My fave paper from my PhD has been published. It& #39;s got:

https://abs.twimg.com/emoji/v2/... draggable="false" alt="🚨" title="Police cars revolving light" aria-label="Emoji: Police cars revolving light">A happy accident
https://abs.twimg.com/emoji/v2/... draggable="false" alt="🌞" title="Sun with face" aria-label="Emoji: Sun with face">Beautiful patterns
https://abs.twimg.com/emoji/v2/... draggable="false" alt="🔬" title="Microscope" aria-label="Emoji: Microscope">Flipbook microscopy
https://abs.twimg.com/emoji/v2/... draggable="false" alt="🧠" title="Brain" aria-label="Emoji: Brain">Smart modelling
https://abs.twimg.com/emoji/v2/... draggable="false" alt="🤯" title="Exploding head" aria-label="Emoji: Exploding head">Disappearing GaN

Here& #39;s a thread of what we did
The paper itself is free to everyone, here: https://aip.scitation.org/doi/10.1063/5.0005770
https://aip.scitation.org/doi/10.10... href="https://twtext.com//hashtag/SciComm"> #SciComm #sciencetwitter

The relationship between the three-dimensional structure of porous GaN distributed Bragg reflectors...

Porous GaN distributed Bragg reflectors offer an opportunity to provide the high reflectance, lattice-matched components required for efficient GaN vertical cavity surface emitting lasers. The bire...

https://aip.scitation.org/doi/10.1063/5.0005770

Before we start, why do we care?

This paper is about how pores in gallium nitride (or GaN) can be used to control an important characteristic of light, the polarisation.
GaN is what makes LEDs and lasers, so finding ways to control light is really useful.

This control factor is called the birefringence.
We made a birefringence of 0.14, which is on par with the biggest hitters in the birefringence championship
Better still we can control how it forms making it easy to use in an LED or laser

But we& #39;re getting ahead of ourselves...

The story starts with https://abs.twimg.com/emoji/v2/... draggable="false" alt="🚨" title="Police cars revolving light" aria-label="Emoji: Police cars revolving light">A happy accident of someone not putting microscope settings back to normal and me not checking whether they had.

I was looking at the surface of a porous gallium nitride distributed Bragg reflector (or porous GaN DBR), which @ProfRachelGaN has already told us all about in this thread https://twitter.com/ProfRachelGaN/status/1245719881867288577">https://twitter.com/ProfRache...

I was expecting a uniform, reflective surface in my sample, but instead I saw these https://abs.twimg.com/emoji/v2/... draggable="false" alt="🌞" title="Sun with face" aria-label="Emoji: Sun with face">Beautiful patterns

The bright, colourful crosses in this image were totally unexpected and confusing...

It turned out the microscope had polarising films in it.
Some googling found other materials with similar features known as "Maltese crosses" (https://abs.twimg.com/emoji/v2/... draggable="false" alt="🇲🇹" title="Flag of Malta" aria-label="Emoji: Flag of Malta">) because of their shape.
These structures are only visible between the polarising films, meaning they are caused by birefringence.

Light is slowed down when travelling through a material and birefringence is where the amount it is slowed down depends on the orientation of the light.
The most famous example is a mineral called calcite. Hyperphysics explains the concept nicely
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/biref.html">https://hyperphysics.phy-astr.gsu.edu/hbase/phy...

Unlike in calcite though, our crosses have localised birefringence in only certain places and have a radial structure.
This made us think that the pores in those places were not simply randomly arranged.
But how do we examine that structure when it& #39;s hidden under the surface?

https://abs.twimg.com/emoji/v2/... draggable="false" alt="🔬" title="Microscope" aria-label="Emoji: Microscope">Flipbook microscopy of course!
This uses an electron microscope to take images and an ion beam to remove a thin layer of material i.e. turn the pages.
First you use the ion beam to remove material to dig a trench then repeat 2 steps
1) take an image
2) turn the page

Making...

Hmm, not very impressive, but using some wonderful open source software (ImageJ) we can convert these side view images into a reconstruction of what the pores look like in each layer.
( @maruf_sarkar is going to do much more interesting analysis of this sort of data in his PhD.)

That image shows the pore structure in one layer of the DBR and we can see the pores are arranged radially around a central pit.
Aha! A radial structure. This might just be the cause of the radial birefringence we saw...

But did it make sense that the pores would do this to light?
We wanted to be sure, so worked with some friends in @DeptofPhysics to do some https://abs.twimg.com/emoji/v2/... draggable="false" alt="🧠" title="Brain" aria-label="Emoji: Brain">Smart modelling, which showed that we could explain the birefringence just using what we knew about the structure. Check.
https://abs.twimg.com/emoji/v2/... draggable="false" alt="✅" title="White heavy check mark" aria-label="Emoji: White heavy check mark">https://abs.twimg.com/emoji/v2/... draggable="false" alt="✅" title="White heavy check mark" aria-label="Emoji: White heavy check mark">https://abs.twimg.com/emoji/v2/... draggable="false" alt="✅" title="White heavy check mark" aria-label="Emoji: White heavy check mark">https://abs.twimg.com/emoji/v2/... draggable="false" alt="✅" title="White heavy check mark" aria-label="Emoji: White heavy check mark">https://abs.twimg.com/emoji/v2/... draggable="false" alt="✅" title="White heavy check mark" aria-label="Emoji: White heavy check mark">https://abs.twimg.com/emoji/v2/... draggable="false" alt="✅" title="White heavy check mark" aria-label="Emoji: White heavy check mark">https://abs.twimg.com/emoji/v2/... draggable="false" alt="✅" title="White heavy check mark" aria-label="Emoji: White heavy check mark">https://abs.twimg.com/emoji/v2/... draggable="false" alt="✅" title="White heavy check mark" aria-label="Emoji: White heavy check mark">

The modelling also allowed us to see how changing the size and shape of the structure could affect the birefringence.
This suggests that the thinner the pores the stronger the effect, which could allow even larger birefringence to be made.

By using different techniques to form the pores, we can create different shapes of birefringence.
For example, https://abs.twimg.com/emoji/v2/... draggable="false" alt="🤯" title="Exploding head" aria-label="Emoji: Exploding head">Disappearing GaN stripes. These disappear when the polarising films are orientated a certain way and are highly reflective when rotated the other way.

So there we have it, a potted version of the latest paper from the GaN centre in @cu_mat at @Cambridge_Uni
For the full story read the paper, free to everyone: https://aip.scitation.org/doi/10.1063/5.0005770">https://aip.scitation.org/doi/10.10...