Femtosecond laser micromachining can be used either to remove materials or to change a material's properties, and can be applied to both absorptive and transparent substances. Over the past decade, this technique has been used in a broad range of applications, from waveguide fabrication to cell ablation. This review describes the physical mechanisms and the main experimental parameters involved in the femtosecond laser micromachining of transparent materials, and important emerging applications of the technology. Interactions between laser and matter are fascinating and have found a wide range of applications. This article gives an overview of the fundamental physical mechanisms in the processing of transparent materials using ultrafast lasers, as well as important emerging applications of the technology.

Femtosecond laser micromachining in transparent materials

The present invention provides nanometer-sized diameter silica fibers that exhibit high diameter uniformity and surface smoothness. The silica fibers can have diameters in a range of a about 20 nm to about 1000 nm. An exemplary method according to one embodiment of the invention for generating such fibers utilizes a two-step process in which in an initial step a micrometer sized diameter silica preform fiber is generated, and in a second step, the silica preform is drawn while coupled to a support element to form a nanometer sized diameter silica fiber. The portion of the support element to which the preform is coupled is maintained at a temperature suitable for drawing the nansized fiber, and is preferably controlled to exhibit a temporally stable temperature profile.

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Subwavelength-diameter silica wires for low-loss optical waveguiding

Micro total analysis systems. Recent developments.

Laser ablation in liquids : Applications in the synthesis of nanocrystals

We report a comprehensive investigation of femtosecond continuum generation in single crystals of several common laser host materials. The absolute spectral energy density, pulse-to-pulse stability, pump threshold, and beam profile are studied in dependence on the focusing conditions, crystal thickness, pump pulse energy, and pump wavelength (775–1600 nm). Continuum generation is shown at repetition rates of up to 80 MHz and for pump pulse durations of up to 350 fs. In yttrium aluminum garnet (YAG), yttrium vanadate (YVO4), gadolinium vanadate (GdVO4), and potassium-gadolinium tungstate (KGW) thresholds below 50 nJ, plateau-like visible and infrared spectra, and higher infrared photon flux as compared to conventional materials like sapphire are found. We discuss the particular advantages of these materials for application in parametric amplification, femtosecond spectroscopy, and carrier-envelope phase stabilization.

Femtosecond continuum generation in bulk laser host materials with sub-μJ pump pulses

Competing nonlinear optical effects are involved in the interaction of femtosecond laser pulses with transparent dielectrics: supercontinuum generation and multiphoton-induced bulk damage. We measured the threshold energy for supercontinuum generation and bulk damage in fused silica using numerical apertures (NAs) ranging from 0.01 to 0.65. The threshold for supercontinuum generation exhibits a minimum near 0.05NA and increases quickly above 0.1 NA. For NAs greater than 0.25, we observe no supercontinuum generation. The extent of the blue broadening of the supercontinuum spectrum decreases significantly as the NA is increased from 0.01 to 0.08, showing that weak focusing is important for generating the broadest supercontinuum spectrum. Using a light-scattering technique to detect the onset of bulk damage, we confirmed bulk damage at all NAs studied. At a high NA, the damage threshold is well below the critical power for self-focusing.

Numerical aperture dependence of damage and supercontinuum generation from femtosecond laser pulses in bulk fused silica

This work describes the use of focused, high-intensity light from a Ti:sapphire laser that generates femtosecond pulses to create topographical structure in a flat surface of poly(di-methylsiloxane) (PDMS), and the use of the PDMS surfaces patterned using this procedure for a range of purposes. PDMS patterned in surface bas-relief is the material most widely used for printing and stamping in soft lithography, [1,2] and a material widely used in microfluidic systems. [3] The bas-relief patterns required in these applications are usually fabricated by casting PDMS against a complementary bas-relief pattern in photoresist, fabricated in turn by photolithography. This process works well, but is not applicable to the preparation of PDMS stamps required for all types of problems; printing on spherical surfaces is an example. The technique described here permits the non-photolitho-graphic generation of surface topography for use in soft lithography and microfluidics; this technique has three useful characteristics: i) It generates features smaller than those generated by standard rapid-prototyping techniques based on printed transparency masks. [1,2] ii) It is applicable to fabrication on non-planar surfaces. iii) It can be used in fabrication of large-area patterns. The rough, recessed features generated by this technique are useful for applications where large surface area to volume ratios are desired, e.g., in catalysis, [7] for super-hydrophobic surfaces, [8±10] in surface-enhanced Raman scattering, [11,12] and as magnetic field concentrators. [13±15] It has the disadvantages that it is a serial process, and that the laser and positioning equipment are relatively specialized. The only requirement for the material is that it must be transparent to 800 nm light: PDMS works well, but other transparent elastomeric polymers should also work. Laser ablation is a common technique for direct writing of patterns into the surfaces of metals, semiconductors, and polymers. [16,17] Most of the techniques that have been described for laser writing work through an absorption mechanism for light that is linear in light intensity. This sort of photochemistry typically requires doping of the material with sensitizers [18±20] and/or the use of UV lasers. [21,22] We have previously described the production of stamps for microcontact printing (lCP) by ablation of a doped polymer with a diode-pumped Nd:YVO 4 laser. [20] Line widths of the features produced through this technique are ~ 5 lm. This technique, as described, is substantially less useful than rapid prototyping. Another process, developed by Hull and co-workers, used a focused ion beam (FIB) to write …

Customization of Poly(dimethylsiloxane) Stamps by Micromachining Using a Femtosecond‐Pulsed Laser

Optical loss measurements in femtosecond laser written waveguides in glass

Thermal 3-D microstructuring of photonic structures is provided by depositing laser energy by non-linear absorption into a focal volume about each point of a substrate to be micromachined at a rate greater than the rate that it diffuses thereout to produce a point source of heat in a region of the bulk larger than the focal volume about each point that structurally alters the region of the bulk larger than the focal volume about each point, and by dragging the point source of heat thereby provided point-to-point along any linear and non-linear path to fabricate photonic structures in the bulk of the substrate. Exemplary optical waveguides and optical beamsplitters are thermally micromachined in 3-D in the bulk of a glass substrate. The total number of pulses incident to each point is controlled, either by varying the rate that the point source of heat is scanned point-to-point and/or by varying the repetition rate of the laser, to select the mode supported by the waveguide or beamsplitter to be micromachined. A wide range of passive and active optical and other devices may be thermally micromachined.

Method and apparatus for micromachining bulk transparent materials using localized heating by nonlinearly absorbed laser radiation, and devices fabricated thereby

The femtosecond laser has become an important tool in the micromachining of transparent materials. In particular, focusing at high numerical aperture enables structuring the bulk of materials. At low numerical aperture and comparable energy, focused femtosecond pulses result in white light or continuum generation. It has proven difficult to damage transparent materials in the bulk at low NA. We have measured the threshold energy for continuum generation and for bulk damage in fused silica for numerical apertures between 0.01 and 0.65. The threshold for continuum generation exhibits a minimum near 0.05 NA, and increases quickly near 0.1 NA. Greater than 0.25 NA, no continuum is observed. The extent of the anti-stokes pedestal in the continuum spectrum decreases strongly as the numerical aperture is increased to 0.1, emphasizing that slow focusing is important for the broadest white light spectrum. We use a sensitive light scattering technique to detect the onset of damage. We are able to produce bulk damage at all numerical apertures studied. At high numerical aperture, the damae threshold is well below the critical power for self-focusing, which allows the breakdown intensity to be determined. Below 0.25 NA, the numerical aperture dependence suggests a possible change in damage mechanism.

Jonathan B. Ashcom

Papers

Numerical aperture dependence of damage and supercontinuum generation from femtosecond laser pulses in bulk fused silica

Customization of Poly(dimethylsiloxane) Stamps by Micromachining Using a Femtosecond‐Pulsed Laser

Optical loss measurements in femtosecond laser written waveguides in glass

Method and apparatus for micromachining bulk transparent materials using localized heating by nonlinearly absorbed laser radiation, and devices fabricated thereby

Numerical aperture dependence of damage and white light generation from femtosecond laser pulses in bulk fused silica