Cross-Talk, Activation, and Ultrashort Pulses for Electron Quantum Degeneracy.

Reaching high quantum degeneracy for free electrons has several technological hurdles. We discuss three such hurdles. (a) The use of two detectors can lead to cross-talk mimicking the antibunching signal. We report a study of this effect including experiment and model. (b) To increase the number of electrons emitted per laser pulse and thus push towards higher degeneracy, the electron source can be "activated" with Cesium. This may lead to the opening of new channels for electron emission using 400 and 800 nm laser pulses. The two-step excitation separates the spin control from the electron emission and may lead to a new way to control electron polarization [1]. This in turn allows for the separation of Pauli forces from Coulomb forces. (c) We are switching from the use of 100 fs to sub-10 fs lasers. This can potentially increase the electron density and thus the quantum degeneracy. Methods for measuring sub-10 fs pulses include non-linear detection of the laser light in an autocorrelator. However, the measurement is not direct and requires fitting. We compare this with a new method using two-photon absorption [2].

[1] William Newman et al., Subadditive femtosecond laser-induced electron emission from a GaAs tip. Submitted to Phys. Rev. B. August 2023.

[2] Rafeeq Syed et al., Rev. Sci. Instrum. 1 October 2023; 94 (10): 103006.