Fluorescent clusters in chloride photo-thermo-refractive glass by femtosecond laser bleaching of Ag nanoparticles

We report photoluminescence in bulk chloride photo-thermo-refractive glass under irradiation with femtosecond laser pulses. The fluorescence originates from the bleaching of silver nanoparticles precipitating in the glass. Similarly to the conventional process of the femtosecond re-shaping of metal inclusions with diameter tens of nanometers, irradiation of the smaller nanoparticles results in a fast shrinking size with an ellipsoidal shape via photofragmentation. Under UV excitation, remaining sub-nanometer silver molecular clusters show visible and near IR fluorescence, which increases with chlorine concentration. The observed bleaching of silver nanoparticles in bulk glass-metal nanocomposite can find applications in data storage and bleaching of volume Bragg gratings. c © 2017 Optical Society of America OCIS codes: (220.0220) Optical design and fabrication; (140.0140) Lasers and laser optics; (350.3390) Laser materials processing; (210.0210) Optical data storage. 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Introduction
Photo-thermo-refractive (PTR) glasses are prospective materials for recording of highly efficient volume phase holograms for the wavelength range 700-2500 nm [1][2][3][4][5].The PTR-glasses possess high absorption coefficient in visible spectral range (λ ma x = 410 -490 nm) due to precipitation of Ag nanoparticles (AgNPs) and formation of the NaF nanocrystallites in the glass host by photo-thermo-induced crystallization [6].Unfortunately, absorption band associated with AgNPs restricts using PTR glass for fabrication holograms for the blue and green parts of the spectrum.However, it is well known that AgNPs can be eliminated by irradiation of glass with intense laser pulses [7][8][9][10][11].It is worth noting that destruction and modification of metal nanoparticles immersed in liquids using pulsed lasers has been demonstrated almost two decades ago [12,13].Bleaching of AgNPs in glass matrix has also been extensively studied [7,9,14] including the interaction with femtosecond laser radiation with relatively large AgNPs (>30 nm) created in glass via ion-exchange process [14,15].This approach has resulted in the development of dichroic filters and can be employed for data storage devices based on the glass-metal nanocomposites [16].Photofragmentation of AgNPs under irradiation with pulsed laser has been studied in phosphate glass [17] and SiO 2 films [18].In PTR glasses, the modification of AgNPs using second and third harmonics of YAG:Nd pulsed laser irradiation has been also studied [19,20].Different regimes of the interaction of ultrashort laser pulses with metal nanoparticles can be found in detailed review [21].
Chloride PTR glass is a new holographic material, which in addition to the remarkable crystallization properties also shows intense visible fluorescence under UV excitation [22].This fluorescence originates from silver molecular clusters (AgMCs) and nanoclusters, which can be created by irradiation of the PTR glass by UV continuous wave laser [23], UV [24] and IR [25] pulsed lasers.It is worth noting that in conventional glass-metal nanocomposites based on sodalime glass, observation of the fluorescence of AgMCs after optical bleaching requires additional thermal treatment [14].Recently, second harmonic generation and multiphoton-absorptioninduced luminescence were shown in the embedded laser-reshaped AgNPs upon picosecond pulsed laser excitation at 1064 nm [26].
In this paper, we report the visible fluorescence of chloride PTR glass after bleaching of AgNPs with femtosecond Ti:Sapphire laser and demonstrate the presence of residue of crystal phase Na 0.9 Ag 0.1 Cl in the irradiated volume.In order to increase the speed of fluorescent centers creation in bulk PTR glass we employ Spatial Light Modulator (SLM) that allows us to split the beam and perform parallel glass bleaching with 25 beams.

Experimental
In the experiment, we use PTR glass that belongs to sodium-alumina-silicate system, Na 2 O -Al 2 O 3 -ZnO -SiO 2 -NaF -NaCl and was activated by CeO 2 (0.007 mol.%),Sb 2 O 3 (0.04 mol.%), and Ag 2 O (0.12 mol.%).Table 1 shows four studied glass compositions with variable chlorine concentration.The glasses were synthesized in fused silica crucibles at 1500 • C in the air environment.Stirring with a Pt thimble was used to homogenize the liquid.After melting, the glasses were cooled down to 500 • C, then annealed at glass transition temperature (T g = 494 • C ) for 1 hour, and cooled down to room temperature with a rate of 0.15 • C/min.Properties of chloride PTR glass were described elsewhere [1].It is worth noting that a finite chlorine concentration in the PTR glass is necessary to observe AgNPs surface plasmon resonance absorption [1].The samples were prepared as 1.5 mm thick polished plates.For the precipitation of AgNPs in the bulk PTR glass samples were irradiated by mercury lamp EFOS Novacure N2001 (Artisian) and thermally treatment in programmable muffle furnaces (Neibotherm) at 560 • C at 90 min.Figure 1 shows experimental setup for glass processing with femtosecond Ti:Sapphire laser (Quantronix Integra-C-3.5)with 790 nm central wavelength, maximum pulse energy 3.5 mJ, pulse duration 120 fs and repetition rate of 1 kHz.The laser beam was split by 25 using a liquid crystal on silicon spatial light modulator (Hamamatsu X10468-02).The diameter of each beam at sample plane was 2.5 µm with energy varying from 0 to 100 µJ per pulse.Minimal number of pulses per spot was 50 due to shutter switching speed.Cameras 1 and 2 served for laser intensity correction and observation of induced modifications, respectively.The processed samples were characterized by measuring transmission and with fluorescence spectra spectrophotometer (Perkin-Elmer Lambda C650) and photoluminescence quantum yield measurement system (Hamamatsu C9920-02G), respectively.The X-ray diffraction (XRD) spectra were obtained using Ultima IV diffractometer (Rigaku).
For AgNPs formation in the PTR glass we employed UV irradiation of the samples and thermal treatment, which was described in details elsewhere [1].Briefly, samples were irradiated by a mercury lamp with UV filter, which cuts down the emission spectrum to 320 nm.One can observe from absorption spectra in Fig. 2 (Curve 1, 2) that there is an absorption band at 305 nm associated with Ce 3+ ions, which absorb a considerable part of the mercury lamp radiation.Due to ionization of Ce 3+ ions, released electrons are trapped by Sb 5+ and Ag + ions and AgMCs Ag m+ [6].The following thermal treatment leads to the growth of AgNPs that are as big as 5 nm in average.Moreover, other nanocrystals are synthesized during the thermal treatment including NaAgCl, AgBr, NaF and CaF 2 when the temperature exceeding the transition glass temperature [1,2,27].The particular assortment of the nanocrystals strongly depends on glass composition and thermal treatment regime.

Bleaching of glass
The parallel glass processing by ultrashort laser pulses was performed using SLM with computer generated hologram (CGH).The calculation of computer generated hologram was performed on commercially available computer using Iterative Fourier Transform Algorithm [28].An initial laser beam can be divided by array of beams with variable intensity.In present work, the size of array was 25 beams, as it shown at Fig. 1.However, it still can be extended up to 2500 beams with current setup [29].The correction of aberrations was also implemented by SLM, so every beam had circular cross-section and equal size.Each beam was focused under glass surface at 150 µm.Similar approach was implemented for data recording in fused silica with high capacity [30].
Figure 2 (Curve 1) shows optical density spectrum of AgCl3 after thermal treatment, which caused formation of AgNPs in glass volume.The absorption band is related with AgNPs and caused by surface plasmon resonance is located at 432 nm pointing out at the presence of dielectric shell around spherical AgNPs [31].According to the previous study, in similar glass composition NaAgCl crystals can be formed, which was also supported by XRD data [1].Such a dielectric shell with relatively high refractive index (up to 2.0) with respect to glass host shifts the absorption band to longer wavelengths [31].
AgNPs in bulk PTR glass were bleached by focused femtosecond laser pulses.Since the central wavelength of the laser beam is in near IR, it interacts with the UV-treated PTR glass through two-photon absorption process, which is enhanced due to the surface plasmon resonance of AgNPs [32]. Figure 2 (Curves 2-7) shows that surface plasmon resonance peak amplitude can be suppressed significantly with increase of laser intensity from 0 to 100 µJ.Minimal dose of 50 pulses was used due to the restriction of shutter switching speed.Effect of metal nanoparticles bleaching is well-known as photodesctruction [21].Red shift of surface plasmon resonance band on Curve 3 can be caused by increase of refractive index of AgNPs surroundings due to silver ions and AgMCs [33].Following photodestruction process most likely accompanied with re-shaping of AgNPs from spherical to ellipsoidal seeing the appearance of two plasmon resonance peaks at 420 and 490 nm (Curves 4-6).However, this assumption should be studied further in details using Transmission Electron Microscopy.

Fluorescence study
Figure 3(a) shows fluorescence spectra of studied samples under excitation at 340 nm.Initial glasses with AgNPs possess only week fluorescence in visible under 340 nm wavelength (for instance AgCl3).However, in all irradiated areas we observed visible fluorescence under UV excitation.One can see that fluorescence maximum is located at 670 nm.The spectra are broad and consist of several fluorescence bands, which are most likely associated with different AgMCs [14,[34][35][36].The identification of the AgMCs size is complicated due to the band overlapping.However, according to the previous studies, molecular clusters Ag m+ n , where n = 2 -10 and m = 0 -2, presumably are presented in glass [22].The intensity of fluorescence grows considerably with the increase of the chlorine concentration from 0.5 to 3 mol.%(Fig. 3(a)).It is well known that chlorine ions make single covalent bond with silicon ions and break the glass network, creating point defects such as non-bridging oxygen hole centers, E'centers etc. [37].In addition, it was shown recently [22] that the fluorescence intensity of PTR glass increases with addition of chlorine ions.Thus, a dependence of fluorescence intensity on chlorine concentration after AgNPs bleaching can be explained by fluorescence quenching of AgMCs in glass with lower chlorine concentration.So, as long as chloride ions promote matrix defects formation, it can lead to further distance among AgMCs after AgNPs bleaching.It reduces the amount of fluorescence quenching channels.Another possible mechanism consist in energy transfer from AgMCs to matrix defects, suggested by Eichelbaum et al. [37], which can also serve as a fluorescent light source.With a number of matrix defects proportional to the chlorine concentration, their fluorescence at 500-700 nm spectral range can also contribute to the enhancement of the fluorescence intensity.
The intensity of the glass fluorescence rises with the increase of UV exposure time before AgNPs formation (Fig. 3(b)).This result can be explained by larger number of AgNPs formed during initial thermal treatment.It allows us to adjust fluorescence intensity after AgNPs bleaching for application.Figure 4(a) presents an array of modified areas in glass volume with variable fluorescence intensity.Different shapes can also be created using SLM and photodestruction process.

XRD study
It is assumed that AgNPs are surrounded by NaAgCl nanocrystal shell before the bleaching, which causes the shift of surface plasmon resonance band [19].According to the previous study of chloride PTR glass, a size of NaAgCl nanocrystals is 27 nm in average using Sherrer's equation after 3 hours of thermal treatment at 530 • C [1,38]. Figure 5 shows XRD spectrum of AgCl3 with the same AgNPs initial size before (dashed) and after (solid) femtosecond laser bleaching.One can see that NaAgCl crystal phase was still presented in glass after bleaching.However, the intensity of peaks at 31 • and 45 • decreased significantly.A precipitation of SiO 2 phase has occurred during the preparation of powder for XRD analysis.Using Sherrer's equation the average size of NaAgCl nanocrystals reduced down to 8 nm.Considerable change of NaAgCl may be related with photosensitivity of similar AgCl crystals to visible light [7].This indicates that under irradiation with intense femtosecomnd pulses, photosensitivity of the NaAgCl nanocrystals may be due to the two-or three-photon absorption

Fig. 1 .
Fig. 1.Scheme of optical setup.F1-F4 -lenses, B -zero order block.Inset circles (a) shows computer generated hologram of laser beams, generated by spatial light modulator; (b) schematic process of AgNPs bleaching with formation of fluorescent AgMCs in focal volume.

Fig. 3 .
Fig. 3. (a) Fluorescence spectra of bleached glass under 340 nm excitation.Inset is an photo of AgCl3 under UV light excitation after bleaching.(b) Dependence of fluorescence intensity on AgNPs concentration before bleaching, which is monotonically increasing with initial dose of UV radiation.

Figure 4 (
Figure 3(a)  shows fluorescence spectra of studied samples under excitation at 340 nm.Initial glasses with AgNPs possess only week fluorescence in visible under 340 nm wavelength (for instance AgCl3).However, in all irradiated areas we observed visible fluorescence under UV excitation.One can see that fluorescence maximum is located at 670 nm.The spectra are broad and consist of several fluorescence bands, which are most likely associated with different AgMCs[14,[34][35][36].The identification of the AgMCs size is complicated due to the band overlapping.However, according to the previous studies, molecular clusters Ag m+ n , where n = 2 -10 and m = 0 -2, presumably are presented in glass[22].The intensity of fluorescence grows considerably with the increase of the chlorine concentration from 0.5 to 3 mol.%(Fig.3(a)).It is well known that chlorine ions make single covalent bond with silicon ions and break the glass network, creating point defects such as non-bridging oxygen hole centers, E'centers etc.[37].In addition, it was shown recently[22] that the fluorescence intensity of PTR glass increases with addition of chlorine ions.Thus, a dependence of fluorescence intensity on chlorine concentration after AgNPs bleaching can be explained by fluorescence quenching of AgMCs in glass with lower chlorine concentration.So, as long as chloride ions promote matrix defects formation, it can lead to further distance among AgMCs after AgNPs bleaching.It reduces the amount of fluorescence quenching channels.Another possible mechanism consist in energy transfer from AgMCs to matrix defects, suggested by Eichelbaum et al.[37], which can also serve as a fluorescent light source.With a number of matrix defects proportional to the chlorine concentration, their fluorescence at 500-700 nm spectral range can also contribute to the enhancement of the fluorescence intensity.The intensity of the glass fluorescence rises with the increase of UV exposure time before AgNPs formation (Fig.3(b)).This result can be explained by larger number of AgNPs formed during initial thermal treatment.It allows us to adjust fluorescence intensity after AgNPs bleaching for application.Figure4(a) presents an array of modified areas in glass volume with variable fluorescence intensity.Different shapes can also be created using SLM and photodestruction process.Figure4(b) demonstrates the image, which was made in glass by 50 femtosecond pulses without scanning process.

Table 1 .
Key components of glass samples