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Femtosecond laser modification of silver doped zinc-phosphate glass

Name
Georgiy
Surname
Shakhgildyan
Scientific organization
D. Mendeleev University of Chemical Technology of Russia
Academic degree
Candidate of Sciences
Position
Researcher
Scientific discipline
New materials, Manufacturing technologies & Processes
Topic
Femtosecond laser modification of silver doped zinc-phosphate glass
Abstract
A high silver-content zinc-phosphate optically transparent glass was modified by femtosecond laser. Micron-sized donut-like structures were formed in the volume of glass. Silver nanoparticles showing surface plasmon resonance effect and intense luminescence were formed on the edges of the structures while birefringent regions were formed in its center. Independent control of luminescence and birefringent in such structures could pave the way for the 6D optical data storage in glass.
Keywords
zinc-phosphate glass, femtosecond laser modification, silver nanoparticles, birefringent nanogratings
Summary

Femtosecond laser modification of silver doped zinc-phosphate glass

Lipatiev A.S., Shakhgildyan G.Y., Lipateva T.O., Vetchinnikov M.P., Popova V.V., Fedotov S.S., Lotarev S.V., Sigaev V.N., Kazansky P.G.

     Using of ultrafast and highly intense laser sources open routes for precise space-selective tuning of properties of the solid matter that can be used for design of nanocomposite and nanostructured materials. Among other techniques, femtosecond laser modification (FLM) represents a maskless patterning process that provides rapid patterning at sub-micrometer resolutions with flexibility, ease of use, and cost effectiveness [1].

     FLM process is most widely used for patterning of optical materials (crystals, polymers and glasses) due to non-linear character of light-matter interactions providing high space selectivity. Significant results in channel waveguides formation and bulk precipitation of metal nanoparticles and nonlinear crystals were achieved during past decades [1]. One of the especially important applications of FLM process is the realization of approach for the 3D optical data storage in glass. This approach potentially enables dramatical increase of data storage density and lifetime. Different parameters of laser modified areas of glass can be used for data encoding (refraction index change, retardance and slow axis of birefringent nanogratings, luminescence of metal nanoclusters etc.) [2, 3]. Formation of birefringent nanogratings was widely demonstrated in fused silica glass and couple of silicate glasses [2]. At the moment, one of the most attractive materials for initiation of light emitting centers in the bulk of glasses is phosphate glass containing photosensitive agents such as noble metals (Ag, Au) [3]. Glass interactions with highly intense laser pulse leads to spatially selected precipitation of metal nanoclusters or nanoparticles which can exhibit luminescence in the visible region. Despite of existence of studies of these parameters alone it is a lack of studies for simultaneous formation in glass both birefringent nanogratings and luminescent emitters that could potentially lead to increase of recording density. The present study reports the possibility of simultaneous formation of silver nanoparticles and birefringent regions in the volume of phosphate glass by means of femtosecond laser modification.

     Silver-doped glass preparation was achieved by the melt-quenching technique for a zinc-phosphate glass matrix of 40P2O5-55ZnO-4Ag2O-1Ga2O3 composition (mol.%). The obtained glass was transparent and optically homogeneous. A setup equipped with NIR femtosecond laser (Nd:YAG, 6W, 100 kHz, 600 fs at 1030 nm, Pharos SP by Light Conversion Ltd.; Olympus objective LCPLNIR 50X NA 0.65) and a high-precision 3D translation stage (Aerotech ABL1000) was used for FLM. Micron sized dots with 10 µm spacing were written by the laser beam about 45 µm below the glass surface. FLM was performed for different laser pulse energies from 38 to 73 nJ and different number of pulses from 1,3 to 10,4 x10^5. For each energy and number of pulses a laser beam polarization angle was varied for 0, 36, 62, 98, 144 degrees relative to the starting position by means of a half-wave plate. Quantitative analysis of birefringence was performed by CRi Abrio system installed in the optical microscope Olympus BX61. Luminescence analysis was carried out using Nikon Eclipse Ni-E epifluorescence microscope.

     As a result, a set of donut-like dots from 3 to 5 µm in diameter increasing with the energy and number of applied laser pulses was formed. The dots had yellowish colour at the edges and were colorless and transparent in the center. The fluorescence microscopy with excitation at 380 nm revealed the noticeable luminescence appearing on the edges of modified regions. Yellow coloration of the edges is the indirect evidence of silver nanoparticle formation due to FLM of glass. Thus, to confirm that fact, a set of 100 µm long lines with the width of 5 µm immediately adjacent to each other was written resulting in yellow coloured rectangular area inside the glass. Absorption microscpectroscopy was performed both in laser modified area of the glass and outside this area revealing the absorption band with a maximum at 445 nm associated with surface plasmon resonance of silver nanoparticles. This clearly confirms the formation of silver nanoparticles in the bulk of the phosphate glass due to FLM. A mean size of nanoparticles calculated in terms of Mie theory was about 3-4 nm. On the other hand, clear observed luminescence from the edges of donut-like dots indicates the presence of silver nanoclusters. Thereby it can be assumed that silver aggregates with a wide size distribution including both plasmonic silver nanoparticles and luminescent silver nanoclusters formed during the FLM of zinc-phosphate glass.

     At the same time, birefringence was clearly observed in the central regions of donut-like dots along with nanoparticles and nanoclusters formation in the edges of dots. The retardance measured after glass irradiation by 2,6x10^5 laser pulses was about 10 nm. Increasing the number of pulses up to 10^6 resulted in the increase of retardance up to 30 nm in the center of dot. It should be noted that the slow axis orientation in the birefringent areas correlated to the linear polatization of the applied femtosecond laser beam being always perpendicular to its polarization plane. This case is typical for birefringent nanogratings in glass and gives and indirectly indicates of appearance of nanogratings which were not reported in phosphate glasses earlier.

     In conclusion, possibility of simultaneous formation of both luminescent silver nanoparticles and nanoclusters and birefringence nanogratings in the bulk of silver doped zinc-phosphate glass by femtosecond laser modification has been demonstrated. Further study is going on to show the possibility of independent control of birefringence and luminescence by means of varying the irradiation conditions. This may open the way for the multilevel encoding for data recording in silver doped zinc-phosphate glass.

     This work was financially supported by the Ministry of Education and Science of the Russian Federation (grant 14.Z50.31.0009), the Grants Council of the President of Russian Federation (grant МК-9290.2016.3) and the Russian Foundation of Basic Research (grant 16-03-00541).

 

1. Phillips K.C. et. al., Adv. Opt. Phot., V.7, 4, 686-705 (2015)

2. Zhang J. et. al., Phys. Rev. Let. 112,3, 033901 (2014)

3. Canioni L. et. al., Opt. Let. 33, 360-362 (2008)