Abstract
We present the theoretical basis for a new photoemission regime, transient work function gating (TWFG), which temporally and energetically gates photoemission and produces near-threshold photoelectrons with thermally limited emittance, percent-level quantum efficiency, and control over temporal coherence. The technique consists of actively gating the work function of a generalized photocathode using a non-ionizing long-wavelength optical field to produce an adiabatic modulation of the carrier density at their surface. We examine TWFG as a means to circumvent the long-standing trade-off between low emittance and high quantum efficiency, untethered to particle source or photocathode specifics. TWFG promises new opportunities in photoemission physics for next generation electron and accelerator-based x-ray photon sources.
Highlights
Today’s electron sources underpin transformational scientific instrumentation, most notably colliders1 and ultrafast electron2 and x-ray free electron lasers (XFELs).3 The latter have enabled a myriad of scientific discoveries in physical chemistry4–6 and biology7–9 with insurmountable societal impact
We present the theoretical basis for a new photoemission regime, transient work function gating (TWFG), which temporally and energetically gates photoemission and produces near-threshold photoelectrons with thermally limited emittance, percent-level quantum efficiency, and control over temporal coherence
The technique consists of actively gating the work function of a generalized photocathode using a non-ionizing long-wavelength optical field to produce an adiabatic modulation of the carrier density at their surface
Summary
Today’s electron sources underpin transformational scientific instrumentation, most notably colliders and ultrafast electron and x-ray free electron lasers (XFELs). The latter have enabled a myriad of scientific discoveries in physical chemistry and biology with insurmountable societal impact. Today’s electron sources underpin transformational scientific instrumentation, most notably colliders and ultrafast electron and x-ray free electron lasers (XFELs).3 The latter have enabled a myriad of scientific discoveries in physical chemistry and biology with insurmountable societal impact. There are two established strategies to increase the photoinjector brightness: increasing the gun accelerating gradient to increase the extractable charge density and reducing the transverse electron energy spread.. There are two established strategies to increase the photoinjector brightness: increasing the gun accelerating gradient to increase the extractable charge density and reducing the transverse electron energy spread.11,12 On the latter, the most salient quantity to characterize the transverse energy spread is the intrinsic— named thermal— emittance (εi,x) of the photocathode. Filled ellipsoidal charge distributions are known to best mitigate space-charge induced phasespace dilution because they produce space-charge fields with linear dependence on the position within the distribution. They are less prone to halo formation, which is attractive for kW-level XFELs. But temporally shaping of photoexcitation lasers suited for high average power XFEL operation, in the ultraviolet (UV) as is the case in conventional photocathode materials, is a complex task undertaken under various linear and nonlinear optics efforts.
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