Friday 12th August 2022

In-plane coherent control of plasmon resonances for plasmonic switching and encoding

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**In-plane coherent control of plasmon resonances for plasmonic switching and encoding
Schematic diagrams of two setups for in-plane coherent control of plasmon resonances. a Fiber-waveguide interferometer. b Dark-field (DF) confocal microscope, where quarter illumination can be satisfied by blocking 3/4 area of the annular aperture. Credit: Light: Science & Applications, doi: https://doi.org/10.1038/s41377-019-0134-1

Light incident on metallic nanoparticles can initiate the collective motion of electrons, causing a strong amplification of the local electromagnetic field. Such plasmonic resonances have significant roles in biosensing with ability to improve the resolution and sensitivity required to detect particles at the scale of the single molecule. The control of plasmon resonances in metadevices have potential applications in all-optical, light-with-light signal modulation and image processing. Reports have demonstrated the out-of-plane coherent control of plasmon resonances by modulating metadevices in standing waves. In optical devices, light can be transferred along the surfaces for the unprecedented control of plasmons. When oscillations in conducting electrons are coupled with light photons, localized surface plasmon resonances (LSPR) can act as information carriers for nano-sized optical sensors and in computers.

In a recent study, Liyong Jiang and co-workers at the Nanjing University of Science and Technology demonstrated two methods for in-plane illumination of LSPRs as a proof-of-principle in gold nanodisks. The results of their work showed that the LSPRs could be switched into different states by adjusting the incident light to encode logical data into chains in a manner that was hitherto not possible with out-of-plane illumination. The results are now published in Light: Science & Applications.

Significant efforts in the past decade were devoted to study light-matter interactions at the nanoscale in plasmonic systems. The ability to control LSPR has led to many practical applications, including pioneering examples such as:

  1. Surface-enhanced Raman Scattering
  2. Plasmon waveguides
  3. Molecular rulers
  4. Biosensing and bioimaging
  5. Nanolasers
  6. Plasmonic holography
  7. Tunnel junctions, and
  8. Metalens.
**In-plane coherent control of plasmon resonances for plasmonic switching and encoding
In-plane coherent control of plasmon resonances in gold nanodisk monomers. a, b Calculated normalized absorption spectra of gold nanodisk monomers with a diameter ranging from 140 to 200 nm for s-polarized in-plane plan wave coming from the right side (dashed line) or both sides (solid line) without phase delay, or with a phase delay of π. “F” and “H” represent fundamental and high-order plasmon resonances. c–e The corresponding spatial distributions of electric-field amplitude |E|, real part Re(Ez), and imaginary part Im(Ez) for the “F” and “H” modes (square and circle signs) of the representative gold nanodisk monomer (D = 160 nm) under asymmetrical and symmetrical in-plane illumination. Under symmetrical in-plane illumination, we can observe phase delay-dependent destructive/constructive interference for the “F” and “H” modes. Credit: Light: Science & Applications, doi: https://doi.org/10.1038/s41377-019-0134-1

During the initial stages of development, scientists focused on controlling LSPR by designing configurations of the plasmonic nanostructures. They understood the size- and shape-dependent LSPR of single plasmonic nanoparticles and coupled plasmonic systems based on the classical Mie theory and well-established plasmonic hybridization models. Additionally, the light beam typically illuminated the sample surface from one direction in conventional optical studies of single and coupled nanoantennas.

Although the ability to control plasmon resonances via out-of-plane illumination has opened a new path to modulate signals, the process has shown limitations. As a result, Jiang et al. reported on in-plane coherent control of plasmon resonances in typical metallic nanoantennas. The scientists provided a proof-of-principle demonstration of plasmonic switching and encoding applications for single and coupled gold nanodisks.

To accomplish in-plane coherent control of plasmon resonances in the lab, the scientists proposed two possible experimental setups. One was based on a fiber-waveguide interferometer, which faced challenges during experiments. In comparison, the second method included a more convenient, widely used dark-field confocal microscopy setup. In this, the condition of completely symmetric in-plane illumination could be satisfied early when the input light focused onto the center of the sample. To construct asymmetric in-plane illumination, the scientists blocked three-fourth of the area of the annular aperture. Jiang et al. showed that the setup was suited to study plasmonic nanostructures with sizes comparable to the focused spot size of the incident light beam.

**In-plane coherent control of plasmon resonances for plasmonic switching and encoding
Demonstration of electrical-field distribution rule for the 200 nm gold nanodisk monomer and dimer by s-SNOM. a Schematic of the s-SNOM measurement for s–s and s–p excitation–collection configurations. The wavelength of the excitation laser is 633 nm and the incidence angle with respect to the plane of the substrate is 30°. b Calculated normalized absorption spectra of 200 nm gold nanodisk monomer and dimer at incidence angle 30° under asymmetrical (dashed line) or symmetrical (solid line) illumination without phase delay. The gap size in the dimer is 30 nm. c Atomic-force microscopic (AFM) images of gold nanodisk monomer and dimer for s–s and s–p measurements. The red arrow represents the incidence direction of the laser and the blue dashed line represents the central axis of the nanodisk. d, e Experimental and simulated spatial distributions of the amplitude |A|, phase ϕ, and real part of electric-field component Ey in s–s measurement and Ez in s–p measurement for 200 nm gold nanodisk monomer and dimer. Credit: Light: Science & Applications, doi: https://doi.org/10.1038/s41377-019-0134-1

To engineer the gold nanodisk samples on silicon dioxide/silica (SiO2/Si) substrates, Jiang et al. used electron-beam lithography (EBL) alongside a lift-off process. They completed the fabrication process by coating the substrate surface with a gold film and an underlying chromium (Cr) adhesion layer using electron-beam evaporation. The scientists then studied in-plane coherent control of plasmon resonances in the gold nanodisks and calculated the absorption spectra of gold nanodisk monomers ranging from diameters of 140 to 200 nm; fabricated on the SiO2/Si substrate surface.

In the work, they established and experimentally verified the distribution rule of electrical-field components to realize destructive and constructive plasmon resonances in an axisymmetric plasmonic nanostructure. They showed how the in-plane coherent control of plasmon resonances strongly relied on the configuration and symmetry of plasmonic nanostructures, compared with out-of-plane coherent control. This feature can allow freedom in tailoring and engineering multiple plasmon resonances in other axisymmetric plasmonic structures, which include nanospheres, nanorod, nano bowtie and nanostructure polymers.

**In-plane coherent control of plasmon resonances for plasmonic switching and encoding
Demonstration of plasmonic switching by dark field (DF) scattering measurement of gold nanodisk monomer and dimer. a Normalized DF scattering spectra of gold nanodisk monomer with a diameter of 200 nm (SEM image) under full and quarter illumination. b The corresponding normalized simulated scattering and absorption spectra. c, d Normalized measured and simulated DF scattering spectra of gold nanodisk dimer with a diameter of 200 nm and a gap size of 30 nm (SEM image) under full and quarter illumination. The red solid curves in c are the smoothing results. The scale bar in SEM images is 200 nm. e, f Polarization diagrams of full and quarter…

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