Abstract

Tight focusing of laser light down to near diffraction limited spot sizes can be easily achieved with high numerical aperture microscope objectives, and state-of-the-art ultrafast laser technology has made possible the generation of sub-two-cycle pulses directly from laser oscillators [1, 2]. In applications such as OCT, the spot size translates into lateral resolution, while techniques relying on nonlinear optical effects, such as CARS spectroscopy and two-photon microscopy, rely on both the spatial and temporal confinement of radiation, which leads to high peak intensities. Space-time focusing of carrier-envelope phase stabilized few-cycle laser pulses plays a crucial role in the study of phase-dependent nonlinear optical phenomena [3]. However, such focusing is not easily achieved, mostly due to the very broad bandwidths of few-cycle pulses and the dispersion introduced by the large amounts of glass present in high-quality microscope objectives. Non-dispersive reflective objectives have been used for this purpose [3], but their intrinsic obscuration usually limits the coupling efficiency to less than 10%, with a consequent decrease in peak intensity. Dispersion compensation of microscope objectives using chirped mirrors has resulted in sub-14-fs pulses [4], and very recently, sub-8-fs pulses were obtained with a 4-f pulse shaper arrangement based on a mechanically deformable mirror [5].

© 2009 IEEE