Two spots in a crystal emit opposite-handed light at different times in a laser pulse.

Tracking electrons in tiny energy pockets called valleys is difficult to do with light alone. Researchers studied this in gapped graphene using laser simulations and an electron-hole recombination model. When hit with an elliptically polarizedelliptically polarizedLight whose electric field traces an ellipse, having both linear and circular components. laser, the material creates high-energy light flashes called harmonics. The two different valleys emit light that twists in opposite directions, known as opposite helicities. Crucially, these emissions happen mainly during different half cycles of the laser pulse. The team also designed a method to measure valley imbalance. A first light pulse creates the imbalance, and a second pulse produces light with unequal twist intensities via Pauli blockingPauli blockingA quantum effect where electrons cannot occupy the same state, affecting how light is absorbed or emitted.. The ratio of these changes directly reveals the valley population.

Why it matters

This could let scientists see how electrons are arranged inside new materials without touching them, using only light. It matters for developing faster electronics or quantum devices where controlling electron behavior is key.

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