Optical Damage Threshold of Silicon for Ultrafast Infrared Pulses Page: 2 of 7
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As discussed in , optical breakdown in dielectric materials occurs in four general
steps: (1) Seed conduction electrons are generated by multiphoton ionization (MPI)
or tunnel ionization (TI), (2) they are accelerated in the laser field and generate an
avalanche by impact ionization, (3) the laser pulse heats the resulting plasma, and (4)
the electron energy is transferred to the lattice, resulting in ablation. This schematic de-
scription raises several questions. First, which process, MPI or TI, generates the seed
electrons? Second, does the seed process or the avalanche dominate the free carrier
generation? As a practical matter for accelerator structure design, can we significantly
increase the damage threshold by lengthening the wavelength beyond a multiphoton
threshold? For instance, the bandgap of silicon at room temperature is 1.12 eV, corre-
sponding to a free-space wavelength of 1107 nm. By using a wavelength longer than
1107 nm, seed electrons can only be generated by two-photon absorption, which has a
much lower cross-section than single-photon absorption. Similarly, operating at wave-
lengths beyond the two-photon threshold at 2214 nm might yield a further increase in
The experiments described in  were conducted at a wavelength of 800 nm on fused
silica (bandgap energy A t 9eV, requiring six-photon absorption) and barium aluminum
borosilicate (BBS, A t 4eV, requiring three-photon absorption). The authors reached
the conclusion that multiphoton ionization dominates as the seed process for FWHM
pulse widths t > 20 fs, while tunneling dominates for shorter wavelengths. Further,
they found that for fused silica, the avalanche dominates carrier generation for pulses
as short as 10 fs, while for the lower-bandgap BBS, multiphoton ionization takes over
for pulses shorter than 100 fs. Other experiments which have been conducted at 527 nm
and 1053 nm are consistent with these conclusions . However, another experiment
measuring breakdown thresholds as a function of polarization has called into question
the significance of MPI in the breakdown process . Also, evidence indicates that TI
is the dominant ionization process in the mid-infrared . Therefore the sustainable
gradient cannot be arbitrarily increased by using longer wavelengths.
EXPERIMENTAL SETUP AND PROCEDURE
As samples for the damage study, we used undoped crystalline silicon cut from a wafer
with a (100) surface orientation. We conducted experiments starting at X = 1550nm and
extending longer in wavelength toward 2200 nm. The infrared pulses were generated by
a commercial Spectra-Physics OPA-800 optical parametric amplifier (OPA) pumped by
a Ti:sapphire laser system. The OPA produced pulses with energy > 20pJ and FWHM
duration between 300 and 700 fs, depending on wavelength. The repetition rate of the
laser was 240 Hz, so the experiment detected multiple-shot damage.
This was a pump-probe measurement in which a CW helium-neon laser was focused
on the same spot on the sample as the infrared pulses and damage was detected by
observing a decrease in reflected HeNe intensity. A schematic of the experiment is shown
in Figure 1. The infrared pulses were normally incident on the sample, focused by a
CaF2 lens to minimize dispersion, while the HeNe beam was incident at an angle. The
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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/m1/2/: accessed July 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.