Light matter interaction in hybrid plasmonic/photonic nanogaps

Viscomi, Francesco

Abstract

The aim of this thesis is to study the processes of light matter interaction at the nanoscale in hybrid nano gaps that are made from both metals and dielectrics. This approach enables the possibility to use both the optical properties of a dielectric, such as low losses and high-quality factor, with the small mode volume typical of a metal. High quality factor and small modal volume together make a high Purcell factor, which is the enhancement of the spontaneous emission rate due to the surrounding cavity environment. Both the size and the time scales involved in this study range in the nanometre and nano second, respectively.

The architecture used for the study of the hybrid nano gaps consists of a substrate containing a Distributed Bragg Reflector (DBR) and a 10 nm thick emitting layer. On top of this layer lies a small concentration of gold nano spheres. Two different emitting dipole orientations have been studied, vertical and horizontal. The vertical orientation is parallel to the nano gap dipole moment while in the case of the horizontal orientation, it is perpendicular to it. These two emitting dipole orientations have been used to perform two different experiments exploiting different properties of the DBR. DBRs have been used for two purposes, reflectors and 1-d photonic crystals. These two applications are used to investigate different properties of the hybrid nano gaps. Indeed, DBRs have a highly reflective spectral region called photonic stopband, outside of it there are some highly localised reflectivity minima called Bragg modes.

The first hybrid nano gap application explored is the directional nano antenna. In this approach the DBR is used as a reflector and the nano cavity is used to control the direction of the emission. Because of the Fermi golden rule, the dipole moment of the emitter and the nano gap must be parallel to achieve the largest coupling possible. The dipole orientation parallel to the cavity dipole moment is called vertical and it has been probed using the emitter Lumogen Red. This dye exhibits a high quantum yield, low photo bleaching and a good vertical orientability when spun on a surface in form of a film. In this configuration, the light is emitted by the layer at very large angle compared to the surface, roughly 60 degrees. The system can measure up to 64 degrees since the objective numerical aperture is 0.9/1. In this nanogap the nanoparticle acts like a directional antenna and 65% of the emitted light gets redirected at angles not accessible by emitters on their own. Spectrally dispersed k-space imaging has been used to perform such a measurement. This study has demonstrated how the light emission cone is a function of the nano particle size. The narrowest emission cone observed was found to occur for a 500 nm diameter particle size. This configuration showed an enhancement emission factor ranging from 30 up to 60.

The second nano gap configuration used the DBR as photonic crystal to achieve localised Tamm plasmon generation. These results are described in chapter 6. The minimum in the reflectivity spectrum of the DBR is called the 1st Bragg mode. In this mode the impinging radiation can penetrate inside the stack and not propagate outside. Tamm states are surface states that can be excited at the interface between a DBR stack and a metal film. Super Tamm are more localised Tamm states that can be excited only by replacing the metallic film with a finite structure such as a micro disk. In this thesis, a new form of localised super Tamm states has been excited. This novelty state has been named Isolated super Tamm modes. The disk has been replaced with a gold nano sphere. Isolated super Tamm modes have been proved to have an intermediate spectral position between the 1st Bragg mode and the super Tamm .

Citation

Viscomi, F. (2020). Light matter interaction in hybrid plasmonic/photonic nanogaps. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4223352

• Thesis Type: Thesis
• Deposit Date: Aug 10, 2021
• Publicly Available Date: Feb 23, 2023
• Keywords: Physics
• Public URL: https://hull-repository.worktribe.com/output/4223352
• Additional Information: Department of Physics and Mathematics, The University of Hull
• Award Date: Aug 1, 2020