Photodiode gate circuit noise
First, use a proper transimpedance amplifier immediately after the output of the diode.  Second, make sure the emitter is as strong as it can be.

You do want to use the photodiode in reverse leakage mode, but you want to hold its voltage constant.  Converting the current signal to a voltage signal right at the diode is a bad idea.

Trying to modify this circuit is pointless.  Start with a real photodiode detector circuit, then see what you have.