Abstract
Planck’s law predicts the distribution of radiation energy, color and intensity, emitted from a hot object at thermal equilibrium. The Law also sets the upper limit of radiation intensity, the blackbody limit. Recent experiments reveal that micro-structured tungsten can exhibit significant deviation from the blackbody spectrum. However, whether thermal radiation with weak non-equilibrium pumping can exceed the blackbody limit in the far field remains un-answered experimentally. Here, we compare thermal radiation from a micro-cavity/tungsten photonic crystal (W-PC) and a blackbody, which are both measured from the same sample and also in-situ. We show that thermal radiation can exceed the blackbody limit by >8 times at λ = 1.7 μm resonant wavelength in the far-field. Our observation is consistent with a recent calculation by Wang and John performed for a 2D W-PC filament. This finding is attributed to non-equilibrium excitation of localized surface plasmon resonances coupled to nonlinear oscillators and the propagation of the electromagnetic waves through non-linear Bloch waves of the W-PC structure. This discovery could help create super-intense narrow band thermal light sources and even an infrared emitter with a laser-like input-output characteristic.
Highlights
A classical theory of light emission from a heated PC has been presented by Luo et al, where the emitting source is a classically described current[8]
In their formulation, the necessary conditions for super-Planckian radiation are (i) a metallic micro-structure that supports slow-light resonances at a frequency above its effective plasma cutoff frequency, (ii) a metal with sufficiently small optical absorption losses, and (iii) a sufficient density of nonlinear oscillators coupled to localized surface plasmons, that can be activated by external pumping and emit light at a slightly lower energy
The observed super-Planckian thermal radiation is attributed to combined effects of non-equilibrium optical pumping of nonlinear oscillators coupled to localized surface plasmons and the multi-dimensional feedback mechanism provided by the underlying hybrid Distributed Bragg Reflector (DBR) cavity/3D W-PC structure
Summary
−5, when the aperture is completely out of the CNT area and aligned only with the PC area, the peak intensity reaches its maximum value This maximum intensity is found to be 8.3 times greater than the blackbody reference taken at pos. The DBR cavity can produce a radiation profile that is more directional than the Lambertian pattern[14], contributes to about 60% enhancement in radiation intensity along the surface normal direction This means that radiative intensities up to 1.6 times the blackbody limit could possibly be accounted for as resulting from the altered emission profile. The temperature in the cavity/PC region would have to be 110K hotter than the blackbody region to account for the observed cavity/PC radiation enhancement This is not likely to occur as our sample’s surface temperature is uniform to within 5K. The hottest T of the system is the heating filament, which is ∼40K hotter than the sample surface
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