M.G. Burdanova, R.J. Kashtiban, Y. Zheng, R. Xiang, S. Chiashi, J.M. Woolley, M. Staniforth, E. Sakamoto-Rablah, X. Xie, M. Broome, J. Sloan, A. Anisimov, E.I. Kauppinen, S. Maruyama and J. Lloyd-Hughes
Nano Lett. 20 5, 3560 (Apr 2020) [ free e-print ] [ preprint pdf ] [ ref ]

Heterostructures built from 2D, atomically thin crystals are bound by the van der Waals force and exhibit unique optoelectronic properties. Here, we report the structure, composition and optoelectronic properties of 1D van der Waals heterostructures comprising carbon nanotubes wrapped by atomically thin nanotubes of boron nitride and molybdenum disulfide (MoS₂). The high quality of the composite was directly made evident on the atomic scale by transmission electron microscopy, and on the macroscopic scale by a study of the heterostructure’s equilibrium and ultrafast optoelectronics. Ultrafast pump–probe spectroscopy across the visible and terahertz frequency ranges identified that, in the MoS₂ nanotubes, excitons coexisted with a prominent population of free charges. The electron mobility was comparable to that found in high-quality atomically thin crystals. The high mobility of the MoS₂ nanotubes highlights the potential of 1D van der Waals heterostructures for nanoscale optoelectronic devices.

A. I Hernandez-Serrano, D. M. Mittleman and E. Pickwell-MacPherson
Optics Letters 45 1208 (Feb 2020) [ pdf ] [ ref ]

In this Letter, we report a broadband frequency/polarization demultiplexer based on parallel-plate waveguides (PPWGs) for terahertz (THz) frequencies. The fabrication and experimental validation of this polarization sensitive demultiplexer is demonstrated for the range from 0.2 to 1 THz. Upgrading the demultiplexer by adding a second demultiplexer stage, a fifty-fifty amplitude splitter is also demonstrated in the same frequency range. The multiplexer is based on a stainless-steel traveling-wave antenna, exhibiting strong mechanical robustness. This unique device exhibits three splitting mechanisms in the same device: amplitude, polarization, and frequency splitting. This is a significant improvement for the next generation of THz passive components for communication purposes.

J. Keller, G. Scalari, F. Appugliese, S. Rajabali, M. Beck, J. Haase, C.A. Lehner, W. Wegscheider, M. Failla, M. Myronov, D.R. Leadley, J. Lloyd-Hughes, P. Nataf, and J. Faist
Physical Review B 101:075301 (Feb 2020) [ pdf ][ ref ]

We probe ultrastrong light-matter coupling between metallic terahertz metasurfaces and Landau-level transitions in high-mobility two-dimensional electron and hole gases. We utilize heavy-hole cyclotron resonances in strained Ge and electron cyclotron resonances in InSb quantum wells, both within highly nonparabolic bands, and compare our results to well-known parabolic AlGaAs/GaAs quantum well systems. Tuning the coupling strength of the system by two methods, lithographically and by optical pumping, we observe a behavior clearly deviating from the standard Hopfield model previously verified in cavity quantum electrodynamics: an opening of a lower polaritonic gap.

K.D.G.I. Jayawardena, R.M.I. Bandara, M. Monti, E. Butler-Caddle, T. Pichler, H. Shiozawa, Z. Wang, S. Jenatsch, S.J. Hinder, M.G. Masteghin, M. Patel, H.M. Thirimanne, W. Zhang, R.A. Sporea, J. Lloyd-Hughes and S. R. P. Silva
J. Mater. Chem. A 8 693 (Jan 2020) [ pdf ] [ ref ]

The performance of all solar cells is dictated by charge recombination. A closer to ideal recombination dynamics results in improved performances, with fill factors approaching the limits based on Shockley-Queisser analysis. It is well known that for emerging solar materials such as perovskites, there are several challenges that need to be overcome to achieve high fill factors, particularly for large area lead-tin mixed perovskite solar cells. Here we demonstrate a strategy towards achieving fill factors above 80% through post-treatment of a lead-tin mixed perovskite absorber with guanidinium bromide for devices with an active area of 0.43 cm². This bromide post-treatment results in a more favourable band alignment at the anode and cathode interfaces, enabling better bipolar extraction. The resulting devices demonstrate an exceptional fill factor of 83%, approaching the Shockley–Queisser limit, resulting in a power conversion efficiency of 14.4% for large area devices.