Organ pipe radiant modes of periodic micromachined silicon surfaces

Abstract

In recent studies on small pyroelectric thermal anemometers with roughened surfaces we showed that one of the most widely used heat transfer models yielded calculated anemometer responses for flow and geometric behaviour that agreed functionally with observations, but were significantly smaller than the experimental data. As the first stage in investigating the role of small structures in heat transfer, we initiated a study of emittance from deep gratings. Here we report measurements at 400 °C of infrared (3 µm⩽λ⩽14 µm), normal, s- and p-polarized spectral emittances of 45 µm deep, near square-wave gratings of heavily phosphorus doped (110) silicon (P content ∼5 × 10 cm). The grating surface repeat scales, Λ, were 10, 14, 18 and 22µm, yielding a range of Λ/λ from 0.14 to 7.33. The s-polarization vector was parallel to the grating slots. Both s and p spectral emittances had pronounced resonant periodicities with a characteristic length of ∼42 µm. A reasonable explanation for this behaviour is the presence of standing waves in the air slots perpendicular to the silicon surface similar to those in an organ pipe. While the resonant amplitude of the s polarization does not depend significantly on Λ it does for the p polarization. No explanation for the Λ dependence of the p polarization is known.

Near-Field Effects in Spatial Coherence of Thermal Sources

“…Thus, such a material is an example of a natural thermal source for which the field correlation in the source plane does not obey the scaling law established by Wolf [15]. Note that temporal and spatial coherence in thermal emission by silicon and SiC gratings has already been observed [5,6]. The analysis presented in this Letter explains the physical origin of this phenomenon.…”

“…Nevertheless, recent interest in microscale and nanoscale radiative transfers [3], together with the development of local-probe thermal microscopy [4] and the observation of coherent thermal emission from doped silicon and silicon carbide (SiC) gratings [5,6], has raised new challenges. In fact, all these topics have in common the substantial role of the nonradiating (evanescent) thermal fields.…”

Spectral selectivity in infrared thermal detection

“…Hence, artificial materials with a structuredependent emissivity will also have a structure-dependent absorptivity. Hesketh et al 40 have demonstrated that the emissivity from deep surface gratings with a period comparable to the wavelength is different from the emissivity of a smooth surface. The emissivity was found to have a dependence on the grating period, which the authors attributed to standing wave modes generated in the deep gratings.…”

Microsecond switchable thermal antenna

“…In 1988, after a pioneering work of Hesketh et al 1 on the radiant modes of periodic micromachined silicon surfaces, the first thermal antenna has been designed using deep gratings. 2 Since then, numerous directional thermal sources and partially coherent sources have been proposed by texturing materials at subwavelength scale.…”