| Keywords [eng] |
Hybrid surface plasmon–phonon polaritons, SPPhP. gallium nitride, GaN surface gratings, thermal emission spectroscopy, FTIR spectroscopy, Reststrahlen band, surface phonon polaritons, RCWA, fill factor, doping concentration, mid-infrared photonics, angular aperture |
| Abstract [eng] |
This thesis investigates hybrid surface plasmon–phonon polaritons (SPPhPs) excited in differently doped gallium nitride (GaN) surface gratings. Infrared photonics is important for applications such as spectroscopy, thermal imaging, chemical sensing, selective thermal emission, radiative cooling, and mid-infrared photonic devices. GaN is suitable for this study because it is a polar semiconductor with infrared-active optical phonons. These phonons allow surface phonon polariton modes to exist in the Reststrahlen-band region. In doped GaN, free carriers add a plasmonic contribution to the dielectric response, allowing optical phonons and plasmons to couple and form hybrid SPPhP modes. The aim of this work is to study how doping level, fill factor, grating geometry, and angular collection conditions influence the spectral behaviour of SPPhP resonances. Two n-type GaN samples with different free-carrier concentrations were investigated. Both samples contained linear and circular surface-relief gratings with a nominal period of 17.5 µm and a height of about 1 µm. Fill factors of 25%, 50%, and 75% were studied. Thermal-emission measurements were performed using a Bruker Vertex 70 FTIR spectrometer. The heated GaN gratings acted as infrared emitters, and the emitted radiation was collected through an external optical setup. Since the setup had a finite angular aperture and no external polarizer, the measured spectra represented angularly averaged and effectively unpolarized emission. This is important because SPPhP modes are mainly TM-polarized and angle-dependent. The experimental spectra were processed using baseline correction and normalization, then compared with theoretical spectra calculated by rigorous coupled-wave analysis (RCWA). RCWA was selected because it is efficient for periodic grating structures and can calculate reflectance, absorption, emissivity, and diffraction effects using the grating geometry and GaN dielectric function. The results show that both samples support SPPhP-related thermal-emission features near the lower edge of the GaN Reststrahlen band. The higher-doped sample generally showed stronger and better-resolved resonances, indicating stronger plasmon–phonon hybridization. The lower-doped sample showed broader and less clearly resolved resonances. The fill factor also had a non-monotonic effect: 25FF often produced clearer resonances, while 50FF and 75FF showed more complex or broader spectral behaviour. Differences between experiment and theory are attributed to finite angular aperture, unpolarized detection, background emission, fabrication imperfections, and idealized assumptions in RCWA modelling. Overall, this work shows that GaN surface gratings are a useful platform for studying tunable hybrid SPPhP modes for future infrared sensing, spectral control, and narrowband thermal-emission applications. |
