Photodiodes as Single Photon Detectors

In this project, we use several photodiodes as Single Photon Avalanche Detectors (SPADs). We use a light bulb as the light source and grating to vary the wavelength of the light source. We will build a passive quenching configuration to get photons detected. Thus, by knowing which light source is working for the LEDs to detect photons, we can determine band gaps of each LEDs.

Setup

Equipment

Power Source

Arduino Uno

Resistors

Breadboard

Jump wires

Variety of LEDs

Oscilloscope

Probe

Procedure

Basic theory

The principle behind using LEDs as single photon detectors lies in their ability to exhibit a phenomenon known as avalanche multiplication. This phenomenon occurs when a single photon interacts with the semiconductor material within the LED, triggering a cascade of electron-hole pairs via the process of impact ionization. 1.Photon Absorption: When a single photon of sufficient energy strikes the semiconductor material of the LED, it can excite an electron from the valence band to the conduction band, creating an electron-hole pair. 2.Impact Ionization: The newly generated electron can gain enough energy to cause further excitations within the semiconductor material through impact ionization. This process leads to the creation of additional electron-hole pairs. 3.Avalanche Effect: If the conditions are right, meaning the electric field within the LED is sufficiently strong, these newly generated electron-hole pairs can also undergo impact ionization, leading to a cascade effect where the number of charge carriers rapidly increases. 4.Detecting the Avalanche: The avalanche of charge carriers results in a measurable current pulse that can be detected by external circuitry connected to the LED. By measuring this current pulse, it's possible to infer the presence of a single photon that initiated the avalanche. 4.Sensitivity and Efficiency: The sensitivity and efficiency of LED-based single photon detectors depend on various factors, including the semiconductor material used, the design of the LED structure, the applied bias voltage, and the temperature. Optimizing these parameters is crucial for maximizing the detector's performance.

Doing experiment

According to the specific experiment, we have designed two circuits. One of them was emitting circuit and the other one was receiving circuit. For the emitting circuit, we also used LED as the light source. By changing the voltage applied to that LED, we can control the intensity of emission. As the same time, we also measured the current through LED. For the receiving part, we also used LED to receive the light. However, by reversing the direction of LED, which would response to different wavelengths of light and produced the current. Then, by measuring the voltage of the resister, which was series connected in the circuit through oscillograph, we could detect the signals.

In the experiment, we have used 4 different colors red, blue, green and orange served as emitter color and receiver color separately. Then recorded the intensity showed in the oscillograph. Controlled the other parameters and concentrated just on the color. Mostly the voltage of signal we found are only around 0.5mV. However, we also discovered that some special LED pairs can reach the voltage about 20mV, which is far large than the others. We thought that situation was very interesting and tried to identify the inner relation between them.