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Abstract Magnon spintronics aims to harness spin waves in magnetic films for information technologies. Color center magnetometry is a promising tool for imaging spin waves, using electronic spins associated with atomic defects in solid-state materials as sensors. However, two main limitations persist: the magnetic fields required for spin-wave control detune the sensor-spin detection frequency, and this frequency is further restricted by the color center nature. Here, we overcome these limitations by decoupling the sensor spins from the spin-wave control fields –selecting color centers with intrinsic anisotropy axes orthogonal to the film magnetization– and by using color centers in diamond and hexagonal boron nitride to operate at complementary frequencies. We demonstrate isofrequency imaging of field-controlled spin waves in a magnetic half-plane and show how intrinsic magnetic anisotropies trigger bistable spin textures that govern spin-wave transport at device edges. Our results establish color center magnetometry as a versatile tool for advancing spin-wave technologies. 

Abstract Electromagnetic hyperbolicity has driven key functionalities in nanophotonics, including super-resolution imaging, efficient energy control, and extreme light manipulation. Central to these advances are hyperbolic polaritons—nanometer-scale light-matter waves—spanning multiple energy-momentum dispersion orders with distinct mode profiles and incrementally high optical momenta. In this work, we report the mode conversion of hyperbolic polaritons across different dispersion orders by breaking the structure symmetry in engineered step-shape van der Waals (vdW) terraces. The mode conversion from the fundamental to high-order hyperbolic polaritons is imaged using scattering-type scanning near-field optical microscopy (s-SNOM) on both hexagonal boron nitride (hBN) and alpha-phase molybdenum trioxide (α-MoO₃) vdW terraces. Our s-SNOM data, augmented with electromagnetics simulations, further demonstrate the alteration of polariton mode conversion by varying the step size of vdW terraces. The mode conversion reported here offers a practical approach toward integrating previously independent different-order hyperbolic polaritons with ultra-high momenta, paving the way for promising applications in nano-optical circuits, sensing, computation, information processing, and super-resolution imaging. 

Abstract Isotope-enriched bulk hexagonal boron nitride (hBN) crystals have enhanced properties that improve the performance of nanophotonic and quantum technologies. Developing methods to deposit epitaxial layers on such crystals enables the exciting prospects of producing isotope-engineered hBN layers and heterostructures. Here, we demonstrate the homoepitaxial growth of hBN with phase-separated¹⁰B and¹¹B isotopes by high-temperature molecular beam epitaxy (HT-MBE). Controlled nucleation, improved surface uniformity, and step-flow growth of an h¹⁰BN epilayer were achieved by etching the h¹¹BN bulk crystals with molecular hydrogen. The alignment of the h¹⁰BN epilayer and host h¹¹BN lattices was confirmed by lattice-resolved atomic force microscopy. Micro-Raman spectroscopy and scattering-type scanning near-field optical microscopy show that the bulk h¹¹BN and h¹⁰BN epilayer have distinct phonon energies, with no intermixing of the van der Waals layers, thus enabling the different boron isotopes to be spatially separated in the heterostructure. This work demonstrates the potential of HT-MBE to produce isotopic heterostructures of hBN to advance future nanophotonic and quantum technologies. 

van der Waals materials support numerous exotic polaritonic phenomena originating from their layered structures and associated vibrational and electronic properties. However, many van der Waals materials' unique properties are most prominent at cryogenic temperatures. This presents a particular challenge for polaritonics research, as reliable optical constant data are required for understanding light-matter coupling. This paper presents a cryogenic Fourier transform infrared microscope design constructed entirely from off-the-shelf components and associated fitting procedures for determining optical constants in the infrared. Data correction techniques were developed to directly quantify systematic errors in the fitting procedure. We use this microscope to present the first temperature-dependent characterization of the optical properties of hexagonal boron nitride enriched with isotopically pure boron. Our full analysis of the infrared dielectric function shows small but significant tuning of the optical constants, which is highly consistent with Raman data from the literature. We then use this dielectric data to perform and analyze the polariton propagation properties, which agree exceptionally well with published cryogenic scattering-type near-field microscopy results. In addition to the insights gained into hyperbolic polaritons in hBN, our paper represents a transferable framework for characterizing exfoliated infrared polaritonic materials and other infrared devices. This could accelerate discoveries in different material systems, especially those that are spatially inhomogeneous or cannot be prepared as large single crystals.