Abstract-Enhanced performance of spintronic terahertz emitters based on defect engineering

Sascha Keller, Dennis M. Nenno, Garik Torosyan, Laura Scheuer, Marco Battiato, Jörg Losch, Alexander Brodyanski, Rolf H. Binder, Hans C. Schneider, Rene Beigang, Evangelos Th. Papaioannou

https://arxiv.org/abs/1901.10011

Spintronic ferromagnetic/non-magnetic heterostructures are novel sources for the generation of THz radiation based on the spin-to-charge conversion in the films. The key technological and scientific challenge of THz spintronic emitters is to increase their low intensity and frequency bandwidth. Our work reveals the factors to engineer both by introducing the scattering lifetime and the inteface transmission for spin polarized, non-equilibrium electrons. We resolve the role played by 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 the emitted as well as the detected THz-pulse shapes and spectra that is essential for future applications of metallic spintronic THz emitters.

Abstract-Efficient Spintronic Terahertz Emitters Based on Epitaxial Grown Fe/Pt Layer Structures

Garik Torosyan, Sascha Keller, Laura Scheuer, Rene Beigang, Evangelos Th. Papaioannou

https://arxiv.org/abs/1707.08894

We report on efficient generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The experimentally determined optimum layer thicknesses were in qualitative agreement with simulations of the spin current induced in the ferromagnetic layer. Our model takes into account spin diffusion and accumulation and electrical as well as optical properties of the bilayer samples. Using the device in a counterintuitive orientation a Si lens was attached to increase the collection efficiency of the emitter. The optimized emitter provided a bandwidth of up to 8 THz which was mainly limited by the low-temperature-grown GaAs (LT-GaAS) photoconductive antenna used as detector and the pulse length of the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at a repetition rate of 80 MHz) have been used for terahertz generation. This and the general performance make the spintronic terahertz emitter compatible with established emitters based on nonlinear generation methods.

Abstract- Precise Determination of Thicknesses of Multilayer Polyethylene Composite Materials by Terahertz Time-Domain Spectroscopy

Norbert Palka, Soufiene Krimi, Frank Ospald, Danuta Miedzinska, Roman Gieleta, Marcin Malek, Rene Beigang

http://link.springer.com/article/10.1007%2Fs10762-015-0156-6

The multilayer structure of an ultra-high molecular weight polyethylene (UHMWPE) composite material was investigated in the terahertz (THz) spectral range by means of time-domain spectroscopy (TDS) technique. Such structures consist of many alternating layers of fibers, each being perpendicular to the other and each having a thickness of about 50 μm. Refractive indices of two composite samples and of a sample composed of four single layers (plies) having the same fiber orientation were determined for two orthogonal orientations of the electric field in a transmission TDS system. The birefringence of a single layer was measured, and the origin of this phenomenon is discussed. Using the TDS system in reflection, the formation of many pulses shifted in time was observed originating from reflections from interfaces of successive layers caused by the periodic modulation of the refractive index along the propagation of the THz radiation. This phenomenon is theoretically described and simulated by means of a transfer matrix method (TMM). A time-domain fitting procedure was used to determine thicknesses of all layers of the composite material. The reconstructed waveform based on the optimized thicknesses shows very good agreement with the measured waveform, with typical differences between measurements and simulations between 3 and 7 μm (depending on the sample). As a result, we were able to determine the thicknesses of all layers of two multilayer (~200 plies) structures by means of the reflection TDS technology with high accuracy.

Abstract-Precise Determination of Thicknesses of Multilayer Polyethylene Composite Materials by Terahertz Time-Domain Spectroscopy

Norbert Palka, 

Soufiene Krimi, 

Frank Ospald, 

Danuta Miedzinska, 

Roman Gieleta, 

Marcin Malek,

Rene Beigang

http://link.springer.com/article/10.1007%2Fs10762-015-0156-6

The multilayer structure of an ultra-high molecular weight polyethylene (UHMWPE) composite material was investigated in the terahertz (THz) spectral range by means of time-domain spectroscopy (TDS) technique. Such structures consist of many alternating layers of fibers, each being perpendicular to the other and each having a thickness of about 50 μm. Refractive indices of two composite samples and of a sample composed of four single layers (plies) having the same fiber orientation were determined for two orthogonal orientations of the electric field in a transmission TDS system. The birefringence of a single layer was measured, and the origin of this phenomenon is discussed. Using the TDS system in reflection, the formation of many pulses shifted in time was observed originating from reflections from interfaces of successive layers caused by the periodic modulation of the refractive index along the propagation of the THz radiation. This phenomenon is theoretically described and simulated by means of a transfer matrix method (TMM). A time-domain fitting procedure was used to determine thicknesses of all layers of the composite material. The reconstructed waveform based on the optimized thicknesses shows very good agreement with the measured waveform, with typical differences between measurements and simulations between 3 and 7 μm (depending on the sample). As a result, we were able to determine the thicknesses of all layers of two multilayer (~200 plies) structures by means of the reflection TDS technology with high accuracy.

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.

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