Could the combination of plasmonics and mechanical actuation be the answer for making tiny reconfigurable optical integrated circuits that are compatible with complementary metal–oxide–semiconductor electronics, can switch rapidly and exhibit low optical loss? The recent findings from a collaboration of scientists from Switzerland, Sweden and the US suggest that this is the case. Writing in Science (Science 366, 806–864; 2019), Christian Haffner and co-workers report how micrometre-scale disk-shaped plasmonic structures that feature a tiny suspended gold membrane that deforms under electrostatic forces can be used to construct electro-optical switches that route infrared light between waveguides. Importantly, the tiny devices (footprint of ~10 μm2) feature a low optical insertion loss (0.1 dB through port, 2 dB drop port), rise and fall switching times of 60 and 100 ns respectively, and an optical contrast of 90% between the on and off state.

Credit: From Haffner, C. et al. Science 366, 806–864 (2019). Reprinted with permission from AAAS.

The switch, fabricated on a silica substrate, consists of a 40nm-thick gold membrane partially suspended over a silicon disk forming a small-air-gap plasmonic waveguide. The switch is located at the junction between two orthogonal silicon waveguides to facilitate coupling to each of them. The switching of light from one silicon waveguide to the other only occurs when the plasmonic switch is resonant with the wavelength of the incoming light signal so that coupling occurs, otherwise the light simply continues along its original waveguide. Changing the voltage applied to the device in the range of 0–1.4 V generates an electrostatic force that causes the gold membrane to deform, thus changing the size of the air-gap waveguide and the device’s resonant wavelength, thus shifting it into either a resonant or off-resonant state. The electro-optical switch operates at telecommunications wavelengths of around 1550–1560 nm and the estimated electrical power consumption is around 600 nW for a driving voltage of 1.4 V and 12 nW at 0.2 V.

The team believe that it should be feasible to fabricate large arrays of such switches and thereby realize a photonic platform for constructing dense optical switch fabrics for telecommunications or optical neural networks for applications in deep learning. “These switches could form the building blocks of optical field-programmable gate arrays and trigger a technological revolution similar to the one enabled over the past few decades by electrical field-programmable gate arrays”, conclude the authors in the final part of their paper. “For instance, 200 switches and their electrical drivers could be integrated on an area as small as the cross section of a single human hair.”