Enhanced Conversion Process in a Sub-wavelength Thin Upconversion Layer by Using Metamaterial Mirror

Because of finite bandgap, useful spectral ranges of semiconductors are limited for photovoltaic applications. The upconversion (UC) process is one of alternatives to extend the useful spectral range by converting long wavelength photons to short wavelength photons. For example, NaYF4 codoped Yb3+/Er3+ converts photons from near-infrared (970 nm) to visible (660 nm). However, in the form of a thin layer on a flat metal mirror, which is a representative structure to add UC layer into photovoltaic devices, the UC material exhibits low efficiency in optical devices because of its low absorption and conversion efficiencies. In this paper, the UC process is highly enhanced by a metamaterial mirror consisting of grooved silver surfaces. The absorption of UC material is improved by a factor of 5.3 due to increased light-matter interaction because the electric field is highly concentrated in the UC layer near the metal surface. The surface plasmon polariton (SPP) modes induced by a dipole emitter to a metal grooved surface enhance the spontaneous emission (SE) rate of the emission wavelength of UC material by a factor of 65 compared with a thin UC layer on flat mirror. Consequently, total efficiency of the UC process is 400 times more enhanced than the reference structure by magnetic mirror property and SPP modes of metamaterial mirror.