Measuring how a surface changes the polarisation of the light it reflects, to work out how thick a film is and how it bends and absorbs light. Polarised light bounced off a coated surface comes back with its two polarisation components changed in strength and shifted against each other, by amounts that depend on the film’s thickness and refractive index. Fitting a model to these changes gives both, down to fractions of a nanometre – without touching the sample, which is why chip factories use it on their .
As the site uses it
For optics, say which gap is quoted and how it was measured, since transmission and ellipsometry give different numbers for BiI3.
What Raman spectroscopy and ellipsometry show for films of Sb2S3, the phase-change member of this family, before and after it crystallises – the same samples measured in several laboratories.
Ellipsometry sends in polarised light and measures how reflection from a film changes its polarisation. A layer model fitted to the measured angles gives the film’s thickness and optical constants. Standard spots are larger than most flakes; imaging versions map flakes layer by layer.
Polarisation as a ruler
Light reflected from a surface can be split into two polarisations, one in the plane of reflection and one across it, and the surface treats them differently. Ellipsometry sends in light of known polarisation and measures how the reflected light’s two components have changed relative to each other – their amplitude ratio and phase difference, the angles Ψ and Δ. Because it measures a ratio rather than an absolute intensity, it is insensitive to fluctuations of the light source.
The angles do not give thickness directly: a model of the layers is fitted to them, with each layer’s thickness and optical constants as parameters. With a good model a film’s thickness comes out to a fraction of a nanometre; with a wrong one, the fit can look good and the answer still be wrong.
Optical constants of a single layer
For , ellipsometry supplies the refractive index and absorption that any optical design needs, from the that makes visible to photonic devices. Spectroscopic ellipsometry of MoS2, MoSe2, WS2 and WSe2 from 0.73 to 6.42 eV and from 4.5 to 500 K found refractive indices settling at 3.5 to 4.0 in the near-infrared at room temperature and falling on cooling as the widens, with thermo-optic coefficients of 6.1 × 10−5 to 2.6 × 10−4 per kelvin at 1200 nm (Liu and colleagues, 2020).
How a value was measured matters. Optical gaps from ellipsometry and from transmission need not agree, and the refractive-index contrast of a film such as Sb2S3 depends on how it was crystallised – about 0.4 or 0.2 at telecom wavelengths after at different temperatures, both below the 0.6 often quoted (Gutiérrez and colleagues, 2022).
From flakes to wafers
A standard ellipsometer illuminates a spot tens of micrometres across, larger than most exfoliated flakes, and imaging versions were slow. Fourier imaging micro-ellipsometry records many angles at once through a microscope, with a lateral resolution below 5 µm and data a thousand times faster than ellipsometers of similar resolution. It mapped the thickness of mono-, bi- and trilayers of graphene, hBN and four dichalcogenides to ångström accuracy, and picked out monolayer hBN, which other characterisation tools struggle to see (Kenaz and colleagues, 2023).
On wafers, ellipsometry is already routine. It measures film thickness and uniformity across 300 mm wafers without contact, which is why it sits beside and mapping in the metrology a 2D-material fab would need.
For specialists
Measurement of the ratio of p- and s-polarised reflection coefficients, tan Ψ · e^(iΔ), usually against wavelength and angle of incidence; thickness and complex refractive index (or function) follow from fitting a layer model. Spectroscopic ellipsometry gave the optical constants of monolayer MoS2, MoSe2, WS2 and WSe2 from 4.5 to 500 K, with near-infrared refractive indices of 3.5–4.0 at room temperature. Standard spots tens of micrometres wide are too large for exfoliated flakes; imaging micro-ellipsometry maps the thickness of mono- to trilayers to ångström accuracy, including monolayer hBN. On wafers it is routine, non-destructive metrology.
media support high-k electromagnetic modes, enabling subdiffractional confinement, anomalous refraction, and enhanced light–matter interactions. While most hyperbolic metamaterials rely on artificial structuring, natural hyperbolicity arises from crystal and direction-dependent plasma frequencies.…
Ultraviolet (UV) exciton-polaritons (EPs) enable , polaritonic lasing, and polariton-mediated photochemistry in the short-wavelength regime, yet progress has been limited due to the scarcity of materials that combine large oscillator strength with stable and narrow UV . Here, we demonstrate UV…