Unintended filtering in a typical photodiode detection system for optical tweezers

We characterize the frequency-dependent response of a photo detection system based on a Si-PIN photodiode and a laser with wavelength 1064 nm, a system commonly used with optical tweezers. We chopped the laser beam with chopper frequencies from 200 Hz to 14 kHz, and found an exponentially delayed response of the detection system with a characteristic delay time of similar to20 mus. The physical mechanism causing this time delay is silicon's transparency to 1064 nm light: Photons are absorbed and create charge carriers not only in the diode's depletion layer, where they are detected within nano-seconds, but predominantly in the n-layer, where they remain undetected till transported out by thermal diffusion. The diode's response is dominated by this delay which can be characterized as a first-order low-pass filter with a 3dB-frequency of 8-9 kHz, depending on laser intensity. Measurements exploiting frequencies near or above this 3dB-frequency must be corrected for this unintended filter effect. We describe how to do this, and how to diagnose other systems which may or may not have the same problem. Explanations are intended for users of photo detection systems, and present the little semi-conductor physics needed to make sense. (C) 2003 American Institute of Physics.