Polarized photocathodes make the grade
Future linear colliders will require high levels of performance from their electron sources. A group at SLAC has recently tested a structure that substantially exceeds current collider polarized electron source pulse-profile requirements.
Surface charge limit in NEA superlattice photocathodes of polarized electron source
The “surface charge limit (SCL)” phenomenon in negative electron affinity (NEA) photocathodes with GaAs–AlGaAs superlattice and InGaAs–AlGaAs strained-layer superlattice structures has been investigated systematically using a 70 keV polarized electron gun and a nanosecond multi-bunch laser. The space-charge-limited beam with multi-bunch structure (1.6 A peak current, 12 ns bunch width and 15 or 25 ns bunch separation) could be produced from the superlattice photocathodes without suffering the SCL phenomenon. From the experimental results, it has been confirmed that the SCL phenomenon is governed by two physical mechanisms at the NEA surface region, the tunneling of conduction electrons against the surface potential barrier (escaping process) and that of valence holes against the surface band bending barrier (recombination process); these effects can be enhanced using the superlattice structure and heavy p-doping at the surface, respectively. We conclude that a superlattice with heavily p-doped surface is the best photocathode for producing the multi-bunch electron beam required for future linear colliders.
Optical absorption and carrier‐induced bleaching effect in quantum wire and quantum box structures
The optical absorption spectra of quantum wires and boxes are studied theoretically to evaluate the carrier‐induced bleaching effect. It is found that the absorption coefficients of these highly confined systems (HCSs) and their changes with the introduction of carriers can be far greater than those of less‐confined systems (LCSs) such as quantum wells (films) and bulk semiconductors. In addition, the carrier‐induced bleaching effect of HCS is found to remain very high at room temperature while that of LCS is substantially weakened by the thermal distribution of carriers.