Deep subwavelength light confinement in disordered bismuth nanorods as a linearly thermal‐tunable metamaterial

Materials with a tunable optical response that can be controllably tailored using external stimuli excitation have undergone considerable research effort for the development of active optical devices, such as thermo‐optical modulators. Although bismuth (Bi) nanodots, embedded into glass matrices, have been proven to have a thermo‐optical response, the recyclability of the structure in solid–liquid phase transitions is a major challenge. Herein, a facile and lithography‐free fabrication method is proposed to realize densely packed stand‐alone Bi nanorods (NRs), with deep subwavelength gaps and a resonance at the midinfrared range (λ ≅ 4.462 μm). Owing to these ultrasmall gaps that support lossy Mie‐like resonances, strong field confinement is achieved, and the resonance wavelength exhibits great sensitivity to temperature, as the thermal sensitivity reaches as high as 1.0316 nm °C−1. This operation is conducted in the moderate temperature interval of 25–85 °C, which is far from the melting point of Bi. Overall, our simple, robust, and high‐performance device is highly promising for realizing optical switches, thermo‐optic modulators, and infrared camouflage.