My First Paper: What’s the best way to make tiny holograms to trap atoms?

Today my first ever publication was put online: “Comparative simulations of Fresnel holography methods for atomic waveguides”.  Although it’s completely open access, it probably needs a physics degree for anyone to get past the abstract so I thought that I’d have a go at explaining it.
 

Within an experiment we’re building, we want to be able to make ‘pipes’ for very very cold atoms to travel along, like the wires of a circuit. A really interesting way of making a ‘pipe’ for atoms is to use light: if the light is the right colour, the atoms will want to sit in bright spots or sections. This means that if we draw a line of light, the atoms could travel along this line, or we could make it more interesting by making the line into a circle.  This means that we want to make patterns made of light, also known as a hologram!

People have been making kinds of holograms for years. In fact, an overhead projector is a form of hologram, as is the projector in a cinema, but these aren’t true 3D holograms, they’re projections. The ‘hologram’ of Princess Leia in Star Wars was an illusion and we’re not quite at the stage of reaching Star Wars technology commercially yet.  A true hologram is a 3D reproduction of the light coming from an object (reflected or emitted).  For our light pipes, we want to make a 3D pipe shape rather than the projection of one.

However, it’s much more complicated than just using an overhead projector because atoms are so small. We need to make the pipes the right size for them, which means that they need to be around 5 μm wide (or a two hundredths of a millimetre), and smooth, so we need to be able to control the light on even tinier length scales.  This is really hard to do, because many hologram techniques use types of LCD screens (just higher spec versions of computer screens), which have pixels which are 12 μm.

My paper compares current LCD screen technology for holograms, called Spatial Light Modulators (SLMs), with a new form of hologram called Fresnel Zone Plates (FZPs), which are microfabricated, so could have pixels as small as 10nm.  An FZP is a microfabricated version of a light house lens (a picture of one that could be used as a lens is shown in figure 1 of the paper).

As SLMs are effectively screens, their patterns can be changed easily and their pixels can be set to a range values (they’re 8-bit), whereas FZPs are fixed at one pattern and are binary, so the pixels can only be 1 of 2 values (on or off). So effectively, the paper looks at whether bit-depth or pixel size is more important when making a hologram.

In the paper, we show pictures of the holograms we want to make in figure 2, and then use simulations find out how well we could make them using SLMs and FZPs (figures 4,5,8), we also show that they’d be 3D in figure 7. The paper concludes that in general pixel size is more important than bit depth for making the tiny holograms we want in the future, but that we need to test this experimentally by having some FZPs made so that means lots more exciting work for my research group!