  Recent experimental studies on near-field thermophotovoltaic (TPV) energy
conversion have mainly focused on enhancing performance via photon tunneling of
evanescent waves. In the sub-micron gap, however, there exist peculiar
phenomena caused by the interference of propagating waves, which is seldom
observed due to the dramatic increase of the radiation by evanescent waves in
full spectrum range. Here, we experimentally demonstrate the oscillatory nature
of near-field TPV energy conversion in the far-to-near-field transition regime
(250-2600 nm), where evanescent and propagating modes are comparable due to the
selective spectral response by the PV cell. Noticeably, it was possible to
produce the same amount of photocurrent at different vacuum gaps of 870 and 322
nm, which is 10\% larger than the far-field value. Considering the great
challenges in maintaining nanoscale vacuum gap in practical devices, this study
suggests an alternative approach to the design of a TPV system that will
outperform conventional far-field counterparts.
