Imran Khan

Experimental Demonstration of Scattering Suppression in AuNP-Coated Dielectric Microspheres

Imran Khan — A proof of concept(POC) experiment

1. Introduction & Motivation

Plasmonic cloaking via nano-assembled AuNP shells has been extensively studied in modeling and simulation. This analysis is a experimental validation of these concepts through the fabrication, characterization, and optical measurement of AuNP-coated dielectric microbeads.

This analysis focuses on:

  • Fabrication of 3D plasmonic metashells
  • SEM analysis and surface coverage estimation
  • Integrating sphere measurements to quantify scattering suppression
  • Comparison of experimental results with simulation predictions
  • Discussion of discrepancies, limitations, and implications

This experimental workflow demonstrates an end-to-end proof-of-concept that the plasmonic cloaking effect predicted by simulations can be observed in physical systems.

schematic
Figure : Schematic of the plasmonic meta-structure. Spherical silica core (blue) decorated with randomly distributed gold nanoparticles (yellow).

2. Fabrication of Plasmonic Metastructures

2.1 Materials

  • Silica microspheres 500nm , and 700nm
  • 20 nm citrate-stabilized AuNPs
  • N-[3-(Trimethoxysilyl)propyl]ethylenediamine (TMSPA) for surface functionalization
  • Ethanol, DI water

2.2 Functionalization Procedure

  1. Silica microspheres were cleaned in ethanol/water mixtures.
  2. Surface functionalization with 1% TMSPA created a positively charged surface.
  3. Functionalized beads were washed to remove unbound silane compounds.

2.3 AuNP Coating

  1. Beads were suspended in AuNP solution.
  2. Electrostatic attraction enabled AuNP self-assembly onto the functionalized surface.
  3. Centrifuge duration controlled filling fraction.
  4. Suspensions were centrifuged and rewashed to remove excess AuNPs.

A monolayer plasmonic shell was formed through this self-assembly mechanism.

3. SEM Imaging & Filling Fraction Analysis

3.1 Sample Preparation

  • A drop of coated beads was placed on glass sides for optical characterization.
  • Samples were dried and sputter-coated with a thin layer (when required for charging mitigation) of a silicon substrate for SEM characterization.

3.2 Characterization

UV-Vis spectroscopy:

  • As-obtained AuNP from the manufacturer showed much higher absorbance compared to the TMSPA functionalized silica core.

SEM was used to:

  • BSD (backscatter detector) was used dominantly
  • verify coating uniformity
  • assess AuNP distribution
  • quantify surface coverage

Images were acquired at several magnifications between 5k× and 40k×.

sem_image_set_1
Figure : Scanning electron micrograph of bare silica cores and AuNP coated silica spheres. [top left] Bare silica sphere with diameter 500 nm. (top right) AuNP coated 500 nm silica spheres. (bottom left) 700 nm silica spheres coated with AuNPs. (bottom right) BSD image of 700 nm silica spheres coated with AuNP. The AuNPs appear brighter with respect to the background in BSD images.

Silica spheres showed varied distribution of AuNPs on the surface.

sem_image_set_2
Figure : Scanning electron micrograph(BSD) of 500 nm silica cores coated with 20nm AuNPs. [left] A Wide range of AuNP surface coverage on the silica surface [right] Single core-shell meta structure.

3.3 AuNP Filling Fraction Using ImageJ

  1. SEM image converted to grayscale.
  2. Thresholding applied to isolate nanoparticles.
  3. Particle counting performed using ImageJ’s Analyze Particles module.
  4. Surface coverage percentage computed as:
    \[ f = \frac{N_{\text{AuNP}} \, \pi r_{NP}^2}{4\pi a^2} \]

Typical filling fractions were 25–32%, consistent with simulation assumptions.

filling fractions
Figure : Surface coverage computation process via image processing using ImageJ. BSD image of a 500 nm core-shell structure with highlighted AuNPs(red). Removal of the background and, finally, obtaining AuNP clusters are shown from left to right.

4. Integrating Sphere Setup for Scattering Measurement

4.1 Measurement Concept

experiment_setup
Figure: Experimental schematic of scattering cross-section measurements.

An integrating sphere spectrophotometer was used to capture the total scattering.

4.2 Optical Setup

  • Tunable laser source
  • Monochromator
  • Integrating sphere with >97% reflectance
  • Sample port designed for drop-cast bead films

4.3 Sample Preparation

  • AuNP-coated and bare beads were drop-cast on glass slides
  • Dried to form thin monolayers/clusters
  • Ensured consistent sample morphology across runs

4.4 Calibration Process

  • Baseline (bare substrate)
  • Reference (BaSO₄ standard)

Ensured accurate scattering extraction independent of substrate/reflection contributions.

5. Scattering Measurement Procedure

  1. Mono chromatic light passed through the sample.
  2. Scattered light integrated by the sphere.
  3. Wavelength scanned from 350–800 nm.
  4. Measurements taken for both bare and coated beads.
  5. Suppression quantified as:
    \[ S(\lambda) = \frac{\sigma_{\mathrm{sca,\,coated}}(\lambda)}{\sigma_{\mathrm{sca,\,bare}}(\lambda)} \]

6. Experimental Results

experiment_set_1
Figure : [left]Measure scattering cross-section of 500 nm bare silica sphere (black dots) and AuNP coated silica sphere (red dots) of the same size. [right]Simulated scattering cross-section of the bare core and core-shell structures for 30% filling fraction.
experiment_set_2
Figure : [left]Measured scattering cross-section (left) of 700 nm bare silica sphere (black dots) and AuNP coated silica sphere (red dots) of the same size. [right]Simulated scattering cross-section of the bare core and core-shell structures for 30% filling fraction.

Key Findings

  • Clear scattering reduction observed in 400–600 nm, corresponding to AuNP plasmon resonance.
  • Suppression magnitude: 10–20% relative to bare beads.
  • Enhancement above 650 nm consistent with simulation predictions.
  • Spectral dip matches the AuNP extinction spectrum, confirming plasmonic contributions.

7. Comparison with Simulation Predictions

Aspect Simulation Experiment
Suppression Window 380–600 nm 420–560 nm
Dip Magnitude 30–40% 10–20%
Angular Redistribution Strong forward bias Weak but present
NP Distribution uniform partial clustering
Filling Fraction controlled measured 25–32%

Agreement: Band location + physical trend.
Differences: Due to fabrication variability + clustering + substrate effects.

8. Physical Interpretation

8.1 Multiple Scattering + Absorption

AuNPs absorb and re-radiate, damping coherent Mie oscillations.

8.2 Cluster-Induced Plasmonic Coupling

Clusters increase local plasmonic interactions → modifies extinction behavior.

8.3 Substrate Interaction

Drop-cast films couple to substrate, shifting scattering minima.

8.4 Non-Uniform Filling Fraction

Ideal monolayer assumption breaks, reducing suppression amplitude.

9. Limitations & Future Work

Limitations

  • Drop-cast geometry not uniform
  • SEM only captures top hemisphere
  • Substrate modifies scattering behavior
  • AuNP clustering not controlled

Future Improvements

  • Microfluidic monolayer assembly
  • Spin-coating for uniform films
  • Angular-resolved dark-field scattering
  • Coupled Maxwell simulations including substrate
  • Machine-learning surrogate models for morphology–scattering mapping

10. Summary

This experimental investigation:

  • Demonstrates scattering suppression using AuNP-coated silica spheres
  • Confirms spectral alignment with plasmonic extinction band
  • Validates simulation predictions qualitatively
  • Proves feasibility of plasmonic cloaking concepts in physical systems

This bridges simulation ↔︎ fabrication ↔︎ measurement, showcasing full-stack experimental and computational capability.

Acknowledgments

I would like to thank Dr.Zachary Petrek from the Chemistry Department for his valuable insight into optimizing AuNP surface functionalization and improving nanoparticle yield during the self-assembly process. His contributions were essential to the fabrication and characterization workflow used in this experimental study.

Citation (PhD Dissertation):

Khan, M. I. (2021). Tuning far-field light–matter interactions using three-dimensional plasmonic meta-structures. Ph.D. Dissertation, University of California, Merced. https://escholarship.org/uc/item/62q0s8f8

© 2025 Imran Khan