Femtosecond Laser Nanoprocessing

Abstract

Abstract Multiphotonmicroscopy based onthe applicationof tight-focused near-infrared (NIR) femtosecond laser beams has been considered a valuable tool for vital cell imaging (Denk et al., 1990). Typically, 80-MHz/90-MHz mode-locked titanium sapphire lasers with about 1 W mean output power have been employed as a laser source to realize two-photon fluorescence imaging and second-harmonic generation (SHG) microscopy (König et al., 2000; Mertz, 2008). The laser beam has to be attenuated to provide =< 10-mW laser power and < 130-pJ pulse energy, respectively, at the sample. However, when increasing the mean power at the sample to the range of 50 mW to 250 mW and 0.5 nJ to 3 nJ pulse energy, respectively, destructive effects occur (König, 2001; König et al., 1997, 1999; Oehring et al., 2000; Tirlapur et al., 2001). At these power and pulse energy levels, the transient laser intensity reaches the TW/cm2 range, which is sufficient to induce multiphoton ionization and plasma formation. When working near the threshold for optical breakdown, material ablation can be realized within the central part of the diffraction-limited subfemtoliter multiphoton interaction volume without any collateral effects (König et al., 1999, 2001, 2002). In fact, nanodissection and hole drilling in human chromosomes with ablation zones as low as sub-70 nm have been realized without out-of-focus effects using a laser wavelength more than 10 times larger (λ 800 nm) (König et al. 2001). Multiphoton femtosecond laser microscopy offers the possibility of breaking the “Abbe barrier” of diffraction-limited nanoprocessing and of enabling laser nanobiotechnology and laser nanomedicine. The possible diffraction-limited minimum beam spot size is inversely proportional to the laser wavelength. So far, most of laser-induced ablation, cutting, and drilling effects inthe submicronrange have beenrealized with ultraviolet (UV) radiation. The ArF excimer laser at 193 nm plays the most