Fed by an optical waveguide, an optical nanoantenna can be used to emit infrared (IR) or visible light in a highly directional manner (just as a radio antenna does for radio waves). However, backreflections originating at the juncture between waveguide and optical antenna sap the efficiency of the configuration. Because scaling the matching-circuit idea for radio antennas down to optical wavelengths is not practical, another approach must be used. Yakir Hada and Ben Steinberg of Tel Aviv University (Tel Aviv, Israel) have come up with a potential solution: a waveguide that ends in a structure that permits only one-way propagation.
The nonreciprocal light transmission happens as a result of the interaction of nonreciprocal optical Faraday (cyclotron) rotation and structural chirality. Faraday radiation occurs when charged particles move in a magnetic field, and so the one-way waveguide contains plasmonic structures (called a subdiffraction chain, or SDC) to which an external magnetic field is applied. For chirality, the researchers design the SDC in a spiral shape (actually a chain of metal ellipsoids spaced 11 nm apart and unidirectionally rotated along the chain). The researchers modeled the structure, showing that a chain length of about three wavelengths produced a directional beam at high efficiency (81% for the antenna itself) for a 0.4 μm wavelength. In addition, changing the strength of the applied magnetic field changed the beam direction, producing a scanner with a 60° deflection range. Contact Steinberg at [email protected].