Schematic
of femtosecond laser fabrication of a monolayer TMDC lens. Inset: (i) AFM image
of a monolayer TMDC single crystal, and (ii) Schematic of femtosecond
laser-induced generation of mox nanoparticles.
Courtesy: Han Lin, Zai-quan xu, Guiyuan Cao, Yupeng Zhang, Jiadong Zhou, Ziyu Wang,
Zhichen Wan, Zheng Liu, Kian Ping Loh, Cheng-Wei Qiu, Qiaoliang Bao, Baohua Jia
Lenses are
one of the most commonly used optical components in daily life, including
eyeglasses, microscopic objectives, magnifying glass, and camera lenses.
Conventional lenses are based on the principle of light refraction, using
different materials, spherical surfaces, and spatial positions to achieve the
control of light. The fabrication of conventional lenses including the
processes of material selection, cutting, rough grinding, fine grinding,
polishing, and testing. In order to minimize the aberrations including the
chromatic aberration, spherical aberration and astigmatisms, it is necessary to
stack multiple layers of lenses to form compound lenses, leading to the
complexity and cumbersomeness of current camera equipment.
Therefore, tremendous effort has been devoted into the development of ultrathin flat lenses. Unlike conventional lenses, flat lenses use nanostructures to modulate light. By controlling the optical properties and the spatial position of each nano-element, advanced functions, such as achromatic and aberration-free focusing, high spatial resolution and special focal intensity distributions can be achieved. However, when the material thickness is reduced to the subwavelength scale, the insufficient phase or amplitude modulation based on the intrinsic refractive index and absorption of the materials results in poor lens performance.
In a new
paper published in Light Science & Application, a team of scientists, led
by Prof. Baohua Jia at Centre for Translational Atomaterials, Swinburne
University of Technology, Australia, Prof. Qiaoliang Bao formerly at Monash
University, Prof. Chengwei Qiu at National University of Singapore and
co-workers have developed an innovative method to fabricate high performance
lenses in monolayer two dimensional transitional metal dichalcogenide (TMDC)
material by using a femtosecond laser to pattern nanoparticles. The lens has a
sub-wavelength resolution and a focusing efficiency of 31%, laying the
foundation for ultimately thin optical devices for use in nano-optics and
on-chip photonic applications.
Although
lenses made from multilayer TMDCs have been demonstrated before, when their
thickness is reduced to the sub-nanometer scale, their insufficient phase or
amplitude modulation results in focusing efficiencies of less than 1%. The
international team discovered that it is possible to generate nanoparticles by
using a femtosecond laser beam to interact with the monolayer TMDC material,
which is significantly different from the process produced by a continuous wave
laser. When the laser pulse is so short that the entire material remains cold
after laser process, the nanoparticles can firmly attach to the substrate. The
nanoparticles show very strong scattering to modulate the amplitude of light.
Therefore, the lens made from the nanoparticles can provide subwavelength
resolution and high efficiency, which allows the team to demonstrate
diffraction-limited imaging by using the lenses.
Monolayer
is the thinnest form of a material, which is the ultimate physical thickness
limit. By using the monolayer for the lens fabrication, the process
demonstrated in this study consumed the least material meeting the theoretical
limitation. More importantly, the femtosecond laser fabrication technique is a
one-step simple process, without the requirements of high vacuum or special
environment, thus it provides the simplest way to fabricate an ultrathin flat
lens. As a result, the lens can be easily integrated into any photonic or
microfluidic devices for broad applications.
“We have
used the thinnest material in the world to fabricate a flat lens, and prove
that the good performance of the ultrathin lens can lead to high resolution
imaging. It shows enormous potential in different applications, such as eyeglasses,
microscopy lenses, telescopes and camera lenses. It is foreseeable that by
using this technique, the weight and size of camera lenses can be significantly
reduced in the near future.” Said Dr Han Lin, the first author from the Centre
for Translational Atomaterials, Swinburne University of Technology.
“We are
excited to see unique outcome from femtosecond laser processing 2D materials.
It opens up new possibility to fabrication photonic devices using scalable
method.” Added by Prof. Baohua Jia, Director of Centre for Translational
Atomaterials.
“We can
integrate the monolayer 2D material lens onto desired devices by simply
attaching the material then using a femtosecond laser to perform fabrication.
The entire process is simple, and the method is flexible and low cost. Thus, we
also see the great application potential of the method.” Commented by Prof.
Qiaoliang Bao formerly at Monash University.
“We
design our lens in such a way that image can be found at different focal
planes, with different magnifications. This mechanism can be readily used to
develop an optical zoom lens that is required in all cellphone cameras.
Currently, lenses with different focal lengths are used to achieve different
zoom function. However, our lenses can achieve different zoom rates simply with
one design.” Prof. Chengwei Qiu from National University of Singapore
forecasts.