• Blue Light Special: an Astrobiological Point of View on the 2014 Physics Nobel Prize

    Anyone who knows me knows that I’ve kind of got a thing for light and lighting.

    When not hunkered down in the science mines I can be found sewing and fixing literally thousands of lights in costumes for the latest Rock Opera that I worked On “The Electric Pharoah”.

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    Also, my doctoral research has been looking at how metabolic responses in land plants respond to changing light conditions over geologic time. Specifically, I have used leaf waxes preserved in fossils and rocks to make interpretations of forest density and the evolution of shade tolerance.

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    When not dissolving rocks or climbing trees I like to think about what life might look like on other planets with stars very different then our own.

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    Of course then, given my interests I was VERY excited by the choice of Nobel Physics laureates this year. Isamu Akasaki, Hiroshi Amano and Shuji Nakamura are credited with building the first light-emitting diode (LED) that can make blue light. This might sound too simple to be worth a Nobel prize – after all just put a blue thing on a light if you want blue light right?

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    Well, light isn’t that easy and some of the most useful things I know about light come from my time in art school, not my time in graduate school! When thinking about light and color you have to remember that there are two ways to mix colors – additive and subtractive. Think back to your coloring books when you were a kids. You know that if you scribbled all the colors together you would get black (well, actually it would probably just be a murky mess) but you could have just made this mess just as easily by mixing just three colors: magenta, yellow, and cyan.

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    Subtractive Color Mixing

     

    This might seem like a contradiction but this is referred to as the “subtractive” mixing of colors. This is because less light is being reflected. As each color is added to the mix more light is being absorbed by each individual color. By contrast, when the primary colors of light are mixed together more color is reflected with each added component. The primary colors of light differ from those of pigments but mixing the three (red, green, and blue) will give you all of the colors – including white!

     

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    Additive Color Mixing

    And this is what made the development of the blue LED so important. The red and green LED were easy to make and had been invented in the 60’s but without a blue LED we could not have cheap, compact, energy-efficient, mercury-free white light. All-colour (white) LEDs are becoming more and more common for everyday lighting, not just for iPhone screens and billboards.

    So why exactly was it so hard to make the color blue in an LED? What is it that Akasaki, Amano, and Nakamura actually did?

     

    LEDs – and really any diodes – are semi-conductors. LEDs have two different types of materials at the cathode and anode. These materials are referred to as “p-type” and “n-type” and this refers to if the material has an excess of electrons (n-type) or an excess of holes for electrons to go in to (p-type). The interface between these materials is called the p-n junction. When energy is applied to these diodes electrons will move across the p-n junction into the holes on the other. When this happens a portion of the energy is released as photons – light! The color of light that is emitted depends on the band gap, the amount of energy that it takes to get an electron to unhinge itself from the atom in the n-type material it is bonded to and go into a higher energy state over in the p-type material. The photon released during this electron hop will have a particular wavelength of light – or color – that corresponds to that energy difference between the high and low states.

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    In the case of blue LEDs that is a much bigger energy difference than other colors and finding that special pair of materials with an energy gap that would produce blue light took decades to figure out. At Nagoya University Akasaki and Amano developed the indium gallium nitride (InGaN) alloy semiconductor. In the rather unpoetically named book “Introduction to Nitride Semiconductor Blue Lasers and Light Emitting Diodes” Steven DenBaars very poetically describes these semiconductors saying

    “Nature has blessed the (InxGal1-x)yAl1-yN alloy system with a continuous and direct bandgap… spanning ultraviolet to blue/green/yellow wavelengths.”

    Does this mean anyone with a periodic table should have been able to figure out the blue LED? Turns out that making a InGaN crystal is really hard and these materials lack a p-n junction, an interface between electrons and electron holes. Akasaki and Amano used sapphire as the base to make crystals of InGaN and GaN that would act as a p-type material and even figured out a way to “p-dope” (add extra holes) by tweaking the amount of indium and aluminum in the diode. Nakamura then figured out how to get the right mix of InN and GaN so band gap between the two materials would result in blue light when an electron moved across it. The development of the blue LED wasn’t just an engineering problem or a technical achievement but a physics breakthrough since it required an understanding of the interatomic distances in the atoms of the alloy, how these distances change in relation to one another with different mixing of materials, and how the crystal structure is essential to manipulating the energy band gap.

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    So, how does all this apply to astrobiology? Well, it turns out it’s even hard for nature to make blue light. Look out in to the night sky and you will see very few blue stars. Of course stars don’t rely on semi-conducting alloys and electron energy gaps to make color and light. The color of a star is temperature dependent and the temperature of stars is usually defined as the “effective temperature”, the temperature that a blackbody (an imaginary ideal material that absorbs ALL light and other electromagnetic radiation) would have to emit the same flux of radiation. Despite what the name implies a blackbody isn’t black but has color depending on it’s temperature and that color can be described by something called the “Planckian locus”, a line that corresponds to the three axes of “color space” – hue, colorfulness and luminance.

    640px-PlanckianLocus

    In this diagram you can see that it would take a REALLY high temperature star to make blue light, so blue stars like Bellatrix (over 4x hotter than the Sun) are very rare!

    Okay, now time for an astrobiological thought experiment – if a star was blue would the plants on any planets around that star not be green? If we look along the Planckian locus and find the temperature of our sun (~5250 K) you will see that it is considered a yellow star. But if take a prism outside and separate sunlight you see it has all the colors of the rainbow.

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    Even if the sun looks yellow it is making electromagnetic radiation (light) all across the spectrum! If we look at a spectrum that shows us the amount of each color of light making it to the surface of the Earth we see that it is less than we would calculate just based on the properties of the Sun since much of the radiation (thankfully a lot of ultraviolet light) is filtered out by the atmosphere. Solar_spectrum_en

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    This means you can’t just use the temperature of a star to determine the quality of light on a planet cause you have to know something about its atmosphere as well.

    Chlorophyll, the major energy-trapping pigment in plants, absorbs all the blue and red light from the sun so the only thing left is green – hence why plants look green. By absorbing red and blue light chlorophyll is taking advantage of a big portion of the electromagnetic radiation coming from the Sun. Plants and other photosynthetic organisms have more than just one pigment though – carotenoids trap some more of blue and green light from the Sun, increasing the amount of the Sun’s radiation to power photosynthesis and make biological energy.

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    Did plants evolve these specific pigments in order to maximize the portion of the Sun’s energy they could trap? Would photosynthetic organisms on other planets develop different pigments in order to capture whatever unique spectrum may be found on the surface of their world?

    These are the kind of things I like to ponder.