Wire grid polarizers are essential optical components used in a wide variety of optical systems, from AR/VR to medical imaging to optical measuring systems. Physical properties of the polarizer, such as groove height and width all impact the polarizer’s efficiency. Here we show how a wire grid polarizer can be characterized non-destructively in the fabrication process with Mueller Matrix ellipsometry and RCWA-based analysis. Using this methodology, physical properties like grating profile, height, pitch, material thicknesses and even grating tilt can be measured non-destructively and inline to provide process control.

In this undergraduate student presentation, we describe measurements of the dielectric function of bulk InSb near the direct band gap using Fourier-transform infrared (FTIR) spectroscopic ellipsometry from 80 to 800 K in an ultra-high vacuum (UHV) cryostat with diamond windows. Indium antimonide (InSb) is the zinc blende compound semiconductor with the smallest direct band gap ( ε₀= 0.18 eV at room temperature) due to its heavy elements and the large resulting spin-orbit splitting and Darwin shifts. It also has a low melting point of 800 K. Previously, the bandgap of InSb has mostly been measured optically up to room temperature and estimated from Hall effect measurements of the effective mass up to 470 K. Ellipsometry measurements of the direct gap of InSb have been described at 300 K. Calculations indicate that InSb should undergo a topological phase transition from semiconductor to semi-metal (and topological insulator) at 600 K. It is interesting to see in the data if this transition occurs below the melting point of InSb.

Thanks to femtosecond pulsed lasers, at ELI Beamlines in the Czech Republic, we developed a time-resolved femtosecond ellipsometry technique, where a pump beam from any wavelength between 200 nm and 2000 nm excites a material; and a second pulse, the probe beam, with a continuous spectrum from 350 nm to 750 nm measures the dielectric function of that material. The pump and the probe beam can be separated in time from femtoseconds to nanoseconds generating a time-scan of the relaxation processes that happens in the material when it returns to its original unexcited state.

This time-resolved pump-probe ellipsometry technique is available to the scientific community through a yearly call for user proposals. We then get the opportunity to measure different type of materials from thin films to crystals of semiconductors and metals including 2D materials and organic samples deposited on metal layers. By imaging ellipsometry, the sample inhomogeneity, roughness and optical properties prior and after the pump-probe measurement are diagnosed. A discussion about newly imaging-analyzed samples that were and will be study by pump-probe ellipsometry will be presented.