Furnaces and refrigerators for photons

In recent years, there has been a resurgence of interest in moving gratings, spurred by technological advancements that enable the modulation of material responses in both space and time.

In this article, it is shown that the refractive index perturbation introduced by a moving step can be used as a mechanism to either compress or decompress radiation, with the specific effect determined by the step's gradient. Notably, near the step, both the frequency and wavenumber of the field are adjusted according to the compression or decompression factor.

This mechanism has tangible implications in the context of thermal light. When the moving step interacts with thermal radiation it can continuously raise or decrease the temperature in its vicinity. Thermal radiation heats up when encountering a step-up and cools down with a step-down. These effects become exponentially strong when the speed of the moving step matches the speed of light at some point along the step. In such a scenario, the fields for a step-up converge irreversibly at the singularity, while for a step-down, they are expelled from it.  Thus, moving steps serve as optical furnaces or refrigerators, offering deep insights into their physics.

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