Gallium (Ga) is not only a metal possessing surface plasmon resonance in the ultraviolet region but also a phase‐change material with very low melting point. Spherical Ga nanoparticles composed of liquid Ga cores and solid Ga2O3 shells, which exhibit surface plasmonic resonances spanning the ultraviolet to near‐infrared spectral region, are fabricated by using femtosecond laser ablation. Under the excitation of femtosecond laser light, only second harmonic generation is observed in the Ga nanospheres placed on a glass substrate. In sharp contrast, white light emission can be generated in such Ga nanospheres when they are placed on a thin silver film. It is revealed numerically and confirmed experimentally that a Fano resonance originating from the interference between the mirror‐image‐induced magnetic dipole mode and the gap plasmon mode is formed in the backward scattering spectra of the Ga nanospheres. Efficient white light emission can be achieved by resonantly exciting either the magnetic dipole resonance or the Fano resonance. Based on spectral analysis, the white light emission is identified as the hot‐electron intraband luminescence. The fabricated core–shell Ga nanospheres provide an ideal platform on which the optical properties and practical applications of liquid Ga nanoparticles can be systematically investigated.
We propose an all-silicon-based nano-antenna that functions as not only a wavelength demultiplexer but also a polarization one. The nano-antenna is composed of two silicon cuboids with the same length and height but with different widths. The asymmetric structure of the nano-antenna with respect to the electric field of the incident light induced an electric dipole component in the propagation direction of the incident light. The interference between this electric dipole and the magnetic dipole induced by the magnetic field parallel to the long side of the cuboids is exploited to manipulate the radiation direction of the nano-antenna. The radiation direction of the nano-antenna at a certain wavelength depends strongly on the phase difference between the electric and magnetic dipoles interacting coherently, offering us the opportunity to realize wavelength demultiplexing. By varying the polarization of the incident light, the interference of the magnetic dipole induced by the asymmetry of the nano-antenna and the electric dipole induced by the electric field parallel to the long side of the cuboids can also be used to realize polarization demultiplexing in a certain wavelength range. More interestingly, the interference between the dipole and quadrupole modes of the nano-antenna can be utilized to shape the radiation directivity of the nano-antenna. We demonstrate numerically that radiation with adjustable direction and high directivity can be realized in such a nano-antenna which is compatible with the current fabrication technology of silicon chips.
We investigated theoretically and numerically the optical pulling and pushing forces acting on silicon (Si) nanospheres (NSs) with strong coherent interaction between electric and magnetic resonances. We examined the optical pulling and pushing forces exerted on Si NSs by two interfering waves and revealed the underlying physical mechanism from the viewpoint of electric- and magnetic-dipole manipulation. As compared with a polystyrene (PS) NS, it was found that the optical pulling force for a Si NS with the same size is enlarged by nearly two orders of magnitude. In addition to the optical pulling force appearing at the long-wavelength side of the magnetic dipole resonance, very large optical pushing force is observed at the magnetic quadrupole resonance. The correlation between the optical pulling/pushing force and the directional scattering characterized by the ratio of the forward to backward scattering was revealed. More interestingly, it was found that the high-order electric and magnetic resonances in large Si NSs play an important role in producing optical pulling force which can be generated by not only s-polarized wave but also p-polarized one. Our finding indicates that the strong coherent interaction between the electric and magnetic resonances existing in nanoparticles with large refractive indices can be exploited to manipulate the optical force acting on them and the correlation between the optical force and the directional scattering can be used as guidance. The engineering and manipulation of optical forces will find potential applications in the trapping, transport and sorting of nanoparticles.
Control and manipulation of radiation direction and directivity is highly desirable in future integrated optical circuits. Here, we investigate theoretically and numerically the scattering properties of a silicon nanosphere dimer illuminated by a focused radially polarized beam. As compared with single silicon nanospheres, a scattering peak with a significantly enhanced intensity and a dramatically reduced linewidth was observed in the scattering spectrum of the silicon nanosphere dimer. Relying on the multipole expansion method, it was revealed that the radiation at the scattering peak originates mainly from the total electric dipole and the magnetic quadrupole excited in the silicon nanosphere dimer. It was found that the radiation direction of the silicon dimer is parallel to its axis, implying a sharp (90°) bending of the radially polarized beam. In addition, the radiation directivity is significantly improved as compared with single silicon nanospheres because of the interference between the total electric dipole and magnetic quadrupole modes. For a homodimer composed of two identical silicon nanospheres, the scattering light is equally distributed in the two radiation directions. In comparison, the incident light is preferentially scattered to the small Si nanosphere for a heterodimer composed of two silicon nanospheres with different diameters. As a result, a unidirectional lateral scattering can be realized by using a single silicon nanosphere displaced appropriately from the focal point. Our findings are helpful for understanding the mode hybridization in silicon nanosphere dimers illuminated by a focused radially polarized beam and useful for designing photonic devices capable of manipulating the radiation direction and directivity of structured light.
To detect the magnetic component of arbitrary unknown optical fields, a candidate probe must meet a list of demanding requirements, including a spatially isotropic magnetic response, suppressed electric effect, and wide operating bandwidth. Here, we show that a silicon nanoparticle satisfies all these requirements, and its optical magnetism driven multiphoton luminescence enables direct mapping of the magnetic field intensity distribution of a tightly focused femtosecond laser beam with varied polarization orientation and spatially overlapped electric and magnetic components. Our work establishes a powerful nonlinear optics paradigm for probing unknown optical magnetic fields of arbitrary electromagnetic structures, which is not only essential for realizing subwavelength-scale optical magnetometry but also facilitates nanophotonic research in the magnetic light–matter interaction regime.
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