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-Generating ultrafast pulses of light from quantum cascade lasers

Feihu Wang, Kenneth Maussang, Souad Moumdji, Raffaele Colombelli, Joshua R. Freeman, Iman Kundu, Lianhe Li, Edmund H. Linfield, A. Giles Davies, Juliette Mangeney, Jérôme Tignon, and Sukhdeep S. Dhillon

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-2-11-944

The generation of ultrashort pulses from quantum cascade lasers (QCLs) has proved to be challenging. It has been suggested that the ultrafast electron dynamics of these devices is the limiting factor for mode locking and, hence, pulse formation. Even so, the clear mode locking of terahertz (THz) QCLs has been demonstrated recently, but the exact mechanism for pulse generation is not fully understood. Here we demonstrate that the dominant factor necessary for active pulse generation is in fact the synchronization between the propagating electronic modulation and the generated THz pulse in the QCL. By using the phase-resolved detection of the electric field in QCLs embedded in metal–metal waveguides, we demonstrate that active mode locking requires the phase velocity of the microwave round-trip modulation to equal the group velocity of the THz pulse. This allows the THz pulse to propagate in phase with the microwave modulation along the gain medium, permitting short-pulse generation. Mode locking was performed on QCLs employing phonon depopulation active regions, permitting the coherent detection of large gain bandwidths (500 GHz) and the generation of 11 ps pulses centered around 2.6 THz when the above “phase-matching” condition is satisfied. This work brings an enhanced understanding of QCL mode locking and will permit new concepts to be explored to generate shorter and more intense pulses from mid-infrared, as well as THz, QCLs.

© 2015 Optical Society of America

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