Mica Waveplates
Mica
can be used to retard the passage of the extra-ordinary ray resulting in a
change of phase difference between the s– and p– states of polarisation.
Mica is a low bi-refringence, laminar crystal, which is easily cleaved.
By controlling the thickness of sheets of mica it is possible to achieve
precise amounts of first-order retardation.
A quarter-wave plate has a net retardation of
p/2 and changes the
polarisation state from linear to circular or
vice-versa. A half-wave plate has a
net retardation of p
and rotates the plane of polarisation by 90°.
These
first-order waveplates are mica sheets with known amounts of retardation at a
specific wavelength. While λ/2 and λ/4 are the most commonly requested
retardations, it is possible to supply these for any chosen retardation at any
wavelength from 400 nm to 2.5μm. The
retardation tolerance is λ/50.
It
is also possible to provide broadband multi-order mica waveplates, which work
over a spectral range of 400-2500 nm with a retardation tolerance of λ/20 @
550 nm.
Mica
waveplates are recommended for low power applications since the material has
high absorption and some inhomogeneity.
They
are normally provided as circular plates of 10, 20, 25, 30, 40 or 50 mm
diameter. The mica sheets are
cemented between protective glass discs. Each
plate has two dots on the face of the protective glass indicating the direction
of the crystalline optic axis. The
nominal thickness of these plates is between 2.5 and 3.5 mm.