Nanolasers operate below the diffraction limit of visible light. Their cavities are smaller than the wavelengths they emit, which means conventional optical microscopy cannot resolve the mode structure inside them. You can measure the light that comes out, but you can't see what's happening inside. Santini, Ngo, Tizei, and colleagues used electrons instead of photons to see inside an operating nanolaser.
Free electrons passing through the near field of a lasing cavity exchange energy with the photons inside it. The energy exchange — gain or loss of quanta — encodes information about the local photon density with nanometer spatial resolution and sub-picosecond temporal resolution. By combining this electron near-field spectroscopy with conventional photon far-field measurements, the team mapped the mode structure of a gallium nitride nanowire laser while it was lasing.
The cavity contains up to 400,000 stimulated photons simultaneously. The mode map reveals that both whispering gallery modes — photons circling the wire's cross-section — and Fabry-Perot modes — photons bouncing between the wire's ends — participate in lasing. These two mode families have fundamentally different spatial distributions and different thresholds, and they coexist in the same cavity at the same time.
The spatial resolution of electron probing reveals how material defects, surface roughness, and structural irregularities modify the local mode density. A scratch or impurity that falls at a mode's antinode affects lasing differently than one at a node. The electron microscope doesn't just image the cavity — it counts the photons at each point inside it, turning a black box into a map.
To see inside a device smaller than light, use something smaller than the device.