During the last decade it has been proven that focused femtosecond laser pulses are an ideal tool for micro- and sub-micro-structuring of all kinds of materials. Due to the high intensities that can be achieved in ultrashort pulses they can be applied for machining transparent media within the volume by means of multi-photon absorption. Besides ablative methods, multi-photon absorption can also lead to photo-polymerization of light-sensitive resins, i.e. two-photon polymerization. In this paper we present our latest results on the fabrication of 3D microstructures by means of two-photon polymerization.
Two-photon polymerization (2PP) of photosensitive inorganic-organic hybrid polymers (ORMOCERs, developed at the Frauenhofer Institut fuer Silicatforschung) is demonstrated as a very promising approach for the fabrication of complicated three-dimensional micro- and nanostructures. These materials are produced by sol-gel synthesis with
molecular level mixing of different components. It is remarkable that properties of the hybrid polymers can be tuned from those that are characteristic for organic polymers to those that are similar to inorganic glasses. They have negative resist behaviour and can be used as storage-stable, liquid photo-polymerizable resins. When Ti:sapphire femtosecond laser pulses are tightly focused into the volume of this resin (which is transparent in the infrared) they can initiate two-photon polymerization process transferring liquid into solid state. This process is confined to a highly localized area at
the focal point due to the quadratic dependence of the two-photon absorption rate on the laser intensity. When the laser focus is moved through the resin in three dimensions, the polymerization occurs along the trace of the focus. This allows to fabricate any computer-generated 3D structure by direct laser "recording" into the volume of the ORMOCER. The non-irradiated liquid resin can be dissolved in alcohol leaving the polymerized copy of the computer model.
Compared to conventional photo-lithography which is a planar processing, two-photon polymerization is a real three-dimensional
volume microfabrication technique. This technology can be used for rapid prototyping and low-cost fabrication of artificial micro- and nanostructured components which are required for different applications in optics, medicine, and biology. Numerous examples such as photonic crystals, micromechanical and microoptical devices are
presented.
Irradiation of 15 nm gold particles with nano- and picosecond laser pulses can create locally temperatures beyond the critical point of water. Due to the short heating times the temperature is localised to the vicinity of the particles. Under irradiation with nano- and picosecond pulses an inactivation of the enzymes alkaline phosphatase and chymotrypsin which were bound to the surface of the particles was observed. As expected by strong temperature gradient caused by
the short irradiation time the protein inactivation is localized within a few tens of nanometers when picosecond pulses were used. This was shown by irradiating gold protein conjugates in which the protein was bound via two anitbodies to the particle.
Two-photon polymerization (2PP) of photosensitive inorganic-organic hybrid polymers (ORMOCERs, developed at the Frauenhofer Institut fur Silicatforschung) is demonstrated as a very promising approach for the fabrication of complicated three-dimensional micro- and nanostructures. These materials are produced by sol-gel synthesis with molecular level mixing of different components. It is remarkable that properties of the hybrid polymers can be tuned from those that are characteristic for organic polymers to those that are similar to inorganic glasses. They have negative resist behaviour and can be used as storage-stable, liquid photo-polymerizable resins.
When Ti:sapphire femtosecond laser pulses are tightly focused into the volume of this resin (which is transparent in the infrared) they can initiate two-photon polymerization process transferring liquid into solid state. This process is confined to a highly localized area at the focal point due to the quadratic dependence of the two-photon absorption rate on the laser intensity. When the laser focus is moved through the resin in three dimensions, the polymerization occurs along the trace of the focus. This allows to fabricate any computer-generated 3D structure by direct laser 'recording' into the volume of the ORMOCER. The non-irradiated liquid resin can be dissolved in alcohol leaving the polymerized copy of the computer model. Compared to conventional photo-lithography which is a planar processing, two-photon polymerization is a real three-dimensional volume microfabrication technique. These technologies can be used for rapid prototyping and low-cost fabrication of artificial micro- and nanostructured components which are required for different applications in optics, medicine, and biology. Numerous examples such as photonic crystals, micromechanical and microoptical devices will be demonstrated in this presentation.
Investigations on two-photon polymerization of inorganic-organic hybrid materials initiated by femtosecond Ti:sapphire laser pulses are performed. The applied resins are designed for ultraviolet photo-lithography and contain photo-initiators sensitive to 390 nm radiation. These resins exhibit exceptionally good mechanical, optical, and chemical properties and can be microstructured by laser-enforced transition from liquid to solid state. These materials are transparent in the infrared, therefore, by tightly focusing femtosecond laser pulses into the volume of a liquid resin, two-photon polymerization can be initiated in a small focal region inside the liquid. First applications of this technique for the fabrication of three-dimensional microstructures and photonic crystals in inorganic-organic hybrid polymers with a structure size down to 200 nm and a periodicity of 450 nm are discussed.
Due to the low energy threshold of photodisruption with fs laser pulses, thermal and mechanical side effects are limited to the sub μm range. The neglection of side effects enables the use of ultrashort laser pulses in a broad field of medical applications. Moreover, the interaction process based on nonlinear absorption offers the opportunity to process transparent tissue three dimensionally inside the bulk.
We demonstrate the feasibility of surgical procedures in different fields of medical interest: In ophthalmology intrastromal cutting and preparing of corneal flaps for refractive surgery in living animals is presented. Besides, the very low mechanical side effects enables the use of fs-laser in otoralyngology to treat ocecular bones. Moreover, the precise cutting quality can be used in fields of cardiovascular surgery for the treatment of arteriosclerosis as well as in dentistry to remove caries from dental hard tissue.
Ultrafast lasers have become a promising tool for micromachining and extremely precise ablation of all kinds of materials. Due to the low energy threshold, thermal and mechanical side effects are limited to the bu micrometers range. The neglection of side effects enables the use of ultrashort laser pulses in a broad field of medical applications. Moreover, the interaction process based on nonlinear absorption offers the opportunity to process transparent tissue three dimensionally inside the bulk. We demonstrate the feasibility of surgical procedures in different fields of medical interest: in ophthalmology intrastromal cutting and preparing of cornael flaps for refractive surgery in living animals is presented. Besides, the very low mechanical side effects enables the use of fs- laser in otoralyngology to treat ocecular bones. Moreover, the precise cutting quality can be used in fields of cardiovascular surgery for the treatment of arteriosklerosis as well as in dentistry to remove caries from dental hard tissue.
Femtosecond lasers have been proven as excellent tools for micromachining of solid targets. In contrast to other existing technologies, this method of laser processing allows structuring with highest precision by minimal damage to the adjacent material. The possibility of structuring nearly any kind of material gives access to new and innovative approaches in the field of optics. Periodic structures with dimensions on a micrometer scale are used for many photonic applications. Conventional ways of producing micrometers -scaled periodic patterns show the drawback of being limited by specific material properties, e.g. hardness, brittleness, which reduce the variety of machinable materials. However, femtosecond laser pulses offer great possibilities for the generation of periodic microstructures independent of the machined material. This includes the ablation of metals, dielectrics as well as the laser induced polymerization of photosensitive resins. Within this paper results on the generation of periodic microstructures by using femtosecond lasers are presented. Results of machining surfaces for applications like anodes and acceleration grids for streak camera tubes are presented, demonstrating a high potential for fs-laser micromachining in the field of optics.
Background: Protein denaturation in the fs-ns time regime is of fundamental interest for high precision applications in laser tissue interaction. Conjugates of colloidal gold coupled to proteins are presented as a model system for investigating ultrafast protein denaturation. It is expected that irradiation of such conjugates in tissue using pico- up to nanosecond laser pulses could result in effects with a spatial confinement in the regime of single macromolecules up to organelles. Materials and Methods: Experiments were done with bovine intestinal alkaline phosphatase (aP) coupled to 15 nm colloidal Gold. This complex was irradiated at 527 nm/ 532 nm with a variable number of pico- and nanosecond pulses. The radiant exposure per pulse was varied from 2 to 50 mJ/cm2 in the case of the picosecond pulses and 10 to 500 mJ/cm2 in the case of the nanosecond pulses. Denaturation was detected as a loss of protein function with the help of the uorescence substrate 4MUP. Results and Discussion: Irradiation did result in a steady decrease of the aP activity with increasing radiant exposures and increasing number of pulses. Inactivations up to 80% using 35 ps pulses at 527 nm with 50 mJ/cm2 and a complete inactivation induced by 16 ns pulses at 450 mJ/cm2 are discussed. The induced temperature in the particles and the surrounding water was calculated using Mie's formulas for the absorption of the nanometer gold particles and an analytical solution of the equations for heat diffusion. The calculated temperatures suggest that picosecond pulses heat a molecular scaled area whereas nanosecond pulses could be used for targeting larger cellular compartiments. It is difficult to identify one of the possible damage mechanisms, i.e. thermal denaturation or formation of micro bubbles, from the dependance of the inactivation on pulse energy and number of applied pulses. Therefore experiments are needed to further elucidate the damage mechanisms. The observed inactivation dependencies on applied energy and radiant power can not be explained with one or two photon photochemistry. In conclusion, denaturing proteins irreversibly via nanoabsorbers using pico-/ nanosecond laser pulses is possible. The expected confinement of the heat to the nanoabsorbers suggests that denaturation of proteins with nanometer precision could be possible with this approach. However, the mechanism of protein inactivation, which is part of present investigations, is crucial for the precision of such nanoeffects.
Background: Laser induced protein denaturation is of fundamental interest for understanding the mechanisms of laser tissue interaction. Conjugates of nanoabsorbers coupled to proteins are presented as a model system for investigating ultrafast protein denaturation. Irradiation of the conjugates using repetitive picosecond laser pulses, which are only absorbed by the nanoabsorbers, could result in effects with a spatial confinement of less than 100 nm. Materials and Methods: Experiments were done with bovine intestinal alkaline phosphates (aP) coupled to 15 nm colloidal gold. This complex was irradiated at 527 nm wavelength and 35 ps pulse width with a varying number of pulses ranging form one up to 104. The radiant exposure per pulse was varied form 2 mJ/cm2 to 50 mJ/cm2. Denaturation was detected as a loss of protein function with the help of the fluorescence substrate 4MUP. Results and discussion: Irradiation did result in a steady decrease of the aP activity with increasing radiant exposures and increasing number of pulses. A maximal inactivation of 80% was reached with 104 pulses and 50 mJ/cm2 per pulse. The temperature in the particles and the surrounding water was calculated using Mie's formulas for the absorption of the nanometer gold particles and ana analytical solution of the equations for heat diffusion. With 50 mJ/cm2, the particles are heated above the melting point of gold. Since the temperature calculations strongly depend on changes in the state of matter of the particles and water, a very sophisticated thermal model is necessary to calculate exact temperatures. It is difficult to identify one of the possible mechanisms, thermal denaturation, photochemical denaturation or formation of micro bubbles from the dependance of the inactivation on pulse energy and number of applied pulses. Therefore, experiments are needed to further elucidate the damage mechanisms. In conclusion, denaturing proteins irreversibly via nanoabsorbers using picosecond laser pulses is possible. The confinement of the heat to the nanoabsorbers when irradiating with picosecond pulses suggests that the denaturation of proteins could be possible with nanometer precision in cells with this approach. However, the mechanism of protein inactivation, which is part of present investigations, is crucial for the precision of such nanoeffects.
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