What’s the world’s smallest laser you can build?

Before we begin this edition of Geekswipe, let me ask you a question! Have you ever tried to play a tuba inside a phone booth? A human with a tuba for reference. No? 

Well, that is essentially the sort of problem physicists run into when they try to build microscopic lasers. While we are obsessed with shrinking technology as much as we can, we want lasers on computer chips, inside blood vessels, lasers powering augmented reality contacts, and what not. 

But you see, light is a wave (and a particle too, yes, but for now it’s a wave as far as its propagation is concerned), and waves need physical space to oscillate.

So, think about it. What happens if you just ignore the rules, keep shrinking the hardware, and build a laser so agonizingly small that it only has room to spit out one single particle of light at a time?

Can you actually build a single photon shooting laser? Well, let’s explore in this edition.

How lasers work?

Let’s first set our theories straight.

A traditional laser works by keeping atoms in an excited state within a medium, typically by an external energy source. When you add two mirrors (one fully reflective and the other partial) on either side, it becomes an optical cavity. When you trap light between the mirrors, it stimulates the excited atoms to emit identical photons (stimulated emission), which rapidly amplifies the light until a concentrated, coherent beam is released through the partially reflective mirror.

So to make the world’s smallest laser, you’ll need the smallest optical cavity.

What are the challenges in making the smallest optical cavity?

It’s a physical roadblock called the diffraction limit. To put it simply, it’s a limit that defines that you cannot easily squeeze light into a cavity smaller than half its own wavelength.

Say, if you want green light, your laser cavity generally needs to be at least a few hundred nanometers wide. Try to make it any smaller, the light would just refuse to stay inside. It leaks out. 

The tuba won’t fit in the booth!

How did scientists overcome the challenge?

To bypass this, scientists stopped using traditional mirrors and started riding on the electrons.

In 2012, a team of researchers managed to build the world’s smallest continuous-wave laser using a material called Gallium Nitride (GaN), which is a semiconductor. It’s the miracle behind high-efficiency LEDs, Blu-ray players, and those hyper-fast charging for your laptops.

I’m simplifying here: To build this microscopic laser, they used a nanorod (a nano scale rod like structure) made of GaN as a medium. They put the GaN rod on a bed of silver and separated it by a gap only a few nanometers thick. And instead of bouncing light back and forth between physical mirrors, they basically coupled the light to the electrons that were already sloshing around on the surface of the silver. (It’s called surface plasmon coupling). This ‘coupling’ effectively compressed the light energy, allowing them to ‘lase’ the GaN atoms at a nanoscale, drastically smaller than the diffraction limit allowed by mirrors.

Incredible achievement no?

Great! Now let’s go deeper. Let’s get down to a single photon level.

How smaller can we go with lasers?

If you take a laser, even a nanoscale GaN laser, and just turn the dial all the way down to dim, do you get a stream of single, orderly photons marching out one by one?

Well, no!

A traditional laser, no matter how small, is fundamentally chaotic on a quantum level. It fires photons in small collections. Like clumps! Sometimes it fires zero, sometimes three, sometimes one. Think of it like a dripping faucet, you can’t predict exactly when the next drop will fall, or if two drops will fall at exactly the same time.

To get exactly one photon at a time, you have to abandon the traditional laser entirely and build a single-photon source.

Instead of a whole rod of Gallium Nitride, you isolate a single “artificial atom” or in other words a tiny quantum dot. You hit that single atomic system with a pulse of laser energy. The atom gets excited, jumps to a higher energy state, and then relaxes. When it relaxes, it emits exactly one packet of energy. One photon.

Because it is a single quantum system, it cannot emit two photons at once. It physically lacks the capacity to hold or release that much energy simultaneously. You press the button, you get one bullet. Every single time.

And that’s how you get towards a single photon system.

So yeah, we can go to the smallest of levels here, but building it at its scale is what is challenging. And such lasers could have a wide variety of applications. From secure low power computer chips to disease identification in cells. Shrinking a laser down to the scale of a single photon is more than just an engineering flex.