Abstract-Ultrafast response of harmonic modelocked THz lasers

Feihu Wang, Valentino Pistore, Michael Riesch, Hanond Nong, Pierre-Baptiste Vigneron, Raffaele Colombelli, Olivier Parillaud, Juliette Mangeney, Jerome Tignon, Christian Jirauschek ,  Sukhdeep S. Dhillon

Fundamental and harmonic modelocking in the a time and b frequency domains

https://www.nature.com/articles/s41377-020-0288-x

The use of fundamental mode locking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair, using quantum cascade lasers (QCLs) as a gain medium. However, unlike classic laser diodes, no demonstrations of harmonic modelocking, active or passive, have been shown in THz QCLs, where multiple pulses per round trip are generated when the laser is modulated at the harmonics of the cavity’s fundamental round-trip frequency. Here, using time-resolved THz techniques, we show for the first time harmonic injection and mode-locking in which THz QCLs are modulated at the harmonics of the round-trip frequency. We demonstrate the generation of the harmonic electrical beatnote within a QCL, its injection locking to an active modulation and its direct translation to harmonic pulse generation using the unique ultrafast nature of our approach. Finally, we show indications of self-starting harmonic emission, i.e., without external modulation, where the QCL operates exclusively on a harmonic (up to its 15th harmonic) of the round-trip frequency. This behaviour is supported by time-resolved simulations of induced gain and loss in the system and shows the importance of the electronic, as well as photonic, nature of QCLs. These results open up the prospect of passive harmonic modelocking and THz pulse generation, as well as the generation of low-noise microwave generation in the hundreds of GHz region.

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.

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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Abstract-Laser-seeding dynamics with few-cycle pulses: Maxwell-Bloch finite-difference time-domain simulations of terahertz quantum cascade lasers

Joshua R. Freeman, Jean Maysonnave, Suraj Khanna, Edmund Linfield, A. Giles Davies, Sukhdeep S. Dhillon, and Jérôme Tignon

http://pra.aps.org/accepted/c107eNceP701751e604f1373d9f2f77c5fdeba29b

We implement a Maxwell-Bloch simulation for a two-level system within the FDTD method to simulate the seeding of lasers by broadband pulse injection. The model does not make the slowly varying envelope approximation and the full electromagnetic field is simulated so that we are able to obtain time resolved seeding by few-cycle pulses. The model is compared to recent results on seeding of THz quantum cascade lasers to aid interpretation of their complex signals. The simulations are found to be in good agreement with the data when gain recovery times of 15ps are used. Furthermore we find that the emission from the laser depends only weakly on the seed used to initiate laser action. The model is readily applicable to any seeded laser system where few-cycle seed pulses are used.