46
3 Rotatable-Polarisation Terahertz Time-Domain Spectroscopy of Anisotropic Media
Alternatively to rotating the sample, the THz polarization state itself may be
rotated, changing the orientation of E relative to D a,b . A number of methods of
achieving this have been reported in the literature: changing the orientation angle of
the generated beam during the emission process; rotating the orientation angle using
a half-waveplate; and projecting the polarisation state using a wire-grid polariser
(WGP). The latter two methods are most commonly used in spectroscopic ellipsometry, with WGPs favored for THz ellipsometry [17]. The low extinction ratio and
frequency-dependent response of many THz wire-grid polarizers can have an influence on the detected polarisation state [5, 18]. The use of WGPs also results in a
reduction of signal size, and extra data analysis is required to extract the orthogonal
electric field components. Further, there is a π/2 phase shift between light parallel to and perpendicular to the wires [19] that introduces a finite ellipticity to the
transmitted beam, which will be demonstrated later in Sect. 3.3.
Ideally, a polarization rotation system should satisfy the following criteria: a
minimal insertion loss for any components used, minimal change in E with rotation
angle in terms of the amplitude |E| and polarisation state (ψ and χ), precise and
accurate determination of the polarisation state (ψ and χ), and uniform operation
over a wide bandwidth. The summary of polarization-rotation methods presented in
this section is by no means exhaustive, but is intended to highlight to the reader that
currently no scheme meets all the criteria set out above.
3.1.3.1 Terahertz Waveplates
Half-waveplates made of a single birefrigent material [1] and metasurface-based
polarization rotators [20] are intrinsically narrowband and have finite loss, and are
thus not ideal for broadband THz spectroscopy with arbitrarily rotatable THz pulses.
Recently however, efforts have been made to produce metamaterial-based devices
that operate over a broader frequency range [21–23]. Internal reflection within a prism
offers broadband polarization rotation: for instance a half-waveplate with retardance
close to π and varying by ±6
◦ with frequency was reported [24]. However these
components are large and require collimated THz beams to function optimally, and
there is an additional loss of signal amplitude due to Fresnel reflection losses on
entering and exiting the prism.
3.1.3.2 Rotation During the Emission Process
A rotation of the generated THz beam has been demonstrated for plasma-based
THz emission [25], however this is reliant upon high pulse energy laser amplifiers
with lower repetition rates. In terms of the more widely-used THz-TDS systems
based on laser oscillators, photoconductive emitters are the THz source of choice for
spectroscopy and imaging applications in custom-made and commercial systems.
The coarse rotation (to 0
◦ , 45
◦ and 90
◦ ) of a wide-area photoconductive emitter has
been reported, to alternate between vertical and horizontal THz emission [13]. This
3 Rotatable-Polarisation Terahertz Time-Domain Spectroscopy of Anisotropic Media
Alternatively to rotating the sample, the THz polarization state itself may be
rotated, changing the orientation of E relative to D a,b . A number of methods of
achieving this have been reported in the literature: changing the orientation angle of
the generated beam during the emission process; rotating the orientation angle using
a half-waveplate; and projecting the polarisation state using a wire-grid polariser
(WGP). The latter two methods are most commonly used in spectroscopic ellipsometry, with WGPs favored for THz ellipsometry [17]. The low extinction ratio and
frequency-dependent response of many THz wire-grid polarizers can have an influence on the detected polarisation state [5, 18]. The use of WGPs also results in a
reduction of signal size, and extra data analysis is required to extract the orthogonal
electric field components. Further, there is a π/2 phase shift between light parallel to and perpendicular to the wires [19] that introduces a finite ellipticity to the
transmitted beam, which will be demonstrated later in Sect. 3.3.
Ideally, a polarization rotation system should satisfy the following criteria: a
minimal insertion loss for any components used, minimal change in E with rotation
angle in terms of the amplitude |E| and polarisation state (ψ and χ), precise and
accurate determination of the polarisation state (ψ and χ), and uniform operation
over a wide bandwidth. The summary of polarization-rotation methods presented in
this section is by no means exhaustive, but is intended to highlight to the reader that
currently no scheme meets all the criteria set out above.
3.1.3.1 Terahertz Waveplates
Half-waveplates made of a single birefrigent material [1] and metasurface-based
polarization rotators [20] are intrinsically narrowband and have finite loss, and are
thus not ideal for broadband THz spectroscopy with arbitrarily rotatable THz pulses.
Recently however, efforts have been made to produce metamaterial-based devices
that operate over a broader frequency range [21–23]. Internal reflection within a prism
offers broadband polarization rotation: for instance a half-waveplate with retardance
close to π and varying by ±6
◦ with frequency was reported [24]. However these
components are large and require collimated THz beams to function optimally, and
there is an additional loss of signal amplitude due to Fresnel reflection losses on
entering and exiting the prism.
3.1.3.2 Rotation During the Emission Process
A rotation of the generated THz beam has been demonstrated for plasma-based
THz emission [25], however this is reliant upon high pulse energy laser amplifiers
with lower repetition rates. In terms of the more widely-used THz-TDS systems
based on laser oscillators, photoconductive emitters are the THz source of choice for
spectroscopy and imaging applications in custom-made and commercial systems.
The coarse rotation (to 0
◦ , 45
◦ and 90
◦ ) of a wide-area photoconductive emitter has
been reported, to alternate between vertical and horizontal THz emission [13]. This