Terahertz Emitters and Detectors for Radially and Azimuthally Polarized Beams

Typical pulsed terahertz (THz) systems operate with freely propagating THz waves of linear polarization and Gaussian beam profile. In the range of visible light and near-infrared radiation, Bessel-Gauss modes have stimulated great interest over last years. The lowest order Bessel-Gauss modes have a donut-like intensity distribution and are either radially or azimuthally polarized. Radially polarized beams have interesting fundamental properties such as smaller beam waists in the focus as compared to Gaussian modes and strong longitudinal field components in the focus. In the THz range, superior coupling properties for radially polarized light to plasmonic guided modes on wires, so called Sommerfeld modes are predicted. While there are many investigations on these guided THz modes, only little work is done on freely propagating modes of radial polarization [1] and no publications exist for azimuthally polarized THz beams. Here we present a concept that allows creating emitters and detectors for any desired mode. The principle is demonstrated by showing results on radially and azimuthally polarized THz beams.
The photoconductive emitter and detector structures are based on a microstructured electrode pattern on a semiconductor substrate. The electrode pattern is inverse to the desired polarization pattern, i.e. emitters for radially polarized modes consist of equidistant concentric ring electrodes, while emitters for azimuthally polarized modes are a circular structure of metallized sectors. To avoid electric fields pointing in the opposite direction, a second metallization layer is used in a similar way as previously described for emitter of linearly polarized radiation [2]. While the emitters were fabricated on semi-insulating GaAs, detectors were prepared on GaAs substrates implanted with N+. A titanium sapphire laser was used for excitation of the emitters and gating of the detectors. Beam profiles were measured for the divergent beam 25 mm behind the emitters, and for the refocused beam after a path of ~0.5 m. In this experiment the detector was sensitive for a linearly polarized component of the radiation. The observed beam profiles agree well with the ones expected for Bessel-Gauss beams. Furthermore radiation with radial polarization was detected with an antenna optimized for radial polarization and one for azimuthal polarization. With the detector for azimuthal polarization no signal is expected. However, due to imperfections of the beam profile of the near-infrared laser, a residual signal of about 10 % compared to the signal measured with the detector for radial polarization is observed. This experiment serves as a proof of principle that the emitter-detector combination well suited for studying changes of the mode structure.

References:

[1] G. Chang, Ch.J. Divin, C.-H. Liu, S.L. Williamson, A. Galvanauskas, and T.B. Norris, Opt. Lett. 32, 433 (2006).
[2] A. Dreyhaupt, S. Winnerl, T. Dekorsy, and M. Helm, Appl. Phys. Lett. 86, 121114 (2005).