The thermoelectric figure of merit ZT measures how efficiently a material converts heat differences into electricity. A ZT of 1 is the practical threshold for useful thermoelectric applications — heat pumps, waste heat recovery, solid-state refrigeration. Most materials fall well below this. The fundamental problem is that the three quantities that determine ZT — electrical conductivity, Seebeck coefficient, and thermal conductivity — are coupled through the electronic band structure in ways that make improving one while maintaining the others extremely difficult. Increasing electrical conductivity typically increases electronic thermal conductivity by the same factor, canceling the benefit. Thermoelectric optimization has been, for decades, a game of incremental improvements against coupled constraints.
Osuala, Choudhary, Biswas, Ganguly, and Maiti (arXiv 2602.22789, February 2026) show that periodic light irradiation can push monolayer WSe2 nanoribbons past ZT = 1 over a broad temperature range, by decoupling the constraints that equilibrium materials cannot escape.
The mechanism works through the Floquet engineering of the electronic band structure. A periodic laser field, treated through the Floquet-Bloch formalism, renormalizes the electronic hopping parameters — the matrix elements that determine how electrons move between neighboring atoms. The renormalization reshapes the transmission spectrum near the Fermi level, modifying the Landauer transport integrals that separately determine the electrical conductance, the Seebeck coefficient, and the electronic thermal conductance. The key insight is that the light field provides a tuning knob that modifies these three quantities with different sensitivities. By choosing the right laser frequency and intensity, the electrical conductivity and Seebeck coefficient can be enhanced while the electronic thermal conductivity grows more slowly.
Simultaneously, the light field enhances anharmonic phonon scattering — it increases the rate at which lattice vibrations dissipate, reducing the lattice thermal conductivity. In equilibrium, electrical and thermal transport are linked through the Wiedemann-Franz law: good electrical conductors are good thermal conductors. The Wiedemann-Franz law applies to the electronic contribution to thermal conductivity. The lattice contribution is separate, and suppressing it through enhanced anharmonic scattering improves ZT without affecting the electrical properties.
The combined effect — Floquet band engineering of the numerator (power factor) and phonon scattering reduction of the denominator (thermal conductivity) — pushes ZT past unity across a broad temperature window. The material is not intrinsically a good thermoelectric. The light makes it one. Remove the laser, and ZT drops back below the useful threshold. The efficiency is a property of the driven system, not the equilibrium material.
The approach requires continuous optical pumping, which costs energy. The thermoelectric device produces power from a heat gradient; the laser consumes power to maintain the Floquet state. Whether the net energy balance favors the light-driven configuration depends on the specific application — waste heat recovery from a source that provides far more thermal energy than the laser consumes could still be net-positive. The physics is clear. The engineering remains to be evaluated. But the principle — using light to break the equilibrium constraints that limit thermoelectric performance — opens a design space that equilibrium materials science cannot access.