Laser recrystallization and inscription of compositional microstructures in crystalline SiGe-core fibres

Abstract

Glass fibres with silicon cores have emerged as a versatile platform for all-optical processing, sensing and microscale optoelectronic devices. Using SiGe in the core extends the accessible wavelength range and potential optical functionality because the bandgap and optical properties can be tuned by changing the composition. However, silicon and germanium segregate unevenly during non-equilibrium solidification, presenting new fabrication challenges, and requiring detailed studies of the alloy crystallization dynamics in the fibre geometry. We report the fabrication of SiGe-core optical fibres, and the use of CO₂ laser irradiation to heat the glass cladding and recrystallize the core, improving optical transmission. We observe the ramifications of the classic models of solidification at the microscale, and demonstrate suppression of constitutional undercooling at high solidification velocities. Tailoring the recrystallization conditions allows formation of long single crystals with uniform composition, as well as fabrication of compositional microstructures, such as gratings, within the fibre core.

Figure 1. Compositional segregation of SiGe.

(a) The phase diagram of SiGe, with (b) a schematic illustration of non-equilibrium cooling in fibre cross-sections resulting in residual Ge-rich regions (green). (c) Compositional variation along the growth axis during directional recrystallization of a thin rod.

Figure 2. Fabrication and processing of fibres.

(a) A schematic of the fibre drawing process used in this work, and (b) schematic of the laser recrystallization set-up. The inset shows an image from the camera. (c) A photomicrograph of a SiGe-core fibre (Scale bar, 1 mm).

Figure 3. Composition and structure of SiGe-core fibres.

(a–c) 25 at% Ge fibre (a) XCT image of dendritic structures, showing a large grain, and red arrows indicating grain boundaries. (b) Cross-sectional BSE image (grey), EDX compositional maps for Ge (green) and Si(red), and (c) X-ray diffraction patterns of a polycrystalline region as a function of axial position (numbers shown are relative position in μm). Note that there are numerous peaks, the placement and number of which varies from frame to frame. (d–f) 6 at% Ge fibre (d) XCT image showing Ge segregation without dendrite formation (e) Cross sectional BSE image (grey), EDX compositional maps for Ge (green) and Si(red); and (f) truncated X-ray diffraction patterns of a fibre as a function of axial position (numbers are relative position in micrometre). Fewer peaks are seen, and the positions are constant over larger distances. The fibre has a grain boundary between 2,000 and 3,000 μm, as revealed by the sudden change in the pattern. Scale bar, 200 μm (a,d), 20 μm (b), 40 μm (e) and 1 Å⁻¹ (c,f). X-ray diffraction patterns in c,f are rotated 90° counter-clockwise to match the shown fibre orientation.

Figure 4. Laser recrystallization of 6 at% Ge fibres.

(a) Frames from CCD video showing Ge-rich liquid flowing from the untreated region to the laser-induced melt zone. Green and blue circles highlight droplet motion. (Frames are stretched 135% in the vertical direction for clarity.) (b) Image of melt zone penetrating the entire core. White spots are emission from interface layer inhomogeneities. (c) Intensity profile from the emission at the location shown by the red dashed line in b, showing a temperature increase towards the centre, and uneven emission from the interface layer. The large drop in emissivity between the solid and the liquid allows observation of the interface. (d) XCT cross-section after recrystallization (background is removed from around the fibre) and (e) XCT side view after recrystallization. Scale bar, 200 μm.

Figure 5. Microstructuring of 6 at% Ge fibres during recrystallization.

(a) XCT cross section of a Ge-rich lobe formed by melting the core and gradually reducing the laser power density, (b) XCT side view of the centre of the Ge-rich region, (c) Ge-rich grating formed in the fibre core by periodically interrupting the laser beam. The angle of the grating is due to the asymmetric heating and resultant tilted solidification boundary. (d) Power versus distance for the germanium grating process. (e) Intensity profile for the grating in f, where the dashed lines highlight the alignment. The upward slope is an unsubtracted background effect. (f) Si-rich regions formed by periodic variation of the velocity during recrystallization and (g) velocity profile during formation of the grating. (h) Taper formed during recrystallization of a fibre with an initial core diameter of 20 μm, made by applying stress to the fibre during laser heating (Scale bar, 200 μm).

Figure 6. Compositional variations in untreated fibres.

(a) BSE micrograph of a region with severe compositional inhomogeneity, with red and blue circles indicating Ge-rich and Ge-poor regions. Scale bar, 10 μm. (b) EBSD crystallographic map of the region inside the turquoise box, with the uniform colour indicating a single crystalline orientation despite large variations in composition. (c) Raman spectra of the regions in the circles; red (Ge-rich) and blue (Ge-poor). The dashed black curve is the result from a homogenized fibre, and the vertical green line is the shift measured for the Si–Si mode on a reference silicon wafer.