Spectroscopic Characterisation of 2D Photonic Microstructures

Thomas F.Krauss, Aline Ribayrol, Richard M. De La Rue and Clivia M. Sotomayor-Torres

The aim of this project is to identify the influence of photonic microstructures (a typical example is shown below) on the emission properties of the semiconductor. The material is a multiple quantum well (MQW) structure which, in the specific case under consideration, has a strong emission peak at a wavelength of around 820nm. The sample is excited by photopumping with an Argon - Ion laser and the resulting emission is spectrally analysed in order to identify changes in the spectrum that could be attributed to the microstructuring of the sample. For example, the two-dimensional grating shown below was designed to suppress propagation of light in the plane of the sample which is expected to result in an enhancement of the out-of plane emission. Because this change is wavelength-dependent, the shape of the spectrum should change significantly.

A particular challenge is to make this measurement angle-dependent, i.e. to detect the spectral changes as a function of emission angle; so far, only the emission normal and at 45 degrees to the surface have been characterised, but no conclusive evidence for spectral changes has been found. It is conceivable, however, that the emission at "odd" angles will be significantly influenced by the grating, which is why the spectrum at any emision angle needs to be assessed.

Detail of a two-dimensional photonic microstructure; the lattice consists of 120nm diameter pillars (bright in the picture) separated by 30nm airgaps (dark). The size of the overall lattice is 150 micrometers square in order to allow characterisation by photoluminescence spectroscopy. The purpose of this structure is to suppress radiation in the plane of the pillars in order to enhance the (useful) out-of plane emission.

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