Microspectroscopy of ultrafast laser inscribed channel waveguides in Yb:tungstate crystals

Abstract

We report microspectroscopy measurements of crystalline channel waveguides fabricated in Yb: KGd͑WO 4 ͒ 2 and Yb: KY͑WO 4 ͒ 2 using the ultrafast laser inscription technique. From these measurements we find that densification of the WO 2 W bridge in the double tungstate crystal lattice is responsible for the refractive index increase, which creates the waveguide confinement. We identified that shifts toward lower energies in the ϳ760 cm −1 Raman mode indicate regions, which guide light polarized along the crystallographic b axis, while higher energy shifts in the 682 and 898 cm −1 Raman lines correspond to guiding regions for light polarized along the crystallographic a axis. ͓Yb: KGd͑WO 4 ͒ 2 and Yb: KY͑WO 4 ͒ 2 ͔, and Li: NbO 3 ͑Li:NbO͒. 7 During ULI, ultrashort-pulse radiation is tightly focused inside a dielectric material, resulting in high electric field intensities, which induce nonlinear processes such as multiphoton absorption and avalanche ionization, and lead to high temperatures and pressures through an induced microplasma. The plasma recombines with the lattice through energy transfer and this induces a modification to the material structure. In crystalline materials, this modification often takes the form of a void structure due to expansion of the material in the focal volume. This expansion induces a strain field in the material surrounding the void, which in turn leads to a refractive index modification via the photoelastic effect. However, the physical changes to the crystalline media and the mechanisms responsible for the resulting stress are not well understood. Two nondestructive techniques, which can be used to investigate the induced microscale modifications in the crystalline material are confocal microluminescence ͑ -PL͒ and micro-Raman ͑ -Raman͒ measurements. 8,9 These techniques involve recording the localized luminescence and Raman spectra from small volumes ͑Ͻ1 m 3 ͒ in and around the waveguide structure. By the observation and interpretation of any changes to the output spectra, such as intensity, peak wavelength, and spectral width, information on localized lattice damage, stress, and disorder can be extracted.Here we report two-dimensional ͑2D͒ microspectroscopy mapping of waveguides fabricated in crystalline Yb 3+ : KGd͑WO 4 ͒ 2 ͑Yb:KGdW͒ and Yb 3+ :KY͑WO 4 ͒ 2 ͑Yb:KYW͒ using ULI. The structural changes to the lattice, which were responsible for the localized refractive index variation, have been identified.Details of the waveguide fabrication and their laser performance can be found in Ref. 6. All structures consisted of two tracks inscribed next to each other at a separation of 20 m. Microspectroscopy mapping was performed on structures written at different pulse energies, which gave different guiding and lasing performance. It was observed that in Yb:KGdW, guiding of light polarized along the crystallographic a axis occurred in regions above and below the central crack ͓Fig. 1͑a͔͒, which was formed during waveguide fabrication. Conversely, guiding of light polarized along the ...

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