3.1 Investigating Anisotropy at Terahertz Frequencies
47
method produced an accuracy in the orientation angle of 5
◦ , a large ellipticity, and a
variation in |E| by 40% on rotating the emitter by 90
◦ .
3.2 Rotatable-Polarisation Terahertz Time-Domain
Spectrometer
As discussed in Sect. 3.1.3, each method of performing spectrocopy on anisotropic
media at THz frequencies comes with its own challenges. The rest of this chapter
will propose and demonstrate a solution to these issues, in the form of a Rotatablepolarisation-THz-TDS (RP-THz-TDS) system. This section will describe the adaptations made to the THz-TDS system detailed in Sect. 2.3, in order to generate an
arbitrarily-rotatable linear polarisation state, and to resolve the full polarisation state
at the detector. The optimisation of the system will also be described, such that all
the criteria for an idealised polarisation rotation system outlined in Sect. 3.1.3 are
satisfied.
3.2.1 Rotating the Terahertz Polarisation State
A schematic diagram of the method used to produce a rotatable, linear THz polarization state in the spectrometer discussed in this chapter is provided in Fig. 3.2a. This
method exploits the polarisation state of THz radiation generated by an interdigitated
photoconductive emitter; a photoconductive emitter produces THz radiation that is
linearly polarised parallel to the electric dipoles generated in the device, hence the
polarisation state will be parallel to the biasing electric field applied between the contacts of the device, signified by the red arrows in Fig. 3.2a. By mounting such a THz
emitter in a motorised rotation stage the orientation of the emitter ψ em , and hence
the orientation angle of the generated THz radiation, can be reliably and repeatably
varied to any arbitrary angle.
For results presented in this chapter, the 800 nm pump beam (red waveform in
Fig. 3.2a) had a power of 200 mW, and the photoexcited charge carriers in the GaAs
substrate were accelerated under an applied bias voltage of ±10 V. The interdigitated
emitter was fabricated (as described below) in the centre of a 25 mm×25 mm semiinsulating GaAs (SI-GaAs) wafer, which was securely glued over the middle of the
clear aperture of a Newport PR50CC motorised rotation stage. The angular range of
the emitter’s rotation was limited to 180
◦ , to ensure the THz generation beam was
never obscured by the emitter’s wiring.
47
method produced an accuracy in the orientation angle of 5
◦ , a large ellipticity, and a
variation in |E| by 40% on rotating the emitter by 90
◦ .
3.2 Rotatable-Polarisation Terahertz Time-Domain
Spectrometer
As discussed in Sect. 3.1.3, each method of performing spectrocopy on anisotropic
media at THz frequencies comes with its own challenges. The rest of this chapter
will propose and demonstrate a solution to these issues, in the form of a Rotatablepolarisation-THz-TDS (RP-THz-TDS) system. This section will describe the adaptations made to the THz-TDS system detailed in Sect. 2.3, in order to generate an
arbitrarily-rotatable linear polarisation state, and to resolve the full polarisation state
at the detector. The optimisation of the system will also be described, such that all
the criteria for an idealised polarisation rotation system outlined in Sect. 3.1.3 are
satisfied.
3.2.1 Rotating the Terahertz Polarisation State
A schematic diagram of the method used to produce a rotatable, linear THz polarization state in the spectrometer discussed in this chapter is provided in Fig. 3.2a. This
method exploits the polarisation state of THz radiation generated by an interdigitated
photoconductive emitter; a photoconductive emitter produces THz radiation that is
linearly polarised parallel to the electric dipoles generated in the device, hence the
polarisation state will be parallel to the biasing electric field applied between the contacts of the device, signified by the red arrows in Fig. 3.2a. By mounting such a THz
emitter in a motorised rotation stage the orientation of the emitter ψ em , and hence
the orientation angle of the generated THz radiation, can be reliably and repeatably
varied to any arbitrary angle.
For results presented in this chapter, the 800 nm pump beam (red waveform in
Fig. 3.2a) had a power of 200 mW, and the photoexcited charge carriers in the GaAs
substrate were accelerated under an applied bias voltage of ±10 V. The interdigitated
emitter was fabricated (as described below) in the centre of a 25 mm×25 mm semiinsulating GaAs (SI-GaAs) wafer, which was securely glued over the middle of the
clear aperture of a Newport PR50CC motorised rotation stage. The angular range of
the emitter’s rotation was limited to 180
◦ , to ensure the THz generation beam was
never obscured by the emitter’s wiring.