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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 (lambda approximately equal to 4.462 mu m). 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 degrees C-1. This operation is conducted in the moderate temperature interval of 25-85 degrees 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.