-transfer reactions. Polymerization rates have also been measured and they show that these two-photon chromophores display increased sensitivity and recording speed over conventional UV photo-initiators. The fabrication of complex, three-dimensional structures by two-photon polymerization is demonstrated and discussed in the context of advanced photonic applications.

Transillumination imaging through scattering media by use of photorefractive polymers

We demonstrate the use of a near-infrared-sensitive photorefractive polymer with high efficiency for imaging through scattering media, using an all-optical holographic time gate. Imaging through nine scattering mean free paths is performed at 800nm with a mode-locked continuous-wave Ti:sapphire laser.

Two-Photon Absorbing Organic Chromophores for Optical Limiting

Strong optical limiting and large two-photon absorptivities are reported for a class of bisdonor diphenylpolyene derivatives with varying polyene bridge lengths. These molecules exhibit strong optical limiting using nanosecond pulses over a broad spectral range. Bis(diphenylamino)stilbene exhibits a 90 nm red shift of its optical limiting band, and only a minimal shift of about 13 nm of its lowest one-photon electronic absorption band relative to bis(di-n-butylamino)stilbene. This suggests a potential for broadband optical limiting with high transparency using mixtures of such compounds. Pulse width dependent nonlinear transmission measurements suggest that two-photon pumped excited-state absorption contributes significantly to the limiting of nanosecond pulses.

Theoretical Design of Organic Chromophores with Large Two-Photon Absorption Cross-Sections

Design strategies and structure-property relationships for two-photon absorption in conjugated molecules are described on the basis of correlated quantum-chemical calculations. We first focus on stilbene derivatives with centrosymmetric structures. We found that derivatization of the conjugated molecule with electroactive groups in a quadrupolarlike arrangement leads to a large increase in the two-photon absorption cross section, δ. Quantum-chemical description provides rich insight into the mechanisms for the two-photon absorption phenomenon.

Two-photon polymerization initiators for efficient three-dimensional optical data storage and microfabrication

Summary form only given. Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has enabled the development of 3D fluorescence imaging, 3D optical data storage, and 3D lithographic microfabrication. Each of these applications takes advantage of the fact that the two-photon absorption probability depends quadratically on intensity, and therefore under tight-focusing conditions, the absorption is confined at the focus to a volume of order /spl lambda//sup 3/, where /spl lambda/ is the laser wavelength. Any subsequent process, such as fluorescence or a photo-induced chemical reaction, is also localized in this small volume. For instance, two-photon excitation can initiate conventional reactions such as side-group deprotection, radical generation, and polymerization, through energy transfer or electron transfer. However, the efficiency of such processes depends critically on the strength of the chromophores two-photon absorptivity. We have developed a wide array of chromophores which hold great promise for 3D optical data storage and 3D microfabrication. These materials are based on donor-/spl pi/-donor, donor-acceptor-donor, or acceptor-donor-acceptor structural motifs.

Two photon absorbing chromophores for broadband optical limiting

Summary form only given. Optical limiters are devices that strongly attenuate intense optical beam while exhibiting high transmittance for low intensity ambient light levels. General requirements for a material to be a good optical limiter are high pulse energy suppression, broad bandwidth, wide temporal response, operation in fast optics, and high ambient transmission. Molecules with large two-photon absorption cross sections that form strongly absorbing excited states have potential for good optical limiting response. We discuss our recent results on obtaining relatively broadband optical limiting response through the use of such two-photon absorbing chromophores.

Two-photon spectroscopy of symmetric donor/acceptor substituted conjugated molecules

Summary form only given. The design of two-photon absorbing chromophores for applications such as optical limiting, fluorescence imaging and microfabrication, relies on the availability of detailed structure-property relationships for the two-photon response. In this presentation we will discuss the two-photon absorption spectra for donor-/spl pi/-donor (D-/spl pi/-D), donor-/spl pi/-acceptor-/spl pi/-donor (D-A-D) and acceptor-/spl pi/-donor-/spl pi/-acceptor (A-D-A) molecules (where /spl pi/ is a biphenyl, a polyenic-type or bis(styryl)benzene-type /spl pi/-bridge), which have been measured using the two-photon induced fluorescence method.