Precise and accurate control of the ellipticity of THz radiation using a photoconductive pixel array

Full control of the ellipticity of broadband pulses of THz radiation, from linear to left- or right-handed circular polarization, was demonstrated via a 4-pixel photoconductive emitter with an integrated achromatic waveplate. Excellent polarization purity and accuracy was achieved, with Stokes parameters exceeding 97% for linear and circular polarization, via a robust scheme that corrected electrically for polarization changes caused by imperfect optical elements. Further, to assess the speed and precision of measurements of the THz polarization we introduced a new figure of merit, the standard error after one second of measurement, found to be 0.047$^{\circ}$ for the polarization angle.