Distributed photomixers
Author(s)
Duerr, Erik Kurt, 1973-
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Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science.
Advisor
Qing Hu.
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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 photomixer which uses an optical waveguide to couple the beat signal to an active area whichis 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 RC rolloff associated with a lumped-element photomixer's intrinsic capacitance 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. (cont.) 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 travelingwave 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.
Description
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2002. Includes bibliographical references (p. 187-197). This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Date issued
2002Department
Massachusetts Institute of Technology. Department of Electrical Engineering and Computer SciencePublisher
Massachusetts Institute of Technology
Keywords
Electrical Engineering and Computer Science.