Open Optics talk at KOALA 2015

Here is the talk I delivered at KOALA 2015 Conference on Optics, Atoms and Laser Applications.

The talk was a tutorial introducing open source tools and also examples of what has been done with open optical instruments from the community and by our group.

Cardboard coffee table

As we are moving in January I couldn't justify buying a new coffee table so I decided to make one out of cardboard boxes. The design comes roughly from Chairigami www.chairigami.com

I found it remarkably sturdy, it can easily hold books and cups however it not strong enough to put my feet on.

Rob's motion activated leaving present

Robert Carter is a good friend from ScienceIT he has helped with my different projects in the lab and outreach from zooming user interfaces to interactive websites for the year of light. Rob is moving out of ScienceIT so a group of colleagues from ScienceIT, Computer science and the Photon Factory came together to design a going away present. Rob likes hacking together art so we came up with something that he would be able to play with.

The case is 3D printed using an inkjet printer with a raspberry pi 2 attached to a 7" touch screen that just slots right in. The case is a Koch fractal which is intersected by the screen. 

An audio fractal was used to colour the outside of the case.

Two ultrasonic sensors on the side of the case detect whether someone is close and a series of recorded thank you messages cycle through.

Leaving gift from Science

A video posted by Robert Carter (@vdu23) on Nov 10, 2015 at 4:23pm PST

It also doubles as a theremin with the left distance sensor giving the volume and the right changing the pitch. The software was written in processing and interfaced to an arduino sending the serial data to the raspberry pi.

Leaving gift from Science

A video posted by Robert Carter (@vdu23) on Nov 10, 2015 at 5:20pm PST

All the best Rob we are going to miss your creativity, enthusiasm and know how in Science.

Scanning electron microscope image of a cats eye

SEM of hi-vis vest retroreflector

link

Our teaching fellow Su (see her blog here) has been working in our lab and is interested in the science around hi-vis vests. Photographing a hi-vis vest with the flash on, you can see the vest light up where the metallic bands are placed (photo above). These metallic looking bands are called retroreflectors, and they have the amazing ability to reflect light back at the same angle that it hit the surface. This is very strange as most surfaces reflect light in one of two ways, diffuse and specular (diagram below).

Link

Specular reflection is where the incident light and reflected light approach and leave the surface at the same angle. Think of a mirror for specular reflection. Diffuse reflection often happens when a surface is rough, and light is reflected in all different directions compared with the incident beam. A retroreflector does something unexpected - the incident beam and the reflected beam go in and come out in the same direction, parallel to each other.

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This ability means that light coming from the flash of a camera bounces straight back into the lens, which is why retroreflectors look so bright under a flash.

Seeing as it is the year of light and we are talking about different light-based technologies (photonics) I thought I would explain how a retroreflector works. 

Corner cube

The most basic retroreflector design is a corner cube where three mirrors are placed at right angles to each other. The incident beam will always be parallel to the reflected beam, as seen below.

Link

By adding many of these together you can make an array of corner cubes. These are often used on bikes.

Link

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Looking close up at the surface you can see the corner cubes which reflect the light back towards the light source. There is even one of these on the moon that was placed by Apollo 11 scientists.

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However, corner cubes is not what we found when we put the hi-vis vest retroreflector surface under the scanning electron microscope.

Cats eyes

This is what we found on the retroreflector surface: small glass balls stuck to the fabric in a single layer.

They are about 60-90 microns across, about the size of a human hair. To give you some scale we put one of Su's hairs on the surface.

On the right is the carbon tape we used to hold the hair down (bottom left corner of the picture). We didn't end up coating the retroreflector with gold, which would have stopped the bright spots you see on the structures. These spots are due to electrons getting stuck to the glass and not being able to drain away; we call this charging of the sample.

The cats eye works in a similar way to the corner cube but instead of two angled mirrors the light is reflected from the back of the sphere.

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This is the same effect that leads to animals' eyes glowing in head lights or camera flash.

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The back of many animals' eyes has a layer called the Tapetum Lucidum. This layer reflects light that is not absorbed by the eyes' light sensors in the retina the first time they pass through. This gives the light a second opportunity to be absorbed by the retina and is found in animals that need good vision in low light conditions, such as nocturnal animals.

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Another fun effect I found when I added water to the surface was iridescence (colour reflected at different angles). I am unsure if this is a photonic crystal effect like opals or due to the effect of rainbows coming from spheres.

What is this, a batman symbol for ants?

We machined the batman symbol in a single laser shot, five hair widths across!

Here are some 3D plots of the Batman symbol. But seriously, how did we machine a Batman symbol using a laser in one shot, and what do we want to use this for?

Using tiny mirrors to machine any shape

Digital Micromirror Devices (DMD) are small chips made up of tiny mirrors that can be tilted using the computer. These are really cool devices - looking at a single mirror, you can see the tiny machinery used to move the mirror.

Applying a voltage to the device pulls on and rocks the mirror.

By reflecting certain pixels and deflecting others, you can get a collection of light and dark spots. Using a lens the Fourier transform can be taken of the pixels and a pattern can be generated (Check out my other blog post on Fourier transforms using lenses). This set of spots diffract at the focus of the lens and form the pattern which is the Fourier transform of the amplitude (intensity). This has been used to machine patterns into materials using a single laser shot before.

10.1088/0960-1317/23/3/035005

One of the problems with machining with these DMDs is that to make the spot pattern you have to deflect much of the light away from the lens and you cannot get good efficiencies.

Slowing light down to make patterns

The method we use in the lab is to change the phase of the light reflected off the mirror by having tunable crystals that we can change the refractive index (speed of light through them) of the pixel. Remember the phase shift is a translation of the wave by an angle theta.

https://cms-assets.tutsplus.com/uploads/users/120/posts/19988/image/1000px-Phase_shift.svg.png

The device we use is a liquid crystal on silicon (LCOS) spatial light modulator (SLM). Starting from the bottom, small electrodes can be turned on and off creating an electric field between the top plate and the bottom. A liquid crystal rotates from being flat to upright when the electric field is turned on as the molecule aligns with the electric field lines.

10.1364/OE.22.016087

http://www.sharpsma.com/lcds/lcd-reference-information/twisted-nematic-displays

How does the phase shift create a pattern? Remember from the double slit experiment that a phase shift between waves can lead to constructive and destructive interference far from the sample. This creates a varying intensity profile far from the two slits.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/mulslid.html

http://www.olympus-ims.com/vi/ndt-tutorials/intro/breif-history/

Instead of using spherical waves as in the case of the double slit to form constructive and destructive interference, we can shift the phase in such a way that it will interfere to form the pattern we want. One helpful illustration is this experiment in Nature journal that used different thicknesses of a material to cause a phase shift to make a lens made for x-rays.

http://www.nature.com/nature/journal/v401/n6756/fig_tab/401895a0_F2.html

You can see how the different phase shifts introduced from the different thicknesses lead to constructive interference at the focal plane. This is a kind of Fresnel lens. A Fresnel lens cuts up a normal lens into a much lighter collection of curves. These were originally used in lighthouses to collimate the light and send it out to sea.

http://www.teara.govt.nz/en/diagram/6675/fresnel-lens

https://upload.wikimedia.org/wikipedia/commons/thumb/e/e8/Fresnel_lens.svg/2000px-Fresnel_lens.svg.png

As the light comes through the lenses the phase shift is different across the lens and this leads to the focusing or collimating of the light as the different phases interfere.

nphoton.2010.143-f1

The LCOS SLM can give you completely controllable phase shifts. It does this by changing the electric field continuously this allows you to rotate the crystals by a given amount. When the molecules are flat they are along the same direction of the electric field of light and light gets slowed down by the maximum amount when the molecules are upright the light does not interact with the molecules as much as they are perpendicular to the electric field of light. By changing the amount of rotation you can shift the phase by a whole wavelength.

If we wanted to do the same Fresnel lens using an SLM you would provide a grayscale picture where the gray value is the amount of rotation of the molecule and therefore the amount of phase shift.

http://opticalengineering.spiedigitallibrary.org/mobile/article.aspx?articleid=1748345

Something quite fun about the SLM is that you don't need a thick piece of curved glass or a complex custom Fresnel lens - you just shift the phase and the SLM will become a lens using interference.

To make an arbitrary shape like the batman symbol we just have to put in the Fourier transform of the symbol we want and use the complex part of this transform which corresponds to the phase.

Doing the Fourier transform and repeating that pattern a few times gives you this phase mask shown below. Often making this phase mask is presented magically appearing. If you simply do the fourier transform of this using an algorithm called the fast Fourier transform FFT you do not get anything good. This is because you do not have the phase information only the intensity to begin with. We use the Gerchberg-Saxton algorithm to iteratively  improve the phase image so that we can get a nice phase mask that reproduces the intensity profile we need (see the wikipedia page and this python code).

We then add the Fresnel lens to this quite simply we just add each pixel together as the phase goes around in a circle this is not a problem.

Below is the final image if you look carefully you can almost see the batman symbol. This is because a lens takes the Fourier transform of the phase only at the focus but you can see it is starting to change the phase into an intensity profile.

What are we using this for?

I have talked a bit about the ultrafast laser pulses we make in the Photon Factory. The light pulses are around 200 femtoseconds which is 0.0000000000002 s long or a millionth of a billionth of a second. They are so brief in time that they are shorter than the time it takes a molecule to vibrate and therefore can be used to remove the electrons in a material (which allow for bonding and hold everything together) without putting energy into vibrations of the nuclei (which is heat). With all the electrons gone the positively charged nuclei repel each other and leave a perfectly clean hole with no heating effects. Here is a picture below comparing femtosecond to nanosecond laser machined features (a nanosecond is 1 million times faster than a femtosecond).

http://www.industrial-lasers.com/articles/2012/06/femtosecond-laser-micromachining-a-back-to-basics-primer.html

Normally we use a lens to focus the light onto the sample and machine a hole. Focusing a laser beam produces a Gaussian or bell shaped intensity profile. 

http://openspim.org/SPIM_Optics_101/Theoretical_basics

http://openspim.org/SPIM_Optics_101/Theoretical_basics

This is great if you want to make parabolic features but not so good if you want to machine really deep holes quickly. This ends up being the major limitation of femtosecond lasers they are very clean but very slow.

The answer to the problem of how to machine very clean deep holes demands a laser beam with a different intensity profile than the Gaussian beam, called a Bessel beam. This function, unlike the Gaussian, is much tighter and has interesting properties such as energy flow towards the centre of the beam which allows self healing of the beam and the ability to machine very deep holes.

The picture below compares the Gaussian (top) to the Bessel beam (bottom) showing the very concentrated intensity in the centre of the Bessel beam.

10.1002/lpor.201100031

These intensity profiles can really cut through materials and make some astonishingly deep and clean holes. Below is a hole in glass that is 400 nanometres across and 43,000 nm deep. 

Normally these beams are made using axicons or cone lenses. However these are very expensive and hard to make.

https://www.st-andrews.ac.uk/physics/news/old_news/selfhealingbessel.htm

Instead we can use the SLM with a phase mask and use the same trick we used for the Fresnel lens. We break down the axicon into small sections that adjust the phase.

Another interesting thing we can do is add phase masks together so we can have an axicon or other pattern and a focusing Fresnel lens add them together on the phase mask and have both operations occur.

http://www.hamamatsu.com/eu/en/technology/innovation/lcos-slm/index.html

We are using these new beams to machine holes in silicon chips for the semiconductor industry and machining of biological tissues for laser assisted surgery.