Nektarios Papadogiannis - Academia.edu (original) (raw)

Papers by Nektarios Papadogiannis

HAL (Le Centre pour la Communication Scientifique Directe), Apr 23, 2012

Applied Physics A

In the current work we report on the generation of acoustic strains in thin ZnO layers using opto... more In the current work we report on the generation of acoustic strains in thin ZnO layers using optoacoustic transduction of ultrashort laser pulses into acoustic waves on an Au thin film transducer. After absorption of energy by the electron system of the metal, energy conversion, thermal expansion and mechanical deformation takes place. The generation and propagation of the induced acoustic strains are monitored in time via a degenerate pump-probe transient reflectivity optical setup at 800 nm, as opposed to most commonly used schemes that employ different wavelengths for the pump and probe beams, mostly in the vicinity of ZnO maximum absorption. The experimental results include energy relaxation times and phonon scattering frequencies and are supported by a thermal vibro-acoustic finite element model. The model is based on the combination of a revised two-temperature approach and elasticity theory, and considers anisotropic properties for the ZnO film and the computation of the elas...

Applied Sciences

Background: Despite intensive research, the ideal protocol applied to maximize the overall benefi... more Background: Despite intensive research, the ideal protocol applied to maximize the overall benefits of antimicrobial photodynamic therapy (aPDT) remains unexplored. Evidence exists that following aPDT, the diffused light beyond the photosensitizer can exert a secondary therapeutic effect known as photobiomodulation (PBM), which stimulates the healing of the surrounding tissues. Therefore, the aim of this study was to examine the attenuation properties of five different photosensitizers activated by their corresponding laser wavelengths. Methods: The illumination of various concentrations of chosen photosensitizers, curcumin, methylene blue, toluidine blue, indocyanine green and a methylene blue derivative, irradiated by their respective laser wavelengths (445 nm, 635 nm, 660 nm and 808 nm) was explored via a spectrophotometric analysis. The onward transmitted light intensities for each combination of a photosensitizer and laser wavelength were assessed. The attenuation percentages o...

Acoustics

String instruments are complex mechanical vibrating systems, in terms of both structure and fluid... more String instruments are complex mechanical vibrating systems, in terms of both structure and fluid–structure interaction. Here, a review study of the modeling and simulation of stringed musical instruments via the finite element method (FEM) is presented. The paper is focused on the methods capable of simulating (I) the soundboard behavior in bowed, plucked and hammered string musical instruments; (II) the assembled musical instrument box behavior in bowed and plucked instruments; (III) the fluid–structure interaction of assembled musical instruments; and (IV) the interaction of a musical instrument’s resonance box with the surrounding air. Due to the complexity and the high computational demands, a numerical model including all the parts and the full geometry of the instrument resonance box, the fluid–structure interaction and the interaction with the surrounding air has not yet been simulated.

Applied Sciences

The rapid growth of nanotechnology has increased the need for fast nanoscale imaging. X-ray free ... more The rapid growth of nanotechnology has increased the need for fast nanoscale imaging. X-ray free electron laser (XFEL) facilities currently provide such coherent sources of directional and high-brilliance X-ray radiation. These facilities require large financial investments for development, maintenance, and manpower, and thus, only a few exist worldwide. In this article, we present an automated table-top system for XUV coherent diffraction imaging supporting the capabilities for multispectral microscopy at high repetition rates, based on laser high harmonic generation from gases. This prototype system aims towards the development of an industrial table-top system of ultrafast soft X-ray multi-spectral microscopy imaging for nanostructured materials with enormous potential and a broad range of applications in current nanotechnologies. The coherent XUV radiation is generated in a semi-infinite gas cell via the high harmonic generation of the near-infrared femtosecond laser pulses. The...

Atoms

The generation of high-order harmonics in a semi-infinite cell by femtosecond laser pulses is a c... more The generation of high-order harmonics in a semi-infinite cell by femtosecond laser pulses is a common practice for reliable coherent and low divergence XUV source beams for applications. Despite the relative simplicity of the experimental method, several phenomena coexist that affect the generated spectral and divergence characteristics of the high harmonic XUV frequency comb. The ionisation degree of the medium and the consequent plasma formation length imposes a spatiotemporal evolution of the fundamental EM field and XUV absorption. Varying the laser pulse chirp and the focusing conditions, as well as the gas density, we measured intense harmonic spectral and divergence variations attributed mainly to self-phase modulations of the laser EM field in the partially ionised medium. Additionally, low-divergence high harmonics are observed for certain laser chirp values attributed to the strong phase matching of only the short electron quantum path. Thus, a tunable, low divergent, and...

The temporal rearrangement of the spectral components of an ultrafast and intense laser pulse, i.... more The temporal rearrangement of the spectral components of an ultrafast and intense laser pulse, i.e., the chirp of the pulse, offers significant possibilities for controlling its interaction with matter and plasma. In the propagation of ultra-strong laser pulses within the self-induced plasma, laser pulse chirp can play a major role in the dynamics of wakefield and plasma bubble formation, as well as in the electron injection and related electron acceleration. Here, we experimentally demonstrate the control of the generation efficiency of a relativistic electron beam, with respect to maximum electron energy, current, and reproducibility, by accurately varying the chirp value of a multi-10-TW laser pulse. We explicitly show that positively chirped laser pulses, i.e., pulses with instantaneous frequency increasing with time, accelerate electrons in the order of 100 MeV much more efficiently in comparison to unchirped or negatively chirped pulses. Corresponding Particle-In-Cell simulati...

Laser-generated plasma filaments in ambient air generate acoustic pulses with characteristic emis... more Laser-generated plasma filaments in ambient air generate acoustic pulses with characteristic emission directivity and frequency spectra. This work studies the effect of the geometrical characteristics of the plasma channel on the generated acoustic radiation. For this purpose, a model based on the classical line source is adopted, which deploys the correlation between light and sound following laser breakdown to predict the acoustic radiation of filaments in air. Through the model, the acoustic directivity and frequency spectra of plasma filaments with different lengths and plasma density distributions are studied. Preliminary results from acoustic measurements of the filament’s sound radiation are presented in the frequency domain. Finally, a complete model based on the wave equation with a heat source is outlined, which allows for the estimation of the filament’s acoustic radiation in the time and frequency domain at any point in space. Preliminary results from a computational sim...

Applied Physics A

Nano-acoustic strain generation in thin metallic films via ultrafast laser excitation is widely u... more Nano-acoustic strain generation in thin metallic films via ultrafast laser excitation is widely used in material science, imaging and medical applications. Recently, it was shown that transition metals, such as titanium, exhibit enhanced photoacoustic transduction properties compared to noble metals, such as silver. This work presents experimental results and simulations that demonstrate that among transition metals tantalum exhibits superior photoacoustic properties. Experiments of nano-acoustic strain generation by femtosecond laser pulses focused on thin tantalum films deposited on Silicon substrates are presented. The nano-acoustic strains are measured via pump-probe transient reflectivity that captures the Brillouin oscillations produced by photon–phonon interactions. The observed Brillouin oscillations are correlated to the photoacoustic transduction efficiency of the tantalum thin film and compared to the performance of titanium thin films, clearly demonstrating the superior ...

This book is based upon work from COST Action TUMIEE (CA17126), supported by COST (European Coope... more This book is based upon work from COST Action TUMIEE (CA17126), supported by COST (European Cooperation in Science and Technology). COST (European Cooperation in Science and Technology) is a funding agency for research and innovation networks. Our Actions help connect research initiatives across Europe and enable scientists to grow their ideas by sharing them with their peers. This boosts their research, career and innovation. www.cost.eu

Physical Review A, 2005

We consider the problem of a helium atom under the radiation field of the DESY vacuum ultraviolet... more We consider the problem of a helium atom under the radiation field of the DESY vacuum ultraviolet ͑VUV͒ free electron laser ͑FEL͒ ͑Phase I, ប Х 13 eV͒. We find by solving numerically the time-dependent Schrödinger equation, that there is a large probability for resonant two-photon excitation from the ground state into a low kinetic energy state just above the first He ionization threshold. From this it is possible to go into another quasi-free state higher up, by resonant absorption of an additional photon. There is no double ionization of He. These results are in general agreement with the He photoelectron and time-of-flight ͑TOF͒ spectra recorded on March 2002, in the last week of the DESY VUV FEL Phase I operation. A detailed report on the experiments is given in a companion paper.

Applied Sciences

An X-pinch scheme of a low-current generator (45 kA, 50 ns rise time) is characterized as a poten... more An X-pinch scheme of a low-current generator (45 kA, 50 ns rise time) is characterized as a potential efficient source of soft X-rays. The X-pinch target consists of wires of 5 μm in diameter—made from either tungsten (W) or gold (Au)-plated W—loaded at two angles of 55° and 98° between the crossed wires. Time-resolved soft X-ray emission measurements are performed to provide a secure correlation with the optical probing results. A reconstruction of the actual photodiode current profile procedure was adopted, capable of overcoming the limits of the slow rising and falling times due to the “slow” response of the diodes and the noise. The pure and Au-plated W deliver an average X-ray yield, which depends only on the angle of the crossed wires, and is measured to be ~50 mJ and ~70 mJ for the 98° and 55° crossed wire angles, respectively. An additional experimental setup was developed to characterize the X-pinch as a source of X-rays with energy higher than ~6 keV, via time-integrated m...

Applied Sciences, 2021

A three-dimensional, thermal-structural finite element model, originally developed for the study ... more A three-dimensional, thermal-structural finite element model, originally developed for the study of laser–solid interactions and the generation and propagation of surface acoustic waves in the macroscopic level, was downscaled for the investigation of the surface roughness influence on pulsed laser–solid interactions. The dimensions of the computational domain were reduced to include the laser-heated area of interest. The initially flat surface was progressively downscaled to model the spatial roughness profile characteristics with increasing geometrical accuracy. Since we focused on the plastic and melting regimes, where structural changes occur in the submicrometer scale, the proposed downscaling approach allowed for their accurate positioning. Additionally, the multiscale simulation results were discussed in relation to experimental findings based on white light interferometry. The combination of this multiscale modeling approach with the experimental methodology presented in thi...

The quantum phases of the electron paths driven by an ultrafast laser in high harmonic generation... more The quantum phases of the electron paths driven by an ultrafast laser in high harmonic generation in an atomic gas depends linearly on the instantaneous cycle-averaged laser intensity. Using high laser intensities, a complete single ionisation of the atomic gas may occur before the laser pulse peak. Therefore, high harmonic generation could be localized only in a temporal window at the leading edge of laser pulse envelope. Varying the laser frequency chirp of an intense ultrafast laser pulse, the centre, and the width of the temporal window, that the high harmonic generation phenomenon occurs, could be controlled with high accuracy. This way, both the duration and the phase of the electron trajectories, that generate efficiently high harmonics, is fully controlled. An accurate and robust method of spectral control and selection of the high harmonic extreme ultraviolet light from distinct quantum paths is experimentally demonstrated. Furthermore, a phenomenological numerical model en...

Applied Physics Letters

Bright betatron x-rays generation from picosecond laser interactions with long-scale near critica... more Bright betatron x-rays generation from picosecond laser interactions with long-scale near critical density plasmas

Optics Express, 2015

The role of thin-film metal transducers in ultrafast laser-generated longitudinal acoustic phonon... more The role of thin-film metal transducers in ultrafast laser-generated longitudinal acoustic phonons in Si (100) monocrystal substrates is investigated. For this purpose degenerate femtosecond pump-probe transient reflectivity measurements are performed probing the Brillouin scattering of laser photons from phonons. The influence of the metallic electron-phonon coupling factor, acoustical impedance and film thickness is examined. An optical transfer matrix method for thin films is applied to extract the net acoustic strain relative strength for the various transducer cases, taking into account the experimental probing efficiency. In addition, a theoretical thermo-mechanical approach based on the combination of a revised two-temperature model and elasticity theory is applied and supports the experimental findings. The results show highly efficient generation of acoustic phonons in Si when Ti transducers are used. This demonstrates the crucial role of the transducer's high electron-phonon coupling constant and high compressive yield strength, as well as strong acoustical impedance matching with the semiconductor substrate.

Applied Physics Letters, 2013

ABSTRACT The three dimensional spatiotemporal response of thin metal films surfaces excited by na... more ABSTRACT The three dimensional spatiotemporal response of thin metal films surfaces excited by nanosecond laser pulses is investigated in both the thermoelastic and the ablation regimes. An experimental laser whole-field interferometric technique allows for the direct monitoring of the dynamic deformation of a macroscopic area on the surface with ultrahigh lateral resolution. A specially developed three dimension finite element model simulates the laser-surface interaction, predicts the experimentally obtained results, and computes key parameters of matter's thermomechanical response. This method provides a powerful instrument for spatiotemporal behavior of thin-film surfaces under extreme conditions demanded for innovative applications.

Dentistry Journal

Background: During in-office bleaching, appropriate light sources are applied in order to enhance... more Background: During in-office bleaching, appropriate light sources are applied in order to enhance the activity of the bleaching gels applied onto teeth. For this method to be effective, a high absorption of light within the gel is necessary. Variation in the light attenuation capability of the gel, the duration of application and light activation can contribute towards safety hazards associated with this procedure. Methods: In this study, seven different gels and hydrogen peroxide have been evaluated for their optical properties by means of spectrophotometry (440–1000 nm). The transmitted light spectrum was used to estimate the intensity loss for each gel. The mean intensity decreases observed were statistically analysed using an analysis of variance (ANOVA). Results: The five more-pigmented gels tested indicated a very similar intensity loss of around 80%, whereas the remaining two gels showed significantly less attenuation (predominantly, p < 10−6). Conclusions: Throughout the ...

Journal of Physics D: Applied Physics

The Journal of the Acoustical Society of America

Plasma filaments in air induced by femtosecond laser pulses lead to the generation of strong shoc... more Plasma filaments in air induced by femtosecond laser pulses lead to the generation of strong shock waves. This letter presents a systematic study, both experimental and theoretical, of the acoustic radiation by femtosecond laser-generated filaments. A theoretical model is developed based on the experimental results and is used to evaluate the directivity of the filament's acoustic radiation within and beyond the audible frequency range. It is shown that the acoustic directivity of plasma filaments can be derived from the model of a weighted acoustic line source, consisting of elementary point sources with N-shaped excitation.

HAL (Le Centre pour la Communication Scientifique Directe), Apr 23, 2012

Applied Physics A

In the current work we report on the generation of acoustic strains in thin ZnO layers using opto... more In the current work we report on the generation of acoustic strains in thin ZnO layers using optoacoustic transduction of ultrashort laser pulses into acoustic waves on an Au thin film transducer. After absorption of energy by the electron system of the metal, energy conversion, thermal expansion and mechanical deformation takes place. The generation and propagation of the induced acoustic strains are monitored in time via a degenerate pump-probe transient reflectivity optical setup at 800 nm, as opposed to most commonly used schemes that employ different wavelengths for the pump and probe beams, mostly in the vicinity of ZnO maximum absorption. The experimental results include energy relaxation times and phonon scattering frequencies and are supported by a thermal vibro-acoustic finite element model. The model is based on the combination of a revised two-temperature approach and elasticity theory, and considers anisotropic properties for the ZnO film and the computation of the elas...

Applied Sciences

Background: Despite intensive research, the ideal protocol applied to maximize the overall benefi... more Background: Despite intensive research, the ideal protocol applied to maximize the overall benefits of antimicrobial photodynamic therapy (aPDT) remains unexplored. Evidence exists that following aPDT, the diffused light beyond the photosensitizer can exert a secondary therapeutic effect known as photobiomodulation (PBM), which stimulates the healing of the surrounding tissues. Therefore, the aim of this study was to examine the attenuation properties of five different photosensitizers activated by their corresponding laser wavelengths. Methods: The illumination of various concentrations of chosen photosensitizers, curcumin, methylene blue, toluidine blue, indocyanine green and a methylene blue derivative, irradiated by their respective laser wavelengths (445 nm, 635 nm, 660 nm and 808 nm) was explored via a spectrophotometric analysis. The onward transmitted light intensities for each combination of a photosensitizer and laser wavelength were assessed. The attenuation percentages o...

Acoustics

String instruments are complex mechanical vibrating systems, in terms of both structure and fluid... more String instruments are complex mechanical vibrating systems, in terms of both structure and fluid–structure interaction. Here, a review study of the modeling and simulation of stringed musical instruments via the finite element method (FEM) is presented. The paper is focused on the methods capable of simulating (I) the soundboard behavior in bowed, plucked and hammered string musical instruments; (II) the assembled musical instrument box behavior in bowed and plucked instruments; (III) the fluid–structure interaction of assembled musical instruments; and (IV) the interaction of a musical instrument’s resonance box with the surrounding air. Due to the complexity and the high computational demands, a numerical model including all the parts and the full geometry of the instrument resonance box, the fluid–structure interaction and the interaction with the surrounding air has not yet been simulated.

Applied Sciences

The rapid growth of nanotechnology has increased the need for fast nanoscale imaging. X-ray free ... more The rapid growth of nanotechnology has increased the need for fast nanoscale imaging. X-ray free electron laser (XFEL) facilities currently provide such coherent sources of directional and high-brilliance X-ray radiation. These facilities require large financial investments for development, maintenance, and manpower, and thus, only a few exist worldwide. In this article, we present an automated table-top system for XUV coherent diffraction imaging supporting the capabilities for multispectral microscopy at high repetition rates, based on laser high harmonic generation from gases. This prototype system aims towards the development of an industrial table-top system of ultrafast soft X-ray multi-spectral microscopy imaging for nanostructured materials with enormous potential and a broad range of applications in current nanotechnologies. The coherent XUV radiation is generated in a semi-infinite gas cell via the high harmonic generation of the near-infrared femtosecond laser pulses. The...

Atoms

The generation of high-order harmonics in a semi-infinite cell by femtosecond laser pulses is a c... more The generation of high-order harmonics in a semi-infinite cell by femtosecond laser pulses is a common practice for reliable coherent and low divergence XUV source beams for applications. Despite the relative simplicity of the experimental method, several phenomena coexist that affect the generated spectral and divergence characteristics of the high harmonic XUV frequency comb. The ionisation degree of the medium and the consequent plasma formation length imposes a spatiotemporal evolution of the fundamental EM field and XUV absorption. Varying the laser pulse chirp and the focusing conditions, as well as the gas density, we measured intense harmonic spectral and divergence variations attributed mainly to self-phase modulations of the laser EM field in the partially ionised medium. Additionally, low-divergence high harmonics are observed for certain laser chirp values attributed to the strong phase matching of only the short electron quantum path. Thus, a tunable, low divergent, and...

The temporal rearrangement of the spectral components of an ultrafast and intense laser pulse, i.... more The temporal rearrangement of the spectral components of an ultrafast and intense laser pulse, i.e., the chirp of the pulse, offers significant possibilities for controlling its interaction with matter and plasma. In the propagation of ultra-strong laser pulses within the self-induced plasma, laser pulse chirp can play a major role in the dynamics of wakefield and plasma bubble formation, as well as in the electron injection and related electron acceleration. Here, we experimentally demonstrate the control of the generation efficiency of a relativistic electron beam, with respect to maximum electron energy, current, and reproducibility, by accurately varying the chirp value of a multi-10-TW laser pulse. We explicitly show that positively chirped laser pulses, i.e., pulses with instantaneous frequency increasing with time, accelerate electrons in the order of 100 MeV much more efficiently in comparison to unchirped or negatively chirped pulses. Corresponding Particle-In-Cell simulati...

Laser-generated plasma filaments in ambient air generate acoustic pulses with characteristic emis... more Laser-generated plasma filaments in ambient air generate acoustic pulses with characteristic emission directivity and frequency spectra. This work studies the effect of the geometrical characteristics of the plasma channel on the generated acoustic radiation. For this purpose, a model based on the classical line source is adopted, which deploys the correlation between light and sound following laser breakdown to predict the acoustic radiation of filaments in air. Through the model, the acoustic directivity and frequency spectra of plasma filaments with different lengths and plasma density distributions are studied. Preliminary results from acoustic measurements of the filament’s sound radiation are presented in the frequency domain. Finally, a complete model based on the wave equation with a heat source is outlined, which allows for the estimation of the filament’s acoustic radiation in the time and frequency domain at any point in space. Preliminary results from a computational sim...

Applied Physics A

Nano-acoustic strain generation in thin metallic films via ultrafast laser excitation is widely u... more Nano-acoustic strain generation in thin metallic films via ultrafast laser excitation is widely used in material science, imaging and medical applications. Recently, it was shown that transition metals, such as titanium, exhibit enhanced photoacoustic transduction properties compared to noble metals, such as silver. This work presents experimental results and simulations that demonstrate that among transition metals tantalum exhibits superior photoacoustic properties. Experiments of nano-acoustic strain generation by femtosecond laser pulses focused on thin tantalum films deposited on Silicon substrates are presented. The nano-acoustic strains are measured via pump-probe transient reflectivity that captures the Brillouin oscillations produced by photon–phonon interactions. The observed Brillouin oscillations are correlated to the photoacoustic transduction efficiency of the tantalum thin film and compared to the performance of titanium thin films, clearly demonstrating the superior ...

This book is based upon work from COST Action TUMIEE (CA17126), supported by COST (European Coope... more This book is based upon work from COST Action TUMIEE (CA17126), supported by COST (European Cooperation in Science and Technology). COST (European Cooperation in Science and Technology) is a funding agency for research and innovation networks. Our Actions help connect research initiatives across Europe and enable scientists to grow their ideas by sharing them with their peers. This boosts their research, career and innovation. www.cost.eu

Physical Review A, 2005

We consider the problem of a helium atom under the radiation field of the DESY vacuum ultraviolet... more We consider the problem of a helium atom under the radiation field of the DESY vacuum ultraviolet ͑VUV͒ free electron laser ͑FEL͒ ͑Phase I, ប Х 13 eV͒. We find by solving numerically the time-dependent Schrödinger equation, that there is a large probability for resonant two-photon excitation from the ground state into a low kinetic energy state just above the first He ionization threshold. From this it is possible to go into another quasi-free state higher up, by resonant absorption of an additional photon. There is no double ionization of He. These results are in general agreement with the He photoelectron and time-of-flight ͑TOF͒ spectra recorded on March 2002, in the last week of the DESY VUV FEL Phase I operation. A detailed report on the experiments is given in a companion paper.

Applied Sciences

An X-pinch scheme of a low-current generator (45 kA, 50 ns rise time) is characterized as a poten... more An X-pinch scheme of a low-current generator (45 kA, 50 ns rise time) is characterized as a potential efficient source of soft X-rays. The X-pinch target consists of wires of 5 μm in diameter—made from either tungsten (W) or gold (Au)-plated W—loaded at two angles of 55° and 98° between the crossed wires. Time-resolved soft X-ray emission measurements are performed to provide a secure correlation with the optical probing results. A reconstruction of the actual photodiode current profile procedure was adopted, capable of overcoming the limits of the slow rising and falling times due to the “slow” response of the diodes and the noise. The pure and Au-plated W deliver an average X-ray yield, which depends only on the angle of the crossed wires, and is measured to be ~50 mJ and ~70 mJ for the 98° and 55° crossed wire angles, respectively. An additional experimental setup was developed to characterize the X-pinch as a source of X-rays with energy higher than ~6 keV, via time-integrated m...

Applied Sciences, 2021

A three-dimensional, thermal-structural finite element model, originally developed for the study ... more A three-dimensional, thermal-structural finite element model, originally developed for the study of laser–solid interactions and the generation and propagation of surface acoustic waves in the macroscopic level, was downscaled for the investigation of the surface roughness influence on pulsed laser–solid interactions. The dimensions of the computational domain were reduced to include the laser-heated area of interest. The initially flat surface was progressively downscaled to model the spatial roughness profile characteristics with increasing geometrical accuracy. Since we focused on the plastic and melting regimes, where structural changes occur in the submicrometer scale, the proposed downscaling approach allowed for their accurate positioning. Additionally, the multiscale simulation results were discussed in relation to experimental findings based on white light interferometry. The combination of this multiscale modeling approach with the experimental methodology presented in thi...

The quantum phases of the electron paths driven by an ultrafast laser in high harmonic generation... more The quantum phases of the electron paths driven by an ultrafast laser in high harmonic generation in an atomic gas depends linearly on the instantaneous cycle-averaged laser intensity. Using high laser intensities, a complete single ionisation of the atomic gas may occur before the laser pulse peak. Therefore, high harmonic generation could be localized only in a temporal window at the leading edge of laser pulse envelope. Varying the laser frequency chirp of an intense ultrafast laser pulse, the centre, and the width of the temporal window, that the high harmonic generation phenomenon occurs, could be controlled with high accuracy. This way, both the duration and the phase of the electron trajectories, that generate efficiently high harmonics, is fully controlled. An accurate and robust method of spectral control and selection of the high harmonic extreme ultraviolet light from distinct quantum paths is experimentally demonstrated. Furthermore, a phenomenological numerical model en...

Applied Physics Letters

Bright betatron x-rays generation from picosecond laser interactions with long-scale near critica... more Bright betatron x-rays generation from picosecond laser interactions with long-scale near critical density plasmas

Optics Express, 2015

The role of thin-film metal transducers in ultrafast laser-generated longitudinal acoustic phonon... more The role of thin-film metal transducers in ultrafast laser-generated longitudinal acoustic phonons in Si (100) monocrystal substrates is investigated. For this purpose degenerate femtosecond pump-probe transient reflectivity measurements are performed probing the Brillouin scattering of laser photons from phonons. The influence of the metallic electron-phonon coupling factor, acoustical impedance and film thickness is examined. An optical transfer matrix method for thin films is applied to extract the net acoustic strain relative strength for the various transducer cases, taking into account the experimental probing efficiency. In addition, a theoretical thermo-mechanical approach based on the combination of a revised two-temperature model and elasticity theory is applied and supports the experimental findings. The results show highly efficient generation of acoustic phonons in Si when Ti transducers are used. This demonstrates the crucial role of the transducer&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s high electron-phonon coupling constant and high compressive yield strength, as well as strong acoustical impedance matching with the semiconductor substrate.

Applied Physics Letters, 2013

ABSTRACT The three dimensional spatiotemporal response of thin metal films surfaces excited by na... more ABSTRACT The three dimensional spatiotemporal response of thin metal films surfaces excited by nanosecond laser pulses is investigated in both the thermoelastic and the ablation regimes. An experimental laser whole-field interferometric technique allows for the direct monitoring of the dynamic deformation of a macroscopic area on the surface with ultrahigh lateral resolution. A specially developed three dimension finite element model simulates the laser-surface interaction, predicts the experimentally obtained results, and computes key parameters of matter&#39;s thermomechanical response. This method provides a powerful instrument for spatiotemporal behavior of thin-film surfaces under extreme conditions demanded for innovative applications.

Dentistry Journal

Background: During in-office bleaching, appropriate light sources are applied in order to enhance... more Background: During in-office bleaching, appropriate light sources are applied in order to enhance the activity of the bleaching gels applied onto teeth. For this method to be effective, a high absorption of light within the gel is necessary. Variation in the light attenuation capability of the gel, the duration of application and light activation can contribute towards safety hazards associated with this procedure. Methods: In this study, seven different gels and hydrogen peroxide have been evaluated for their optical properties by means of spectrophotometry (440–1000 nm). The transmitted light spectrum was used to estimate the intensity loss for each gel. The mean intensity decreases observed were statistically analysed using an analysis of variance (ANOVA). Results: The five more-pigmented gels tested indicated a very similar intensity loss of around 80%, whereas the remaining two gels showed significantly less attenuation (predominantly, p < 10−6). Conclusions: Throughout the ...

Journal of Physics D: Applied Physics

The Journal of the Acoustical Society of America

Plasma filaments in air induced by femtosecond laser pulses lead to the generation of strong shoc... more Plasma filaments in air induced by femtosecond laser pulses lead to the generation of strong shock waves. This letter presents a systematic study, both experimental and theoretical, of the acoustic radiation by femtosecond laser-generated filaments. A theoretical model is developed based on the experimental results and is used to evaluate the directivity of the filament's acoustic radiation within and beyond the audible frequency range. It is shown that the acoustic directivity of plasma filaments can be derived from the model of a weighted acoustic line source, consisting of elementary point sources with N-shaped excitation.