Nonlinear Nanocomposite Optical Waveguide

We report on the fabrication and characterization of a Kerr nonlinearity waveguide constructed of a nanocomposite core of 1.3% volume concentration CdSe quantum dots in PFCB polymer. Cytop polymer was used for the WG cladding. PFCB and Cytop were chosen due to their low loss at telecom wavelength and also their high index contrast for tight confinement of the optical mode (refractive index of 1.49 and 1.34 for PFCB and Cytop respectively). PFCB has also been proven a good host for nanoparticles (NPs) to create nonlinear nanocomposite. For the NPs we used CdSe QDs since it has high bulk third order susceptibility of c⁽³⁾ =4·10^-19 m²/V² as well as a sufficiently large band gap (~2ev) to avoid TPA in telecom wavelength. The WG fabrication included standard clean room processes, such as spin coating, lithography, reactive ion etching and plasma ashing. Next we characterized the WG in term of linear loss, i.e. propagation and coupling loss. 1.4 dB/cm and 2.5 dB/facet were calculated respectively. To prove the WG nonlinearity a Four-Wave-Mixing experiment was conducted. Two CW pumps around 1549nm spaced 0.65 nm apart were combined, amplified by an EDFA and coupled into the WG. Both pumps had the same power and polarization. The measurements were done with and without the WG using the same conditions. To distinguish the difference of of two measurements, we used a Fiber Bragg Grating notch filter with 20dB attenuation around 1550 nm, into which the generated red shifted FWM harmonic falls into. The calculated nonlinear coefficient γ is 0.2 (W·m)^-1 and 0.27 (W·m)^-1 for TE and TM modes respectively. In addition, when inserting different intensities to the NL-WG and the insertion loss didn’t change which indicate the WG doesn’t possess TPA. The low coupling and propagation losses, the high γ and lack of TPA suggest that NL-WG constitute a very useful platform for nonlinear-optical signal processing.