Conformal surface plasmons propagating on ultrathinand flexible filmsXiaopeng Shena,1, Tie Jun Cuia,1,2, Diego Martin-Canob, and Francisco J. Garcia-Vidalb,c,2aState Key Laboratory of Millimetre Waves, School of Information Science and Engineering, Southeast University, Nanjing 210096, China;bDepartamentode Fisica Teorica de la Materia Condensada, Universidad Autonoma de Madrid, E-28049 Madrid, Spain; andcDonostia International Physics Center, 20018San Sebastian/Donostia, SpainEditedby Federico Capasso, Harvard University, Cambridge, MA, and approved November 15, 2012 (received for review June 18, 2012)Surface plasmon polaritons (SPPs) are localized surface electromag- netic waves that propagate along the interface between a metaland a dielectric. Owing to their inherent subwavelength confine- ment, SPPs have a strong potential to become building blocks of a type of photonic circuitry built up on 2D metal surfaces; however,SPPs are difficult to control on curved surfaces conformably andflexibly to produce advanced functional devices. Here we propose the concept of conformal surface plasmons (CSPs), surface plasmonwaves that can propagate on ultrathin and flexible films to long distances in a wide broadband range from microwave to mid-infrared frequencies. We present the experimental realization of these CSPsin the microwave regime on paper-like dielectric films with a thick-ness 600-fold smaller than the operating wavelength. The flexiblepaper-like films can be bent, folded, and even twisted to mold theflow of CSPs.metamaterials|plasmonics|waveguiding Surface plasmon polaritons (SPPs) are highly localized surface waves (1) that propagate along the interface between two materials whose real parts of electric permittivity have opposite signs, and decay exponentially in the transverse direction. At op- tical frequencies, metals behave like plasma with negative permit- tivity, and thus SPPs exist on metalair interfaces (2, 3). Owing totheir ability to confine light in a subwavelength scale with high in- tensity, SPPs can be used to overcome the diffraction limit, mini- aturize photonic components, and build highly integrated optical componentsandcircuits.Thus,theyhavefound(orhavepotential)applications in biomedical sensing, near-field microscopy, opto- electronics, photovoltaics, and nanophotonics (411). In the far-infrared, terahertz, and microwave frequency bands, metals behave akin to perfectly electrical conductors (PECs), and thus SPPs cannot be supported by a metal surface. Although some designs based on metal wires or strips are able to support surfaceleaky modes that have some degree of lateral confinement at terahertz frequencies (12, 13), the concept of plasmonic meta- materials has proven very useful in the production of highly con-fined surface electromagnetic (EM) waves at low frequencies (14 27). Early work in this area can be traced back to the 1950s and 1960s, when corrugated metal structures were used to generate surface EM waves at microwave frequencies (14, 15). Generally, plasmonic metamaterials consist of metal surfaces decorated with 1D arrays of subwavelength grooves, 2D arrays of subwavelength holes/dimples,or3Dmetalwiresinwhichaperiodicarrayofradial grooves is drilled (1626). Recently, an alternative “spoof” SPP structureusingcomplementarysplit-ringresonatorsastheunitcell elements has been proposed theoretically (27). The surface EM modes decorated by all of these plasmonic metamaterials are called spoof SPPs, or designer SPPs, because their properties are very similar to those of SPPs at optical frequencies. An important advantage of this metamaterial approach is that the dispersioncharacteristics and spatial confinement of the spoof SPPs can be controlled simply by geometrical means. However, all of the aforementioned plasmonic metamaterials have a major limitation associated with their inherent 3D geometry.For the production of advanced plasmonic functional devices, itis very important to realize surface EM waves that can be confinedin the subwavelength scale and can propagate on flexible and curved surfaces. Electronic and photonic circuits, devices, andsystems integrated on flexible, stretchable, and biocompatible curved substrates have numerous applications, including elec- tronic eyeball cameras (28), personal health monitors and bio- medical devices (29), deformable light-emitting displays (30), adaptive photonic systems (31), human body sensors (32), paper-like electronic displays (33), artificial skin sensors (34), and elec- tronic vivo brain monitoring devices (35). More recently, nano-scale stencils fabricated on flexible and stretchable substrate have beenusedasplasmonicnanoantennasatinfraredfrequencies(36).Because flexible and stretchable photonic structures can be wrappedoncurvedsurfacesandobjects,theyhaveadvantagesover devices integrated on traditional rigid wafer-based substrates. Forboth SPPs and spoofSPPs, much of theresearch todate hasconcentrated on flat surfac