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Article Dans Une Revue Physical Review Letters Année : 2000

Near-Field Spectral Effects due to Electromagnetic Surface Excitations

Résumé

We demonstrate theoretically that the spectra of electromagnetic emission of surface systems can display remarkable differences in the near and the far zones. The spectral changes occur due to the loss of evanescent modes and are especially pronounced for systems which support surface waves. PACS numbers: 78.20. – e, 05.40. – a, 44.40. + a, 87.64.Xx Spectroscopy of electromagnetic radiation is perhaps the most powerful exploration tool employed in natural sciences: astronomy, atomic and molecular physics and chemistry, materials science, biology, etc. The central question considered in this paper—whether the spectral content of the radiation emitted by an object can change on propagation to the observer—usually does not arise, since it seems natural that nothing can happen to waves travel-ing through empty space. Surprisingly similar failure of common sense was put forward by the recent progress of near-field optical microscopy [1,2], which achieves sub-wavelength resolution exactly because evanescent modes carrying subwavelength spatial information do not propagate far away from the object. However, a great deal of work devoted to such an irreversible change of spatial information on propagation has been accompanied with significantly lesser interest in the possibility of the change of spectral information. The subject of spectral changes on propagation has, however, been addressed. In the 1980's, Wolf [3] predicted that the spectrum of light can be changed on propagation from the source to the observer, even through empty space. This effect, whose origin lies in the fluctuating nature of the source, has been intensively studied in a variety of systems [4,5]. Typically, the Wolf effect is manifested in small spectral shifts and can be viewed as a redistribution of the weights of different spectral components. In this Letter, we demonstrate spectral changes, whose physical origin is very different from that of Wolf spectral shifts and lies in the presence of the evanescent component in the emitted field. We show that the near-field and far-field spectra of emitted electric fields can display drastic differences. For a broadband emission, such as thermal emission which is considered in detail in this paper, the near-field spectrum dominated by evanescent modes can be entirely different from the far-field spectrum of propagating modes. These spectral changes occur not due to the statistical nature of the source (as in the Wolf effect) but due to the loss of evanescent components on propagation. We analyze how such spectral changes can be enhanced by electromagnetic surface waves (SW) near the interface. These SW are known to play an important role in the enhancement of interaction between nanoparticles near the surface [6], in localization effects on random surfaces [7], in surface-enhanced Raman scattering [8], in extraordinary transmission of light through subwavelength holes [9], etc. In this Letter, we show that SW provide the leading contribution to the density of energy in the near-field zone of electromagnetic emission. We now proceed with analyzing near-field effects in the spectra of thermal emission, which provides an easy way to excite both propagating and evanescent electromagnetic modes in a wide range of frequencies (at least, in the infrared [10–12]). Thermal emission is frequently associated with the textbook example of equilibrium black-body radiation. The Planck spectrum I BB v of such radiation is obtained by multiplying the thermal energy uv, T ¯ hvexp ¯ hvk B T 2 1 of a quantum oscil-lator by the density of oscillations (modes) per unit volume Nvdv v 2 dvp 2 c 3 in the frequency interval v, v 1 dv, and dividing the result by dv [13], I BB v uv, T Nv ¯ h p 2 c 3 v 3 exp ¯ hvk B T 2 1. (1) Here T is the body temperature, k B is Boltzmann's constants , ¯ h is Planck's constant divided by 2p, and c is the speed of light in vacuum. A well-known representation of blackbody radiation is the equilibrium radiation in a closed cavity with lossy walls when only propagating modes of the field are taken into account. To demonstrate the importance of near-field effects, we consider a somewhat more sophisticated example of thermal emission from a semi-infinite z , 0 slab of homogeneous , nonmagnetic material held in local thermodynamic equilibrium at a uniform temperature T , into the empty half-space z. 0. We will describe the macroscopic dielec-tric properties of the material by a frequency-dependent, complex dielectric function´v´0function´vfunction´v´function´v´0 v 1 i´00i´00 v. The 1548 0031-90070085(7)1548(4)$15.00
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Dates et versions

hal-01624761 , version 1 (26-10-2017)

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Andrei V Shchegrov, Karl Joulain, Rémi Carminati, Jean-Jacques Greffet. Near-Field Spectral Effects due to Electromagnetic Surface Excitations. Physical Review Letters, 2000, 85 (7), pp.1548 - 1551. ⟨10.1103/PhysRevLett.85.1548⟩. ⟨hal-01624761⟩
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