Nanofocusing in a metal-insulator-metal gap plasmon waveguide with a three-dimensional linear taper

The development of techniques for efficiently confining photons on the deep sub-wavelength spatial scale will revolutionize scientific research and engineering practices. The efficient coupling of light into extremely small nanofocusing devices has been a major challenge in on-chip nanophotonics because of the need to overcome various loss mechanisms and the on-chip nanofabrication challenges. Here, we demonstrate experimentally the achievement of highly efficient nanofocusing in an Au-SiO2-Au gap plasmon waveguide using a carefully engineered three-dimensional taper. The dimensions of the SiO2 layer, perpendicular to the direction of wave propagation, taper linearly below 100 nm. Our simulations suggest that the three-dimensional linear-tapering approach could focus 830 nm light into a 235 nm(2) area with <= 3 dB loss and an intensity enhancement of 3.0x10(4). In a two-photon luminescence measurement, our device achieved an intensity enhancement of 400 within a 14380 nm(2) area, and a transmittance of 74%.