Longitudinal THz electric fields in the focus of radially polarized beams


Longitudinal THz electric fields in the focus of radially polarized beams

Winnerl, S.; Hubrich, R.; Peter, F.; Helm, M.; Schneider, H.

While typical pulsed terahertz (THz) systems operate with freely propagating THz waves of linear polarization and Gaussian beam profile, modes of different polarization can also be attractive. In particular, radially polarized beams exhibit longitudinal fields at the focus and smaller spot sizes as compared to linearly polarized beams. These key properties of radially polarized beams have been demonstrated indirectly for visible light [1]. Recently radially polarized single cycle THz beams have been generated by velocity-mismatched optical rectification [2] and photoconductive antennas [3,4]. However, longitudinal THz fields have been observed so far only in the near field of plasmon-polariton excitations in a metal tip [5].

We report on longitudinal THz fields in the focus of freely propagating waves. Single cycle THz pulses with radial polarization are generated by photoexcitation of emitters with an electrode structure consisting of concentric rings [4]. By means of electro-optic detection we determine the phase relation between transverse and longitudinal field components. The radiation is focused onto ZnTe sensing crystals oriented along the (110) and (100) axis, respectively, in order to detect transverse and longitudinal THz fields. While the THz beam is kept fixed, the electro-optic detection system is scanned along the x-axis, which denotes the horizontal direction perpendicular to the axis of propagation. For comparison, emitters for linearly polarized beams are operated under the same conditions.

The transverse field changes sign, as the detector is moved across the propagation axis located at x = 0, as expected for a radially polarized wave. The longitudinal field, however, is located around the propagation axis and does not change sign as x changes from negative to positive. At t = 0 where the transverse fields exhibits extrema (dashed line), the longitudinal field is zero. This phase difference of pi/2 has been predicted theoretically as a consequence of Maxwell’s equation div E = 0. To our knowledge, this phase relation has not been measured previously in the entire range of the electromagnetic spectrum. Furthermore we show that the spot size of the longitudinal component is smaller than the diffraction limited spot of a linearly polarized THz beam. The results are analyzed by calculations based on the vector Helmholtz equation beyond the paraxial approximation.

[1] R. Dorn, S. Quabis, G. Leuchs, Sharper focus for a radially polarized light beam, Phys. Rev. Lett. 91, 233901 (2003).
[2] G. Chang, Ch.J. Divin, C.-H. Liu, S.L. Williamson, A. Galvanauskas, T.B. Norris, Generation of radially polarized terahertz pulses via velocity-mismatched optical rectification, Opt. Lett. 32, 433 (2006).
[3] J.A. Deibel, K. Wang, M.D. Escarra, D. Mittleman, Enhanced coupling of terahertz radiation to cylindrical wire waveguides, Opt. Express. 14, 279 (2006).
[4] S. Winnerl, B. Zimmermann, F. Peter, H. Schneider, M. Helm, Thz Bessel-Gauss beams of radial and azimuthal polarization from microstructured photoconductive antennas, Opt. Express 17, 1571 (2009).
[5] N.C.J. van der Valk and P.C.M. Planken, Electro-optic detection of subwavelength terahertz spot sizes in the near field of a metal tip, Appl. Phys. Lett. 81, 1558 (2002).

Keywords: terahertz waves; longitudinal and azimuthal fields; radial polarization

  • Lecture (Conference)
    5th International Symposium on Ultrafast Phenomena & Terahertz Waves (ISUPTW'2010), 12.-16.09.2010, Xi'an, China

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Publ.-Id: 14774