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Diffraction and Guiding Dynamics

We are currently working on dielectric waveguides and other planar dielectric components to be used at frequencies in the visible region of the electromagnetic spectrum. Other research is continuously done into light focusing elements, including tapers, microdisks and Fresnel Zone Plates (FZPs), which can be easily integrated into the waveguide structure. At the moment the main focus is the characterisation of waveguides which have been fabricated by the group.

The different optical elements used in this part of the group consist of a 400 nm thick layer of silicon nitride (Si3N4) grown on top of 2 μm thick SiO2 which sits on a silicon wafer (Si). In all cases the Si3N4 is the layer of interest (e.g. the core of the waveguides) due to its quite high refractive index (2.05) and low absorption for visible light. All of the optical elements that we test are fabricated by etching patterns into this Si3N4 layer. Etching masks are patterned by electron beam lithography (EBL) before an inductively coupled plasma (ICP) etch is used to transfer the pattern to the Si3N4 layer. Waveguides with sub-micron dimensions are fabricated by this method along with tapers, bends and multimode interferometers as well as microdisks and other focusing elements.

Characterisation of the waveguides is an important step before the capabilities of the other elements can be determined. Characterisation consists mainly of finding the number of modes supported by the waveguide, first by calculation and then verified by experiment, and also finding the propagation losses of the waveguide. The focusing elements are designed to work for a single incident mode and are less effective when multiple modes are present.

Light focusing elements working by means of refraction and diffraction have been studied. These elements are either fabricated as part of the waveguide structure, e.g. the tapers and microdisks, or are later introduced to the structure, the FZPs which are milled by ion beam. Research into some of these elements (e.g. microdisk) has been carried out in this group for a number of years and so there is already a strong understanding and plenty of experimental data on their performance. It has been shown that microdisks can focus light to spot sizes of about one quarter of the wavelength. Work is ongoing for the tapers, where focusing depends greatly on the taper angle, and also on the FZPs, which do not gather as much light to the focus but can still produce sub-wavelength spots.

iagram of the materials used for waveguides and focusing elements (in this case a microdisk). The top layer is 400 nm Si3N4 with refractive index 2.05 which acts as the core, the middle layer is 2 μm SiO2 with refractive index 1.45 and sits on a Si wafer.

Example of a focusing element, a Fresnel Zone Plate, in a slab waveguide. (a) SEM image taken with the sample slightly tilted showing holes milled into the waveguide core which act as scatterers to form a focal spot, scale bar is 1 µm. (b) Simulated intensity map (false colour, red=high intensity, blue=low). Light incident from the left forms a focus 5 µm beyond the holes and is scattered out of the waveguide by a trench milled across the waveguide. (c) The intensity profile at the focus for the FZP. In contrast to simulations, from experiment the central focus was not as intense relative to the outer regions.