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virtues make the polaritons promising candidates for superlensing, [5] super-Planckian heat transfer, [6] wavefront control, [7,8] and other novel applications. Besides hexagonal boron nitride (hBN), [9][10][11][12][13][14][15] phonon polaritons have been investigated in SiC, [5,16] GaAs, [17,18] LiTaO 3 , [19] MoO 3 , [20,21] as well as in metamaterials. [8,22] In these systems, phonon polaritons span a broad range of frequencies, from terahertz to mid-infrared (mid-IR).An intriguing aspect of hBN in the context of phonon polariton physics and applications is its optical hyperbolicity, [9,10,23] i.e., the existence of a frequency band between transverse optical (TO) mode at ω TO and longitudinal optical (LO) mode at ω LO : ω TO < ω < ω LO. In this latter frequency region, the basal-plane permittivity of hBN Re ε t < 0 whereas the z-axis permittivity is positive Re ε z > 0. Theory predicts [14] that the polariton dispersion within the hyperbolic frequency region consists of multiple branches whose number is equal to the number N of atomic layers. In experiment, only the so-called principal branch is typically observed, as is the case here. The theory further predicts that the Phonon polaritons in van der Waals materials reveal significant confinement accompanied with long propagation length: important virtues for tasks pertaining to the control of light and energy flow at the nanoscale. While previous studies of phonon polaritons have relied on relatively thick samples, here reported is the first observation of surface phonon polaritons in single atomic layers and bilayers of hexagonal boron nitride (hBN). Using antennabased near-field microscopy, propagating surface phonon polaritons in monoand bilayer hBN microcrystals are imaged. Phonon polaritons in monolayer hBN are confined in a volume about one million times smaller than the freespace photons. Both the polariton dispersion and their wavelength-thickness scaling law are altered compared to those of hBN bulk counterparts. These changes are attributed to phonon hardening in monolayer-thick crystals. The data reported here have bearing on applications of polaritons in metasurfaces and ultrathin optical elements. Phonon PolaritonsPhonon polaritons are collective modes formed by hybridization of free-space photons with lattice vibrations in polar insulators. These modes exhibit a high density of states, a strong confinement of the electric field, [1,2] and a relatively low loss comparable to that of state-of-the-art plasmonic structures. [3,4] These

Phonon polaritons provide useful opportunities, complementary to those provided by plasmon polaritons, in the study of the interaction of light with matter at small scales. The focus of this review is on phonon polaritons in low‐dimensional van der Waals (vdW) materials and heterostructures. Phonon polaritons confined in vdW materials exhibit large electromagnetic localization and are easy to hybridize with other collective modes. The extreme optical anisotropy in vdW systems produces the natural hyperbolic dispersion, enabling the access to deep subdiffractional optics and often yielding improved figures of merit over hyperbolic metamaterials. These virtues hold promises for practical nanophotonic applications, including optical sensing, super‐resolution imaging, energy and emission engineering, quantum optics, and a next generation of optical circuit elements.

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Tumor-derived exosomes play a vital role in the process of cancer development. Quantitative analysis of exosomes and exosome-shuttled proteins would be of immense value in understanding cancer progression and generating reliable predictive biomarkers for cancer diagnosis and treatment. Recent studies have indicated the critical role of exosomal programmed death ligand 1 (PD-L1) in immune checkpoint therapy and its application as a patient stratification biomarker in cancer immunotherapy. Here, we present a nanoplasmonic exosome immunoassay utilizing gold−silver (Au@Ag) core−shell nanobipyramids and gold nanorods, which form sandwich immune complexes with target exosomes. The immunoassay generates a distinct plasmonic signal pattern unique to exosomes with specific exosomal PD-L1 expression, allowing rapid, highly sensitive exosome detection and accurate identification of PD-L1 exosome subtypes in a single assay. The developed nanoplasmonic sandwich immunoassay provides a novel and viable approach for tumor cell-derived exosome detection and analysis with quantitative molecular details of key exosomal proteins, manifesting its great potential as a transformative diagnostic tool for early cancer detection, prognosis, and post-treatment monitoring.