4.4 Rapid Modulation of Circular Polarisation States for Circular Dichroic Spectroscopy
87
to produce circularly polarised THz pulses, the bias voltages on the two contacts
were varied according to Eqs. 4.6 and 4.7. This calibration procedure identified an 8
◦
misalignment of the device from its intended 45
◦ orientation. The optimal voltages
were found to be V H = 9.903 V and V V = 1.392 V to produce a right-hand circularly
polarised THz pulse, corresponding to a linear THz pulse being emitted at an angle
relative to the orientation of the emitter of ψ R = 8
◦ , and V H = −1.392 V and V V =
9.903 V to produce a left-hand circularly polarised THz pulse, corresponding to a
relative emission angle of ψ R = 98
◦ .
The resulting optimised right-hand circularly polarised and left-hand circularly
polarised THz pulses are presented in Fig. 4.7a and b, respectively. The purity of the
circular polarisation states were analysed using the Stokes parameter V normalised
by the total intensity I , as described in Sect. 4.4.2. Figure 4.7c shows V /I for both circular THz pulses in Fig. 4.7a and b over the experimental bandwidth of 0.3−2.5 THz.
Bandwidth was limited in this setup due to transmission losses on entering and exiting the silicon prism. For both chiralities, V /I remains close to the target values
of ±1, demonstrating the purity of the circular states created by this method. Small
deviations away from V /I = ±1 at different frequencies may occur because of the
different divergence at different frequencies giving a small frequency dependence to
θ i ; this could be avoided by collimating the THz emission from the device before the
prism. As in the linear emitting case presented in Sect. 4.3 the purely electrical nature
of the polarisation state switching allows rapid modulation between the two states,
which will permit fast broadband circular dichroism spectroscopic measurements to
be performed at THz frequencies using these multi-pixel devices.
4.5 Summary
To summarise, a novel photoconductive emitter geometry for broadband THz polarisation rotation, consisting of separate interdigitated pixels for emission of horizontal
and vertical polarisation states, has been proposed and tested. The generated orientation angle of the THz pulse was shown to be controllable by varying the relative bias
voltages applied to the horizontally and vertically emitting contacts, and remained
close to the target values over a 360
◦ rotation of the polarisation state. Simulations of
the emitted radiation pattern showed that in the far-field beams from the individual
pixels have overlapped, and demonstrated that the calculated gaussian beam quality of the emitter is higher at lower frequency. The wide bandwidth (0.3−5.0 THz),
scalable design, and simple polarisation rotation method (varying the applied bias
voltage to each contact) make this emitter geometry an attractive concept for applications in spectroscopy and imaging systems, in which the THz polarisation state
may be rotated arbitrarily without resorting to any physically moving parts. Since the
polarisation angle of the THz radiation generated by this device depends only upon
the applied bias voltages, the THz polarisation can be modulated at speeds much
faster than systems relying on mechanically rotating components, such as polarizers,
which are limited to low frequencies (e.g. 15 Hz) [22]. In addition, an experimental
87
to produce circularly polarised THz pulses, the bias voltages on the two contacts
were varied according to Eqs. 4.6 and 4.7. This calibration procedure identified an 8
◦
misalignment of the device from its intended 45
◦ orientation. The optimal voltages
were found to be V H = 9.903 V and V V = 1.392 V to produce a right-hand circularly
polarised THz pulse, corresponding to a linear THz pulse being emitted at an angle
relative to the orientation of the emitter of ψ R = 8
◦ , and V H = −1.392 V and V V =
9.903 V to produce a left-hand circularly polarised THz pulse, corresponding to a
relative emission angle of ψ R = 98
◦ .
The resulting optimised right-hand circularly polarised and left-hand circularly
polarised THz pulses are presented in Fig. 4.7a and b, respectively. The purity of the
circular polarisation states were analysed using the Stokes parameter V normalised
by the total intensity I , as described in Sect. 4.4.2. Figure 4.7c shows V /I for both circular THz pulses in Fig. 4.7a and b over the experimental bandwidth of 0.3−2.5 THz.
Bandwidth was limited in this setup due to transmission losses on entering and exiting the silicon prism. For both chiralities, V /I remains close to the target values
of ±1, demonstrating the purity of the circular states created by this method. Small
deviations away from V /I = ±1 at different frequencies may occur because of the
different divergence at different frequencies giving a small frequency dependence to
θ i ; this could be avoided by collimating the THz emission from the device before the
prism. As in the linear emitting case presented in Sect. 4.3 the purely electrical nature
of the polarisation state switching allows rapid modulation between the two states,
which will permit fast broadband circular dichroism spectroscopic measurements to
be performed at THz frequencies using these multi-pixel devices.
4.5 Summary
To summarise, a novel photoconductive emitter geometry for broadband THz polarisation rotation, consisting of separate interdigitated pixels for emission of horizontal
and vertical polarisation states, has been proposed and tested. The generated orientation angle of the THz pulse was shown to be controllable by varying the relative bias
voltages applied to the horizontally and vertically emitting contacts, and remained
close to the target values over a 360
◦ rotation of the polarisation state. Simulations of
the emitted radiation pattern showed that in the far-field beams from the individual
pixels have overlapped, and demonstrated that the calculated gaussian beam quality of the emitter is higher at lower frequency. The wide bandwidth (0.3−5.0 THz),
scalable design, and simple polarisation rotation method (varying the applied bias
voltage to each contact) make this emitter geometry an attractive concept for applications in spectroscopy and imaging systems, in which the THz polarisation state
may be rotated arbitrarily without resorting to any physically moving parts. Since the
polarisation angle of the THz radiation generated by this device depends only upon
the applied bias voltages, the THz polarisation can be modulated at speeds much
faster than systems relying on mechanically rotating components, such as polarizers,
which are limited to low frequencies (e.g. 15 Hz) [22]. In addition, an experimental