Observed circuit limits to time resolution in correlation measurements with Si-on-sapphire, GaAs, and InP picosecond photoconductors

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Abstract
We report cross-correlation measurements of the response of photoconductor pulsers and sampling gates excited by a femtosecond laser. The photoconductors were fabricated in microstrip transmission line structures on Si-on-sapphire, semi-insulating GaAs, and semi-insulating InP wafers. The photoconductor sampling gates were ion beam damaged to produce short carrier lifetimes (<3 ps in one case). Damage was introduced with 6 MeV 20Ne on the Si-on-sapphire, 2 MeV 2H on the GaAs, and 2 MeV 4He on the InP. Doses in the range 1012–1015 cm−2 were used. Our results show circuit limits to the time resolution in correlation measurements from two sources: (a) RC time constants due to photoconductor gap capacitance and transmission line characteristic impedance and (b) dispersion in microstrip transmission lines.