Optical gain in nanocrystalline silicon: Comparison of planar waveguide geometry with a non-waveguiding ensemble of nanocrystals

Stimulated emission from nanocrystalline silicon in the visible has become a hot topic during the past years. Various forms of silicon nanostructures are being exploited, among them planar optical waveguides made of silicon nanocrystals, silicon superlattices and tiny silicon nanoparticles. We report on optical gain measurements using the variable-stripe-length and the shifting-excitation-spot methods in two different types of nanocrystalline samples: a planar nanocrystalline waveguide prepared by silicon-ion implantation (400 keV, 4 × 10¹⁷ cm^-2) into a silica substrate and a layer of porous silicon grains embedded in a sol-gel derived SiO ₂ matrix. The latter does not exhibit any waveguiding. At a first sight one would expect much more favorable conditions for the stimulated emission occurrence in the former type of samples. We observed in fact small optical gain in both types (modal gain of 12 cm^-1 at ∼760 nm in ion implanted sample and of 25 cm^-1 at ∼650 nm in sol-gel embedded sample, respectively), however, under different pumping conditions. We discuss advantages and disadvantages of both nanostructures, referring in particular to leaky-mode emission that may occur in planar waveguides on transparent substrates.