In contrast to interband excitons in undoped quantum wells, doped quantum wells do not display sharp resonances due to excitonic bound states. The effective Coulomb interaction between electrons and holes in these systems typically leads to only a depolarization shift of the single-electron intersubband transitions¹. Non-perturbative light–matter interaction in solid-state devices has been investigated as a pathway to tuning optoelectronic properties of materials^2,3. A recent theoretical work⁴ predicted that when the doped quantum wells are embedded in a photonic cavity, emission–reabsorption processes of cavity photons can generate an effective attractive interaction that binds electrons and holes together, leading to the creation of an intraband bound exciton. Here, we spectroscopically observe such a bound state as a discrete resonance that appears below the ionization threshold only when the coupling between light and matter is increased above a critical value. Our result demonstrates that two charged particles can be bound by the exchange of transverse photons. Light–matter coupling can thus be used as a tool in quantum material engineering, tuning electronic properties of semiconductor heterostructures beyond those permitted by mere crystal structures, with direct applications to mid-infrared optoelectronics.

与未掺杂的量子阱中的带间激子相反，掺杂的量子阱由于激子束缚态而不会显示出尖锐的共振。在这些系统中，电子与空穴之间的有效库仑相互作用通常仅导致单电子子带间跃迁¹的去极化位移。固态器件中的非扰动光-质相互作用已被研究为调节材料^2,3的光电特性的途径。最近的理论著作⁴据预测，当掺杂的量子阱嵌入光子腔中时，腔光子的发射-重吸收过程可以产生有效的吸引相互作用，将电子和空穴结合在一起，从而导致带内结合激子的产生。在这里，我们在光谱上观察到只有当光和物质之间的耦合增加到临界值以上时才会出现在电离阈值以下的离散共振这样的束缚状态。我们的结果表明，两个带电粒子可以被横向光子交换束缚。因此，光-质耦合可以用作量子材料工程中的工具，将半导体异质结构的电子特性调整到仅晶体结构所允许的范围之外，并直接应用于中红外光电。