Optical Characterization Techniques for Two-Dimensional Materials

Two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDCs) exhibit unique electronic and optical properties arising from strong quantum confinement and reduced dielectric screening. Optical characterisation techniques probe these properties non-invasively, offering insight into band structure, excitonic behaviour and anisotropy. Common approaches include differential reflectance and transmittance spectroscopy, which reveal excitonic resonances and layer-dependent bandgap shifts; photoluminescence spectroscopy, which maps emission from excitons and defects; and Raman spectroscopy, which diagnoses lattice vibrations and strain. Ellipsometry, both spectroscopic and imaging variants, quantifies refractive index and dielectric function across a broad spectral range, while photocurrent and photomodulation spectroscopy probe light–matter interaction under applied bias. Emerging methods such as hyperspectral phase microscopy and near-field scanning optical microscopy extend spatial resolution to the nanoscale, enabling local mapping of refractive index and optical anisotropy. Together, these techniques underpin the design of 2D-material-based photonic and optoelectronic devices, from modulators and sensors to on-chip waveguides and metamaterials.

Research from Nature Portfolio

Studies have revealed giant optical anisotropy in layered TMDCs, demonstrating birefringence values up to three in the visible range. Correlative far-field and near-field measurements, supported by first-principles calculations, show that strong intralayer covalent bonds and weak interlayer van der Waals forces produce unprecedented birefringence for on-chip photonic applications.

Temperature-dependent spectroscopic ellipsometry of monolayer TMDCs has mapped refractive indices from the visible to near-infrared, revealing abnormal dispersion features below 800 nm and constant permittivity in the mid-infrared. Thermo-optic coefficients and shifts in optical bandgap with temperature have been quantified, providing essential parameters for devices operating across varied thermal environments.

Systematic measurement of the dielectric function in atomically thin MoS₂ films shows that excitonic contributions dominate for films below six layers. Layer-dependent exciton binding energies and Bohr radii have been extracted, highlighting the transition from exciton-governed dielectric response in few-layer films to band-structure-dominated behaviour in thicker samples.

Optical Characterization Techniques for Two-Dimensional Materials publication trend

The graph below shows the total number of articles in optical characterization techniques for two-dimensional materials across all publications each year (not limited to Nature Index journals).

Technical terms

Exciton: A bound electron–hole pair whose optical transitions dominate in 2D semiconductors.

Refractive index: A complex quantity (n + ik) representing phase velocity and absorption of light in a material.

Birefringence: The difference in refractive index for light polarised along different crystallographic axes.

Spectroscopic ellipsometry: A polarisation-based technique to determine optical constants over a wide wavelength range.

Hyperspectral phase microscopy: A wide-field imaging method that maps refractive index by measuring phase shifts across wavelengths.

Dielectric function: A frequency-dependent response describing how a material polarises in an electromagnetic field.

References

1. Giant optical anisotropy in transition metal dichalcogenides for next-generation photonics. Nature Communications (2021).
2. Temperature-dependent optical constants of monolayer MoS2, MoSe2, WS2, and WSe2: spectroscopic ellipsometry and first-principles calculations. Scientific Reports (2020).
3. Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films. Scientific Reports (2015).
4. Facile Projection of Spatially Resolved Refractive Index Modulation in Monolayer MoS2 via Light Phase Changes. Small (2025).
5. Optical Constants of Several Multilayer Transition Metal Dichalcogenides Measured by Spectroscopic Ellipsometry in the 300–1700 nm Range: High Index, Anisotropy, and Hyperbolicity. ACS Photonics (2022).
6. Measuring the optical permittivity of two-dimensional materials without a priori knowledge of electronic transitions. Nanophotonics (2018).
7. Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS2, MoSe2, WS2 and WSe2. Nanomaterials (2018).
8. Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy. Scientific Reports (2014).