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A main goal of this thesis was the development of an efficient, continuous-wave, room-temperature operating, tunable n-i-pn-i-p superlattice Terahertz (THz) photomixer. Photomixers generate a THz AC current by absorbing the beat signal of two lasers that are offset by the THz frequency. The current is fed into an antenna for radiation of a free space THz signal. Photomixers are limited at high frequencies by the time required for separation of optically generated carriers and by their electrical antenna (R) and device (C) RC time constant. An n-i-pn-i-p photomixer consists of a stack of N high speed p-i-n diodes. The optimization is carried out by transit-time optimisation of the intrinsic layer length using the quasi-ballistic electron transport concept and by shifting the RC time constant to smaller values by growing a number of p-i-n superlattice periods in series. In the first thesis part, the n-i-pn-i-p concept is demonstrated for GaAs and InGaAs based photomixers, independent transit and RC optimization is shown and balistically enhanced electron transport is demonstrated for GaAs based emitters. In the further development of telecom-wavelength compatible InGaAs based emitters, the transit-time 3 dB frequency was increased from 0.26 to 0.72 THz, and plasma processing allowed to fabricate photomixers with a cross section of 27 square micron to reduce capacitance. The thermal management was improved for high power operation, and layout was adapted for more homogenous current distribution. The maximum recombination current density was increased by a factor of about 3.5 by using higher doping concentrations and lower aluminium contents. The highest doping level in a complete photomixing structure was ND(Si) = 1*1019 1/cm3 and NA(Be) = 2*1019 1/cm3 for an In(0.53)Ga(0.39)Al(0.08)As mixer. This resulted in a maximum current density of 12 kA/cm2 for a homogeneously illuminated device. These improvements and increased detection efficiency of the setup resulted in an output power level of 0.18 mW around 66 GHz and 0.27 microW at 1 THz for photocurrents around 9.4 mA. This is an increase of about two orders of magnitude compared to the values at the beginning of this thesis. A new set of high current ErAs:InGaAs recombination diodes with doping levels up to ND(Si) = 3*1019 1/cm3, NA(C) = 1*1020 1/cm3 was investigated. They support an increase of the maximum recombination current density by a factor of 15 with respect to the former recombination diodes. No fundamental theoretical limit concerning the THz power is reached yet. In the second part of the thesis, n-i-pn-i-p superlattice photomixers were used for THz experiments. THz optical elements such as wire grid polarizers, a blazed grating and waveguides were characterized. Their performance agreed well with the respective theoretical expectations. As the next step, THz whispering gallery mode resonators were investigated. Interference within the cavity generates very narrow resonances that can be utilized for frequency-selective filter structures. Compared to the optical domain, the resonator features are by about a factor of 1000 larger due to the wavelength used. Therefore, the THz frequency range can act as a test bed for shape related theories. We demonstrated mode splitting by coupled resonator systems of 2 and 3 cavities with matched mode spectra. Excellent agreement between experiment and theory was found. The scalability of electromagnetism allows for directly transferring these results to other frequency domains. The coherence of the THz signal was investigated. First, the Young's double slit experiment in the THz frequency range was demonstrated. The coherence length was determined by a THz Mach-Zehnder interferometer. A coherence length of 13.6 m was deduced. This corresponds to a linewidth of 8.3 MHz (HWHM), close to that of the optical beams. We exploited the mutual coherence of THz emitters that are driven by the same pair of lasers for a chip-sized 3x3 photomixer array. The photomixers were emitting in phase, increasing the directivity and intensity. Their phase relation was controlled by including a small angle of a few mrad between the two lasers. This allowed for steering the THz beam. A layout for a tens-of-cm scale, fiber-based THz array was presented and supported by numerical calculations. High power, intense spots with sizes in the centimeter range should be feasible at an imaging distance of 30 m by using apertures in the range of 4 inch (101.6 mm) in diameter.