Photo: Mikal Schlosser

Microscopic laser attracting attention

Monday 25 Jun 18


Jesper Mørk
Professor, Group Leader
DTU Fotonik
+45 45 25 57 65

A tiny new laser is revealing surprising unforeseen properties.

A laser too small to be seen by the naked eye has taken the research world by storm. Locally at DTU Fotonik, where it was developed, and internationally, where the laser has been acclaimed by the Optical Society of America as one of the most exciting inventions of 2017.

The laser is only one micron in size (one thousandth of a millimetre).

It is so small that it is not even visible using a normal microscope. You need the assistance of an advanced electron microscope in order to see the laser’s internal structures. And yet, it works. Not only that—it is self-pulsating. This is the first time ever that a self-pulsating laser has been demonstrated at nanoscale.

Laser with unforeseen properties

A self-pulsating laser is one that automatically emits its light as pulses. You could say that it flashes all by itself.

 Normally lasers emit their light as a steady beam, and if you want them to pulse, you have to switch them rapidly on and off.

The nanolaser’s self-pulsating effect was not something that the research group at DTU Fotonik had in any way foreseen.

"We’ve used the same mathematical models that explain the self-pulsation to calculate the speed of our laser. If we’ve understood things correctly, we believe we have a laser that can be up to 100 times faster than the lasers we have today. But we haven’t demonstrated this yet"
Jesper Mørk, Professor, DTU Fotonik

“We had described the theoretical physics behind the laser as early as 2014 and published our findings in Physical Review Letters. But it was only after we constructed the laser in 2016 and began measuring it that we discovered that it is self-pulsating,” says Professor Jesper Mørk, leader of the research group for Quantum and Laser Photonics at DTU Fotonik.

After the discovery, the research team were able to explain the effect using their theoretical models and calculations.

“New, undiscovered effects often occur when we work experimentally at nanoscale. This is a completely new area of physics, because we’re investigating things on a scale where no one yet fully understands what’s going on. What we’re dealing with here is technological basic research,” says Jesper Mørk speaking about the latest research developments which were published in Nature Photonics in 2017.

A self-pulsating laser is exciting because the pulses can be ‘translated’ into ones and zeroes: A pulse can represent a one, while a missing pulse is a zero.

Ones and zeroes are the foundation of all electronic communication, and hence everything that takes place on a computer, mobile phone, or the Internet.

Lasers are already being used in these technologies. These are bigger than DTU Fotonik’s new laser. They can be as small as 300 microns—300 times the size of the laser Jesper Mørk’s group has been tinkering with.

The primary role of the laser is to convert our communication from the electron’s ones and zeros to the equivalent in light. By converting current (electrons) to light, researchers can speed up how quickly data is transferred between our mobile phones and computers. It’s the speed that decides whether you can go home tonight and watch a movie on Netflix without interruptions.

More surprises

Enough about the self-pulsating effect. Jesper Mørk’s research group has more surprises up its sleeve: The microscopic laser has the potential to be 100 times faster than the lasers we have today. Theoretically at least.

 “We’ve used the same mathematical models that explain the self-pulsation to calculate the speed of our laser. If we’ve understood things correctly, we believe we have a laser that can be up to 100 times faster than the lasers we have today. But we haven’t demonstrated this yet,” says Jesper Mørk.

High-speed Internet is something we appreciate, and ever since the Internet was developed, speeding it up has been a constant focus.

A laser that breaks with conventions

Enough about speed. Jesper Mørk wants to share another surprise about his nanolaser: It breaks with all the conventions for how a laser should be built.

 Normally, lasers consist of two mirrors. You insert a material that emits light when it is energized between the mirrors. The light is reflected back and forth between the two mirrors, creating the laser light as the very powerful monochrome light beam we are familiar with.

However, Jesper Mørk’s research group has dropped one of the mirrors. Instead they exploit a physical principle called ‘Fano resonance’, that can make up for the missing mirror.

The principle involves a particular form of resonance which was described for the first time in 1961 by Italian physicist Ugo Fano, whom the phenomenon was named after. To understand the details of Fano resonance you need to have read and understood considerably more than normal high-school physics. But the phenomenon is well known among physicists and is used in many contexts, according to Jesper Mørk.

The utilization of Fano resonance in Mørk’s nanolaser is also reflected in its name. This new type of nanolaser is known as the Fano laser in the circles that follow developments in laser physics and advanced optical communication technology.

Even faster Internet

Jesper Mørk and his colleagues are the first to have come up with the idea of using Fano resonance in a laser. And it’s a good idea because:

“We can change the laser’s intensity much faster than is currently possible using conventional lasers. If we want even higher Internet speeds in the future, it is crucial that the laser that converts the electrical signal into light can do so much faster than conventional lasers,” says Jesper Mørk.

The fastest lasers have a capacity of 40 gigabits per second today. Jesper Mørk predicts that the information society of the future will need to be able to transmit 1,000 gigabits (1 terabit) per second.

“Using the Fano laser we can transcend the physical limitations that put a cap on the speed of conventional lasers. We have not demonstrated the Fano laser’s high speed yet. We are still working to understand all the physics, and to develop the necessary nanotechnology to refine the design of these microscopic lasers,” says Jesper Mørk.

Fano laser

The Fano laser is just a few microns long, and is therefore not visible to the naked eye. In comparison, a human hair is about 50-100 microns thick.

1. The laser contains a photonic crystal

2. The laser’s single mirror is located at the left end of the crystal.

3. Numerous holes have been drilled in the crystal using electron beams. The holes prevent the light from spreading. It is forced instead to follow the ‘smooth’ path in the crystal.

4. The light does this until it encounters the nanocavity. The nanocavity is simply a small deviation in the hole pattern. A single hole may have a different size, or its position may differ slightly from the others.

5. The light gets caught in the nanocavity and is reflected back.  The nanocavity thus behaves like a mirror. Since the behaviour of the light is based on the principles of Fano resonance, this area in the laser is called a Fano mirror.

6. However, some of the trapped light escapes from the nanocavity. It escapes in regular pulses with a steady intensity.

7. Using electrodes around the nanocavity, the wavelength (colour) of the light reflected from the Fano mirror can be changed. This allows the light signal that the laser emits to be changed, and this feature can be used to convert an electrical signal into a light signal that can be transmitted over the Internet and to the receiver.

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