This paper deals with the microstructure of the generated crystals in borate glass by femtosecond laser irradiation, Raman spectroscopy was used to study the distribution of the high temperature and low temperature phases of barium metaborate crystals produced in the borate glass, and the mechanism was discussed.
Femtosecond laser has been widely used in microscopic modifications to materials due to its ultra-short laser pulse and ultrahigh light intensity. When a transparent material e.g. glass is irradiated by a tightly focused femtosecond laser, the photo-induced reaction is expected to occur only near the focused part of the laser beam inside the glass due to the multiphoton processes. We observed various induced localized microstructures e.g. color center defects, refractive index change, micro-void and micro-crack, in glasses after the femtosecond laser irradiation, and discussed the possible applications of the microstructures in the fabrication of various micro-optical components, e.g. optical waveguide, micro-grating, micro-lens, fiber attenuator, 3-dimensional optical memory. In this paper, we review our recent investigations on single femtosecond laser-beam induced nanostructures. We introduce the space-selective nanoscale valence state manipulation of active ions, precipitation and control of metal nanoparticles and observation of polarization-dependent permanent nanostructures, and discuss the mechanisms and possible applications of the observed phenomena.
We report on multiphoton-induced periodic microstructures inside bulk polymers by multibeam interference of femtosecond laser pulses at a wavelength of 800 nm, which is out of the absorption range of the polymer sample. Two-dimensional structures and three-dimensional periodic microstructures were fabricated inside bulk azodye-doped polymethylmethacrylate by four-beam interference and five-beam interference, respectively. The Raman spectra measurements for the photoinduced area of the sample before and after irradiation with the writing light showed that the periodic microstructures probably arose from photoinduced decomposition reaction of azodye molecules through multiphoton absorption. Two-dimensional and three-dimensional periodic microstructures were also fabricated in photopolymerization by four-beam and five-beam laser interference, respectively.
We report on precipitation, control and erasure of Au nanoparticles inside transparent glasses. Au3+-doped glass is first irradiated by an 800-nm femtosecond laser at room temperature and then annealed at various temperatures. Transmission electron microscopy shows Au nanoparticles precipitates near the focal point of the laser beam after the irradiation and successive annealing at temperatures above 450°C. We suggest that multiphoton ionization leads to reduction of Au ions, and subsequent nucleation from a high-temperature annealing results in precipitation of Au nanoparticles. We show that the size of the Au nanoparticles could be controlled by the irradiation conditions, and demonstrate that the precipitated Au nanoparticles could be broken by the second irradiation of the femtosecond laser. The similar phenomena have also been observed in Ag+-doped glasses. The observed phenomenon has promising applications for the fabrication of 3-dimensional multi-colored images inside transparent materials, rewriteable optical memory and for integrated micro-optical switches.
Femtosecond laser is under intense study as a new laser processing tool which can induce refractive index change and color center, etc. inside of transparent materials. Here, we report on the first observation of the formation of polarization-dependent nano-grating by irradiation of only single femtosecond laser beam inside silica glasses. This nano-grating was formed by the self-organization at intervals of 200 nm of generated oxygen defect in the laser focal point. The direction of nano-grating was controllable by laser polarization direction. Additionally, the periodicity was variable according to the irradiation laser light of energy and pulse number. We proposed a mechanism of this novel phenomenon containing the interference between the laser light (photon) and the generated plasma wave on the focal point.
Femtosecond laser has been widely used in materials micro-processing when high accuracy and small structure size are required. When a transparent material like glass is irradiated by a tightly focused femtosecond laser, the photoinduced reaction is expected to occur only near the focused part of the laser beam due to the multiphoton processes. We observed various induced structures e.g. color center defects, refractive index change, micro-void and micro-crack, in glasses after the femtosecond laser irradiation. Here, we demonstrate the fabrication of various micro-optical components, e.g. optical waveguide, microgating, microlens, fiber attenuator, 3-dimensional optical memory by using the femtosecond laser-induced structures. The femtosecond laser direct writing technique is very promising in the fabrication of micro-optical components with various optical functions.
We show two kinds of photoinduced molecular orientations in bulk azodye-doped polymers: photoinduced molecular orientation with centrosymmetry and photoinduced molecular orientation with noncentrosymmetry. For the photoinduced molecular orientation with centrosymmetry, the samples were irradiated with a linearly polarized
femtosecond laser whose wavelength (800 nm) was out of the absorption range of the azodye molecules. This photoinduced molecular orientation with centrosymmetry resulted in a birefringence in the sample, based on which an optical image storage was demonstrated. The probe transmittance for the induced birefringence was estimated to be 92%. For the photoinduced molecular orientation with noncentrosymmetry, the sample was irradiated simultaneously by coherent superposition of the 1500-nm fundamental and 750-nm second-harmonic light of a femtosecond laser. The second-harmonic generation (SHG) conversion efficiency of a 105-μm thick film was estimated to be about 2%.
Two kinds of photoinduced periodic microstructures in azodye-doped polymethylmethacrylate were fabricated by interference of two coherent beams of a nonresonant femtosecond laser. One is volume holographic gratings induced by interference of two fs-laser beams with same frequency; the other is molecular polar orientation induced by dual-frequency coherent fs-laser excitation at fundamental and second harmonic frequencies. The photoinduced holographic gratings consist of two surface relief gratings and refractive index modulated gratings in the interior of the polymers. Diffraction efficiency up to 90% of the first-order Bragg for the gratings was obtained. For the photoinduced molecular polar orientation, three kinds of noncentrosymmetries of the polymer films were optically tailored using appropriate combinations of the writing beam polarizations.
Nonlinear optical properties of femtosecond laser-induced micro-structures containing gold nanoparticles in silicate glasses were investigated by using Z-scan technique with 8ns pulses at 532nm. Optical limiting effects of such structures were also measured. The experimental results were found to be significantly variable for the microstructures under different induction conditions. Strong optical nonlinearities in these yielded structures can be attributed to the surface plasmon absorption of gold nanoparticles precipitated in glasses. These microstructures exhibiting large optical nonlinearities are inferred to be useful for the fabrication of integrated ultrafast optical switching and laser protection devices.
The Optical nonlinearities and optical limiting (OL) properties of Sol-gel solidified gold nanorods were investigated by using Z-scan technique and OL measurement, respectively, with nanosecond pulses at 532nm. The experimental results show that their strong nonlinear optical performances depend on the sizes of nanorods. The surface plasmon absorption is the main mechanism.
Three-dimensionally manipulated gold nanoparticles were precipitated in silicate glass containing gold ions by irradiation with femtosecond pulsed laser and successive heat treatment. Influence of the heat treatment conditions on the gold nanoparticles precipitation behaviors was investigated. For the tested silicate glass, the treatment temperature threshold of irradiated area for color appearance was obviously lower than that of non-irradiated area. Along with increasing the treatment temperature from such threshold, the irradiated area turned to be colorful. And finally the whole glass became multicolored corresponding to a certain high temperature. Absorption spectrum measurement indicated that, under the given irradiation condition, the sizes of Au nanoparticles were obviously affected by the heat treatment temperature.
The optical properties of rare-earth organic complexes-doped poly(methyl methacrylates)(PMMA) have been studied because of their great prospect of application in optical devices, such as optical fibers, fiber lasers, amplifiers and waveguides. Eu3+, Pr3+ and Tb3+ ions were encapsulated in tri(dibenzolymethane)(DBM) and tri(thenoyltrifuoroacetone)(TTA) chelates, then Eu3+ complexes were doped in MMA before the polymerization, and stirred thoroughly during the polymerization. These complexes were characterized by elementary analysis, FT-IR, XPS and ICP, and fluorescent properties of rare earth complexes-doped PMMA were measured by fluorescence spectrometer. Analysis reveals that Eu(TTA)3 is a promising dopant for use in rare-earth-doped PMMA in the research, the complex can be doped to high concentrations in PMMA systems without quenching, providing means for polymer amplification devices. For the rare-earth-doped PMMA samples, an effective energy transfer between the ligands and rare-earth ions is observed and discussed. To our knowledge, the effect of molecular weight of PMMA on the fluorescent properties of rare-earth-doped PMMA has not been reported, and the result indicates that the larger the molecular weight of PMMA is, the better the fluorescent property of the system.
New phenomena of anisotropic light scattering, anisotropic Cherenkov third-harmonic generation, photoinduced birefringence and anisotropic reflection from femtosecond direct-write structures are reviewed. The phenomena reveal new mechanism of light-matter interaction.
We report on the observation of photostimulated long-lasting phosphorescence in a Tb3+-doped glass sample. After irradiation by an ultraviolet (UV) light at 254 nm or an 800 nm femtosecond pulsed laser, we find that the glass sample emits bright and long-lasting phosphorescence. Long-lasting phosphorescence is observed once again after further excitation by 365 nm UV light when the 254 nm UV light or the femtosecond laser-induced long-lasting phosphorescence cannot be detected. The intensity of the photostimulated long-lasting phosphorescence decreases in inverse proportion to the time. Based on absorption, emission spectra and thermoluminescence curves, we consider the photostimulated long-lasting phosphorescence to be due to the light-induced re-arrangement and thermostimulated recombination of electrons and holes in traps induced by the 254 nm UV light or the femtosecond laser at room temperature. We infer that the observed phenomenon is useful in the fabrication of rewriteable three-dimensional optical memory with ultrahigh storage density and ultrafast storage speed.
Femtosecond laser is a perfect laser source for materials processing when high accuracy and small structure size are required. Due to the ultra short interaction time and the high peak power, the process is generally characterized by the absence of heat diffusion and, consequently molten layers. Various induced structures have been observed in glasses after the femtosecond laser irradiation. Here, we report on space-selective valence state manipulation of active ions, long-lasting phosphorescence and photostimulated long-lasting phosphorescence phenomena in the femtosecond laser-irradiated glasses, and recent development of direct writing of optical waveguide in glasses with femtosecond laser pulses.
We report on the observation of space-selective precipitation of silver nanoparticles in the Ag2O doped silicate glass in a method of irradiation with an 800nm femtosecond laser and then heat-treatment. The irradiated portion of the glass first became gray in color after irradiation of the femtosecond laser and then turned into yellow after further heat-treatment at around 400 °C. An absorption peak at 402 nm observed in the absorption spectra of the irradiated and heat-treated glass indicates that silver nanoparticles have precipitated in the irradiated region of the glass. A possible mechanism has been suggested that the Ag+ ions in the region irradiated are first reduced to Ag atoms by femtosecond laser through multiphoton absorption and then accumulate to form silver nanoparticles when glass sample was heat-treated. The observed phenomenon should have potential applications in fabrication of ultrafast all-optical switch.
Volume holographic gratings and two-dimension periodic microstructures in azodye-doped polymethylmethacrylate were fabricated, respectively, by interference of two coherent beams of a femtosecond laser and by interference of three coherent beams of a nanosecond laser. The volume holographic gratings consist of two parts, surface relief gratings on both surfaces and refractive index modulated volume gratings in the interior of the polymers. The diffraction efficiency ofthe first-order Bragg for the gratings was estimated to be 91%. In the experiments for interference ofthree beams, the period of two-dimension periodic microstructures was changed by adjustment of the angle between the three writing beams. Experimental results showed that two-dimension periodic microstructures with O.69-µm period were formed on the surface ofpolymer samples.
All-optical control of the noncentrosymmetry of azodye-doped polymer films was demonstrated by nonresonant dual-frequency coherent excitation. Three kinds of noncentrosymmetries of the polymer films were optically tailored using appropriate combinations of the writing beam polarizations. The characteristic kinetics of the photoinduced molecular polar orientation and the tensor analysis were performed by monitoring SHG of the films. The SHG conversion efficiency of a 105)mum thick film was estimated to be about 2%. It was observed that when polarizations of the writing beams were changed from the linear polarization to the circular polarization the saturated SHG signals were enhanced even at the same readout condition. A relaxation retardation effect of the photoinduced (chi) (2) was also observed in the thick films. The quadratic dependence of SHG on film thickness showed that a (chi) (2)grating satisfying the phase-matching condition for SHG was optically induced in the polymer films, indicating this nonresonant all-optical poling can be used to tailor the symmetry of bulk samples.
We examined the feasibility of making various microscopic modifications in glasses by ultra-short pulses. It was confirmed that permanent refractive index changes, photo- reduction of samarium ions, the phenomenon of long-lasting phosphorescence, and creation of microcrystals with second- order nonlinear optical functions can be produced with a femtosecond pulse laser only in selective internal areas in glasses. By using a femtosecond laser with a high repetition rate, permanent optical waveguides can be successfully written in various glasses, where refractive index changes are continuously induced along a path traversed by focal point. We also confirmed that only rare earth ions of the core region in the waveguides are reduced by the laser irradiation.
We report on the observations of novel phenomena in rare- earth-doped glasses induced by an 800 nm femtosecond pulsed laser. 3D long-lasting phosphorescence in various colors was observed in calcium aluminosilicate glasses doped with Ce3+, Tb3+, and Pr3+. After irradiation by the femtosecond laser, the focused part of the laser in the glasses emits bright and long-lasting phosphorescence able to be clearly seen with the naked eye in the dark even one hour after the removal of the activating laser. We also observed permanent photoreduction of Sm3+ to Sm2+ inside a transparent and colorless Sm3+- doped sodium aluminoborate glass. After irradiation by the femtosecond laser, the focused part of the laser in the glass became orange in color. Absorption and confocal photoluminescence spectra showed that a part of Sm3+ was reduced to Sm2+ after the laser irradiation. Photochemical hole burning phenomenon has also been confirmed in the femtosecond laser photoreduced part after the further irradiation by a DCM dye laser. Electron spin resonance spectra of the glasses before and after the laser irradiation have been measured to clarify the mechanisms of these phenomena. Microscopic modifications using a femtosecond pulsed laser is promising to open new field in science and technology of rare-earth-doped materials.
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