Photodiodes for Applications in Quantum Information and Microwave Photonic Systems

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Zang, Jizhao, Electrical Engineering - School of Engineering and Applied Science, University of Virginia
Campbell, Joe, En-Elec/Computer Engr Dept, University of Virginia

Photodiodes are widely used in quantum optical and microwave photonic systems. This dissertation focuses on three topics: high quantum efficiency photodiodes, a photovaractor for optically modulated scatterer, and investigation of AM-to-PM noise conversion.
For the high-efficiency photodiodes, we achieved 98% ±0.8% external quantum efficiency at 1064 nm. At bias voltage of -5 V, the 3 dB bandwidth is 7 GHz and the dark current is ~10 nA. The dark current at low bias voltage is dominated by generation-recombination with 0.23 eV activation energy.
For the photovaractor, we designed III-V photovaractors whose capacitance change is based on the decrease of depletion width and the steep increase of the effective area after illumination. At photocurrent of 12 mA, the reactance at 60 GHz is changed by 280 times relative to the dark condition. S11 parameter fitting shows that the capacitance of a 50 μm-diameter device increases by 37 times after illumination. The photovaractor is further flip-chip bonded with matching network to enhance the scattering. The S11 can be changed by ~ 40 dB by modulating it optically or electrically. Further, the proposed photovaractor is used as an optically-controlled microwave attenuator. By changing the photocurrent, a dynamic range of up to 40 dB is achieved. At photocurrent < 3 mA, the phase variation is less than 10 degrees in the frequency range of 5 GHz to 55 GHz.
For the investigation of AM-to-PM, we have optimized the epitaxial layer design of the CC-MUTC photodiodes to reduce the AM-to-PM noise conversion. Measurement results show that the range of phase change at different photocurrent is reduced by more than three times and the AM-to-PM conversion is greatly suppressed, especially at large photocurrent. For 10 µm devices, the 3-dB bandwidth is 35 GHz at bias voltage of -6 V and the maximum RF output power at 15 GHz is 13.7 dBm after optimization. The AM-to-PM in large (>10 ) devices is also investigated. Measurement shows that not only the carrier transit time, but also the RC phase response will influence the AM-to-PM in large devices.

PHD (Doctor of Philosophy)
Photodiode, AM-to-PM, Photovaractor, Quantum efficiency
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