Nuclear spin polarization and control in hexagonal boron nitride.

Electron spins in van der Waals materials are playing a crucial role in recent advances in condensed-matter physics and spintronics. However, nuclear spins in van der Waals materials remain an unexplored quantum resource. Here we report optical polarization and coherent control of nuclear spins in a van der Waals material at room temperature. We use negatively charged boron vacancy ([Formula: see text]) spin defects in hexagonal boron nitride to polarize nearby nitrogen nuclear spins. We observe the Rabi frequency of nuclear spins at the excited-state level anti-crossing of [Formula: see text] defects to be 350 times larger than that of an isolated nucleus, and demonstrate fast coherent control of nuclear spins. Further, we detect strong electron-mediated nuclear-nuclear spin coupling that is five orders of magnitude larger than the direct nuclear-spin dipolar coupling, enabling multi-qubit operations. Our work opens new avenues for the manipulation of nuclear spins in van der Waals materials for quantum information science and technology.

Enhancing the graphene photocurrent using surface plasmons and a p-n junction.

The recently proposed concept of graphene photodetectors offers remarkable properties such as unprecedented compactness, ultrabroadband detection, and an ultrafast response speed. However, owing to the low optical absorption of pristine monolayer graphene, the intrinsically low responsivity of graphene photodetectors significantly hinders the development of practical devices. To address this issue, numerous efforts have thus far been made to enhance the light-graphene interaction using plasmonic structures. These approaches, however, can be significantly advanced by leveraging the other critical aspect of graphene photoresponsivity enhancement-electrical junction control. It has been reported that the dominant photocarrier generation mechanism in graphene is the photothermoelectric (PTE) effect. Thus, the two energy conversion mechanisms involved in the graphene photodetection process are light-to-heat and heat-to-electricity conversions. In this work, we propose a meticulously designed device architecture to simultaneously enhance the two conversion efficiencies. Specifically, a gap plasmon structure is used to absorb a major portion of the incident light to induce localized heating, and a pair of split gates is used to produce a p-n junction in graphene to augment the PTE current generation. The gap plasmon structure and the split gates are designed to share common key components so that the proposed device architecture concurrently realizes both optical and electrical enhancements. We experimentally demonstrate the dominance of the PTE effect in graphene photocurrent generation and observe a 25-fold increase in the generated photocurrent compared to the un-enhanced cases. While further photocurrent enhancement can be achieved by applying a DC bias, the proposed device concept shows vast potential for practical applications.

Stable emission and fast optical modulation of quantum emitters in boron nitride nanotubes.

Atom-like defects in two-dimensional (2D) hexagonal boron nitride (hBN) have recently emerged as a promising platform for quantum information science. Here, we investigate single-photon emissions from atomic defects in boron nitride nanotubes (BNNTs). We demonstrate the first, to the best of our knowledge, optical modulation of the quantum emission from BNNTs with a near-infrared laser. This one-dimensional system displays a bright single-photon emission, as well as high stability at room temperature, and is an excellent candidate for optomechanics. The fast optical modulation of a single-photon emission shows multiple electronic levels of the system and has potential applications in optical signal processing.

Visible light biophotosensors using biliverdin from Antheraea yamamai.

We report an endogenous photoelectric biomolecule and demonstrate that such a biomolecule can be used to detect visible light. We identify the green pigment abundantly present in natural silk cocoons of Antheraea yamamai (Japanese oak silkmoth) as biliverdin, using mass spectroscopy and optical spectroscopy. Biliverdin extracted from the green silk cocoons generates photocurrent upon light illumination with distinct colors. We further characterize the basic performance, responsiveness, and stability of the biliverdin-based biophotosensors at a photovoltaic device level using blue, green, orange, and red light illumination. Biliverdin could potentially serve as an optoelectric biomolecule toward the development of next-generation implantable photosensors and artificial photoreceptors.