Is it possible to make a laser out of wood?

Wood is a precious material. It has been used for thousands of years as fuel, in construction, for tools and weapons, as well as for furniture and paper. It is also important for biochemistry in the production of purified cellulose and its derivatives, such as cellophane and cellulose acetate. Wood has unarguably been the most important raw material that allowed civilizations to flourish and it will probably remain that way.

But quite recently the usefulness of wood has expanded to a completely new area: photonics and optics. When you think about it, it sounds improbable, or even down right stupid to use wood as a material in optical physics, but that's really what has been going on!

Let me be clear, this is not the type of wood you would traditionally use, but chemically treated so that it transmits light. The process is simple in principle, you just soak some wood in a chemical that dissolves lignin. Then you take the delignified wood and add epoxy to it, and voila! You now have your very own transparent piece of wood.

But if you watched the above video, you may have noticed that the result is not crystal clear, and more like fog glass. The quality is nowhere near the level required to create lenses and other optical elements from it. Maybe you can make some wooden windows (which is still quite cool), but that's not very useful in photonics research, so what good is it?

Two words: wood - laser. That's right, some of my colleagues from KTH in Sweden actually made a working wood based laser! They used the same process as for transparent wood, but with an extra twist, they added some laser dye to the epoxy resin. Laser dye is a colorant that has the property of absorbing certain wavelengths and then emitting others (usually longer wavelengths).

If you think about it, a laser has only three components, the pump, the gain material and the cavity. The pump can be some external energy source, which is easily arranged, the laser dye is the gain medium and the wood fibers act as small laser cavities. So we have everything we need, right there, in that small piece of transparent wood.

I had the honor of measuring the spatial coherence of this particular laser, and all of the results we got suggests that the wood fibers indeed do work as uncoupled miniature resonators. Now, the interesting thing about this type of light source is that it has some properties from normal lasers, as well as random lasers. The wood has a certain degree of order, since all of the fibers grow towards the same direction, but since they twist and turn somewhat, there is some randomness. We call it hence a quasi-random laser.

Of course, we can make such lasers from a wide variety of materials, wood just happens to be a prime candidate for this process. Tweaking the gain material and pumping process can even lead to commercial lighting applications. Although it's still ways off, I think it would be really cool to have wooden light bulbs in my ceiling.

I went to the lab and made a quick video of the wood laser, which you can see below. Hope you enjoy my awkward Finnish accent!