Non-perturbative coupling of photons and excitons produces hybrid particles, exciton–polaritons, which have exhibited a variety of many-body phenomena in various microcavity systems. However, the vacuum Rabi splitting (VRS), which defines the strength of photon–exciton coupling, is usually a single constant for a given system. Here, we have developed a unique architecture in which excitons in an aligned single-chirality carbon nanotube film interact with cavity photons in polarization-dependent manners. The system reveals ultrastrong coupling (VRS up to 329 meV or a coupling-strength-to-transition-energy ratio of 13.3%) for polarization parallel to the nanotube axis, whereas VRS is absent for perpendicular polarization. Between these two extremes, VRS is continuously tunable through polarization rotation with exceptional points separating crossing and anticrossing. The points between exceptional points form equienergy arcs onto which the upper and lower polaritons coalesce. The demonstrated on-demand ultrastrong coupling provides ways to explore topological properties of polaritons and quantum technology applications.

光子和激子的非扰动耦合产生了混合粒子，激子-极化子，它们在各种微腔系统中表现出多种多体现象。但是，真空拉比分裂（VRS）定义了光子-激子耦合的强度，对于给定的系统通常是一个常数。在这里，我们开发了一种独特的体系结构，其中排列的单手性碳纳米管薄膜中的激子以偏振相关的方式与腔体光子相互作用。该系统显示出平行于纳米管轴的极化具有超强耦合（VRS高达329 meV或耦合强度与跃迁能量之比为13.3％），而垂直极化则不存在VRS。在这两个极端之间，VRS可通过极化旋转连续调谐，并具有将交叉点和反交叉点分开的特殊点。例外点之间的点形成等极化弧，上极化子和下极化子在其上合并。演示的按需超强耦合为探索极化子的拓扑特性和量子技术应用提供了方法。