Distributed photomixers

Although the terahertz domain has been explored scientifically, components, especially sources, are needed to enable further exploration of the frequency range. A photomixer generates coherent THz radiation through optical heterodyne down-conversion. A terahertz-frequency beat signal on an optical carrier illuminates an ultrafast photoconductor, modulating the conductance. The time-varying conductance together with a constant voltage bias generates time-varying current at the beat frequency. Low-temperature-grown gallium arsenide (LTG-GaAs) is the photoconductive material of choice, because its short carrier lifetime allows the conductance to be efficiently modulated at THz frequencies. The distributed photomixer described in this thesis is a new style of LTG-GaAs photo-mixer which uses an optical waveguide to couple the beat signal to an active area which is large relative to the terahertz wavelength. This large, traveling-wave mode active area distributes the heat load from absorbed optical power and ohmic heating from photocurrent and avoids the ¡ £ ¢ rolloff associated with a lumped-element photomixer's intrinsic capac-itance and the load resistance. The distributed photomixer structure consists of coplanar strips (CPS) fabricated on top of a dielectric strip-loaded waveguide that guides the optical beat. The conductance of a thin layer of LTG-GaAs between the CPS and optical guide is modulated by the weakly coupled optical signal. The THz conductance wave between the dc-biased CPS creates a THz electromagnetic wave which propagates along the CPS. To velocity match the THz and optical waves, the CPS are periodically loaded with thin electrodes that add a small shunt capacitance to the line. The CPS are terminated in a planar antenna that radiates the THz wave. This thesis describes the design, fabrication, and testing of waveguide-coupled distributed photomixers. The photomixers demonstrated in this thesis operated in traveling-wave mode and produced 100 nW of power at 0.3 THz and the power output rolled off at 6 dB/octave until 1.4 THz. A model which qualitatively and quantitatively predicts the device performance as a function of frequency, illumination and voltage bias is developed. A general design methodology, detailed discussion of fabrication steps and possible methods to increase output power are also presented.