Multiphoton Process in Nanofabrication and Microscopy Imaging

Abstract

Under the intense intensity of femtosecond laser pulse, various nonlinear optical effects between laser and matter can occur. Among them, one nonlinear optical effect, multiphoton absorption (MPA), is easily induced by femtosecond laser pulse, which provides an opportunity to achieve the spatial resolution smaller than optical diffraction limit. Multiphoton process using femtosecond laser has provided a powerful technique for fabricating three-dimensional (3D) micro/nanostructures and microscopy imaging due to its intrinsic 3D penetration depth and high spatial resolution at nanometric scale. As an emerging micro/nanofabrication technique, multiphoton polymerization (MPP) based on multiphoton process has been used in fabricating various micromachines and microdevices with conventional photoresists and polymer nanocomposites, such as microneedles, micropumps, microgear sets, and 3D photonic crystals. Aside from MPP based on multiphoton process, microscopy imaging is another research field employing multiphoton process. Two-photon-excited fluorescence (TPEF) microscopy, an established nonlinear microscopy technique, is more practically useful for noninvasive live cell imaging because of the deeper penetration, less photobleaching, and higher spatial resolution. Second-harmonic generation (SHG) is a nonlinear optical process, in which photons with the same frequency interacting with a nonlinear material are effectively "combined" to generate new photons with twice the energy, and therefore twice the frequency and half the wavelength of the initial photons. SHG, as an even-order nonlinear optical effect, is only allowed in