Overview of Phase Modulation Technology

The UVISEL range of horiba scientific spectroscopic ellipsometers use photoelastic modulators to perform polarization modulation at a high frequency (50 kHz) without any mechanical movement. Owing to this technology these systems have the advantage of being very fast, having no moving parts and providing high accuracy measurements over a wide spectral range without the need for extra optical elements. Phase modulation allows the achievement of higher sensitivity for the characterization of thin film thickness and optical constants when compared with conventional ellipsometers.

How does a Phase Modulated Ellipsometer Work?

Optical Setup

The light source is a Xenon lamp that covers a large spectral range from 190 to 2100 nm. After passing through the first polarizer which establishes a linear polarization, the light reflects at oblique angle (generally 70°) from the sample under study. The output head comprises a photoelastic modulator and an analyzing polarizer that resolves the polarization state of the reflected beam.

Both polarizers are held fixed during the measurement while the photoelastic modulator is used to induce a modulated phase shift of the reflected beam. The light is analyzed by a grating monochromator that directs sequentially the light for each individual wavelength onto the detector. Two types of detectors are employed: photomultipliers for FUV-VIS applications, and InGaAs photodiodes for NIR applications.

What is a Photoelastic Modulator?

The photoelastic modulator is a fused silica bar exhibiting isotropic behavior when no stress is applied. The photoelastic modulator is an optical element that can be described as a birefringence modulator.

If a mechanical strain is applied to the quartz bar, such as by a piezoelectric transducer attached to the end of the bar, the modulator becomes birefringent (n0 ≠ ne). This means that light travels along one axis faster than the other when passing through it which produces a different phase velocity for each, and a modulated phase shift is induced to the light beam.