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).
