The name is absent

many other values of t. To make these repeated measurements, we use many identical
copies of the THz electric field. Sampling techniques of this sort, which rely critically
on the precise synchronization between the terahertz field and the pulse used to gate
the detector, are widely used in ultrafast optics and optoelectronics.

Optical sampling only works if every THz pulse in the pulse train is identical.
If the shape of the THz pulse is evolving on a time scale comparable to (or shorter
than) the measurement time, it is not possible to sample the waveform accurately.
In addition to this fundamental issue, there are some other disadvantages to optical
sampling. Like any sampling technique, it takes time to obtain the data. In principle,
the acquisition time cannot be less than N ■ δt, where N is the number of measured
values of the electric field required in order to characterize the terahertz pulse, and
δt is the pulse-to-pulse spacing of the pulse train. Because we usually take advantage
of signal averaging, the acquisition time is usually much longer than this minimum
value. Another problem inherent to sampling measurements is that they require a
method for varying the delay of the sampling gate (i.e. the probe pulse) relative to
the terahertz pulse. This is most often accomplished using a mechanical delay line,
moving a mirror to vary an optical path length.

It is important to note that, using either photoconductive sampling [14, 22] or
free-space electro-optic sampling [23-25], one obtains a direct measurement of the
terahertz electric field E(t), not merely intensity I(t). In both cases, the measurement
is sensitive only to coherent radiation, and moreover only to radiation which is phase-
locked to the repetition rate of the femtosecond oscillator. As a result, both of
these sampling techniques are blind to thermal radiation, and therefore the detection
operates at room temperature without degradation of performance. This eliminates
the requirement for liquid cryogens which had previously limited the broader use of

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