  We suggest and study theoretically a novel type of optical Yagi-Uda
nanoantennas tunable via variation of the free-carrier density of a
semiconductor disk placed in a gap of a metallic dipole feeding element. Unlike
its narrowband all-metal counterparts, this nanoantenna exhibits a broadband
unidirectional emission and demonstrates a bistable response in a preferential
direction of the far-field zone, which opens up unique possibilities for
ultrafast control of subwavelength light not attainable with dipole or bowtie
architectures.
