Surface Phonon Polaritons Mediated Energy Transfer between Nanoscale Gaps Sheng Shen, † Arvind Narayanaswamy,* ,‡ and Gang Chen* ,† Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, Department of Mechanical Engineering, Columbia UniVersity, New York, New York 10027 Received April 16, 2009; Revised Manuscript Received June 18, 2009 ABSTRACT Surface phonon polaritons are electromagnetic waves that propagate along the interfaces of polar dielectrics and exhibit a large local-field enhancement near the interfaces at infrared frequencies. Theoretical calculations show that such surface waves can lead to breakdown of the Planck’s blackbody radiation law in the near field. Here, we experimentally demonstrate that surface phonon polaritons dramatically enhance energy transfer between two surfaces at small gaps by measuring radiation heat transfer between a microsphere and a flat surface down to 30 nm separation. The corresponding heat transfer coefficients at nanoscale gaps are 3 orders of magnitude larger than that of the blackbody radiation limit. The high energy flux can be exploited to develop new radiative cooling and thermophotovoltaic technologies. Although Planck’s blackbody radiation is often considered as the maximum of heat radiation between two surfaces, Planck himself recognized that the law bearing his name is not valid when the characteristic length scales are comparable to the wavelength of thermal radiation. 1 The theoretical foundation of near-field radiation was established based on fluctuational electrodynamics theory 2,3 and has been em- ployed to study near-field radiation between the surfaces of metals, 3 dielectrics 4-6 and semiconductors. 7 In particular, theory has predicted that near-field radiation between polar dielectric materials (SiO 2 , SiC, BN, etc.), which support resonant surface phonon polaritons, is dominated by the surface phonon-polariton contribution and can be enhanced by several orders of magnitude beyond the Planck’s black- body radiation limit. 4 So far, however, such significant enhancement of energy transfer mediated by surface phonon polaritons has not been experimentally demonstrated. In this letter, we clearly show this enhancement by measuring near- field thermal radiation between a glass sphere and different substrate materials with a sensitive bimaterial atomic force microscope cantilever. 8,9 Surface phonon-polariton, which originates from the resonant coupling between the electromagnetic field and optical phonons in polar dielectrics, is an infrared counterpart of surface plasmon-polariton that usually exists on metal surfaces in the visible and ultraviolet range. In both cases, these surface waves share the following properties: they are modes of the system that can be resonantly excited; and they are characterized by large energy densities at the interface, which decay rapidly with distance from the surface. 10,11 Figure 1A shows the calculated local density of states (LDOS) 12 in vacuum at 50 nm above an interface between vacuum and three different materials considered in our experiments. Silicon dioxide (glass) is a polar dielectric material that can support surface phonon polaritons, although compared to crystalline polar materials such as SiC, the resonance is broadened due to stronger damping in amor- phous materials. The large peaks in LDOS are observed near the surface of polar materials at certain wavelengths (λ ≈ 8.5 μm and λ ≈ 20.3 μm for glass) that correspond to surface phonon-polariton resonances. Silicon and gold surfaces, however, do not exhibit any strong resonant excitation peaks in the spectral region under consideration. These surface waves on SiO 2 surface decay rapidly away from the interface. Hence, despite the high energy density near the interface (Figure 1A), these surface waves do not lead to far-field emission. When another surface is brought close by, the surface waves can tunnel from one side to the other, contributing significantly to heat transfer. Figure 1B,C shows, respectively, the spectral and total radiative heat transfer coefficients defined as the net radiative flux (per unit wavelength interval for spectral heat transfer coefficients) divided by the temperature difference between two parallel plates made of different material combinations (SiO 2 -SiO 2 , * To whom correspondence should be addressed. (G.C.) Phone: 617- 253-0006. Fax: 617-258-5802. E-mail: gchen2@mit.edu. (A.N.) Phone: 212- 854-0303. E-mail: arvind.narayanaswamy@columbia.edu. † Massachusetts Institute of Technology. ‡ Columbia University. NANO LETTERS 2009 Vol. 9, No. 8 2909-2913 10.1021/nl901208v CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009