Strongly enhanced and directionally tunable second-harmonic radiation from a plasmonic particle-in-cavity nanoantenna

© 2016 American Physical Society.Second-harmonic (SH) generation is tremendously important for nonlinear sensing, microscopy, and communication systems. One of the great challenges of current designs is to enhance the SH signal and simultaneously tune its radiation direction with a high directivity. In contrast to the linear plasmonic scattering dominated by a bulk dipolar mode, a complex surface-induced multipolar source at the doubled frequency sets a fundamental limit to control the SH radiation from metallic nanostructures. In this work, we harness a plasmonic hybridization mechanism together with a special selection rule governing the SH radiation to achieve the high-intensity and tunable-direction emission by a metallic particle-in-cavity nanoantenna (PIC-NA). The nanoantenna is modelled with a first-principle, self-consistent boundary element method, which considers the depletion of pump waves. The giant SH enhancement arises from a hybridized gap plasmon resonance between the small particle and the large cavity that functions as a concentrator and reflector. Centrosymmetry breaking of the PIC-NA not only modifies the gap plasmon mode boosting the SH signal, but also redirects the SH wave with a unidirectional emission. The PIC-NA has a significantly larger SH conversion efficiency compared to existing literature. The main beam of the radiation pattern can be steered over a wide angle by tuning the particle's position.published_or_final_versio

Sum-frequency and second-harmonic generation from plasmonic nonlinear nanoantennas

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Enhanced second harmonic generation in a plasmonic particle-in-cavity nanoantenna

© 2016 IEEE.Plasmonic nanostructures that support surface plasmon resonances potentially provide a route for the development of nanoengineered nonlinear optical media. A strong enhancement of second harmonic generation (SHG) in a particle-in-cavity nanoantenna (PIC-NA) is proposed and theoretically demonstrated in this work. The fundamental and second harmonic responses of the PIC-NA are calculated using a self-consistent boundary element method. Through a comparison with the linear field enhancement spectra, the SHG from PIC-NA is boosted by the strongly enhanced fundamental and SH fields arising from the cascaded focusing of large optical energy into the small gap. The proposed 3D plasmonic PIC-NA functions as the new synthetic second-order nonlinear optical material that has promising applications in the nonlinear sensing, spectroscopy and frequency generating.Link_to_subscribed_fulltex

A new Coulomb gauge based electric field integral equation method

The electric field integral equation (EFIE) method is one of the most commonly-adopted computational electromagnetic methods. Its popularity stems from the efficient surface triangulation, excellent numerical precision, and the powerful capability of handling open and complex geometries. However, when the frequency tends to zero, the method-of-moment (MoM) solution of EFIE using the Rao-Wilton-Glisson (RWG) basis functions suffers from the low-frequency breakdown, where the contribution from the vector potential is extremely imbalanced with that from the scalar potential. As a result, the matrix representation of EFIE operator is highly ill-conditioned and cannot be inverted reliably and efficiently. © 2013 IEEE.Link_to_subscribed_fulltex

A novel beam-steering nonlinear nanoantenna with surface plasmon resonance

© 2016 IEEE.Plasmonic nanostructures that support surface plasmon (SP) resonance potentially provide a route for the development of nanoengineered nonlinear optical media. In this work, a novel plasmonic particle-in-cavity nanoantenna (PIC-NA) is proposed. The second harmonic generation (SHG) of the PIC-NA under strong localized SP resonance is systematically analyzed by a self-consistent numerical solution based on boundary element method (BEM). The developed method solves the fundamental field and second harmonic (SH) field together iteratively to capture their mutual coupling. Strong enhancement of SHG from PIC-NA is achieved. The SHG enhancement factor is around four orders of magnitude, which is much higher than those achieved in literatures. Moreover, unidirectional SH radiation for the asymmetric PIC-NA is realized. Its emission direction can be controlled by the location of the nanosphere. The proposed novel plasmonic PIC-NA functions as the new synthetic second-order nonlinear optical material that has promising applications in the nonlinear sensing, spectroscopy and frequency generation.Link_to_subscribed_fulltex

Carotenoids and Vitamin A in Breastmilk of Hong Kong Lactating Mothers and Their Relationships with Maternal Diet

Carotenoids and vitamin A are nutrients crucial to infants’ development. To date, there is limited data on their availability in breastmilk and the associated dietary factors, especially in Hong Kong, where people follow a westernized Chinese diet. This study determined the selected breastmilk’s carotenoid and vitamin A (retinol) contents by ultraperformance liquid chromatography with photodiode detection (UPLC-PDA) and the dietary intakes by three-day food records in 87 Hong Kong lactating mothers, who were grouped into tertiles based on their daily carotenoid intake. Low vitamin A intake (530.2 ± 34.2 µg RAE/day) and breastmilk retinol level (1013.4 ± 36.8 nmol/L) were reported in our participants, suggesting a poor vitamin A status of the lactating participants having relatively higher socioeconomic status in Hong Kong. Mothers in the highest tertile (T3) had higher breastmilk carotenoid levels than those in the lowest (T1) (p < 0.05). There were significant associations between maternal carotenoid intakes and breastmilk lutein levels in the linear regression models (p < 0.05) regardless of dietary supplement intake. Furthermore, maternal dark green vegetable intakes were associated with breastmilk retinol, lutein, and β-carotene levels. These findings can serve as dietary references for lactating mothers to enhance breastmilk carotenoid and vitamin A contents for the benefits of child growth and development