Ok, i will try again.
We plan to investigate the sample by stimulating it with high intensity light from an LED. We expect only a fraction of this light to make it through the sample, and we expect there to be a very slight delay in the transmitted signal related mostly to distance but also to the refractive properties of the sample.
I understand that a perfect square wave does not exist, and if it did the harmonics probably be a big problem. My goal mainly is to have an LED swith on and off symmetrically, and with a minimum rise/fall time. I dont think there is a huge difficulty with the switch on, but it is my understanding that when the 'logic high' is removed from the LED there is a slight delay in the extinguishing of the light emitted. The junction "slowly" loses its bias and the light emitted falls off, due to the discharge of the junction capacitance.
I was thinking if there may be some nice circuit to switch the LED off. Perhapps to apply a negative bias, discharge the junction, and extinguish the light quickly.
To give an idea of rise/fall times: say the switching signal is 100KHz, which we want to measure to an accuracy of 0.01 %. 100k/0.01 = 10M. This would be our required sampling frequency, and it would be desirable to have the LED go from fully on to fully off in one sample time; 100 ns