Improvements in ultra-thin film photocathode technology
Metal photocathodes are widely used as electron sources by the accelerator community, due to them offering several advantages: Metal photocathodes present a fast response time, a relative insensitivity to the vacuum environment and a relatively low intrinsic emittance, which is defined as the position and energy spread of the photoemitted particle bunch, in the three orthogonal axes.
However, metal photocathodes usually have low quantum efficiency (QE) and photoemission only occurs in the short UV wavelengths due to the relatively high work function (WF) of metals. Due to these restrictions, accelerator design is limited when using metal photocathodes, due to the requirement for using high power deep UV lasers, which drives up the cost and complexity of design.
(a) 0.5 x 0.5 μm2 AFM topograph. (b) 100 x 100 nm2 LFM image overlaid with a contour height map (0.3, 0.6, 0.9, and 1.2 nm represented by black and increasingly lighter grey lines, respectively). Vertical scale represents the lateral force. (Credit: Chris Benjamin)
In an open access paper lead by Chris Benjamin of the University of Warwick and published in the Journal of Applied Physics, Liam Soomary of the Quasar Group has been driving the advancement of photocathode technology alongside Hugh Churn and others of the Photocathode Group at Daresbury Laboratory, being involved in exciting research looking to enhance photocathode performance through ultra-thin MgO film deposition on Ag (100).
It has been shown experimentally that the inclusion of an ultra-thin MgO film dramatically enhances the photoemissive properties of a Ag (100) photocathode. The MgO-enhanced photocathode exhibited a WF reduction close to 1 eV relative to clean Ag (100), as well as an increase in the QE of a factor of 8 relative to clean Ag (100). Alongside this, the MgO film shows an enhancement in the robustness of the photocathode against gas exposure.
MTE measurements for the Ag (100) photocathode (green points) and MgO-enhanced photocathode at room temperature (red points) and 175 K (blue points). (Credit: Hugh Churn)
These results shown in the paper indicate potential application for thin-film enhanced metal surfaces in the future. With large reductions in the WF alongside notable increases in the QE, there is potential for this material to be used as a UV-A sensor with a measurable photocurrent response at wavelengths below 360 nm. Alongside this, the possibility of changing the drive lasers used for current metal photocathodes to less complex and lower power variants is an exciting prospect for future accelerator design.