A new multilayer mirror has enabled the generation of light pulses suitable for studying extremely fast charge dynamics in semiconductors. Electrons move around in matter on incredibly short timescales of hundreds of attoseconds (1 as = 10¯¹⁸ s). The way electrons redistribute when two molecules approach each other, or when a molecule interacts with light, lies at the heart of chemistry and biology. Photonic and electronic technologies rely on processes such as photoexcitation and electron transport within semiconductor materials. Electron dynamics can be studied in their native attosecond timescale provided a short enough probe - typically an attosecond light pulse at extreme ultraviolet photon energies - is available.

Heisenberg’s uncertainty principle tells us that certain pairs of observables cannot be measured with arbitrary precision. The better the accuracy with which a particle’s position is known, for example, the greater the uncertainty in its momentum. In optics, the minimum possible duration of a light pulse is linked in a similar way to the spread of photon energies. To measure the fastest electron dynamics in simple atoms and molecules, the highest possible time resolution is often needed, meaning the photons should cover as broad a range of energies as possible.

But electronic states in solids can be closely spaced in energy. This means that disentangling contributions from different states sometimes requires a balance to be struck between time resolution and energy resolution. In this Optics Letters article, Guggenmos et al. have developed a narrowband multilayer mirror to select long attosecond pulses with high spectral resolution, and have demonstrated the suitability of the pulses for performing attosecond measurements on semiconductors. Measurements using the new pulses could aid the development of next generation electronic devices by providing new insights into the fundamental behaviour of electrons in semiconductors.

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