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Direct laser writing of waveguiding crystalline channels in lanthanum borogermanate glasses

Name
Sergey
Surname
Lotarev
Scientific organization
D. Mendeleyev University of Chemical Technology of Russia
Academic degree
PhD in chemistry
Position
senior researcher
Scientific discipline
New materials, Manufacturing technologies & Processes
Topic
Direct laser writing of waveguiding crystalline channels in lanthanum borogermanate glasses
Abstract
Femtosecond laser-induced crystallization of lanthanum borogermanate glasses with chemical composition around stoichiometry of the ferroelectric LaBGeO5 phase has been studied. Applying a focused beam with elliptical cross-section of the waist is shown to improve homogeneity of the laser-written LaBGeO5 crystalline channels. An efficient technique of seed crystal growth using a beam with gradually increasing pulse energy which stabilizes and reduces time required to grow a seed is suggested.
Keywords
direct laser writing, space-selective crystallization, femtosecond laser, lanthanum borogermanate glass, LaBGeO5
Summary

Direct laser writing of waveguiding crystalline channels in lanthanum borogermanate glasses

S.V. Lotarev, S.S. Fedotov, T.O. Lipateva, A.S. Lipatiev, A.S. Larkin, V.N. Sigaev, P.G. Kazansky

Femtosecond laser direct writing of crystalline channels possessing waveguide properties in glasses gives an opportunity to combine functionality of crystals (e.g. luminescent or nonlinear optical properties) and flexibility, manufacturability and low cost of oxide glasses. This technique based on high repetition rate femtosecond regenerative amplifiers (typically 250 kHz) developed during the latest decade is now regarded as a promising way for active components of integrated optical devices [1-3]. Formation of quasi single-crystalline channels typically consists of two steps including initiation of a seed microcrystal inside the glass by the tightly focused stationary femtosecond laser beam and growth of a crystalline channel from the obtained seed by moving the waist of the laser beam along the predetermined path with a constant speed. Best results have been so far obtained in 25La2O3·25B2O3·50GeO2 glass in which femtosecond laser-written multimode waveguiding channels consisting of the ferroelectric stillwellite-like LaBGeO5 phase and showing optical losses of about 2.5-3 dB/cm were reported [3]. Recently, we have demonstrated that femtosecond laser-induced growth of oriented crystalline channels in this glass can be realized at much smaller pulse repetition rate starting from ~25 kHz [4].

In the present study, femtosecond laser-induced crystallization of lanthanum borogermanate glasses with chemical composition around stoichiometry of the ferroelectric LaBGeO5 phase has been investigated using a femtosecond ytterbium laser TETA (wavelength 1030 nm, pulse duration 290 fs, pulse energy up to 0.11 mJ, pulse repetition rate up to 100 kHz).

A technique of seed crystal growth using a beam with gradually increasing pulse energy is suggested. It stabilizes and reduces time required to grow a seed crystal and allows obtaining it in glasses with lower crystallization ability as compared to the beam with a constant pulse energy. This technique also allowed us crystallization of glasses with a chemical composition shifted from the stoichiometric composition of the precipitating LaBGeO5 phase. It has been demonstrated that lower crystallization ability of these glasses as compared to 25La2O3·25B2O3·50GeO2 glass enables growth of more homogeneous crystalline channels presumably due to lower concentration of undesirable crystallites on the way of the growing channel but at the same time drastically hinders precipitation of the seed crystal with a conventional beam with a constant pulse energy. Thus the glasses with shifted composition could be crystallized only using a beam with an increasing pulse energy.

Another enhancement of the earlier experimental technique has been applying a focused femtosecond beam with elliptical cross-section of the waist aligned along the writing direction enabled further improvement of the homogeneity of the laser-written LaBGeO5 crystalline channels.

As a result, crystalline channels obtained in the bulk of 25La2O3-30B2O3-45GeO2 glass noticeably deviating from the chemical composition of the crystal showed a waveguide effect and optical losses measured at 1030 nm were not higher than 1.6 dB/cm which is somewhat less that was reported earlier for the analogous channels in 25La2O3·25B2O3·50GeO2 glass [3].

1. X. He et al. J. Opt. Ceram. Soc. B, 2014, V. 31, P. 376.

2. A. Stone et al. Optics express, 2009, V. 17, P. 23284-23289.

3. A. Stone et al. Scientific reports, 2015, V. 5, P. 10391.

4. T. Lipateva et al. Proc. SPIE, 2015, V. 9450, P. 945018.