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The blackbody theory is revisited in the case of thermal electromagnetic fields inside uniaxial anisotropic media in thermal equilibrium with a heat bath. When these media are hyperbolic, we show that the spectral energy density of these fields radically differs from that predicted by Plancks blackbody theory. We demonstrate that the maximum of their spectral energy density is shifted towards frequencies smaller than Wiens frequency making these media apparently colder. Finally, we derive Stefan-Boltzmanns law for hyperbolic media which becomes a quadratic function of the heat bath temperature.
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Nanomechanical systems are generally embedded in a macroscopic environment where the sources of thermal noise are difficult to pinpoint. We engineer a silicon nitride membrane optomechanical resonator such that its thermal noise is acoustically driven by a spatially well-defined remote macroscopic bath. This bath acts as an acoustic blackbody emitting and absorbing acoustic radiation through the silicon substrate. Our optomechanical system acts as a sensitive detector for the blackbody temperature and for photoacoustic imaging. We demonstrate that the nanomechanical mode temperature is governed by the blackbody temperature and not by the local material temperature of the resonator. Our work presents a route to mitigate self-heating effects in optomechanical thermometry and other quantum optomechanics experiments, as well as acoustic communication in quantum information.
Nearly all thermal radiation phenomena involving materials with linear response can be accurately described via semi-classical theories of light. Here, we go beyond these traditional paradigms to study a nonlinear system which, as we show, necessarily requires quantum theory of damping. Specifically, we analyze thermal radiation from a resonant system containing a $chi^{(2)}$ nonlinear medium and supporting resonances at frequencies $omega_1$ and $omega_2approx 2omega_1$, where both resonators are driven only by intrinsic thermal fluctuations. Within our quantum formalism, we reveal new possibilities for shaping the thermal radiation. We show that the resonantly enhanced nonlinear interaction allows frequency-selective enhancement of thermal emission through upconversion, surpassing the well-known blackbody limits associated with linear media. Surprisingly, we also find that the emitted thermal light exhibits non-trivial statistics ($g^{(2)}(0) eq 2$) and biphoton intensity correlations (at two distinct frequencies). We highlight that these features can be observed in the near future by heating a properly designed nonlinear system, without the need for any external signal. Our work motivates new interdisciplinary inquiries combining the fields of nonlinear photonics, quantum optics and thermal science.
Hyperbolic metamaterials (HMMs) are highly anisotropic optical materials that behave as metals or as dielectrics depending on the direction of propagation of light. They are becoming essential for a plethora of applications, ranging from aerospace to automotive, from wireless to medical and IoT. These applications often work in harsh environments or may sustain remarkable external stresses. This calls for materials that show enhanced optical properties as well as tailorable mechanical properties. Depending on their specific use, both hard and ultrasoft materials could be required, although the combination with optical hyperbolic response is rarely addressed. Here, we demonstrate the possibility to combine optical hyperbolicity and tunable mechanical properties in the same (meta)material, focusing on the case of extreme mechanical hardness. Using high-throughput calculations from first principles and effective medium theory, we explored a large class of layered materials with hyperbolic optical activity in the near-IR and visible range, and we identified a reduced number of ultrasoft and hard HMMs among more than 1800 combinations of transition metal rocksalt crystals. Once validated by the experiments, this new class of metamaterials may foster previously unexplored optical/mechanical applications.
Hyperbolic metamaterials (HMMs) support propagating waves with arbitrarily large wavevectors over broad spectral ranges, and are uniquely valuable for engineering radiative thermal transport in the near field. Here, by employing a rational design approach based on the electromagnetic local density of states, we demonstrate the ability of HMMs to substantially rectify radiative heat flow. Our idea is to establish a forward-biased scenario where the two HMM-based terminals of a thermal diode feature overlapped hyperbolic bands which result in a large heat current, and suppress the reverse heat flow by creating spectrally mismatched density of states as the temperature bias is flipped. As an example, we present a few high-performance thermal diodes by pairing HMMs made of polar dielectrics and metal-to-insulator transition (MIT) materials in the form of periodic nanowire arrays, and considering three representative kinds of substrates. Upon optimization, we theoretically achieve a rectification ratio of 324 at a 100 nm gap, which remains greater than 148 for larger gap sizes up to 1 um over a wide temperature range. The maximum rectification represents an almost 1000-fold increase compared to a bulk diode using the same materials, and is twice that of state-of-the-art designs. Our work highlights the potential of HMMs for rectifying radiative heat flow, and may find applications in advanced thermal management and energy conversion systems.
The optical properties of some nanomaterials can be controlled by an external magnetic field, providing active functionalities for a wide range of applications, from single-molecule sensing to nanoscale nonreciprocal optical isolation. Materials with broadband tunable magneto-optical response are therefore highly desired for various components in next-generation integrated photonic nanodevices. Concurrently, hyperbolic metamaterials received a lot of attention in the past decade since they exhibit unusual properties that are rarely observed in nature and provide an ideal platform to control the optical response at the nanoscale via careful design of the effective permittivity tensor, surpassing the possibilities of conventional systems. Here, we experimentally study magnetic circular dichroism in a metasurface made of type-II hyperbolic nanoparticles on a transparent substrate. Numerical simulations confirm the experimental findings, and an analytical model is established to explain the physical origin of the observed magneto-optical effects, which can be described in terms of the coupling of fundamental electric and magnetic dipole modes with an external magnetic field. Our system paves the way for the development of nanophotonic active devices combining the benefits of sub-wavelength light manipulation in hyperbolic metamaterials supporting a large density of optical states with the ability to freely tune the magneto-optical response via control over the anisotropic permittivity of the system.
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