D.J.L. Coxon, M. Staniforth, B.G. Breeze, S.E. Greenough, J.P. Goss, M. Monti, J. Lloyd-Hughes, V.G. Stavros, and M.E. Newton
J. Phys. Chem. Lett. XX, XXXX (July 2020) [ pdf ] [ ref ]

Atomic-scale defects can control the exploitable optoelectronic performance of crystalline materials, and several point defects in diamond are emerging functional components for a range of quantum technologies. Nitrogen and hydrogen are common impurities incorporated into diamond, and there is a family of defects that includes both. The N₃VH⁰ defect is a lattice vacancy where three nearest neighbor carbon atoms are replaced with nitrogen atoms and a hydrogen is bonded to the remaining carbon. It is regularly observed in natural and high-temperature annealed synthetic diamond, and gives rise to prominent absorption features in the mid-infrared. Here, we combine time- and spectrally-resolved infrared absorption spectroscopy to yield unprecedented insight into the N₃VH⁰ defect’s vibrational dynamics following infrared excitation of the C–H stretch. In doing so, we gain fundamental information about the energies of quantized vibrational states, and corroborate our results with theory. We map out, for the first time, energy relaxation pathways, which include multiphonon relaxation processes and anharmonic coupling to the C–H bend mode. These advances provide new routes to quantify and probe atomic-scale defects.

W. Dong, J.J.P. Peters, D. Rusu, M. Staniforth, A. Brunier, J. Lloyd-Hughes, A.M. Sanchez and M. Alexe
Nano Lett. 20 X, XXX (July 2020) [ pdf ] [ ref ]

Ferroelectric–paraelectric superlattices show emerging new states, such as polar vortices, through the interplay and different energy scales of various thermodynamic constraints. By introducing magnetic coupling at BiFeO₃–La_0.7Sr_0.3MnO₃ interfaces epitaxially grown on SrTiO₃ substrate, we find, for the first time in thin films, a sub-nanometer thick lamella-like BiFeO₃. The emergent phase is characterized by an arrangement of a two unit cell thick lamella-like structure featuring antiparallel polarization, resulting an antiferroelectric-like structure typically associated with a morphotropic phase transition. The antipolar phase is embedded within a nominal R3c structure and is independent of the BiFeO₃ thickness (4–30 unit cells). Moreover, the superlattice structure with the morphotropic phase demonstrates azimuth-independent second harmonic generation responses, indicating a change of overall symmetry mediated by a delicate spatial distribution of the emergent phase. This work enriches the understanding of a metastable state manipulated by thermodynamic constraints by lattice strain and magnetic coupling.

R. I. Stantchev, X. Yu, T. Blu and E. Pickwell-MacPherson
Nature Communications 11 2535 (May 2020) [ pdf ] [ ref ]

Terahertz (THz) radiation is poised to have an essential role in many imaging applications, from industrial inspections to medical diagnosis. However, commercialization is prevented by impractical and expensive THz instrumentation. Single-pixel cameras have emerged as alternatives to multi-pixel cameras due to reduced costs and superior durability. Here, by optimizing the modulation geometry and post-processing algorithms, we demonstrate the acquisition of a THz-video (32 × 32 pixels at 6 frames-per-second), shown in real-time, using a single-pixel fiber-coupled photoconductive THz detector. A laser diode with a digital micromirror device shining visible light onto silicon acts as the spatial THz modulator. We mathematically account for the temporal response of the system, reduce noise with a lock-in free carrier-wave modulation and realize quick, noise-robust image undersampling. Since our modifications do not impose intricate manufacturing, require long post-processing, nor sacrifice the time-resolving capabilities of THz-spectrometers, their greatest asset, this work has the potential to serve as a foundation for all future single-pixel THz imaging systems.

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.