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Radiation as a heat transfer mode inside a bulk material is usually negligible in comparison to conduction. Here, the contribution of radiation to energy transport inside a hyperbolic material, hexagonal boron nitride (hBN), is investigated. With hyperbolic dispersion, i.e., opposite signs of dielectric components along principal directions, phonon polaritons contribute significantly to energy transport due to a much greater number of propagating modes compared to that in a normal material. A many‐body model is developed to account for radiative heat transfer in a material with a nonuniform temperature distribution. The total radiative heat transfer through hBN is found to be largely contributed by the high‐κ modes within the Reststrahlen bands, and is comparable to phonon conduction. Experimental measurements of temperature‐dependent thermal transport also show that radiative contribution to thermal transport is of the same order as that from phonons. Therefore, this work shows, for the first time, radiative heat transfer inside a material can be comparable to phonon conductive heat transfer.

Printing of high‐resolution three‐dimensional nanostructures utilizing two‐photon polymerization has gained significant attention recently. In particular, isopropyl thioxanthone (ITX) has been implemented as a photoinitiator due to its capability of initiating and depleting polymerization on demand, but new photoinitiating materials are still needed in order to reduce the power requirements for the high‐throughput creation of 3D structures. To address this point, a suite of new thioxanthone‐based photoinitiators were synthesized and characterized. Then two‐photon polymerization was performed using the most promising photoinitiating molecule. Importantly, one of the initiators, 2,7‐bis[(4‐(dimethylamino)phenyl ethynyl)‐9H‐thioxanthen‐9‐one] (BDAPT), showed a fivefold improvement in the writing threshold over the commonly used ITX molecule. To elucidate the fundamental mechanism, the excitation and inhibition behavior of the BDAPT molecule were evaluated using density functional theory (DFT) calculations, low‐temperature phosphorescence spectroscopy, ultra‐fast transient absorption spectroscopy, and the two‐photon Z‐scan spectroscopic technique. The improved polymerization threshold of this new photoinitiator presents a clear pathway for the modification of photoinitiators in 3D nanoprinting. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2019,57, 1462–1475