3.1 Investigating Anisotropy at Terahertz Frequencies
45
this method the optical properties of a sample can be extracted via a lock-in detection
scheme [6], which simultaneously measures two components of the complex Jones
transfer matrix. This demonstrates an advantage over the static polariser method,
which requires two measurements with WGP 2 at different orientations to achieve
the same results. However, since the detectors used in these methods are only sensitive
to polarisation in one direction, only two components of the transfer matrix can be
extracted for a given orientation of the detector, e.g. only the ˜
t xx and ˜
t yx components
can be extracted when the detector is oriented along the x-axis in Fig. 3.1. To extract
the other two components, ˜
t xy and ˜
t yy , a rotation of 90
◦ is required in either the
sample, or a simultaneous rotation of the THz emitter, detector and WGPs 1 and 3.
Using this rotatable WGP method, Morris et al.. were able to achieve a precision
in the measured polarisation angle of ∼ 0.02
◦ for frequencies below 1.5 THz after
20 min of measurement time. This is in contrast to detection schemes based on multicontact PCEs [13–15] and static wire-grid polarisers [16] which have a precision on
the order of 0.2
◦ .
3.1.2.3 Terahertz Ellipsometry
Ellipsometry is often performed in the reflection geometry, and makes use of the
different reflectivity of the s- and p-polarised components of light from the surface
of, or interface between, materials. As such, a WGP is often used to project the
generated THz polarisation to 45
◦ , in order to obtain equal horizontal and vertical
components of the THz pulse incident on the sample. A rotatable WGP then selects
between sensitivity to the reflected s- or p-component, before a final analyser WGP
projects the polarisation state to −45
◦ before detection.
Many of the experimental techniques used in THz ellipsometry are similar to those
performed in THz polarimetry, except measurements are performed in reflection,
rather than transmission. Many methods of rotating the sample or projecting the
polarisation state via WGPs are similar. Further information can be found in reference
[17].
3.1.3 Methods of Terahertz Polarisation Rotation
The majority of anisotropic THz spectroscopy methods make use of rotating the
sample, changing the directions of D a,b relative to E, in order to investigate the
anisotropy. While simple to perform at room temperature, at cryogenic temperatures
or in high external magnetic fields rotating the sample is challenging. Also, if the
sample is not perfectly aligned with the axis of rotation, different numbers of grains
or domains in the sample may be probed at different angles. This is an important
consideration in inhomogeneous materials such as LaAlO 3 , where the transmitted
THz polarization state depends strongly on the size and number of domains probed
[1].
45
this method the optical properties of a sample can be extracted via a lock-in detection
scheme [6], which simultaneously measures two components of the complex Jones
transfer matrix. This demonstrates an advantage over the static polariser method,
which requires two measurements with WGP 2 at different orientations to achieve
the same results. However, since the detectors used in these methods are only sensitive
to polarisation in one direction, only two components of the transfer matrix can be
extracted for a given orientation of the detector, e.g. only the ˜
t xx and ˜
t yx components
can be extracted when the detector is oriented along the x-axis in Fig. 3.1. To extract
the other two components, ˜
t xy and ˜
t yy , a rotation of 90
◦ is required in either the
sample, or a simultaneous rotation of the THz emitter, detector and WGPs 1 and 3.
Using this rotatable WGP method, Morris et al.. were able to achieve a precision
in the measured polarisation angle of ∼ 0.02
◦ for frequencies below 1.5 THz after
20 min of measurement time. This is in contrast to detection schemes based on multicontact PCEs [13–15] and static wire-grid polarisers [16] which have a precision on
the order of 0.2
◦ .
3.1.2.3 Terahertz Ellipsometry
Ellipsometry is often performed in the reflection geometry, and makes use of the
different reflectivity of the s- and p-polarised components of light from the surface
of, or interface between, materials. As such, a WGP is often used to project the
generated THz polarisation to 45
◦ , in order to obtain equal horizontal and vertical
components of the THz pulse incident on the sample. A rotatable WGP then selects
between sensitivity to the reflected s- or p-component, before a final analyser WGP
projects the polarisation state to −45
◦ before detection.
Many of the experimental techniques used in THz ellipsometry are similar to those
performed in THz polarimetry, except measurements are performed in reflection,
rather than transmission. Many methods of rotating the sample or projecting the
polarisation state via WGPs are similar. Further information can be found in reference
[17].
3.1.3 Methods of Terahertz Polarisation Rotation
The majority of anisotropic THz spectroscopy methods make use of rotating the
sample, changing the directions of D a,b relative to E, in order to investigate the
anisotropy. While simple to perform at room temperature, at cryogenic temperatures
or in high external magnetic fields rotating the sample is challenging. Also, if the
sample is not perfectly aligned with the axis of rotation, different numbers of grains
or domains in the sample may be probed at different angles. This is an important
consideration in inhomogeneous materials such as LaAlO 3 , where the transmitted
THz polarization state depends strongly on the size and number of domains probed
[1].