Immersion Lens Focuses Radially Polarized Beam for Laser Nanoprocessing

High spatial resolution in ultrafast laser processing is increasingly important in semiconductor fabrication, the automotive parts industry, medical device manufacturing, and other fields. In laser processing, precision and spatial resolution are primarily influenced by the focal spot size of the laser beam. The diffraction of light typically limits the achievable spot size, depending on the numerical aperture (NA) of the lens and the wavelength of the focused laser beam.

A research team at Tohoku University investigated the use of radially polarized laser beams, also known as vector beams, to enhance processing accuracy and resolution in ultrafast laser processing. A radially polarized beam generates a longitudinal electric field at the focus spot. Compared to conventional beams with linear or circular polarization, a radially polarized beam produces a small focal spot, especially when it is tightly focused using a high-NA lens.

Although radially polarized beams show promise for increasing precision and resolution in laser nanoprocessing, the radially polarized beam’s longitudinal field has been found to weaken inside the material, due to light refraction at the air-material interface.

A conceptual illustration of single-shot laser processing by an annular-shaped radially polarized beam, focused on the back surface of a glass plate. Courtesy of Y. Kozawa et al.

To better understand how the interface affects the longitudinal field at the focus, the researchers implemented a single-shot laser ablation of a transparent glass sample using a radially polarized beam that was tightly focused on the surface of the glass. They examined the impact of the boundary conditions at the interface under high-NA conditions.

When the researchers focused the radially polarized beam on the back surface of the glass from the inside, using an immersion lens, the longitudinal electric field at the focus was significantly enhanced. The researchers were able to produce a small focal spot, which they attributed to the enhanced longitudinal field on the glass surface, and enable direct laser processing.

The researchers used an oil immersion objective lens like those found in biological microscopes, professor Yuichi Kozawa said. “Because the immersion oil and glass have nearly identical refractive indices, the light that passes through them does not bend,” Kozawa said.

By focusing a radially polarized beam on the back surface of the glass from the inside, the researchers could enhance the longitudinal field even inside the material, directly inducing a light-matter interaction.

The researchers created a small focal spot using an annular-shaped, radially polarized beam. The small focal spot formed by the beam’s longitudinal field allowed the researchers to fabricate a fine, spot-shaped, 67-nm ablation hole, about 1/16 of the laser beam’s wavelength, through total internal reflection.

Fabrication of an ablation crater with a size corresponding to about 1/16 of the wavelength by single-shot laser irradiation of the back surface of a glass with an annular-shaped, radially polarized beam. Courtesy of Y. Kozawa et al.

The experimental results demonstrate a potential approach to shrinking the scale of laser material processing and achieving laser nanoprocessing using a radially polarized beam. The findings could advance the development of laser ablation processes using radially polarized beams in high-NA environments.

Laser machining is widely used across industries to produce electronic components, machine parts, precision machinery, and medical devices. Ultrashort laser pulses, with widths from picoseconds to femtoseconds, enable precise processing at the μm scale, but laser machine processing below 100 nm is challenging to achieve with existing methods.

“This breakthrough enables direct material processing with enhanced precision using the enhanced longitudinal electric field,” Kozawa said. “It offers a simple approach to realize processing scales below 100 nm and opens new possibilities for laser nanoprocessing in various industries and scientific fields.”

The research was published in Optics Letters (www.doi.org/10.1364/OL.517382).