LEDs as Single Photon Detectors

With the current great interest in single photon experiments in undergraduate physics labs, it can be useful for students to have the opportunity to work with a very inexpensive version of a single photon detector in order to understand how they work.  In this experiment, an LED is reverse-biased in order to have it act as an avalanche photodiode. Students build a simple comparator circuit to produce a digital pulse whenever light is detected.

With this simple setup, students are able to investigate a large ‘physics phase space’ of phenomena and techniques including:

• Op-amp circuits
• Comparators (which help students understand the triggering circuits of oscilloscopes)
• Statistics of photon counting
• Semiconductor physics of avalanche photodiodes
• Quenching of pulses
• Time constants and dead time (and how to measure deadtime using time-between-events distributions)
• Temperature dependence and spectral response of the LED/SPAD
• Afterpulsing in avalanche photodiodes

Participants do not need to bring anything besides themselves, though they may want to bring some means of taking notes about their procedures, and of storing the data files they generate.

Estimated Cost: 
This is a very inexpensive experiment, with the LED, comparator, and resistors costing less than a dollar per setup.  An oscilloscope, circuit breadboard, and 2-3 standard lab power supplies are also needed.  A decade resistor is convenient, but definitely not required.  The data can be acquired using any number of data acquisition devices.