Atomically smooth optical fibers made of polycrystalline silicon are advancing microscale optoelectronics. In the July 1 issue of Optics Letters (DOI:10.1364/OL.36.002480; p. 2480), N. Healy of the University of Southampton, F.R. Sparks of Pennsylvania State University, and their colleagues detail their process for creating optical waveguides with an unprecedentedly low level of surface scattering.

The research team used 500°C silane and helium gas to deposit silicon in the pores of 5.6 μm and 1.3 μm internal diameter silica capillaries before annealing the constructs at 1325°C. The resulting waveguides possessed a surface roughness of σ = 0.1 nm RMS with z-direction resolution of 0.01 nm RMS. This surface roughness is an order of magnitude less than what has been previously measured in polycrystalline waveguides, and is also lower than that of a competing fabrication process, etchless silicon-on-insulator. The high degree of smoothness reduces surface scattering at the fiber-cladding interface, reducing scattering-based transmission loss to negligible levels.

(a) Optical microscope image of the 5.6 μm polysilicon core fiber as prepared for optical coupling, scale bar 50 μm. (b) Helium ion microscope image of the 1.3 μm core, scale bar 400 nm. Reproduced with permission from Opt. Lett. 36(13)v(2011), DOI:10.1364/OL.36.002480; p. 2480. © 2011 Optical Society of America.

Further measuring transmission losses over an extended telecom wavelength band, the researchers suggest the resulting λ^-4 dependence is associated with Rayleigh scattering in the bulk. By optimizing the annealing process, scientists could create inexpensive optical waveguides with less loss than currently available materials, according to the research team.

Polycrystalline silicon is an attractive material to use for optoelectronics and these advances only make it more so. In addition to their low cost, polycrystalline waveguides are easily integrated vertically on-chip and retain some of the electronic functionality of crystalline silicon. New, loss-minimized waveguides using this method could open the door to faster, more capable fiber communication devices.