Conventionally, the theory of radiative energy transport is based on the concept of blackbody, which is known to emit the largest amount of radiative energy between objects. However, when two objects are separated within a sub-wavelength distance, the radiative energy transfer can exceed the blackbody limit by orders of magnitude owing to near-field coupling of the electromagnetic waves radiated by the objects. This near-field enhancement of radiative energy transport can be beneficially used for emerging nanoscale heat engines. In addition, a nanoscale gap can also make a significant effect on the charge transport phenomena by suppressing the space charge effect, creating image charges in the electrodes, concentrating electric fields, and allowing electron tunneling across the gap. The coupling of such near-field charge transport with near-field radiative energy transfer will be discussed along with our experimental efforts to demonstrate its viability for energy applications.
Electromagnetic (EM) behaviors of photonic crystals (PhCs) and nanoantenna (NA) arrays have been extensively studied and applied to a myriad of applications, including light absorption, surface-enhanced Raman scattering, light trapping in photovoltaics, and spectral narrowing of thermal emission. However, not many works have studied the integration of three-dimensional (3-D) PhCs and NA arrays into one structure mainly due to technical challenges in manufacturing 3-D PhCs. The present article reports the design analysis of a hybrid optical structure that has a gold rectangular NA array aligned on a 3-D silicon-on-nothing (SON) PhC substrate. By applying a continuous phase field model, we numerically simulate the formation of SON-PhC structures (i.e., a 3-D periodic array of spherical voids in silicon) during the high-temperature annealing process of a silicon substrate having vertical trenches. Photonic behaviors of the NA-on-SON PhC structure are computed using the finite-difference time-domain method. The obtained results exhibit the resonant absorption of midinfrared (mid-IR) light in the stopping bands of the SON-PhC (3.0 μm < λ < 7.5 μm) by photon coupling with the free electron oscillations in each NA structure. This PhC-mediated NA resonance is manifested by highly concentrated electric fields at NA corners; the corresponding local field enhancement factor is one order of magnitude greater than that of the NA array on a bare silicon substrate.
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