  Direct access to the electric field of light provides a subcycle view of the
polarization response of matter, thus enabling sensitive metrology in physics,
chemistry, and medicine. Optical-field sampling techniques take advantage of
some process that lasts much less than an optical cycle of the measured light
wave. Most of them rely on the generation of free electrons either by a weak
extreme ultraviolet pulse or by a strong optical pulse. The ability of such
techniques to detect weak signals is limited by undesirable effects associated
with ionization. An alternative is all-optical methods, where the fast process
is a nonlinear wave mixing. Such photonic methods may rely on extreme
ultraviolet or optical pulses. They typically have better sensitivity, but
smaller bandwidth. Here we propose generalized heterodyne optical-sampling
techniques (GHOSTs), which overcome the bandwidth limit through controlling
light-pulse waveforms.
