The name is absent

Visible image

Figure 2.1 : One of the first images acquired using a time-domain system, adapted
from the original paper by Hu and Nuss. This shows a transmission image of a semi-
conductor integrated circuit, through the black epoxy package which is transparent
to terahertz radiation. The metallization inside the package is clearly visible, as is
the semiconductor wafer in the center. The spatial resolution of this image is roughly
250 microns. The inset shows an optical image of the sample.

numerous features of this spectrometer which have proven advantageous for imaging.

A THz-TDS system starts with a femtosecond laser, producing a train of pulses
of typically ~100 fs duration, at a repetition rate which is usually near 100 MHz.
We split this pulse train into two using a beam splitter. One half is used to gen-
erate the terahertz radiation, by exciting a photoconductive antenna or by optical
rectification, for example. The second half is used to gate a detector, typically either
via a photoconductive or electro-optic interaction. Ideally the detector is sensitive to
the incoming terahertz field only for a brief period of less than one picosecond. We
can use this brief temporal window to sample the terahertz field at various delays,
relative to the arrival of the terahertz pulse at the detection point. In other words, we
determine the terahertz electric field as a function of time by measuring the value of
EτHz(t) at a particular value of t , and then repeat the measurement many times, at

<<   22   23   24   25   26   27   28   >>

More intriguing information