Tailoring optical pulling force on gain coated nanoparticles with nonlocal effective medium theory

Tailing Optical Pulling Force on a Metal-Dielectric Hybrid Dimer with Electromagnetic Coupling

In this work, we demonstrate that optical pulling forces (OPFs) can be induced by a hybrid dimer consisting of a Si nanoparticle (NP) and a coated nanoparticle with a gain core and Au shell under normal plane wave illumination. Analytical theory reveals that the underlying physical mechanism relies on interactions between the electric dipole (ED) modes excited in the NPs. As compared with the individual NP, it is found that the magnitude of optical force can be enlarged by almost three orders for the Si NP and one order for the coated gain NP in the coupled dimer. In addition, we find that the OPFs exerted on the NPs are heavily dependent on the gain level of the core materials, the incident polarization angle and the sizes of the NPs. More interestingly, we find that the OPF can also be exerted on a trimer system consisting of two identical Si NPs and a coated NP arranged in a line. The related results could be used to propose a versatile platform for manipulating NPs.

Nonlocal composite metamaterial in calculation of near-field thermal rectification

We consider the near-field radiative heat transfer (NFRHT) between a semi-infinite polar dielectric plate and a composite metamaterial containing semiconductor nanoparticles with spatial dispersion or nonlocality. We show that the enhancement of near-field heat radiation is found owing to the strong coupling of surface phonon polaritons (SPhPs) excited between polar dielectric plate and air, and localized surface plasmon polaritons (LSPPs) in the composite metamaterials. The introduction of nonlocality is helpful to enhance the NFRHT and even the rectification efficiency significantly when the temperature difference is small. For a large volume fraction of semiconductor nanoparticles, the thermal rectification efficiency can be larger than 90% at a small temperature difference within 150 K. Our findings may pave a way for thermal devices based on composite metamaterial containing semiconductor nanoparticles.

Optical Pulling Using Chiral Metalens as a Photonic Probe

Optical pulling forces, which can pull objects in the source direction, have emerged as an intensively explored field in recent years. Conventionally, optical pulling forces exerted on objects can be achieved by tailoring the properties of an electromagnetic field, the surrounding environment, or the particles themselves. Recently, the idea of applying conventional lenses or prisms as photonic probes has been proposed to realize an optical pulling force. However, their sizes are far beyond the scope of optical manipulation. Here, we design a chiral metalens as the photonic probe to generate a robust optical pulling force. The induced pulling force exerted on the metalens, characterized by a broadband spectrum over 0.6 μm (from 1.517 to 2.117 μm) bandwidth, reached a maximum value of −83.76 pN/W. Moreover, under the illumination of incident light with different circular polarization states, the longitudinal optical force acting on the metalens showed a circular dichroism response. This means that the longitudinal optical force can be flexibly tuned from a pulling force to a pushing force by controlling the polarization of the incident light. This work could pave the way for a new advanced optical manipulation technique, with potential applications ranging from contactless wafer-scale fabrication to cell assembly and even course control for spacecraft.

Effective optical properties of nanoparticle-mediated surface plasmon resonance sensors

We investigate effective medium properties of nanoparticles (NPs) in surface plasmon (SP) resonance detection. Attention was paid to the effective medium characteristics with respect to the particle distribution in equally spaced, aggregated, and intermediate models, although effects of other parameters such as size, material, and concentration were also explored. The results suggest that the distribution may cause significant measurement deviation by as much as 20% for gold NPs and less than 5% for silica. Particle concentration showed complicated dependence in the effective medium. Different mechanisms were observed to govern effective medium properties of dielectric and metal NPs, SP mode transition and multiple scattering for silica NPs. In contrast, metal damping dominated resonance characteristics for gold NPs. The results are expected to provide fresh insights on how to apply an effective medium and interpret measured data in SP resonance sensors and beyond.

Tunable optical pulling force mediated by resonant electromagnetic coupling

In terms of the law of momentum conservation, the optical pulling force (OPF) is a counterintuitive phenomenon for optical manipulation. We investigate analytically and numerically the tunable OPF exerted on the low refractive index nanoparticle (NP) in a hybrid dimer system when it is illuminated by a plane wave based on the coupled dipole approximation method and the finite-difference time-domain method, respectively. The underlying physical mechanism relies on the near-field electromagnetic coupling between the low refractive index dielectric NP and the plasmonic NP. We further evaluate the dependence of the OPF on the geometrical parameters of the system. It is also numerically demonstrated that a Gaussian beam can be used to achieve pure OPF with no transverse force component. The proposed OPF offers an additional degree of freedom for optical sorting, transport, and trapping of NPs.

Giant enhancement of reflectance due to the interplay between surface confined wave modes and nonlinear gain in dielectric media

We study theoretically the interplay between the surface confined wave modes and the linear and nonlinear gain of the dielectric layer in the Otto configuration. The surface confined wave modes, such as surface plasmons or waveguide modes, are excited in the dielectric-metal bilayer by obliquely incident p waves. In the purely linear case, we find that the interplay between linear gain and surface confined wave modes can generate a large reflectance peak with its value much greater than 1. As the linear gain parameter increases, the peak appears at smaller incident angles, and the associated modes also change from surface plasmons to waveguide modes. When the nonlinear gain is turned on, the reflectance shows very strong multistability near the incident angles associated with surface confined wave modes. As the nonlinear gain parameter is varied, the reflectance curve undergoes complicated topological changes and sometimes displays separated closed curves. When the nonlinear gain parameter takes an optimally small value, a giant amplification of the reflectance by three orders of magnitude occurs near the incident angle associated with a waveguide mode. We also find that there exists a range of the incident angle where the wave is dissipated rather than amplified even in the presence of gain. We suggest that this can provide the basis for a possible new technology for thermal control in the subwavelength scale.