How to get a window of signal data before and after its peak?

>> [pks, locs] = findpeaks(voltage);

Then, from all the peaks, determine the global peak:

>> [vPeak, iPeak] = max(pks);
>> locPeak = locs(iPeak);

Finally, use the "find" function to find the indices of when your  signal reaches 0.2*Vpeak and drops to 0.4*Vpeak of its peak:

>> idx_start = find(voltage(1:locPeak) < 0.2*vPeak, 1, 'last');
>> idx_end = locPeak + find(voltage(locPeak:end) < 0.4*vPeak, 1, 'first');