Guiding of electromagnetic energy in subwavelength periodic metal structures

Dissertation / Doktorarbeit von Stefan Maier

Abstract:

The ultimate miniaturization of optical devices requires structures that guide electromagnetic energy with a lateral confinement below the diffraction limit of light. In this thesis, the possibility of employing plasmon-polariton excitations in „plasmon waveguides“ consisting of closely spaced metal nanoclusters for this purpose is examined. The feasibility of energy transport with mode sizes below the diffraction limit of visible light over distances of several hundred nanometers is demonstrated.

As a macroscopic analogue to plasmon waveguides, the transport of electromagnetic energy in the microwave regime along closely spaced centimeter-scale metal rods is examined. Full-field electrodynamic simulations show that information transport occurs at a group velocity of 0.65c for fabricated structures consisting of copper rods excited at 8 GHz. A variety of passive routing structures and an all-optical modulator are demonstrated.

The possibility of guiding electromagnetic energy at visible frequencies with mode sizes below the diffraction limit using plasmon waveguides is analyzed using a point-dipole model and finite-difference time-domain simulations. It is shown that energy transport occurs via near-field coupling between metal nanoparticles, which leads to coherent propagation of energy. For spherical gold particles in air, group velocities up to 0.06c are demonstrated, and a change in particle shape to spheroidal particles shows up to a threefold increase in group velocity. Pulses with transverse polarization are shown to propagate with negative phase velocities antiparallel to the energy flow.

Plasmon waveguides consisting of gold and silver nanoparticles were fabricated using electron beam lithography. The key parameters that govern the energy transport are determined for various interparticle spacings and particle chain lengths using far-field measurements of the collective plasmon modes. Spherical gold nanoparticles with a diameter of 50 nm and an interparticle spacing of 75 nm show an energy attenuation of 6 dB/30 nm. This loss can be reduced by one order of magnitude by a geometry change to spheroidal particles. Using the tip of a near-field optical microscope as a local excitation source and fluorescent nanospheres as detectors, experimental evidence for energy transport over a distance of 0.5 µm is presented for plasmon waveguides consisting of silver rods with a 3:1 aspect ratio.

Table of Contents:

Chapter 1	Introduction	1
1.1	Towards nanoscale optical devices	1
1.2	Surface plasmons as a way to overcome the diffraction limit	2
1.3	Road map through this thesis	5
Chapter 2	Yagi waveguides	8
2.1	Introduction	8
2.2	The dispersion of a Yagi waveguide	9
2.3	Guiding along linear and corner Yagi arrays: Experiments and simulations	14
2.4	Towards active devices: A three-terminal modulator	19
2.5	The link to nanoscale plasmon waveguides	22
2.6	Conclusions and outlook	23
Chapter 3	Going nanoscale: Point-dipole theory of plasmon waveguides	25
3.1	Plasmon resonances in small metal clusters	25
3.2	Near-field particle interactions in plasmon waveguides	32
3.3	Routing and switching of electromagnetic energy in plasmon waveguides	37
3.4	Conclusions and limitations of the dipole model	39
Chapter 4	FDTD simulations of plasmon waveguides	41
4.1	Introduction	41
4.2	Collective far-field excitation of plasmon waveguides	41
4.3	Locally excited plasmon waveguides	49
4.4	Tailoring of the guiding properties by particle design	55
4.5	Conclusions and outlook	57
Chapter 5	Fabrication and far-field properties of plasmon waveguides	62
5.1	Introduction	62
5.2	Fabrication of plasmon waveguides	63
5.3	Far-field characterization of interparticle coupling in plasmon waveguides	67
5.4	Conclusion and outlook: Decrease of waveguide loss by particle design	75
Chapter 6	Local excitation of plasmon waveguides	79
6.1	Introduction	79
6.2	Transmission NSOM analysis of plasmon waveguides: Facts and artifacts	83
6.3	Molecular fluorescence as a probe for localized electromagnetic fields	89
6.4	Local excitation and detection of energy transport in plasmon waveguides	96
6.5	Conclusions and outlook	103
Chapter 7	Conclusions and outlook	109
	Bibliography	116