Two plates at temperatures Te (hot emitter, top) and Tc (cooler PV cell, bottom) exchange thermal radiation across a vacuum gap of width d, drawn here as a 2D cross-section you can pan (drag) and zoom (scroll). When d is large compared with the thermal wavelength, only propagating photon modes reach the cell, and the flux is capped by the Stefan-Boltzmann blackbody limit:
q_bb = σ (T_e⁴ − T_c⁴) σ = 5.67×10⁻⁸ W/m²K⁴
Once the gap shrinks to nanometre scale — below the thermal wavelength λ_T = b/T (Wien's law, b ≈ 2898 µm·K) — evanescent modes that would normally decay to nothing before reaching the other side can instead tunnel across the gap (frustrated total internal reflection / coupled surface-polariton resonances). This near-field photon tunneling lets the flux exceed q_bb by orders of magnitude:
d_c = λ_T / 2π (crossover gap)
F(d) = 1 + (d_c / d)² (near-field enhancement)
q(d) = F(d) · q_bb
This simulator uses that simplified but qualitatively correct d⁻² scaling law (the real Polder–van Hove theory integrates transmission over all evanescent wavevectors; published nanogap experiments on polar-dielectric pairs like SiC show 2–3 orders of magnitude enhancement at few-nanometre gaps, consistent with this trend). The PV cell then converts a share of the incoming flux to electricity, bounded by the thermodynamic Carnot ceiling and a bandgap-match quantum-efficiency factor ηQE:
η_Carnot = 1 − T_c / T_e
P_elec = η_QE · η_Carnot · q(d)
- Gap slider — log-mapped from 1 nm (near-contact) to 10 µm (far field); the glowing particle stream between the plates thickens and speeds up as tunneling turns on.
- Te / Tc — set the emitter and cell temperatures; the emitter plate's glow color follows a blackbody-style warm-to-white ramp.
- Bandgap match — how well the PV cell's bandgap is tuned to the emitted spectrum at this Te, scaling the fraction of near-field heat actually converted to electricity.
- Drag / scroll — pan and zoom the gap cross-section; the log-log curve in the corner plots flux enhancement vs. gap distance with a marker at the current operating point.
Real-world relevance: near-field thermophotovoltaics is an active nanotechnology research area — MEMS-actuated nanogap emitters and micro-machined vacuum chip stacks aim to harvest waste heat at power densities unreachable by any far-field device, because it is only the sub-wavelength gap that unlocks the evanescent channel.