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.

$\{\begin{array}{c}hf<\varphi \to E_{electron}<0\to \text{No electrons emitted}\\ hf=\varphi \to E_{electron}=0\to \text{Surface electrons}\\ hf>\varphi \to E_{electron}>0\to \text{Electrons are emitted with kinetic energy}{E}_{electron}\end{array}$

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.One additional note regarding this experiment is that the amount of current detected is not proportional to the frequency, but rather the intensity of light. While a higher frequency of light would provide the electrons with higher kinetic energies, this does not lead to a rise in current. However, a larger intensity, corresponding to a larger number of photons, would mean that more electrons are emitted, causing higher currents.

Pulsed Lasers

Pulsed lasers differ from the more commonly found continuous wave (cw) lasers in that they emit light in a discrete manner. The difference between the two can be understood from the following picture:

In addition to the usual parameters that characterises a laser; the wavelength and the power, pulsed lasers have two additional important numbers. The first is the pulse duration, which refers to how long each pulse lasts in the time domain. The second is the repetition rate, or how many pulses are emitted per second.

Experiment

Consider a situation where one has a pulsed laser split into two beam paths, which are then directed to coincide on a single metal plate. The material of the metal plate is chosen such that a single photon from one of the paths of the pulsed laser is insufficient to produce an electron ($=hf<\varphi$).

Project Breakdown

This project has been broken down into the following steps:

  1) Design and Construction of DIY electron multiplier
  2) Design and Consturction of vaccum chamber (mainly an issue of getting things to fit inside nicely)
  3) Evacuation of vaccum chamber
  4) Start experiments.

But to ensure that the experiment would work, we first perform some simple simulations to estimate what would be the resources required to generate a high enough signal that may be detected.

Simulations

This project is generally heavily limited by the resources that we have on hand. As such, we initially perform some simple simulations to ensure that whatever resources we have are sufficient for the actual experiment to work.