Jeffrey E. Ehrlich

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

Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Three-dimensional microfabrication using two-photon polymerization

Photopolymerization initiated by the simultaneous absorption of two photons is unique in its ability to produce complex three-dimensional (3D) structures from a single, thick photopolymer film. Strong 3D confinement of the polymerization process is not possible in other polymer microfabrication techniques such as LIGA, rapid prototyping, and conventional photoresist technology. Two-photon polymerization also permits the fabrication of 3D structures and the definition of lithographic features on non-planar surfaces. We have developed a wide array of chromophores which hold great promise for 3D microfabrication, as well as other applications, such as two-photon fluorescence imaging and 3D optical data storage. These materials are based on a donor- (pi) -donor, donor-acceptor-donor, or acceptor-donor-acceptor structural motif. The magnitude of the two-photon absorption cross-section, (delta) , and the position of the two-photon absorption maximum, (lambda) (2)max, can be controlled by varying the length of the conjugated bridge and by varying the strength of the donor/acceptor groups. In this way, chromophores have been developed which exhibit strong two- photon absorption in the range of 500 - 975 nm, in some cases as high as 4400 X 10-50 cm4 s/photon-molecule. In the case of donor-(pi) -donor structures, quantum-chemical calculations show that the large absorption cross-sections arise from the symmetric re-distribution of charge from the donor end-groups to the conjugated bridge, resulting in an electronic excited-state which is more delocalized than the ground state. For many of these molecules, two-photon excitation populates a state which is sufficiently reducing that a charge transfer reaction can occur with acrylate monomers. The efficiency of these processes can be described using Marcus theory. Under suitable conditions, such reactions can induce radical polymerization of acrylate resins. Polymerization rates have been measured, and we show that these two-photon chromophores display increased sensitivity and recording speed over conventional photoinitiators. Complex 3D structures can be fabricated in acrylate films doped with these chromophores using tightly focused near-infrared femtosecond laser pulses. A 3D periodic array of polymeric columns has been produced for use in photonic bandgap applications. Tapered waveguide structures for interconnecting disparate-sized optical components have been constructed. More traditional MEMS structures, such as cantilevers, have also been produced. Such structures may be useful for organic vapor sensors. The two-photon photopolymerization process can be extended to other material systems, such as metallic, ceramic, and composite materials, by templating the photopolymer structures.

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.

Molecules with enhanced two-photon absorptivities for two-photon optical recording in photopolymers

Summary form only given. Molecules exhibiting strong two-photon absorption hold great potential for a wide range of applications, including 3D optical data storage. In particular, two-photon initiated polymerization holds tremendous promise for ultra high-density optical memory. One can selectively polymerize very small volumes of polymer near the focus of a laser beam, thus creating a change in refractive index and permitting high data storage densities. We have observed large two-photon absorptivities in bis-donor diphenylpolyene derivatives that appear to be correlated to simultaneous charge transfer from the end groups to the /spl Pi/-conjugated bridge in the molecule.

Structure/property relationships for two-photon absorbing fluorophores

In very recent work we have developed design strategies that allow us to independently control the strength, position, fluorescence quantum yield, and intersystem crossing rates for two-photon absorbing dyes.