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Ilya Alexeev

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Papers by Ilya Alexeev

New approaches in teaching laser engineering classes: modeling and building up a laser

14th Conference on Education and Training in Optics and Photonics: ETOP 2017, Aug 16, 2017

We present a simulation tool to model performance of a bulk solid state laser and propose several... more We present a simulation tool to model performance of a bulk solid state laser and propose several ways how this tool can be used to enhance educational experience of the student studying laser technology. In one of the possible approaches, the ASLD software can complement available educational laser kits to provide the students with more universal practical training. In the second approach, which is the primary focus of this contribution, the ASLD software can be used as a development tool that allows students to verify their understanding of the subject as well as to propose and verify their own design ideas. The software can be extremely helpful if an experimantal setup has to be built from already present optical components in order to reduce the cost of training or if specific design objectives have to be attained.

A simple laser beam characterization apparatus based on imaging

Journal of Applied Research and Technology, 2021

The development of an original low-cost hardware/software apparatus to optically control the lase... more The development of an original low-cost hardware/software apparatus to optically control the laser beam and measure quality parameters by image analysis using a monochrome matrix sensor is described, along with experimental validation. Laser parameters, namely beam waist, divergence, Rayleigh range and M2, which are the most relevant in manufacturing operations (like optical measurement and laser processing), have been physically and mathematically characterized. Strategies for the apparatus size reduction are also presented, including the use of a prism to reduce by half the optical path, and a camera model to simulate the sensor resolution.

Fabrication and Application of Phase only Holograms for High Power Laser Beam Shaping

Journal of Laser Micro/Nanoengineering, 2016

Fabrication of phase only holograms via spatially selective ablation of an ITO nanoparticulate la... more Fabrication of phase only holograms via spatially selective ablation of an ITO nanoparticulate layer coated on an optical quality transparent substrate appears to be very attractive due to the simplicity and flexibility of this approach. A pre-calculated binary pattern can be directly written on such a layer in order to produce a desired intensity distribution in the focal plane of a focusing lens. In this contribution we analyze the performance of the fabricated holograms and their suitability for high power beam shaping as well as propose a way to improve the diffraction efficiency of such structures by transitioning from a single to multi-layer holograms.

Diagnostic of Laser-Plasmas: Single-shot Supercontinuum Spectral Interferometry

AIP Conference Proceedings, 2004

Ultrafast optical diagnostics play a vital role in probing the dynamics in laser-matter interacti... more Ultrafast optical diagnostics play a vital role in probing the dynamics in laser-matter interactions, including those observed in the high intensity ultrashort laser pulse regime. We developed a new femtosecond optical diagnostic, single-shot supercontinuum spectral interferometry (SSSI), which measures ultra-rapid transients in the complex index of refraction induced by an intense laser pulse. This measurement provides a direct view on how the laserproduced perturbations evolve in time and space. To date, SSSI has been successfully used to diagnose femtosecond dynamics in the interaction of intense laser pulses with gases, nanometersized atomic or molecular clusters, and plasmas, including plasma waveguides.

Positioning Accuracy in Optical Trap Assisted Nanostructuring

Journal of Laser Micro/Nanoengineering, 2013

The well-known optical diffraction limit prevents laser radiation from being focused to arbitrary... more The well-known optical diffraction limit prevents laser radiation from being focused to arbitrary small spots. Utilizing spherical micro-particles as near-field lenses provides an easy and robust way to overcome this limitation and to generate nanoscale surface structures. To achieve this, microspheres are positioned and translated with an optical trap and then used to focus pulsed laser radiation. The paper at hand presents the factors which influence the positioning accuracy in optical trap assisted nanostructuring. Theoretical considerations are backed up with experimentally obtained values of the surface structure accuracy in dependence of optical trap stiffness and particle position.

Application of Bessel beams for ultrafast laser volume structuring of non transparent media

Physics Procedia, 2010

The use of ultrashort laser pulses has been shown to be an attractive option for high quality mic... more The use of ultrashort laser pulses has been shown to be an attractive option for high quality micromachining. In these applications low energy laser pulses are tightly focused to achieve high light intensities. While this approach has been demonstrated to be successful, its application is challenged by the short Rayleigh range of a tightly focused laser beam. Precise positioning of material samples becomes crucial imposing extra requirements on sample flatness, tilt, and irregularities. To overcome some of these limitations an application of non-diffractive Bessel beams for laser material processing has been attempted recently. The non-diffractive focus of Bessel beams can have the depth of field significantly longer than the Rayleigh range of a Gaussian beam of a comparable diameter. With sub mJ laser pulse energy the light intensity along the line focus can reach and exceed the required threshold levels making laser Bessel beams suitable for material processing. Preliminary numerical simulations show that truncated by a micron size aperture Bessel beam still has the propagation depth sufficiently long to be suitable for volumetric micro structuring of non transparent media. The paper aims to demonstrate feasibility of ultrafast laser structuring of non transparent media using Bessel beam focusing geometry.

Laser Melting of Nanoparticulate Transparent Conductive Oxide Thin Films

Journal of Laser Micro/Nanoengineering, 2013

Layers of ZnO nanoparticles with thicknesses of about 40 nm were prepared on silicon substrates. ... more Layers of ZnO nanoparticles with thicknesses of about 40 nm were prepared on silicon substrates. The layers were irradiated by single pulses of a 248 nm excimer laser, which proofed suitable for consolidation and significant densification of the particle layers under ambient conditions. Experiments as well as simulations have confirmed that the application of a SiO 2 particle layer between the substrate and the ZnO particle layer can be used to hamper heat transfer towards the substrate. Thus the ZnO layer can be thermally insulated from the substrate while heating the ZnO up to its extremely high melting point. Consequently, such a layer stack could enable the application of consolidated particle layers on temperature-sensitive carrier substrates such as polymer foils which are to be used in low-cost mass production of devices like displays or solar cells.

High Speed Pump-Probe Apparatus for Observation of Transitional Effects in Ultrafast Laser Micromachining Processes

Micromachines, 2015

A pump-probe experimental approach has been shown to be a very efficient tool for the observation... more A pump-probe experimental approach has been shown to be a very efficient tool for the observation and analysis of various laser matter interaction effects. In those setups, synchronized laser pulses are used to create an event (pump) and to simultaneously observe it (probe). In general, the physical effects that can be investigated with such an apparatus are restricted by the temporal resolution of the probe pulse and the observation window. The latter can be greatly extended by adjusting the pump-probe time delay under the assumption that the interaction process remains fairly reproducible. Unfortunately, this assumption becomes invalid in the case of high-repetition-rate ultrafast laser material processing, where the irradiation history strongly affects the ongoing interaction process. In this contribution, the authors present an extension of the pump-probe setup that allows to investigate transitional and dynamic effects present during ultrafast laser machining performed at high pulse repetition frequencies.

X-ray and extreme ultraviolet emission induced by variable pulse-width irradiation of Ar and Kr clusters and droplets

Physical Review E, 2000

Measurements are presented of x-ray ͑Ͼ1.5 keV͒ and extreme ultraviolet ͑EUV, ϭ2-44 nm͒ emission f... more Measurements are presented of x-ray ͑Ͼ1.5 keV͒ and extreme ultraviolet ͑EUV, ϭ2-44 nm͒ emission from argon and krypton supersonic gas jets at room (Tϭ300 K) and cryogenic (Tϭ173 K) temperatures irradiated with constant energy ͑50 mJ͒, variable width laser pulses ranging from 100 fs to 10 ns. Two regimes of jet operation are explored: cluster formation (radiusϽ100 nm) and droplet formation (radiusϾ1 m). The results for both clusters and droplets can be understood in terms of two time scales: a short time scale for optimal resonant absorption at the critical density layer in the expanding plasma, and a longer time scale for the plasma to drop below critical density.

Tubular plasma generation with a high-power hollow Bessel beam

Physical Review E, 2000

A high power, hollow Bessel beam (J 5) is generated using an axicon and a phase plate in combinat... more A high power, hollow Bessel beam (J 5) is generated using an axicon and a phase plate in combination. The optical breakdown of a gas target and generation of a tubular plasma fiber with such a beam is realized. Hydrodynamic simulations of the hollow beam-plasma breakdown and heating are in reasonable agreement with interferometric measurements of the plasma time evolution.

$Research paper thumbnail of 7. Optical trap assisted sub diffraction limited laser structuring$

7. Optical trap assisted sub diffraction limited laser structuring

Biomedical and Technical Applications

The ability to structure surfaces on a submicron scale is becoming increasingly important for a b... more The ability to structure surfaces on a submicron scale is becoming increasingly important for a broad range of technical and biological applications. Such structures can be produced via irradiating micron sized transparent particles placed over a substrate of interest with ultrafast laser pulses. To enhance the structuring flexibility of this method particles can be optically trapped and spatially manipulated relative to the structured surface in order to achieve arbitrary patterns. In this contribution the authors describe the principle of optical trap assisted nanostructuring and present simulated and experimental results demonstrating the potential and limitations of this innovative nanoscale optical material processing technology.

Ultrashort Pulse Laser Cutting of Intraocular Lens Polymers

Journal of Laser Micro/Nanoengineering, 2014

Material processing by ultra-short pulsed laser ablation is an attractive approach for various ap... more Material processing by ultra-short pulsed laser ablation is an attractive approach for various applications. In this paper we present our studies on cutting of medical polymers used for intraocular lenses (IOL). To achieve best results the ablation characteristics and the ablation thresholds are identified for single and multiple irradiation of the lens material by picosecond pulses. The results are used for quantification of the incubation effect. Furthermore, cutting experiments are carried out and thereby different damage types can be observed. A process window is developed containing the regions of the respective types of damages and backed up by theoretical considerations.

Ultrafast Laser Surface Structuring of Intraocular Lens Polymers

Journal of Laser Micro/Nanoengineering, 2013

Ultrafast laser material processing is an attractive approach for surface structuring of medical ... more Ultrafast laser material processing is an attractive approach for surface structuring of medical polymers used for intraocular lenses. To achieve best results in terms of efficiency and quality, the wavelength and pulse energy dependence on the ablation behavior of such materials has to be studied. Ablation thresholds have been determined for a polymer with different dyes which is used for intraocular lens (IOL) manufacturing. Furthermore, the ablation efficiency, achievable precision and surface roughness have been quantified to optimize the parameters for an efficient intraocular lens manufacturing process by surface microstructuring.

Resonant self-trapping of high intensity Bessel beams in underdense plasmas

Physical review. E, Statistical, nonlinear, and soft matter physics, 2002

We present a comprehensive report based on recent work [Phys. Rev. Lett. 84, 3085 (2000)] on reso... more We present a comprehensive report based on recent work [Phys. Rev. Lett. 84, 3085 (2000)] on resonant self-trapping and enhanced absorption of high power Bessel beams in underdense plasmas. The trapping resonance is strongly dependent on initial gas pressure, Bessel-beam geometry, and laser wavelength. Analytic estimates, and simulations using a one-dimensional Bessel-beam-plasma interaction code consistently explain the experimental observations. These results are for longer, moderate intensity pulses where the self-trapping channel is induced by laser-heated plasma thermal pressure. To explore the extension of this effect to ultrashort, intense pulsed Bessel beams, we perform propagation simulations using the code WAKE [Phys. Rev. E 53, R2068 (1996)]. We find that self-trapping can occur as a result of a plasma refractive index channel induced by the combined effects of relativistic motion of electrons and their ponderomotive expulsion.

Focus Alignment Method for Laser Manufacturing at Sub-micron Positional Accuracy

Physics Procedia, 2012

Accurate positioning of a sample is one of the major challenges in the laser micro manufacturing-... more Accurate positioning of a sample is one of the major challenges in the laser micro manufacturing-especially if the requirements on tolerances are high as in ultrafast laser micromachining. There are a number of methods that allow detection of the surface position, however only few of them use the beam of the processing laser as a basis for the measurement. These methods have an advantage that any changes in the structuring beam will be inherently accommodated for. This work describes a direct contact free method to accurately determine the surface position with respect to the structuring beam focal plane. The method makes alignment of unique samples precise and time efficient due to ease of automation and provides a reproducibility of surface detection of less than 1μm.

Direct Waveguide Writing with High Energy High Repetition Rate Picosecond Laser Pulses

Physics Procedia, 2012

Direct femtosecond pulse laser waveguide writing based on laser induced localized modification of... more Direct femtosecond pulse laser waveguide writing based on laser induced localized modification of the refractive index of bulk glass can be a versatile fabrication method for many micro applications. Although the method's potentials have been realized its application is still rather limited, either due to the slow writing speeds caused by low laser pulse repetition rates of amplified systems or due to the shallow structuring depths restricted by low pulse energies of nonamplified lasers. In this work we investigate the direct waveguide writing process using a picosecond laser that allows fast and deep waveguide writing due to high pulse energies available at high pulse repetition frequencies.

Defect formation in glass welding by means of ultra short laser pulses

Physics Procedia, 2010

Many applications require a joining of several glass components. However, the established process... more Many applications require a joining of several glass components. However, the established processes for glass joining have certain disadvantages. By contrast, laser fusion welding by ultrafast lasers exhibits in principle excellent versatility, if the nonlinear interaction is localized enough so that plasma induced stresses can be tolerated and the temperature is high enough to melt the glass adjacent to the focal spot. However, defect generation during welding was observed for a broad range of welding conditions. This paper reports the observed defects and analyzes their origins.

Practical Aspects of Surface Generated Third Harmonic for Highly Precise Beam-workpiece Alignment

Journal of Laser Micro/Nanoengineering, 2013

Precise beam-workpiece alignment is highly important in laser processing. Though many methods exi... more Precise beam-workpiece alignment is highly important in laser processing. Though many methods exist for this task few allow high-precision measurements using the processing laser itself. One such method detects the laser's backreflection from the workpiece's surface using a confocal setup and yields ideally an accuracy of about 10% of the Rayleigh length of the focused beam. Nevertheless, under certain conditions the accuracy of this method may suffer from a mismatch of beam divergences between laser and confocal detection optics. This becomes important when transparent materials are processed in three dimensions using optics with very high numerical apertures or if expensive materials or samples have to be processed where any misalignment would be costly. A possible solution to this is surface based third harmonic generation that allows for absolute surface detection while having the same accuracy as the confocal method. However, this method requires a complex setup while exhibiting low conversion efficiency. Both reasons make a proper alignment of this measurement scheme difficult. Herein we present results showing a clear conversion efficiency dependence on the reflection coefficient of the generating interface for a number of material combinations. This makes the selection of suitable materials more straightforward and reduces the alignment effort. The highest conversion efficiency was achieved for air/silicon which exhibits the highest reflection coefficient of the investigated materials.

Generation of phase-only holograms by laser ablation of nanoparticulate ITO layers

Journal of Optics, 2014

Phase holograms offer great potential e.g. for laser beam shaping and imaging applications. Howev... more Phase holograms offer great potential e.g. for laser beam shaping and imaging applications. However, the generation of these structures typically requires multi-step processes which tend to be time-consuming and expensive, especially if high-quality structures are required. In this work we demonstrate a flexible and inexpensive method which allows for the production of binary phase-only holograms by laser ablation of ITO nanoparticle layers. Since these layers are ablated free of residues, accurately defined phase shifts can be achieved. While arbitrarily shaped structures can be generated, the ITO layer thickness can be adjusted in order to freely tune the phase shift for the desired wavelength. In the diffraction patterns generated by the holograms we observed an excellent zero order suppression.

Microsphere near-field nanostructuring using picosecond pulses

Physics Procedia, 2010

In laser material processing the use of nonlinear multi-photon and near-field effects allows to p... more In laser material processing the use of nonlinear multi-photon and near-field effects allows to produce nanostructures smaller than the wavelength of the processing laser. In the near-field of small particles locally restricted field enhancements occur, allowing a focusing of pulsed laser radiation far beyond the diffraction limit. In this contribution a basic study of near-field and beam-matter interaction phenomena in microsphere near-field nanostructuring using picosecond laser pulses shall be presented.

New approaches in teaching laser engineering classes: modeling and building up a laser

14th Conference on Education and Training in Optics and Photonics: ETOP 2017, Aug 16, 2017

We present a simulation tool to model performance of a bulk solid state laser and propose several... more We present a simulation tool to model performance of a bulk solid state laser and propose several ways how this tool can be used to enhance educational experience of the student studying laser technology. In one of the possible approaches, the ASLD software can complement available educational laser kits to provide the students with more universal practical training. In the second approach, which is the primary focus of this contribution, the ASLD software can be used as a development tool that allows students to verify their understanding of the subject as well as to propose and verify their own design ideas. The software can be extremely helpful if an experimantal setup has to be built from already present optical components in order to reduce the cost of training or if specific design objectives have to be attained.

A simple laser beam characterization apparatus based on imaging

Journal of Applied Research and Technology, 2021

The development of an original low-cost hardware/software apparatus to optically control the lase... more The development of an original low-cost hardware/software apparatus to optically control the laser beam and measure quality parameters by image analysis using a monochrome matrix sensor is described, along with experimental validation. Laser parameters, namely beam waist, divergence, Rayleigh range and M2, which are the most relevant in manufacturing operations (like optical measurement and laser processing), have been physically and mathematically characterized. Strategies for the apparatus size reduction are also presented, including the use of a prism to reduce by half the optical path, and a camera model to simulate the sensor resolution.

Fabrication and Application of Phase only Holograms for High Power Laser Beam Shaping

Journal of Laser Micro/Nanoengineering, 2016

Fabrication of phase only holograms via spatially selective ablation of an ITO nanoparticulate la... more Fabrication of phase only holograms via spatially selective ablation of an ITO nanoparticulate layer coated on an optical quality transparent substrate appears to be very attractive due to the simplicity and flexibility of this approach. A pre-calculated binary pattern can be directly written on such a layer in order to produce a desired intensity distribution in the focal plane of a focusing lens. In this contribution we analyze the performance of the fabricated holograms and their suitability for high power beam shaping as well as propose a way to improve the diffraction efficiency of such structures by transitioning from a single to multi-layer holograms.

Diagnostic of Laser-Plasmas: Single-shot Supercontinuum Spectral Interferometry

AIP Conference Proceedings, 2004

Ultrafast optical diagnostics play a vital role in probing the dynamics in laser-matter interacti... more Ultrafast optical diagnostics play a vital role in probing the dynamics in laser-matter interactions, including those observed in the high intensity ultrashort laser pulse regime. We developed a new femtosecond optical diagnostic, single-shot supercontinuum spectral interferometry (SSSI), which measures ultra-rapid transients in the complex index of refraction induced by an intense laser pulse. This measurement provides a direct view on how the laserproduced perturbations evolve in time and space. To date, SSSI has been successfully used to diagnose femtosecond dynamics in the interaction of intense laser pulses with gases, nanometersized atomic or molecular clusters, and plasmas, including plasma waveguides.

Positioning Accuracy in Optical Trap Assisted Nanostructuring

Journal of Laser Micro/Nanoengineering, 2013

The well-known optical diffraction limit prevents laser radiation from being focused to arbitrary... more The well-known optical diffraction limit prevents laser radiation from being focused to arbitrary small spots. Utilizing spherical micro-particles as near-field lenses provides an easy and robust way to overcome this limitation and to generate nanoscale surface structures. To achieve this, microspheres are positioned and translated with an optical trap and then used to focus pulsed laser radiation. The paper at hand presents the factors which influence the positioning accuracy in optical trap assisted nanostructuring. Theoretical considerations are backed up with experimentally obtained values of the surface structure accuracy in dependence of optical trap stiffness and particle position.

Application of Bessel beams for ultrafast laser volume structuring of non transparent media

Physics Procedia, 2010

The use of ultrashort laser pulses has been shown to be an attractive option for high quality mic... more The use of ultrashort laser pulses has been shown to be an attractive option for high quality micromachining. In these applications low energy laser pulses are tightly focused to achieve high light intensities. While this approach has been demonstrated to be successful, its application is challenged by the short Rayleigh range of a tightly focused laser beam. Precise positioning of material samples becomes crucial imposing extra requirements on sample flatness, tilt, and irregularities. To overcome some of these limitations an application of non-diffractive Bessel beams for laser material processing has been attempted recently. The non-diffractive focus of Bessel beams can have the depth of field significantly longer than the Rayleigh range of a Gaussian beam of a comparable diameter. With sub mJ laser pulse energy the light intensity along the line focus can reach and exceed the required threshold levels making laser Bessel beams suitable for material processing. Preliminary numerical simulations show that truncated by a micron size aperture Bessel beam still has the propagation depth sufficiently long to be suitable for volumetric micro structuring of non transparent media. The paper aims to demonstrate feasibility of ultrafast laser structuring of non transparent media using Bessel beam focusing geometry.

Laser Melting of Nanoparticulate Transparent Conductive Oxide Thin Films

Journal of Laser Micro/Nanoengineering, 2013

Layers of ZnO nanoparticles with thicknesses of about 40 nm were prepared on silicon substrates. ... more Layers of ZnO nanoparticles with thicknesses of about 40 nm were prepared on silicon substrates. The layers were irradiated by single pulses of a 248 nm excimer laser, which proofed suitable for consolidation and significant densification of the particle layers under ambient conditions. Experiments as well as simulations have confirmed that the application of a SiO 2 particle layer between the substrate and the ZnO particle layer can be used to hamper heat transfer towards the substrate. Thus the ZnO layer can be thermally insulated from the substrate while heating the ZnO up to its extremely high melting point. Consequently, such a layer stack could enable the application of consolidated particle layers on temperature-sensitive carrier substrates such as polymer foils which are to be used in low-cost mass production of devices like displays or solar cells.

High Speed Pump-Probe Apparatus for Observation of Transitional Effects in Ultrafast Laser Micromachining Processes

Micromachines, 2015

A pump-probe experimental approach has been shown to be a very efficient tool for the observation... more A pump-probe experimental approach has been shown to be a very efficient tool for the observation and analysis of various laser matter interaction effects. In those setups, synchronized laser pulses are used to create an event (pump) and to simultaneously observe it (probe). In general, the physical effects that can be investigated with such an apparatus are restricted by the temporal resolution of the probe pulse and the observation window. The latter can be greatly extended by adjusting the pump-probe time delay under the assumption that the interaction process remains fairly reproducible. Unfortunately, this assumption becomes invalid in the case of high-repetition-rate ultrafast laser material processing, where the irradiation history strongly affects the ongoing interaction process. In this contribution, the authors present an extension of the pump-probe setup that allows to investigate transitional and dynamic effects present during ultrafast laser machining performed at high pulse repetition frequencies.

X-ray and extreme ultraviolet emission induced by variable pulse-width irradiation of Ar and Kr clusters and droplets

Physical Review E, 2000

Measurements are presented of x-ray ͑Ͼ1.5 keV͒ and extreme ultraviolet ͑EUV, ϭ2-44 nm͒ emission f... more Measurements are presented of x-ray ͑Ͼ1.5 keV͒ and extreme ultraviolet ͑EUV, ϭ2-44 nm͒ emission from argon and krypton supersonic gas jets at room (Tϭ300 K) and cryogenic (Tϭ173 K) temperatures irradiated with constant energy ͑50 mJ͒, variable width laser pulses ranging from 100 fs to 10 ns. Two regimes of jet operation are explored: cluster formation (radiusϽ100 nm) and droplet formation (radiusϾ1 m). The results for both clusters and droplets can be understood in terms of two time scales: a short time scale for optimal resonant absorption at the critical density layer in the expanding plasma, and a longer time scale for the plasma to drop below critical density.

Tubular plasma generation with a high-power hollow Bessel beam

Physical Review E, 2000

A high power, hollow Bessel beam (J 5) is generated using an axicon and a phase plate in combinat... more A high power, hollow Bessel beam (J 5) is generated using an axicon and a phase plate in combination. The optical breakdown of a gas target and generation of a tubular plasma fiber with such a beam is realized. Hydrodynamic simulations of the hollow beam-plasma breakdown and heating are in reasonable agreement with interferometric measurements of the plasma time evolution.

$Research paper thumbnail of 7. Optical trap assisted sub diffraction limited laser structuring$

7. Optical trap assisted sub diffraction limited laser structuring

Biomedical and Technical Applications

The ability to structure surfaces on a submicron scale is becoming increasingly important for a b... more The ability to structure surfaces on a submicron scale is becoming increasingly important for a broad range of technical and biological applications. Such structures can be produced via irradiating micron sized transparent particles placed over a substrate of interest with ultrafast laser pulses. To enhance the structuring flexibility of this method particles can be optically trapped and spatially manipulated relative to the structured surface in order to achieve arbitrary patterns. In this contribution the authors describe the principle of optical trap assisted nanostructuring and present simulated and experimental results demonstrating the potential and limitations of this innovative nanoscale optical material processing technology.

Ultrashort Pulse Laser Cutting of Intraocular Lens Polymers

Journal of Laser Micro/Nanoengineering, 2014

Material processing by ultra-short pulsed laser ablation is an attractive approach for various ap... more Material processing by ultra-short pulsed laser ablation is an attractive approach for various applications. In this paper we present our studies on cutting of medical polymers used for intraocular lenses (IOL). To achieve best results the ablation characteristics and the ablation thresholds are identified for single and multiple irradiation of the lens material by picosecond pulses. The results are used for quantification of the incubation effect. Furthermore, cutting experiments are carried out and thereby different damage types can be observed. A process window is developed containing the regions of the respective types of damages and backed up by theoretical considerations.

Ultrafast Laser Surface Structuring of Intraocular Lens Polymers

Journal of Laser Micro/Nanoengineering, 2013

Ultrafast laser material processing is an attractive approach for surface structuring of medical ... more Ultrafast laser material processing is an attractive approach for surface structuring of medical polymers used for intraocular lenses. To achieve best results in terms of efficiency and quality, the wavelength and pulse energy dependence on the ablation behavior of such materials has to be studied. Ablation thresholds have been determined for a polymer with different dyes which is used for intraocular lens (IOL) manufacturing. Furthermore, the ablation efficiency, achievable precision and surface roughness have been quantified to optimize the parameters for an efficient intraocular lens manufacturing process by surface microstructuring.

Resonant self-trapping of high intensity Bessel beams in underdense plasmas

Physical review. E, Statistical, nonlinear, and soft matter physics, 2002

We present a comprehensive report based on recent work [Phys. Rev. Lett. 84, 3085 (2000)] on reso... more We present a comprehensive report based on recent work [Phys. Rev. Lett. 84, 3085 (2000)] on resonant self-trapping and enhanced absorption of high power Bessel beams in underdense plasmas. The trapping resonance is strongly dependent on initial gas pressure, Bessel-beam geometry, and laser wavelength. Analytic estimates, and simulations using a one-dimensional Bessel-beam-plasma interaction code consistently explain the experimental observations. These results are for longer, moderate intensity pulses where the self-trapping channel is induced by laser-heated plasma thermal pressure. To explore the extension of this effect to ultrashort, intense pulsed Bessel beams, we perform propagation simulations using the code WAKE [Phys. Rev. E 53, R2068 (1996)]. We find that self-trapping can occur as a result of a plasma refractive index channel induced by the combined effects of relativistic motion of electrons and their ponderomotive expulsion.

Focus Alignment Method for Laser Manufacturing at Sub-micron Positional Accuracy

Physics Procedia, 2012

Accurate positioning of a sample is one of the major challenges in the laser micro manufacturing-... more Accurate positioning of a sample is one of the major challenges in the laser micro manufacturing-especially if the requirements on tolerances are high as in ultrafast laser micromachining. There are a number of methods that allow detection of the surface position, however only few of them use the beam of the processing laser as a basis for the measurement. These methods have an advantage that any changes in the structuring beam will be inherently accommodated for. This work describes a direct contact free method to accurately determine the surface position with respect to the structuring beam focal plane. The method makes alignment of unique samples precise and time efficient due to ease of automation and provides a reproducibility of surface detection of less than 1μm.

Direct Waveguide Writing with High Energy High Repetition Rate Picosecond Laser Pulses

Physics Procedia, 2012

Direct femtosecond pulse laser waveguide writing based on laser induced localized modification of... more Direct femtosecond pulse laser waveguide writing based on laser induced localized modification of the refractive index of bulk glass can be a versatile fabrication method for many micro applications. Although the method's potentials have been realized its application is still rather limited, either due to the slow writing speeds caused by low laser pulse repetition rates of amplified systems or due to the shallow structuring depths restricted by low pulse energies of nonamplified lasers. In this work we investigate the direct waveguide writing process using a picosecond laser that allows fast and deep waveguide writing due to high pulse energies available at high pulse repetition frequencies.

Defect formation in glass welding by means of ultra short laser pulses

Physics Procedia, 2010

Many applications require a joining of several glass components. However, the established process... more Many applications require a joining of several glass components. However, the established processes for glass joining have certain disadvantages. By contrast, laser fusion welding by ultrafast lasers exhibits in principle excellent versatility, if the nonlinear interaction is localized enough so that plasma induced stresses can be tolerated and the temperature is high enough to melt the glass adjacent to the focal spot. However, defect generation during welding was observed for a broad range of welding conditions. This paper reports the observed defects and analyzes their origins.

Practical Aspects of Surface Generated Third Harmonic for Highly Precise Beam-workpiece Alignment

Journal of Laser Micro/Nanoengineering, 2013

Precise beam-workpiece alignment is highly important in laser processing. Though many methods exi... more Precise beam-workpiece alignment is highly important in laser processing. Though many methods exist for this task few allow high-precision measurements using the processing laser itself. One such method detects the laser's backreflection from the workpiece's surface using a confocal setup and yields ideally an accuracy of about 10% of the Rayleigh length of the focused beam. Nevertheless, under certain conditions the accuracy of this method may suffer from a mismatch of beam divergences between laser and confocal detection optics. This becomes important when transparent materials are processed in three dimensions using optics with very high numerical apertures or if expensive materials or samples have to be processed where any misalignment would be costly. A possible solution to this is surface based third harmonic generation that allows for absolute surface detection while having the same accuracy as the confocal method. However, this method requires a complex setup while exhibiting low conversion efficiency. Both reasons make a proper alignment of this measurement scheme difficult. Herein we present results showing a clear conversion efficiency dependence on the reflection coefficient of the generating interface for a number of material combinations. This makes the selection of suitable materials more straightforward and reduces the alignment effort. The highest conversion efficiency was achieved for air/silicon which exhibits the highest reflection coefficient of the investigated materials.

Generation of phase-only holograms by laser ablation of nanoparticulate ITO layers

Journal of Optics, 2014

Phase holograms offer great potential e.g. for laser beam shaping and imaging applications. Howev... more Phase holograms offer great potential e.g. for laser beam shaping and imaging applications. However, the generation of these structures typically requires multi-step processes which tend to be time-consuming and expensive, especially if high-quality structures are required. In this work we demonstrate a flexible and inexpensive method which allows for the production of binary phase-only holograms by laser ablation of ITO nanoparticle layers. Since these layers are ablated free of residues, accurately defined phase shifts can be achieved. While arbitrarily shaped structures can be generated, the ITO layer thickness can be adjusted in order to freely tune the phase shift for the desired wavelength. In the diffraction patterns generated by the holograms we observed an excellent zero order suppression.

Microsphere near-field nanostructuring using picosecond pulses

Physics Procedia, 2010

In laser material processing the use of nonlinear multi-photon and near-field effects allows to p... more In laser material processing the use of nonlinear multi-photon and near-field effects allows to produce nanostructures smaller than the wavelength of the processing laser. In the near-field of small particles locally restricted field enhancements occur, allowing a focusing of pulsed laser radiation far beyond the diffraction limit. In this contribution a basic study of near-field and beam-matter interaction phenomena in microsphere near-field nanostructuring using picosecond laser pulses shall be presented.