Carbon nanostructures produced by pyrolysis under high pressure inside a nanosize silica matrix

In this work, the pyrolysis under high pressure of hydrocarbons dispersed inside a nanosized silica matrix (Aerosil) was investigated. The samples consisted of hydrophobic nanometric silica powder terminated by methyl groups with carbon contents ranging from 0.7 to 4 wt%. The pyrolysis was carried out in the temperature range from 1000 to 1600 °C under high pressure (1.25 up to 7.7 GPa) to keep the two‐dimensional distribution of carbon atoms originally at the silica grain boundaries. Evidences from Raman spectroscopy and transmission electron microscopy suggested that the resulting carbon nanostructures were actually graphene‐like nanoflakes. The size of the nanostructures calculated from the I_D/I_G ratio increased from 6 to 30 nm for processing temperatures increasing from 1000 to 1600 °C under pressure, respectively. The results revealed that the very good dispersion of the methyl groups inside the nanosize silica matrix, and the confinement under high pressure during the pyrolysis, played both a relevant role in the resulting carbon nanostructures. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

Laser-induced fabrication of randomly distributed nanostructures in fused silica surfaces

The nanostructuring of dielectrics is a big challenge for laser patterning methods. In this study a novel laser structuring method for the fabrication of randomly distributed nanostructures, called laser-induced front side etching using in situ pre-structured metal layers (IPSM-LIFE), is presented. The pulsed laser irradiation of a thin metal film deposited onto a dielectric substrate with fluences below the ablation threshold results in the formation of randomly distributed metal structures by self-assembly processes. Further pulsed laser irradiation of these metal structures with higher or equal laser fluences causes the formation of complex patterns at the surface of the dielectric due to localized ablation and melting processes of the dielectric surface induced by the absorption of the laser energy by the metal structures and the local energy transfer into the dielectric surface. The pattern formation observed in the film and the dielectrics substrate after irradiation of 10 nm chromium layers on fused silica, with laser pulses (Δt _p =25 ns, λ=248 nm), was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Different features with a lateral size down to a few tens of nanometers, like concentric ring patterns, donut-like structures, and bar patterns were observed at the dielectric.     

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.     

The polarization-dependence of femtosecond laser damage threshold inside fused silica

The laser damage threshold inside fused silica is dependent not only on the numerical aperture (NA) of the focusing objective, but also on the polarization of the incident femtosecond laser pulses. The damage threshold for circularly polarized beams is higher than that for linearly polarized beams when NA>0.4, but the former was lower than the latter when NA<0.4. The reverse might be due to different damage processes: laser induced damage at high NA and the self-focusing induced breakdown at low NA.     

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.     

Femtosecond laser erasing and rewriting of self-organized planar nanocracks in fused silica glass

Tightly focused, linearly polarized, femtosecond laser radiation can produce highly birefringent nanograting structures inside fused silica glass. Here we report that when the polarization direction of the femtosecond light is changed, old nanogratings are erased and simultaneously replaced with new ones whose orientation is solely determined by the polarization of the rewrite beam. We also show that these volume nanogratings can be rewritten 1000 times with little degradation in their quality.     

Numerical model for light propagation and light intensity distribution inside coated fused silica capillaries

Numerical simulations of light propagation through capillaries have been reported to a limited extent in the literature for uses such as flow-cell design. These have been restricted to prediction of light path for very specific cases to date. In this paper, a new numerical model of light propagation through multi-walled cylindrical systems, to represent coated and uncoated capillaries is presented. This model allows for light ray paths and light intensity distribution within the capillary to be predicted. Macro-scale (using PMMA and PC cylinders) and micro-scale (using PTFE coated fused silica capillaries) experiments were conducted to validate the model's accuracy. These experimental validations have shown encouragingly good agreement between theoretical predictions and measured results, which could allow for optimisation of associated regions for monolith synthesis and use in fluidic chromatography, optical detection systems and flow cells for capillary electrophoresis and flow injection analysis.     

Material response during nanosecond laser induced breakdown inside of the exit surface of fused silica

The material response following nanosecond, UV laser induced breakdown inside of the exit surface of fused silica is investigated using multimodal time resolved microscopy. The study spans up to about 75 ns delay from the onset of material modification during the laser pulse through the observation of material ejection. A number of distinct processes were identified, including: a) the onset of optical absorption in the material arising from the buildup of an electronic excitation, b) the expansion of the hot modified region (plasma) along the surface and inside the bulk, c) the formation of radial and circumferential cracks, d) the swelling of the affected region on the surface and, e) the onset of ejection of material clusters at about 30 ns delay and its progression to a well‐defined jet by about 75 ns delay. Limited theoretical modeling is used to aid the interpretation of the data.     

Optical waveguide arrays induced in fused silica by void-like defects using femtosecond laser pulses

We report a new approach to the microfabrication of permanent optical waveguide arrays inside fused silica induced by focusing infrared femtosecond laser pulses with microjoule energy. These arrays consist of waveguides limited by void-like damage zones with very loose coupling among adjacent guides, thus allowing the excitation of a single one. The proposed method shows the possibility of using created void-like structures for both the fabrication of integrated optical devices as well as for the control of previously induced refractive index change regions. PACS 42.65.Re; 42.82.Et; 42.79.-e     

Optically bistable arrays and chaotic dynamics

An analogy between transverse coupling of bistable optical devices and nonlinear dynamics is exploited to demonstrate that chaos in the dynamics is necessary for independent operation of the elements of a bistable array.     

Photoluminescence spectra of copper-based planar nanostructures

We have studied the luminescence of planar nanostructures based on amorphous copper, excited by a low-intensity source in the UV region of the spectrum. We have shown that it is dependent on the packing density of copper granules on the surface of the quartz substrate, the presence of chains of granules, the optical properties of the surrounding medium, and the oxidation time. The observed maxima at the wavelengths of 400 nm and 520 nm correspond to luminescence of the quartz and copper oxide Cu₂O. The maximum at 650 nm is located in the region of plasma resonances of the oxidized copper chains and aggregates, and is enhanced in the “hot spot” region near the surface of interacting copper particles. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 510–515, July–August, 2006.     

Ultrafast two-color ablation of fused silica

Two ultrafast laser pulses at the fundamental Ti:sapphire laser wavelength of 800 nm and the second harmonic at 400 nm were used to study the temporal evolution of the transmissivity in fused silica and resulting material ablation. It was observed that there was a sharp drop in the transmissivity of the probe pulse at zero delay between the two pulses, indicating that there was enhanced absorption/reflection due to the creation of defect states or free electron plasma by the pump pulse. Subsequent atomic force microscopy measurements of the ablated holes revealed that the ablated volume increased by about 50% when the separations of the two pulses are within 300 fs. Two-color machining of channels at the surface also showed a similar increase in the machined depth and width when the pulses are overlapped in time. PACS 52.38.Mf; 78.47.+p; 79.20.Ds     

Arresting ultraviolet-laser damage in fused silica

The 351-nm laser-damage initiation threshold for surface damage in conventionally polished fused silica is demonstrated to be stress dependent. By circumferential application of modest loads to a sample, a controllable stress field can be established within the clear aperture of a fused-silica specimen, in response to which both the damage-initiation fluence and the crack-propagation fluence requirements are increased above those for unstressed conditions.     

Optical modulation of repaired damage site on fused silica produced by CO_2 laser rapid ablation mitigation

CO_2 laser rapid ablation mitigation(RAM) of fused silica has been used in high-power laser systems owing to its advantages of high efficiency, and ease of implementing batch and automated repairing. In order to study the effect of repaired morphology of RAM on laser modulation and to improve laser damage threshold of optics, an finite element method(FEM) mathematical model of 351 nm laser irradiating fused silica optics is developed based on Maxwell electromagnetic field equations, to explore the 3D near-field light intensity distribution inside optics with repaired site on its surface. The influences of the cone angle and the size of the repaired site on incident laser modulation are studied as well. The results have shown that for the repaired site with a cone angle of 73.3°, the light intensity distribution has obvious three-dimensional characteristics. The relative light intensity on z-section has a circularly distribution, and the radius of the annular intensification zone increases with the decrease of z. While the distribution of maximum relative light intensity on y-section is parabolical with the increase of y. As the cone angle of the repaired site decreases, the effect of the repaired surface on light modulation becomes stronger, leading to a weak resistance to laser damage. Moreover, the large size repaired site would also reduce the laser damage threshold. Therefore, a repaired site with a larger cone angle and smaller size is preferred in practical CO_2 laser repairing of surface damage. This work will provide theoretical guidance for the design of repaired surface topography, as well as the improvement of RAM process.     

Second harmonic generation in planar chiral nanostructures

The effect of the chirality of planar spiral nanostructures on optical second harmonic generation (SHG) is studied in the paper. It is shown that the intensity of left and right circularly polarized SHG differs for structures of different handedness and the direction of the SHG polarization rotation plane is opposite for enantiomers.     

Giant optical activity in quasi-two-dimensional planar nanostructures

We examine the spectral dependence in the visible frequency range of the polarization rotation of two-dimensional gratings consisting of chiral gold nanostructures with subwavelength features. The gratings, which do not diffract, are shown to exhibit giant specific rotation (approximately 10(4) degrees/mm) of polarization in direct transmission at normal incidence. The rotation is the same for light incident on the front and back sides of the sample. Such reciprocity indicates three dimensionality of the structure arising from the asymmetry of light-plasmon coupling at the air-metal and substrate-metal interfaces. The structures thus enable polarization control with quasi-two-dimensional planar objects. However, in contradiction with recently suggested interpretation of experiments on larger scale but otherwise similar structures, the observed polarization phenomena violate neither reciprocity nor time-reversal symmetry.