Possibly the laser activated neurons that has high fear or pleasure thus there is a change in fear levels, causing stronger memory in order to avoid the imagined fear or to make the imagined hope come true.

Using shorter wavelength has no effect since it cannot penetrate the skull, while using laser on the other side has less chance to activate any high pleasure or fear neuron since right handed people will have more intense memories associated with their right hand and in turn, the neurons are formed there.

So if such is correct, by having the participants be all left handed, the laser should have no effect on the right prefrontal cortex.