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MeV electron pulses induce sub-10 ps optical changes in semiconductors via bandgap modulation

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First Direct Observation of Strong Ultrafast Nonlinear Optical Modulations Induced by MeV Electron Ionization in Bulk Semiconductors at Room Temperature

Key Findings

  • Researchers utilized 4.2 MeV electron pulses (150 fs duration) from a radio-frequency photocathode gun at SLAC National Accelerator Laboratory to excite bulk II-VI semiconductors (CdS, CdSe, ZnO, ZnSe, ZnTe).

  • Visible laser pulses (1.65–2.83 eV, ~75 fs) probed optical changes using a common-path interferometric setup with crossed polarizers and birefringent BBO crystals, achieving sub-10 ps temporal resolution.

  • A blueshift of the absorption edge by approximately 52 meV was observed in CdSe, attributed to the Burstein–Moss effect. Estimated carrier densities reached ~10¹⁸ cm⁻³, roughly 100 times higher than predictions based solely on deposited energy.

  • Time-resolved measurements revealed rising and falling edges in optical signal corresponding to carrier arrival and recombination, with arrival-time variances below 5 ps for most samples and below 1 ps for CdSe and ZnTe.

  • The ionization cascade time remained under 10 ps across all semiconductors, matching Monte Carlo simulations within 3 ps.

  • Modulation amplitude depended on electron bunch charge and sample thickness, due to geometric effects of ionization-track spreading and probe-volume overlap.

  • CdSe exhibited induced transparency (bandgap increase), while ZnTe showed induced opacity (excitonic effects).

Significance

This is the first direct observation of strong ultrafast nonlinear optical modulations induced by MeV electron ionization in bulk semiconductors at room temperature.

The findings demonstrate sub-10 ps changes in transmission linked to bandgap widening from high-density localized carriers, providing a foundation for compact, room-temperature, laser-based radiation sensors with enhanced temporal and spatial resolution.

Journal Reference

Jeong D., Hopper T.R., et al. (2026). Strong ultrafast nonlinear optical response from megaelectronvolt electrons in semiconductors. Nature Photonics. DOI: 10.1038/s41566-026-01894-3.