Ultrahigh Purcell factor achieved in a single sphere-gap-cone hybrid nanoantenna

Hybrid nanostructures exhibit extraordinary optical properties combining near field enhancement and inherent magnetic and electric response induced by metal and dielectric components, respectively. Here, we study theoretically... [ view full abstract ]

Hybrid nanostructures exhibit extraordinary optical properties combining near field enhancement and inherent magnetic and electric response induced by metal and dielectric components, respectively. Here, we study theoretically optical properties of a novel type of hybrid sphere-gap-cone nanostructure. We demonstrate, this type of nanostructure provides strong magnetic and electric response in the visible range. Especially, looking into single resonant wavelength indicates unidirectional scattering of this nanostructure. Besides, ultrahigh Purcell factor is achieved by placing a discrete dipole in the gap in different orientations.

A schematic of sphere-gap-cone nanostructure is presented in Fig. 1a. As can be seen, the gold sphere is located on the truncated crystalline silicon cone with a 10 nm gap in the middle. The bottom base’s diameter as well as the height of the cone is 190 nm. The diameter of the cone upper base is half of the bottom base, i.e., 95 nm. Fig.1b indicates the scattering cross section of the novel sphere-gap-cone, in which points A, B, C are the resonances, respectively. Fig. 1c are the electric field distributions corresponding to points A, B, C. Obviously, resonances A, B covers dipole resonances. Moreover, the inset in Fig. 1b shows high directivity in one direction. Consequently, this nanostructure supports strong magnetic and electric response as well as unidirectional scattering in the visible range.

Fig. 2 represents the ultrahigh Purcell factor achieved in the sphere-gap-cone nanostructure. As usual, when dipole orientates along the z axis, we can obtain much higher Purcell factor (over 10000). If we change the dipole orientation to the x axis, the corresponding Purcell factor will reduce 40 times. Besides, this Purcell factor spectrum has a wide bandwidth resonance, which is promising for emission enhancements. In summary, we propose a novel hybrid sphere-gap-cone nanostructure, corresponding scattering cross section and near field distribution indicate this nanostructure provide strong magnetic and electric response as well as unidirectional scattering in the visible range. Besides, ultrahigh Purcell factor is achieved by placing a discrete dipole in the gap in various orientations.  Thus this proposed type of hybrid nanostructure can be applied for sensing as well as optical modulators.