Jie's Current Introduction

Light is made up of tiny packets of energy called photons; so small, in fact, that it is almost impossible to measure directly. The amount of energy that is deposited when one photon is absorbed is equal to _________________________. Therefore, to detect single photons, physicists built a machine that magnifies the energy of the photon. This machine is called an avalanche photodiode.

The normal diode is a device that allows electricity to flow one way, but not the other. It is used in a whole bunch of very common electrical appliances, from computers to toasters. Normally, photodiodes produce a single electron from each photon that hits the detector area. But since physicists are trying to detect single photons, that is not nearly enough. They had to create a device that releases many electrons for every single photon that hits the detector. (Discussion on PMT?) They could do just that with the avalanche photodiode. Diodes only allow electricity to flow one way. So if voltage is applied in the opposite direction of the way that the electrons were meant to flow, no electricity would cross. Yet every single diode has a breaking point. If enough voltage, or electrical force, is put across a diode, it could suddenly allow all the electricity through, like a dam breaking. The voltage that is applied in the reverse direction is called reverse bias voltage. Physicists take advantage of that effect by applying enough reverse bias voltage that the Avalanche photodiode that anything, even the energy from a single photon is sufficient to cause it to break down. This is called the breakdown voltage. If a photon were to hit this diode, it would cause a huge surge of electricity to go through the diode and therefore the entire curcuit, one that could be measured by the scientist.

Normally, when a diode breaks down, it allows all the electricity to go through, causing it to heat up immediatly and burn up in an instant. But in the interests of preserving the photodiode for reuse, a resister is introduced in the circuit. This resister limits the amount of aperage, or the number of electrons from going through the circuit, and therefore prevents the photodiode from burning up.

Once the current is limited by the resister, the voltage drops below the breakdown voltage and the diode again starts to restrict the flow of electrons and the diode resets. (don’t know how to explain the capacitor)

A new innovation that scientists have developed is the SiPM. It is a photon detector that is an array of avalanche photodiodes. ***(Should I even mention the SiPM? Since dark rate works the same way with the Avalanche photodiode and the SiPM)

The photon detector's sensitivity also poses