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Abstract
Abstract Shaping the emission pattern of second harmonic (SH) generation from plasmonic nanoparticles is important for practical applications in nonlinear nanophotonics but is rendered challenging by the complex second-order nonlinear-optical processes. Here, we theoretically and experimentally demonstrate that a pair of V- and Y-shaped gold nanoparticles directs the SH emission perpendicularly to an incident light direction. Owing to spatial overlap of two orthogonal plasmonic dipole modes at the fundamental and SH wavelengths of the individual particles, surface SH polarizations induced by the fundamental field is efficiently near-field coupled to the SH plasmon mode, resulting in dipolar SH emission from the individual particles. Moreover, the phase of this emission can be tuned simply by altering the part of the Y-particle shape, which changes the SH plasmon resonance while keeping the fundamental resonance. Our approach is a promising platform for engineering not only directional nonlinear nanoantennas but also nonlinear metamaterials.
Highlights
Controlling the nonlinear optical processes in nanoparticles is critical for practical applications in imaging [1,2,3,4], labelfree sensing [5], and the selective identification [6, 7] of second-order nonlinear-optical effects are forbidden in centrosymmetric materials such as metals within the electric-dipole approximation [23], a second harmonic (SH) polarization that radiates the electromagnetic field with doubled frequency of the incident light is generated at the metal surface where this symmetry is locally broken
From the selection rules for SH generation of isolated single metal nanoparticles, which are derived from parity conservation laws [33, 34], the general SH-emission patterns are based on quadrupoles and clearly differ from the dipolar scattering pattern in linear response [19, 22, 35, 36]
We experimentally demonstrated that a pair of V- and Y-shaped gold nanoparticles enables unidirectional SH emission perpendicularly to the incident light direction
Summary
Controlling the nonlinear optical processes in nanoparticles is critical for practical applications in imaging [1,2,3,4], labelfree sensing [5], and the selective identification [6, 7] of second-order nonlinear-optical effects are forbidden in centrosymmetric materials such as metals within the electric-dipole approximation [23], a SH polarization that radiates the electromagnetic field with doubled frequency of the incident light is generated at the metal surface where this symmetry is locally broken. As SH polarizations are primarily induced by the component of the electric field normal to the surface at the fundamental wavelength [24, 25], the surface shape and roughness directly affect the orientation of the SH polarizations. This wayward orientations of the SH polarizations are a common cause of uncontrollable SH emission patterns from metal nanoparticles. The localized plasmon resonances of nanoantennas can be engineered to shape the emission patterns of nanoscale light emitters. Some theoretical studies [30, 31] have shown that SH emission from a metal nanosphere with no plasmon resonance at the SH wavelength can be directionally controlled by applying other passive structures, such as dielectric nanoparticles or a large nanocup cavity as the reflector and
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