Early in my training, I learned one rule: loss is not your friend. In laser physics, loss means that every photon that goes missing is a photon that no longer stimulates emission. And, with every lost photon, it becomes just that little bit harder to keep a laser going. So, when Science published a paper showing that this rule doesn't always hold, I was intrigued.

Also it gives me the chance to talk about lasers, which I never tire of.

Gain, loss, and lasers

Before we get to the experiment, let's talk about lasers in general. Lasers emit light through a process called stimulated emission. Stimulated emission only dominates under two conditions: there have to be more emitters ready to emit light instead of to absorb light. This is referred to as population inversion and provides the gain (or the source of light amplification). The other requirement is that there is light present to stimulate emission. To put it slightly incorrectly, the amplifier needs something to amplify.

So, let's go through the start up process for a laser to see how this all comes together. We have a medium that we are going to turn into our amplifier by pumping it. The pumping process places emitters in an excited state. From the excited state, the emitter will relax to a slightly lower, yet still energetic state. We hope to get all the emitters into this state to obtain a large population inversion. But the emitters can also relax back to the ground state by emitting a photon. To obtain a population inversion, we have to pump the emitters into the excited state faster than they relax back to the ground state.

So, we throw lots of energy into the medium and get a population inversion. At some random time, an emitter will relax and emit a photon. As the photon travels through the medium, it will encounter other excited emitters. When it does, there is a certain chance that the presence of the photon will stimulate the emitter to emit a photon as well—that will become a fellow traveler. The important thing is that this probability depends on the number of photons in the light field. That is, if there are two photons traveling together, their chance of stimulating an emitter is greater.

This sets up a competition. When you have something like 1023 emitters, more than one is going to emit spontaneously. These spontaneously emitted photons are all different: they have different colors, different phases, and different directions. They can all cause stimulated emission though. This is where the mirrors that form an optical cavity around the gain medium come into play.

The mirrors act to provide selectivity to the stimulated emission process. Photons that are emitted along the axis of the mirror alignment will be reflected back through the gain medium several times. This provides an advantage to specific emission directions: there are more photons traveling along that direction, so the light field created by these photons grows faster.

Furthermore, the photon is not just a particle, but also a wave. So, as the photon cycles back and forth between the mirrors, it interferes with itself and other photons in the field. If, after a roundtrip between the two mirrors, the electric field peaks are aligned, then the electromagnetic field associated with that particular color will grow. Otherwise, the fields experience destructive interference and will be lost.

While all this is going on, we're also losing photons. At each surface, a small amount of light is lost; at each mirror, a small amount of light leaks through (this is the output of the laser); and all the matter, including the emitters, may absorb photons. All of this reduces the intensity of the light field, which reduces the chance of stimulated emission. For anything to happen, the number of photons emitted into the field by stimulated emission must be at least equal t/o the loss of photons due to all possible processes.

Hence, if the number of photons lost per roundtrip goes up, the gain—or number of photons stimulated to emit—must go up to compensate. For most lasers, the population inversion is not perfect—that is, not all emitters are in the correct excited state at the same time. While it is usually possible to simply increase the amount of energy we put into the medium, or increase the size of the medium, in general, we fight losses at every step of the process.

Turn up the loss

When I encountered the idea behind the new Science paper—that loss might make a laser easier to switch on—it was as if someone had suggested that house cleaning is quicker if you start by emptying a septic tank in the living room.

To understand this, we need to take a look at the experimental setup. You can picture it like this: imagine three mirrors in a row, forming two optical cavities. Light can be reflected back and forth between the first mirror and the second mirror, or between the second mirror and the third mirror. Light can leak between the two cavities via the imperfect reflectivity of the center mirror. Light can also leak into and out of the cavities via the end mirrors. Finally, the loss that occurs in the second cavity can be varied over a wide range by inserting a material that absorbs light into the cavity.

What the researchers observed is that, initially, when the loss was low, the laser would turn on at some expected pumping power. As the loss was increased in the second cavity, the power to turn the laser on would go up. But when a certain level of loss was reached, the power needed to turn the laser on would drop again.

To get an idea of how strange it is, let's look at it another way. Imagine that the two optical cavities have the same loss. Now if I put a bit of paper in front of the third mirror—so the loss is 100 percent for the second cavity—a laser in the first optical cavity can be turned on. Let's say that the power required to start the laser is one Watt. I can transfer the paper from the third mirror to the first mirror, a laser based in the second cavity will also turn on when I pump with one Watt of power. If I take the paper out, the threshold power will drop to, say, a quarter of a Watt and the laser will use both cavities, with light leaking from one cavity to another as if the center mirror were barely there. The total loss is low, and both gain media contribute to reduce the required power.

But here's the weird bit: if I add some loss of the second cavity (less than 100 percent), the power threshold to get a laser to start goes up until it is above one Watt. In other words, even though the loss is less than in the case of completely blocking one of the cavities, the laser doesn't want to start.

He pulls, she pulls, we all fall over

This can be understood by looking at the light in each cavity. Remember, I said that each photon interferes with itself on each roundtrip. It will only stay in the cavity if it adds up in phase—that is, that the distance between the mirrors is an integer multiple of half the wavelength.

There is, however, some freedom in this. Imagine that there is a 50 percent chance of a photon leaving the cavity every roundtrip. The interference between the light from different roundtrips is rather weak, so the destructive interference doesn't effectively remove light from the cavity, and the constructive interference doesn't build light up in the cavity very strongly. That means that, although the cavity prefers light of exactly the right wavelength, it is prepared to accept light that is only a little bit wrong. But the cavity becomes more and more picky as the loss goes down, so the band of acceptable colors that's centered on the correct color shrinks to a narrower and narrower selection.

Imagine that we have our two cavities that have different losses, but both losses are relatively low. The light from the low-loss cavity can easily fit into the high loss cavity because it has exactly the right color. Even though both cavities might have the same total circulating power, that power is spread over a larger range of colors in the high-loss cavity.

But a significant fraction of the light generated in the high-loss cavity cannot enter the low-loss cavity because it is the wrong color (remember, with low losses, the cavity is pickier about its light). As a result, the low-loss cavity keeps feeding light into the high-loss cavity to balance intensities, but the high-loss cavity absorbs more of the light. As a result, the high-loss cavity acts as a kind of vacuum cleaner, sucking light out of the low-loss cavity and preventing the laser from turning on. To compensate, even more power than that required for the single cavity is needed to turn the laser on.

As we increase the loss, however, the interference of the light field in the high-loss cavity gets weaker and weaker. As a result, the first cavity starts to behave as if the high-loss cavity doesn't exist. Because of that, the second cavity doesn't remove light from the first cavity as efficiently, which means that the power required to turn the laser on starts to go down again. This continues until the loss reaches 100 percent in one cavity, and the power required to turn the laser on returns to the value required to turn on the laser based on the first cavity.

This is one of those counterintuitive oddities that make us all think a little bit harder. They are, in fact, one of the reasons that I am still a scientist.

Science, 2014, DOI: 10.1126/science.1258004