Transverse thermophotovoltaics from nonreciprocal plasmon drag in metal

Transverse thermophotovoltaics has been conceptually proposed as a paradigm distinct from conventional junction-based photovoltaics, but has so far lacked a theoretical foundation. In this Letter, we establish a microscopic formalism of this effect in which a transverse electric current emerges in a two-dimensional metal sheet via nonreciprocal surface plasmon polaritons driven by near-field thermal radiation. This theoretical formalism incorporates the electron-photon interaction by integrating electronic transition factor governed by energy-momentum conservation, the photon flux factor encoding the nonreciprocal surface modes, and their directional coupling. Our approach quantitatively confirms the plasmon-drag mechanism and reveals the role of impurity scattering. This work provides a rigorous theoretical foundation for transverse thermophotovoltaic devices and opens avenues for active nanoscale thermal energy conversion.