This publication in prestigious journal Nature Communications represents the most important milestone within the research topic (1) Optodynamic aspects of light-matter interaction. The members of the program group, dr. Tomaž Požar, dr. Jernej Laloš and dr. Aleš Babnik in collaboration with other foreign research groups have designed an original optodynamic experiment. They used it to measure the electromagnetic momentum carried by light through the observation of the mechanical effects exerted on illuminated object due to radiation pressure. Momentum conversion from electromagnetic fields to elastic waves within a solid object proceeds through a string of electrodynamic and elastodynamic phenomena, collectively bound by momentum and energy continuity. The details of this conversion predicted by theory have yet to be validated by experiments, as it is difficult to distinguish displacements driven by momentum from those driven by heating due to light absorption. Here, we have measured temporal variations of the surface displacements induced by laser pulses reflected from a solid dielectric mirror. Ab initio modelling of momentum flow describes the transfer of momentum from the electromagnetic field to the dielectric mirror, with subsequent creation/propagation of multicomponent elastic waves. Complete consistency between predictions and absolute measurements of surface displacements offers compelling evidence of elastic transients driven predominantly by the momentum of light. The results of this achievement have a potential to clarify the century-old Abraham-Minkowski dilemma about the momentum of the light in matter. The results of this achievement will feed the research topics (1) Optodynamic aspects of light-matter interaction and (2) Optodynamics of laser manufacturing and other processes.
COBISS.SI-ID: 16198683
This achievement was conducted by the program group member dr. Peter Gregorčič. It presents the most important milestone within the research topic (2) Optodynamics of laser manufacturing and other processes. The results clearly demonstrate that laser surface engineering can be effectively used to produce surfaces that enable enhanced heat transfer by nucleate boiling. Nucleate boiling enables effective cooling and heat transfer at low temperature differences between a heated surface and the surrounding fluid. It is utilized in many applications, ranging from large power plants to small microelectronics. To enhance the boiling process by minimization of the surface temperature and increase the maximum attainable heat flux, several approaches for surface modifications were recently developed. However, each of them has at least one important drawback, including challenging and expensive production, mechanical and/or thermal instability or problematic scale-up. In this achievement, a straightforward, robust and flexible method using a nanosecond fiber laser for production of surfaces with multi-scale micro-cavities (with diameters ranging from 0.2 to 10 um) is developed. Examination of these surfaces in two very contrasting fluids - water, which is polar, has high surface tension and high latent heat of vaporization; and non-polar, dielectric tetradecafluorohexane (FC-72) with low surface tension and much lower latent heat - confirms that such surfaces enable enhanced heat transfer and controlled boiling in combination with diverse fluids. This demonstration suggests that the developed method has the potential to overcome the current limitations for further miniaturization of microelectronic devices and to increase performance and safety in high heat flux systems. In the next steps, these results will be used to develop appropriate prototypes of laser system and technology that will enable production of such surfaces. The results of this achievement will feed the research topics Optodynamics of laser manufacturing and other processes.
COBISS.SI-ID: 16034331
The leader of the program group dr. Matija Jezeršek and the program group members, Dr. Jernej Laloš and dr. Peter Gregorčič performed an optical study of elastic wave propagation inside skin phantoms consisting of agar gel as induced by an Er:YAG laser pulse. This achievement presents the most important milestone in the program topic (3) Optodynamics of laser medical applications. A laser-beam-deflection probe was used to measure ultrasonic propagation and a high-speed camera was used to record displacements in ablation-induced elastic transients. These measurements were further analyzed with a custom developed image recognition algorithm utilizing the methods of particle image velocimetry and spline interpolation to determine point trajectories, material displacement and strain during the passing of the transients. The results indicate that the ablation-induced elastic waves propagate with a velocity of 1 m/s and amplitudes of 0.1 mm. Compared to them, the measured velocities of ultrasonic waves are much higher, within the range of 1.42-1.51 km/s, while their amplitudes are three orders of magnitude smaller. This proves that the agar gel may be used as a rudimental skin and soft tissue substitute in biomedical research, since its polymeric structure reproduces adequate soft-solid properties and its transparency for visible light makes it convenient to study with optical instruments. The results presented provide an insight into the distribution of laser-induced elastic transients in soft tissue phantoms, while the experimental approach serves as a foundation for further research of laser-induced mechanical effects deeper in the tissue. The participating research organization Fotona d.d. will use the findings of the research in the further development of novel approaches in laser dermatology. The results of this achievement will feed the research topics (1) Optodynamic aspects of light-matter interaction and (3) Optodynamics of laser medical applications.
COBISS.SI-ID: 15967771
The program group members, dr. Peter Gregorčič and dr. Matija Jezeršek have investigated the impact of different optical fiber tip geometry on the dynamics of vapor bubbles which are formed when erbium laser pulses are led into water through the fiber. Pulses of this type of lasers have an extremely short absorption length. The absorbed laser energy overheats the water, and this is followed by the formation of vapor bubbles. The described optodynamic mechanism is used in dentistry for laser dental treatment of root canals, where different geometries of fiber tips are in use. This geometry plays a key role in both the shape of the vapor bubbles, as well as on the amount of light energy that is converted into mechanical energy of the bubble. The authors defined the ratio between the mechanical energy of the bubble and the optical energy of the laser pulse as optodynamic efficiency of energy conversion. This efficiency is of utmost importance, because higher efficiency means less unwanted damage and less heating of the surrounding tissue. In these studies, the authors used high-speed shadow photography. The participating research organization Fotona d.d. will use the findings of the research in the further development of laser systems for cleaning hard tissue (teeth and bones). The results of this achievement will feed the research topics (1) Optodynamic aspects of light-matter interaction and (3) Optodynamics of laser medical applications.
COBISS.SI-ID: 12358939
The achievement was conducted by the program group member dr. Peter Gregorčič within the collaboration between program group and University of Suthampton (UK). It represents an important contribution to micro-/nano structuring of arbitrary patterns by using a digital micromirror device and laser-induced mass transfer. Within this achievement, we have theoretically and experimentaly studied the transfer regimes and dynamics of polymer flyers from laserinduced backward transfer (LIBT) via time-resolved shadowgraphy. Imaging of the flyer ejection phase of LIBT of 3.8 um and 6.4 um thick SU-8 polymer films on germanium and silicon carrier substrates was performed over a time delay range of 1.4-16.4 us after arrival of the laser pulse. The experiments were carried out with 150 fs, 800 nm pulses spatially shaped using a digital micromirror device, and laser fluences of up to 3.5 J/cm2 while images were recorded via a CCD camera and a ultra-high-speed spark discharge lamp. Velocities of flyers found in the range of 6-20 m/s, and the intact and fragmented ejection regimes, were a function of donor thickness, carrier and laser fluence. The crater profile of the donor after transfer and the resulting flyer profile indicated different flyer ejection modes for Si carriers and high fluences. The results contribute to better understanding of the LIBT process, and help to determine experimental parameters for successful LIBT of intact deposits and to use this approach for laser microstructuring. It represents an important basis for further modelling of laser-induced mass transfer and for development of new monitoring methods that will improve the performance of laser-induced mass transfer applications. The results of this achievement will feed the research topics (1) Optodynamic aspects of light-matter interaction and (2) Optodynamics of laser manufacturing and other processes.
COBISS.SI-ID: 15103003
The program group member, dr. Tomaž Požar and co-authors have proven that optodynamic methods represent very useful tool also for answering the important questions outside the scope of Optodynamics. One of such remaining opened questions is related to cavitation erosion. Here, the influence and underlying mechanisms of the microjet and the shock wave on the formation of the pit was still not well understood. Up until now, no successful attempt has been made to study this in detail, mainly because the damage could only be detected and evaluated after several successive bubble collapses. However, by using an optodynamic approach, it was possible to create a bubble with a maximum diameter of up to 3.3 mm by photoionization using a Nd:YAG laser. The damage was observed on a 9 um thick aluminum foil attached to a glass substrate. Two high-speed cameras were simultaneously used. One captured the dynamics of the bubble, while the other recorded the damage of the foil. In this way, we were able to observed the collapse of a bubble in the presence of shear flow, where most of the damage is created by the microjet mechanism. Sometimes, the collapse of the bubble rim, at the rebound of the initial bubble causes pits in a well-known circular pattern. From the recordings at the very fastest acquisition rate, we determined that the material deforms and then partially relaxes, while a significant deformation remains. The whole process is only 2-3 us long. The results have a significant impact on further development of all of three program research topics. The results of this achievement will feed the research topics (1) Optodynamic aspects of light-matter interaction, (2) Optodynamics of laser manufacturing and other processes and (3) Optodynamics of laser medical applications.
COBISS.SI-ID: 16342555
The program group member, dr. Peter Gregorčič and co-authors within collaboration with Brandenburg University of Technology Cottbus-Senftenberg (Germany) made an important contribution to understanding the physical mechanisms for laser-induced periodic surface structures (LIPSS) formation. We investigated LIPSS that are produced on cold work tool steel by irradiation with a low number of picosecond laser pulses. As expected, the ripples, with a period of about 90% of the laser wavelength, are oriented perpendicular to the laser polarization. Subsequent irradiation with the polarization rotated by 45° or 90° results in a corresponding rotation of the ripples. This is visible already with the first pulse and becomes almost complete – erasing the previous orientation - after as few as three pulses. The phenomenon is not only observed for single-spot irradiation but also for writing long coherent traces. The experimental results strongly defy the role of surface plasmon-polaritons as the predominant key to LIPSS formation and endorse the model of self-organized structure formation from a laser-induced thermal instability (dynamic melting). The results of this achievement will feed the research topics (1) Optodynamic aspects of light-matter interaction and (2) Optodynamics of laser manufacturing and other processes.
COBISS.SI-ID: 14711835
The program group member, dr. Peter Gregorčič and co-authors represented an important proof that surface functionalization can be performed by nanosecond laser pulses and investigates the influence of laser texturing on the surface corrosion behavior. We used a nanosecond Nd:YAG laser to texture superhydrophobic surfaces expressing self-cleaning effect on stainless steel 316L. This achievement confirms that surface wettability can be successfully modified also by nanosecond-laser systems that are approximately 10 times cheaper as ultrafast (ps, fs) pulsed laser systems, representing current state of the art in surface functionalization. Further, this work investigates the influence of evolution from superhydrophilic to superhydrophobic surface state on corrosion behaviour. Results confirm perfect correlation among wettability and corrosion, hence superhydrophobic surface with a contact angle of 168°+/-3.0° reflects in enhanced passivity, lower anodic dissolution and corrosion current reduction. Characterization of the corrosion attack by 3D microscopy reveals high sensitivity of superhydrophilic surfaces on corrosion propagation direction in regard to the laser beam passage (90°/0°). However, this trend completely diminishes with superhydrophobic development. Further, direct laser texturing also completely prohibits intergranular corrosion detected with the non-processed sample. The results of this achievement will feed the research topic (2) Optodynamics of laser manufacturing and other processes.
COBISS.SI-ID: 15482907
The leader of the program group dr. Matija Jezeršek and the program group member, Dr. Nejc Lukač developed new optodynamic method for amplification of pressure waves in laser-assisted endodontics. When attempting to clean surfaces of dental root canals with laser-induced cavitation bubbles, the resulting cavitation oscillations are significantly prolonged due to friction on the cavity walls and other factors. Consequently, the collapses are less intense and the shock waves that are usually emitted following a bubble's collapse are diminished or not present at all. A new technique of synchronized laser-pulse delivery intended to enhance the emission of shock waves from collapsed bubbles in fluid-filled endodontic canals was developed. The following optodynamic methods: laser beam deflection probe, a high-speed camera, and shadow photography were used to characterize the induced photoacoustic phenomena during synchronized delivery of Er:YAG laser pulses in a confined volume of water. A shock wave enhancing technique was employed which consists of delivering a second laser pulse at a delay with regard to the first cavitation bubble-forming laser pulse. Influence of the delay between the first and second laser pulses on the generation of pressure and shock waves during the first bubble's collapse was measured for different laser pulse energies and cavity volumes. Results show that the optimal delay between the two laser pulses is strongly correlated with the cavitation bubble's oscillation period. Under optimal synchronization conditions, the growth of the second cavitation bubble was observed to accelerate the collapse of the first cavitation bubble, leading to a violent collapse, during which shock waves. The achievement represent an important basis for further development of modalities in laser endodontics. The results of this achievement will feed the research topics (1) Optodynamic aspects of light-matter interaction and (3) Optodynamics of laser medical applications.
COBISS.SI-ID: 15860251
The leader of the program group, dr. Matija Jezeršek and the program group members, Dr. Boris Cencič and Dr. Peter Gregorčič have developed novel optodynamic methods for monitoring laser ablation during laser tattoo removal. Efficiency of the laser-tissue interaction varies even within a single tattoo because of the inhomogeneous distribution of the tattoo pigment embedded in the skin. An optodynamic monitoring system was therefore developed for simultaneous monitoring of the laser tattoo removal process based on acoustical and optical techniques. A laser-beam-deflection probe was used for measuring the acoustical signals accompanying the breakdown, and a CCD camera captured the level and the spatial distribution of the plasma radiation. Using these methods, we examined the degree of excitation-pulse absorption within the pigment and the degree of the structural changes of the skin. A Nd:YAG laser with a top-hat beam profile, designed for tattoo removal by the participating research organization Fotona d.d., was used as the excitation source in our experiments. Special attention was given to structural changes in the skin, which depend on the applied fluence. Tattoo removal with multiple pulses was also analyzed. Experiments were made in vitro (skin phantoms) and ex vivo (marking tattoos on the pig skin). The presented results are important for the understanding and optimization of the process used in medical therapies. The results of this achievement will feed the research topics (1) Optodynamic aspects of light-matter interaction and (3) Optodynamics of laser medical applications.
COBISS.SI-ID: 12427291