CC BY
9LM-I-35
Ultrafast-laser writing of birefringent nanogratings in alkali-
containing glasses
S.V. Lotarev, S.S. Fedotov, A.I. Pomigueva, A.S. Lipatiev, V.N. Sigaev
Mendeleev University of Chemical Technology, 125047, Miusskaya sq. 9, Moscow, Russia
Main author email address: [email protected]
Birefringent periodical nanostructures inscribed by a series of ultrafast-laser pulses in the inside of glass which are known as nanogratings have become the first optically induced phenomena with regular period substantially smaller than the wavelength of light inducing it. First shown in fused silica [1], nanogratings drew strong attention due to their form birefringence and the possibility to control their orientation and optical retardance by the parameters of laser treatment. They have already been used in applications such as polarization converters, ultrastable data storage and microfluidics [2]. Most of the studies of this phenomenon were carried out in fused silica. Some years ago, the inscription of nanogratings was also demonstrated in several crystals and multicomponent glasses. The phenomenon of nanogratings occured to be much more common than it was first considered to be. Due to presence of several types of elements with different diffusion properties, the formation of nanogratings in multicomponent glasses was shown to be a more complicated process relative to fused silica and could be accompanied by nanoperiodical elemental redistribution, partial crystallization [3,4], photodarkening [5] or could produce more than one nanostructure with different periods [6]. However, there are still only few studies of effect of glass composition and the content of various components on the possibility, conditions and mechanism of nanograting inscription.
In this study, we investigated formation and optical properties of nanogratings in binary alkali-containing glasses as a simple model system allowing for understanding of the role of the type and content of monovalent cations in this context. Glasses under study included a set of alkali silicate glasses xR2O-(100-x)SiO2 (R = Li, Na, K) and sodium germanate glasses with various Na2O content. FEMTOLAB setup for laser processing based on PHAROS SP femtosecond laser (Light Conversion Ltd.) operating at 1030 nm wavelength was used in the performed experiments. Nanogratings could be inscribed in all glasses under study. The content of alkali oxide was shown not to be limiting factor for the possibility to form a nanograting in the studied range but had a drastic effect on the conditions of their inscriptions. In the case of sodium oxide, a number of ultrafast-laser pulses required to induce form birefringence manifested the exponential growth with its content. The existence of the upper limit of the peak intensity providing the formation of nanogratings was also revealed.
By the example of 22Na2O-78GeO2 glass, an effect of the pulse repetition rate and laser-induced heal accumulation on the nanograting formation and unexpected behavior of their optical retardance are described. It is also shown that, depending on other parameters of the laser pulses, nanogratings and laser-induced crystallization can be realized simultaneously or in non-overlapping ranges of the pulse repetition rate.
This study was financially supported by the Russian Foundation for Basic Research (grant 21-5312026).
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