MeV electron pulses induce sub-10 picosecond optical changes in semiconductors through bandgap modulation. This enables high-resolution radiation detection and advances ultrafast optoelectronic sensing technologies.
Study: Strong ultrafast nonlinear optical response from megaelectronvolt electrons in semiconductors. Image Credit: FOTOGRIN/Shutterstock
In a recent article published in the journal Nature Photonics, researchers explored the ultrafast optical response of semiconductors upon excitation with megaelectronvolt (MeV) electron pulses.
Ultrafast Radiation–Matter Interaction Challenges and Opportunities
Traditional scintillators convert ionizing radiation to visible photons, but the scintillation process inherently restricts timing resolution. Prior attempts to achieve sub-picosecond timing have explored alternative mechanisms such as Cherenkov radiation and Purcell enhancement, yet these methods often face challenges in yield or complexity.
Advanced synchronized radiation sources, such as ultrafast X-ray lasers, have revealed femtosecond-scale optical modulations associated with electron–hole plasma formation but require extremely high photon fluxes and produce homogeneous carrier distributions. MeV electron pulses, by contrast, generate sparse, track-like ionization pathways with longer trajectories and lower particle counts, resulting in more localized and heterogeneous carrier distributions. Until now, it remained unclear whether optical nonlinearities, particularly bandgap-related refractive index changes, could be observed on ultrafast timescales under MeV electron excitation.
Tailoring II–IV Semiconductor Crystals and Interferometric Detection Setup
The experiment utilized ultrafast, 4.2 MeV electron pulses generated by a radio-frequency photocathode gun at the SLAC National Accelerator Laboratory, with pulse durations around 150 femtoseconds. These electron pulses acted as the pump source, exciting bulk semiconductor samples from the II–VI family, including CdS, CdSe, ZnO, ZnSe, and ZnTe, chosen for their sizeable bandgaps compatible with visible-light probing.
To probe the ultrafast optical changes induced, the researchers employed visible laser pulses with tunable photon energies between 1.65 and 2.83 eV (480–750 nm) and durations of approximately 75 fs. The temporal delay between the electron pump and optical probe was precisely controlled in 0.2-ps increments using a mechanical delay stage, achieving sub-10-ps temporal resolution of the induced optical response.
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A key innovation was the use of a common-path interferometric setup with crossed polarizers and birefringent BBO crystals. This configuration allowed the generation of an optical probe composed of two pulses delayed by 3.3 ps, producing distinct rising and falling edges in the detected signal. This pulse shape provided enhanced timing accuracy by minimizing variance in arrival-time measurements. The probe transmission was measured in a transmission geometry to resolve changes in complex refractive index, exploiting bandgap modulation effects.
Revealing Sub-10-Picosecond Bandgap Modulations Driven by Localized Carrier Cascades
A key observation linking optics to carrier dynamics was a clear blueshift of the absorption edge by approximately 52 meV in CdSe samples, consistent with the Burstein–Moss effect arising from high carrier densities filling conduction-band states. This bandgap widening modulated the refractive index rapidly, resulting in visible changes in transparency on sub-10-ps timescales. The estimated carrier densities reached ~1018 cm-3, approximately 100 times higher than those predicted purely by deposited energy, underscoring the extreme spatial localization of carriers at ionization sites along the electron tracks.
Time-resolved measurements revealed distinct rising and falling edges in the optical signal, associated with the arrival and recombination of charge carriers, respectively. The variances in arrival times were below 5 ps for most samples and even below 1 ps for some (CdSe, ZnTe), indicating excellent temporal resolution suitable for sub-10-ps radiation detection.
The ionization cascade time, defined as the duration of energy deposition through ionizing collisions, consistently remained under 10 ps across all semiconductor types. These experimental timings closely matched Monte Carlo simulation results, with differences under 3 ps, confirming the predictive power of modeling on carrier dynamics.
Exploring the modulation amplitude dependence on electron bunch charge and sample thickness revealed non-linearities arising from geometric effects of ionization-track spreading and probe-volume overlap. Increasing electron charge led to a larger effective beam spot due to space-charge effects, causing many ionization tracks to extend beyond the probe laser region and reducing effective modulation strength per electron. Similarly, thicker samples allowed ionization tracks to diverge more, again diminishing the number of carriers interacting with the probe volume.
The study distinguishes two forms of bandgap modification across different materials: CdSe exhibited induced transparency as band filling increased the bandgap, whereas ZnTe showed induced opacity, consistent with excitonic effects and complex band-structure interactions. This material-dependent optical behavior points towards versatile nonlinear responses achievable by choosing semiconductor materials tailored to specific detection requirements.
Implications for Precise Spatiotemporal Ionizing Radiation Detection
This study provides the first direct observation of strong ultrafast nonlinear optical modulations induced by MeV electron ionization in bulk semiconductors at room temperature. By using synchronized ultrafast optical probes near the bandgap, the team revealed sub-10-ps changes in transmission linked to bandgap widening due to high-density localized carrier populations.
The insights into carrier localization along ionization tracks and the interplay with nonlinear optical properties provide a foundation for engineering compact, room-temperature, laser-based radiation sensors with enhanced temporal and spatial resolution. Future work could extend these principles to lower-energy radiation types and thin-film semiconductor platforms to fully exploit ultrafast bandgap modulation phenomena in practical detection technologies.
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, https://www.nature.com/articles/s41566-026-01894-3