FEMTOSECOND LASER WRITING IN HIGH-PURITY TELLURITE GLASSES

FEMTOSECOND LASER WRITING IN HIGH-PURITY TELLURITE GLASSES

M.P. Smayev¹, V.V. Dorofeev², A.G. Okhrimchuk¹

¹International Centre of Laser Technology, D. Mendeleyev University of Chemical Technology of Russia, Miusskaya sq. 9, 125047 Moscow, Russia

²G.G. Devyatykh Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences, Tropinina st. 49, 603950 Nizhny Novgorod, Russia

Glasses based on tellurium dioxide are characterized by low phonon energy and can be obtained with an extremely low 3d-transition metals and hydroxyl groups impurities content, that provides transparency in the near and mid infrared regions. At the same time, viscosity parameters, sufficient mechanical strength and high nonlinear refractive index make them a very attractive matter for bulk and waveguide devices.

We studied the nonlinear optical properties and femtosecond laser writing in high-purity zinc- and tungstate- tellurite glasses modified with molybdenum, lanthanum or bismuth oxides.

The glasses were produced by melting the oxides in gold or platinum crucibles inside a sealed silica chamber filled with purified oxygen. TeO₂, WO₃, MoO₃ fabricated in our laboratory by original techniques and commercial high-purity ZnO, La₂O₃, Bi₂O₃ were used for glass synthesis. The total content of the 3d-transition metals impurities in raw materials and in glasses did not exceed 1-2 ppm wt. High stability of glasses against crystallization was confirmed by DSC at a heating rate 10 K/min and there were no thermal effects of crystallization and melting of crystals. The hydroxyl groups absorption at ~3 μm, being calculated from transmission spectra by the Beer–Lambert–Bouguer law equation, was as low as 0.001-0.01 cm^-1 in the glass samples prepared.

Using Z-scan technique [1] we have experimentally determined Kerr nonlinear coefficient that characterizes refractive index change Dn due to response of electronic shells:

  n =  × I,                                                 (1)

where I is a light intensity. The femtosecond laser beam was focused by a lens with focal distance of 250 mm. Beam waist radius in focal plane was of 50 microns. Laser operation wavelength was of 1030 nm, pulse duration was about 180 fs, and repetition rate equaled to 1 kHz. A glass sample with thickness of 1.5 mm was installed on a motorized translation stage, and translated along the beam axis with respect to focal plane on the distance of 120 mm.

All samples exhibit positive nonlinearity. The dependences of nonlinear coefficient  on the energy in femtosecond pulse for tungstate- tellurite glasses are presented on Figure 1. The maximal  values were obtained for bismuth oxide modified glasses:  = 5.6 ´ 10^-6 cm²/GW, lowest nonlinear coefficients among glasses investigated were obtained for zinc-tellurite glasses ( » 10^-6 cm²/GW).

Figure 1. The dependences of Kerr nonlinearity coefficient  on pulse energy for various tungstate-tellurite glasses: 1 - bismuth oxide modified glasses; 2 – molybdenum and lanthanum oxides modified glasses; and 3 - lanthanum oxide modified glasses.

Tungstate-tellurite glasses modified with bismuth, molybdenum or lanthanum oxides were tested for femtosecond direct writing of waveguide tracks. To this end, fs-laser beam was focused in the bulk of glass plate with objective of numerical aperture NA=0.85 and series of tracks were inscribed under wide range of pulse energies, repetition rates, and scan speeds.

Figure 2. The series of tracks with modified refractive index inscribed in molybdenum and lanthanum oxides modified tungstate-tellurite glass by femtosecond pulses with different energies: 1 – E_p = 30 nJ; 2 – E_p = 60 nJ; 3 – E_p = 90 nJ.

All obtained tracks exhibit decreased refractive index with respect to non-modified region of sample. The highest value of refractive index change was about Dn = - 0.002 produced in molybdenum and lanthanum oxides modified tungstate-tellurite glass. The series of tracks in this sample recorded for different pulse energies is shown in Figure 2. Threshold energy required for glass modification was E_tr = 14 nJ. The tracks were smooth and homogeneous at wide energy range E = 14 ÷ 160 nJ.

The results of the study evidence high potential of tellurite oxide based glasses for realization of nonlinear waveguide devices.

[1] M. Sheik-Bahae, Ali A. Said, T.H. Wei, D.J. Hagan, E.W. Stryland, IEEE J. Quantum Electron. 26, 1990, p. 760.