(a) Schematics of the QD-loaded nanoantenna excited by a polarization-controlled light beam. (b) Simulated spectral dispersions and spatial distributions of the local-field responses under x-polarized and y-polarized excitation. (c,d) Simulated spectral dispersions of local-field responses under elliptically polarized excitation. The spectra exhibit Fano lineshapes with tunable Fano asymmetry parameter q and nearly vanishing Fano dips. Local-field distributions show that at the Fano dips the hot spot at the nanogap can be turned into a cold spot. Credit: Juan Xia, Jianwei Tang, Fanglin Bao, Yongcheng Sun, Maodong Fang, Guanjun Cao, Julian Evans, and Sailing He

Optical nanoantennas can convert propagating light to local fields. The local-field responses can be engineered to exhibit nontrivial features in spatial, spectral and temporal domains. Local-field interferences play a key role in the engineering of local-field responses. By controlling local-field interferences, researchers have demonstrated local-field responses with various spatial distributions, spectral dispersions and temporal dynamics. Different degrees of freedom of the excitation light have been used to control local-field interferences, such as polarization, beam shape and beam position, and incidence direction. Despite remarkable progress, achieving fully controllable local-field interferences remains a major challenge. A fully controllable local-field interference should be controllable between a constructive interference and a complete destructive interference. This would bring unprecedented benefit for the engineering of local-field responses.

In a new paper published in Light Science & Applications, a team of scientists from China, led by Professor Sailing He from Zhejiang University and Professor Jianwei Tang from Huazhong University of Science and Technology, have experimentally demonstrated that based on a fully controllable local-field designed in the nanogap of a nanoantenna, a local-field hot spot can be turned into a cold spot, and the spectral dispersion of the local-field can exhibit dynamically tunable Fano lineshapes with nearly vanishing Fano dips. By simply controlling the excitation , the Fano asymmetry parameter q can be tuned from negative to positive values, and correspondingly, the Fano dip can be tuned across a broad wavelength range. At the Fano dips, the local-field intensity is strongly suppressed by up to ~50-fold.

The nanoantenna is an asymmetric dimer of colloidal gold nanorods, with a nanogap between the nanorods. The local-field response in the nanogap has the following features: First, a local field can be excited by both orthogonal polarizations; second, the local-field polarization has a negligible dependence on the excitation polarization; third, the local-field response is resonant for one excitation polarization, but nonresonant for the orthogonal excitation polarization. The first two features make the local-field interferences fully controllable. The third feature further enables Fano-shaped local-field responses.

For experimental study of the local-field responses, it is crucial to probe the local fields at specified spatial and spectral positions. The scientists use a single quantum dot as a tiny sensor to probe the local-field spectrum in the nanogap of the nanoantenna. When the quantum dot is placed in the local field, it is excited by the local field, and its photoluminescence intensity can reveal the local-field response through comparison with its photoluminescence intensity excited directly by the incident light.

(a) AFM image of the fabricated QD-loaded nanoantenna. Upper inset: TEM image of GNRs (scale bar, 50 nm); Lower inset: TEM image of a silica-encapsulated QD (scale bar, 30 nm). (b) Local-field spectral responses under x- and y-polarized excitation. (c) Local-field spectral responses under elliptically polarized excitations. The spectra can be fitted by Fano lineshapes with q values given. The Fano dips have nearly vanishing responses, where the local-field intensity can be strongly suppressed by up to ~50-fold, implying that the hot spot in the nanogap can be turned into a cold spot. Credit: Juan Xia, Jianwei Tang, Fanglin Bao, Yongcheng Sun, Maodong Fang, Guanjun Cao, Julian Evans, and Sailing He

Superb fabrication techniques are needed to fabricate such a tiny nanoantenna and put the tiny quantum dot sensor into the nanogap. The scientists use the sharp tip of an atomic force microscope (AFM) to do this job, pushing nanoparticles together on a glass substrate.

The scientists summarized the relevance of their work: "Turning a local-field hot spot into a cold spot significantly expands the dynamic range for local-field engineering. The demonstrated low-background and dynamically tuneable Fano-shaped local-field responses can contribute as to the toolbox for spatial, spectral and temporal local-field engineering."

"More importantly, the low background and high tunability of the Fano lineshapes indicate that local-field interferences can be made fully controllable. Since local-field interferences play a key role in the spatial, spectral and temporal engineering of the local-field responses, this encouraging conclusion may further inspire diverse designs of local-field responses with novel spatial distributions, spectral dispersions and temporal dynamics, which may find application in nanoscopy, spectroscopy, nano-optical quantum control and nanolithography."

More information: Juan Xia et al, Turning a hot spot into a cold spot: polarization-controlled Fano-shaped local-field responses probed by a quantum dot, Light: Science & Applications (2020). DOI: 10.1038/s41377-020-00398-1

Journal information: Light: Science & Applications