Reststrahlen band optics for the advancement of far-infrared optical architecture

The dissertation aims to build a case for the benefits and means of investigating novel optical materials and devices operating in the underdeveloped far-infrared (20 - 60 microns) region of the electromagnetic spectrum. This dissertation and the proposed future investigations described here have the potential to further the advancement of new and enhanced capabilities in fields such as astronomy, medicine, and the petrochemical industry. The first several completed projects demonstrate techniques for developing far-infrared emission sources using selective thermal emitters, which could operate more efficiently than their simple blackbody counterparts commonly used as sources in this wavelength region. The later projects probe the possible means of linking bulk optical phonon populations through interaction with surface modes to free space photons. This is a breakthrough that would enable the development of a new class of light sources operating in the far-infrared. Chapter 1 introduces the far-infrared wavelength range along with many of its current and potential applications. The limited capabilities of the available optical architecture in this range are outlined along with a discussion of the state-of-the-art technology available in this range. Some of the basic physical concepts routinely applied in this dissertation are reviewed; namely, the Drude formalism, semiconductor Reststrahlen bands, and surface polaritons. Lastly, some of the physical challenges that impede the further advancement of far-infrared technology, despite remarkable recent success in adjacent regions of the electromagnetic spectrum, are discussed. Chapter 2 describes the experimental and computational methods employed in this dissertation. Spectroscopic techniques used to investigate both the mid-infrared and far-infrared wavelength ranges are reviewed, including a brief description of the primary instrument of infrared spectroscopy, the Fourier Transform Infrared (FTIR) spectrometer. Techniques for measuring infrared reflection and thermal emission at fixed and variable angles are described. Finally, the two computational methods most commonly employed in this dissertation are outlined; namely, the transfer matrix method (TMM) and rigourous coupled wave analysis (RCWA) techniques for calculating reflection and transmission spectra for layered materials. The later technique employs the first one in a Fourier space in order to efficiently calculate spectra from layered periodic structures. Chapter 3 is the first of five to present experimental work carried out in the current course of study and describes a tunable selective thermal emitter made from a thin-film metamaterial composed of germanium deposited upon a layer of highly doped silicon. The structure is essentially an interference filter with an anti-reflection coating (the germanium film) that is significantly thinner than the typical quarter wavelength thickness used in such filters -- an effect enabled by the plasmonic properties of the highly doped silicon. The strong absorption band observed in reflection measurements was shown to be selective, tunable by choice of germanium thickness, and largely independent of polarization and angle of incidence. Subsequent heating of the devices demonstrated selective, tunable thermal emission Chapter 4 describes a different approach to achieving selective, tunable thermal emission; moreover, one that operates in the far-infrared. These devices are made of gold 1D gratings patterned atop aluminum nitride films with molybdenum ground planes beneath. These devices exhibited strong selective absorption that could be tuned by choice of gold grating width. This single parameter was shown to provide absorption resonance tuning across a wide range of the far-infrared with marginal change in the strength and quality factor of the resonance. Subsequent heating of the devices with 2D gratings demonstrated polarization independent selective thermal emission. Computational models of the emission indicated the samples had significantly higher power efficiency than a blackbody at the same temperature in the same wavelength band. Chapter 5 presents selective thermal emission in the far-infrared from samples of patterned gallium phosphide. The selective absorption of the samples occurs in the material's Reststrahlen band and can be attributed to surface phonon polariton modes. The surfaces of the samples were grated via wet etching to provide the additional momentum necessary for free space photons to couple into and out of the surface phonon polariton modes. Upon heating the samples, selective thermal emission of the surface phonon polariton modes was observed. Chapter 6 investigates a potential means of linking lattice vibrations to free space photons. Lightly doped films of gallium arsenide were grown by molecular beam epitaxy and wet etched with 1D gratings. The light doping served to modify the material's intrinsic permittivity and extend the region of its Reststrahlen band. Though the extension of the region with negative real permittivity was small, it extended beyond the longitudinal optical phonon energy of the material, which stands as the high energy boundary of the unmodified material's Reststrahlen band. Hybrid surface polariton modes were observed at energies near the longitudinal optical phonon energy where they are not supported on the surface of the intrinsic material -- offering a potential bridge between bulk optical phonon populations and free space photons. Chapter 7 presents preliminary results exploring the prospect of exploiting an absorption resonance known as the Berreman mode as a mechanism to link optical phonons to free space photons. The Berreman mode is a strong absorption resonance that occurs near the longitudinal optical phonon energy at moderate angles of incidence in polar semiconductors. Preliminary results demonstrate selective thermal emission consistent with the expected spectral position of the Berreman mode in aluminum nitride (AlN), while Raman spectroscopy confirmed the spectral proximity of the longitudinal optical phonon. The final chapter summarizes the findings and outlines several suggestions for additional research directions that may further advance the pursuit of new technological capabilities in the far-infrared

Platinum germanides for mid- and long-wave infrared plasmonics

Platinum germanides (PtGe) were investigated for infrared plasmonic applications. Layers of Pt and Ge were deposited and annealed. X-ray diffraction identified PtGe2 and Pt2Ge3 phases, and x-ray photoelectron spectroscopy determined vertical atomic composition profiles for the films. Complex permittivity spectra were measured by ellipsometry over the 2 to 15 mu m wavelength range. Surface plasmon polariton (SPP) characteristics such as propagation length and field penetration depth were calculated. Photon-to-SPP couplers in the form of 1D lamellar gratings were fabricated and characterized in the range 9 - 10.5 mu m via wavelength-dependent specular reflection spectra for multiple angles of incidence. The observed resonances compare well with calculated spectra for SPP excitation on PtGe2. Platinum germanides are CMOS compatible and may serve as SPP hosts for on-chip mid-IR plasmonic components with tighter field confinement than noble-metal hosts