Electron density oscillations with acoustic dispersions and sustained at boundaries between different media provide information about surface and interface properties of heterostructures. In ultrathin metallic films these plasmonic excitations are heavily damped. Superconductivity is predicted to reduce dissipation allowing detection of these resonances. Emerging low-loss interface Cooper-pair waves have been studied before, however, the observation of surface-confined Josephson plasmons in highly anisotropic superconductors has remained elusive. Here, we report on generation and coupling to these excitations in an ultrathin single-crystal film of high-temperature superconductor La_1.85Sr_0.15CuO₄. The film becomes brighter than Au below the critical temperature when probed with sub-gap THz photons. We show that the enhanced signal in the superconducting state, which can be visualized with a spatial resolution better than λ/3000, originates from near-field coupling of light to surface Josephson plasmons. Our results open a path towards non-invasive investigation of enhanced superconductivity in artificial multilayers, buried interface states in topological heterostructures, and non-linear phenomena in Josephson devices.

具有声弥散并维持在不同介质之间边界的电子密度振荡提供了有关异质结构的表面和界面特性的信息。在超薄金属膜中，这些等离激元激发被严重抑制。预测超导会减少耗散，从而可以检测这些共振。以前已经研究了新兴的低损耗界面库珀对波，但是，在高度各向异性的超导体中表面受限的约瑟夫森等离子体激元的观察仍然难以捉摸。在这里，我们报道了高温超导体La _1.85 Sr _0.15 CuO ₄的超薄单晶膜中这些激发的产生和耦合。。当用亚间隙太赫兹光子探测时，薄膜在临界温度以下变得比Au亮。我们表明，在超导状态下的增强信号可以通过比λ/ 3000更好的空间分辨率进行可视化，它源自光到表面约瑟夫森等离子体激元的近场耦合。我们的结果开辟了一条通往非侵入性研究的途径，该研究旨在提高人工多层中超导性，拓扑异质结构中的掩埋界面状态以及约瑟夫森装置中的非线性现象。