Enhanced thermal conduction by surface phonon-polaritons

Long Propagating Polaritonic Heat Transfer: Shaping Radiation to Conduction

Surface phonon polaritons (SPhPs) originate from the coupling of mid-IR photons and optical phonons, generating evanescent waves along the polar dielectric surface. The emergence of SPhPs gives rise to a phase of quantum matter that facilitates long-range energy transfer (100s μm-scale). Albeit of the recent experimental progress to observe the enhanced thermal conductivity of polar dielectric nanostructures mediated by SPhPs, the potential mechanism to present the high thermal conductivity beyond the Landauer limit has not been addressed. Here, we revisit the comprehensive theoretical framework to unify the distinct pictures of two heat transfer mechanisms by conduction and radiation. We first designed our experimental platform to distinguish contributions of two distinct fundamental modes of SPhPs, resulting in far-field radiation and long propagating conduction, respectively, by tuning the configuration of a nanostructured heat channel integrated into the thermometer. We could effectively control the transmission of long-propagating SPhPs to influence the apparent thermal conductivity of the nanostructure. This study reveals the high thermal conductance of 1.63 nW/K by a fast SPhP mode comparable to that by classical phonons, with measurements showing apparent conductivity values of up to 2 W/m·K at 515 K. The origin of the enhanced thermal conductivity was exploited by observing the interference of dispersive evanescent waves by double heat channels. Furthermore, our experimental observations of length-dependent thermal conductance by SPhPs are in good agreement with the revisited Landauer formula to illustrate a polaritonic mode of heat conduction, considering the dispersive nature of radiation not limited to the physical boundaries of a solid object yet directionally guided along the surface.

Enhanced Far-Field Thermal Radiation through a Polaritonic Waveguide

We experimentally demonstrate the enhancement of the far-field thermal radiation between two nonabsorbent Si microplates coated with energy-absorbent silicon dioxide (SiO_{2}) nanolayers supporting the propagation of surface phonon polaritons. By measuring the radiative thermal conductance between two coated Si plates, we find that its values are twice those obtained without the SiO_{2} coating. This twofold increase results from the hybridization of polaritons with guided modes inside Si and is well predicted by fluctuational electrodynamics and an analytical model based on a two-dimensional density of polariton states. These findings could be applied to thermal management in microelectronics, silicon photonics, energy conversion, atmospheric sciences, and astrophysics.

Probing Hyperbolic Shear Polaritons in β-Ga2O3 Nanostructures Using STEM-EELS

Phonon polaritons, quasiparticles arising from strong coupling between electromagnetic waves and optical phonons, have potential for applications in subdiffraction imaging, sensing, thermal conduction enhancement, and spectroscopy signal enhancement. A new class of phonon polaritons in low-symmetry monoclinic crystals, hyperbolic shear polaritons (HShPs), have been verified recently in β-Ga2O3 by free electron laser (FEL) measurements. However, detailed behaviors of HShPs in β-Ga2O3 nanostructures still remain unknown. Here, by using monochromatic electron energy loss spectroscopy in conjunction with scanning transmission electron microscopy, the experimental observation of multiple HShPs in β-Ga2O3 in the mid-infrared (MIR) and far-infrared (FIR) ranges is reported. HShPs in various β-Ga2O3 nanorods and a β-Ga2O3 nanodisk are excited. The frequency-dependent rotation and shear effect of HShPs reflect on the distribution of EELS signals. The propagation and reflection of HShPs in nanostructures are clarified by simulations of electric field distribution. These findings suggest that, with its tunable broad spectral HShPs, β-Ga2O3 is an excellent candidate for nanophotonic applications.

Molecular Polaritons for Chemistry, Photonics and Quantum Technologies

Molecular polaritons are quasiparticles resulting from the hybridization between molecular and photonic modes. These composite entities, bearing characteristics inherited from both constituents, exhibit modified energy levels and wave functions, thereby capturing the attention of chemists in the past decade. The potential to modify chemical reactions has spurred many investigations, alongside efforts to enhance and manipulate optical responses for photonic and quantum applications. This Review centers on the experimental advances in this burgeoning field. Commencing with an introduction of the fundamentals, including theoretical foundations and various cavity architectures, we discuss outcomes of polariton-modified chemical reactions. Furthermore, we navigate through the ongoing debates and uncertainties surrounding the underpinning mechanism of this innovative method of controlling chemistry. Emphasis is placed on gaining a comprehensive understanding of the energy dynamics of molecular polaritons, in particular, vibrational molecular polaritons─a pivotal facet in steering chemical reactions. Additionally, we discuss the unique capability of coherent two-dimensional spectroscopy to dissect polariton and dark mode dynamics, offering insights into the critical components within the cavity that alter chemical reactions. We further expand to the potential utility of molecular polaritons in quantum applications as well as precise manipulation of molecular and photonic polarizations, notably in the context of chiral phenomena. This discussion aspires to ignite deeper curiosity and engagement in revealing the physics underpinning polariton-modified molecular properties, and a broad fascination with harnessing photonic environments to control chemistry.

Scale-Dependent Heat Transport in Dissipative Media via Electromagnetic Fluctuations

We develop a theory for heat transport via electromagnetic waves inside media, and use it to derive a spatially nonlocal thermal conductivity tensor, in terms of the electromagnetic Green's function and potential, for any given system. While typically negligible for optically dense bulk media, the electromagnetic component of conductivity can be significant for optically dilute media, and shows regimes of Fourier transport as well as unhindered transport. Moreover, the electromagnetic contribution is relevant even for dense media, when in the presence of interfaces, as exemplified for the in-plane conductivity of a nanosheet, which shows a variety of phenomena, including absence of a Fourier regime.

STEM in situ thermal wave observations for investigating thermal diffusivity in nanoscale materials and devices

Practical techniques to identify heat routes at the nanoscale are required for the thermal control of microelectronic, thermoelectric, and photonic devices. Nanoscale thermometry using various approaches has been extensively investigated, yet a reliable method has not been finalized. We developed an original technique using thermal waves induced by a pulsed convergent electron beam in a scanning transmission electron microscopy (STEM) mode at room temperature. By quantifying the relative phase delay at each irradiated position, we demonstrate the heat transport within various samples with a spatial resolution of ~10 nm and a temperature resolution of 0.01 K. Phonon-surface scatterings were quantitatively confirmed due to the suppression of thermal diffusivity. The phonon-grain boundary scatterings and ballistic phonon transport near the pulsed convergent electron beam were also visualized.

Remarkable heat conduction mediated by non-equilibrium phonon polaritons

Surface waves can lead to intriguing transport phenomena. In particular, surface phonon polaritons (SPhPs), which result from coupling between infrared light and optical phonons, have been predicted to contribute to heat conduction along polar thin films and nanowires1. However, experimental efforts so far suggest only very limited SPhP contributions2-5. Through systematic measurements of thermal transport along the same 3C-SiC nanowires with and without a gold coating on the end(s) that serves to launch SPhPs, here we show that thermally excited SPhPs can substantially enhance the thermal conductivity of the uncoated portion of these wires. The extracted pre-decay SPhP thermal conductance is more than two orders of magnitude higher than the Landauer limit predicted on the basis of equilibrium Bose-Einstein distributions. We attribute the notable SPhP conductance to the efficient launching of non-equilibrium SPhPs from the gold-coated portion into the uncoated SiC nanowires, which is strongly supported by the observation that the SPhP-mediated thermal conductivity is proportional to the length of the gold coating(s). The reported discoveries open the door for modulating energy transport in solids by introducing SPhPs, which can effectively counteract the classical size effect in many technologically important films and improve the design of solid-state devices.

Near- and Far-Field Observation of Phonon Polaritons in Wafer-Scale Multilayer Hexagonal Boron Nitride Prepared by Chemical Vapor Deposition

Polaritons in layered materials (LMs) are a promising platform to manipulate and control light at the nanometer scale. Thus, the observation of polaritons in wafer-scale LMs is critically important for the development of industrially relevant nanophotonics and optoelectronics applications. In this work, phonon polaritons (PhPs) in wafer-scale multilayer hexagonal boron nitride (hBN) grown by chemical vapor deposition are reported. By infrared nanoimaging, the PhPs are visualized, and PhP lifetimes of ≈0.6 ps are measured, comparable to that of micromechanically exfoliated multilayer hBN. Further, PhP nanoresonators are demonstrated. Their quality factors of ≈50 are about 0.7 times that of state-of-the-art devices based on exfoliated hBN. These results can enable PhP-based surface-enhanced infrared spectroscopy (e.g., for gas sensing) and infrared photodetector applications.

Ultrafast and Nanoscale Energy Transduction Mechanisms and Coupled Thermal Transport across Interfaces

The coupled interactions among the fundamental carriers of charge, heat, and electromagnetic fields at interfaces and boundaries give rise to energetic processes that enable a wide array of technologies. The energy transduction among these coupled carriers results in thermal dissipation at these surfaces, often quantified by the thermal boundary resistance, thus driving the functionalities of the modern nanotechnologies that are continuing to provide transformational benefits in computing, communication, health care, clean energy, power recycling, sensing, and manufacturing, to name a few. It is the purpose of this Review to summarize recent works that have been reported on ultrafast and nanoscale energy transduction and heat transfer mechanisms across interfaces when different thermal carriers couple near or across interfaces. We review coupled heat transfer mechanisms at interfaces of solids, liquids, gasses, and plasmas that drive the resulting interfacial heat transfer and temperature gradients due to energy and momentum coupling among various combinations of electrons, vibrons, photons, polaritons (plasmon polaritons and phonon polaritons), and molecules. These interfacial thermal transport processes with coupled energy carriers involve relatively recent research, and thus, several opportunities exist to further develop these nascent fields, which we comment on throughout the course of this Review.

Thermal Conductivity of 3C/4H-SiC Nanowires by Molecular Dynamics Simulation

Silicon carbide (SiC) is a promising material for thermoelectric power generation. The characterization of thermal transport properties is essential to understanding their applications in thermoelectric devices. The existence of stacking faults, which originate from the "wrong" stacking sequences of Si-C bilayers, is a general feature of SiC. However, the effects of stacking faults on the thermal properties of SiC are not well understood. In this study, we evaluated the accuracy of Tersoff, MEAM, and GW potentials in describing the thermal transport of SiC. Additionally, the thermal conductivity of 3C/4H-SiC nanowires was investigated using non-equilibrium molecular dynamics simulations (NEMD). Our results show that thermal conductivity exhibits an increase and then saturation as the total lengths of the 3C/4H-SiC nanowires vary from 23.9 nm to 95.6 nm, showing the size effect of molecular dynamics simulations of the thermal conductivity. There is a minimum thermal conductivity, as a function of uniform period length, of the 3C/4H-SiC nanowires. However, the thermal conductivities of nanowires weakly depend on the gradient period lengths and the ratio of 3C/4H. Additionally, the thermal conductivity of 3C/4H-SiC nanowires decreases continuously from compressive strain to tensile strain. The reduction in thermal conductivity suggests that 3C/4H-SiC nanowires have potential applications in advanced thermoelectric devices. Our study provides insights into the thermal transport properties of SiC nanowires and can guide the development of SiC-based thermoelectric materials.

Boosting Thermal Conductivity by Surface Plasmon Polaritons Propagating along a Thin Ti Film

We experimentally demonstrate boosted in-plane thermal conduction by surface plasmon polaritons (SPPs) propagating along a thin Ti film on a glass substrate. Due to the lossy nature of metal, SPPs can propagate over centimeter-scale distances even along a supported metal film, and the resulting ballistic heat conduction can be quantitatively validated. Further, for a 100-nm-thick Ti film on a glass substrate, a significant enhancement of in-plane thermal conductivity compared to bulk value (∼25%) is experimentally shown. This Letter will provide a new avenue to employ SPPs for heat dissipation along a supported thin film, which can be readily applied to mitigate hot-spot issues in microelectronics.

Modeling effective thermal conductivity enhanced by surface waves using the Boltzmann transport equation

The thermal management of semiconductors at the device level has become a crucial issue owing to the high integration density and miniaturization of microelectronic systems. Because surface phonon polaritons (SPhPs) exhibit long propagation lengths, they are expected to contribute significantly to the heat dissipation in microelectronic systems. This study aims to numerically estimate the heat transfer due to SPhPs in a thin SiO₂ film. The one-dimensional Boltzmann transport equation (BTE) is solved using the estimated propagation length based on the SPhP dispersion curves. The temperature profiles and heat fluxes are predicted and demonstrate the size effect of the film on the effective in-plane thermal conductivity of the SiO₂ film. The results indicate that the temperature distribution was constant regardless of the film length and thickness because the propagation length was much longer than the film length. In addition, the heat flux increased with decreasing film thickness owing to the depth-averaged energy transfer. The effective thermal conductivities predicted using the BTE differed by ~ 16.5% from the values obtained from the analytical expression. The numerical results of this study can provide valuable data when studying the thermal behavior of SPhPs.

Near-Isotropic Polariton Heat Transport along a Polar Anisotropic Nanofilm

The heat transport of surface phonon-polaritons propagating along a polar uniaxial anisotropic nanofilm is studied for different orientations of its optical axis, film thicknesses, and temperatures. For an hBN nanofilm, it is shown that i) the propagation of polaritons can be described in terms of even and odd modes that generalize the transverse magnetic and transverse electrical ones that typically appear in isotropic films. ii) The frequency spectrum of polaritons can efficiently be tuned with the angle between the film optical axis and their propagation direction. iii) The polariton thermal conductivity takes higher values for a thinner or hotter nanofilm. iv) The even and odd modes have a remarkable contribution to the total polariton thermal conductivity, which takes a value higher than 5.6 Wm⁻¹K⁻¹ for a 25-nm-thick nanofilm at 500 K. The obtained results thus uncover some key features of the propagation and heat transport of polaritons in uniaxial nanofilms.

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Polariton thermal conductivity of a uniaxial anisotropic nanofilm

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Polariton heat transport driven by the optical axis of a polar anisotropic nanofilm

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Higher thermal conductivities for thinner or hotter nanofilms

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Near-isotropic thermal response of a strongly anisotropic nanofilm

Silver Peroxide Nanoparticles for Combined Antibacterial Sonodynamic and Photothermal Therapy

Metal peroxide nanoparticles designed to elevate the oxidative stress are considered a promising nanotherapeutics in biomedical applications, including chemotherapy, photodynamic therapy, and bacterial disinfection. However, their lack of specificity towards the therapeutic target can cause toxic side effects to healthy tissues. Here, silver peroxide nanoparticles (Ag₂ O₂ NPs) capable of controlled reactive oxygen species (ROS) release are synthesized. The release of bactericidal Ag⁺ ions and ROS is strictly regulated by external stimuli of ultrasound (US) and near-infrared (NIR) light. In vitro and in vivo investigations show that the Ag₂ O₂ NPs present enhanced antibacterial and antibiofilm capabilities with a killing efficiency >99.9999% in 10 min, significantly accelerate multi-drug resistant Staphylococcus aureus infected skin wound closure with excellent cytocompatibility and hemocompatibility. This work not only provides the first paradigm for fabricating silver peroxide nanoparticle but also introduces a highly efficient noninvasive and safe therapeutic modality for combating bacterial infectious diseases.

Spatiotemporal imaging of 2D polariton wave packet dynamics using free electrons

Coherent optical excitations in two-dimensional (2D) materials, 2D polaritons, can generate a plethora of optical phenomena that arise from the extraordinary dispersion relations that do not exist in regular materials. Probing of the dynamical phenomena of 2D polaritons requires simultaneous spatial and temporal imaging capabilities and could reveal unknown coherent optical phenomena in 2D materials. Here, we present a spatiotemporal measurement of 2D wave packet dynamics, from its formation to its decay, using an ultrafast transmission electron microscope driven by femtosecond midinfrared pulses. The ability to coherently excite phonon-polariton wave packets and probe their evolution in a nondestructive manner reveals intriguing dispersion-dependent dynamics that includes splitting of multibranch wave packets and, unexpectedly, wave packet deceleration and acceleration. Having access to the full spatiotemporal dynamics of 2D wave packets can be used to illuminate puzzles in topological polaritons and discover exotic nonlinear optical phenomena in 2D materials.