Forward to Backward Scattering Ratio of Dielectric–Metal Heterodimer Suspended in Almost Free-Space

摘要

A designed nanoantenna exhibiting unidirectional scattering is a promising building block for novel nanophotonic devices. For device applications using nanoantennas, quantitative information on the directional scattering property is indispensable. However, experimental determination of the forward to backward scattering intensity ratio of a nano-object is not straightforward. Here, a scheme to determine the ratio quantitatively is proposed. A key technology of the scheme is placing a nano-object on a thin SiO2 membrane. Because of the low refractive index and the small thickness of the membrane, the nano-object can be treated as being suspended in free-space. This allows us to use an ideal Mie scatterer such as a spherical silicon (Si) nanocrystal as a reference to calibrate optical setups for the measurements of forward and backward scattering. Using the developed scheme, the directional scattering property of a heterodimer composed of Si and gold nanospheres synthesized by a solution process is investigated. By combining transmission electron microscopy and scattering measurements, systematic studies on the directional scattering property of a heterodimer by changing the size of a Si nanosphere is performed. It is demonstrated that strong Kerker-type forward scattering is achieved by tuning the resonance wavelength of the magnetic dipole resonance of a Si nanosphere.