Unintended filtering in a typical photodiode detection system for optical tweezers

Author :Kirstine Berg-Sørensen, Lene Oddershede, Ernst-Ludwig Florin, and Henrik Flyvbjerg
Publication :Journal of Applied Physics
Publisher :American Institute of Physics
Volume :93
Number :6
Pages :3167-3176
Year :2003

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 ;20ms.
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.