Monday, 8 December 2014

10 billion frames per second videos of single pulses of light

There has been quite some buzz about doing ultrafast photography. This involves using ultra short pulses of light to illuminate a scene and take images at billions of frames per second. However there is always a trick and in this case the pulse of light is not the same pulse of light throughout the movie. What you do is repeat the measurement and delay the opening of the shutter on the camera. This is the same way we take ultra-fast spectrum of molecules we rely on the repeatability of the measurement and change when we observe the molecules after firstly exciting them. Here is a video made up of hundred of different pulses that when you put them altogether you can see how a pulse of light hits an apple.
An exciting new paper by Liang Gao et al. in nature shows a way to take a movie of a single pulse of light. This opens up the ability to see non-repeatable events such as nuclear explosions, optical rogue waves or gravitational waves. These events happen on the femtosecond (10-15s) timescale that are one off events.

Before we get into the nuts and bolts of how it works here are the amazing movies they captured of a single pulse of light.

Light hitting a mirror will be reflected at the same angle it approaches the mirror. Something else you can see if the evanescent field (light tunnels through the material) which decays away exponentially over time. We can use this to perform spectroscopy on a surface.
Light will travel slower in a lower refractive index material here is a comparison of air and resin where the light in the resin is travelling slower.
Refraction occurs at the surface of a lower refractive index material and a higher refractive index material.
This is my favourite, Seeing the glow of molecules after they have been excited. They capture a phenomenon called fluorescence where light of a higher energy (green in this case) is absorbed by the molecule (Rhodamine) the excited molecules loses some of that energy to vibrations and then drops down to the ground state emitting lower energy light (red).

Here are all of the different experiments.

So how does it work. Here is my basic explanation feel free to correct me in the comments. Firstly an ultrafast laser pulse is needed these are generated using special laser cavities that bounce a laser pulse inside a cavity hundreds of times each time they destructively interfere inbetween pulses and constructively interfere only for a few femtoseconds - if you have ever made a diffraction pattern in on a wall (spatially) think of this as a diffraction pattern in time (temporally). The camera used is quite similar to the old CRT TVs but in reverse instead of electrons being scanned over a phosphorous screen that converts the electrons to light. Light hits a phosphorous screen where the photons of light are converted to electrons these are accelerated towards a camera sensor like in a cellphone and an image is created. However you apply a voltage ramp across the path of the electrons so they are deflected depending on when they arrive (you reduce the voltage as a function of time). This means electrons from light that arrived earlier will be deflected more by the electric field than electrons from light that arrives near the end. You can think of this as sophisticated light painting where you get a blur of colour by keeping the shutter open on a camera.
Here is a schematic of the optics used.

Now how do you extract out a video from a blured image on the image sensor. You need some sort of pattern that will be the same at every time step. The way they do this is using a coded mask that randomly patterns the image of the object. You can see this in the diagram below.

Using matrix methods on the computer you can reconstruct a whole video from a single blurred image. What do you want to observe at 10 billion frames per second?

Gao, L.; Liang, J.; Li, C.; Wang, L. V. Nature 2014, 516, 74–77.