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 CO 2 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.

Semiconductor waveguide fabrication for photonics applications is usually performed in a planar geometry. However, over the past decade a new fi eld of semiconductor-based optical fi ber devices has emerged. The drawing of soft chalcogenide semiconductor glasses together with low melting point metals allows for meters-long distributed photoconductive detectors, for example. [ 1 , 2 ] Crystalline unary semiconductors (e.g., Si, Ge) have been chemically deposited at high pressure into silica capillaries, [ 3 , 4 ] allowing the optical and electronic properties of these materials to be exploited for applications such as all-fi ber optoelectronics. [ 5 -7 ] In contrast to planar rib and ridge waveguides with rectilinear cross sections that generally give rise to polarization dependence, the cylindrical fi ber waveguides have the advantage of a circular, polarization-independent cross section. Furthermore, the fi ber pores, and thus the wires deposited in them, are exceptionally smooth [ 8 ] with extremely uniform diameter over their entire length. The high-pressure chemical vapor deposition (HPCVD) technique is simple, low cost, and fl exible so that it can be modifi ed to fi ll a range of capillaries with differing core dimensions, while high production rates can be obtained by parallel fabrication of multiple fi bers in a single deposition. It can also be extended to fi ll the large number of micro-and nanoscale pores in microstructured optical fi bers (MOFs), providing additional geometrical design fl exibility to enhance the potential application base of the fi ber devices. [ 9 ] Semiconductor fi bers fabricated via HPCVD in silica pores also retain the inherent characteristics of silica fi bers, including their robustness and compatibility with existing optical fi ber infrastructure, thus presenting considerable advantages over fi bers based on multicomponent soft glasses.However, one key challenge in this rapidly developing fi eld is to realize low-optical-loss crystalline, direct-bandgap compound semiconductor fi bers. High-performance optoelectronic devices are almost exclusively fabricated from crystalline compound semiconductors owing to their superior light emission effi ciency, excellent electronic properties (e.g., high carrier mobility), and large optical nonlinearities. [ 10 ] In particular, compound semiconductors can have high second-order nonlinear optical coeffi cients, χ (2) , [ 11 ] not found in centrosymmetric crystalline unary semiconductors or in amorphous semiconductors. These second-order nonlinearities allow for effi cient frequency conversion on which devices such as optical parametric oscillators are typically based. [ 10 ] Compound semiconductors can also be optically transparent over a wider wavelength range than unary semiconductors, with ZnSe, for example, having excellent optical transmission at wavelengths from 500 to 22 000 nm. [ 12 ] An additional advantage of crystalline compound semiconductors is their ability to host transition metal ions; Cr 2 + -doped ZnSe functions as an ...

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