A remarkable phenomenon in ultrafast laser processing of transparent materials, in particular, silica glass, manifested as a change in material modification by reversing the writing direction is observed. The effect resembles writing with a quill pen and is interpreted in terms of anisotropic trapping of electron plasma by a tilted front of the ultrashort laser pulse along the writing direction.

No abstract

Control of structural modifications inside silica glass by changing the front tilt of an ultrashort pulse is demonstrated, achieving a calligraphic style of laser writing. The phenomena of anisotropic bubble formation at the boundary of an irradiated region and modification transition from microscopic bubbles formation to self-assembled form birefringence are observed, and the physical mechanisms are discussed. The results provide the comprehensive evidence that the light beam with centrosymmetric intensity distribution can produce noncentrosymmetric material modifications.

Self-assembled, sub-wavelength periodic structures are induced in fused silica by a tightly focused, linearly polarized, femtosecond laser beam. Two different types of periodic structures, the main one with period (Lambda(E)) in the direction of the laser beam polarization and the second with period (Lambda(k)) in the direction of the light propagation, are identified from the cross-sectional images of the modified regions using scanning electron microscopy. We demonstrate the spatial coherence of these nanogratings in the plane perpendicular to the beam propagation direction. The range of effective pulse energy which could produce nanogratings narrows when the pulse repetition rate of writing laser increases. The period Lambda(E) is proportional to the wavelength of the writing laser and period Lambda(k) in the head of the modified region remains approximately the wavelength of light in fused silica.

The authors describe the fabrication of buried waveguides in a highly nonlinear chalcogenide glass, gallium lanthanum sulfide, using focused femtosecond laser pulses. Through optical characterization of the waveguides, they have proposed a formation mechanism and provide comparisons to previous work. Tunneling has been identified as the dominant nonlinear absorption mechanism in the formation of the waveguides. Single mode guidance at 633 nm has been demonstrated. 3 This is because of its ability to cause nonlinear phase shifts over much shorter interaction lengths than conventional ͑silica based͒ devices. Various waveguiding structures such as fibers, proton beam written waveguides, continuous wave laser written waveguides, and femtosecond laser written waveguides could be used to realize such devices. Of these femtosecond laser writing is particularly attractive because as well as having rapid processing times waveguiding structures can be formed below the surface of the glass enabling three-dimensional structures to be fabricated. There have been several studies detailing the fabrication and characterization of waveguides using focused femtosecond laser pulses in phosphate glass, 4 chalcogenide glass, 5 and heavy metal oxide glass. 6 Of these chalcogenide glasses are especially attractive because they have a high nonlinear refractive index and enhanced IR transmission coupled with low maximum phonon energy. Of the chalcogenide glasses gallium lanthanum sulfide ͑GLS͒ is probably the most notable with respect to optical nonlinear devices as it has the highest nonlinear figure of merit ͑FOM͒ of any glass reported to date, 7 FOM= n 2 / ͑2␤͒, where is the wavelength, n 2 is the real part of the nonlinear refractive index, and ␤ is the two-photon absorption coefficient. In this letter we report the fabrication and characterization of buried waveguides written into GLS glass using focused femtosecond laser pulses.A sample of GLS was prepared by mixing 65% gallium sulfide, 30% lanthanum sulfide, and 5% lanthanum oxide ͑mol %͒ in a dry-nitrogen purged glove box. Gallium and lanthanum sulfides were synthesised in-house from high purity gallium and lanthanum precursors in a flowing H 2 S gas system; the lanthanum oxide was purchased commercially and used without further purification. The glass was melted in a dry-argon purged furnace at 1150°C for 24 h before being quenched and annealed at 400°C for 12 h, it was then cut and polished into a 12ϫ 12ϫ 5 mm 3 slab. To write the waveguides a Ti:sapphire laser ͑Coherent RegA͒ emitting a train of pulses with a duration of 150 fs, a repetition rate of 250 kHz, and a central wavelength of 800 nm was used. Pulse energy was varied using a variable neutral density filter. The laser beam was focused via a 50ϫ objective ͓numerical aperture ͑NA͒ = 0.55͔ around 200 m below the surface of the sample and had a focus spot diameter of around 2 m. The sample was mounted on a computer controlled linear motor translation stage which could move in three axes with a resolution of a few nanomete...