Femtosecond laser processing of Sapphire moth-eye structures
Electromagnetic waves with frequencies up to 300 GHz (millimeter waves) are expected to be the main frequency band used in the coming 6G communication, and research and development will be actively promoted in the future. In the field of radio astronomy, the frequency of the cosmic background radiation belongs to this frequency band, and there is a need for optical elements (lenses, windows, waveplates, etc.) to control the electromagnetic waves in this frequency band. ICCPT are developing elements to control such electromagnetic waves using femtosecond laser processing technology in collaboration with the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) at the University of Tokyo.
The prevention of electromagnetic wave reflection is one of the important functionalities required, and a structure called a moth-eye, which is a periodic array of microscopic protrusions with the size of a wavelength, is known as a structure that suppresses reflection in a wide frequency range. However, moth-eye structures for millimeter-wave applications are difficult to fabricate using conventional processing methods because the characteristic structure size is on the order of several hundred meters. We have succeeded in fabricating millimeter-wave moth-eye structures on sapphire substrates by using a femtosecond laser with a central wavelength of 1030 nm, pulse width of 290 fs, and average power of 15 W as a light source and manipulating the beam spot with a galvanometer mirror (Fig. 1). A moth-eye structure with a height of about 2 mm and a period of about 540 m has been fabricated uniformly over a wide area. Figure 2 shows the electromagnetic wave transmission spectrum and simulation results of a sapphire plate with moth-eye structures fabricated on both sides. The transmittance is almost unity over a wide frequency range, i.e., no reflection, which is in good agreement with the result expected from the simulation.
We are now developing a technique to fabricate large-area sapphire moth-eye structures on the order of several tens of centimeters by further increasing the average power of the femtosecond laser. In the future, we aim to install the moth-eye structure fabricated by this technology in a satellite for observing the polarization state of the cosmic microwave background radiation.
Figure 1: Laser microscope image of the fabricated sapphire moth-eye structure.
Figure ２: Transmittance spectrum of the fabricated moth-eye structure (blue dots: experiment, red line: simulation)
Published paper:
R. Takaku, S. Hanany, H. Imada, H. Ishino, N. Katayama, K. Komatsu, K. Konishi, M. Kuwata-Gonokami, T. Matsumura, K. Mitsuda, H. Sakurai, Y. Sakurai, Q. Wen, N. Y. Yamasaki, K. Young, J. Yumoto,
"Broadband, millimeter-wave anti-reflective structures on sapphire ablated with femto-second laser",
Journal of Applied Physics, 128, 225302 (2020).
