Thermal Tuning of Plasmofluidic Disk Resonators Filled With a Liquid Crystal: Its Narrow-Trench Filling and Arrangement

Plasmonic waveguides are hybrid plasmonic waveguides based on strong light confinement in a fluid filling narrow trenches bounded by silicon and metal surfaces. They might be expected to work as a tunable device with a large phase change if (1) a liquid crystal (LC) fills such trenches, (2) the LC molecules are ideally aligned in the direction normal to the cross-sections of the trenches, and (3) they are electrically oriented in the direction normal to the surfaces. However, to our knowledge, even the first two assumptions have never been confirmed to be correct. Here, we investigate LC-filled plasmofluidic disk resonators to check the correctness of the assumptions. We show that their resonance wavelengths decrease as the LC temperature increases and that the resonance wavelengths decrease rapidly near the clearing point of the LC. This indicates that the 30-nm-wide trenches are really filled with the LC but that the LC molecules are oriented toward the direction normal to the surfaces. The real arrangement of the LC molecules in the trenches may make the phase change obtainable from the plasmofluidic waveguide about two times smaller than expected from the ideal arrangement. This work may lead to development of phase-tunable LC-filled plasmofluidic waveguide devices.