Abstract-Routing of strongly confined terahertz spoof surface plasmon polaritons on metasurfaces along straight and curved pathways with subwavelength width

Sven Becker, Tassilo Fip, and Marco Rahm

Field maps of the 𝑧$z$-component of the SSPP electric field 𝐸𝑧$E_z$ in the 𝑥$x$-𝑦$y$-plane at 𝑧$z$ = 50µm away from the metasurface. The close-ups indicate the geometric relation between the SSPP electric field distribution and the subjacent metasurface structure. The SSPP electric field is monitored at frequencies of (a) 0.48 THz for the straight 1-cut-wire metasurface, (b) 0.34 THz for the 2-cut-wire metasurface and (c) 0.28 THz for the 3-cut-wire metasurface. (d) Absolute value of 𝐸𝑧$Ez$ along an intersection line in 𝑦$y$-direction at 𝑥=$x=$ 1000 µm transverse to the propagation direction of the SSPPs for (1) the 1-cut-wire, (2) the 2-cut-wire and (3) the 3-cut-wire metasurface.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-5-6766

In search of new technologies for optimizing the performance and space requirements of electronic and optical micro-circuits, the concept of spoof surface plasmon polaritons (SSPPs) has come to the fore of research in recent years. Due to the ability of SSPPs to confine and guide the energy of electromagnetic waves in a subwavelength space below the diffraction limit, SSPPs deliver all the tools to implement integrated circuits with a high integration rate. However, in order to guide SSPPs in the terahertz frequency range, it is necessary to carefully design metasurfaces that allow one to manipulate the spatio-temporal and spectral properties of the SSPPs at will. Here, we propose a specifically designed cut-wire metasurface that sustains strongly confined SSPP modes at terahertz frequencies. As we show by numerical simulations and also prove in experimental measurements, the proposed metasurface can tightly guide SSPPs on straight and curved pathways while maintaining their subwavelength field confinement perpendicular to the surface. Furthermore, we investigate the dependence of the spatio-temporal and spectral properties of the SSPP modes on the width of the metasurface lanes that can be composed of one, two or three cut-wires in the transverse direction. Our investigations deliver new insights into downsizing effects of guiding structures for SSPPs.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Modification of spintronic terahertz emitter performance through defect engineering

Dennis M. Nenno, Laura Scheuer, Dominik Sokoluk, Sascha Keller, Garik Torosyan, Alexander Brodyanski, Jörg Lösch, Marco Battiato, Marco Rahm, Rolf H. Binder, Hans C. Schneider, René Beigang,  Evangelos Th. Papaioannou

https://www.nature.com/articles/s41598-019-49963-8

Spintronic ferromagnetic/non-magnetic heterostructures are novel sources for the generation of THz radiation based on spin-to-charge conversion in the layers. The key technological and scientific challenge of THz spintronic emitters is to increase their intensity and frequency bandwidth. Our work reveals the factors to engineer spintronic Terahertz generation by introducing the scattering lifetime and the interface transmission for spin polarized, non-equilibrium electrons. We clarify the influence of the electron-defect scattering lifetime on the spectral shape and the interface transmission on the THz amplitude, and how this is linked to structural defects of bilayer emitters. The results of our study define a roadmap of the properties of emitted as well as detected THz-pulse shapes and spectra that is essential for future applications of metallic spintronic THz emitters.

Abstract-Electrically Reconfigurable Micromirror Array for Direct Spatial Light Modulation of Terahertz Waves over a Bandwidth Wider Than 1 THz

Jan Kappa, Dominik Sokoluk, Steffen Klingel, Corey Shemelya, Egbert Oesterschulze, Marco Rahm,

https://www.nature.com/articles/s41598-019-39152-y

We report the design, fabrication and experimental investigation of a spectrally wide-band terahertz spatial light modulator (THz-SLM) based on an array of 768 actuatable mirrors with each having a length of 220 μm and a width of 100 μm. A mirror length of several hundred micrometers is required to reduce diffraction from individual mirrors at terahertz frequencies and to increase the pixel-to-pixel modulation contrast of the THz-SLM. By means of spatially selective actuation, we used the mirror array as reconfigurable grating to spatially modulate terahertz waves in a frequency range from 0.97 THz to 2.28 THz. Over the entire frequency band, the modulation contrast was higher than 50% with a peak modulation contrast of 87% at 1.38 THz. For spatial light modulation, almost arbitrary spatial pixel sizes can be realized by grouping of mirrors that are collectively switched as a pixel. For fabrication of the actuatable mirrors, we exploited the intrinsic residual stress in chrome-copper-chrome multi-layers that forces the mirrors into an upstanding position at an inclination angle of 35°. By applying a bias voltage of 37 V, the mirrors were pulled down to the substrate. By hysteretic switching, we were able to spatially modulate terahertz radiation at arbitrary pixel modulation patterns.

Abstract-Confined terahertz surface waves on meta-surfaces and Goubau lines

Sven Becker,  Tassilo Fip, Corey Shemelya,  Marco Rahm,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10719/107191H/Confined-terahertz-surface-waves-on-meta-surfaces-and-Goubau-lines/10.1117/12.2320602.short

Integrated circuits revolutionized electronics long time ago and paved the way towards minimized microprocessors today. In analogy, plasmonics aims at the creation of highly integrated optical networks on a small chip that enable the implementation of ultra-small sensors or optical processors. In the terahertz frequency regime, we investigate the propagation of tightly bound pure surface waves on specifically designed meta-surfaces. While most presented metasurfaces on a thin film in the literature support waveguide mode propagation in the thin film substrate, whose evanescent electromagnetic fields form the surface waves at the waveguide boundaries, we observed pure surface waves that are not coupled to a waveguide mode in the thin film. Such meta-surfaces are particularly advantageous for use as surface sensors, since the surface waves carry most of their energy in the space between the surface and air and almost no energy in the thin film substrate. This is in strict contrast to most of the presented meta-surfaces in literature so far, which guide a significant part of unusable energy in the inaccessible region of the substrate. Furthermore, we study structures of Goubau lines and meta-surfaces that combine excellent spectrally broadband terahertz surface wave guiding with frequency-selective meta-surface areas and meta-surface sub-wavelength resonators on a chip. In detail, we investigate the coupling efficiency between Goubau lines and meta-surfaces.

© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Abstract-Towards loss compensated and lasing terahertz metamaterials based on optically pumped graphene

Peter Weis, Juan Luis Garcia-Pomar, Marco Rahm
http://arxiv-web3.library.cornell.edu/abs/1312.2882

It is evidenced by numerical calculations that optically pumped graphene is suitable for compensating inherent loss in terahertz (THz) metamaterials. In a first step, the complex conductivity of graphene under optical pumping is calculated and the proper conditions for terahertz amplification in single layer graphene are determined. It is shown that amplification in graphene occurs for temperatures up to room temperature and for moderate pump intensities when pumped at a telecommunication wavelength λ=1.5 μm. Furthermore, the amplification properties of graphene are evaluated and discussed at a temperature as low as T=77 K and a pump intensity I=300 mW/mm2 to investigate the coupling between graphene and a plasmonic split ring resonator (SRR) metamaterial. The contributions of ohmic and dielectric loss mechanisms are studied by full wave simulations. As a result, it is found that the loss of a split-ring resonator metamaterial can be compensated by optically stimulated amplification in graphene. Moreover, it is shown that a hybrid material consisting of asymmetric split-ring resonators and optically pumped graphene can exceed the laser threshold condition and can emit coherent THz radiation at minimum output power levels of 60 nW/mm2. The use of optically pumped graphene is well suited for loss compensation in THz metamaterials and paves the way to new kinds of coherent THz sources.

Abstract-Metamaterial-based gradient index beam steerers for terahertz radiation

Jens Neu¹, René Beigang², and Marco Rahm¹

¹Department of Electrical and Computer Engineering and Research Center OPTIMAS, University of Kaiserslautern, 67663 Kaiserslautern, Germany
²Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, 67663 Kaiserslautern, Germany 

http://apl.aip.org/resource/1/applab/v103/i4/p041109_s1?bypassSSO=1

We designed, fabricated, and optically characterized single and double layer metamaterial-based gradient index beam steerers for terahertz radiation. We measured a maximal deflection angle of 6°. The operation bandwidth of the beam steerers was 300 GHz around a center frequency of 1.3 THz. Within this bandwidth, the amplitude transmission was higher than 50%. Due to a thickness of only 100 μm or below, the implemented beam steerers are ideally suited for integration in compact terahertz measurement systems.

© 2013 AIP Publishing LLC

Abstract-In-plane focusing of terahertz surface waves on a gradient index metamaterial film

 

Martin F. Volk, Benjamin Reinhard, Jens Neu, René Beigang, andMarco Rahm  »View Author Affiliations

http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-38-12-2156

We designed and implemented a gradient index metasurface for in-plane focusing of confined terahertz (THz) surface waves. We measured the spatial propagation of the surface waves by two-dimensional mapping of the complex electric field using a THz near-field spectroscope. The surface waves were focused to a diameter of 500 μm after a focal length of approximately 2 mm. In the focus, we measured a field amplitude enhancement of a factor of 3.

© 2013 Optical Society of America