Near-field radiative heat transfer allows heat to propagate across a small vacuum gap in quantities that are several orders of magnitude greater then the heat transfer by far-field, blackbody radiation. Although heat transfer via near-field effects has been discussed for many years, experimental verification of this theory has been very limited. We have measured the heat transfer between two macroscopic sapphire plates, finding an increase in agreement with expectations from theory. These experiments, conducted near 300 K, have measured the heat transfer as a function of separation over mm to µm and as a function of temperature differences between 2.5 and 30 K. The experiments demonstrate that evanescence can be put to work to transfer heat from an object without actually touching it.PACS numbers: 44.40.+a,78.20.Ci Humans knew of radiative heat transfer at least as early as the discovery of fire, and physicists have investigated this process for centuries, culminating in the blackbody theory of Planck and the birth of the quantum theory. Planck's equation for black-body radiation contains only the temperature and some fundamental constants. When actual materials are involved, their emissivities enter the discussion, but little else. For example, the heat transfer per unit area between two semi-infinite planes is set by their temperatures and integrated emissivities but does not depend on their separation or other geometrical quantities. When the two planes approach each other closely the situation changes. In this near-field regime, each material interacts with exponentially decaying evanescent electromagnetic fields generated in and existing outside the other material; these fields can drive currents and generate heat. [1][2][3] This near-field radiative heat transfer can be several orders of magnitude greater than far-field blackbody radiation.Much like the Casimir and van der Waals force, nearfield heat transfer deals with fluctuations that only exist over small distances. The first in-depth theory for nearfield heat transfer between planar surfaces was derived by Polder and Van Hove,[2] building on the work of Rytov [1]. There have been several other theoretical approaches, and in general the theory seems complete, except perhaps at distances comparable to atomic dimensions. [4] Although heat transfer via near-field effects has been discussed for many years, experimental verification of the theory for heat transfer between two planar surfaces has been limited. Hargreaves[5] has presented room temperature observations for two Cr surfaces at distances as small as 1 µm. Domoto et al. [6] reported results at cryogenic temperatures but for relatively large (50 µm) separations, where near-field effects were barely observable. Neither study compared experiment to theory. A comparison at a fixed spacing has been put forward, but the plates were separated by polyethylene spacers, so the distance could not be varied. [7] There have also been several recent results using a sphere-plane geometry. [4,[7][8][9] There ...
The next generation of interferometric gravitational wave detectors will employ laser powers approaching 200 W to increase shot-noise limited sensitivity. Optical components that transmit the laser light will exhibit increased thermal lensing induced by bulk absorption and concomitant changes in the material refractive index, resulting in significant changes in the modal characteristics of the beam. Key interferometer components such as electro-optic modulators and Faraday isolators are particularly at risk, since they possess relatively large absorption coefficients. We present a method for passive correction of thermally induced optical path length () changes induced by absorption in transmissive optical components. Our method relies on introducing material in the optical path that possesses a negative index temperature derivative, thereby inducing a compensating opposite. We experimentally demonstrate a factor of 10 reduction in higher order spatial mode generation for terbium gallium garnet, a Faraday isolator material.
Context. A comprehensive framework for comparing spectral data from different planets has yet to be established. This framework is needed for the study of extrasolar planets and objects within the solar system. Aims. We completed observations to compile a library of planet spectra for all planets, some moons, and some dwarf planets in the solar system to study their general spectroscopic and photometric natures. Methods. During May and November of 2008, we acquired spectra for the planets using TRISPEC, which is capable of simultaneous three-band spectroscopy across a wide wavelength range of 0.45−2.5 μm with low resolving power (λ/Δλ ∼ 140−360). Results. Patterns emerge when comparing the spectra. By analyzing their general spectroscopic and photometric natures, we show that it is possible to distinguish between gas planets, soil planets, and ice planets. These methods can be applied to extrasolar observations acquired using low resolution spectrography or broad-band filters.Conclusions. The present planet spectral library is the first library to contain observational spectra for all of the solar system planets, based on simultaneous observations at visible and near infrared wavelengths. This library will be a useful reference for analyzing extrasolar planet spectra and calibrating planetary data sets.
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