According to the laws of physics, you cannot focus a laser beam to a size fundamentally smaller than its wavelength. That’s why researchers are constantly looking for ways to loosen up the way they interpret these laws – and Dr. Ulf Quentin, Assistant to the Managing Director International Sales and Services at TRUMPF, is no exception. He has succeeded in focusing laser light with a wavelength of 500 nanometers on a tiny spot measuring just 100 nanometers, structuring the substrate surface in the process.
The trick he uses is to place a tiny glass sphere with a one micrometer diameter on the substrate surface. He bombards this tiny sphere with ultrashort laser pulses, creating what is known as an ‘optical near field’ at the bottom of the sphere, as he explains: “An optical near field always occurs at an interface where light is transferred from one medium into another – in this case from the glass particle to the ambient medium close to the substrate surface.”
In the optical near field, the beam of light ‘re-forms’ itself and has different properties at the interface than it does in the ‘normal’ far field. “I exploit this effect to narrow the focus of the beam more than would normally be possible. That allows me to ablate an area with a diameter five times smaller than the wavelength of the laser light,” Quentin explains.
As well as incorporating small points in the substrate, Quentin also uses this method to ablate lines and areas. He does this by using a second, continuous laser beam as optical tweezers, guiding the tiny sphere over the substrate.
“In the future, people might use this technique to carve prototype circuits for microchips or gently cut open living cells in order to manipulate them,” says Quentin. “It’s another way for us to increase the already impressive precision of laser machining.”