Femtosecond (fs) laser beams may be shaped into Bessel beam (BB) profiles by spatial light modulators or axicon lenses. Temporally reshaping laser pulses by chirping and stretching further alters the spatio-temporal intensity pattern within the elongated focal volume. Such beam shaping applied to high-power pulses is useful for numerous materials processing applications, enabling fabrication of very high aspect ratio columns in optically transparent materials. We report the development of a compact, adaptable microscope turret-mounted assembly containing an axicon and a high numeric aperture aspheric lens imaging system, and the use of temporal reshaping studies in various fs laser machining applications. Depending on axicon angle, lens separations, and the refractive index of the substrate, the central lobe diameter of the BB may be less than 1 μm but extending over 500 μm long, effectively forming a narrow, long cylindrical column. Moreover, because an entire column can be machined with a single, energetic pulse, high processing rates are possible. Materials such as fused silica and polymers are found to be good candidates for directly formed voids. Microchannels in silica can be used in single-molecule recycling experiments, while permeable membranes in thin plastics are sought for cellular studies, passing nutrients but not cells. In rigid crystalline structures like diamond and sapphire, the substrate material is transformed in place. In particular, a BB column machined through electrically insulating diamond can become conductive graphite, which is of interest for developing radiation-hard detectors of highenergy particles.
A freely diffusing single fluorescent molecule may be scrutinized for an extended duration within a confocal microscope
by actively trapping it within the femtoliter probe region. We present results from computational models and ongoing
experiments that research the use of multi-focal pulse-interleaved excitation with time-gated single photon counting and
maximum-likelihood estimation of the position for active control of the electrophoretic and/or electro-osmotic motion
that re-centers the molecule and compensates for diffusion. The molecule is held within a region with approximately
constant irradiance until it photobleaches and/or is replaced by the next molecule. The same photons used for
determining the position within the trap are also available for performing spectroscopic measurements, for applications
such as the study of conformational changes of single proteins. Generalization of the trap to multi-wavelength excitation
and to spectrally-resolved emission is being developed. Also, the effectiveness of the maximum-likelihood position
estimates and semi-empirical algorithms for trap control is discussed.