BJT driven LED lengthy fall time
My suggestion is that the light is stopping within hundreds of ns and your "photo receptor" is almost 100% of the perceived problem. 

A negatively biased PIN photodiode into a GHz transimpedance amplifier is one standard way to get response times down, for somewhat less performance you can also use a negatively biased PIN photodiode into a low resistance (like 100 ohms) and crank up the gain on your 'scope. Higher sensitivity means more PD acreage, which means more capacitance, which means you need a better circuit to keep it fast.