1. Introduction

Femtosecond ultrashort pulse lasers have become powerful tools for the fabrication of functional optical devices as they can induce various controllable microstructures by microscopic modifications in transparent materials. Different optical functions have been demonstrated, such as fabrication of optical waweguide, coupler and photonic crystal, three-dimensional optical storage and rewritable optical memory [1–6]. Recently, ultrashort laser interactions with metal ion-doped glasses, especially noble metal ions such as gold and silver ions doped glass, have been extensively studied for the reasons of developing their applications in functional optical devices [7–11]. M. Kaempfe et al. [7] irradiated intense femtosecond laser pulses on a glass sample containing silver nanoparticles, resulting in permanent colour changes when the laser wavelength was in the region of the particles’ surface plasmon resonance. Y. Watanabe et al. [8] demonstrated a photosensitivity in soda-alumino-phosphate glass doped with Ag⁺ upon exposure to near-ultraviolet femtosecond laser pulses and found the formation of color centers such as Ag⁰ and Ag²⁺ in the glass. Y. Cheng et al. [9] investigated the refractive index change in Ag⁺ doped Foturan glass by femtosecond laser irradiation followed by postbaking. Qiu et al. [10,11] developed an effective method to space selectively precipitate metal nanoparticles in glass with a femtosecond laser irradiation. Metal nanoparticles with the size of several nanometers could be obtained in selected area inside the glass by using this approach.

In this paper, we report the precipitation and dissolution of silver nanoparticles in Ag⁺-doped phosphate glass by irradiation with a near-IR 800-nm femtosecond laser pulses and further annealing at various temperature. The mechanisms of the observed phenomena are discussed. We show that the process can be controlled by adjusting the laser parameters and annealing parameters.

2. Experimental

A glass sample with composition of 25Na₂O·5Al₂O₃·70P₂O₅·0.15Ag₂O (in wt%) was prepared from reagent grade Na₂CO₃, Al(PO₃)₃, and AgNO₃. The mixture of raw materials was melt in an alumina crucible at 1450°C for 1 hour under the ambient atmosphere. Then the melt was quenched into transparent and colorless glass by pouring it onto a stainless steel module at room temperature. Finally the glass was cut and polished into the glass samples with thickness of 3 mm.

A commercial regeneratively amplified 800-nm Ti: Sapphire laser (Spitfire, Spectra- Physics) that emits 120 fs, 1 kHz, mode-locked pulses was used in our experiments. The laser beam with an average power of 3 mW was focused by a 10× objective lens with a numerical aperture of 0.3, and the position of the focal point was 30 µm beneath the sample surface with the help of a computer controlled 3D XYZ stage at the scanning rate of 1000 µm/s.

Grating structures with a period of 20 µm were inscribed into the glass samples by laser direct writing. The size of grating was 3 mm×3 mm to measure the absorption spectrum. After exposure to the femtosecond laser, the sample was subjected to annealing at 150°C, 300°C, 400°C, 450°C, 500°C for 30 min. Optical absorption and photoluminescence spectra of the glass sample were measured using a Jasco V-570 spectrophotometer and Jasco FP-6500 spectrofluorometer, respectively. The precipitation and dissolution of silver nanoparticles were observed by a JEOL JEM-2010 transmission electron microscope (TEM). Also, the refractive index change of laser-irradiated area was evaluated by examining the diffraction efficiencies of grating using a He-Ne laser beam with the wavelength of 633 nm.

3. Results and discussion

Figure 1(a) shows absorption spectra for the Ag⁺-doped glass sample before (A) and after (B) femtosecond laser irradiation, and subsequent annealing at various temperatures for 30 min. As seen in Fig. 1(a), no apparent absorption (curve A) was observed at the wavelength longer than 350 nm for the unirradiated glass sample. It should be noted that there is no intrinsic absorption at the wavelength of 800 nm for the glass sample. After irradiation, there was an apparent increase in the absorbance as shown in curve B. With increasing the annealing temperature, the absorption for the irradiated glass sample gradually decreased, and the tendency of decrease was weaker after annealed at 500°C, as shown in curves C to G. Figure 1(b) is the difference in absorption spectra of the glass samples in Fig. 1(a) before and after the femtosecond laser irradiation and after subsequent annealing at various temperatures for 30 min. In the difference spectra, there was an apparent absorption band ranging from 250 to 400 nm (curve H), peaking at 320 nm, which was attributed to the formation of color centers associated with Ag⁺, i. e., trapped electron center Ag⁰ and trapped hole center Ag²⁺ in the glass sample [8,12]. After annealing at 150oC, the absorption band decreased slightly (curve I). While, the absorption band disappeared after annealing at 300°C, indicating that electrons/holes at traps were released by thermal stimulation and recombine with holes/electrons again. In addition, a new absorption band peaking at 450 nm appeared (curve J) which can be assigned to the absorption due to the surface plasmon of the metallic silver nanoparticle [13]. Base on the Mie theory [14], the calculated average radii of silver nanoparticles are about 5 nm.

 

Fig. 1. (a) Absorption spectra of Ag⁺-doped glass sample before (A) and after (B) femtosecond laser irradiation, and subsequent annealing at various temperatures of 150°C (C), 300°C (D), 400°C (E), 450°C (F) and 500°C (G) for 30 min. (b) The difference in absorption spectra of the glass samples in Fig. 1(a) before and after (H=B-A) the femtosecond laser irradiation and after subsequent annealing at various temperatures of 150°C (I=C-A), 300°C (J=D-A), 400°C (K=EA), 450°C (L=F-A) and 500°C (M=G-A) for 30 min. The inset is the amplification of curves J and K.

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Preliminary observation with a TEM showed that the precipitation of silver nanoparticles with diameters ranging from 10 to 20 nm in the glass sample as shown in Fig. 2. Composition analysis using energy dispersive spectroscopy (EDS) in TEM confirms that these spherical nanoparticles are metallic silver. After annealing at 400°C, the silver nanopartice surface plasmon absorption peak decreased (curve K). The surface plasmon absorption peak disappeared when the annealing temperature reached 450°C (curve L). Also, the TEM observation confirmed the dissolution of silver nanoparticles.

 

Fig. 2. TEM micrograph of silver nanoparticles precipitated in Ag₂O-doped phosphate glass after femtosecond laser irradiation and annealing at 300°C for 30 min.

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It is well known that both Ag⁰ and Ag²⁺ centers possess bright orange fluorescence against excitation with UV light [8]. Figure 3 shows the excitation and emission spectra for the glass sample after femtosecond laser irradiation and further annealing at different temperatures. Curve A represents the excitation spectrum obtained with fluorenscence at 565 nm, while curve B represents the fluorescence spectrum obtained with excitation at 320 nm. Curves C and D represent the excitation and emission spectra after annealing at 150°C. After annealing at 300°C, no emission was detected for the glass sample.

 

Fig. 3. Excitation and emission spectra of Ag-associated color centers (λ_excitation=320nm, λ_emission=565nm) after femtosecond laser irradiation (A, B) and subsequent annealing at 150°C (C, D). After annealing at 300°C, no emission was detected.

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We also investigated the effect of precipitation and dissolution of silver nanoparticles at different annealing temperature on refractive index change in laser-irradiated area by measuring the first-order diffraction efficiencies of the grating written inside the glass sample. Based on the coupled wave theory of Kogelnik’s [15], we can deduce the refractive index change (Δn) of the laser-irradiated area. The details for the calculation of refractive index change are reported in Reference 16.

Figure 4 shows both the refractive index change and the diffraction efficiency in the laser-irradiated area as functions of annealing temperature. After femtosecond laser irradiation, there was an increase of 6.12×10^-4 for the refractive index with diffraction efficiency of 9.51%. After annealing at 150°C, the diffraction efficiency of the grating and the refractive index change in the irradiated area decrease slightly, while after annealing at 300°C, the diffraction efficiency of the grating and the refractive index change in the irradiated area have an increase compared to those at the state after laser irradiation, indicating that the precipitation of silver nanoparticles plays a role for the increase of refractive index. After annealing at 400°C, the diffraction efficiency of the grating and the refractive index change in the irradiated area decrease again. After annealing at 450°C, it appears that both diffraction efficiency and refractive index change nearly decreases to zero. These changes were also observed from the diffractive patterns, i. e., the brighter diffraction pattern of the first order can be seen after the glass sample was annealed at 300°C in comparison with the irradiation state, while the brightness of diffraction pattern of the first order waken and finally disappeared after annealing at 400°C and 450°C.

 

Fig. 4. The diffraction efficiency and the refractive index change in the laser-irradiated area as functions of annealing temperature.

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We have observed Ag-associated process of coloration and decoloration, precipitation and dissolution of in the femtosecond laser irradiated glass sample with different annealing temperatures. Then how did the process evolve and what were the mechanisms of the evolution are the main points. Previous studies have explained the process of the formation of Ag-associated color centers in ionizing radiation experiments, as a consequence of capturing “radio-activated” electron/hole onto Ag⁺ within the glass [8,12]. We deduce that the present case is as follows. Firstly, Ag-associated color centers Ag⁰ and Ag²⁺ were formed by capturing photo-excited electron or hole onto Ag⁺ via femtosecond laser irradiation. This should be a nonlinear optical process as no intrinsic absorption at the wavelength of 800 nm for the glass sample. Free electrons are generated by the multiphoton absorption of the incident photon and consequent avalanche ionization when the sample irradiated by femtosecond laser. Such avalanche ionization produces high absorptive and dense plasma, facilitating the transfer of energy from the laser to the sample. The silver ions capture photo-excited electron or hole to form color centers. In the meantime, plasma expansion induces the local densification, resulting in the permanent structural changes. Secondly, with increasing annealing temperature, Ag⁰ centers gradually aggregate to form nanoparticles and Ag²⁺ centers released holes at traps by thermal stimulation and recombine with electrons again leading to the increase of isolated Ag⁺. This change can be represented by the fluorescence change of isolated Ag⁺, as shown in Fig. 5. As seen in this figure, we can confirm silver in the unirradiated glass sample was in the monovalent state, Ag⁺, by observing the characteristic blue fluorescence peaked at 390 nm (curve B) under the UV excitation peaked at 237 nm (curve A). After the glass sample irradiated by femtosecond laser, the intensities of fluorescence decrease (curve D), indicating that the monavalent Ag⁺ was partly consumed to form Ag-associated centers. While the annealing temperature reaches 300°C, the intensity of fluorescence increases (curve F) in comparison with the state after laser irradiation, indicating the increase of isolated Ag⁺ coincides well with the decrease of absorption in 320 nm after annealing at 300°C. Finally, when the annealing temperature is above 400°C, the aggregated silver nanoparticles dissolve into the glass matrix.

 

Fig. 5. Excitation and emission spectra of isolated Ag⁺ (λ_excitation=237nm, λ_emission=390nm) before (A, B) and after (C, D) femtosecond laser irradiation and subsequent annealing at 300°C (E, F).

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It is worthy to mention that the refractive index change for the laser-irradiated area in the glass sample is effected by color centers, permanent structural changes and precipitation of silver nanoparticles induced by femtosecond laser. In the present results, the refractive index change is mainly caused by permanent structural changes such as local densification, local structural rearrangement, etc. Decoloration of color centers lead to some little decrease of refractive index, while the precipitation of silver nanoparticles can increase the refractive index. Therefore we can selectively increase the refractive index by precipitation of metallic nanopartices in expected region in this kind of glasses. For the present laser power of 3 mW, the refractive index has a slight increase after the precipitation of silver nanoparticles, In order to get higher refractive index change, increasing the laser power density and the doped concentration of silver ions should be a feasible method.

4. Conclusions

In summary, we have reported the formation and decoloration of Ag-associated color centers and precipitation and dissolution of silver nanoparticles in Ag⁺-doped phosphate glass. The process can be controlled by adjusting the laser irradiation parameters and annealing parameters. The above results should be applicable in the fields such as ultrafast all-optical switching devices and high-density optical data storage systems, etc.