An electric power cable, such as an overhead power line that connects your light bulbs to a power plant, determines the direction of electricity transport. Similarly, light transport may be confined in an optical cable, which can route light from the outdoors to a subterranean parking lot or deliver the Internet to your home from across the oceans. Most forms of energy, including electricity and light, can be easily controlled and guided. But there is an exception.
Heat, arguably the most ubiquitous form of energy, is notoriously difficult to control due to its diffusive nature. Planck's Law of radiation, which lays the fundamentals of quantum theories, describes heat radiation as an incandescent process covering a wide range of wavelengths and angles.
However, as noted by Max Planck himself (after whom the law of radiation is named), the incandescent radiation is not always true. The radiation energy distribution will diverge considerably from the law if the size of the item emitting the radiation is less than the wavelength of the thermal radiation, which is roughly 10 micrometers at ambient temperature.
In Planck's day, making such a gadget was nearly impossible. Sunmi Shin, a former UC San Diego Ph.D. student who is now an Assistant Professor at the National University of Singapore, discovered a means to make Planck's prediction come true. She devised a series of novel setups that demonstrated the capability to enhance and directly measure the coherent thermal emission from nanoscale emitters with exceedingly low emitting power from cryogenic to room temperature. This achievement provides new insights into the realization of spatial and spectral distribution control for thermal emission in the far-field. Then, she further strengthened the directionality of radiative emission. In another word, she fabricated a "heat wire" and controlled the direction of heat flow in a similar fashion to an optical fiber.
Her study brought together two disciplines of sciences: heat transfer and photonics. These new ideas allowed for the control and directing of heat along a surface, similar to how optical waves are guided. It dramatically extends the degree of freedom to engineer heat transfer.