Ultrafast laser micromachining has been extensively researched for its “clean, cold” cutting potential in fields from microelectronics to dentistry. It is clear that the mechanism of laser ablation with pulses shorter than about 500 fs differs significantly different from the light-to-heat dominated processes with longer pulsed (ns, ps) and CW laser machining. However, the details of the femtosecond laser ablation mechanism remain incompletely understood. The ablation threshold (J/cm^2) is widely used for characterizing laser machining efficiency. Unfortunately, it is not entirely clear what the ablation threshold means in the ultrashort pulse regime. For example, our diameter regression measurements of the ablation thresholds of several materials using 800 nm, 120 fs laser pulses reveal multiple distinct ablation regimes, each characterized by a different effective beam waist. Evidence of similar behavior can be found in the literature, however it is often unremarked upon. In this paper, we present thorough characterization of the ultrafast laser ablation for a diverse collection of materials (undoped silicon, sapphire, stainless steel and cortical bone). For example, for undoped silicon we find three ablation regimes each characterized by a different ablation threshold and apparent beam waist: (1) 1.56 J/cm^2, 11.8 µm; (2) 1.21 J/cm^2, 51.9 µm; and (3) 0.85 J/cm^2, 159.9 µm. We show the presence of up to three different ablation regimes that vary depending on the type of material. Using computational modeling, we address the mechanistic underpinnings of these observations, particularly the dependence upon pulse energy and spatial beam shape.

The generation of multiple beams is an established method to increase the processing speed of advanced laserbased material processing techniques. This multi-foci approach has shown promising results in laser micromachining, scanning microscopy, optical data storage, or optical tweezer arrays. Of particular interest is its application to manufacturing bigger objects where laser treatment is required over surfaces on the centimetre scale or larger. Using a Spatial Light Modulator (SLM), such parallelisation has been studied over the last few years by splitting the input beam into multiple beams (up to 1000 beams [1]) for simultaneous machining on a plane (x-y) perpendicular to the propagation axis (z). This method is primarily based on holographic femtosecond laser processing [2] where computer-generated holograms (CGH) are displayed on an SLM. Further applications include phase distortion and lens aberration correction [3], or for four-dimension light shaping [4]. The SLM provides unique features for dynamic pulse shaping, more efficient use of the laser energy, or the machining of complex shapes.

Ultrafast laser micromachining that utilises pulses on a femtosecond timescale is a rapidly growing area of research with applications in a wide variety of fields, from microelectronics to microsurgery. Femtosecond pulses are often praised for their ability to perform precise cutting of materials through a ‘cold-cutting’ mechanism which avoids mechanical and thermal collateral damage to the surrounding area. However, the high precision and clean ablation features associated with ultrafast laser micromachining can be counteracted through the intense plasma in air that is generated at high pulse energies. The highly reflective plasma generated above the sample surface can result in a distorted beam profile at the target machining plane, producing machined features with reduced edge quality and accuracy. In addition, the highly reflective plasma results in underutilised portions of the incident pulse energy, therefore decreasing machining efficiency. We present the ablation threshold data and trends for a variety of materials including undoped silicon, stainless steel and sapphire laser machined under vacuum and other ambient conditions. Ablation thresholds were determined using the diameter regression technique with 130 fs, 800 nm laser pulses at a repetition rate of 500 Hz. Ablation features are analysed extensively to observe the impact of the ambient conditions on the resulting feature quality.

Laser micromachining with ultrashort pulses has shown great promise for clean, safe surgical treatment of bone tissue. However, comparisons of performance and development of "best practice" have been hampered by the difficulty of comparing results across a wide variety of experimental approaches and under surgically irrelevant conditions (e.g., dried, dead bone). Using a femtosecond (fs) pulsed laser system (τ = 140 fs, repetition rate = 1 kHz, λ = 800 nm), a comprehensive study of femtosecond laser microsurgery using the standard metrics of laser micromachining (ablation threshold, incubation effects, ablation rates, effect of focal point depth within the material and heat affected zone (HAZ)) was conducted on live, freshly harvested bovine and ovine cortical bone. Three important points of optimism for future implementation in the surgical theatre were identified: (1) the removal of material is relatively insensitive to the focal point depth within the material, removing the need for extreme depth precision for excellent performance; (2) femtosecond laser ablation of fresh bone demonstrates very little incubation effect, such that multiple passes of the laser over the same region of bone removes the same amount of material; and (3) the complete absence of collateral damage, heat- or shock-induced, on both the macro- and microscopic scales can be achieved readily, within a broad parameter range. Taken together, these results indicate a handheld or robotic deployed fiber laser platform for femtosecond laser microsurgery is a very viable prospect.

We review some of the latest advances in beam delivery of high-energy femtosecond pulses. We focus on custom beam delivery via spatial beam shaping or via hollow core fibres for ultra-short pulse transportation.

Femtosecond laser micromachining holds significant promise for advanced manufacturing, however uptake has been limited by the low processing speed. Altering the beam shape from its typical Gaussian profile has been attempted to improve efficiency, however virtually all reliable methods for quantifying the efficiency assume a Gaussian beam shape. Here, we describe an approach for quantifying the ablation threshold fluence - the key parameter for comparing efficiency - suitable for weakly focused non-Gaussian beams. We successfully demonstrate this method for Bessel and vortex beams, finding that the ablation threshold depends not just on the material, but the beam shape as well.

In femtosecond laser micromachining, the ablation threshold is a key processing parameter that characterises the energy density required to cause ablation. Current techniques for measuring the ablation threshold such as the diameter regression and diagonal scan methods are based on the assumption of a Gaussian spatial profile, however no techniques currently exist for measuring the ablation threshold using a non-Gaussian beam shape. Here we present a formalism of the diagonal scan method for determining the ablation threshold and pulse superposition for femtosecond vortex pulses. To the authors' knowledge this is the first ablation threshold technique developed for pulses with non-Gaussian spatial profiles. Using this method, the ablation threshold can be calculated using measurement of a single feature (the maximum damage radius ρ_max), which allows investigations of ablation threshold and incubation effects to be carried out quickly and easily. Extending this method to non-Gaussian beams will allow exploration of new avenues of research, enabling characterisation of the ablation threshold and incubation behaviour for a material when ablated with femtosecond vortex pulses.

Using a femtosecond pulsed laser system (pulse width = 100fs, repetition rate = 1 kHz, lambda=800nm), a zero-order Bessel beam was generated using a LCOS-Spatial light modulator (LCOS-SLM) with an effective cone angle of 4.56 degrees. Ablation threshold studies of fresh bovine and ovine load bearing cortical bone was identified using the method of least damage and found to be identical at. phi(th) = 0.15 +/- 0.03 J cm(-2), irrespective of the target species. The ablation threshold is significantly reduced compared to the ablation threshold determined for Gaussian beams in bovine and ovine cortical bone (Load Bearing: phi(th) = 0.91 +/- 0.03 J cm(-2), Skull: phi(th) = 1.19 +/- 0.06 J cm(-2)). Incubation effects were investigated and the incubation coefficient was determined to be zeta= 0.93 +/- 0.06, indicating no incubation effects are present. The relationship between tissue removal and the number of pulses applied was explored. By altering the translation rate of the sample under the Bessel region of the incident laser, the number of pulses applied at each point along the linear ablation features was varied. Cross sections of ablation features were measured using scanning electron microscopy (SEM) and maximum depths of the ablation features measured. The ablation rate of bovine and ovine cortical was found to be 2.69 - 13.21 +/- 0.05 mu m pulse(-1) and 2.49 - 12.79 +/- 0.03 mu m pulse(-1) respectively for fluence values ranging from 2.5 - 25.0 Jcm(-2), significantly higher than those of Gaussian beams. Structural analysis of the ablation features using SEM and optical microscopy showed no signs of heat affected zone (HAZ) in the form of thermal shockwave cracking, molten debris deposition or charring of the tissue.

Ultrashort pulse laser micromachining is an advanced materials processing technique that allows "cold cutting" of almost any material. This is especially of interest in the semiconductor industry, where mechanical cutting of wafers generates large amounts of waste - if laser micromachining could be applied here, there is the potential for huge increases in the efficiency and flexibility of semiconductor manufacturing. The biggest barrier to industrial application of this technology is the cutting speed, however by tailoring the pulse properties, we hypothesise that machining speed can be increased greatly. The commonly used metric for evaluation of laser micromachining is the ablation threshold - the energy density required to cause material ablation. In this study the effect of incident laser wavelength on the ablation threshold for materials of interest for microfabrication (e.g., silicon) was investigated. This was achieved using a Ti: Sapphire pumped optical parametric amplifier (TOPAS-C) producing femtosecond pulses ( tau = 110 fs, repetition rate = 1 kHz) with wavelengths ranging from 400 nm to 1200 nm using the D-Scan technique. Future work will employ advanced beam shaping technology to tailor pulses in both the spatial and temporal domains to further improve machining efficiency.

We quantify the efficiency of femtosecond micromachining of quartz and silicon as a function of the spatial beam shape (Bessel, vortex, Gaussian). A > 15-fold range of ablation thresholds was observed. Incubation effects were significant.

Refractive index gratings have been inscribed in polymer thin films by permanently photobleaching the organic chromophore PYR-3 dopant. The grating inscription process was investigated in detail for the purpose of improving the diffraction efficiency (η) of the PYR-3 doped polymer gratings. Three processes were identified that contributed to the η of the first diffracted order: a periodic change in the refractive index due to photobleaching of the PYR-3, formation of the surface relief grating as a consequence of free volume change during bleaching, and the introduction of periodic, strain-induced changes in the refractive index.

In laser micromachining, the ablation threshold (minimum fluence required to cause ablation) is a key performance parameter and overall indicator of the efficiency of material removal. For pulsed laser micromachining, this important observable depends upon material properties, pulse properties and the number of pulses applied in a complex manner that is not yet well understood. The incubation effect is one example. It manifests as a change in the ablation threshold as a function of number of laser pulses applied and is driven by photoinduced defect accumulation in the material. Here, we study femtosecond (800 nm, 110 fs, 0.1-1 mJ/pulse) micromachining of a material with well-defined initial defect concentrations: doped Si across a range of dopant types and concentrations. The single-pulse ablation threshold (Fth,1) was observed to decrease with increasing dopant concentration, from a maximum of 0.70 J/cm2 (+/-0.02) for undoped Si to 0.51 J/cm2 (+/-0.01) for highly N-type doped Si. The effect was greater for N-type doped Si than for P-type, consistent with the higher carrier mobility of electrons compared to holes. In contrast, the infinite-pulse ablation threshold (Fth,inf) was the same for all doping levels and types. We attribute this asymptotic behaviour to a maximum defect concentration that is independent of the initial defect concentration and type. These results lend insight into the mechanism of multipulse, femtosecond laser ablation.

Fast, accurate cutting of technical ceramics is a significant technological challenge because of these materials' typical high mechanical strength and thermal resistance. Femtosecond pulsed lasers offer significant promise for meeting this challenge. Femtosecond pulses can machine nearly any material with small kerf and little to no collateral damage to the surrounding material. The main drawback to femtosecond laser machining of ceramics is slow processing speed. In this work we report on the improvement of femtosecond laser cutting of sintered alumina substrates through optimisation of laser processing parameters. The femtosecond laser ablation thresholds for sintered alumina were measured using the diagonal scan method. Incubation effects were found to fit a defect accumulation model, with Fth,1=6.0J/cm2 (±0.3) and Fth,∞=2.5J/cm2 (±0.2). The focal length and depth, laser power, number of passes, and material translation speed were optimised for ablation speed and high quality. Optimal conditions of 500mW power, 100mm focal length, 2000µm/s material translation speed, with 14 passes, produced complete cutting of the alumina substrate at an overall processing speed of 143µm/s – more than 4 times faster than the maximum reported overall processing speed previously achieved by Wang et al. [1]. This process significantly increases processing speeds of alumina substrates, thereby reducing costs, making femtosecond laser machining a more viable option for industrial users.

We investigate ultrashort laser pulses for orthopedic surgery upon two species of fresh, unaltered bone. Ablation thresholds and rates are determined and we explore non-conventional beam types to optimize ablation efficiency and feature properties.

Ultrafast laser machining of ceramic and crystalline substrates offers many benefits versus mechanical dicing. We optimized femtosecond laser parameters for cutting industry sintered alumina and quartz wafers, yielding drastic improvements in cutting speed and quality.

Using a femtosecond pulsed laser system (pulse width = 100fs, repetition rate = 1kHz, lambda = 800nm), ablation threshold studies of freshly culled bovine and ovine cortical bone samples were identified using the diameter regression technique. Using the D-2 technique, the ablation threshold was found to lie within a range of 0.83 - 0.96 Jcm(-2) and 0.89 - 0.95 Jcm(-2) for ovine and bovine cortical bone respectively indicating that laser ablation of bone is irrespective of target species.The relationship between cortical bone tissue removal and the number of applied pulses was explored. By altering the laser spot translation rate, we varied the number of pulses at each point along scribed linear cuts. Optical Coherence Tomography (OCT) and PDMS casting indicates that cut depth is linearly dependent on the number of pulses applied to the tissue, irrespective of donor species. For single pulse ablation of ovine and bovine cortical bone, we determined that the ablation rates were 0.41 - 0.75 mu m per pulse and 0.28 - 0.90 mu m per pulse when pulses of fluences in the range 0.52 - 2.63 Jcm(-2) were applied to ovine and bovine cortical bone tissue, respectively.Structural analysis of the ablation features using environmental scanning electron microscopy and optical microscopy were utilized to assess the ablation features and identify signs of damage to surrounding tissue. We observed no structural indications of thermal shockwave cracking, molten debris deposition or charring of the tissue whilst leaving hydroxyapatite crystal structure intact.

Femtosecond laser pulses are used to understand fading of art chromophores across more than 20 orders of magnitude in time, provide sophisticated approaches for material processing, and as a relatively straightforward tool for device development.