Abstract-Large-area photoconductive switches as emitters of terahertz pulses with fully electrically controlled linear polarization

Kenneth Maussang, José Palomo, Juliette Mangeney, Sukhdeep S. Dhillon, and Jérôme Tignon

Fig. 1 (a) Cut view of an interdigitated photoconductive switch. Interdigitated gold electrodes on top of the GaAs layer consist of 4μm wide electrodes, equally spaced by a distance 𝛥$\Delta$ = 4μm. A second metallic layer is composed of metallic fingers covering gaps with a periodicity double that of the first, isolated from the first metallic layer by a 300nm thick layer of SiO2. The femtosecond excitation pulse is focused on the front face of the photoconductive switch generating carriers in the GaAs layer (electrons in blue and holes in red). (b) Top view of the intermixed geometry principle (only the first metallic layer is represented). The pairs of digits share a common ground potential VG, but can be polarized independently with two different electrical potentials VH and VV, resulting is respectively horizontal and vertical polarization. (c) Large area implementation investigated experimentally (only the first metallic layer is represented). The total area of the gold finger electrodes is 450μm × 450μm. (d) Orientation of the wire-grid polarizer 𝑢→$\overrightarrow{u}$with respect to the interdigitated structure directions(𝑢→𝐻,𝑢→𝑉)$({\overrightarrow{u}}_H,{\overrightarrow{u}}_V)$for the emitted field experimental characterization.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-10-14784

Polarimetric measurements in the terahertz (THz) range have a wide range of applications in material science and physico-chemistry. Usually performed using mechanically controlled elements, such measurements are inherently limited in precision and acquisition rate. Here, we propose and realize an innovative concept of a THz pulse emitter, linearly polarized, which allows electrical continuous control of the polarization direction and modulation ability up to several tens of kHz. It consists in an interdigitated photoconductive switch with an intermixed sickle geometry, where the vertical and horizontal components of the electric field are intermixed at a subwavelength scale. We demonstrate that such an emitter permits control of the direction and amplitude emitted with an excellent degree of polarization up to 4 THz, which is estimated to be experimentally better than 98%. This work opens perspectives for sensitivity improvements in THz polarimetry with lock-in detection schemes.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-THz surface plasmon polariton modes coupled to complementary metasurfaces tuned by inter meta-atom distance

Janine Keller, Curdin Maissen,   Johannes Haase,  Gian Lorenzo Paravicini-Bagliani,  Federico Valmorra, José Palomo, Juliette Mangenev,  Jérôme Tignon,   Sukhdeep S. Dhillon,  Giacomo Scalari, Jérôme Faist

http://ieeexplore.ieee.org/document/8087672/

Tailoring the electro-magnetic response of materials beyond naturally occurring properties is possible with the concept of meta-materials [1]. Subwavelength elements which are usually closely spaced can influence the electro-magnetic response and form a fundamental building block of modern optics. The influence of the spacing of the meta-atoms has been investigated for direct meta-materials [2] but only little for complementary metamaterials [3], which are of interest e.g. in ultra-strong coupling experiments at THz frequencies [4]. The effective medium condition is changing due to the presence of a metal sheet in between the meta-atoms which has a very high refractive index in the THz.THz time domain spectroscopy was performed for 15 samples with a constant frequency of the complementary split ring resonator (cSRR) and varying inter meta-atom distances from 40 μm to 160 μm [6], a sample sketch is shown in Fig. 1 a). For spacings of the cSRR that are not strictly subwavelength anymore, a regime of resonant coupling to THz surface plasmon polaritons (SPPs) [5] is entered. We observe an anti-crossing (shown in Fig. 1 b)) of the cSRR LC-mode and the SPP-mode, leading to a strong coupling with a normalized coupling ratio of 3.5% at the resonance frequency of 1.07 THz. Finite element simulations with CST MWS show the characteristic field distribution of the two modes in the plane of the resonator and the intermixing of the LCmode with the SPP-mode very clearly as well as simulations of the mode extensions into the substrate (see Fig. 1 c)). Analytical modeling with a simple two oscillator model describes the coupling well and yields an effective relative permittivity of 11.6 for the coupled system. Measurements of the broader, dipole-like resonance of the cSRR in orthogonal polarization direction show a Fano-like lineshape when the SPPs tune across. Utilizing rectangular array configurations we show that the excitation direction lies along the polarization of the exciting THz pulse. Additionally, we measured the dependence of the incident angle on the frequency of the measured SPPs, where we see a splitting of the SPP. We demonstrate that the understanding of the SPP modes is fundamental for research and applications in which the metasurface has to be designed for special needs.