Lasers with a lot of power need diamond mirrors

Illustration of a high-powered continuous laser hitting nanostructures on a diamond mirror. Credit: Loncar Lab/Harvard SEAS

Since 1970, almost every car, train, and plane has been made with the help of high-powered lasers that shoot a steady beam of light. These lasers are strong enough to cut through steel, accurate enough to do surgery, and powerful enough to send messages into deep space. In fact, they are so powerful that it is hard to make strong, long-lasting parts that can control the powerful beams that lasers send out.

Most mirrors used in high-power continuous wave (CW) lasers today to direct the beam are made by layering thin coatings of materials with different optical properties. But if there is even one tiny flaw in any of the layers, the powerful laser beam will burn through, causing the whole device to fail.

If you could make a mirror out of just one material, there would be a lot less chance of problems and the laser would last longer. But what strong material could be used?

Now, scientists at Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) have made a mirror out of diamond, which is one of the strongest substances on Earth. By etching nanostructures into a thin sheet of diamond, the research team made a highly reflective mirror that could withstand tests with a 10-kilowatt Navy laser without getting damaged.

Marko Loncar, the Tiantsai Lin Professor of Electrical Engineering at SEAS and the paper’s senior author, said, “Our one-material mirror approach gets rid of the thermal stress problems that happen with traditional mirrors made of stacks of different materials when they are hit with a lot of light.” “This method could make high-power lasers better or open up new uses for them.”

Nature Communications has written about the research.

The technique was first made by Loncar’s Laboratory for Nanoscale Optics to etch nanoscale structures into diamonds for use in quantum optics and communications.

Haig Atikian, the first author of the paper and a former graduate student and postdoctoral fellow at SEAS, said, “We thought, why not use what we developed for quantum applications for something more classical?”

Using this method, which involves using an ion beam to etch the diamond, the researchers made a 3-millimeter-by-3-millimeter sheet of diamond into a surface with columns in the shape of golf tees. Because the golf tees are wide at the top and narrow at the bottom, the diamond’s surface is 98.9 percent reflective.

Neil Sinclair, a research scientist at SEAS and co-author of the paper, said, “You can make reflectors that are 99.999 percent reflective, but they have 10 to 20 layers. This is fine for low power lasers, but they wouldn’t be able to handle high powers.”

The team worked with people at the Pennsylvania State University Applied Research Laboratory, a U.S. Navy University Affiliated Research Center, to test the mirror with a high-power laser.

There, the researchers put their mirror in front of a 10-kilowatt laser, which is strong enough to burn through steel, in a room made just for that purpose and locked so that dangerous levels of laser light can’t leak out and blind or burn people in the next room.

The mirror came out without any damage.

Atikian said, “The most interesting thing about this research is that we put a 10-kilowatt laser on a 3-by-3-millimeter diamond and focused it on a 750-micron spot. This is a lot of energy focused on a very small spot, and we didn’t burn the diamond.” “This is important because laser systems are getting more and more power hungry, so you need to think of new ways to make the optical parts stronger.”

Researchers think that these mirrors will be used in the future for defence, making semiconductors, making things for industry, and talking to people in deep space. This method could also be used with cheaper materials, like fused silica.

Harvard OTD has protected the intellectual property of this project and is looking into ways to make money off of it.

Further information: Haig A. Atikian et al, Diamond mirrors for high-power continuous-wave lasers, Nature Communications (2022). DOI: 10.1038/s41467-022-30335-2

Journal information: Nature Communications

Source: Harvard John A. Paulson School of Engineering and Applied Sciences