Martin Guy Salt

Tuning the resonance of a photonic crystal microcavity with an AFM probe

We present theoretical and experimental results on switching and tuning of a two-dimensional photonic crystal resonant microcavity by means of a silicon AFM tip, probing the highly localized optical field in the vicinity of the cavity. On-off switching and modulation of the transmission signal in the kHz range is achieved by bringing an AFM tip onto the center of the microcavity, inducing a damping effect on the transmission resonance. Tuning of the resonant wavelength in the order of several nanometers becomes possible by inserting the AFM tip into one of the holes of the Bragg mirror forming the microcavity in the propagation direction.

Application of the boundary-element method to the interaction of light with single and coupled metallic nanoparticles

The boundary-element method is applied to the interaction of light with resonant metallic nanoparticles. At a certain wavelength, excitation of a surface plasmon takes place, which leads to a resonantly enhanced near-field amplitude and a large scattering cross section. The resonance wavelength for different scatterer geometries is determined. Alteration of the scattering properties in the presence of other metallic nanoparticles is discussed. To treat this problem, a novel formulation of the boundary-element method is presented that solves the interaction problem for all the coupled particles.

High Resolution Interference Microscopy: A Tool for Probing Optical Waves in the Far-Field on a Nanometric Length Scale

High Resolution Interference Microscopy (HRIM) is a technique that allows the characterization of amplitude and phase of electromagnetic wave-fields in the far-field with a spatial accuracy that corresponds to a few nanometers in the object plane. Emphasis is put on the precise determination of topological features in the wave-field, called phase singularities or vortices, which are spatial points within the electromagnetic wave at which the amplitude is zero and the phase is hence not determined. An experimental tool working in transmission with a resolution of 20 nm in the object plane is presented and its application to the optical characterization of various single and periodic nanostructures such as trenches, gratings, microlenses and computer generated holograms is discussed. The conditions for the appearance of phase singularities are theoretically and experimentally outlined and it is shown how dislocation pairs can be used to determine unknown parameters from an object. Their corresponding applications to metrology or in optical data storage systems are analyzed. In addition, rigorous diffraction theory is used in all cases to simulate the interaction of light with the nano-optical structures to provide theoretical confirmation of the experimental results.

Measuring optical phase singularities at subwavelength resolution

We will present experimental and theoretical studies of optical fields with subwavelength structures, in particular phase singularities and coherent detection methods with nanometric resolution. An electromagnetic field is characterized by an amplitude, a phase and a polarization state. Therefore, experimental studies require coherent detection methods, which allow one to measure the amplitude and phase of the optical field with subwavelength resolution. We will present two instruments, a heterodyne scanning probe microscope (heterodyne SNOM) and a high resolution interference microscope (HRIM). We will review some earlier work using the heterodyne SNOM, in particular the measurement of phase singularities produced by a 1 μm pitch grating with 10 nm spatial sampling. Using the HRIM we have investigated the intensity and phase distributions (with singularities) in the focal region of microlenses. The measurements are compared with the results calculated by rigorous diffraction theory.

Modification of polymer light emission by lateral microstructure

We report the use of wavelength-scale microstructure to control the intensity, spectrum and polarisation of light-emission from thin polymer films. It is shown that periodic corrugation of the emissive layer can substantially increase the efficiency of light-emission. Detailed photoluminescence (PL) studies of the angle dependence of the emission together with theoretical modelling show that the observed emission enhancement is associated with Bragg-scattering of waveguided light out of the polymer layer. The application of this approach to increase the efficiency of a light-emitting diode (LED) is demonstrated.

Measuring amplitude and phase distribution of fields generated by gratings with sub-wavelength resolution

In this paper, we intend to gain an understanding of the interaction of light with microstructures. Measurements of amplitude and phase in the diffracted field close to gratings using a heterodyne scanning probe are presented. Coherent light diffracted by microstructures produces periodic features and can give birth to phase dislocations, also called phase singularities. Phase singularities are isolated points where the amplitude of the field is zero. We present measurements of such phase singularities with 10 nm spatial sampling and compare them with theoretical results obtained from rigorous diffraction calculations. The observed polarization effects reveal also important information about the vectorial field conversion by the fiber tip.

Analyzing the scattering properties of coupled metallic nanoparticles

We apply the boundary element method to the analysis of the plasmon response of systems that consist of coupled metallic nanoscatterers. For systems made of two or more objects, the response depends strongly on the individual particle behavior as well as on the separation distance and on the configuration of the particles relative to the illumination direction. By analyzing the behavior of these systems, we determine the smallest interaction distance at which the particles can be considered decoupled. We discriminate the two cases of particle systems consisting of scatterers with the same and different resonance wavelengths.

Photonic band gaps in guided modes of textured metallic microcavities

We present a detailed study of the effect of periodic texture upon the dispersion of guided modes supported by a metal-clad microcavity, in the visible. Specifically, we look at how a periodic corrugation, a diffraction grating, produces a band gap in the propagation of such modes and how the central frequency of this band gap depends upon the propagation direction of the mode. We compare experimental results with those from a theoretical model and find good agreement.

Optically tunable microcavity in a planar photonic crystal silicon waveguide buried in oxide.

We present all-optical tuning and switching of a microcavity inside a two-dimensional photonic crystal waveguide. The photonic crystal structure is fabricated in silicon-on-insulator using complementary metal-oxide semiconductor processing techniques based on deep ultraviolet lithography and is completely buried in a silicon dioxide cladding that provides protection from the environment. By focusing a laser onto the microcavity region, both a thermal and a plasma dispersion effect are generated, allowing tuning and fast modulation of the in-plane transmission. By means of the temporal characteristics of the in-plane transmission, we experimentally identify a slower thermal and a fast plasma dispersion effect with modulation bandwidths of the order of several 100 kHz and up to the gigahertz level, respectively.

Observation of amplitude and phase in ridge and photonic crystal waveguides operating at 1.55 μm by use of heterodyne scanning near-field optical microscopy

We apply heterodyne scanning near-field optical microscopy (SNOM) to observe with subwavelength resolution the amplitude and phase of optical fields propagating in several microfabricated waveguide devices operating around the 1.55 microm wavelength. Good agreement between the SNOM measurements and predicted optical mode propagation characteristics in standard ridge waveguides demonstrates the validity of the method. In situ observation of the subwavelength-scale distribution and propagation of optical fields in straight and 90 degrees bend photonic crystal waveguides facilitates a more detailed understanding of the optical performance characteristics of these devices and illustrates the usefulness of the technique for investigating nanostructured photonic devices.