Electrooptic and Photoconductive Techniques for Probing and Imaging of THz Electric Signals

Abstract

Since femtosecond lasers have become available, they have been are applied to measure, in the time domain, the electric response of ultrahigh-speed devices with bandwidths in the GHz and THz frequency range. Such laser-based measurement systems exhibit a superior time resolution (bandwidth coverage) owing to the short duration of the optical pulses. Because of their complexity, femtosecond optical techniques remained confined to a small community working in ultrahigh-speed-device development where measurement alternatives are sparse. In recent years, however, optical probing has become more and more attractive because it has developed into a flexible test tool that allows us to choose from a variety of methods to solve a specific measurement problem. Here, we describe a modular measurement system that offers a number of alternatives to synchronize to, generate, and detect high-frequency electric signals in microelectronic devices and circuits [1]. A frequency range spanning more than three orders of magnitudes from 1 GHz to 4 THz is covered. For stroboscopic measurements on circuits driven by electronically generated clock signals, the system is configured to lock onto periodic signals of arbitrary frequency. Alternatively, impulsive time-domain mesurements can be performed on chip by signal injection with freely positionable photoconductive probes or by direct optical excitation of active devices. With respect to detection, the following approaches are available: sampling with freely positionable electrooptic and photoconductive probe tips, and probe-tip-free testing based on the field-dependent optical nonlinearity of the circuit’s substrate material.