Learn by driving

From a green photon to a designed nanoantenna: why metal nanoparticles have a color, what that color can do, and how to engineer it.

1. 1

   The energy scale of visible light

   Plasmons in gold nanoparticles live in the visible. Start by fixing the scale: green light at 532 nm is a ~2.3 eV photon oscillating at ~560 THz.

   Look for: Note where 532 nm sits between the UV and NIR bands — everything in this lesson happens within ±1 eV of it.

   Open Photon Calculator →
2. 2

   The plasmon resonance of a gold sphere

   A gold sphere of 50 nm radius in water scatters and absorbs light resonantly: the conduction electrons oscillate collectively, driven by the field. Mie theory gives the exact cross-sections.

   Look for: The extinction peak near ~570 nm — red-shifted from the small-particle ~530 nm by retardation — and that σ can exceed the geometric cross-section (here ~7×). Switch σ_sca ↔ σ_abs: at this size absorption still competes with scattering.

   Open Mie Scattering →
3. 3

   Resonant absorption becomes heat

   Whatever the particle absorbs, it dissipates. Under a focused CW laser at the resonance, a gold nanoparticle is a nanoscale heater — the basis of photothermal therapy.

   Look for: ΔT at 532 nm, then move the laser to 800 nm: off resonance the same particle barely warms. Resonance is the switch.

   Open Photothermal Calculator →
4. 4

   The particle as an antenna for emitters

   Put a quantum emitter next to the same sphere: its decay rate is enhanced (Purcell effect), but part of the emission is absorbed by the metal (quenching). The gap distance decides which wins.

   Look for: Slide the gap d from 50 nm down to 2 nm: F_tot keeps rising while q′ collapses. There is an optimal gap — q′ peaks near d ≈ 30 nm before quenching takes over — and that trade-off is the core of nanoantenna design.

   Open Purcell Factor →
5. 5

   Design: compare materials on the workbench

   Silver is less lossy than gold — for small spheres its resonance is sharper and strongly blue-shifted. Wire the comparison yourself on the node workbench: two materials feeding two Mie solvers, side by side.

   Look for: Load the “Au vs Ag” example. Ag peaks far to the blue — but at this 60 nm radius its spectrum splits into two overlapping resonances and reads broader. Shrink the radius to ~20 nm to see the textbook sharp Ag line; then swap one material for the Drude model and see an idealized metal.

   Open Material Workbench →