Researchers Find New Way To Improve Laser Processing Resolution

Recently, a team of researchers from Tohoku University in Japan has successfully used a customized radially polarized laser beam to focus on the inside of a material to produce tiny spots of light, which in turn significantly improves the resolution of laser material processing.
This innovative approach, detailed in the journal Optics Letters, revolutionizes laser processing technology.
Laser processing technology plays a vital role in a number of industries, including automotive, semiconductor and pharmaceuticals, especially in precision machining such as drilling and cutting. Although ultrashort pulsed laser sources have been able to achieve precise processing at the micron to tens of microns scale, modern industry and scientific research have seen a growing demand for smaller scale processing, with accuracy below 100 nanometers becoming an insurmountable obstacle for current technology.
Researchers at Tohoku University focused on radially polarized laser beams, special vector beams that generate longitudinal electric fields at the focal point, resulting in a smaller spot than conventional beams. Although this property shows great processing potential, photorefraction at the air-material interface causes the spot to weaken inside the material, limiting its application.
To overcome this challenge, the research team creatively employed the oil immersion objective technique, which is commonly used in biomicroscopy. By applying the oil-immersion objective to a laser-processed glass substrate, the light does not bend as it passes through the immersed oil and glass because the oil and glass have similar refractive indices, thus ensuring spot stability and precision.
The researchers further delved into the behavior of radially polarized beams and found that the longitudinal field is greatly enhanced when the beam is focused and combined with a ring display. This enhancement effect results from the high convergence angle total reflection at the glass-air interface. Using this annular radially polarized beam, the team succeeded in creating a tiny focal point.
They then applied the technique to the processing of glass surfaces with an ultrashort pulsed laser beam. The converted pulse is fired once at the back of the glass substrate to create a 67-nanometer-diameter hole in the material, a size that is about 1/16th of the wavelength of the laser beam, significantly improving processing accuracy.
This breakthrough not only improves the precision of direct material processing using an enhanced longitudinal electric field, but also provides us with a simple way to realize processing scales of less than 100 nanometers," said Yuichi Kozawa, an associate professor at Tohoku University's Institute for Multidisciplinary Research in Advanced Materials (IMRAM), and a co-author of the paper. This will open up new possibilities for laser nanofabrication in various industrial and scientific fields."