Spinning thermal radiation from symmetry-broken metasurfaces

Spinning thermal radiation is a unique phenomenon observed in condensed astronomical objects, including the Wolf-Rayet star EZ-CMa and the red degenerate star G99-47, due to the existence of strong magnetic fields. Here, by designing symmetry-broken metasurfaces, we demonstrate that spinning thermal radiation with a nonvanishing optical helicity can be realized even without applying a magnetic field. We design nonvanishing optical helicity by engineering a dispersionless band that emits omnidirectional spinning thermal radiation, where our design reaches 39% of the fundamental limit. Our results firmly suggest that metasurfaces can impart spin coherence in the incoherent radiation excited by thermal fluctuations. The symmetry-based design strategy also provides a general pathway for controlling thermal radiation in its temporal and spin coherence.

Symmetry-based optical spin control of thermal radiation. (A) Thermal radiation originates from fluctuating dipoles and is thus considered an incoherent signal. It is naturally broadband and omnidirectional and carries no SAM. (B and C) Recent research efforts aim to impart temporal (B) and spatial (C) coherence in thermal radiation, where narrow-band and directional thermal radiation are demonstrated. (D) In this work, by imparting spin coherence, we achieve effective tailoring of thermal emission in its spectral and spin properties. (E) Schematic demonstrates that the photon spin characteristics are governed by the symmetries in the 2D system. When both inversion (î) and mirror (m) symmetries are preserved, the photon spin of thermally radiated photons is degenerate in energy-momentum space. Thus, the spin and helicity both vanish. (F) When inversion symmetry is broken, spinning thermal radiation arises at oblique angles. However, the antisymmetric spin pattern guarantees the spin degeneracy at surface normal and a total-zero optical helicity. (G) In this work, we show that photon spin arises in an asymmetric pattern when both inversion and mirror symmetries are broken, and the nonvanishing optical helicity is observed.

Symmetry-broken metasurfaces for thermal radiation engineering. (A to I) The optical images (left; scale bar, 5 μm), averaged emissivity spectra (middle), and DoCP (right) are plotted for devices with mirror symmetry (A to C), inversion symmetry (D to F), and fourfold rotational symmetry (G to I), respectively. The average emissivity is symmetric along k = 0 in all three cases, which is a manifestation of the reciprocity. The DoCP is zero for the mirror-symmetric device and symmetric for the inversion and C4 devices. The results demonstrate that the intertwined spectral and spin properties of thermal radiation can be effectively tailored through symmetry engineering.

Reference:

Observation of nonvanishing optical helicity in thermal radiation from symmetry-broken metasurfaces, Science Advances, 619, 743-748, 2023

Wang, X., Sentz, T., Bharadwaj, S., Ray, S.K., Wang, Y., Jiao, D., Qi, L. and Jacob, Z.

High-Temperature Polaritons in Ceramic Nanotube Antennas

Spectral emissivity of BNNT system on tungsten thin film (solid red) at 938 K. 

High-temperature thermal photonics presents unique challenges for engineers as the database of materials that can withstand extreme environments are limited. In particular, ceramics with high temperature stability that can support coupled light-matter excitations, that is, polaritons, open new avenues for engineering radiative heat transfer. Hexagonal boron nitride (hBN) is an emerging ceramic 2D material that possesses low-loss polaritons in two spectrally distinct mid-infrared frequency bands. The hyperbolic nature of these frequency bands leads to a large local density of states (LDOS). In 2D form, these polaritonic states are dark modes, bound to the material. In cylindrical form, boron nitride nanotubes (BNNTs) create subwavelength particles capable of coupling these dark modes to radiative ones. In this study, we leverage the high-frequency optical phonons present in BNNTs to create strong mid-IR thermal antenna emitters at high temperatures (938 K). Through direct measurement of thermal emission of a disordered system of BNNTs, we confirm their radiative polaritonic modes and show that the antenna behavior can be observed even in a disordered system. These are among the highest-frequency optical phonon polaritons that exist and could be used as high-temperature mid-IR thermal nanoantenna sources.

Reference:

High-Temperature Polaritons in Ceramic Nanotube Antennas, Nano Letters 19(12), 2019.

Starko-Bowes, R., Wang, X., Xu, Z., Pramanik, S., Lu, N., Li, T., and Jacob, Z.

(online cover)

High-Temperature Thermal Photonics, Annual Review of Heat Transfer, 23, Chapter 9, 2020.

Wang, X., Starko-Bowes, R., Khandekar, C., and Jacob, Z.