Optical Damage Threshold of Silicon for Ultrafast Infrared Pulses Page: 4 of 7
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FIGURE 2. Photo of the damage threshold measurement experiment.
and infrared pulse energy and reflected HeNe power were then acquired on a shot-to-
shot basis. Since the data acquisition rate was limited to ~ 100 Hz, not all samples were
acquired. The acquisition was stopped, and the beam stop reinserted, when either the
HeNe power decreased, indicating damage, or a certain amount of time, usually > 100 s,
had elapsed with no damage. Events were taken both above and below the damage
A sample event is plotted in Figure 3. We notice that the reflected HeNe power
increases for a fraction of a second before falling off. This is ostensibly due to additional
focusing from the silicon surface as the damage morphology develops. Indeed, we
observed using a CCD image of the reflected HeNe light that the mode pattern changes
for about a second during damage before finally disappearing.
From the multiple-shot data in this event and others like it, it is not immediately clear
how to compute the measured damage threshold. We assume that damage was initiated
by a single pulse, with further damage occurring in each subsequent shot due to the
field enhancement resulting from the initial deformation of the surface. That pulse may
or may not have been acquired, and the data do not tell us how long before the visible
onset of damage the pulse occurred. However, we can make a maximum likelihood
estimate for the damage threshold based on a few assumptions. First, the damage process
is deterministic, as reported in , so damage did occur after a pulse that exceeded the
threshold and did not otherwise. Second, at least one pulse with energy above threshold
therefore must have occurred within the 1000 acquired shots before the visible onset
of damage. Third, no such shot occurred more than 1000 shots before the visible onset
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Cowan, B. Optical Damage Threshold of Silicon for Ultrafast Infrared Pulses, article, September 7, 2006; [Menlo Park, California]. (digital.library.unt.edu/ark:/67531/metadc882699/m1/4/: accessed July 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.