Northeastern University of Japan: Breakthrough Laser Technology for Nanoscale Laser Processing

In the fields of optics and micro/nano processing, precise manipulation of lasers to meet the growing demand for miniaturization is an important challenge in driving the development of modern electronic and biomedical equipment. Recently, researchers from Tohoku University in Japan successfully demonstrated the use of interference technology to enhance the longitudinal electric field of radially polarized beams, in order to improve the accuracy of laser ablation technology and achieve fine feature processing with a diameter of less than 100 nm. This technology not only has potential application value in the field of semiconductor manufacturing, but also may revolutionize micro operations in the medical field, while providing a new method for achieving nanoscale accuracy in laser processing technology.

Figure 1: Researchers shape a laser beam to induce total internal reflection, focusing radially polarized light from the laser onto a very small point on the far side of the glass sheet, thereby enhancing its longitudinal electric field

Reducing the size of laser focus: achieving fine-grained feature processing
Using laser pulses of several hundred femtoseconds (10-15 seconds), micrometer level features can be finely carved without generating much heat. However, modern electronic and biomedical devices typically require fine-grained features of 100 nm or lower. Meeting these size requirements is mainly achieved by reducing the size of the laser focal point, however, this goal is often constrained by the wavelength of the laser and the numerical aperture of the lens used for collimating light.

One method to reduce the size of the spot is to use a radially polarized beam, where the electric field vector of the beam is all directed towards its center. This type of beam can improve resolution by generating a longitudinal electric field at the focal point, which is an improvement compared to traditional linear or circularly polarized light. Radial polarization has been applied in a specific form of microscopy technology, which also has the potential to improve ultrafast laser processing technology.

However, generating a sufficiently strong longitudinal electric field at the interface between different materials is a major challenge. The strength of the longitudinal electric field varies with the square ratio of the refractive index of the two materials involved. Therefore, when light enters materials such as glass from air, its strength may be severely weakened.

Innovation focus strategy: radially polarized beams and interference enhancement
Recently, Yuichi Kozawa and colleagues from Tohoku University in Japan demonstrated how to overcome this problem by focusing radially polarized beams on the far surface inside transparent materials, rather than the closer outer surface. They also enhance the intensity of the longitudinal field by utilizing the interference between the incident and reflected waves.

The researchers first studied the changes in the longitudinal field intensity and spot size of radially polarized beams when focused through a high numerical aperture lens through computer simulation. They found that in the air, the intensity reached its peak at the focal point, forming a compact circular spot. The simulation results show that if the beam is focused on the surface of the glass block, the difference in refractive index will reduce the intensity and form a circular spot.

However, Yuichi Kozawa and his team demonstrated their ability to restore smaller spot sizes by focusing light on the far side of the glass and placing the lens in oil. Due to the same refractive index of oil as glass, the influence of the upper interface is removed, its boundary conditions are eliminated, and the intensity of the longitudinal field is regained.

These simulation results were validated through experiments, using a laser pulse of approximately 300 fs with a wavelength of 1040 nm, which was focused on the front or back of a borosilicate glass plate after passing through a segmented half wave plate. Using three different lenses and azimuthal light, they found that only when using a radially polarized beam and focusing the pulse on the back of the glass plate with a lens with a numerical aperture of 1.4, circular ablation pits with a diameter of approximately 200 nm could be generated instead of circular pits.

Figure 2: Using a single laser irradiation on the back of the glass, a circular radially polarized beam is used to create an ablation pit with a size of approximately 1/16 of the wavelength
Enhancing longitudinal field intensity: improving spatial resolution in laser processing
In addition, researchers also explored how to enhance the longitudinal field intensity by maximizing the interference between the incident wave and the wave reflected from the back of the glass. As explained in their paper, this phenomenon occurs when all light rays reflect rather than refract, i.e. when the beam is incident at a critical angle of glass and air. Therefore, they reasoned that it should be possible to achieve this by shaping the beam profile into a narrow ring to limit the angle of the incident wave.

Through experiments, they confirmed this idea. In the experiment, they used a spatial light modulator to transmit laser pulses to generate circular patterns with different parameters, then applied radial polarization to these pulses and focused them on the far end of the glass plate. The experimental results indicate that the annular parameters can vary within a range of values and can still generate ablation pits in the shape of light spots. But the experiment also confirmed that the minimum spot can only be obtained when these parameters are close to the values required to achieve the critical angle. The width of the spot is only 67 nm, which is about 1/16 of the laser wavelength.

Yuichi Kozawa and colleagues believe that these results demonstrate that the magnitude of ablation characteristics can be controlled by manipulating the longitudinal electric field of a radially polarized laser beam. Therefore, they stated that this work has the potential to improve the spatial resolution of laser processing and achieve nanoscale processing technology.

Researchers explain that this idea of utilizing total internal reflection can be applied to many existing technologies that involve focusing laser beams on the far surface of transparent materials. They added that by changing the laser wavelength, this method can also be applied to other materials, such as processing silicon with lasers of 1100 nm or longer.

Source: Sohu