4GLS photoinjector problems:
10ps response decreases QE
Low (30 meV) energy spread decreases QE
100 mA on cathode needs 50 W laser, needs cooling
100 mA gives short lifetime due to ion back-bombardment
Solve the first 3 using new cathode heterostructure design - the response time depends on the number of random walks through the thickness of the cathode. A cathode with a thin active later is therefore better. But QE drops because of incomplete light absorption, and mean transverse energy can increase.
In reflection mode (RM), can use Distributed Bragg Reflector
In transmission mode (TM), can use Graded Band Gap Layer - can increase QE by maybe 10x.
Band bending near surface can increase transverse energy spread via momentum scattering (and 'acceleration'), up to value of negative electron affinity. This is not the longitudinal energy spread, however, the longitudinal and transverse energy spreads are similar (difference between conduction band minimum and vacuum energy level, around 150 meV at 300 K and 250 meV at 100 K.
If active layer thickness is smaller than the characteristic thermalisation length (of the order of 100 nm) then you get bigger energy spread.
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