Above-Bandgap Surface-Emitting Frequency Conversion in Semiconductor Nanoribbons With Ultralow Continuous-Wave Pump Power

Semiconductors have large optical nonlinear susceptibilities especially in the spectral range above material bandgaps. However, optical frequency conversion encounters large absorption above bandgaps and difficulty using common phase-matching techniques. The frequency conversion bandwidths are thus limited. Here, frequency up-conversion far above the bandgaps using surface emissions from semiconductor nanoribbons is demonstrated, wherein nanoscale waveguiding tightly confines fundamental waves for decreasing pump powers, and above-bandgap absorption is greatly decreased in nanoscale waveguide thickness. By using CdSe nanoribbons, efficient 532- and 404-nm second-harmonic and 459-nm sum-frequency generations are obtained with the continuous-wave pump power less than 100 μW. The normalized efficiency of 532-nm second-harmonic generation is about 2 × 10^-5 mm^-1 at pump power of 300 μW. Attractive features such as tunable spatial distribution and highly polarization are observed. A broadband emission with a full width half maximum of ∼10 nm is attained by frequency summing a continuous-wave laser and an amplified spontaneous emission source.