Coincidence Time Measurement of Pulsed Lasers & "Useful" Applications

Members

Tan Wen Yi, Kim Mu Young

Abstract

Given two paths through a which a pulsed laser propagates, one might be interested in ensuring that the pulses coincide on the same spatial point at exactly the same time. One possible situation where such a coincidence may be useful is in the driving of Raman transitions via a pulsed laser. We intend to perform such a coincidence time measurement using commonly-available metals. By choosing a material with a work function on the order of the energies of two 355nm photons, the detection of a photocurrent through a metal would then indicate that both laser pulses arrived at the metal at approximately the same time. The project first aims to find such a metal and design a set-up to perform this experiment; after which, we intend to measure the timing resolution such a method is limited to. If time permits, we then intend to try to use this method to perhaps characterise the density/surface smoothness and other possible features of the metal object used.

Theory

Photoelectric Effect

The photoelectric effect is a simple yet revolutionary phenomena, being one of the first experiments to show the quantisation of energy. Every metal surface has a characteristic feature known as the work function ($\varphi$) which describes the amount of energy required for an electron to be ejected from its crystal structure into the continuum. If one illuminates such a metal surface using some light source, the energies of the electrons that are emitted are characterised by a simple equation:

$E_{electron}=hf-\varphi$

Where $h$ is the Planck's constant and $f$ is the frequency of light used.

There are three possible results based on the frequency of light.

$\{\begin{array}{c}hf<\varphi \to E_{electron}<0\\ hf=\varphi \to E_{electron}=0\\ hf>\varphi \to E_{electron}>0\end{array}$

If ${\displaystyle }$, which is impossible. This refers to the case where the light has insufficient energy to overcome the work function of the material, and thus no electrons are emitted. Instead, if ${\displaystyle }$, this would be a free electron within the crystal structure of the metal. And finally, if ${\displaystyle }$, then the electrons are ejected into free space with a kinetic energy given by $E_{electron}$.

A typical set-up used to test the model described above goes as the following (obtained from https://physicscatalyst.com/chemistry/photoelectric-effect.php, but will make our own later on):

A light source is used to illuminate a metal surface. If the frequency of the light is high enough, electrons are ejected from the surface. These ejected electrons are accelerated onto a detector via the application of an external potential. Once these electrons fall upon the detector, they travel through the wires, causing a current to be detected. As such, depending on the lack or presence of a current, one could tell if the frequency of light used is sufficiently high to overcome the work function of the metal.