Abstract

Plasmonic nanoantennas have been extensively explored to boost nonlinear optical processes due to their capabilities to confine optical fields on the nanoscale. In harmonic generation, nanoantenna array architectures are often employed to increase the number of emitters in order to efficiently enhance the harmonic emission. A small laser focus spot on the nanoantenna array maximizes the harmonic yield since it scales nonlinearly with the incident laser intensity[1, 2]. However, the nonlinear yield of the nanoantennas lying at the boundary of a focused beam may exhibit significant deviations in comparison to those at the center of the beam due to the Gaussian intensity distribution of the beam. This spatial beam inhomogeneity can cause power instability of the emitted harmonics when the lateral beam position is not stable which we observed in plasmon-enhanced third-harmonic generation (THG). Hence, we propose a method for deterministically designing the density of a nanoantenna array to decrease the instability of the beam position-dependent THG yield. This method is based on reducing the ratio between the number of ambiguous nanoantennas located at the beam boundary and the total number of nanoantennas within the beam diameter to increase the plasmon-enhanced THG stability, which we term as the ratio of ambiguity (ROA)[3]. We find that the coefficient of variation of the measured plasmonic THG yield enhancement decreases with the ROA. Our results provide a useful parameter for judging system reliability by revealing the maximum allowable pitch distance to satisfy the desired power stability. Then, the minimum pitch distance can be determined by considering other design requirements such as coupling effect, cost efficiency, or fabrication capability. For example, the unexpected coupling effect can occur when the pitch distance between nanoantennas is too short. Previous research reported that the plasmonic coupling effect decays over a length roughly 0.2 times the particle’s length[4, 5]. Thus, our method is beneficial for designing reliable sensors or nonlinear optical devices consisting of nanoantenna arrays for enhancing output signals.

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