A spaser (surface plasmon amplification by stimulated emission of radiation) swaps the mirrored cavity of an ordinary laser for a metal nanoparticle. Optically or electrically pumped dye or quantum-dot molecules in the surrounding gain shell transfer their excitation into the particle's surface plasmon — the collective sloshing of its free electrons — instead of into a photon. Below threshold, excited molecules decay randomly and the plasmon field flickers incoherently. Above threshold, stimulated transfer dominates: every new excitation joins the same oscillation phase, the plasmon field locks into one coherent mode, and the emitted light output turns sharply upward while its spectral linewidth collapses — exactly the L-L kink and line-narrowing that define lasing, but confined to a cavity only tens of nanometers across.
Nₚₗ(t+1) = Nₚₗ + [gain·Npump − loss]·Nₚₗ·dt + spontaneous
threshold when gain·Npump = loss → L-L kink
Δν ∝ 1/Nₚₗ (linewidth narrows as coherent population grows)
- Pump power — drives excited-state population in the gain shell; cross the threshold (~100%) and stimulated plasmon emission overtakes spontaneous decay.
- Metal core radius — sets the plasmon cavity size and its resonance; this is also the whole "laser cavity" — no mirrors, no wavelength-scale resonator.
- Gain shell thickness — more gain medium means more excited molecules available to feed the plasmon, lowering the pump power needed to reach threshold.
- Diffraction limit sphere — an ordinary focused laser spot can't be made smaller than roughly half its wavelength (~200-300 nm for visible light); toggle it to see the spaser cavity sitting far inside that limit.
Real-world relevance: spasers are being explored as ultra-compact coherent light sources for on-chip optical interconnects, single-molecule sensing and super-resolution nanoscopy, anywhere a light source has to fit inside a space no ordinary laser cavity ever could.