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.
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.
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.
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.
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