THz technology, THz vector beams
Photoconductive emitters excited with femtosecond laser pulses are a widely used source for single-cycle terahertz (THz) radiation pulses. By application of an interdigitated electrode geometry we have improved the emitter efficiency and THz output power significantly. To prevent destructive interference of THz wavelets generated by electrons that are accelerated in opposite directions, a second metallization (green in figure) prevents photogeneration of carriers in every second gap between the electrodes. Emitters based on this patented design are commercially available.
Semi-insulating GaAs is the standard material for photoconductive emitters excited with near infrared radiation from titanium-sapphire lasers. Modification of the material by ion beam irradiation results in the generation of trapping centers, which efficiently reduce the carrier lifetime. Such materials are well suited for photoconductive terahertz detectors. Materials exhibiting a lower band gap (such as InGaAs) and at the same time high resistivities are of interest for devices with can be excited with compact fiber lasers operating at 1.55 µm.
Scalable THz emitters offer freedom with respect to the electrode geometry. This can be used to generate modes that differ from the commonly used linearly polarized Gauss beams. These more general modes are called vector beams since they are solutions of the vector Helmholz equation. We generated radially and azimuthally polarized THz beams by photoconductive emitters. Especially radially polarized beams have interesting properties. They can be focussed to smaller spot sizes as compared to linearly polarized beams and they exhibit longitudinal field components in the focus. Furthermore they are well suited for exciting guided modes on metal wires, so called Sommerfeld modes.
K. J. Kaltenecker, J. C. Otto, M. Mittendorff, S. Winnerl, H. Schneider, M. Helm, H.P. Helm, M. Walther, and B. M. Fischer, Gouy phase shift of a radially polarized Gaussian beam, Optica 3, 35 (2016)
M. Xu, M. Mittendorff, R. J. B. Dietz, H. Künzel, B. Sartorius, T. Göbel, H. Schneider, M. Helm, S. Winnerl, THz generation and detection with InGaAs-based large-area photoconductive devices excited at 1.55 µm, Appl. Phys. Lett. 103, 251114 (2013)
M. Mittendorff, M. Xu, R. J. B. Dietz, H. Künzel, B. Sartorius, H. Schneider, M. Helm, S. Winnerl, Large area photoconductive THz emitter for 1.55 µm excitation based on an InGaAs heterostructure, Nanotechnology 24, 214007 (2013)
S. Winnerl, R. Hubrich, M. Mittendorff, H. Schneider, M. Helm, Universal phase relation between longitudinal and transverse fields observed in focused terahertz beams, New J. Phys. 14, 103049 (2012)
S. Winnerl, B. Zimmermann, F. Peter, H. Schneider, M. Helm, Terahertz Bessel-Gauss beams of radial and azimuthal polarization from microstructured photoconductive antennas, Optics Express 17, 1571-1576 (2009).
F. Peter, S. Winnerl, H. Schneider, M. Helm, K. Köhler, Terahertz emission from a large-area GaInAsN emitter, Appl. Phys. Lett. 93, 101102 (2008).
F. Peter, S. Winnerl, S. Nitsche, A. Dreyhaupt, H. Schneider, M. Helm, Coherent terahertz detection with a large-area, non-resonant photoconductive THz antenna, Appl. Phys. Lett. 91, 081109 (2007).
A. Dreyhaupt, S. Winnerl, M. Helm, T. Dekorsy, Optimum excitation conditions for the generation of high-electric-field THz radiation from an oscillator-driven photoconductive device, Optics Lett. 31, 1546 (2006).
A. Dreyhaupt, S. Winnerl, T. Dekorsy, M. Helm, High-intensity THz radiation from a microstructured large area photoconductor, Appl. Phys. Lett. 86, 121114 (2005).