Van der Waals heterostructures have emerged as promising building blocks that offer access to new physics, novel device functionalities, and superior electrical and optoelectronic properties [1][2][3][4][5][6][7]. Applications such as thermal management, photodetection, light emission, data communication, high-speed electronics and light harvesting [8][9][10][11][12][13][14][15][16] require a thorough understanding of (nanoscale) heat flow. Here, using time-resolved photocurrent measurements we identify an efficient out-of-plane energy transfer channel, where charge carriers in graphene couple to hyperbolic phonon polaritons [17][18][19] in the encapsulating layered material. This hyperbolic cooling is particularly efficient, giving picosecond cooling times, for hexagonal BN, where the high-momentum hyperbolic phonon polaritons enable efficient near-field energy transfer. We study this heat transfer mechanism through distinct control knobs to vary carrier density and lattice temperature, and find excellent agreement with theory without any adjustable parameters. These insights may lead to the ability to control heat flow in van der Waals heterostructures.Owing to its large in-plane thermal conductivity, graphene has been suggested as material for the thermal management of nanoscale devices [8]. At the same time, graphene is well-known for its ability to convert incident light into electrical heat, i.e. hot electrons that can be used to generate photocurrent, with applications in photodetection, data communication and light harvesting [10,20,21]. Understanding, and ultimately controlling, heat flow in graphene-van der Waals heterostructures is therefore of paramount importance. For example, a short cooling time of graphene hot carriers is advantageous for thermal management and for high switching rates of photodetectors (PDs) for data communication, whereas a long cooling time is favorable for photodetection sensitivity [10,20,21]. Of particular relevance are heterostructure devices that contain high-quality graphene encapsulated by layered materials, such as hexagonal BN (hBN) and MoS 2 , which have the potential to crucially improve the performance of electronic and optoelectronic devices [1,2]. * Electronic address: Correspondence: klaas-jan.tielrooij@icfo.eu, frank.koppens@icfo.eu ‡ Equal contribution.A number of cooling pathways for graphene carriers have been proposed, involving among others strongly coupled optical phonons [22][23][24], acoustic phonons [25][26][27][28], substrate phonons [29] and plasmons [30] (see also Appendix 1). Here, using several experimental approaches, we show that cooling in graphene encapsulated by hBN is governed by out-of-plane coupling of graphene electrons to special polar phonon modes that occur in layered materials (LMs): hyperbolic phonon polaritons, where xx zz < 0, with xx and zz the permittivity parallel and perpendicular to the LM plane. Owing to this property, these materials carry deep sub-wavelength, raylike optical phonon polaritons. For hBN, within the two Rest...

Radiative heat transfer (RHT) between macroscopic bodies at separations that are much smaller than the thermal wavelength is ruled by evanescent electromagnetic modes and can be orders of magnitude more efficient than its far-field counterpart, which is described by the Stefan-Boltzmann law. In this Letter we present a microscopic theory of RHT in van der Waals stacks comprising graphene and a natural hyperbolic material, i.e. hexagonal boron nitride (hBN). We demonstrate that RHT between hot carriers in graphene and hyperbolic phonon-polaritons in hBN is extremely efficient at room temperature, leading to picosecond time scales for the carrier cooling dynamics.Introduction.-The cooling stages of the temperature dynamics of hot carriers in a crystal typically proceed via energy transfer to phonons [1]. In the case of pristine graphene, ultra-long cooling times, on the order of nanoseconds, have been theoretically predicted [2,3]. Such slow cooling dynamics is due to energy transfer to graphene acoustic phonons via collisions that conserve momentum. If realized experimentally, this intrinsic relaxation dynamics would imply notable figures of merit for graphene-based photodetectors [4]. Unfortunately, the cooling dynamics in "first-generation" graphene samples [5], i.e. samples deposited on SiO 2 , is believed to be dominated by far more efficient disorder-assisted momentum-non-conserving collisions between electrons and graphene acoustic phonons [6][7][8][9]. According to theory [8,9], such "supercollisions" are due to short-range (rather than long-range) disorder.It is therefore not clear yet how to reach the intrinsic regime [2,3]. In this respect, a natural question arises: What is the fate of the temperature dynamics of hot carriers in "second-generation" samples [10], where graphene is encapsulated between hexagonal boron nitride (hBN) crystals [11][12][13][14][15][16]? On the one hand, these samples have shown nearly ideal transport characteristics [11][12][13][14][15][16], whereby momentum-conserving electronacoustic phonon scattering [17,18] fully determines dc transport times at room temperature, at least for sufficiently large carrier densities. On the other hand, hBN crystal slabs are known to support low-loss standing Fabry-Pérot phonon-polaritons [20][21][22]. These modes occur because hBN is a uniaxial crystal with intrinsic hyperbolic character [19], i.e. with in-( x ) and out-of-plane ( z ) components of the dielectric tensorˆ having opposite signs in the so-called "reststrahlen" frequency bands.Could radiative heat transfer (RHT) to hyperbolic phonon-polaritons in hBN significantly affect the late stages of the cooling dynamics of hot carriers in graphene? In this Letter we answer this question affirmatively.

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Graphene-based moiré superlattices have recently emerged as a unique class of tuneable solid-state systems that exhibit significant optoelectronic activity. Local probing at length scales of the superlattice should provide deeper insight into the microscopic mechanisms of photoresponse and the exact role of the moiré lattice. Here, we employ a nanoscale probe to study photoresponse within a single moiré unit cell of minimally twisted bilayer graphene. Our measurements reveal a spatially rich photoresponse, whose sign and magnitude are governed by the fine structure of the moiré lattice and its orientation with respect to measurement contacts. This results in a strong directional effect and a striking spatial dependence of the gate-voltage response within the moiré domains. The spatial profile and carrier-density dependence of the measured photocurrent point towards a photo-thermoelectric induced response that is further corroborated by good agreement with numerical simulations. Our work shows sub-diffraction photocurrent spectroscopy is an exceptional tool for uncovering the optoelectronic properties of moiré superlattices.

Conducting materials typically exhibit either diffusive or ballistic charge transport. When electron–electron interactions dominate, a hydrodynamic regime with viscous charge flow emerges1–13. More stringent conditions eventually yield a quantum-critical Dirac-fluid regime, where electronic heat can flow more efficiently than charge14–22. However, observing and controlling the flow of electronic heat in the hydrodynamic regime at room temperature has so far remained elusive. Here we observe heat transport in graphene in the diffusive and hydrodynamic regimes, and report a controllable transition to the Dirac-fluid regime at room temperature, using carrier temperature and carrier density as control knobs. We introduce the technique of spatiotemporal thermoelectric microscopy with femtosecond temporal and nanometre spatial resolution, which allows for tracking electronic heat spreading. In the diffusive regime, we find a thermal diffusivity of roughly 2,000 cm2 s−1, consistent with charge transport. Moreover, within the hydrodynamic time window before momentum relaxation, we observe heat spreading corresponding to a giant diffusivity up to 70,000 cm2 s−1, indicative of a Dirac fluid. Our results offer the possibility of further exploration of these interesting physical phenomena and their potential applications in nanoscale thermal management.

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A conventional optical cavity supports one or more modes, which are confined since they are unable to leak out of the cavity. Bound state in continuum (BIC) cavities are an unconventional alternative, based on confinement by destructive interference 1,2 , even though optical leakage channels are available. BICs are a general wave phenomenon 2-8 , of particular interest to optics 6,9-14 , but BICs have never been demonstrated at the nanoscale level. Nanoscale BIC cavities are more challenging to realize, however, as they require destructive interference at the nanometer scale. Here, we demonstrate the first nanophotonic cavities based on BIC and find an unprecedented combination of quality factors and ultrasmall mode volume. In particular, we exploit hyperbolic media, HyM, as they can support large (in principle unlimited) momentum excitations, which propagate as ultra-confined rays, so that HyM cavities can in principle be extremely small. However, building a hyperbolic BIC (hBIC) cavity presents a fundamental challenge: an hBIC has an infinite number of modes, which would all need to interfere simultaneously. Here, we bring the BIC concept to the nanoscale by introducing and demonstrating a novel multimodal reflection mechanism of the ray-like optical excitations in hyperbolic materials. Using near-field microscopy, we demonstrate mid-IR confinement in BIC-based nanocavities with volumes down to 23x23x3 and quality factors above 100a dramatic improvement in several metrics of confinement. This alliance of HyM with BICs yields a radically novel way to confine light and is expected to have far reaching consequences wherever strong optical confinement is utilized, from ultra-strong light-matter interactions, to mid-IR nonlinear optics and a range of sensing applications.The general premise of nanophotonics involves shrinking light to the subwavelength nanometric scale, for example by compressing light into the tiny volume of a nanocavity which dramatically enhances its interaction with matter. Innovations in nanocavity design 15 have allowed even single emitters to be strongly bound to cavity polaritons [16][17][18] . Likewise, nanocavity coupling strengths can become so large as to reach the onset of ultrastrong 19 and deep 20 coupling regimes, where bound states entangle with virtual excitations 21,22 , challenging the basic understanding of light matter interaction. However, shrinking light typically comes at a costabsorption losses, which plague all existing nanocavity designs. Fig. 1a visually summarizes the state of the art in nanocavity research [23][24][25][26][27][28][29][30][31][32][33][34][35][36] and shows that cavity performance progressively worsens beneath the 100 nm scale (i.e. for < 10 −4 λ 0 3 , with 0 the

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