Femtosecond laser induced thermophoretic writing of waveguides in silicate glass

Abstract

Here in, the fs-laser induced thermophoretic writing of microstructures in ad-hoc compositionally designed silicate glasses and their application as infrared optical waveguides is reported. The glass modification mechanism mimics the elemental thermal diffusion occurring in basaltic liquids at the Earth's mantle, but in a much shorter time scale (10⁸ times faster) and over a well-defined micrometric volume. The precise addition of BaO, Na₂O and K₂O to the silicate glass enables the creation of positive refractive index contrast upon fs-laser irradiation. The influence of the focal volume and the induced temperature gradient is thoroughly analyzed, leading to a variety of structures with refractive index contrasts as high as 2.5 × 10^-2. Two independent methods, namely near field measurements and electronic polarizability analysis, confirm the magnitude of the refractive index on the modified regions. Additionally, the functionality of the microstructures as waveguides is further optimized by lowering their propagation losses, enabling their implementation in a wide range of photonic devices.

Figure 1

Characterization of optical waveguides produced with 530 nJ pulses and UO lens focusing at 250 µm of depth. (a) Transmission and (b) reflection cross-section optical microscopy, (c) near field image of the guided mode at 1640 nm, (d) longitudinal (top view) optical microscopy, (e) EDX elementary mapping and (f) EDX profile measured along the vertical line indicated in (e). Scale bar on (c) applies to all images. Laser beam direction is indicated by the red arrow in (a).

Figure 2

Cross-section transmission optical microscopy images of structures produced with 530 nJ pulses and objective lenses (UO, AL and CO) as labeled at the indicated depths. Scale bars apply to the images of each objective lens.

Figure 3

(a) Mode field diameter, (b) step refractive index and (c) propagation losses as a function of depth at 1640 nm for waveguides written with UO, AL and CO lenses.

Figure 4

(a) Beam intensity for AL lens along XZ and YZ planes for the labeled focusing depths. (b) Electron density profiles for the labeled focusing depths. Spectra are horizontally shifted so the maximum electron density is at 0 for all the cases. (c) Temperature gradient (main graph) and activated length (inset) versus the focusing depth.

Figure 5

(a) Mode fields (top and bottom panels) and optical transmission microscopy (middle panel) of irradiations at the indicated pulse energies, (b) step refractive index and (c) propagation losses as a function of pulse energy at 1640 nm for waveguides written with AL objective lens.