Jinhai Si

Photoinduced birefringence of azodye-doped materials by a femtosecond laser

Birefringence in azodye-doped polymethylmethacrylate and azodye-doped silica hybrid materials was induced by the two-photon excitation of a femtosecond laser. The growth and the relaxation characteristics of the photoinduced birefringence in the two types of material were investigated, and the probe transmittance for the induced birefringence was estimated to be 92%. Furthermore, an optical image storage based on this photoinduced birefringence was demonstrated.

Space-selective precipitation and control of metal nanoparticles inside glasses

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.

Photoinduced microstructures inside bulk azodye-doped polymers by the coherent field of a femtosecond laser

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.

Induced periodic structures in bulk polymers by interference of laser beams

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.

Phase-matched second-harmonic generation in azodye-doped polymer films by nonresonant all-optical poling

Phase-matched second-harmonic generation (SHG) in azodye-doped polymer films was demonstrated using the nonresonant optical poling technique. A quadratic dependence of SHG on the film thickness and a relaxation retardation effect of the photoinduced ?(2) were observed in the films. The SHG conversion efficiency of a 105-?m thick film was estimated to be about 2%.

Ultrafast Induction of Electronic Structures by Ultrashort Laser Pulses

The availability of ultrashort laser pulses has recently stimulated much interest in both fast photonic devices and materials that can be applied in optical communication and optical computing. Third-order optical nonlinearity is the most important property for realization of photonic devices, such as alloptical switching, all-optical bistability, and optical limiting devices. Because of their large third-order nonlinearity, there has been significant interest in semiconductors (quantum-well structures). This resonant-type material, however, show large optical absorption and a slower response time in the limited wavelength range where large third-order nonlinearity is obtained.

Computer Simulation of Induced Structures

Silica (SiO2) has various polymorphs; quartz, cristobalite, tridymite, stishovite, coesite and an amorphous one [221]. Experimental [222–226] and theoretical [227–243] studies have been performed on the crystal forms of SiO2 to determine their electronic structure. It is well known that amorphous SiO2 is very common and useful materials in microelectronic devices: optical fibers, semiconductor transistors, etc. Although there has been less theoretical work on amorphous SiO2 [238, 244–249] than on crystal [227–243], theoretical quantum-mechanical calculation has proven valuable to the understanding of the detailed electronic structure of amorphous SiO2.

Generation of Induced Structures in Rare-Earth-Ions-Doped Glasses

Rare earths have been used widely in various fields, such as phosphor, magnetic material and superconductor. The effect of rare-earth ions can be divided into two parts: one due to the electron configuration of their 4f orbit, and the other due to their chemical bonding state, e.g. ion radius and chemical bond length. The most important properties of rare-earth-containing materials come from the special 4f electronic structure of the rare earth. As was pointed out in the Introduction, a large amount of rare-earth ions can be stuffed into glass. Glass is an excellent matrix for rare-earth ions to realize various optical functions. The optical properties of glasses doped with rareearth ions have been investigated intensively, and these glasses have been used as materials for lasers, optical amplifiers and so on. However, only a few studies have been made of the induced structure in rare-earth-ion-doped glasses.

Active Glasses for Functional Devices

In Parts 1 and 2, ultrafast induction of electronic structures and induction of permanent structure by ultrashort laser pulses were introduced. Three-dimensional optical waveguides were written and single-crystal-like structures were grown in glasses using a focused femtosecond laser. Various novel phenomena in rare-earth-ions-doped glasses were observed, including photostimulated luminescence, long-lasting phosphorescence and space-selective manipulation of valence state of rare-earth ions. Methods such as X-rayabsorption fine structure analysis and, microscopic Raman scattering spectroscopy were used to analyze the induced structures in glasses. Computer simulation was also performed for induced structures in glasses. In this chapter, the following functional devices using active glasses are introduced: microsphere lasers [269] , rare-earth-ion-doped glasses for UV and blue upconversion lasers [270] , semiconductor-microcrystal-doped films emitting electroluminescence [271], dye-doped polymers exhibiting SHG [272, 273], Faraday effect glass which can be used as a magneto-optical switch [152, 274], and long-period optical fiber gratings [275]. They vary in form from microsphere, film, bulk to fiber, and in any of the function devices, induced structure takes an important role in the realization of optical functions.

Development of Analytical Methods for Induced Structures

In Part 1 of this book, external electromagnetic field induced structures in glasses and other materials and promising applications in various fields such as integrated optics and optical communications were introduced. From the viewpoint of material design, it is important to clarify the formation mechanism of the induced structures. In this chapter a novel method for the direct observation of excited-state absorption of rare-earth ions is introduced [203] . The valence state and chemical bond state of rare-earth ions in glasses showing room-temperature spectral hole burning and photostimulated luminescence were examined by using X-ray absorption fine structures (XAFS) [205] . Microscopic Raman scattering spectroscopy was used to study the focused femtosecond infrared laser induced structural change in SiO2 glass in Sect. 4.4.