It is shown experimentally that the result of the UV nanosecond laser irradiation of the silver-containing photo-thermo-refractive (PTR) glasses depends on the pre-history of glass. If the PTR glass was preliminary irradiated by the UV radiation by UV mercury lamp into the absorption band of Ce3+ ions the laser irradiation results in the silver molecular clusters (MCs) luminescence quenching. If the PTR glass was not preliminary irradiated by the UV radiation the UV nanosecond laser irradiation action results in the silver MCs luminescence appearance. The subsequent thermal treatment below the glass transition temperature results to the increase of the luminescence intensity. The thermal treatment above the glass transition temperature led to the silver nanoparticles formation and to luminescence quenching.
© 2015 Optical Society of America
Since the last quarter of the 20th century photo-thermo-refractive (PTR) glasses have been attracting a lot of attention due to their unique optical properties . PTR glass is a photosensitive glass that changes its refractive index after the exposure by the near UV radiation (300–350 nm) followed by a thermal treatment in the vicinity of 500 °C. PTR glass has been used for holographic recording of various types of volume diffractive optical elements , and finds a wide range of applications for laser technique and photonics. PTR glasses are the complicated systems with a lot of dopants such as cerium, antimony and silver that play a key role in the luminescence centers formation and changing of the glass refractive index . Moreover, they are very suitable materials for the investigation of optical properties of silver molecular clusters (MCs) because the as-prepared glass is transparent at the near UV range and after the glass melting silver is presented in the glass in ionic state and charged MCs state. According to the explanation of U. Kreibig molecular cluster it is a form of matter that consists of a few amount of atoms or ions . The notation charged MC means that such a molecular cluster consists of neutral atoms and charged ions. As long as silver ions possess high mobility in the silicate glass matrix it is possible to combine them into MCs of various sizes. As was shown before  such sub-nanosized objects possess quite different luminescent bands under UV radiation excitation. A lot of scientific groups investigated silver MCs behavior in noble gases [5,6], solutions , zeolites , polymers  and glasses [3,10,11]. For the glass matrix there are several approaches for the formation of silver MCs. Basically they consist in the reduction of silver ions and charged MCs followed by activation of the thermal diffusion of silver atoms and ions . The most effective approaches for the starting of the clusterization process are the laser [10,12,13] or the electron beam [14,15] irradiation and the subsequent thermal treatment . Laser glass modification is one of the most prevalent ways to the manipulation of silver ions, atoms and MCs due to the spreading of powerful laser systems with various wavelengths in last decades. Non-linear processes that are involved in the interaction between light and matter result in peculiar changes of glass matrix and MCs themselves, such as ion exchange in a focal point , waveguide recording , nanoparticles formation [18,19] etc. Simo and associates have considered a process of silver nanoparticles growth going by silver MCs . They demonstrated that there is a threshold temperature below that nanoparticles can’t be raised in some glass matrix. The luminescent properties of silver MCs was also widely investigated in various glass matrixes [3,10,20]. The influence of UV nanosecond laser radiation on silver-doped zincphosphate glass was studied with a comparison of such an impact with femtosecond laser pulses . The ability of the PTR glasses to change the luminescent or the absorption properties under the laser radiation action makes them promising as a media for optical information recording.
In our previous work  we have shown that the UV nanosecond laser pulses irradiation of the luminescent PTR glasses, which were preliminary irradiated to the absorption band of Ce3+ ions (λ = 305 nm) by the mercury lamp, results in the luminescence quenching. The subsequent irradiation of the samples by the UV mercury lamp, or the thermal treatment led to the recovery of the luminescence. The present work is the continuation of . The aim of the present work is the study of effect of the UV nanosecond laser pulses irradiation on the similar glasses, which were not preliminary irradiated by the UV mercury lamp, and initially possess very weak luminescence. The effect of the thermal treatment below and above glass transition temperature after the laser irradiation is also investigated.
PTR silicate glasses based on the Na2O–ZnO–Al2O3–SiO2–NaF–NaCl system and doped with Ag2O (0.12 mol. %), the photosensitizer CeO2 and the reducer Sb2O3 were synthesized in ITMO University (St. Petersburg, Russia). These glasses were similar to the used in . The second group of the samples was not doped with CeO2 and Sb2O3. These glasses possess the PTR properties, but a mechanism of refractive index changing is different and consists in the formation of defect centers by femtosecond laser irradiation . The glass transition temperature of the samples was measured by the differential scanning calorimeter STA 449 F1 Jupiter (NETZSCH-Gerätebau GmbH) and was equal 495 °C. It must be noted, that during the glass synthesis part of Ce ions change the valence from IV to III and as well as Sb ions – from III to V . The samples to be investigated were prepared in a form of a plane-parallel polished plates 1.0 mm thick. The as-prepared glasses were transparent, colorless and had a weak luminescence in a visible spectral range. For the irradiation of the samples we used the third harmonic (λ = 355 nm) of YAG:Nd laser LS-2131M (Lotis TII) with the maximum energy of 30 mJ, pulse duration of 5 ns and repetition rate 1 Hz. The focus of laser beam was located at the distance of 3 cm from the backward surface of the sample with the beam diameter of 1.0 mm on the forward surface. The dose of the irradiation was varied by the change of the number of laser pulses. UV mercury lamp wasn’t used in current experiment but has been used in previous one concerning with luminescence quenching . For the thermal treatment of the samples at 350 °C and 500 °C the muffle furnace (Nabertherm) was used. The optical density spectra of the samples were recorded in the 300–800 nm spectral region using Lambda 650 (Perkin–Elmer) spectrophotometer. The luminescence and excitation spectra were measured by the MPF-44A (Perkin–Elmer) spectrofluorimeter. The luminescence spectra were corrected taking into account the spectral sensitivity of the spectrofluorimeter photodetector. For the excitation spectra, no correction was conducted. The spectral measurements were performed at room temperature The photos of the irradiated zones luminescence were obtained by the luminescence microscope MSFU-K (LOMO) with the excitation at λ = 400 nm.
The as-prepared PTR glasses have the very weak luminescence in a visible spectral region. Figure 1 shows the luminescence spectrum of the as-prepared PTR glass for the excitation wavelength 320 nm. It can be seen that for this excitation wavelength the two luminescence bands are present with the maxima at 380 nm and 540 nm. The contribution to the shorter wavelength band makes mainly Ce3+ ions .The contribution to the longer wavelength band makes Ag2+ MCs  and Ag32+ MCs .
Figure 2 shows the difference of the luminescence of the PTR and silver-containing glass samples after the UV nanosecond laser action with and without preliminary irradiation by the UV mercury lamp into the absorption band of Ce3+ ions. It can be seen that in the first case (Fig. 2(a)) the luminescence bleaching takes place. The luminescence around photobleached areas are caused by UV mercury lamp irradiation . In the second case (Fig. 2(b)) the laser action results in the intense luminescence appearance in the irradiated zones. The same result was obtained for the glass, which did not contain Ce and Sb ions (Fig. 2(c)). The following results are described for the PTR glasses which were not irradiated by the UV mercury lamp.
After the UV nanosecond laser irradiation the weak yellow color in the irradiated zones of the samples appear. Figure 3 shows the optical density spectra of the PTR glass samples after UV nanosecond laser irradiation with the different doses. The absorption band at λ = 305-310 nm is related to the Ce3+ ions. It can be seen that the laser irradiation led to the increase of the absorption in the spectral range 300-550 nm. This effect is caused by the transformation of the silver ions and charged silver MCs to the neutral state [3,28,29]. The contribution to the absorption rise at λ = 300-350 nm makes the increase of the concentration of structural defects of a glass network [29,30].
The luminescence spectra of the irradiated zones of the PTR glass are shown at Fig. 4(a). It can be seen that the luminescence band consists at least of three bands. The short wavelength band with the maximum at 380 nm is related to the Ce3+ ions, Ag0, and Ag3 MCs [24,28,31]. The luminescence at the 450-500 nm spectral region is caused by the Ag0, Ag2 and Ag4 MCs [28,32]. The luminescence band at 580-660 nm is related to the Ag3 MCs . It was shown previously that Ce3+ ions increasing of its concentration increase a luminescence intensity of silver MCs . Figure 4(b) shows the difference between the luminescence excitation spectra of PTR glass after the UV mercury lamp irradiation and after the UV nanosecond laser irradiation. It can be seen that the excitation spectra differ depending on the irradiation source. This indicates that the wavelength, the duration and the radiation intensity effect on the concentration of silver neutral MCs of different types. At the same time absorption spectra almost do not depend on the irradiation source .
Figures 5(a) and 5(b) show the luminescence and excitation spectra of PTR glass samples after the thermal treatment at temperature less than the glass transition temperature. It can be seen that the luminescence spectrum in the visible range changes considerably with respect to Fig. 4(a): the half-width of the luminescence band became narrow and its maximum shifts to 460 nm. The luminescence intensity increases, and its color changes from the orange to green (Fig. 5(c) and 5(d)). The reason of this can be the increase of the concentration of Ag4 MCs which have the luminescence band at 458 nm  and decrease of the concentration of Ag3 MCs which have the luminescence band at 616 nm [33,34] by the following process:Fig. 5(b)).
The thermal treatment at the temperature above the glass transition temperature led to the non-reversible quenching of the luminescence in the centers of the irradiated zones (Fig. 6, Fig. 7). But at the perimeters of the irradiated zones the luminescence intensity increases. The optical density spectra (Fig. 8), measured in the centers of the irradiated zones show the appearance of the wide absorption band in the 380-550 spectral range typical to the plasmon resonance of silver nanoparticles . The color of the centers of the irradiated zones changes from weak yellow to brown (see inset in Fig. 8). The described changes indicate that during the thermal treatment the neutral silver MCs transform to the silver nanoparticles by the trapping of Ag atoms. The decrease of silver MCs concentration and the increase of the absorption result in the luminescence quenching. It can be seen from Fig. 8 that the appeared absorption band is the superposition of the at least two absorption bands. This can be caused by the two reasons : (i) the presence in a glass of non-spherical silver nanoparticles, and (ii) the electromagnetic interaction between nanoparticles located close to each other. Similar situation was considered for PTR glass with silver nanoparticles after UV light with wavelength λ = 325 nm and heat treatment .
The similar results were obtained for the samples which did not contain Ce and Sb ions and were not previously irradiated by UV mercury lamp. This indicates that the contribution of these ions to the effects described above is negligible.
The presented results and their comparison with the results, described in , show that the pre-history of the glass influences on the final effects considerably. The preliminary irradiation of glass by continuous UV radiation into the absorption band of Ce3+ ions led to the intense luminescence appearance, and the subsequent action of UV nanosecond laser pulses results in the luminescence quenching  (first case). In the glass, not irradiated by the UV mercury lamp, the action of UV nanosecond laser results in the luminescence appearance (second case). Let us compare the main processes in the both cases.
During the preliminary UV irradiation of PTR glass in the first case the photoionization of Ce3+ ions takes place, and Ce3+ ions transform to the Ce4+ state:3] and of the broad absorption band in 340-470 nm spectral range, similar to the band, shown at Fig. 3 (curves 2-4). The irradiation of such glass by UV nanosecond laser pulses led to the luminescence quenching  throw the photoionization of neutral silver MCs:Fig. 3). So the photoionization of silver MCs and point defects of glass network during UV nanosecond laser irradiation, at least for the first laser pulse, can be caused by the following processes: (i) by the nonlinear two-photon absorption, and (ii) by the two-step absorption. The probability of the first effect is high because of the high intensity of laser radiation in the irradiated zone (I > 80 MW/cm2). The two-step absorption consists of two absorption acts: the absorption of one photon with the creation of the metastable excited state, and the absorption of the other photon in the excited state, resulting in photoionization:Fig. 3).
The probability of non-resonant photoionization of Ce3+ ions by the mentioned processes is very low. So Ce ions during the laser pulses keep in the three-valence state, and, in contradiction to Ce4+ ions, cannot trap electrons. After the laser pulse the free electrons can be trapped by the Ag and Sb ions and by the charged silver MCs (processes (2-4)). As the free electrons are not trapped in this case by the Ce ions (process (5)), the concentration of free electrons is higher, than in the first case. This results in the increase of the luminescence intensity in the irradiated zone. The negligible role of Ce ions in the second case is confirmed by the experiments with the glasses without Ce.
The thermal treatment of the irradiated glass at temperature below Tg led to the loose of the trapped electron by the [Sb5+] ̶ complex (process (6)) in PTR glasses and by the charged defects in the glasses without Sb ions. The additional free electrons are trapped by the mentioned silver ions and charged MCs and coarse the further increase of the luminescence intensity. The thermal treatment of the irradiated glass at temperature above Tg led to the growth of the silver nanoparticles with silver MCs being the centers of growth. When the silver nanoparticles appear in a glass the concentration of the luminescent MCs decreases, the absorption of glass increase because of the plasmon absorption band appearance, and the luminescence intensity in the center of the irradiated zone is bleached (Fig. 5, Fig. 6). In the contradiction to the central part of the irradiated zone no nanoparticles are formed and the luminescence intensity increases at its perimeter. The reason of this is the low intensity of laser radiation on the laser beam perimeter. This results in the low concentration of the free electrons after the laser pulse, and low efficiency of MCs transformation from the charged to neutral state. During the thermal treatment the loose of electrons by the [Sb5+] ̶ complex or charged defects led to the increase of the neutral MCs, but the thermal treatment duration is not enough for the silver nanoparticles formation.
The preliminary treatment of the PTR glasses makes possible to form in them the positive or the negative images by the UV nanosecond laser irradiation through the mask or by the laser beam scanning. Figure 9 shows the luminescent PTR glass plates irradiated through the mask by the UV nanosecond laser radiation with and without the preliminary irradiation by the UV mercury lamp.
It should be mentioned also that optical information can be also recorded in the PTR glasses by the local absorption change under the action of UV nanosecond laser and the subsequent thermal treatment (see inset in Fig. 8).
It is shown experimentally that the result of the UV nanosecond laser irradiation of the PTR glasses depends on the pre-history of glass. If the PTR glass was preliminary irradiated by the UV radiation into the absorption band of Ce ions the UV nanosecond laser irradiation results in the silver MCs luminescence quenching. If the PTR glass was not preliminary irradiated by the UV radiation into the absorption band of Ce ions the UV nanosecond laser irradiation results in the neutral silver MCs luminescence appearance. The subsequent thermal treatment above the glass transition temperature results in the silver nanoparticles growth in the irradiated zones. The described effects can be used for the optical information recording by the local change of glass luminescence or absorption.
This work was financially supported by Ministry of Education and Science during the scientific-research work in the frame of the project part of state task in the scientific work area for the task # 11.1227.2014/K
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