Experimental observations of near-field radiative energy transfer

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Due to difficulties in maintaining the nanoscale gap distance between the emitter and the receiver, experimental investigations of near-field thermal radiation have been rather limited. Cravalho et al. [1], Domoto et al. [2], and Hargreaves [3,4] were among the first to measure the radiative flux of two parallel plates at cryogenic temperatures. Domoto et al. [2] measured the radiative heat transfer at cryogenic temperatures between two copper plates at gaps from 1 to 10 μm. While the near-field heat transfer was 2.3 times greater than that of far-field, the measured heat flux was only 3% of the energy transfer between blackbodies. Hargreaves [3,4] measured the near-field heat transfer between two chromium plates separated by vacuum gaps from 6 to 1.5 μm. At 1.5 μm vacuum gap, the near-field heat transfer at room temperature was five times greater than that in the far field. However, the measured heat flux was still only 40% of that between two blackbodies. In 1994, Xu et al. [5] tried to measure near-field radiative heat transfer through a sub-micrometer vacuum gap by using an indium needle of 100 μm in diameter and a thin-film thermocouple on a glass substrate, but could not observe a substantial increase of radiative heat transfer. On the other hand, Muller-Hirsch et al. [6] found that near-field radiation plays an important role in heat transfer between a STM thermocouple probe and a substrate. However, due to the limit of their experimental setup, they were not able to determine the absolute value of near-field thermal radiation. This limit was overcome in their following work [7] by successfully calibrating the STM thermocouple probe, demonstrating the d-3 dependence in the near-field thermal radiation from the surface to the thermocouple tip of R = 60 nm when the gap is larger than 10 nm. However, for gaps less than 10nm, the measured heat flux saturates and differs from the divergent behavior of the predicted results. The authors attributed this difference to the spatial dependence of the dielectric function of materials.

Continuous efforts have been made to experimentally demonstrate the near-field enhancement of the energy transfer for other relatively simple geometries, such as parallel plates separated by micro-particle spacers [8] and microsphere-plate geometry [9-11]. They successfully measured the thermal conductance of near-field radiation for a gap distance as small as 30 nm by using a vertically aligned bimetallic AFM cantilever having a silica microsphere at the free end. The plate was heated to produce a temperature difference ΔT between the sphere and the plate, typically on the order of 10–20 K, leading to the near-field radiative heat flux of the order of nanowatts. In order to measure such small heat flux, the measurement was conducted in a vacuum condition (~ 10-6 mbar). The near-field thermal radiation was measured by monitoring the deflection of the bimetallic cantilever, which has a minimum measurable temperature of 10-4 – 10-5 K and a minimum detectable power of 5×10-10 W [9]. Comparison of their measurement with the Derjaguin approximation confirms that the near-field thermal radiation between the microsphere and flat surface is more than two orders of magnitude larger than that of blackbody radiation and has the d-1 dependence. However, they could not measure near-field thermal radiation below 30 nm gap distances due to the surface roughness of the sphere.

Highly enhanced near-field thermal radiation between a tip and substrate has been used to develop novel scanning probe microscopies. De Wilde et al. [12] has developed the thermal radiation scanning tunneling microscopy (TRSTM), which is the infrared, apertureless near-field scanning optical microscopy (NSOM) [13] that operates without any external illumination. When a gold-coated tip scans over a heated SiC samples with gold patterns, thermally excited surface waves in the infrared, i.e., surface plsamons on gold and surface phonon polaritons on SiC, are near-field interacted and scattered by the tip. By measuring the scattered thermal emission with a HgCdTe detector, they could achieve the near-field image of the sample along with the AFM topographic image. In fact, TRSTM can the electromagnetic local density of states at a frequency that can be defined by a suitable filter: this is analogous to the scanning tunneling microscopy that probes the electronic local density of states [14].


References

[1] Cravalho, E. G., Tien, C. L., and Caren, R. P., 1967, “Effect of Small Spacings on Radiative Transfer between Two Dielectrics,” Journal of Heat Transfer, 89, pp. 351–358.

[2] Domoto, G. A., Boehm, R. F., and Tien, C. L., 1970, “Experimental Investigation of Radiative Transfer between Metallic Surfaces at Cryogenic Temperatures,” Journal of Heat Transfer, 92, pp. 412–417.

[3] Hargreaves, C. M., 1969, “Anomalous Radiative Transfer Between Closely-Spaced Bodes,” Physics Letters A, 30, pp. 491–492.

[4] Hargreaves, C. M., 1973, “Radiative Transfer Between Closely Spaced Bodies,” Philips Research Report, 5, pp. 1–80.

[5] Xu, J. B., Lauger, K., Moller, R., Dransfeld, K., and Wilson, I. H., 1994, “Heat Transfer between Two Metallic Surfaces at Small Distances,” Journal of Applied Physics, 76, pp. 7209–7216.

[6] Muller-Hirsch, W., Kraft, A., Hirsch, M. T., Parisi, J., and Kittel, A., 1999, “Heat Transfer in Ultrahigh Vacuum Scanning Thermal Microscopy,” Journal of Vacuum Science and Technology A, 17, pp. 1205–1210.

[7] Kittel, A., Muller-Hirsch, W., Parisi, J., Biehs, S. A., Reddig, D., and Holthaus, M., 2005, "Near-Field Heat Transfer in a Scanning Thermal Microscope," Physics Review Letters, 95, p. 224301.

[8] Hu, L., Narayanaswamy, A., Chen, X., and Chen, G., 2008, “Near-Field Thermal Radiation between Two Closely Spaced Glass Plates Exceeding Planck's Blackbody Radiation Law,” Applied Physics Letters, 92, p. 133106.

[9] Narayanaswamy, A., Shen, S., and Chen, G., 2008, “Near-Field Radiative Heat Transfer between a Sphere and a Substrate,” Physical Review B, 78, p. 115303.

[10] Shen, S., Narayanaswamy, A., and Chen, G., 2009, “Surface Phonon Polaritons Mediated Energy Transfer between Nanoscale Gaps,” Nano Letters, 9, pp. 2909–2913.

[11] Rousseau, E., Siria, A., Jourdan, G., Volz, S., Comin, F., Chevrier, J., and Greffet, J.-J., 2009, “Radiative Heat Transfer at the Nanoscale,” Nature Photonics, 3, pp. 514–517.

[12] De Wilde, Y., Formanek, F., Carminati, R., Gralak, B., Lemoine, P. A., Joulain, K., Mulet, J.-P., Chen, Y., and Greffet, J.-J., 2006, "Thermal Radiation Scanning Tunnelling Microscopy," Nature, 444, pp. 740-743.

[13] Hillenbrand, R., Taubner, T., and Keilmann, F., 2002, “Phonon-Enhanced Light-Matter Interaction at the Nanometer scale,” Nature, 418, pp. 159–162.

[14] Tersoff, J., and Hamann, D. R., “Theory of the Scanning Tunneling Microscope,” Physical Review B, 31, pp. 805–813.