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Abstract
Photosensitivity in photo-thermo-refractive (PTR) glass can be triggered by UV and near-infrared fs laser irradiation. Here we focus on the nonlinear photochemical process triggered by ultrashort laser Gaussian-Bessel beams. The transmission and absorption spectra show that the primary difference between UV and fs laser exposure is the formation of color centers and kinetic process of silver nanoparticles growth. It is contributed to the nonlinear ionization of PTR glass matrix and thermal effects during interaction of glass matrix and ultrashort laser pulses. Transmission electron microscopy verifies the generation of nanoscale crystals in the irradiated region, and X-ray diffraction shows the existence of quartz crystal and NaF after laser irradiation and thermal treatment. Moreover, the dependence of photochemical reaction on laser parameters is investigated, as well as the tailoring of silver nanoparticles. On this basis, volume Bragg gratings with ultrashort laser Gaussian-Bessel beams are inscribed as an application which possess good diffraction characteristics.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photo-thermo-refractive (PTR) glass, which is a photosensitive multicomponent silicate glass doped with silver, cerium, and other elements, owns a large number of advantages such as photosensitiveness, environment robustness, high laser damage threshold, long-term stability and wide transparency range [1,2]. These advantages make it to be rapidly developed for recording highly efficient volume diffraction optical elements [3–5]. The traditional method for recording such optical elements is the continuous-wave (CW) UV irradiation and successive two-step thermal treatment. During these processes, the main physicochemical reactions going on are as follows. The Ce3+ ions in PTR glass as the photosensitizer absorb actinic radiation and after release electrons that will be captured by silver ions. Subsequent thermal development agglomerates silver atoms to silver molecular clusters (SMCs), and then the crystalline phase precipitation grows on it, which leads a localized refractive index decrement [6] or increment [7,8].
In recent years, ultrashort laser pulses micromachining as a remarkable efficient method has achieved spatial localized modification with high spatial resolution in transparent materials [9–11]. In this regime, high intensity laser pulses can induce permanent changes into the volume or on the surface of materials with the nonlinear effects. Three-dimensional microstructures could be manufactured, which differ from the original material by the refractive index, crystalline structure, morphology and so on [12,13]. Therefore, such techniques can offer an ability to fabricate optical elements in PTR glass in three dimensions. A study by Glebov et al. revealed the change of refractive index and laser-induced damage after exposure to fs laser pulses with Gaussian beams in PTR glass. They concluded that the nonlinear photosensitivity results from a complex interaction combining multi-photon and tunneling ionization [14,15]. A phase Fresnel lens was also successfully designed using fs laser Gaussian beams in PTR glass [16]. Compared with Gaussian beams, Gaussian-Bessel (GB) beams [17] have a higher degree of nonlinear stability and aspect ratio. In our previous work, we have fabricated transmission volume phase holographic gratings in PTR glass by fs laser GB beams, and also observed the microstructures composed of nano-sized crystals in the exposed regions [18,19]. However, the detailed mechanism of the nonlinear photochemical reaction and the generation of the nanocrystals induced by ultrashort laser pulses remain unclear at present.
In this paper, we focus on the nonlinear photochemical process of PTR glass during fs GB beams irradiation, which triggers the formation of the core-shell nanocrystals with an average size of 5 nm. The influences of pulse energy and pulse width on the photochemical reaction are investigated by using the characterization of absorption spectra. Especially, the concentration and size of silver nanoparticles (NPs) responding to laser irradiation are analyzed. Furthermore, the inscription of volume Bragg gratings with high diffraction efficiency (DE) inside PTR substrate is demonstrated.
2. Experimental
PTR glass samples were prepared based on sodium-alumina-silicate system (composition, mol%, SiO2 73, Na2O 11, ZnO + Al2O3 7, BaO + La2O3 3, NaF 5, KBr 1), including four dopants (SnO2 0.02, CeO2 0.02, Sb2O3 0.08, AgNO3 0.01). The raw materials were melted and homogenized at 1440°C for 5 h using a platinum crucible in an electric furnace with a mechanical stirring process under ambient atmosphere. All the glass melts were cast into a copper mold and annealed at 500 °C (near the glass transition temperature) for 6 h. After annealing, the bulk glass was obtained and the composition was found a slight variation due to high volatility of bromine [20]. It was cut with a thickness of 2 mm and polished to optical quality with cerium oxide.
Laser exposure was conducted using a Yb: KGW laser system (1028 nm, Pharos, Light Conversion Ltd.). An axicon with an apex angle = 176° was used to transform Gaussian beam into GB beam, and a 4f demagnification optical system composed of a convex lens (f1 = 200 mm) and a microscope objective (10 ×, NA = 0.26, f2 = 20 mm, Mitutoyo NIR) to decrease the initial size of GB beam by 10 times. Further details of generation and characterization of the GB beam were described in [17]. Then resulted GB beam was imaged inside PTR glass sample that was mounted on a high-precision 3D motorized translation stage (ANT130, Aerotech). By translating the sample perpendicularly to the GB beam, linear structures, with length of 2 mm and interval of 4 μm were written 150 μm below the surface and at speed of 200 μm/s, resulting in square patterns. After laser exposure, the sample was first nucleated for 3 h at 460 °C and then thermally developed at 540 °C for 1 h in a muffle furnace with program control.
The optical transmission measurements were performed in the 200-800 nm spectral region using an UV-VIS-NIR spectrophotometer (UV-3101, Shimadzu). Crystallographic characterization of the samples was done with an X-ray diffractometer (XRD, D8 Discover, Bruker) using CoKα (λ=0.1789 nm) radiation with step size 0.05° (2θ) from 20° to 90°. Transmission electron microscopy (TEM, Talos F200X, FEI) was used for investigating the microstructure of nanocrystals in the structured zones. The samples with laser irradiation and thermal annealing as volume Bragg gratings were demonstrated using a CW 532 nm laser diode (LD).
3. Results and discussion
3.1 Nonlinear photo-thermo mechanism
Photochemical process of fs laser exposure is the major difference in comparison with UV exposure. Nonlinear ionization induced by fs laser can produce plenty of free electrons in glass matrix, which plays an important role in the succeeding process. In order to distinguish this difference, transmission measurements were performed at each procedure during the fs laser-based manufacturing process as shown in Fig. 1(a). The corresponding absorption spectra are illustrated in Fig. 1(b). The laser irradiation conditions are 8 μJ and 220 fs. It is shown that fs laser irradiation results in an obvious decline of the transmittance in the range of 300 nm-600 nm. The broad absorption band with the shoulder centered at 350 nm (curve B), is probably caused by the generation of color centers and photoreduction of silver ions [11,13,21]. After thermal treatment, a well-marked decrease of the transmittance at the range of 350 nm-600 nm is observed. The absorption band with a peak at about λ=466 nm (curve C), is known to be associated with the surface plasmon resonance (SPR) of silver NPs [8].
Fig. 1. (a) Measured optical transmission spectra and (b) calculated absorption spectra of PTR glass. (c) Decomposition of absorption spectra with Gussian functions (A: initial untreated, B: after irradiation, C: after irradiation and thermal treatment).
To perform a better analysis of the absorption spectra, Gaussian multi-peak fitting method was used and detailed in Fig. 1(c). Cerium ions occur both in Ce3+ and Ce4+ states in pristine PTR glass, which have strong absorption in the UV region [22]. In our experiments (curves A), two bands located at λ=229 nm (E=5.42 eV) and λ=302 nm (E=4.11 eV) are attributed to Ce4+ and Ce3+, respectively. During laser irradiation, a large number of free electron-hole pairs are produced in PTR glass matrix through nonlinear ionization [9]. Due to nonlinear ionization associated with successive pulses cumulative thermal effects at the focal volume, electrons are easily trapped by silver ions in the optical field, resulting in generation of silver atoms and diffusion of SMCs such as Ag2, Ag2+, Ag3+, etc. These SMCs can serve as nucleation centers for large entities such as NPs after subsequent aggregation [23]. In our experiments (curves B), the bands at λ=392 nm (E=3.17 eV) and λ=441 nm (E=2.82 eV) are attributed to the SMCs. The free electrons and holes also can be captured by the lattice defects or impurities, the absorption band around λ=350 nm (E=3.55 eV) is attributed to intrinsic color centers [24,25]. In general, it is a challenging task to study the structure of this band and separate the contribution of the type of color centers (electron or hole centers) being induced by fs laser irradiation, resulting from the overlap of their absorption bands. In the subsequent thermal treatment, the exposed area initiates the growth of nanocrystals at the photoinduced nuclei sites and causes a change in refractive index. A strong SPR absorption band (curves C) of silver NPs around λ=463 nm (E=2.68 eV) is observed. Meanwhile the color centers band decreases but not disappears with the thermal bleaching shows that color centers induced by fs laser irradiation are much more stable than those induced by UV exposure. For instance, the bleaching temperature of almost all the induced color centers go to 750 °C in fused silica [26]. Moreover, a new weak band near 600 nm appears at the stage of laser exposure, and becomes stronger after thermal treatment, can be associated with some unknown species that will be our future work.
The sample was ground into powder and analyzed by XRD shown in Fig. 2(a). The initial untreated PTR glass presents in the amorphous state. Nevertheless, the PTR glass after fs laser irradiation and heat treatment possesses apparent crystal structure. The diffraction peak at 2θ = 31.03° corresponds to the (101) reflections from the crystalline structure of SiO2 (ICSD 79-1910). This phase transition could be induced by the melting during the interaction between fs laser pulses and PTR glass. During the irradiation process, the incubation effect in PTR glass matrix leads to high temperature and pressure conditions that prompts the phase transition of amorphous fused silica to crystalline quartz [27]. The rest three crystalline diffraction peaks at 2θ = 45.15°, 66.11°, and 83.75° can be indexed to the (200), (220), and (222) reflections from the cubic Fm-3m crystalline structure of NaF (ICSD 89-2956). The above results suggest that such treatment of PTR glass leads to the formation of the core with silver NPs while the shell consists of NaF.
Fig. 2. XRD patterns and TEM images of the PTR glass after fs laser irradiation and thermal treatment: (a) XRD patterns; (b) SAED image; (c) bright-field image; (d) dark-field image; (e) size distribution histogram of the nanoparticles produced.
The crystalline phase was also analyzed using TEM. The selected area electron diffractogram (SAED, Fig. 2(b)) verifies the precipitation of nanocrystals, nevertheless, proves to be of little help in establishing the nature of core-shell morphology of the nanocrystals. The nanocrystals display distinct ring patterns demonstrating their crystallinity. The bright-field and dark-field TEM images of the sample are presented in Fig. 2(c) and 2(d), respectively. An inset in Fig. 2(c) shows the high-resolution TEM image that reveals the nanocrystals are highly crystalline. Size histogram of the nanocrystals distribution is plotted on the right part of Fig. 2(d). The nanocrystals keep a nearly spherical shape and have a uniform size of 2.5-7.5 nm and distribute randomly in the matrix.
To describe the evolution of the silver-containing nanocrystals after fs laser irradiation and thermal treatment, we propose a three-step mechanism and compare it with linear interactions (Fig. 3). For silicate glass such as PTR glass, the basic structural unit is the tetrahedral, where silicon is in 4-fold coordination by oxygen. The silicon-centred tetrahedral structural species are designated as Qn, where Q refers to silicon atom and n denotes the number of bridging oxygens in the structural unit. First, ultrashort laser GB beams irradiation can induce the nonlinear effect in PTR glass. The band gap of the PTR glass sample is 4.25 eV according to Fig. 1(b). In this case, for the writing photon energy (1.21 eV) four-photon absorption is possible to excite electrons across the band gap. The strong electric field is presented in an instant, which results in great changes even disorder of Qn. Then plenty of free electrons and hole pairs can be produced, accompanied with some induced defects which have strong absorption in UV range. The photoinduced electrons mostly are captured by adjacent silver ions to form SMCs. Quartz crystal is also generated during this process, and we assume that a small amount of silver NPs are probably formed because of heat deposition. While in the case of UV exposure, the electrons originate from the linear photosensitivity of Ce3+ ions and can be trapped partially by silver ions to generate silver atoms and SMCs, but mostly by Sb5+ ions [28]. Therefore, the second stage (Heating I) for UV exposure leads to releasing the trapped electrons from (Sb5+)- ions with further formation of SMCs and colloidal particles. While in the case of fs laser exposure, the formation of SMCs is done in the focal zone at the first stage, this process promotes the generation of silver NPs and more uniform. Then, further heating results in the growth of NaF crystalline phase on the nucleation centres for both cases. The nanocrystals are confined to the vicinity of the focus because of the relatively low repetition rate (50 kHz) used in our experiment, thermal effects are diminished [11,29], allowing three-dimensional confinement of silver-containing nanocrystals formation at the sub-micrometer scale in glass.
Fig. 3. Schematic for the linear [2] and nonlinear photo-thermo-induced crystallization mechanism in PTR glass: (a) the photoinduced process; (b) crystal nucleation process; (c) NaF crystallization process.
3.2 Role of pulse energy and pulse width
The absorption spectra change substantially when different pulse energy and pulse width act on PTR glass. Figure 4(a) demonstrates the absorption spectra of PTR glass under different pulse energy with a fixed pulse width of 220 fs. Laser irradiation below 2 μJ leads to no significant change of the absorption, which can be ascribed to the generation of small SMCs [30]. The absorption continuously increases when pulse energy exceeds 2 μJ, but decreased notably at 12 μJ. It is assumed that the decrease results from the fs laser-induced fragmentation and size reduction of SMCs by three possible mechanism of photothermal evaporation, Coulomb explosion and near-field ablation [31]. After thermal treatment, the corresponding absorption spectra are depicted in Fig. 4(b). With increasing pulse energy, the absorption increases. The absorption reaches the peak at 12 μJ with the maximum absorption at 457 nm. Their decomposition is explained in Fig. 4(c). The absorption band of color centers and silver NPs increases with the increasing pulse energy. This phenomenon is thought to be due to the number of photo-electrons induced by nonlinear ionization grows with the increasing pulse energy, and thus more silver ions are reduced into silver atoms, and also more color centers are produced. With the thermal treatment, growth of silver NPs are preferred at higher pulse energy and color centers are repaired to some degree. Figure 4(d) presents the variety of the absorption peak’s area of silver NPs to illustrate the changes in the concentration of silver NPs in the glass samples. For those different writing pulse energy, the concentration of silver NPs increases continuously. Based on Mie-Drude theory [32], the mean diameter of silver NPs can be calculated by using the equation below:
where vf (=1.39×108 cm/s) is the Fermi velocity, Δω1/2 is the full width at half maximum (FWHM). Figure 4(e) reveals the dependence of the silver NPs size on pulse energy. It is plotted on an increasing trend (2 μJ -8 μJ) and then shows a decline (12 μJ), which is in agreement with laser-induced fragmentation. The maximum mean diameter of silver NPs is calculated to be about 2.54 nm with the pulse energy of 8 μJ.Fig. 4. Absorption spectra of PTR glass under different pulse energy: (a) after irradiation; (b) after irradiation and thermal treatment. (c) Decomposition of absorption spectra after irradiation and thermal treatment. (d) Line chart displaying the change in the absorption peak’s area of silver NPs under different pulse energy. (e) Calculated average diameter of silver NPs under different pulse energy.
The absorption spectra of PTR samples as a function of the pulse width are summarized in Fig. 5. The pulse width varies from 220 fs to 4 ps at a fixed pulse energy of 8 μJ, the absorption in Fig. 5(a) exhibits an obviously downward trend with the increase of pulse width. Subsequent thermal treatment results in the change of the absorption are shown in Fig. 5(b). Their decomposition is illustrated in Fig. 5(c). Peak power decreases with the increase in the pulse width, and the probability of multiphoton ionization becomes lower resulting in the decrease of photo-electrons and color centers. Figure 5(d) reveals the variety of the absorption peak’s area of silver NPs. For those different pulse width, the concentration of silver NPs decreases continuously. Figure 5(e) presents the dependence of the silver NPs size on pulse width. The size of silver NPs decreases with the increase of pulse width (220 fs-2 ps). Further increase in pulse width seems to have no effect on the size of NPs, we assume that the average diameter less than 1.9 nm for silver NPs in PTR glass above 3 ps according to the absorption spectra with small nanoparticles leads to a slight increase of absorbance below 350 nm for exhibiting the fading plasmonic properties [33]. We have shown that laser pulse energy and pulse width play a crucial role in the concentration and size of silver NPs. Higher energy or shorter pulse width leads to higher concentration, but not necessary for obtaining larger size of silver NPs. Nonetheless, laser parameters have great influence on concentration while slight for size. Laser exposure is the trigger point and thermal treatment is responsible for the growth. More research work is urgent to identify the kinetic process during the thermal treatment of the crystallization after ultrashort laser irradiation.
Fig. 5. Absorption spectra of PTR glass under different pulse width: (a) after irradiation; (b) after irradiation and thermal treatment. (c) Decomposition of absorption spectra after irradiation and thermal treatment. (d) Line chart displaying the change in the absorption peak’s area of silver NPs under different pulse width. (e) Calculated average diameter of silver NPs under different pulse width.
3.3 Diffraction properties of volume Bragg grating
The samples with laser irradiation and heat treatment were further used as volume Bragg gratings. The experimental setup is illustrated in Fig. 6(a). Most energy of GB beams consists in zero-order which is surrounded by plenty of high-order Bessel rings (Fig. 6(a1)). Using GB beams a longer grating thickness can be produced [34]. A top view of optical transmission micrograph image of volume Bragg gratings is presented in Fig. 6(a2), and the darkened lines represent the exposed region that the diameter was about 1 μm. The region of optical modification was about 1.8 mm long from a side view. The grating period was 4 μm and the total grating size was 2 mm. Based on the grating geometry, a grating with the thickness of 1.8 mm and the aperture of 2 × 2 mm2 could be produced. The DE was measured using a 532 nm laser beam. Figure 6(b) and 6(c) displays the DE with different pulse energy and pulse width, respectively. The DE value rises greatly at first with the increase of the pulse energy and reaches the maximum value when the pulse energy is 2 μJ. Then the DE value decreases with the increase of pulse energy. Figure 6(c) shows a gradual growth with the expanding of the pulse width and reaches the maximum value when the pulse width reaches 3 ps. Then a sharp decline appears. The highest DE value reaches 95.1% when the pulse energy is 2 μJ and pulse width is 220 fs. Based on the above results, the diffraction properties of volume Bragg gratings in PTR glass can be customised by controlling pulse energy and pulse width to serve as spectral and angular selectors in integrated optics. Moreover, laser parameters generated smaller silver NPs and lower concentration within a certain scope are more easily to obtain higher DE value. Additional, it is valuable to increase the aperture of gratings compared with the commercial available volume Bragg gratings. Approaches such as multi-foci parallel processing, high-speed translation stage, or scanning galvanometer could be used to improve the efficiency in our future work.
Fig. 6. (a) Schematic of volume Bragg grating fabrication in PTR glass: (a1) the lateral intensity distribution of the generated GB beam; (a2) the optical transmission micrograph of fabricated volume Bragg grating. (b) DE versus pulse energy. (c) DE versus pulse width.
4. Conclusion
In this work, we have studied the nonlinear photochemical process and the evolution of the silver-containing nanocrystals in the multicomponent PTR glass after being irradiated and thermal treated through analyzing the changes of their transmission and absorption spectra. The thermal treatment of laser irradiated PTR glass results in the precipitation of NaF nanocrystals by XRD analysis, and TEM images also confirms the presence of nanocrystals with an average size of 5 nm that below the diffraction limit. In addition, we show the effect of the pulse energy and pulse width on the photochemical reaction by using the characterization of absorption spectra. The results show that the concentration and size of silver NPs is associated with the number of photo-electrons. The electrons accelerate with the increase of the pulse energy, while pulse width change inversely. The trend in concentration is same, but only suitable for obtaining larger size in a certain range. Furthermore, the samples are utilized as volume Bragg gratings. The maximum DE of 95.1% is obtained with a 532 nm LD. We believe that PTR glass with ultrashort laser pulses irradiation is full of future applications such as high efficiency volume diffraction optical elements, microfluidic devices, and optical waveguides.
Funding
National Key Research and Development Program of China (2018YFB1107401); National Natural Science Foundation of China (61775236); Natural Science Foundation of Shaanxi Province (2020JQ-830).
Disclosures
The authors declare no conflicts of interest.
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