Unlike conventional optics, scanning near-field optical microscopy (SNOM) overcomes the Rayleigh criterion and can therefore achieve better resolutions than conventional optical microscopes. This feature is utilized to measure the optical properties of different silver particle distributions on a glass surface. This paper mainly lays focus on intensity correction of the optical data due to topographical artifacts, analysis of plasmonic behavior and a tentative representation of the optical data.
The simple approach for optical artifact correction has been shown to yield qualitative success, with necessity of improvement for quantitative results. Given the conditions of the experiment, it has also been observed that plasmonic coupling seems to have a greater impact on the small observed particles. The tentative representation of the optics suggests that the larger particles are able to emit light by absorption of electromagnetic energy from their surrounding.
Table of Contents
1 Introduction
2 Physical Principles
2.1 Near and Far Field
2.1.1 Limitation of Far-Field Measurement
2.1.2 Evanescent Wave Measurement via Slit
2.2 Evanescent Waves and Plasmons
2.3 Plasmon Resonance from Theory
2.4 SNOM Theory
2.4.1 Aperture-Mode
2.4.2 Shear Force Feedback
2.5 Topographic Artifacts
3 Setup
4 Samples
5 Experimental
5.1 Preparation of Measurement
5.2 Measurement Procedure
5.3 Calibration and Tip Properties
5.4 Results
6 Discussion
6.1 Optical Correction by Height
6.2 Correlation between Topography & Near-Field Optics
6.3 Analysis of Contractive Effects - Emission / Absorption
7 Summarizing Conclusion
8 Outlook
9 Appendices
9.1 Equipment
9.2 Program for Data Correction and Display
Research Objectives and Core Topics
The primary objective of this thesis is to characterize the optical properties of metallic nanoparticle distributions on a glass substrate using Scanning Near-field Optical Microscopy (SNOM). By simultaneously measuring topography and optical signals, the research aims to overcome the far-field resolution limit imposed by the Rayleigh criterion and analyze the plasmonic behavior of silver nanoparticles, with a specific focus on correcting topographical artifacts in the optical data.
- Theoretical analysis of near-field detection and the Rayleigh resolution limit.
- Experimental implementation of simultaneous reflection and transmission mode SNOM.
- Development of methods for topographical artifact correction and intensity normalization.
- Characterization of plasmonic coupling and resonance effects in silver nanoparticles.
- Correlative analysis between topographical features and near-field optical intensity.
Excerpt from the Thesis
6.2 Correlation between Topography & Near-Field Optics
To make statements on the plasmonic behavior, it is beneficial to know the average particle resonance. This can be determined from figure (6) - the average particle radius R and used laser frequency of 532 nm have been plotted. The particles of sample C6 have an average radius of (110±25) nm thus having an average dipole resonance wavelength of about (800±200) nm in air or (1200±300) nm in glass. The quadrupole resonance wavelengths in air are approximately (420 ± 60) nm and (600 ± 100) nm in glass. It follows that only quadrupole excitations of relatively large (2 standard deviations) particles should occur for single particles. But for the most part, the present particles lie in between dipole and quadrupole resonance making behavioral forecast difficult. The particles can behave red-shifted towards quadrupole and blue shifted towards dipole resonance. The behavior due to plasmonic coupling with the surrounding may therefore be of more significance.
First, the four large particles of figure (25), located at (x/y) = (0.5,0.5), (0.2,0.6), (1.3,0.1), (1.7,0.7) shall be observed. It can be seen that all of these particles scatter constructively, creating high transmission where they are located. This behavior can also be noticed in the reflection image. This is typical for large particles driven by red shifted light [22]. The lowest transmission regions are all located near these large particles indicating them to absorb light from these areas. Thus indicating a large distance of interaction.
Summary of Chapters
1 Introduction: Provides an overview of SNOM as a technique for transcending the Rayleigh criterion and outlines the study's focus on analyzing silver nanoparticle distributions.
2 Physical Principles: Covers the theoretical background of near-field optics, evanescent waves, plasmon resonance, and the operation of the SNOM, including topographical artifact formation.
3 Setup: Details the experimental hardware configuration, including the laser source, fiber-based aperture tip, and the scanning apparatus.
4 Samples: Describes the preparation and properties of the silver nanoparticle samples on glass, analyzed via scanning electron microscopy.
5 Experimental: Documents the practical aspects of the research, including measurement preparation, procedural steps, calibration using test gratings, and presentation of the raw results.
6 Discussion: Analyzes the experimental data, focusing on height-based optical correction, the correlation between physical topography and optical signals, and plasmonic emission/absorption effects.
7 Summarizing Conclusion: Reviews the main findings, noting the successful correlation of optical and topographical images and identifying areas for improvement in quantitative accuracy.
8 Outlook: Suggests future directions, including analyzing individual particle structures and improving light intensity coupling to refine experimental resolution.
9 Appendices: Lists the specific laboratory equipment used and provides the source code for the data correction and visualization programs.
Keywords
Scanning Near-field Optical Microscopy, SNOM, Plasmon Resonance, Evanescent Waves, Rayleigh Criterion, Silver Nanoparticles, Topographical Artifacts, Intensity Correction, Near Field, Far Field, Shear Force Feedback, Nanophotonics, Spectroscopy, Light Scattering, Optical Resolution.
Frequently Asked Questions
What is the core focus of this bachelor thesis?
The thesis investigates the optical characterization of metallic nanoparticle distributions using SNOM to surpass the conventional far-field resolution limit.
What are the primary technical challenges mentioned in the work?
Key challenges include detecting weak evanescent signals, mitigating topographical artifacts in optical images, and managing consistent tip performance.
What is the main scientific objective?
The goal is to map the optical properties, specifically plasmonic behaviors like absorption and emission, of silver nanoparticles relative to their physical dimensions.
Which methodology is employed for imaging?
The study utilizes aperture-mode SNOM in simultaneous transmission and reflection configurations, employing shear force feedback to maintain a constant tip-sample distance.
What does the discussion section emphasize?
It concentrates on sample C6 to demonstrate an empirical height-based correction method for optical data and analyzes the correlation between nanoparticle positioning and light scattering.
Which keywords best describe the research?
The core concepts are SNOM, plasmon resonance, evanescent waves, nanoparticle characterization, and optical resolution enhancement.
How is the tip radius determined in the study?
The tip radius is estimated using test gratings and further validated by analyzing observed particle sizes in the topographical images against theoretical geometric models.
Why are standard optical images insufficient for this study?
Standard microscopes are limited by the Rayleigh criterion, preventing the resolution of sub-wavelength features that the SNOM can detect via near-field interactions.
What is the significance of the "Aperture-Mode" mentioned in the setup?
Aperture-mode uses a metal-coated fiber tip to restrict light emission to an area smaller than the wavelength, which is essential for capturing evanescent waves.
- Quote paper
- Steven Kämmer (Author), 2014, Characterisation of metallic particle distributions by scanning near-field optical microscopy (SNOM) in simultaneous reflection and transmission mode, Munich, GRIN Verlag, https://www.grin.com/document/281073
