Laser-based defect characterization and removal process for manufacturing fused silica optic with high ultraviolet laser damage threshold

Residual processing defects during the contact processing processes greatly reduce the anti-ultraviolet(UV)laser damage performance of fused silica optics,which significantly limited development of high-energy laser systems.In this study,we demonstrate the manufacturing of fused silica optics with a high damage threshold using a CO_(2)laser process chain.Based on theoretical and experimental studies,the proposed uniform layer-by-layer laser ablation technique can be used to characterize the subsurface mechanical damage in three-dimensional full aperture.Longitudinal ablation resolutions ranging from nanometers to micrometers can be realized;the minimum longitudinal resolution is<5 nm.This technique can also be used as a crack-free grinding tool to completely remove subsurface mechanical damage,and as a cleaning tool to effectively clean surface/subsurface contamination.Through effective control of defects in the entire chain,the laser-induced damage thresholds of samples fabricated by the CO_(2)laser process chain were 41%(0%probability)and 65.7%(100%probability)higher than those of samples fabricated using the conventional process chain.This laser-based defect characterization and removal process provides a new tool to guide optimization of the conventional finishing process and represents a new direction for fabrication of highly damage-resistant fused silica optics for high-energy laser applications.

Heat accumulation effects in femtosecond laser-induced subwavelength periodic surface structures on silicon

High-repetition rate femtosecond lasers are shown to drive heat accumulation processes that are attractive for femtosecond laser-induced subwavelength periodic surface structures on silicon.Femtosecond laser micromachining is no longer a nonthermal process,as long as the repetition rate reaches up to 100 kHz due to heat accumulation.Moreover,a higher repetition rate generates much better defined ripple structures on the silicon surface,based on the fact that accumulated heat raises lattice temperature to the melting point of silicon(1687 K),with more intense surface plasmons excited simultaneously.Comparison of the surface morphology on repetition rate and on the overlapping rate confirms that repetition rate and pulse overlapping rate are two competing factors that are responsible for the period of ripple structures.Ripple period drifts longer because of a higher repetition rate due to increasing electron density;however,the period of laser structured surface is significantly reduced with the pulse overlapping rate.The Maxwell–Garnett effect is confirmed to account for the ripple period-decreasing trend with the pulse overlapping rate.

Ultrafast z-scanning for high-efficiency laser micro-machining

High-throughput laser micro-machining demands precise control of the laser beam position to achieve optimal efficiency,but existing methods can be both time-consuming and cost-prohibitive.In this paper,we demonstrate a new high-throughput micromachining technique based on rapidly scanning the laser focal point along the optical axis using an acoustically driven variable focal length lens.Our results show that this scanning method enables higher machining rates over a range of defocus distances and that the effect becomes more significant as the laser energy is increased.In a specific example of silicon,we achieve a nearly threefold increase in the machining rate,while maintaining sharp side walls and a small spot size.This method has great potential for improving the micro-machining efficiency of conventional systems and also opens the door to applying laser machining to workpieces with uneven topography that have been traditionally difficult to process.

Facile preparation of silver nanoparticles in bulk silicate glass by high-repetition-rate picosecond laser pulses

One-step precipitation of Ag nanoparticles in Ag+-doped silicate glasses was achieved through a focused picosecond laser with a high repetition rate. Absorption spectra and transmission electron microscopy(TEM) confirmed that metallic Ag nanoparticles were precipitated within glass samples in the laser-written domain. The surface plasmon absorbance fits well with the experimental absorption spectrum. The nonlinear absorption coefficient β is determined to be 2.47 × 10-14 m/W by fitting the open aperture Z-scan curve, which originated from the intraband transition in the s-p Ag band. The formation mechanism of Ag-glass nanocomposites is discussed as well.

High-aspect 3D two-photon polymerization structuring with widened objective working range (WOW-2PP)

We developed a novel two-photon polymerization(2PP)configuration for fabrication of high-aspect three-dimensional(3D)structures,with an overall height larger than working distance of the microscope objective used for laser beam focusing into a photosensitive material.This method is based on a modified optical 2PP setup,where a microscope objective(1003 high N.A.),immersion oil and cover glass can be moved together into the photosensitive material,resulting in an effective higher and wider objective working range(WOW-2PP).The proposed technique enables the fabrication of high-aspect structures with sub-micrometer process resolution.3D structures with a height of 7 mm are demonstrated,which could hardly be built with the conventional 2PP set-up due to refractive index mismatch and laser beam disturbances.

Four-dimensional light shaping: manipulating ultrafast spatiotemporal foci in space and time

The spectral dispersion of ultrashort pulses allows the simultaneous focusing of light in both space and time,which creates socalled spatiotemporal foci.Such space–time coupling may be combined with the existing holographic techniques to give a further dimension of control when generating focal light fields.In the present study,it is shown that a phase-only hologram placed in the pupil plane of an objective and illuminated by a spatially chirped ultrashort pulse can be used to generate threedimensional arrays of spatio-temporally focused spots.By exploiting the pulse front tilt generated at focus when applying simultaneous spatial and temporal focusing(SSTF),it is possible to overlap neighboring foci in time to create a smooth intensity distribution.The resulting light field displays a high level of axial confinement,with experimental demonstrations given through two-photon microscopy and the non-linear laser fabrication of glass.