Optical nonlinearities in microstructures produced by ultrashort laser pulses

Biological hard tissues, such as those found in bone and teeth, are complex tissues that build a strong mineral structure over an organic matrix framework. The laser-matter interaction for bone hard tissues holds great interest to laser surgery and laser dentistry; the use of short/ultrashort pulses, in particular, shows interesting behaviors not seen in continuous wave lasers. High laser energy densities in ultrashort pulses can be focused on a small irradiated surface (spot diameter is 10-50 μm) leading to rapid temperature rise and thermal ablation of the bone tissue. Ultrashort pulses, specifically those in the picosecond and femtosecond ranges, impose several challenges in modeling bone tissue response. In the present paper we perform time-dependent thermal simulations of short and ultrashort pulse laser-bone interactions in singlepulse and multipulse (set of ultrashort pulses) modes of laser heating. A comparative analysis for both radiation modes is discussed for laser heating of different types of the solid bone on the nanosecond, picosecond and femtosecond time scales. It is shown that ultrashort laser pulses with high energy densities can ablate bone tissue without heating tissues bordering the ablation creator. This reaction is particularly desirable as heat accumulation and thermal damage are the main factors affecting tissue regrowth rates, and thus patient recovery times.

Ultrashort pulse (USP) laser processing has set a new standard in the manufacturing industry due to its high precision and its capacity to engrave nearly all materials. This technology has been introduced in sectors such as automotive, aerospace and electronics for tasks such as micro-drilling or surface structuring. However, some challenges to the widespread use of USP laser processing include improving fabrication speed and exibility to meet industrial demands. Beam shaping techniques, which involves altering the amplitude, phase, or polarization of laser beams, have emerged as a convenient solution to address these issues, as they allow for more ecient, versatile, and high-resolution material processing. In this thesis, various beam shaping strategies are explored to boost the throughput and applicability of USP laser manufacturing processes. In particular, the experimental work is divided into three main parts, each focusing on a dierent type of beam shaping. The rst part investigates the optimization and application of Bessel beams for generating volume diraction gratings in dierent glasses. It includes the study of nonlinear laser-matter interactions during irradiation of each material and examines its eect on the morphological and optical properties of the resulting volume modications. By ne-tuning the laser writing parameters, ecient phase elements suitable for multiple applications are produced in all types of glass in a single laser pass. The second part examines the use of triangular beams to streamline asymmetrical surface machining. This novel manufacturing method was employed to engrave grooves with a wide range of geometries in a single step. Additionally, it enabled the straightforward fabrication of blazed gratings on stainless steel, with features similar to those manufactured using conventional methods. The third part focuses on the generation of multilevel at-top beams and its application in USP laser polishing. To optimize light structuring, several diractive optical elements (DOEs) were manufactured using a femtosecond (fs) laser-assisted lithographic process. The results of this line of work demonstrate the importance of the design and fabrication of the DOEs to ensure homogeneous beam shaping and uniform processing using an USP laser source. Overall, this thesis presents new strategies to enhance the precision and eciency of ultrashort pulse (USP) laser material processing. The ndings contribute to the broader eld of laser microfabrication and have clear implications in industrial applications.

The ongoing increase of peak and average power of diode lasers makes the consideration of thermo-optical effects due to absorption in the optical system necessary. Otherwise, this local increase in temperature might lead to a significant reduction of the entire functionality or a complete failure. Here, we consider this issue in volume Bragg gratings (VBG) as they are typically used for wavelength stabilization of the laser diodes. Conventionally, these gratings are fabricated very effectively by inscription using ultraviolet light into special photosensitive glasses. However, the use of these special glasses might compromise the transmission and, thus, limit the application. As an alternative, we inscribe VBGs in pure fused silica using ultrashort laser pulses and the phase mask scanning technique. Applying these gratings for external stabilization of high power diode lasers already demonstrated an outstanding performance in sense of spectral drift due to load change compared to conventional VBGs. Here, we present investigations to further optimize the residual absorption of the gratings especially in the NIR. Apart from analyzing the influence of the processing parameters on the residual absorption, we also studied the possibility for a thermal post-processing to anneal absorbing defects. The resulting gratings exhibit a residual absorption close to the intrinsic absorption of fused silica, making them ideally suited for high power applications. Further applications in ultrashort pulse laser systems for pulse stretching and compression are discussed.

Summary form only given. The investigation of strong-field light-matter interactions is increasingly being applied to surfaces and nanostructures. Local enhancement and sub-wavelength confinement facilitate highly nonlinear optics in tailored optical near-fields. Using ultrashort pulses at visible and near-infrared frequencies, metallic nanotips have been shown to allow for enhanced nonlinear photoelectron emission confined to nanometric volumes; numerous effects regarding strong-field emission and carrier-envelope sensitivity were recently identified [1-3]. Currently, there is increasing interest in the impact of spatial inhomogeneity on electron dynamics in nanoscale systems[4]. Here, we investigate localized photoelectron emission from single gold nanotips with ultrashort laser pulses at near- and mid-infrared frequencies. We observe localized optical field emission throughout a wavelength range from 800 nm to 8 μm. The emission is found to be polarization sensitive, and photoelectron energies of hundreds of electronvolts are recorded. Simulations within a two-step photoemission model reveal that the dynamics are governed by the spatial confinement of the driving field. The talk will disseminate these recent results [5,6] and will present novel applications in time-resolved electron imaging based on such localized electron sources. Figure 1a displays the general experimental scenario, in which electrons are field-emitted from a nanometric gold tip using femtosecond laser pulses at various wavelengths. In the low frequency limit, the physical process underlying strong-field infrared photoemission is transient tunnelling through a barrier generated by the driving field. It can be characterized using the intensity dependence of the emission yield, which agrees well with a simple Fowler-Nordheim type tunnelling model (Fig. 1c). Alongside the transition to the strong-field regime, electron acceleration in the locally enhanced optical fields becomes important. Specifically, the kinetic energy spectra consist of a plateau region and an intensity-dependent cutoff. Because of the low absorption of gold at mid-infrared wavelengths and the scaling of the Keldysh-parameter with frequency [5], we can more deeply access the strong-field regime than in previous experiments on nanostructures. The intensities of the local driving fields are substantial enough to accelerate electrons to kinetic energies of hundreds of electronvolts (Fig. 1b). This causes a wavelength scaling of the observed cutoffs that directly reflects the strong spatial localization of the driving field at the nanotip apex [5]. Specifically, the strong-field acceleration at such high energies leads to conditions, in which electrons escape the enhanced near-field within much less than an optical cycle (cf. Fig. 1a). In conclusion, the combination of ultrashort mid-infrared pulses and optical field enhancement allows for unique access to a parameter regime in strong-field physics, in which the spatial and temporal scales of the driving field become coupled. This illustrates new prospects for ultrafast control of electron dynamics in ways that are exclusive to nanostructures.

In this paper, methods of welding and sealing optically transparent materials using an ultrashort pulsed (USP) fiber laser are demonstrated which overcome the limit of small area welding of optical materials. First, the interaction of USP fiber laser radiation inside glass was studied and single line welding results with different laser parameters were investigated. Then multiline scanning was used to obtain successful area bonding. Finally, complete four-edge sealing of fused silica substrates with a USP laser was demonstrated and the hermetic seal was confirmed by water immersion test. This laser microwelding technique can be extended to various applications in the semiconductor industry and precision optic manufacturing.

Mid-infrared ultrafast pulses are useful to drive nonlinear effects towards supercontinuum generation. These mid-infrared supercontinuum light sources are useful in label-free spectral imaging on biological tissues for disease identification. Other potential applications include spectroscopy and spectral imaging for food safety and quality inspection. A high photon flux is also desirable especially in single-shot applications enabling high signal-to-noise ratios or when probing moving targets such as living cells. Fiber lasers are particularly attractive as laser sources as they offer compactness and excellent beam quality. Furthermore, their efficient heat dissipation along long fiber lengths allows high pulse repetition rates and consequently high output powers. However, there are challenges to scale the output powers in fiber lasers due to the deleterious effects of nonlinearities and modulation instability. High-power ultrafast mid-infrared lasers are also useful for material processing of semiconductors and clear polymers, and for interactions with water-rich biological tissues and biomaterials. These applications will benefit from the flexible delivery of the ultrafast pulses with low losses whilst maintaining good beam quality. However, there are challenges in delivering ultrafast mid-infrared pulses, due to silica absorption, peak power damage thresholds in glass and dispersion which distorts the pulse temporal profile. These applications will also benefit from a shorter pulse width which eliminates thermal-related effects. In this thesis, some strategies useful for power scaling in thulium-based fiber lasers namely spectral gain shaping, resonant pumping and chirped pulse amplification will be discussed. The spectral bandwidth and shape of the propagating laser light may suppress nonlinear effects. Thus, a design method to shape the spectral gain of thulium-based sources based on cascaded segments of fibers, variable pumping schemes and other parameter optimization was proposed. Experimentally, a 3-dB bandwidth of 178 nm centered at 1944.75 nm through the cascaded two-segment TDF system was achieved. The use of resonant pumping at 1940 nm in a pulsed thulium fiber amplifier system was investigated. An output power of 40 W was attained with 53 W launched pump power from the 1940 nm CW fiber laser, corresponding to an efficiency of 87%. The gain fiber 5 did not require active cooling due to the low quantum defect, thus resonant pumping method eases the cooling requirements and may potentially reduce the onset of transverse modulation instability (TMI). A compact megawatt picosecond thulium-based fiber laser system based on the chirped pulse amplification (CPA) technique was constructed. The CPA technique allowed amplification at high pulse energies, and we achieved an output pulse energy of 46.3 μJ after compression from a multipass CVBG-based compressor at 2 μm wavelengths. The megawatt picosecond pulses were then used for processing of hydrogel, a temperature-sensitive biomaterial commonly used in cell culture studies. Microchannels, pores and surface foaming were created by direct writing without the need for additives or additional processing. A relatively new hollow-core fiber, the antiresonant hollow-core fiber (AR-HCF), was studied through finite element method analysis and experimental work as a potential candidate for the delivery and compression of high-power ultrafast mid-infrared pulses. The use of a cone type AR-HCF for flexible single-mode delivery of up to 39.1 W average power of 2 μm nanosecond pulses was demonstrated, with transmission efficiencies of up to 77%. The bending performance of the AR-HCF was evaluated and the fiber maintains a Gaussian-like beam profile up to 15 cm bending diameter. The transmission of picosecond and femtosecond 2 μm pulses using the AR-HCF was studied and verified its usefulness for ultrashort pulse delivery. Finally, the use of a gas-filled AR-HCF for nonlinear pulse compression was studied through numerical modelling and experimental demonstration. The pulse propagation in a gas-filled AR-HCF was modelled based on the numerical implementation of the nonlinear Schrödinger equation. Then, the gas cell to contain each fiber end was designed and fabricated, allowing separate pressure control at both fiber ends. The shortest pulse duration obtained was 55.4 fs using a 48 cm-long AR-HCF.

Black-body irradiation method can be utilized for measuring the instantaneous temperatures of electrons and lattice in dielectric machined by the ultrashort laser. One ultrashort laser pulse, of which the pulse energy and pulse duration are 240 J and 599 fs respectively, is focused into the fused silica by objective lenses with a magnification of 10 times. The focal point is at the position of 874 m. The microstructure induced by laser near the focal point is 16 m wide and 104 m long. The central region of the microstructure is heavily damaged, and the marginal region is slightly modified. The black-body irradiation spectra are recorded by the system that is composed of objective lenses, a fiber with two lenses, a spectrometer and an intensified charge coupled device (ICCD). Furthermore, other imaging elements can also be used as alternative to objective lenses, for measuring black-body spectra. The image point, which is conjunctive with the machined region due to the imaging effect of the objective lenses, is coupled into the fiber by one lens. Another lens collimates the diverging light beam from the fiber. The collimated light is incident into the spectrometer and dispersed on the ICCD. Because the minimum gate width of ICCD is much larger than the coupled time of electron and lattice, the temperature of electron equals that of lattice when they are characterized by the black-body irradiation method. The temperatures of the electrons and the lattice are regarded as the temperature of dielectric. When the system acquires the reflection peak of incident ultrashort laser, the delay is set to be 0 ns, and the central wavelength of the peak is 784 nm. Therefore, to eliminate the reflection peak, the second harmonic and supercontinuum spectra, the delay for black-body irradiation acquirement is set to be above 6 ns and the machined region should be confined inside the dielectric. The system collects the black-body spectra emitted by the heat-affected zone in fused silica 981 ns after the fused silica has been irradiated by single ultrashort laser pulse. And then the spectra are fitted by the Planck formula to obtain the temperature of dielectric. After the dielectric is processed by the ultrashort laser pulse, the valence electrons of the dielectric transit to the conduction band via strong filed ionization and avalanche ionization. The plasma with high temperature and pressure moves outward in the form of shockwave. The shockwave transfers energy by convection after fused silica has been machined by laser pulse. Due to inverse Bremsstrahlung effect during the avalanche ionization, nearly all the incident laser energy is absorbed by the fused silica. The irradiated energy is only 1.3% of the absorbed energy, so the ways of heat transfer are mainly convection and heat diffusion. 21 ns later the shock wave turns into acoustic wave, so central gaseous fused silica affects the surrounding region through heat diffusion and the temperature of fused silica decreases slowly. The temperature of fused silica is 5333 exp(-t/1289) K at time t (unit: ns). The temperature drops down to room temperature 3.72s after the fused silica has been irradiated by one ultrashort laser pulse. If another laser pulse arrives at fused silica before 3.72s, the temperature rises on the basis of the previous laser pulse. In other words, the heat accumulation effect cannot be ignored if the repetition rate of ultrashort laser is more than 269 kHz.

When periodic structures are illuminated by a monochromatic continuous light source, the exact images of themselves at certain distances can be observed in the Fresnel diffraction region, and this is the so-called Talbot effect. The Talbot effect has attracted more and more attention because of its wide ranging applications in fields which include optical measurement, optical array illumination, and optical interconnections. Following the rapid development in the techniques of ultrashort laser, therefore, it is necessary to study behaviors of the Talbot effect under ultrashort laser pulses illumination. In this paper, the Talbot effect under illuminated by ultrashort laser pulses with complex spectral distribution has been studied. By using a Michelson interferometer, ultrashort laser pulses with complex spectral distribution can be generated based on the spectral interference of ultrashort laser pulses. In experiments, the Talbot images of a grating under illuminated by ultrashort laser pulses with different complex spectral distributions are obtained. Experimental results are in good agreement with the theoretical analysis. We believed that the behaviors of the Talbot effect should have potential applications in optical measurement.

The interactions of ultrashort laser pulses with planar metal targets are investigated theoretically. The electron temperature as a function of time is derived based on the one-dimensional, two-temperature diffusion model. The valid range of laser duration under the approximation of ultrashort pulses and an expression of the attainable highest electron temperature for the ultrashort pulses laser heating have been obtained.

In the course of increasing miniaturization of components in minimal access surgery the superelastic properties of nickel titanium shape memory alloys (NiTi-SMA) find more and more attention. However, only a few processes are available for machining of miniaturized NiTi-components. Changes of the mechanical properties due to heat input or mechanical tensions have to be avoided. Especially for complex geometries with dimensions in the submillimeter-range, these requirements are hardly to meet. Finding new methods for manufacturing micro-instruments of NiTi wires with high geometrical resolution and superelastic mechanical properties for applications in endoscopic surgery are the main goals of the investigations presented here. Because of the precise focussing properties and the possibility of an excellent non-tactile energy coupling, material processing by lasers is a suitable alternative to conventional machining-processes. Comparing investigations with laser systems with different wavelengths and pulse duration show the suitability of ultra-short pulse Ti:Sapphire lasers for this machining process. Only by the use of ultra-short laser pulses it is possible to structure micro-components of NiTi almost without thermal influence. Results of mechanical and metallographic examinations show that the special properties of miniaturized SMA-components can be maintained. Experimental results as well as example geometries produced with ultra-short pulse lasers are presented in the paper.

In a recent paper [Phys. Rev. A 83, 032106 (2011)], Mackenroth and Di Piazza studied photoemission spectra of an electron driven by intense ultrashort laser pulses. Using kinematic principles, they argued that an electron experiences no mass dressing in an ultrashort pulse. They also proposed a method by which one might experimentally verify their claim. We argue that the scattering kinematics do not imply this conclusion nor do they justify the proposed experiment.

Reliable phase-sensitive time-resolved interferometry with ultrashort pulsed lasers is performed with the use of a closed scanning Michelson interferometer in combination with a fixed MachZehnder interferometer at the front end. The technique is based on measuring the full phase and frequency properties of the pulse distortion of an ultrashort optical pulse introduced by linear or nonlinear interaction with a sample. The necessary stability and reproducibility to perform an interferometric measurement is provided by a commercially available Fourier transform spectrometer enabling time-resolved measurements from the IR well into the visible part of the optical spectrum. The feasibility of the technique is demonstrated by measuring the distortion introduced by an etalon and a surface-plasmon polariton. 0 199.5 American Institute of Physics. We report a method for performing phase-sensitive timeresolved interferometry, using a commercially available Fourier transform spectrometer operating from the IR up to the UV part of the spectrum in combination with a fixed MachZehnder (MZ) at the front end. The technique provides a reliable spectroscopic tool for performing time-resolved interferometry over a wide spectral range. The approach is similar to the method described in Refs. l-5, and is, in principle, identical to the method already used by Michelson.! All these methods are based on measuring the interferometric cross-correlation signal between an undistorted and a distorted light field. After Michelson, the method reported here was again of interest in Fourier tranform infrared spectroscopy.7 Recent interest in the method is in the field of ultrashort pulse generation,le5 due to the role of precise pulse evaluation of both intensity and chirp after they have traveled through optical components. From the cross-correlation signal, both absorption and dispersion in the sample are measured simultaneously, enabling the determination of the group velocity dispersion in materials, for example. In particular, for the distortion of an ultrashort optical laser pulse, the chirp and the sign of the chirp of the pulse distortion can be obtained in a straightforward way without any assumptions. In a pump-probe type of setup, the time resolution is limited by the time duration of the pulses. The shortest reliable time scale in this type of experiment is the time duration of the pulses. However, in our reported method no such limitation occurs, but the shortest reliable time .scale .is in the order of the duration of one cycle of the light field used in the experiments. In the experiment a pulse from a laser is sent through a fixed-length MZ interferometer (Fig. 1) producing two pulses with a separation AL. The sample is placed in one of the arms of the MZ interferometer and this laser pulse is distorted by the sample. The pulse in the other arm remains unaltered and will be used as a reference pulse. The electric field of the reference pulse with light frequency w, may in general be described in the frequency domain by A(o)exp(iwt), with A(o) being a complex slowly varying amplitude function. The complex amplitude function A ( o j describes both the power spectrum of the reference pulse and the relative phase relation between the frequency components. The electric fields of the sum of the distorted pulse and the reference pulse is given by

Recently, a so-called “directly induced” laser ablation effect has been reported, where an ultra-short laser pulse (660 fs and 1053 nm) irradiates a thin Mo film through a glass substrate, resulting in a “lift-off” of the irradiated layer in form of a thin, solid, cylindrical fragment. This effect provides a new and very energy-efficient selective structuring process for the Mo back electrode in thin-film solar cell production. To understand the underlying physical mechanisms, a 3D axisymmetric finite element model was created and numerically solved. The model is verified by a direct comparison of experimental and numerical results. It includes volume absorption of the laser pulse, heat diffusion in the electron gas and the lattice, thermal expansion of the solid phase and further volume expansion from phase transition to fluid and gas, and finally the mechanical motion of the layer caused by the resulting stress wave and the interaction with the substrate. The simulation revealed that irradiation of the molybdenum layer with an ultra-short pulse causes a rapid acceleration in the direction of the surface normal within a time frame of a hundred picoseconds to a peak velocity of about 100 m/s. The molybdenum layer continues to move as an oscillating membrane, and finally forms a dome after about 100 ns. The calculated strain at the edges of the dome exceeds the tensile stress limit at fluences that initiate the “lift-off” in experimental investigations. In addition, the simulation reveals that the driving mechanism of the “lift-off” is the ultrafast expansion of the interface layer and not the generated gas pressure.

Laser applications, such as cutting of medical devices (e.g. stents) and watch components (e.g. gear wheels, hands), demand highest cutting accuracy in combination with extremely high cutting edge quality and minimum cutting kerf. For single pass laser cutting of small components with high precision the application of fiber lasers is the common industrial practice [1]. In this fusion cutting process, molten material in the cutting kerf is ejected by the process gas. Besides the formation of a heat affected zone (HAZ), the weakness of the fusion cutting process is the formation of striations on the cutting surface, which increases the surface roughness of laser cut components [2].Ultra-short pulse lasers provide the possibility of material processing with minimum thermal impact on the remaining material structure [3,4]. Thus laser cutting with very small feature sizes (several microns) and negligible HAZ can be realized. This laser vaporization process using ultra-short laser pulses can replace the common fusion cutting process, if an optimization of the cutting quality and cutting precision is developed. Specifically, the influence of pulse duration on the cutting characteristics must be characterized.In this study, the laser cutting process of different materials is examined by reducing the pulse duration from 6 ps to 900 fs, then down to 400 fs. The results of the micro-cutting process of thin metal sheets with regard to pulse duration, pulse energy, laser repetition rate and cutting velocity have been determined. Quality criteria were identified for evaluating the influence of cutting parameters on the cutting results. The HAZ and micro-structural changes, the striation formation and surface roughness and the taper lead angle of the cutting kerf and surface deposition of material were chosen as criteria and have been analyzed in detail.The results show that ultra-short laser pulses enable high quality cutting with improved roughness and minimized HAZ. The highest quality and cutting speed are reached with sub-ps pulses. Furthermore the processing window for laser vaporization cutting is enlarged by using femtosecond pulses. Finally, with shorter pulses under optimal cutting conditions, an improvement of cutting efficiency and cutting quality is possible.Laser applications, such as cutting of medical devices (e.g. stents) and watch components (e.g. gear wheels, hands), demand highest cutting accuracy in combination with extremely high cutting edge quality and minimum cutting kerf. For single pass laser cutting of small components with high precision the application of fiber lasers is the common industrial practice [1]. In this fusion cutting process, molten material in the cutting kerf is ejected by the process gas. Besides the formation of a heat affected zone (HAZ), the weakness of the fusion cutting process is the formation of striations on the cutting surface, which increases the surface roughness of laser cut components [2].Ultra-short pulse lasers provide the possibility of material processing with minimum thermal impact on the remaining material structure [3,4]. Thus laser cutting with very small feature sizes (several microns) and negligible HAZ can be realized. This laser vaporization process using ultra-short laser pulses can replace the common ...

Optical biochips may incorporate both optical and microfluidic components as well as integrated light emitting semiconductor devices. They make use of a wide range of materials including polymers, glasses and thin metal films which are particularly suitable if low cost devices are envisaged. Precision laser micromachining is an ideal flexible manufacturing technique for such materials with the ability to fabricate structures to sub-micron resolutions and a proven track record in manufacturing scale up. Described here is the manufacture of a range of optical biochip devices and components using laser micromachining techniques. The devices employ both microfluidics and electrokinetic processes for biological cell manipulation and characterization. Excimer laser micromachining has been used to create complex microelectrode arrays and microfluidic channels. Excimer lasers have also been employed to create on-chip optical components such as microlenses and waveguides to allow integrated vertical and edge emitting LEDs and lasers to deliver light to analysis sites within the biochips. Ultra short pulse lasers have been used to structure wafer level semiconductor light emitting devices. Both surface patterning and bulk machining of these active wafers while maintaining functionality has been demonstrated. Described here is the use of combinations of ultra short pulse and excimer lasers for the fabrication of structures to provide ring illumination of in-wafer reaction chambers. The laser micromachining processes employed in this work require minimal post-processing and so make them ideally suited to all stages of optical biochip production from development through to small and large volume production.