Abstract Thermal diodes, which allow heat to flow in a preferential direction, are an essential component of thermal circuitry and will find applications in many fields ranging from thermal photon controls to quantum information. The quality of a thermal diode is defined by the thermal rectification ratio (TRR). Here, an ultrahigh TRR exceeding 10 5 at a small temperature difference (1–5 K) is presented based on near‐field thermal photon (NFTP) manipulation, by utilizing asymmetric gap‐variated dual terminals based on graphene/Si heterostructures and materials with contrasting thermal expansion coefficients. Analyses of the photon tunneling probability show that the colossal TRR comes from coupled and decoupled graphene surface plasmon polaritons under opposite temperature biases, remaining robust across a wide operating temperature range. Further enhancement can be achieved by tuning the Fermi level of graphene. This work demonstrates the significance of asymmetry in NFTP manipulation for thermal diode performance, achieving an ultrahigh TRR, which is at least three orders higher than the state‐of‐the‐art structures at the same small temperature difference (1–5 K), and is comparable to state‐of‐the‐art electronic diodes.

The negatively-charged boron vacancy (V - B ) center in hexagonal boron nitride (hBN) is currently garnering considerable attention for the design of two-dimensional (2D) quantum sensing units. Such developments require a precise understanding of the spin-dependent optical response of V - B centers, which still remains poorly documented despite its key role for sensing applications. Here we investigate the spin-dependent photodynamics of V - B centers in hBN by a series of time-resolved photoluminescence (PL) measurements. We first introduce a robust all-optical method to infer the spin-dependent lifetime of the excited states and the electron spin polarization of V - B centers under optical pumping. Using these results, we then analyze PL time traces recorded at different optical excitation powers with a seven-level model of the V - B center and we extract all the rates involved in the spin-dependent optical cycles, both under ambient conditions and at liquid helium temperature. These findings are finally used to study the impact of a vector magnetic field on the optical response. More precisely, we analyze PL quenching effects resulting from electron spin mixing induced by the magnetic field component perpendicular to the V - B quantization axis. All experimental results are well reproduced by the seven-level model, illustrating its robustness to describe the spin-dependent photodymanics of V - B centers. This work provides important insights into the properties of V - B centers in hBN, which are valuable for future developments of 2D quantum sensing units.

The polarized photoluminescence emitted on the edge of a series of aluminum‐rich (Al,Ga)N‐AlN quantum wells (QWs) grown by molecular beam epitaxy on AlN templates deposited by metal organic chemical vapor deposition on c‐plane sapphire is measured. The contrast and the principal axis of the emission diagrams for 2 nm‐thick (Al,Ga)N QWs grown for aluminum compositions between 40% and 90% are studied. The light is emitted on the edge of the QWs at wavelengths going from 280 nm down to 209 nm. The emission diagram, a change from oblate to prolate with respect to the in‐plane orientation, for an aluminum composition is found to occur around 72%, that is, at an emission wavelength of about 235 nm. The orientations and shapes of the edge‐emission diagrams indicate that the fluctuations of the composition of the (Al,Ga)N confining layer are deep enough for producing intravalence band mixings. This property, that acts in concert with the built‐in strain and quantum‐confined Stark effect, contributes to the anisotropy of the light emission when the aluminum composition reaches 60–70%, that is, for an emission wavelength of 260–235 nm.