Anomalous correlation between quantum efficiency and transverse momentum spread in semiconductor cathode photoemission

The next generation of accelerator based scientific facilities, such as the megahertz repetition rate free electron laser and electron-ion collider, relies on photoelectron sources of high peak and high average brightness. Therefore, photocathodes with both low transverse momentum spread (TMS) and high quantum efficiency (QE) are required. The conventional positive correlation between QE and TMS leads to a tradeoff between peak and average brightness. In this study, a negative correlation between QE and TMS is experimentally observed for the first time on a semiconductor photocathode (${\mathrm{Cs}}_2\mathrm{Te}$). Two models are proposed to explain such an anomalous correlation. One model considers the junction between the semiconductor cathode and the metal substrate in the case of slightly excessive cesium deposition, and the other model considers different Cs-Te composites. Our studies inspire a new way to break the conventional correlation between QE and TMS, and open a new perspective to advance the beam brightness frontiers.

Figure 1

Schematic of the experimental setup.

Figure 2

(a) Minimization of rf induced emittance growth for the off axis beam by a special rf phase. The off axis laser position is (1,1) relative to the cathode center with units of millimeter. (b) Off axis electron beam images measured at gun phases of MMMG and $\mathrm{MMMG}+8.8$ degrees.

Figure 3

QE map (a) and TMS map (b) of the ${\mathrm{Cs}}_2\mathrm{Te}$ cathode #678.1. The QE map is measured with the laser diameter of 0.25 mm and the step size of 0.2 mm. The TMS map is measured with laser diameter of 0.5 mm and the step size of 0.5 mm.

Figure 4

The correlation of TMS and QE of four ${\mathrm{Cs}}_2\mathrm{Te}$ cathodes.

Figure 5

The schematic of the Schottky barrier model in the junction between ${\mathrm{Cs}}_2\mathrm{Te}$ and Mo substrate.

Figure 6

The schematic of the multiple Cs-Te phases model. The $x_1$, $x_2$ and $x_3$ in the schematic refer to the chemical ratio. The cathode sample contains several Cs-Te phases, assuming from ${\mathrm{Cs}}_{x1}\mathrm{Te}$ to ${\mathrm{Cs}}_{x3}\mathrm{Te}$ the QE increases and the TMS decreases. This can also lead to the negative correlation between QE map and TMS map.