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Herein, the complex pseudodielectric function of Ge and Si from femtosecond pump–probe spectroscopic ellipsometry with 267, 400, and 800 nm pump–pulse wavelengths is analyzed by fitting analytical lineshapes to the second derivatives of the pseudodielectric function with respect to energy. This yields the critical point parameters (threshold energy, lifetime broadening, amplitude, and excitonic phase angle) of E1 and E1+Δ1 in Ge and E1 in Si as functions of delay time. Coherent longitudinal acoustic phonon oscillations with a period of about 11 ps are observed in the transient critical point parameters of Ge. From the amplitude of these oscillations, the laser‐induced strain is found to be on the order of 0.03% for Ge measured with the 800 nm pump pulse, which is in reasonable agreement with the strain calculated from theory. [ABSTRACT FROM AUTHOR]

We report on terahertz (THz) electron paramagnetic resonance generalized spectroscopic ellipsometry (THz-EPR-GSE). Measurements of field and frequency dependencies of magnetic response due to spin transitions associated with nitrogen defects in 4H-SiC are shown as an example. THz-EPR-GSE dispenses with the need of a cavity, permits independently scanning field and frequency parameters, and does not require field or frequency modulation. We investigate spin transitions of hexagonal (h) and cubic (k) coordinated nitrogen including coupling with its nuclear spin (I = 1), and we propose a model approach for the magnetic susceptibility to account for the spin transitions. From the THz-EPR-GSE measurements, we can fully determine polarization properties of the spin transitions, and we can obtain the k coordinated nitrogen g and hyperfine splitting parameters using magnetic field and frequency dependent Lorentzian oscillator line shape functions. Magnetic-field line broadening presently obscures access to h parameters. We show that measurements of THz-EPR-GSE at positive and negative fields differ fundamentally and hence provide additional information. We propose frequency-scanning THz-EPR-GSE as a versatile method to study properties of spins in solid state materials. [ABSTRACT FROM AUTHOR]

We present a setup for time-resolved spectroscopic ellipsometry in a pump–probe scheme using femtosecond laser pulses. As a probe, the system deploys supercontinuum white light pulses that are delayed with respect to single-wavelength pump pulses. A polarizer–sample–compensator–analyzer configuration allows ellipsometric measurements by scanning the compensator azimuthal angle. The transient ellipsometric parameters are obtained from a series of reflectance-difference spectra that are measured for various pump–probe delays and polarization (compensator) settings. The setup is capable of performing time-resolved spectroscopic ellipsometry from the near-infrared through the visible to the near-ultraviolet spectral range at 1.3 eV–3.6 eV. The temporal resolution is on the order of 100 fs within a delay range of more than 5 ns. We analyze and discuss critical aspects such as fluctuations of the probe pulses and imperfections of the polarization optics and present strategies deployed for circumventing related issues. [ABSTRACT FROM AUTHOR]

The insulator‐to‐metal transition (IMT) in vanadium dioxide (VO2) can enable a variety of optics applications, including switching and modulation, optical limiting, and tuning of optical resonators. Despite the widespread interest in VO2 for optics, the wavelength‐dependent optical properties across its IMT are scattered throughout the literature, are sometimes contradictory, and are not available at all in some wavelength regions. Here, the complex refractive index of VO2 thin films across the IMT is characterized for free‐space wavelengths from 300 nm to 30 µm, using broadband spectroscopic ellipsometry, reflection spectroscopy, and the application of effective‐medium theory. VO2 films of different thicknesses are studied, on two different substrates (silicon and sapphire), and grown using different synthesis methods (sputtering and sol–gel). While there are differences in the optical properties of VO2 synthesized under different conditions, these differences are surprisingly small in the ≈2–11 µm range where the insulating phase of VO2 also has relatively low optical loss. It is anticipated that the refractive‐index datasets from this article will be broadly useful for modeling and design of VO2‐based optical and optoelectronic components, especially in the mid‐wave and long‐wave infrared. [ABSTRACT FROM AUTHOR]