<p>The optical properties of matter are determined by the coupling of the incident electromagnetic radiation to oscillators within the material. The oscillators can be electrons, ions or molecules. Close to a resonance the dielectric function exhibits strong dispersion and may be negative. A negative dielectric function gives rise to a complex wave vector which is associated with no allowed states for photons, i.e. high extinction and bulk reflectance, as well as the possibility to support surface waves. </p><p>It is possible to manufacture a dielectric material that generates a complex wave vector. Such materials are called photonic crystals and they may exhibit a frequency range without allowed states for photons, i.e. an energy gap. A photonic crystal has a periodically varying dielectric function and the lattice constant is of the same order of magnitude as the wavelengths of the gap. </p><p>In this thesis, two optical phenomena causing a complex wave vector are combined. Polar materials, which have lattice resonance in the thermal infrared causing strong dispersion, are studied in combination with a periodic structure. The periodicity introduced is achieved using another material, but also by structuring of the polar material. One, two and three dimensional structures are considered. The polar materials used are silicon dioxide and silicon carbide. It is shown, both by calculations and experiments that the two optical phenomena can co-exist and interact, both constructively and destructively. A possible application for the combination of the two phenomena is discussed: Selective emittance in the thermal infrared. It is also shown that a polar material can be periodically structured by a focused ion beam in such way that it excites surface waves.</p>

The optical properties of matter are determined by the coupling of the incident electromagnetic radiation to oscillators within the material. The oscillators can be electrons, ions or molecules. Close to a resonance the dielectric function exhibits strong dispersion and may be negative. A negative dielectric function gives rise to a complex wave vector which is associated with no allowed states for photons, i.e. high extinction and bulk reflectance, as well as the possibility to support surface waves. It is possible to manufacture a dielectric material that generates a complex wave vector. Such materials are called photonic crystals and they may exhibit a frequency range without allowed states for photons, i.e. an energy gap. A photonic crystal has a periodically varying dielectric function and the lattice constant is of the same order of magnitude as the wavelengths of the gap. In this thesis, two optical phenomena causing a complex wave vector are combined. Polar materials, which have lattice resonance in the thermal infrared causing strong dispersion, are studied in combination with a periodic structure. The periodicity introduced is achieved using another material, but also by structuring of the polar material. One, two and three dimensional structures are considered. The polar materials used are silicon dioxide and silicon carbide. It is shown, both by calculations and experiments that the two optical phenomena can co-exist and interact, both constructively and destructively. A possible application for the combination of the two phenomena is discussed: Selective emittance in the thermal infrared. It is also shown that a polar material can be periodically structured by a focused ion beam in such way that it excites surface waves.

The concept of photonic crystals was born in the late 1980's when two important letters were published that showed the possibility to control light propagation by a periodic structure. A photonic crystals consists of two or more materials with different dielectric functions periodically arranged on the length scale of light. If the conditions are favorable, a gap will open in the dispersion relation, often called photonic band structure, and electromagnetic waves with frequency in the gap range cannot propagate through the photonic crystal. In this thesis, mainly two types of structures and their properties have been numerically investigated: two-dimensional structures that are either square or triangular. In the calculations, both dielectric and polaritonic materials have been used. Polaritonic materials have an interval of high reflectance in the IR range, due to strong lattice resonances. Within such an interval, the real part of the dielectric function is negative, which causes a metal-like behavior. A polaritonic material, BeO has been introduced in photonic crystals to study the coexistence of structure and polaritonic gaps. Band structures and for some cases transmission spectra have been calculated to study the existence of complete gaps, i.e. energy intervals in which an incoming electromagnetic wave is totally reflected regardless of polarization and angle of incidence. A brief discussion on signature management and thermal emission, and calculations for low-emittance coatings is included. It is shown that a 50-60µm layer of a 3D photonic crystal can be sufficient to achieve a thermal emittance of 20%.

In this work, homojunction interfacial workfunction internal photoemission (HIWIP) detectors based on GaAs, and heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors based mainly on the Gallium Arsenide/Aluminum Gallium Arsenide material system are presented. Design principles of HIWIP and HEIWIP detectors, such as free carrier absorption, photocarrier generation, photoemission, and responsivity, are discussed in detail. Results of p-type HIWIPs based on GaAs material are presented. Homojunction detectors based on p-type GaAs were found to limit their operating wavelength range. This is mainly due to band depletion arising through carrier transitions from the heavy/light hole bands to the split off band. Designing n-type GaAs HIWIP detectors is difficult as it is strenuous to control their workfunction. Heterojunction detectors based on Gallium Arsenide/Aluminum Gallium Arsenide material system will allow tuning their threshold wavelength by adjusting the alloy composition of the Aluminum Gallium Arsenide/Gallium Arsenide barrier, while keeping a fixed doping density in the emitter. The detectors covered in this work operate from 1 to 128 micron (300 to 2.3 THz). Enhancement of detector response using resonance cavity architecture is demonstrated. Threshold wavelength extension of HEIWIPs by varying the Al composition of the barrier was investigated. The threshold limit of approximately 3.3 THz (92 micron), due to a practical Al fraction limit of approximately 0.005, can be overcome by replacing GaAs emitters in Gallium Arsenide/Aluminum Gallium Arsenide HEIWIPs with Aluminum Gallium Arsenide/Gallium Arsenide emitters. As the initial step, terahertz absorption for 1 micron-thick Be-doped Aluminum Gallium Arsenide epilayers (with different Al fraction and doping density) grown on GaAs substrates was measured. The absorption probability of the epilayers was derived from these absorption measurements. Based on the terahertz absorption results, an Aluminum Gallium Arsenide/Gallium Arsenide HEIWIP detector was designed and the extension of threshold frequency (f0) to 2.3 THz was successfully demonstrated. In a different study, switching in Gallium Arsenide/Aluminum Gallium Arsenide heterostructures from a tunneling dominated low conductance branch to a thermal emission dominated high conductance branch was investigated. This bistability leads to neuron-like voltage pulses observed in some heterostructure devices. The bias field that initiates the switching was determined from an iterative method that uses feedback information, such as carrier drift velocity and electron temperature, from hot carrier transport. The bias voltage needed to switch the device was found to decrease with the increasing device temperature.