Invasiveness of LiTaO3 and GaAs probes in external E-O sampling

Disturbances of signals on a coplanar waveguide (CPW) induced by the presence of LiTaO₃ and GaAs electrooptic probes in external electrooptic (EO) sampling have been simulated and compared quantitatively. The finite-difference-time-domain method is used to simulate the full wave field around a coplanar waveguide on a GaAs substrate in an external EO sampling configuration. The results indicate that the induced signal disturbance, or invasiveness, of a LiTaO₃ probe is almost ten times that of a GaAs probe in terms of the magnitude of S₁₁, but that LiTaO₃ yields about two times the EO response for a given S₁₁ and optical probing wavelength. The transparency of LiTaO₃ to shorter wavelengths, however, allows an even higher sensitivity for this material relative to GaAs. The results suggest that these probes do not exhibit significant invasiveness (magnitude of S₁₁ smaller than -40 dB), if they are removed from contact by the distance of CPWs center conductor width.