Temperature-dependent effective mass in AlGaN/GaN heterostructures was experimentally observed via THz time domain spectroscopy of 2D plasmons in the range of 80–300 K. Grating couplers with different periods and filling factors were developed in order to monitor the behavior of plasma resonances in transmission spectra in the frequency range of 0.5–3.5 THz. For the grating with a 50% filling factor, the fundamental modes were excited and observed at temperatures below 225 K. The change of the filling factor to 80% led to the excitation of the fundamental and second order plasma harmonics observable up to 300 K and 220 K, respectively. Moreover, with an increase in temperature, the 2D plasmons experienced the red-shift in transmission power and phase spectra of all samples. This phenomenon was explained by the renormalization of effective mass, which started distinctly to deviate at 134 K temperature and at 295 K increased up to 55% of its nominal value. The THz spectroscopy of 2D plasmons further confirms a temperature-dependent effective mass in AlGaN/GaN heterostructures as reported previously in optical Hall effect studies.

Two-dimensional plasmons were investigated by terahertz time domain spectroscopy observing experimentally the distinctive minima and inflection points in the transmission power amplitude and phase spectra, respectively. Gratings of different periods (600, 800, and 1000 nm) and filling factors (50 and 80%) were provided to the two-dimensional electron gas in AlGaN/GaN heterostructures in order to measure the plasmon dispersion and the coupling efficiency with THz radiation. Comparative analysis of experimental data revealed that the resonant plasmon features in the amplitude spectrum are related to those in the phase spectrum by a simple integral relation, paving the way for phase spectroscopy of the plasmon phenomena in fields of THz physics and engineering.

Terahertz time-domain spectroscopy and Fourier-transform infrared spectroscopy were developed as the method for the investigation of high-frequency characteristics of two-dimensional electron gas and GaN:C buffer layers in AlGaN/AlN/GaN heterostructures grown on a semi-insulating SiC substrate. The reflectance and transmittance spectra of the selected heterostructure layers were studied after the top layers were removed by a reactive ion etching. Results were numerically analyzed using the transfer matrix method taking into account the high-frequency electron conductivity via a Drude model and complex dielectric permittivity of each epitaxial layer via a one-phonon-resonance approximation. Good agreement between the experiment and theory was achieved revealing the temperature dependent electron effective mass in AlGaN/AlN/GaN high electron mobility transistor structures and the small damping factors of optical phonons due to high crystal quality of the epitaxial layers fabricated on the SiC substrate.

In this study, polarized reflection and emission spectroscopy was employed in order to describe experimentally and numerically the coherent behavior of surface plasmon phonon polaritons in shallow n-type gallium nitride grating. For the grating with a $1\hspace{0.17em}\mu$ m depth and an $11\hspace{0.17em}\mu$ m period, polariton coherence was observed measuring the reduction of the linewidth and the redshift of the resonance position in both reflection and emission spectra with the increase in the incident angle. Maximum coherence conditions were found at a frequency of 562 cm⁻¹ and an angle of 37°, demonstrating the coherence length value up to 724 μm (41λ).

Dispersion characteristics of hybrid surface plasmon-phonon-polaritons (SPPhPs) on the air/polar semiconductor interface were investigated by means of shallow surface relief grating using emission spectroscopy methods. A set of grating structures with optimal 1 µm depth and periods from 8 to 22 µm was developed on a heavily-doped GaN crystal. The SPPhPs were excited by thermal heating or electrical biasing of the samples which radiated directive polarized features in an extremely narrowband spectrum range. Detailed analysis of damping factors and propagation losses revealed maximum values of quality factor and spatial coherence of hybrid SPPhPs modes. Highest quality factor was found to be practically independent on the period of the shallow grating, as it was always detected near the frequency of transverse optical phonon, demonstrating values as high as 88 and 200 in experiment and theory, respectively. Meanwhile, the largest values of coherence length strongly depended on the grating as the propagation losses of hybrid SPPhP modes showed a tendency to accumulate with the wavevector increase. The sample with 22 µm grating period demonstrated the highest coherence of hybrid polaritons with the experimental (theoretical) coherence length values as high as 1.6 mm (2.3 mm).

A nearly perfect absorber across an ultrabroad frequency range from 1 to 1200 THz is reported that combines a silicon moth‐eye structure with a thin layer of pyrolytic carbon (PyC). The moth‐eye design provides a perfect impedance match with air via gradual change of the refractive index at low frequencies and simultaneously serves as a multiple scatterer at high frequencies while coating the moth‐eye structure with PyC enables ultrabroad band absorption. The combination of the strong absorption in the PyC conductive film and moth‐eye surface allows to create a material capable of absorbing over 98% incident radiation across the entire THz–deep ultraviolet (DUV) frequency range. This makes this moth‐eye/PyC structure a promising material for a wide range of applications including energy harvesting, electromagnetic shielding, and the development of an ideal blackbody radiation source.