Femtosecond laser writing of porous capillaries inside fused silica glass

We demonstrate that within a restricted optical pulse duration-pulse energy parameter space tightly focused femtosecond laser radiation can be used to fabricate porous capillaries in bulk fused silica glass by simply moving the laser focus through the material. We show that the rate of penetration of liquids into the porous capillaries can be controlled by the laser polarization, which determines their morphology. The fluid propagation is measured using the form birefringence of nanocrack/nanovoid structures produced inside the capillaries. We also demonstrate the nanofiltration capabilities of the capillaries by separating the relatively small molecules of Rhodamine 6G dye from their solvent.     

Femtosecond laser fabrication of nanostructures in silica glass

A femtosecond laser beam focused inside fused silica and other glasses can modify the refractive index of the glass. Chemical etching and atomic-force microscope studies show that the modified region can have a sharp-tipped cone-shaped structure with a tip diameter as small as 100 nm. Placing the structure near the bottom surface of a silica glass sample and applying a selective chemical etch to the bottom surface produces clean, circular, submicrometer-diameter holes. Holes spaced as close to one another as 1.4 microm are demonstrated.     

Femtosecond laser fabrication of phase-shifted Bragg grating waveguides in fused silica

Phase-shifted Bragg grating waveguides (PSBGWs) were formed in bulk fused silica glass by femtosecond laser direct writing to produce narrowband (22±3) pm filters at 1550 nm. Tunable π and other phase shifts generated narrow passbands in controlled positions of the Bragg stopband, while the accurate placement of multiple cascaded phase-shift regions yielded a rectangular-shaped bandpass filter. A waveguide birefringence of (7.5±0.3)×10(-5) is inferred from the polarization-induced spectral shifting of the PSBGW narrowband filters.     

Femtosecond Bessel-beam-assisted high-aspect-ratio microgroove fabrication in fused silica

A simple and repeatable method to fabricate high-aspect-ratio(HAR) and high-quality microgrooves in silica is reported. The method consists of two steps:(1) formation of laser-modified regions by femtosecond Bessel beam irradiation, and(2) removing laser-modified regions through HF etching. Uniform, straight microgrooves can be fabricated and the highest aspect ratio that can be reached is ～52. The phenomenon is attributed to the uniform energy distribution in the long propagation distance, which leads to the long and uniform laser-modified regions and subsequent HF acid etching of laser-modified regions with high selectivity. This method will have potential applications in fabrication of HAR microgrooves in transparent materials.     

Femtosecond laser-induced damage and filamentary propagation in fused silica

Bulk damage induced by fs IR laser pulses in silica is investigated both experimentally and numerically. In a strong focusing geometry, a first damage zone is followed by a narrow track with submicron width, indicating a filamentary propagation. The shape and size of the damage tracks are shown to correspond to the zone where the electron density created by optical field ionization and avalanche is close to 10(20) cm(-3). The relative role of avalanche and photoionization is studied. The plasma density produced in the wake of the pulse is shown to saturate around 2-4x10(20) cm(-3).     

Wavelength-tunable multicolored femtosecond laser pulse generation in a fused silica glass plate

We obtained an array of multicolored femtosecond laser pulses with as many as 17 different colors that are spatially isolated. The mechanism of generation was proved to be cascaded four-wave mixing and with the following procedure. The output beam from a femtosecond laser was split into two. One of the two beams was pulse-compressed with a hollow core fiber and the intensity of the other was reduced. The two beams were synchronized and combined with a small crossing angle in a plate of fused silica glass plate. The wavelengths of the sidebands are continuously tunable from near-ultraviolet to near-infrared. The pulse duration, spatial mode, spectrum, and energy stability of the sidebands were studied. As many as fifteen spectral up-shifted pulses and two spectral downshifted pulses were obtained with spectral bandwidths broader than 1.8 octaves. Properties such as pulse energy as high as 1 μmJ, 45 fs pulse duration, smaller than 1.1 times of the diffraction limit Gaussian spatial profile, and better than 2% RMS power stability of the generated sidebands make it can be used in various experiments. The characterization showed that the sidebands have sufficiently good qualities to enable application to for various multicolor femtosecond laser experiments, for example, a multicolor pump-probe experiment.     

Femtosecond laser-assisted etching of three-dimensional inverted-woodpile structures in fused silica

Three-dimensional inverted-woodpile (WP) structures were embedded in a microchannel by femtosecond laser direct-writing of fused silica followed by chemical etching with diluted hydrofluoric acid. We show the hole size is linearly dependent on laser-scanning depth for various pulse energies, permitting the control of laser exposures to facilitate close 5 μm periodic packing of uniform microcapillary arrays. Exposure compensation for depth-dependent etching rate and optical beam aberrations yielded stable and crack-free uniform inverted-WP structures. The direct formation of the inverted-WP structure together with microchannels in an all-fused silica substrate, offers chemical stability and inertness, and biocompatibility to be exploited as new microfluidic systems for chromatography and electro-osmotic pumps.     

Fabrication and stitching of embedded multi-layer micro-gratings in fused silica glass by fs laser pulses

Fabrication and stitching of internal 2D, 1D and multi-layer micro-gratings in fused silica glass using amplified Ti:sapphire femtosecond laser were reported. These gratings have the pitch of 4 μm and the size of 400 μm×400 μm. For a two-layer 1D micro-grating where a second-layer grating was overwritten on a first-layer grating at the exact X,Y position and the different Z depth, the diffraction efficiency can reach more than 25% due to the grating thickness increase. If a second-layer grating was stitched with a first-layer by the shift of 2 μm in the X direction and at the different Z depth, the diffraction angle was doubled but the diffraction efficiency was about 9%. The last result has the potential application for fabricating high-density micro-/nano-structures beyond the diffraction limit through 3D stitching. PACS 42.79.Dj; 42.40.Lx; 42.62.Cf     

Ultrafast time-resolved photography of femtosecond laser induced modifications in BK7 glass and fused silica

The dynamics of absorption after excitation of fused silica and BK7 glass with femtosecond laser radiation are visualized by transient absorption spectroscopy. Focusing laser radiation with pulse durations in the picosecond time regime in BK7 glass generates free electrons with relaxation by emission of radiation or by formation of defects. The temporal and spatial emission characteristics are observed by high-speed photography in the streak mode. The beam waist moves within the pulse duration towards the incoming laser radiation by self-focusing and with the laser radiation absorbed by multi-photon processes. The dynamics of the long lasting stress formation is visualized by time-resolved Nomarski-Photography. The modification of the glass is investigated during and after irradiation with ultra-short pulsed laser radiation (100 fs<t_p<3 ps) at the wavelength =810 nm. The formation of a sound wave in fused silica and BK7 glass is observed and the mechanical stress, depending on the excitation pulse duration, is measured. PACS 06.60.Jn; 42.50.Md; 78.47.-p; 81.16.-c; 82.53.-k     

Micro-channel etching characteristics enhancement by femtosecond laser processing high-temperature lattice in fused silica glass

A fused silica glass micro-channel can be formed by chemical etching after femtosecond laser irradiation, and the successful etching probability is only 48%. In order to improve the micro-channel fabrication success probability,the method of processing a high-temperature lattice by a femtosecond laser pulse train is provided. With the same pulse energy and scanning speed, the success probability can be increased to 98% by optimizing pulse delay.The enhancement is mainly caused by the nanostructure, which changes from a periodic slabs structure to some intensive and loose pore structures. In this Letter, the optimum pulse energy distribution ratio to the etching is also investigated.     

Damage characteristics of laser plasma shock wave on rear surface of fused silica glass

The damage to the rear surface of fused silica under the action of high power laser is more severe than that incurred by the front surface, which hinders the improvement in the energy of the high power laser device. For optical components,the ionization breakdown by laser is a main factor causing damage, particularly with laser plasma shock waves, which can cause large-scale fracture damage in fused silica. In this study, the damage morphology is experimentally investigated, and the characteristics of the damage point are obtained. In the theoretical study, the coupling and transmission of the shock wave in glass are investigated based on the finite element method. Thus, both the magnitude and the orientation of stress are obtained. The damage mechanism of the glass can be explained based on the fracture characteristics of glass under different stresses and also on the variation of the damage zone's Raman spectrum. In addition, the influence of the glass thickness on the damage morphology is investigated. The results obtained in this study can be used as a reference in understanding the characteristics and mechanism of damage characteristics induced by laser plasma shock waves.     

Femtosecond laser induced defects in various silica glasses

The mechanism of near infrared (IR) focused femtosecond (fs) laser induced defects in silica glasses produced by different methods is systematically investigated through measurements of absorption,fluorescence, and electronic spin resonance (ESR) spectra. The influence of impurities and hydroxyl groups on defects is discussed. The results show that ES silica glasses containing high OH and few defects are much stable under fs laser irradiation. It is also verified that Si E'δ center formation has no direct relation with chloride ions.     

Femtosecond ultraviolet autocorrelation measurements based on two-photon conductivity in fused silica

We utilize the two-photon conductivity of a fused-silica substrate to produce a photoconductive switch for use in an intensity autocorrelator for ultraviolet ultrashort pulses. We perform measurements at 267 nm with pulse durations in the range of 110-330 fs and with energies as weak as 10 nJ. Based on the bandgap of fused silica, this device can potentially operate in the wavelength range of 140-280 nm.     

Optically produced arrays of planar nanostructures inside fused silica

Linearly polarized femtosecond light pulses, focused inside fused silica to an intensity that leads to multiphoton ionization, produce arrayed planes of modified material having their normal parallel to the laser polarization. The planes are < or = 10 nm thick and are spaced at approximately lambda/2 in the medium for free space wavelengths of both 800 and 400 nm. By slowly scanning the sample under a fixed laser focus, order is maintained over macroscopic distances for all angles between the polarization and scan direction. With the laser polarization parallel to the scan direction we produce long-range Bragg-like gratings. We discuss how local field enhancement influences dielectric ionization, describe how this leads to nanoplane growth, why the planes are arrayed, and how long-range order is maintained.     

Creation of chiral structures inside fused silica glass

Circularly polarized (CP) femtosecond laser light focused into fused silica produces a permanent recording of optical helicity. Material modification is in the form of highly ordered submicrometer chiral structures whose handedness follows the handedness of the CP light. The evolution of the ordered structures from chaotic modification is described.     

Quasi-phase-matched second-harmonic generation in ultraviolet-assisted periodically poled planar fused silica

First-order quasi-phase-matched second-harmonic generation of 1064 to 532 nm in a thermally poled planar fused-silica plate with periodic UV erasure of the second-order nonlinearity was successfully implemented. We obtained a 1:2.9 ratio of d31:d33 for UV-grade fused silica in support of the proposed mechanism for electric-field-induced second-order nonlinearity in this material.     

Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses

Atomic-scale structural changes have been observed in the glass network of fused silica after modification by tightly focused 800-nm, 130-fs laser pulses at fluences between 5 and 200 J cm^-2. Raman spectroscopy of the modified glass shows an increase in the 490 and 605-cm^-1 peaks, indicating an increase in the number of 4- and 3-membered ring structures in the silica network. These results provide evidence that densification of the glass occurs after exposure to fs pulses. Fluorescence spectroscopy of the modified glass shows a broad fluorescence band at 630 nm, indicating the formation of non-bridging oxygen hole centers (NBOHC) by fs pulses. Waveguides that support the fundamental mode at 633 nm have been fabricated inside fused silica by scanning the glass along the fs laser beam axis. The index changes are estimated to be approximately 0.07×10^-3. Received: 17 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-925/423-2463, E-mail: dmkrol@ucdavis.edu