V. V. Kononenko

Laser ablation of dental materials using a microsecond Nd:YAG laser

The action of microsecond laser pulses with a wavelength of 1064 nm on dental tissues (enamel and dentin) and various dental materials used for tooth replacement and filling (ceramics, metal alloys, and composites) is studied. It is demonstrated that the ablation thresholds of all of the dental materials are significantly lower than the threshold laser fluences for the dental tissue (Ethr = 200–300 J/cm2). At the laser fluences that do not allow ablation and damage of the dental tissues, the dental materials are effectively removed at a rate of no greater than 40 μm per pulse. It is shown that the laser ablation of the materials under study involves two processes (evaporation and volume explosion) depending on the optical density. The results obtained indicate that the laser radiation with a wavelength of 1064 nm and the microsecond pulse duration is promising for dental applications, since it allows effective cleaning of the tooth surface from various dental materials in the absence of the damages of dental tissues.

Laser-induced modification of bulk fused silica by femtosecond pulses

The features of the local nondestructive modifications of the bulk fused silica are experimentally studied. A method for the measurement of the refractive index in a transparent sample irradiated with femtosecond laser pulses is developed. The accuracy of the refractive-index measurements achieves about 10−5, and a spatial resolution is several microns. The quantitative data on the dynamics of the permanent modification of glass as a function of the radiation energy are obtained for the first time. The spatial localization of the modified area that is predominantly determined by the laser-beam divergence is analyzed. The femtosecond interferometry is used to investigate the laser-pulse propagation in amorphous silica.

CVD diamond transmissive diffractive optics for CO2 lasers

Growing applications of CO2-lasers in many cases demand transmission optical elements (widows, lenses, etc.) which could reliably operate at high beam powers. In the present work we propose to produce CVD diamond transmissive diffractive optics. To realize this idea surface microstructuring by laser ablation was applied. Computer controlled selected area ablation of preliminary polished (plane) diamond plates with thickness of 0.3 mm was performed by scanning focused beam of KrF laser. As a test sample for radiation with (lambda) equals 10.6 micrometers a cylindrical lens with the aperture of 4 X 4 mm2 and focal length of 25 mm was produced. Numerical simulation of the lens was made for the fundamental mode TEM00. For tests of the diamond lens 20 W CO2 laser was used.

Activation of color centers in bismuth glass by femtosecond laser radiation

The activation of color centers in the bismuth aluminum-boron-phosphate glass by high-intensity femtosecond laser radiation is experimentally studied. The dynamics of the laser-induced emitting centers in the bulk of sample is characterized. The photoactivation of bismuth glasses is possibly related to the recharging of structural precursors, which serve as effective electron traps and whose possible configuration is discusses. The effect of the femtosecond irradiation involves the initiation of the nonlinear ionization of the glass matrix and the generation of plasma with the concentration of carriers that is sufficiently high to provide almost complete recharging of precursors over several laser pulses.

Laser-induced phase transitions in ion-implanted diamond

Abstract Results are reported on the study of phase transformations in D + (deuterium) ion-implanted diamond single crystals induced by nanosecond pulses of a KrF excimer laser (λ=248 nm). Multipulse laser irradiation at fluences lower than the graphitization thresholds resulted in progressive annealing, pronounced in an increase of the optical transmission and surface contraction. A non-linear defect distribution in a near-surface layer was found to strongly affect the annealing and graphitization processes; higher annealing efficiency and higher graphitization thresholds were observed under irradiation conditions when a laser beam was incident onto a buried defective layer through diamond, i.e., onto a ‘back’ side of the diamond sample opposite to the ion-implanted side. The influence of non-uniform laser-induced heating of defective diamond material on characteristic features of the phase transitions is discussed.

Laser shaping of diamond for IR diffractive optical elements

Recently a new technique for laser-induced generation of phase relief to manufacture diamond diffractive lenses for the mid IR range has been proposed. In the present paper the realization of more complicated diamond diffractive optical elements (DOEs) is considered, able to transform a CO2 laser beam into arbitrary pre-given focal domains. Two DOEs for completely different tasks of laser beam focusing have been manufactured and finally investigated by means of various optical techniques. Measured intensity distributions in the DOEs focal planes as well as diffraction efficiencies have been compared with related results of computer simulation, and have been found to be in a good mutual concordance. The obtained results indicate that laser ablation technique can be effectively used to manufacture high quality diamond DOEs for laser beam focusing. Special attention is paid to the diamond surface graphitization in the process of laser ablation. Main parameters of excimer laser ablation are investigated and density of laser-induced graphite-like layer is defined. It was demonstrated experimentally that graphitized layer formed at different regimes of irradiation remains almost constant in thickness, but has different crystal structure.

Excimer laser micromachining for fabrication of diamond diffractive optical elements

Results of the development of multilevel diamond diffractive optical elements for high power CO2 lasers are reported. For this purpose, the technique of selected-area laser ablation was applied to create a given phase microrelief on mechanically polished CVD polycrystalline diamond plates. High precision micromachining of the diamond plates was performed using a KrF excimer-laser-based system. The spherical and cylindrical Fresnel lenses with an aperture up to 6 by 6 mm6 were produced and tested with a cw CO2 laser. The diffraction efficiency of the developed DOE measured in the case of the cylindrical lens was found to be only 1-2 percent lower than the theoretical value. Also a diffractive shaper transforming a Gaussian beam of a CO2 laser into a uniformly filled-in rectangle was designed and fabricated.

Diamond DOEs for focusing IR laser beams into pregiven focal domains

Recently a new technique has been proposed for laser-assisted generation of phase microrelief to manufacture diamond diffractive lenses for the far IR range. In the present paper the realization of diamond diffractive optical elements (DOEs) is considered, able to focus an incoming CO2 laser beam into certain pregiven focal domains. Exemplarily, two completely different DOEs for different tasks of laser beam focusing have been designed by different methods, manufactured and finally investigated by means of optical experiment and computer simulation. Measured intensity distributions in the DOEs’focal planes as well as measured diffraction efficiencies have been compared with related results of computer simulation, and have been found to be in good mutual concordance. Obtained first results indicate that technique of laser-assisted ablation can be effectively used for manufacturing of high quality diamond DOEs for laser beam focusing.

Effect of laser radiation parameters on the conductivity of structures produced on the polycrystalline diamond surface

Graphitized structures are fabricated on the polycrystalline diamond surface using an excimer KrF (λ = 248 nm, τ = 20 ns) and a Ti:Al2O3 (λ = 400 nm, τ = 120 fs) lasers. It is shown that the conductivity of formed structures is independent of the energy density and the number of pulses per surface point in the case of the excimer laser, whereas such a dependence was observed for femtosecond pulses. The causes of the dependence of the conductivity of surface structures on laser irradiation parameters are discussed.

Laser structuring of the diamond surface in the nanoablation regime

The possibility of using nanoablation (graphitisation-free photoinduced etching) for precise micro- and nanostructuring of the surface of a diamond single crystal has been experimentally investigated. The processing has been performed by femtosecond third-harmonic pulses of a Ti : sapphire laser . The specific features of the formation of a surface nanorelief (regularity of the newly formed structures and reduced nanoablation rate near craters) are discussed. The possibility of fabricating a diamond phase diffraction grating with a relief depth of is demonstrated and the diffraction pattern recorded at is compared with the results of theoretical analysis.