Todor Petrov

Measurement of nonlinear refractive index and multiphoton absorption by the subpicosecond z-scan method of tellurite multicomponent glassy matrixes having nonlinear susceptibility

Abstract New multicomponent tellurite glassy matrixes possessing high nonlinear susceptibility weredeveloped and their optical properties were studied. The nonlinear refractive index n 2 andmultiphoton absorption β were measured by using the subpicosecond z -scan method.Second-harmonic generation was observed in poled glassy samples.PACS numbers: 61.43.−j, 42.70.Ce, 42.65.Re, 42.65.Ky(Some figures may appear in color only in the online journal) 1. Introduction Vigorousinvestigationsofpoledglasseshavebeencarriedoutsince the observation of unusually efficient second-harmonicgeneration (SHG) in germanium–silica glass fibers [1] andthermally–electrically poled [2] fused silica glasses. Thereason for the intensive studies of such disordered materialsand related centrosymmetrical crystals is the ability to changetheir nonlinear optical susceptibility by poling (optical,thermal, electrical, etc). This phenomenon could be usedin optical memory storage, optic switching and some otheroptical communication applications [3, 4].The poling of glasses and centrosymmetrical crystals isbased on breaking of the macroscopic matrix symmetry. Theobtained deformations range in size from several nanometerstomicrometers.Themainadvantageofglassescomparedwithsingle crystals and polycrystalline materials is the possibilityof obtaining samples with the desired shape (almost anyshape). Additionally, glasses possess high transparency in abroad wavelength range and their poling may be achievedrelatively easily when compared with that applied to singlecrystals.Tellurite glasses are promising materials for lasertechnologybecauseoftheirextendedvisibleandinfrared(IR)transmission range as well as their high refractive linear andnonlinearindices[5,6],andamongoxideglassestheypossessone of the highest third-order nonlinear susceptibilities [7].Moreover, they are relatively easily producible due to theirrelatively low melting temperatures. Tellurite glasses withheavy metal oxides were found to show remarkable physicaland optical properties [8–12].SHG in thermally assisted electro-poled tellurite glassmatrixes was reported in [10, 13–14]. Sum frequencygeneration in poled glassy matrixes was reported by Fujii

Precise femtosecond laser ablation of dental hard tissue: preliminary investigation on adequate laser parameters

This work aims at evaluating the possibility of introducing state-of-the-art commercial femtosecond laser system in restorative dentistry by maintaining well-known benefits of lasers for caries removal, but also in overcoming disadvantages such as thermal damage of irradiated substrate. Femtosecond ablation of dental hard tissue is investigated by changing the irradiation parameters (pulsed laser energy, scanning speed and pulse repetition rate), assessed for enamel and dentin. The femtosecond laser system used in this work may be suitable for cavity preparation in dentin and enamel, due to the expected effective ablation and low temperature increase when using ultra short laser pulses. If adequate laser parameters are selected, this system seems to be promising for promoting a laser-assisted, minimally invasive approach in restorative dentistry.

Hydroxyapatite kinetic deposition on solid substrates induced by laser-liquid-solid interaction

Hydroxyapatite (HA) is present in the human body as a mineral constituent of the bones and teeth, as well as a major or minor component of kidney stones. HA deposited on different solid substrates can find applications including biomaterials and biosensors. This work deals with the kinetics of the HA growth by applying a novel method of laser-liquid-solid-interaction (LLSI) process on three types of materials (stainless stell, silicon and silica glass). The method allows interaction between a pulsed laser and a substrate immersed in a solution (simulated body fluid, SBF). By a scanning system, a design of seven squares at a distance of 200 μm was created at the end of each sample. In this way the center of the substrate (about 6x6 mm) was no irradiated. Following the LLSI process, the samples were left in the irradiated SBF for various intervals of time. Light microscopy (LM) showed surfaces seede with randomly distributed transparent and white particles. The surface seeding increased with the immersion time and was dependent on the substrate type. Fourier transform infrared (FTIR) spectrsocopy showed that in the first stage of soaking (up to 6 h) the observed white particles were calcium phosphate containing. Energy dispersive X-ray (EDX) spectrsocopy revealed that the transparent particles were NaCl. In the next stage (after 12 h) vibrational modes typical for HA were clearly observed. Detailed observation with scanning electron microscopy (SEM) after 12 h showed morphology of sphere-like aggregates, grouped in a porous network. Raman spectroscopy, X-ray diffraction (XRD) and EDX confirmed that after 12 h the grown layer was HA. It was found that in comparison to the traditionally empoyed prolonged soaking in SBF, the applied LLSI process yielded a synergistic effect due to the simultaneous use of the solid substrate, the aqueous solution and the laser energy.

Enhancement of hydroxyapatite formation by laser-liquid-solid interaction

In this paper the use of stainless steel, silicon and silica glass substrates for the growth of hydroxyapatite (HA, widely used as artificial bone material), induced by a laser-liquid-solid-interaction is reported. The method allows growing of HA layer by using the interaction between a laser beam and a liquid precursor solution, as well as laser irradiation of the substrate during the laser-liquid interaction. The scanned laser beam (pulsed CuBr laser) is directed at the solid substrate, which is immersed in the solution, which resembles the ion composition and concentrations of the human blood plasma (simulated body fluid, SBF). The set-up includes an open deposition system, which allows the introduction of the laser beam led by a scanning system. It is shown that the proposed method enhances the HA formation, in comparison with the traditional methods of prolonged soaking in SBF. The HA layers grown in this manner are investigated by Light Microscopy (LM), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) Spectroscopy, X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR) and Raman Spectroscopy.

Results of the trials and light delivery evaluation on low-level laser therapy of acute and chronic pain

Although the low-level laser therapy (LLLT) has been accepted in the clinical practice, its efficiency is still questionable because of the unclear mechanisms of LLLT action. This work presents the results of LLLT applied to volunteers who need recovery from trauma or suffer from rheumatic diseases, inflammatory disorders, etc. The control group we used for comparison consisted of patients being treated by conventional therapy that included massage and acupuncture needles. The effectiveness of the LLLT was graded under four categories. Short-term and long-term side effects as well as conditions responding only to LLLT were recorded. The successful treatments were up to 70%, which coincided with the result of the control group. The LLLT was performed with a GaAs laser system provided with additional set of six light CW emitting diodes to scan a larger area of a tissue surface. To evaluate the light delivery inside the tissue, the spatial energy distribution within the laser beam was measured with a CCD camera. On its basis, the light dose absorbed in the tissue was calculated both in the boundary layer under the surface and in depth using a reduced variance Monte-Carlo code.

Ion-beam-modified surfaces as substrates for hydroxyapatite growth induced by laser-liquid-solid interaction

In this study stainless steel, silicon and silica glass are used as representatives of metal, semiconductor and isolator with the purpose to create an experimental model for studying the formation of minerals like hydroxyapatite (HA, the bone and teeth mineral part) from aqueous solutions. The samples are Na+ implanted and consequently subjected to thermal treatment in air at 873 K. Implantation with Na+ is known to lead to formation of hydroxylated surface, i.e. formation of metal- or Si-OH- groups upon immersion in a liquid, simulating the human blood plasma (simulated body fluid, SBF). The negatively charged hydroxylated surfaces induce HA formation in SBF. The samples are immersed in SBF, irradiated through the solution with a scanning laser beam (CuBr pulsed laser equipped with a scanning system) and subsequently soaked in the solution at 37°C for a shorter time, comparing to the traditional methods for HA growing. The grown HA layers are investigated by Fourier Transform Infrared (FTIR) and Raman Spectroscopies, X-ray Diffraction (XRD), Light Microscopy (LM), Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray (EDX) Spectroscopy to evaluate the effect of the surface modification by the thermal treatment following the ion implantation, as well as the effect of the laser irradiation on the process of HA formation.

Results of the trials and light-delivery evaluation at low-level laser therapy of acute and chronic pain

Although the low-level laser therapy (LLLT) is accepted in the clinical practice, its efficiency is still questionable because of the unclear mechanisms of LLLT action. This work presents the results of LLLT applied to volunteers who need recovery from trauma or suffer from rheumatic diseases, inflammatory disorders, etc. The control group we used for comparison consisted of patients being treated by conventional therapy that included massage and acupuncture needles. The effectiveness of the LLLT was graded under four categories. Short-term and long-term side effects as well as conditions responding only to LLLT were recorded. The successful treatments were up to 70%, which coincided with the result of the control group. The LLLT was performed with a GaAs laser system SIX LASER IR - Bulgaria provided with additional set of six light CW emitting diodes to scan a larger area of a tissue surface. To evaluate the light delivery inside the tissue, the spatial maps of the light spot at the laser output in different operating regimes were measured. On their basis, the absorbed dose was calculated both in the boundary layer under the tissue surface and in depth using a reduced variance Monte-Carlo code.