an excited state. The 2PA process is weak relative to one-photon excitation for intensities below about 10 GW/cm2 and nominal values for the absorption cross Materials for sections. The probability of a molecule absorbing one photon is proportional to the intensity of the excitation beam. In Multiphoton 3D contrast, the probability of a molecule absorbing two photons simultaneously is proportional to the square of the intensity of the excitation beam. Microfabrication The nonlinear intensity dependence of 2PA gives rise to some very interesting and useful effects. First, 2PA enables the excita- Seth R. Marder, Jean-Luc Brédas, tion of molecules with a very high degree of spatial confinement in three dimen- and Joseph W. Perry sions,12 leading to feature widths of ~100 nm and excited volumes as small as ~0.003 µm3 or 3 attoliters. This 3D control Abstract of the excitation arises from focusing an Two-photon/multiphoton lithography (MPL) has emerged as a versatile technique for intense laser beam into a material and the the fabrication of complex 3D polymeric, hybrid organic/inorganic, and metallic fact that the beam intensity decreases structures. This article reviews some recent advances in the development of molecules roughly quadratically with the distance and materials that enable two-photon and multiphoton 3D micro- and nanofabrication. from the focal plane. Because the probabil- Materials that exhibit high sensitivity for the generation of reactive intermediates are ity of 2PA is proportional to the square of described, as are various materials systems that enable functional devices to be made intensity of the laser beam, the probability and in some cases enable structures to be replicated. The combination of advances of 2PA falls off roughly as the fourth illustrates the opportunities for MPL to have a significant impact in the areas of power of the distance from the focus. photonics, microelectromechanical systems, and biomedical technologies. Consequently, 2PA at appreciable dis- tances from the focal plane (say, at roughly twice the Rayleigh length, or 2.5 times the beam radius at the focus) is negligible (5%), relative to that at the focal plane, Introduction leading to a much greater confinement of There is a growing interest in the fabri- have limitations in terms of being quite excitation along the direction of propaga- cation of truly three-dimensional micro - restricted in the range or the spatial reso- tion and at the focus of the beam for 2PA as structures and devices for potential lution of 3D structures that can be formed, compared with one-photon absorption applications in photonics, microelectro- or are consuming in terms of the number (Figure 1a). Second, with 2PA it is possible mechanical systems, microfluidics, and of masks and amounts of solvent to excite molecules at increased depth, rel- tissue engineering, among other fields.1–3 required. Many new or emerging tech- ative to one-photon absorption, in a nomi- In 2D lithography, major advances have nologies demand fabrication of 3D struc- nally high-absorbing medium, because the been made in decreasing the feature sizes tures with submicron feature sizes and photon energy lies well below that at in devices. For example, lithographic great flexibility in shape and material which the medium absorbs via one- processes for making complex integrated composition. Two-photon/multiphoton photon absorption (Figure 1b). With these circuits with 65-nm features are now used lithography (MPL) offers the potential for features of higher resolution and the abil- in the commercial production of micro- micro- and nanofabrication of arbitrary ity to “write” at different depths, 2PA has processors,4 but these processes are 3D structures in a single coat/expose/ enabled the realization of 3D optical data restricted to 2D patterning in a given develop process cycle. In this article, we storage,13–16 lithographic microfabrica- exposure step. Progress also has been review the development of two-photon tion,15,17 and imaging12 via pulsed laser made in using molecular, nanotube, and absorbing materials that can be used for excitation. By computer-controlled scan- nanoparticle self-assembly to build struc- 3D microfabrication of a variety of com- ning of the focus of a laser beam within a tures with unique properties in a bottom- plex structures. photochemically active precursor material, up approach.5–7 Such self-assembly many complex 3D structures can be fabri- processes, however, are limited predomi- Two-Photon/Multiphoton cated with submicron resolution (see nantly to the formation of periodic struc- Lithography Figure 1c for a schematic illustrations of an tures. Fabrication of 3D objects of Two-photon absorption (2PA) provides MPL system). arbitrary shapes with feature sizes a means of activating chemical or physical MPL is helping to revolutionize the fab- approaching those achievable by either processes with high spatial resolution in rication of 3D structures on the submicron photon or electron-beam lithography tra- three dimensions and feature widths as scale, because of the following advan- ditionally has been an extremely difficult small as ~100 nm. With sufficiently tages: task. Some types of 3D structures can be intense light, such as from a pulsed laser 1. The numerous types of 3D structures fabricated using sequential layer-by-layer beam, it is possible for a molecule to that can be fabricated, including ones with lithographic processes,8 direct-writing simultaneously absorb two photons, each interlocking or movable parts; with inks,9 stereolithography,10 and multi- approximately half the energy (twice the 2. The ability to directly pattern various beam interference lithography,11 which wavelength λ) normally required to reach materials ranging from transparent MRS BULLETIN • VOLUME 32 • JULY 2007 • www/mrs.org/bulletin 561 Materials for Multiphoton 3D Microfabrication a b Accordingly, schemes that seek to exploit the use of 2PA would be aided by the development of molecules with large δ. One key to the design of molecules with high sensitivity in 2PA is a detailed under- standing of how δ depends on molecular structure. Molecules with a large δ would be more efficiently excited than molecules with a small δ at the same laser intensity, enabling the use of lower-power and less costly pulsed lasers19 and increasing the speed with which scanning laser expo- sures could be made, thus making 2PA applications more economically feasible. Recent advances in two-photon materials and processing methods suggest the promise for MPL to become a mainstream, commercially viable fabrication technol- ogy, as we briefly describe here. c Before 1998, the largest value of δ reported was roughly 300 GM for the laser Nd: VO laser Mode-locked dye rhodamine B (where 1 GM ≡ 1 × 10−50 4 Ti:sapphire laser cm4 s).20 We discovered that E-4,4′-bis(di- n-butyl)aminostilbene, 1, had a peak two- δ photon excitation cross section max = 210 GM at 605 nm, which is almost 20 times Sample Varible optical greater than the peak cross section for XYZ translation attenuator trans-stilbene.21 The δ value for 1 was stage among the largest values of δ reported for organic compounds at that time. High-speed shutter Quantum-chemical calculations were per- High-NA formed to gain insight into the much objective δ larger max value for 1 compared with trans-stilbene. The 2PA spectra were XY raster obtained from the dispersion of the imag- scanner inary part of the second hyperpolarizabil- ity using the sum-over-states expression from perturbation theory that reproduced the experimentally observed order-of- Figure 1. (a) Fluorescence/optical image demonstrating localized excitation in three δ magnitude enhancement in max upon dimensions by two-photon excitation. Top spot, two-photon excitation; bottom streak, one substitution of trans-stilbene with termi- photon excitation. (b) Demonstration of the limited penetration depth of one-photon nal dialkylamino groups. The calculations excitation in a strongly absorbing medium (bottom spot at right edge of cuvette); improved penetration depth via two-photon excitation is clearly seen by the intense spot in the central showed that the lowest singlet excited upper portion of the cuvette. (c) Schematic illustration of a system for two- state (e) is a one-photon-allowed state photon/multiphoton 3D microfabrication. with Bu symmetry, whereas the second singlet excited state (e′) is two-photon- allowed with Ag symmetry and corre- sponds to the observed 2PA peak. The calculations also suggested that the polymers to inorganic–organic hybrids to Two-Photon Absorption Materials ground-state to excited-state (g → e) exci- metals; and Processes tation is accompanied by a substantial 3. The possibility to create “micromolds” To exploit these advantages, it is benefi- charge transfer from the amino groups to that can be used for templated growth of a cial to use specially designed and engi- the central vinylene group, leading to a range of materials, enabling the integra- neered photoactive materials within which large change in quadrupole moment upon tion of disparate materials into devices; it is possible to localize photochemical excitation; a similar sense and magnitude 4. The capability to fabricate features reactions within the excitation volume. of charge transfer is calculated for the g → from the nanoscale (<100 nm) to the Whereas various efforts to exploit 2PA in e′ transition. This pronounced redistribu- microscale, providing a technology for 3D microfabrication and imaging have tion of the π-electronic density was corre- integrating nanostructures with well- been reported, many of these schemes lated with a significant increase in the established microscale components. have employed conventional photoinitia- e→e′ transition dipole moment, which is Thus, two-photon/multiphoton 3D tors or fluorescent chromophores that the major contributor to the enhanced lithography is a disruptive technology were developed for use under one-photon two-photon cross section of 1 with respect that fundamentally changes what 3D excitation conditions.