a photoresponsive pulser / sampler system for investigating electrical signals and responses in vlsi systems employs a microtip which communicates electrically and mechanically with the device under test , and electrically with an electrode via a photoresponsive gate . the photoresponsive gate is formed of interdigitated electrodes which have interdigital spacings therebetween on the order of 1 μm . the structure of the invention is operable in the dual modalities of probe and pulser which share a common probe tip . simultaneous implementation of the pulser and sampler functions is achieved with the use of a lock - in amplifier and the technique of difference frequency mixing , where the pulser optical pulse frequency is f 1 and the gate optical pulse frequency is f 2 . multiple photoresponsive gates coupled to a common probe tip and having respective electrodes are disclosed for achieving respective pulsing and sampling functions , and also for effecting s - parameter measurements . the lock - in amplifier is tuned to the difference between the two frequencies . piezoelectric apparatus is employed for translating the probe tip over the device under test .