Hybrid optical antenna with high directivity gain.

Coupling of a far-field optical mode to electronic states of a quantum absorber or emitter is a crucial process in many applications, including infrared sensors, single molecule spectroscopy, and quantum metrology. In particular, achieving high quantum efficiency for a system with a deep subwavelength quantum absorber/emitter has remained desirable. In this Letter, a hybrid optical antenna based on coupling of a photonic nanojet to a metallo-dielectric antenna is proposed, which allows such efficient coupling. A quantum efficiency of about 50% is predicted for a semiconductor with volume of ~λ³/170. Despite the weak optical absorption coefficient of 2000 cm(-1) in the long infrared wavelength of ~8 μm, very strong far-field coupling has been achieved, as evidenced by an axial directivity gain of 16 dB, which is only 3 dB below of theoretical limit. Unlike the common phased array antenna, this structure does not require coherent sources to achieve a high directivity. The quantum efficiency and directivity gain are more than an order of magnitude higher than existing metallic, dielectric, or metallo-dielectric optical antenna.