Lab equipment includes:
Ellipsometer
Ellipsometry measures a change in polarization as light reflects or transmits from a material structure. The polarization change is represented as an amplitude ratio, Ψ, and the phase difference, Δ. The measured response depends on optical properties and thickness of individual materials. Thus, ellipsometry is primarily used to determine optical constants or film thickness. However, it is also applied to characterize composition, crystallinity, roughness, doping concentration, and other material properties associated with a change in optical response. The ellipsometer in our lab is from J. A. Woollam Inc. and covers the spectral range between 0.5 eV and 6.5 eV.
FTIR Spectrometer
FTIR stands for Fourier Transform InfraRed, the preferred method for infrared spectroscopy. This technique uses infrared light to probe material properties in transmittance and/or reflectance geometries. The resulting spectra can be analyzed to obtain the frequency-dependent complex dielectric function which contains information about the electronic and lattice dynamics of the material. This makes infrared spectroscopy a very powerful tool for investigating low energy excitations in a system. We have a Bruker Vertex 80v FTIR spectrometer in our lab. At present, it covers the spectral range between ~ 2 meV and 3 eV in transmission and reflectance. We can perform reflectance measurements in a home-built high-vacuum, cryogenic system between 5 K and 400 K.
Near-field Infrared Microscope
Apertureless scanning near-field optical microscopy (SNOM) uses a tip of an atomic force microscope (AFM) along with an optical interferometric setup to extract the near-field signal. This technique depends on the fact that an illuminated sharp tip can exhibit enhanced optical fields in its neighborhood. Such near fields are modified by the presence of a sample. Due to this near-field interaction between the tip and the sample, the scattered light measured in the far field carries information on the sample’s local optical properties. This near-field scattering is the basis of the scattering-type scanning near-field optical microscopy (s-SNOM). The radius of curvature of the tip apex determines both the mechanical and the optical resolution of about 20 nm. The apertureless s-SNOM is particularly advantageous in the infrared spectral regime. The infrared signal intensity and spatial resolution are not compromised because optical fibers are not required.