When we first heard the term “random laser,” we did a double-take. After all, most ordinary sources of light are random. One defining characteristic of a traditional laser is that it emits coherent light. By coherent, in this context, that usually includes temporal coherence and spatial coherence. It is anything but random. It turns out, though, that random laser is a bit of a misnomer. The random part of the name refers to how the device generates the laser emission. It is true that random lasers may produce output that is not coherent over long time scales or between different emission points, but individually, the outputs are coherent. In other words, locally coherent, but not always globally so.

That is to say that a random laser might emit light from four different areas for a few brief moments. A particular emission will be coherent. But not all the areas may be coherent with respect to each other. The same thing happens over time. The output now may not be coherent with the output in a few seconds.

Baseline

A conventional laser works by forming a mirrored cavity, including a mirror that is only partially reflective. Pumping energy into the gain medium — the gas, semiconductor, or whatever — produces more photons that further stimulate emission. Only cavity modes that satisfy the design resonance conditions and experience gain persist, allowing them to escape through the partially reflecting mirror.

The laser generates many photons, but the cavity and gain medium favor only a narrow set of modes. This results in a beam that is of a very narrow band of frequencies, and the photons are highly collimated. Sure, they can spread over a long distance, but they don’t spread out in all directions like an ordinary light source.

So, How does a Random Laser Work?

Random lasers also depend on gain, but they have no mirrors. Instead, the gain medium is within or contains some material that highly scatters photons. For example, rough crystals or nanoparticles may act as scattering media to form random lasers.

The scattering has photons bounce around at random. Some of the photons will follow long paths, and if the gain exceeds the losses along those paths, laser emission occurs. Incoherent random lasers that use powder (to scatter) or a dye (as gain medium) tend to have broadband output. However, coherent random lasers produce sharp spectral lines much like a conventional laser. They are, though, more difficult to design and control.

Random lasers are relatively new, but they are very simple to construct. Since the whole thing depends on randomness, defects are rarely fatal. The downside is that it is difficult to predict exactly what they will emit.

There are some practical use cases, including speckle-free illumination or creating light sources with specific fingerprints for identification.

It’s Alive!

Biological tissue often can provide scattering for random lasers. Researchers have used peacock feathers, for example. Attempts to make cells emit laser light are often motivated by their use as cellular tags or to monitor changes in the laser light to infer changes in the cell itself.

The video below isn’t clearly using a random laser, but it gives a good overview of why researchers want your cells to emit laser light.

You may be thinking: “Isn’t this just amplified spontaneous emission?” While random lasers can resemble amplified spontaneous emission (ASE), true random lasing exhibits a distinct turn-on threshold and, in some cases, well-defined spectral modes. ASE will exhibit a smooth increase in output as the pump energy increases. A random laser will look like ASE until you reach a threshold pump energy. Then a sharp rise will occur as the laser modes suddenly dominate.

We glossed over a lot about conventional lasers, population inversion, and related topics. If you want to know more, we can help.