1. Laboratoire EM2C

2. Near-field radiative heat transfer : application to energy conversion Jean-Jacques Greffet Ecole Centrale Paris, CNRS.

3. Laboratoire EM2C Collaborators Rémi Carminati, O. Chapuis, K. Joulain, F. Marquier, J.P. Mulet, M. Laroche, S. Volz C. Henkel ( Potsdam) A. Shchegrov ( Rochester) Y. Chen, S. Collin, F.Pardo, J.L. Pelouard ( LPN, Marcoussis) Y. de Wilde, F. Formanek, P.A. Lemoine ( ESPCI)

4. Laboratoire EM2C Density of energy above a SiC surface at temperature T Temperature T z

5. Laboratoire EM2C T=300 K z Density of energy near a SiC-vacuum interface PRL, 85 p 1548 (2000)

6. Laboratoire EM2C T=300 K z Density of energy near a SiC-vacuum interface PRL, 85 p 1548 (2000)

7. Laboratoire EM2C T=300 K z Density of energy near a SiC-vacuum interface PRL, 85 p 1548 (2000)

8. Laboratoire EM2C T=300 K z Density of energy near a Glass-vacuum interface

9. Laboratoire EM2C What is the physical mechanism responsible for this huge enhancement ? The density of energy is the product of -the density of states, -the energy h -the Bose Einstein distribution. The density of states can diverge due to the presence of surface waves : Surface phonon-polaritons.

10. Laboratoire EM2C -++++++----

11. + +++++ - -- - - - -

12. Dispersion relation of a surface phonon-polariton It is seen that the number of modes diverges for a particular frequency. This happens only close to the surface. PRB, 55 p 10105 (1997)

13. Laboratoire EM2C Derivation of the thermal emission of a hot body i) A volume element below the interface contains currents due to the random thermal motion of charges. ii) Each volume element is equivalent to a dipolar antenna that emits radiation. iii) The mean field is null. PRL, 82 p 1660 (1999)

14. Laboratoire EM2C iv) Derivation of the intensity v) The only quantity needed is the correlation function of the random current. This is given by the fluctuation-dissipation theorem. PRL, 82 p 1660 (1999)

15. Laboratoire EM2C Advantages of the electromagnetic approach -It is valid in the near field - It yields the value of the emissivity - It yields physical insight in Kirchhoff law.

16. Laboratoire EM2C Direct proof of the coherence of thermal radiation in the near field. Application to the measurement of the EM LDOS

17. Laboratoire EM2C Direct experimental evidence of the spatial coherence of thermal radiation in near field de Wilde et al. to be published in Nature

18. Laboratoire EM2C Direct experimental evidence of the spatial coherence of thermal radiation in near field de Wilde et al. to be published in Nature

19. Laboratoire EM2C Fabrication of a coherent source of infrared radiation : Infrared antenna

20. Laboratoire EM2C The thermally emitted fields may be spatially coherent along the interface ! PRL 82, 1660 (1999) T=300 K z  M P

21. Laboratoire EM2C Fabricating an infrared antenna with a microstructured semiconductor. Thermal currents radiates surface waves A grating ruled on the surface scatters the surface wave. The scattered wavevector is related to the surface wave wavevector by the relationship : 

22. Laboratoire EM2C Image of the SiC grating taken with an atomic force microscope. Nature 416, p 61 (2002)

23. Laboratoire EM2C The emission pattern looks like an antenna emission pattern. The angular width is a signature of the spatial coherence. Emission pattern of a SiC grating Green line : theory (300K) Red line : measurement (800K). Nature 416, p 61 (2002)

24. Laboratoire EM2C Comparison between theory and measurements Nature 416, p 61 (2002)

25. Laboratoire EM2C Thermal emission by a tungsten grating Opt.Lett. 30 p 2623 (2005) Angular width : 14 mrad

26. Laboratoire EM2C Emission mediated by surface waves 1.Excitation of a surface wave. 2. Scattering by a grating.

27. Laboratoire EM2C Coherent thermal emission T Source : current thermal fluctuations Greffet et al., Nature (London) 416, 61 (2002), Marquier et al. PRB 69, 155412 (2004) Emission mediated by surface waves

28. Laboratoire EM2C The interface as an antenna (1) What is an antenna ? i) Increases the emitted power. ii) Modifies the emission pattern. How does it work ? Antenna = Intermediate resonator between the source and vacuum : i) More energy is extracted from the source because the LDOS is enhanced (Purcell effect) ii) The resonator is a secondary source.

29. Laboratoire EM2C The interface as an antenna (2) Example of antenna: a guitar Source : the string Resonator Optical analog : microcavity

30. Laboratoire EM2C The interface as an antenna (3) Source : current fluctuations T Resonator : the interface + the grating i)The output is increased because the LDOS is increased (Purcell effect) ii) The angular pattern of the antenna depends on the decay length of the SPP.

31. Laboratoire EM2C Electromagnetic heat transfer in the near field

32. Laboratoire EM2C Application to radiative heat transfer between two half-spaces Temperature T1 Temperature T2>T1. d  Poynting vector yields the radiative enregy flux.

33. Laboratoire EM2C Radiative heat transfer coefficient, T=300 K. d

34. Laboratoire EM2C Monochromatic radiative heat transfer coefficient, d=10 nm, T=300K. d Microscale Thermophysical Engineering 6, p 209 (2002)

35. Laboratoire EM2C AuGaN Kittel et al., PRL 95 p 224301 (2005) Experimental data

36. Laboratoire EM2C Implications of near-field heat transfer for thermophotovoltaics

37. Laboratoire EM2C thermal source T= 2000 K TPV cell T= 300 K d << rad thermal source T= 2000 K TPV cell T= 300 K PV cell T= 300 K Photovoltaics Thermophotovoltaics Near-field thermophotovoltaics T= 6000K

38. Laboratoire EM2C potential improvement on the output electric power and efficiency of near-field thermophotovoltaic devices : necessity of a quantitative model thermal source T= 2000 K TPV cell T= 300 K d << rad P R ( W.m -2 ) d (m)  400 enhanced radiative power transfer Why near field ?

39. Laboratoire EM2C Near-field I-V characteristic of a TPV cell z enhanced radiative power (Mulet 2002, Whale 2002, Chen 2003) modification of the electron-hole pairs lifetime (Baldasaro 2001) hot source T= 2000 K TPV cell T= 300 K d << rad

40. Laboratoire EM2C Near-field radiative power transfer  (rad.s -1 ) P R (W. m -2. Hz -1 ) d = 10  m W T= 2000 K GaSb cell T= 300 K d  (rad.s -1 ) P R (W. m -2. Hz -1 ) d = 30 nm (near field) (far field) 1.10 -10 3.5.10 -10 evanescent waves contribution in the near field enhancement by a factor 3

41. Laboratoire EM2C Near-field effects on the radiative power transfer d = 30 nm d = 10  m  (rad.s -1 ) P R (W. m -2. Hz -1 )  (rad.s -1 ) P R (W. m -2. Hz -1 ) Drude Metal T= 2000 K GaSb cell T= 300 K d (far field) (near field) 9.10 -12 6.10 -10 evanescent waves contribution in the near field enhancement by two orders of magnitude monochromaticity degraded by the presence of the TPV converter

42. Laboratoire EM2C Enhanced radiative transfer and photogeneration current in the near field d (m) P R ( W.m -2 ) tungsten source quasi-monochromatic source P R ( W.m -2 ) d (m) I ph ( A.m -2 )  50  40  400  1000

43. Laboratoire EM2C Near-field electron-hole pairs lifetime hot source d << rad vacuum GaSb z for both sources : near-field effect on the radiative recombination lifetime of electron-hole pairs negligible

44. Laboratoire EM2C Near-field output electric power output electric power enhanced by at least one order of magnitude tungsten sourcequasi-monochromatic source d (m)  50 far field :3.10 4 W/m 2 near field :15.10 5 W/m 2 P el (W. m -2 ) d (m) near field : 2.5.10 6 W/m 2 far field : 1.4.10 3 W/m 2  3000 P el (W. m -2 ) BB 2000 K

45. Laboratoire EM2C Near-field TPV converter efficiency  (%) d (m)  (%) near field : 27% far field : 21 % near field : 35% significant increase of the efficiency far field : 8 % tungsten source quasi-monochromatic source BB 2000 K

46. Laboratoire EM2C Summary ?

47. Laboratoire EM2C Heat transfer between two nanoparticles

48. Laboratoire EM2C Heat transfer between two nanoparticles PRL 94, 85901, (2005)

49. Laboratoire EM2C PRL 94, 85901, 2005

50. Laboratoire EM2C Radiative heat transfer between a small sphere and an interface d Appl.Phys.Lett, 78, 2931 (2001)

51. Laboratoire EM2C Power absorbed by a SiC sphere as a function of the distance. Diameter = 10 nm, SiC substrate. Appl.Phys.Lett, 78, 2931 (2001)

52. Laboratoire EM2C Emission mediated by surface plasmons QW luminescence A. Scherer Nature Materials 3, p 601 (2004)

53. Laboratoire EM2C Conclusions * The existence of surface modes of electromagnetic waves modifies drastically the emission. * Radiative heat transfer can be increased by four orders of magnitude between two plates. * Radiative heat transfer can be very local. * Radiative heat transfer is almost monochromatic at nanoscale. * Radiation emitted by a thermal source is temporally coherent (monochromatic) close to an interface that supports a surface wave. * Radiation emitted by a thermal source is spatially coherent (narrow emitted beams). * Highly directional infrared thermal antennas can be designed.

54. Laboratoire EM2C Dispersion relation of the surface-phonon polariton Nature 416, p 61 (2002)

55. Laboratoire EM2C Introduction  Measurement of the coherence length

56. Laboratoire EM2C Comparison of calculated and measured emissivity Calculation with optical data at 300 K Calculation with optical data at 800 K Phys.Rev.B (2004)

57. Laboratoire EM2C Application to local heating. The peak power deposited per unit volume is 100 MWm -3. A SiC sphere (a=5 nm) is located at a distance 100 nm above a SiC surface. Contours line are in log scale. The power decreases as R -6. R T=300 K Appl.Phys.Lett, 78, 2931 (2001)

58. Laboratoire EM2C Thermal emission by photonic crystals PRL 96, 123903 (2006)

59. Laboratoire EM2C 2D photonic crystal PRL 96, 123903 (2006)

60. Laboratoire EM2C Thermal emission assisted by surface waves Transmission Absorption by the crystal Absorption by the truncated crystal

61. Laboratoire EM2C (a)slab, (b) photonic crystal, (c) truncated PC, (d) amplitude of the surface wave

62. Laboratoire EM2C Design of an isotropic source