First carbon-nanotube-based optical component performs wavelength conversion

The interaction of carbon nanotubes (CNTs) with the evanescent field of light in the core of a silicon planar waveguide is the basis for the first CNT-based integrated optical component developed by researchers at the University of Tokyo and Alnair Labs Corporation (both in Tokyo, Japan). The CNT material—fabricated using a high-pressure carbon-oxygen conversion process creating nanotube diameters and distribution parameters optimized for strong absorption near 1550 nm—is deposited in a spraying process over the 7-µm-wide core of a cladding-removed waveguide fabricated using standard silicon-based planar lightwave circuit (PLC) techniques. The CNTs are believed to have a nonlinear refractive index several orders of magnitude higher than silica, originating from interband transitions of p-electrons causing nonlinear polarization like other highly nonlinear organic optical materials.

Due to strong CNT nonlinearity, pump and signal wavelengths from tunable laser sources input to the device undergo a four-wave mixing process; in effect, the device is an effective wavelength converter, with a nonlinear coefficient as high as 5.64 × 103 W^-1km^-1. For a 10 Gbit/s data stream, the power penalty was 3 dB for a bit-error rate (BER) of 10^-9. The ability to deposit CNTs on a variety of waveguide patterns makes possible devices such as Mach-Zehnder interferometers, while their fast response time (less than 500 fs) enables the development of ultrahigh-speed structures for all-optical signal processing and optical logic gates. Contact Kin Kee Chow at [email protected].