1,720,957 research outputs found
Theoretical treatments of ultrashort pulse laser processing of transparent materials: towards explanations of extraordinary phenomena
No full textThe dynamics of ultrashort-laser-induced generation of free electron plasma inside bulk glass (fused silica as an example) is analyzed. The results of modeling are presented for typical glass modification regimes, obtained on the basis of the Maxwell equations supplemented with the equations describing electron plasma formation and the laser-induced electric current. We demonstrate that the model allows revealing important features of laser beam propagation in the regimes of dense electron plasma generation such as strong scattering up to complete displacing of light from the plasma region followed by beam refocusing. The results are compared with those obtained in the frames of the non-linear Schrödinger equation (NLSE). It is demonstrated that a unidirectional approximation for laser beam propagation under the laser breakdown conditions does not provide an adequate description. Moreover, the NSLE may lead to overestimating the absorbed laser energy up to several times as compared to a more rigorous approach based on the Maxwell equations. A controversial issue of the density level of free electron plasma generated inside bulk glass by ultrashort laser pulses is discussed. The energy balance of excited matter is considered with introducing an "Ee - ne" diagram which matches the level of transient excitation with the maximum temperature of the glass matrix. From the simulation data on the geometry of the laser energy absorption zone, the glass temperature is mapped which may be foreseen at the end of electron - glass matrix relaxation. This, in turn, allows calculations of the laser-induced stress levels and making conclusions on the routes of glass modification. Based on the modeling results, we propose the mechanisms responsible for two unusual effects, formation of volume nanogratings in a number of transparent solids under the action ultrashort laser pulses and laser direct writing anisotropy observed for laser pulses with a tilted front. Finally we address the question on the material properties which are responsible for nanograting imprinting to the glass matrix
Ultrashort-pulse laser modification of transparent materials: Insight from inside
No full textThe processes initiated in transparent dielectrics by ultrashort pulse irradiation upon focusing inside the bulk are analyzed. The analysis is based on the modeling in the frames of Maxwell's equations supplemented with the equations for electron plasma formation and laser-induced electric current. On the case of fused silica, important features of laser beam propagation in the regimes of dense electron plasma generation are revealed such as complete displacing of laser light from the plasma region followed by beam refocusing. Density levels of free electron plasma generated inside bulk glass by ultrashort laser pulses are discussed. The energy balance of excited matter is considered with introducing a diagram matching the level of transient excitation with the maximum temperature of material matrix. The temperature and laser-induced stress upon material excitation are mapped which allow making conclusions on modification routes. We propose the mechanisms responsible for the formation of volume nanogratings in a number of transparent solids under the action ultrashort laser pulses. Finally the question on the material properties favorable for nanograting imprinting is addressed
Theoretical treatment of ultrashort pulse laser processing of transparent materials: What is energetically and mechanically meaningful?
No full textUltrashort laser pulses are a powerful tool for modifying the structure and properties of transparent materials. Depending on material properties and irradiation conditions, a wide variety of modifications can be induced such as surface and bulk periodic structures (nanogratings), densification with associated refractive index change, microvoids and void chains, phase transitions, etc. This gives rise to numerous technological applications based on 3D photonic structures in bulk optical materials (waveguides, Bragg gratings, Fresnel zone plates, rewritable optical memories, and others). Among transparent materials, optical glasses are of prime importance for optoelectronics and photonics due to their relatively low cost, processability, and possibility of governing refractive index and inducing optical anisotropy. The physics behind laser-induced glass modifications is extremely rich and involves the multiplicity of the consecutive processes initiated by radiation absorption during the laser pulse and extending to millisecond time scales when the final structure becomes “frozen” in the glass matrix. While tremendous achievements have been made toward laser-writing techniques and assembling integrated optics, the physical mechanisms underlying glass modifications have not been fully understood. The exigency of controllable generation of desired structures requires deeper insight into of the mechanisms and spatiotemporal dynamics of laser-induced glass transformations.In this report, we will review the physical processes and mechanisms responsible for various forms of glass modification. Different approaches for modeling ultrashort laser pulse propagation in transparent materials will be critically assessed. The dynamics of laser-induced creation of free electron plasma inside bulk glass will be analyzed, depending on the irradiation conditions. A contradictory issue on the free electron density generated in glass materials upon laser irradiation will be addressed with reviewing the existing theoretical results and experimental evaluations based on application of the Drude theory. The results of modeling will be presented obtained on the basis of the Maxwell’s equations supplemented with the equations describing electron plasma generation and the laser-induced electric current. We will demonstrate that the model allows following important features of laser beam propagation in the regimes of tight focusing and dense electron plasma generation when unidirectional approximations such as the non-linear Schrödinger equation do not provide adequate description. Based on this model we have studied spatiotemporal dynamics of laser beam propagation with self-focusing, free electron generation, and plasma-induced defocusing on the example of fused silica glass under particular irradiation regimes employed for laser direct writing. As a result, the geometry of the laser energy absorption zone is determined and the glass temperature is mapped which may be foreseen at the end of electron – glass matrix relaxation. This, in turn, allows estimating the laser-induced stress levels and making conclusions on the routes of glass modification. Finally, based on the performed analysis, we consider the energy balance, matching the free electron energy and temperature with several threshold values (melting, plastic deformation, material failure with void formation, sublimation).*This research is supported by Marie Curie International Incoming Fellowship grant of the corresponding author, No. 272919
Ultrafast laser modification in glasses: basic and novel aspects and applications
No full textUltrashort laser pulses are a powerful tool for modifying the structure and properties of transparent materials. Depending on material properties and irradiation conditions, a wide variety of modifications can be induced such as surface and bulk periodic structures (nanogratings), densification with associated refractive index change, microvoids and void chains, phase transitions, etc. This gives rise to numerous technological applications based on 3D photonic structures in bulk optical materials (waveguides, Bragg gratings, Fresnel zone plates, rewritable optical memories, and others). Among transparent materials, optical glasses are of prime importance for optoelectronics and photonics due to their relatively low cost, processability, and possibility of governing refractive index and inducing optical anisotropy. The physics behind laser-induced glass modification is extremely rich and involves the multiplicity of the consecutive processes initiated by radiation absorption during the laser pulse and extending to millisecond time scales when the final structure becomes .frozen. in the glass matrix. While tremendous achievements have been made toward laser-writing techniques and assembling integrated optics, the physical mechanisms underlying glass modifications have not been fully understood. However, further development of laser-writing techniques for controllable generation of desired modifications in transparent materials is impossible without deep understanding of the governing mechanisms of modifications. In this report, we will review the physical processes and mechanisms responsible for various forms of glass modification. Different routes of photoexcitation with creation of dense plasma inside glass bulk will be analyzed depending on the irradiation conditions. Relative contributions of multi-photon and tunneling ionizations and the avalanche process will be considered. The thermodynamic and thermomechanical processes following plasma recombination at different space and time scales will be discussed. A number of intriguing experimental evidences will be canvassed with their detailed theoretical analysis. The results of modeling based on the Maxwell.s equations will be presented for the particular irradiation regimes when different modifications are observed such as nanograting structures and microvoids. The levels of plasma densities and glass matrix temperature distributions upon plasma recombination will be compared and a criterion of glass rupture will be discussed. On the basis of the theory of thermoelastoplastics, we will show that void formation is not obligatory involves the TPa or GPa pressure levels. A special attention will be paid to the collective plasma effects whose role in formation of exotic structures inside and on the surface of transparent materials is highly underestimated. In particular, we will show that the standing waves in confined plasmas can strongly influence the dynamics of laser light absorption. Finally, the most intriguing effects upon laser-induced glass modification which still require explanations and adequate theoretical descriptions will be outlined
Ultrashort laser modification of transparent materials: synergy of excitation/relaxation kinetics, thermodynamics and mechanics
No full textUltrafast laser modification of transparent materials is an important technique enabling production of 3D photonic structures whose practical applications are rapidly widening. The physics behind laser-induced modifications is extremely rich and involves a variety of consecutive processes initiated by radiation absorption during the laser pulse and extending to millisecond timescales when the final structure becomes "frozen" in the material matrix. The quality of the final structures depends of the synergetic action of excitation of confined electron plasma, its relaxation with drawing matter into different thermodynamic states from soft heating to extreme conditions, generation of GPa pressures resulting in shock-induced material deformations, re-forming of covalent bonds upon photo-excitation of the material network. In this report, we will review the physical processes responsible for various forms of laser-induced modification in wide-bandgap materials, including volume nanograting formation. We will present the modeling results obtained on the basis of the Maxwell’s equations supplemented with equations describing the dynamics of the laser-induced electron plasma on the example of silica glass for typical experimental conditions. The temperature and associated stress levels are mapped in the laser energy absorption zone which may be foreseen at the end of electron - glass matrix relaxation, enabling to make conclusions on the routes of glass modification. Finally, the energy balance is considered, matching the free electron density and temperature with several threshold values (melting, plastic deformation, material failure with void formation, sublimation)
Ultrashort laser modification of transparent materials: synergy of excitation/relaxation kinetics, thermodynamics and mechanics
No full textUltrafast laser modification of transparent materials is an important technique enabling production of 3D photonic structures whose practical applications are rapidly widening. The physics behind laser-induced modifications is extremely rich and involves a variety of consecutive processes initiated by radiation absorption during the laser pulse and extending to millisecond timescales when the final structure becomes "frozen" in the material matrix. The quality of the final structures depends of the synergetic action of excitation of confined electron plasma, its relaxation with drawing matter into different thermodynamic states from soft heating to extreme conditions, generation of GPa pressures resulting in shock-induced material deformations, re-forming of covalent bonds upon photo-excitation of the material network. In this report, we will review the physical processes responsible for various forms of laser-induced modification in wide-bandgap materials, including volume nanograting formation. We will present the modeling results obtained on the basis of the Maxwell's equations supplemented with equations describing the dynamics of the laser-induced electron plasma on the example of silica glass for typical experimental conditions. The temperature and associated stress levels are mapped in the laser energy absorption zone which may be foreseen at the end of electron - glass matrix relaxation, enabling to make conclusions on the routes of glass modification. Finally, the energy balance is considered, matching the free electron density and temperature with several threshold values (melting, plastic deformation, material failure with void formation, sublimation)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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