Monolithic Multifocal Diamond Metalens for High-Power Laser Systems

Imagine you are trying to cook a giant pizza for a party. You could use one tiny oven, but it would take forever. So, you decide to use a fleet of tiny, super-fast ovens to cook the whole thing at once. This is the dream of "high-power laser systems" in the world of manufacturing and science: instead of burning a single hole in a material, they want to slice, drill, or weld many spots simultaneously to get the job done faster. But here's the catch: these laser ovens get incredibly hot. If you try to use a standard glass lens to focus all that energy, the glass acts like a sponge, soaking up the heat, warping, and eventually melting. It's like trying to hold a blowtorch with a pair of ice cubes; they just can't handle the temperature. Scientists have been looking for a material that is as tough as a diamond (literally) and as good at spreading heat as a metal, so they can build lenses that don't melt under the pressure of extreme light.

This is where a team of researchers from Westlake University and their partners stepped in. They decided to build a lens out of the hardest material on Earth: a single crystal of diamond. But they didn't just make a simple magnifying glass; they engineered a "metalens," which is a flat, ultra-thin surface covered in microscopic pillars that act like a team of tiny traffic cops, directing light beams exactly where they need to go. The team created a "monolithic" (meaning it's all one solid piece) diamond lens that can split a powerful laser into two beams at the same time. They tested it against a standard commercial lens and found that while the glass lens got hot, warped, and lost its focus, the diamond lens stayed cool, steady, and sharp. They even blasted it with a laser so powerful (8.25 kW) that it would have melted a glass lens in a second, and the diamond didn't even flinch. This work suggests that we might finally have a way to build laser tools that are small, simple, and tough enough to handle the extreme heat of industrial manufacturing without falling apart.

The Story of the Unmeltable Lens

The Problem: The Melting Glass
Think of high-power lasers as a river of pure energy. When you want to cut through tough materials like silicon carbide (a super-hard ceramic used in electronics), you need a lot of water—er, energy. But when that energy hits a lens, some of it gets absorbed as heat. In a normal glass lens, this heat builds up like steam in a closed kettle. The glass expands, warps, and the focus point drifts away, like trying to aim a water hose while the nozzle is twisting in your hand. To fix this, engineers usually stack bulky mirrors and special glass pieces together, but these are heavy, hard to align, and still struggle when the power gets too high.

The Solution: The Diamond Super-Lens
The researchers asked: "What if we built the lens out of diamond?" Diamond is famous for being hard, but it's also a thermal superstar. It conducts heat about five times better than silicon carbide and spreads it out so fast that it never gets a chance to build up and warp. The team carved a 7.2 mm wide diamond disk (about the size of a large coin) and etched thousands of tiny, truncated-cone pillars onto its surface. These pillars are so small (500 nanometers wide) that you need a microscope to see them, but they act like a sophisticated steering wheel for light.

Instead of just focusing the laser to one spot, this "multifocal" metalens splits the beam into two separate spots, separated by 200 micrometers. This allows the laser to cut two lines at once, doubling the speed of the work. The shape of the pillars is tapered (wider at the bottom, narrower at the top) to let the light pass through efficiently without bouncing back, achieving a 90% transmission rate.

The Test: The Heat Battle
To see if this diamond lens was truly tough, the team put it in a "death match" against a standard commercial lens equipped with a beam-splitter. They blasted both with a 25-watt laser for one hour.

• The Commercial Lens: It started to heat up, rising by 34.7°C. As it got hotter, the focus drifted wildly, moving by 121.9 micrometers. When they tried to use it to cut a groove into a silicon carbide sample, the depth of the cut changed drastically, going from 130.8 micrometers to 449.9 micrometers. It was like trying to write with a pen that keeps changing its ink flow.
• The Diamond Lens: It barely warmed up, only rising by 9.0°C. Because it stayed cool, its focus only drifted by 25.5 micrometers. When cutting the same material, the depth of the cut stayed incredibly consistent, varying by only 33.2 micrometers. It was like a laser scalpel that never lost its grip.

The Extreme Stress Test
But the team didn't stop there. They wanted to see if the diamond could handle a "nuclear" level of heat. They exposed the lens to a continuous-wave laser with a power of 8.25 kW for 30 seconds. For comparison, they tested a standard glass lens coated with titanium dioxide. The glass lens melted and failed in less than one second. The diamond lens? It survived the full 30 seconds with no visible damage, no melting, and no structural collapse.

They also measured how much energy it takes to damage the material (the Laser-Induced Damage Threshold, or LIDT). The diamond withstood 2.45 Joules per square centimeter, which is nearly four times better than the silicon carbide reference (0.65 J/cm²) and vastly superior to the commercial lens (0.2 J/cm²).

What This Means
This paper doesn't just show a cool diamond lens; it proves that we can make large, flat, transmissive lenses out of diamond that can handle the extreme heat of industrial lasers. By combining the hardness of diamond with the smart design of a metalens, the researchers have created a tool that is smaller, simpler, and far more stable than anything currently available. While there is still work to be done—like adding special coatings to the back of the lens to stop light from reflecting off the other side—this study opens the door for a new generation of laser systems that can work faster, hotter, and more reliably than ever before. It suggests that the future of high-power manufacturing might just be written in diamond.