A thermophotovoltaic (TPV) converter includes spectrally-selective metamaterial emitters disposed on peripheral walls of an all-metal box-like enclosure, and associated photovoltaic (PV) cells configured to efficiently convert in-band photons having optimal conversion spectrums into electricity. The peripheral walls surround a substantially rectangular interior cavity having an inlet opening through which heat energy (e.g., concentrated sunlight) is supplied, and an outlet opening through which waste heat exits the cavity. Concentrated sunlight passing through the box-like enclosure heats the peripheral walls to a high temperature (i.e., above 1000° K), causing thermally excited surface plasmons generated on the emitters' concentric circular ridges to produce highly-directional radiant energy beams having a peak emission wavelength roughly equal to a fixed grating period separating the ridges. The metamaterial emitter is optionally provided with multiple bull's eye structures in a multiplexed (overlapping) pattern and with different grating periods to produce a broad emission spectrum overlapping the optimal conversion spectrum.
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