50
3 Rotatable-Polarisation Terahertz Time-Domain Spectroscopy of Anisotropic Media
Fig. 3.3 a–e Resistance of the interdigitated photoconductive emitter as it is raster scanned, under
illumination, relative to the photoexcitation beam. The angle the emitter was rotated to in each scan
is displayed above the panel. f Centre of the device at 10 ◦ steps over a 180 ◦ rotation of the emitter
select angles are shown in Fig. 3.3a–e, together with the centre of the device (white
dots).
The centre of the emitter was obtained by performing a centre of mass-type calculation on the resultant resistance maps; the centre coordinates x c and y c are found
using
x c (ψ em ) =
1
R tot (ψ em )
i, j
1
R(ψ em , x i , y j )
x i (ψ em ),
(3.2)
y c (ψ em ) =
1
R tot (ψ em )
i, j
1
R(ψ em , x i , y j )
y i (ψ em ),
(3.3)
where R tot =
i, j R(ψ em , x i , y j ), and i, j are the map coordinates. The co-ordinates
of the emitter’s centre are shown in Fig. 3.3f for all 10
◦ steps of ψ em over the full
180
◦ range, revealing that the emitter was off-centred from the stage’s rotation axis
by about 0.3 mm. For higher precision scans, i.e. scans with a smaller angular step
size than 10
◦ , the centre position was found by interpolating the data in Fig. 3.3f. A
larger emitter area or more precise mounting on the rotation stage’s axis could avoid
this x-y calibration step.
The amplitude of the horizontal and vertical components of the THz electric field
are shown in Fig. 3.4 versus ψ em by the red and blue data points, respectively. When
the applied electric field in the emitter was vertical, close to ψ em = 0 and 180
◦ ,
the amplitude of E y was a maximum, while E x was greatest near ψ em = 90
◦ . A
3 Rotatable-Polarisation Terahertz Time-Domain Spectroscopy of Anisotropic Media
Fig. 3.3 a–e Resistance of the interdigitated photoconductive emitter as it is raster scanned, under
illumination, relative to the photoexcitation beam. The angle the emitter was rotated to in each scan
is displayed above the panel. f Centre of the device at 10 ◦ steps over a 180 ◦ rotation of the emitter
select angles are shown in Fig. 3.3a–e, together with the centre of the device (white
dots).
The centre of the emitter was obtained by performing a centre of mass-type calculation on the resultant resistance maps; the centre coordinates x c and y c are found
using
x c (ψ em ) =
1
R tot (ψ em )
i, j
1
R(ψ em , x i , y j )
x i (ψ em ),
(3.2)
y c (ψ em ) =
1
R tot (ψ em )
i, j
1
R(ψ em , x i , y j )
y i (ψ em ),
(3.3)
where R tot =
i, j R(ψ em , x i , y j ), and i, j are the map coordinates. The co-ordinates
of the emitter’s centre are shown in Fig. 3.3f for all 10
◦ steps of ψ em over the full
180
◦ range, revealing that the emitter was off-centred from the stage’s rotation axis
by about 0.3 mm. For higher precision scans, i.e. scans with a smaller angular step
size than 10
◦ , the centre position was found by interpolating the data in Fig. 3.3f. A
larger emitter area or more precise mounting on the rotation stage’s axis could avoid
this x-y calibration step.
The amplitude of the horizontal and vertical components of the THz electric field
are shown in Fig. 3.4 versus ψ em by the red and blue data points, respectively. When
the applied electric field in the emitter was vertical, close to ψ em = 0 and 180
◦ ,
the amplitude of E y was a maximum, while E x was greatest near ψ em = 90
◦ . A