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Technology Update
Introduction
In sum-frequency mixing interactions
near the band edge of BBO (below 200 nm) thermal de-phasing
effects can significantly reduce conversion efficiency and cause
power instability. The de-phasing is a result of
heating from absorption of the generated sum-frequency signal.
The performance of the mixer can be greatly improved by cooling the crystal. In
a previously published paper, IBM Almaden Research Center
and Actinix described an OPO sum-frequency mixing system
that utilized thermo-electrically cooled BBO to provide stable,
moderate power at 193 nm for the exposure of millimeter
scale
fields in a liquid immersion interference lithography testbed. This technology update briefly reviews
and describes new work in this area.
Experimental Apparatus
The laser system used to generate 193.4
nm was the Model 3193, a
5kHz solid-state diode pumped laser source designed especially to provide a
high quality spatial mode for interference lithography. The flux-grown, uncoated BBO crystal was
enclosed in a sealed housing filled with a dry
atmosphere. The housing cover accommodated chilled water cooling of the TEC
heat sink. Using a four-stage thermoelectric cooler, the BBO crystal could be cooled to a
minimum temperature of 223K.
Phase matching angle versus
temperature
We measured the phase matching angle
for 193.4 nm generation as a function of crystal
temperature, for fixed UV and IR wavelengths, for a BBO
crystal cut at 78.5 degrees. Low powers were applied to
reduce absorption-induced thermal effects. The change in
external angle (the required crystal rotation angle)
relative to room temperature is shown for temperatures
down to ~260K. The slope of the resulting curve is 0.93
mrad/K. Using an average index of refraction of 1.7
(no(266) = 1.76, no(710) = 1.66, ne(193) = 1.73) this
corresponds to a variation in the internal
phase-matching angle of ~0.55 mrad/K. This value is
about an order of magnitude greater than that predicted
by Sellmeier curve fitting based on available harmonic
generation data and thermal coefficients of the indices
of refraction. One problem faced in using a model to
predict tuning behavior near the band edge in BBO
is the high absorption of the generated far-UV
radiation. The Sellmeir equations are derived typically
from data obtained in the infrared, visible and near UV
where the indices of refraction are not perturbed by
generated self-absorption.
Absorption versus temperature
Absorption of ~193.4-nm light in a
flux-grown BBO crystal was measured as a function of
crystal temperature at low incident power (< 5 mW)
between 225K and 300K (blue curve). Measured window
transmission and calculated normal-incidence Fresnel
reflection losses were removed in order to determine the
bulk power-absorption coefficient via T = exp(-aL).
For comparison, the red data points are bulk absorption
values in a Cz-grown BBO crystal, as measured by Kouta
and Kuwano (Opt. Lett. 24, 1230 ‘99).
5 kHz 193 nm generation: 300K and
226K
Power generation and phase-matching
properties were investigated using a room-temperature,
78.5-deg-cut 8x8x6-mm BBO crystal. From 800 mW of 266-nm
light and 1.2-W of 710-nm light incident on the crystal
(each beam with a TEM00 mode ~0.3 mm FWHM diameter),
over 20 mW of 193-nm light was produced. The high
absorption of the generated 193-nm light results in dramatic
hysteretic phase-matching behavior. Blue data points
(shown in the figure at the left) were taken for increasing crystal angle, and red data
points for the opposite direction. Each data point
represents the final power value that stabilized after
20 sec; each error bar indicates the initial power value
achieved immediately upon crystal rotation. In this case
the effective angular
tolerance is difficult to determine precisely, but the
external half-width at half-maximum is approximately 1.0 mrad,
corresponding to an internal FWHM of about 1.2 mrad.
Using the
SNLO
non-linear optics modeling program (AS Photonics), we
calculate that for this interaction the FWHM internal
angular tolerance should be 0.38 mrad. The large
discrepancy is due chiefly to a phase mismatch (delta K)
being imposed by the self heating (the temperature
bandwidth for this interaction is about 3 degrees
Kelvin).
A 76-degree-cut 8x8x6 mm BBO crystal
was then cooled to 226K and the experiment repeated with
all other factors constant. The maximum output
power increased to just under 40 mW.
The hysteresis is much reduced relative to the
room-temperature case, and the temperature-stabilized
crystal exhibits much greater long-term power stability.
The FWHM angular tolerance is approximately 1 mrad external,
0.6 mrad internal, which is one-half of the room-temperature value
and 1.5 times theoretical. The self-heating is far less
severe than at room temperature. The twice narrower
angular tolerance indicates that phase-matching is
preserved over twice the interaction length as it is at
300K.
Tunable 193-196 nm generation at 271K
A 10Hz, high energy pump laser (SP
Lab-150) was
used to drive a similar OPO/mixer architecture as the
5 KHz system described above. In this case the emphasis
is on generating mJ level pulses and tuning from 193 to
196 nm to enable optical studies near the band edge
of UV materials. The system (Model 1000)
employs a single stage TEC in the final mixing stage which can lower the BBO
temperature to just under 0 degrees centigrade. The BBO
is mounted within a dry nitrogen purged, sealed chamber
that is air cooled. The enhancement in energy efficiency at
minus 2 degrees C compared to room temperature is about
50%.
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