Woollam M2000X Spectroscopic Ellipsometer
Ellipsometry measures the change in polarisation as light reflects from a material or layered sample.
The change in both the amplitude and phase of the reflected polarised light compared to the incoming linearly polarised beam depends on the film thickness and optical constants of the material being measured.
Our Woollam M2000X spectroscopic ellipsometer arrived in the Spring of 2024. It has a wide spectral range and fast data acquisition over a range of angles allowing measurements in a matter of minutes. The data can be modelled using the Woolam CompleteEASE software. It can be used for characterising thin films or bulk materials provided there is adequate reflection, allowing film thicknesses and optical constants to be determined.
The optical constants determine how light interacts with the materials and are typically described as the complex refractive index - including both the refractive index n and extinction co-efficient k. A different convention may also describe it as the complex dielectric function, and the two conventions can be related.
The refractive index describes the phase velocity of light as it travels in a material compared to the speed of light in a vacuum. Light must slow as it enters a material of higher refractive index, but as the frequency of the light cannot change, the wavelength must shorten. The extinction coefficient describes the loss of wave energy to the material and is related to the absorption coefficient. The optical constants vary as a function of wavelength depending on the mechanism causing the absorption energy from the light wave - whether that's molecular vibration in the infrared region or electronic transitions for ultraviolet absorption.
The optical constants and film thickness values are determined from the collected data using regression analysis, i.e. a model is constructed to describe the sample and the polarisation change predicted by the model is calculated and compared against the experimental data collected. The model is then optimised to reduce the difference between the calculated and measured change in polarisation and minimise the Mean Squared Error (MSE).
Fitted experimental polarisation change data from bulk cuprous oxide
