Second harmonic generation by femtosecond Yb-doped fiber laser source based on PPKTP waveguide fabricated by femtosecond laser direct writing

The frequency doubling of femtosecond pulses from an Yb-doped fiber laser source was demonstrated in a PPKTP waveguide fabricated by femtosecond laser direct writing. The PPKTP waveguide contains a fixed period of 8.9 μm and the feomtosecond fundamental pulses have a central wavelength of 1044 nm. A maximum SHG power of 406 mW was produced, yielding a conversion efficiency of 5.6%. Numerical simulations were carried out to investigate the property of frequency doubling for femtosecond pulses. The results show that the SHG process proceeds even the quasi-phase-matching (QPM) condition is not well satisfied, which is significantly different from that of “long” pulses or CW light and is accorded with the experimental results.     

Optical studies of two dimensional gratings in fused silica, GE 124, and Foturan glasses fabricated using femtosecond laser pulses

Herein we present results on the femtosecond laser direct writing and optical characterization studies of two dimensional gratings in fused silica, GE 124, and Foturan^™ glasses. Varieties of structures were achieved with varying input energy and spatial orientation of the samples. Various characterization techniques including fluorescence spectroscopy, micro-Raman spectroscopy, and laser confocal microscopy were employed to analyze the structural and physical modifications at the focal volume resulting in change of refractive index. Diffraction efficiencies of 9–12% were observed from the grating structures. A broad-band emission was observed in the laser-modified region of the Foturan glass. The obtained results are analyzed in the light of recent work in similar glasses and exploring the applications of such structures in the fields of photonics.     

Alcohol-assisted photoetching of silicon carbide with a femtosecond laser

Femtosecond lasers have proved to be effective tools for micromachining silicon carbide material. In the drilling process, however, when the debris around the hole was not removed efficiently, the depth of hole would not increase further. In this paper, alcohol-assisted photoetching of 6H silicon carbide was investigated using a femtosecond laser. Machining in the presence of alcohol was beneficial to the debris ejection from the hole. The alcohol flow and volatilization was also helpful to further carry away the ablation debris and reduce the ablated material redeposition. The experiment showed that photoetching assisted by alcohol produced cleaner ablation effect and deeper hole than in ambient air. Moreover, alcohol assistance would not produce additional thermal damage around the hole. Vias were formed in a 250 μm thick wafer with alcohol-assisted photoetching technique using a femtosecond laser, which demonstrated the potential for this processing technique.     

3D integration of microcomponents in a single glass chip by femtosecond laser direct writing for biochemical analysis

3D integration of microcomponents in a single glass chip by femtosecond laser direct writing followed by post annealing and successive wet etching is described for application to biochemical analysis. Integration of microfluidics and microoptics realized some functional microdevices like a μ-fluidic dye laser and a biosensor. As one of practical applications, we demonstrate inspection of living microorganisms using the microchip with 3D microfluidic structures fabricated by the present technique.     

Soda-lime glass microlens arrays fabricated by laser: Comparison between a nanosecond and a femtosecond IR pulsed laser

We present the manufacturing of microlens arrays on soda-lime glass substrates by using two different IR pulsed lasers: a nanosecond Nd:YVO₄ laser (1064 nm) and a femtosecond laser based on Ytterbium crystal technology (1030 nm). In both cases, the fabrication technique consists of the combination of a direct-write laser process, followed by a post-thermal treatment assisted by a CO₂ laser. Through the analysis of the morphological characteristics of the generated microlenses, the different physical mechanisms involved in the glass ablation process with a nanosecond and a femtosecond laser are studied. In addition, by analyzing the optical features of the microlenses, a better result in terms of the homogeneity and quality of the spot focuses are observed for those microlenses fabricated with the Nd:YVO₄ nanosecond laser. Microlens arrays with a diameter of 80 and 90 µm were fabricated.     

Micromachining of a thin film by laser ablation using femtosecond laser with masks

A gold thin film was machined by laser ablation using a femtosecond laser with mask patterns in the shape of lines and numbers. The patterns were successfully transferred with proper focusing and laser fluence. The optimal femtosecond laser fluence to keep the line width was about 5.2 mJ/cm² on the mask, and 99 mJ/cm² on the film. The processing resolution was 13 μm, and the narrowest line width was about 4 μm.     

Micromachining of circular ring microstructure by femtosecond laser pulses

In this paper, integration of interference phenomenon into femtosecond laser micromachining was reported as the femtosecond laser pulses were reshaped spatially to perform ablation. The generation of circular interference pattern was demonstrated by overlapping infrared femtosecond laser pulses. The interference pattern was subsequently focused on a copper substrate to ablate microstructures of concentric circular rings. The present technique is expected to open up new applications in the areas of rapid fabrication of micro-Fresnel lenses, hybrid microlenses and lens arrays.     

Large negative refractive index modification induced by irradiation of femtosecond laser inside optical glasses

We report on the refractive index modification (Δn) and its cross sectional profile of the created lines inside the different types of optical glasses, containing BaO, TiO₂, or La₂O₃ as a metal oxide. The lines were fabricated by scanning a stage and focusing the femtosecond laser pulses, 800 nm wavelength, a 250 kHz repetition rate and 200 fs pulse duration, from the Ti:sapphire regenerative amplifier system. The Δn measurements were performed with the qualitative phase microscopy technique. As a result, it was found that the Δn and its sign are different depending on glass types. For example, in the glasses containing TiO₂, the Δn became smaller in the modified region and some of them showed relatively large decrease of the Δn, Δn < −0.01, with about 10 μm width. Such a glass material could be useful for the compact optics and optical devices.     

Surface ablation of lithium tantalate by femtosecond laser

We report measurements of the laser induced breakdown threshold in lithium tantalate with different number of pulses delivered from a chirped pulse amplification Ti: sapphire system. The threshold fluences were determined from the relation between the diameter D² of the ablated area and the laser fluence F₀. The threshold of lithium tantalite under single-shot is found to be 1.84 J/cm², and the avalanche rate was determined to be 1.01 cm²/J by calculation. We found that avalanche dominates the ablation process, while photoionization serves as a free electron provider.     

Laser induced reverse transfer of gold thin film using femtosecond laser

Laser induced reverse transfer (LIRT) has been executed first time using a Mega Hertz pulse frequency femtosecond laser radiation under ambient conditions. Research has been conducted to understand the evolution of deposited structures with regard to pulse energy. Solid deposition of gold could be deposited on quartz substrate only if pulse energy falls within a certain range. For the experiment conducted in this work, it is 36-40 nJ. For energies above this range, crests appear in the middle of the deposition. There is a threshold in maximum applicable pulse energy, 82 nJ in this exepriment, beyond which further increase in pulse energy results in only traces of deposited material. Results also show that the width of deposited line increases with the increase in pulse energy and decreases with the increase in scan speed. These observations have been explained using the dynamics of ablated plume.     

Spectral and temporal characteristics of metallic nanoparticles produced by femtosecond laser pulses

We study the time of flight optical emission from titanium and tungsten nanosized particles, generated through femtosecond laser-matter interaction in vacuum, in the wavelength spectral range from 300 to 900 nm. Typical spectra consist of broadband structureless signals similar to black body emission from a macroscopic object. Nanoparticles temperature, deduced from their emission spectra, decreases drastically as a function of their time of arrival at a given distance from the target. This behaviour is seen to be independent of individual particle velocities.     

Geometric-optical reconstruction of a wavefront experimental realization with femtosecond laser pulses

The mechanism of wavefront reconstruction by geometric-optical reflection of reconstruction radiation from surfaces with constant phase differences between the object and reference waves has been investigated. The main difference between this mechanism and a holographic one is the absence of diffraction of the reconstructing radiation by the periodic structure and as a consequence the achromatism of the reconstruction process. Incoherent continuous radiation and ultrashort laser pulses were used in the experiments. The effect of achromatic reconstruction has been obtained after recording the interference of counterpropagating 30–40 fs pulses from an Al₂O₃:Ti³⁺ laser in bulk media.     

Investigation of nanoparticle generation during femtosecond laser ablation of metals

The production of nanoparticles via femtosecond laser ablation of gold and copper is investigated experimentally involving measurements of the ablated mass, plasma diagnostics, and analysis of the nanoparticle size distribution. The targets were irradiated under vacuum with a spot of uniform energy distribution. Only a few laser pulses were applied to each irradiation site to make sure that the plume expansion dynamics were not altered by the depth of the laser-produced crater. Under these conditions, the size distribution of nanoparticles does not exhibit a maximum and the particle abundance monotonously decreases with size. Furthermore, the results indicate that two populations of nanoparticles exist within the plume: small clusters that are more abundant in the fast frontal plume component and larger particles that are located mostly at the back. It is shown that the ablation efficiency is strongly related to the presence of nanoparticles in the plume.     

Generation of regular femtosecond pulses in AlGaInP semiconductor laser

It is shown experimentally that stable ~ 200 fs regular pulses and optical frequency comb spectrum containing ~ 70 discrete frequencies can be achieved using semiconductor injection laser working in the conditions of superfluorescence under dc pumping current, which is lower than the threshold of laser action.     

Metal thin film ablation with femtosecond pulsed laser

Micromachining thin metal films coated on glass are widely used to repair semiconductor masks and to fabricate optoelectrical and MEMS devices. The interaction of lasers and materials must be understood in order to achieve efficient micromachining. This work investigates the morphology of thin metal films after machining with femtosecond laser ablation using about 1 μm diameter laser beam. The effect of the film thickness on the results is analyzed by comparing experimental images with data obtained using a two-temperature heat transfer model. The experiment was conducted using a high numerical aperture objective lens and a temporal pulse width of 220 fs on 200- and 500-nm-thick chromium films. The resulting surface morphology after machining was due to the thermal incubation effect, low thermal diffusivity of the glass substrate, and thermodynamic flow of the metal induced by volumetric evaporation. A Fraunhofer diffraction pattern was found in the 500-nm-thick film, and a ripple parallel to the direction of the laser light was observed after a few multiple laser shots. These results are useful for applications requiring micro- or nano-sized machining.     

Picosecond and femtosecond pulsed laser ablation and deposition of quasicrystals

A Nd:glass laser with pulse duration of 250 fs and 1.3 ps has been used to evaporate a Al₆₅Cu₂₃Fe₁₂ quasicrystalline target. The gaseous phase obtained from the ablation process has been characterised by several techniques such as emission spectroscopy, quadrupole mass spectrometry and ICCD imaging, used to study the plume composition, energy and morphology. The results show that the ablation processes in the short-pulse regimes are very different to the nanosecond one. In particular the plume angular distribution shows a characteristic high cosine exponent and the composition is completely stoichiometric and independent from the laser fluence. Furthermore the mass spectra indicate the presence of clusters, both neutral and ionised and the emission from the target suggest a rapid thermalisation leading to the melting of the surface. To clarify the ablation process some films have been deposited, on oriented silicon, at different experimental conditions and analysed by scanning electron microscopy, atomic force microscopy, energy dispersive X-ray analysis and X-ray diffraction. The analyses show the presence of nanostructured films retaining the target stoichiometry but consisting of different crystalline and non crystalline phases. In particular the nanostructure supports the hypothesis of the melting of the target during the ablation and a mechanism of material ejection is proposed for both picosecond and femtosecond regimes.     

Direct patterning of microelectrode arrays using femtosecond laser micromachining

Micron-resolution microelectrodes and microelectrode arrays are commonly used in Lab-on-a-chip applications, typically for particle and fluidic sensing or pumping applications. The common method of fabricating such structures is to use conventional photolithography, which involves several steps, any of which can affect the quality and dimension of the final structure. Here we present an alternative method of creating microelectrodes using direct laser patterning. This is a significantly simpler and faster fabrication route requiring a single patterning step, and which also allows design changes to be implemented quickly, without having to re-manufacture a photolithographic mask. To confirm the suitability of this approach to fabricating microelectrodes, a complex multielectrode array design for neuron stimulation was directly laser-patterned.     

Surface texturing of the carbon steel AISI 1045 using femtosecond laser in single pulse and scanning regime

Surface texturing of the metals, including steels, gained a new dimension with the appearance of femtosecond lasers. These laser systems enable highly precise modifications, which are very important for numerous applications of metals. The effects of a Ti:sapphire femtosecond laser with the pulse duration of 160 fs, operating at 775 nm wavelength and in two operational regimes - single pulse (SP) and scanning regime, on a high quality AISI 1045 carbon steel were studied. The estimated surface damage threshold was 0.22 J/cm² (SP). Surface modification was studied for the laser fluences of 0.66, 1.48 and 2.37 J/cm². The fluence of 0.66 J/cm², in both working regimes, induced texturing of the material, i.e. formation of periodic surface structures (PSS). Their periodicity was in accordance with the used laser wavelength. Finally, changes in the surface oxygen content caused by ultrashort laser pulses were recorded.     

Micromachining of semiconductor by femtosecond laser for integrated circuit defect analysis

The latest International Technology Roadmap for Semiconductors (ITRS) has highlighted the detection and analysis of defects in Integrated Circuits (IC) as a major challenge faced by the semiconductor industry.Advanced tools used today for defect cross sectioning include dual beams (focused ion- and electron-beam technologies) with resolution down to the sub-Angstrom level. However ion milling an IC with a FIB is time consuming because of the need to open wide cavities in front of the cross-sections that need to be analyzed. Therefore the use of a femtosecond laser as a tool for direct material removal is discussed in this paper. Experiments were performed on IC structures to reveal the different layers of fabrication: selective or total ablation can occur depending on the laser energy density, without delamination of the layers. Different laser irradiation conditions like pressure (air, vacuum), polarization, beam shaping, and scanning parameters have been used to produce different types of cavities. The femtosecond laser engraving of silicon-based structures could be useful for cross-sectioning devices but also for other applications like direct-write lithography, photomask repair, maskless implantation or reverse engineering/restructuring.