Tantalate Substrates for Pyroelectric Detectors
Lithium Tantalate ( LiTaO3 ) belongs
to class of ferroelectric crystals which possess intrinsic spontaneous
electric polarization. Usually crystal volume is divided into many microscopic
domains with randomly oriented polarizations. However ,it is possible to
align all domains in one direction parallel to so called polar axis by
applying high external electric field near the Curie temperature. After
such poling procedure crystal will stay in single domain state with uniform
polarization direction if operation temperature does not exceed Curie temperature.
Pyroelectric effect is change of polarization with temperature resulting
in a polarization current within a crystal. In practice, the effect can
be measured if electrodes are applied to crystal surfaces normal to polar
axis and connected through an external circuit. So pyroelectric substrate
( thin plate of pyroelectric material with proper electrodes) is a heart
of pyroelectric detectors featuring very broad operation wavelength with
extremely fast response time. LiTaO3 is the most popular material for pyroelectric
detector substrates due to itís very high pyroelectric coefficient, chemical
and mechanical stability and wide temperature range defined by high Curie
ELAN produces standard and custom substrates made of high optical quality,
fully poled LiTaO3 crystals. Standard specs are as follows:
Orientation: Z-cut +/- 15 arc. min.
Standard: 10 mm +/- 0.1 mm
Available: from 2x2 mm to 30x30 mm, depending on thickness
Standard: 50 microns +/- 5 microns
Available: from 20 microns to 200 microns , depending on diameter
Polish: both sides polished with 20/10 Scratch/Dig
Parallelism < 5 arc. Sec.
Electrodes: 25-30 nm Cromium
or 25-30 nm Cromium overcoated with 50-60 nm gold
Other specs are available on request.
Pyroelectric substrates made of Lithium Niobate (LiNbO3) are available
This crystal has less pyroelectric coefficient as compared to LiTaO3 but
itís Curie temperature is even higher ( 1150 deg. C as compared to 610 deg.
of LiTaO3) and better temperature stability of the detector. This
material is cheaper and available in larger sizes.