Lens-on-lens microstructures
QING YANG,
1,2
SIYU TONG,
1,2
FENG CHEN,
1,3,
*ZEFANG DENG,
1,3
HAO BIAN,
1,3
GUANGQING DU,
1,3
JIALE YONG,
1,3
AND XUN HOU
3
1
State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China
2
School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China
3
Key Laboratory of Photonics Technology for Information, School of Electronics & Information Engineering, Xi’an Jiaotong University,
Xi’an 710049, Shaanxi, China
*Corresponding author: chengfeng@mail.xjtu.edu.cn;
Received 18 September 2015; revised 22 October 2015; accepted 22 October 2015; posted 22 October 2015 (Doc. ID 249778);
published 10 November 2015
Microlenses with multiple focal lengths play an important
role in three-dimensional imaging and the real-time detec-
tion of unconfined or fluctuating targets. In this Letter, we
present a novel method of fabricating lens-on-lens micro-
structures (LLMs) using a two-step femtosecond laser wet
etching process. A 3 × 3 LLM array was made with a diam-
eter of 129.0 μm. The fabricated LLM has two focal lengths,
80.4 and 188.7 μm, showing excellent two-level focusing
and imaging abilities. Its size and focal length can be
controlled by adjusting laser power and etching time. Its
surface roughness remains about 61 nm. This simple and
efficient method for large-scale production of LLMs has
potential applications in diverse optical systems.
© 2015
Optical Society of America
OCIS codes: (220.0220) Optical design and fabrication; (230.0230)
Optical devices; (220.3630) Lenses; (140.3330) Laser damage.
http://dx.doi.org/10.1364/OL.40.005359
Microlenses are crucial optical devices because of their extensive
applications in optical sensing technology, optical waveguides,
optical fiber coupling, artificial compound eye structures,
micro-manufacturing, biochemical systems, and lab-on-a-chip
systems [1–7]. In the recent years, microlenses with special
structures have played more important roles than simple lenses
in many applications; an example of the former type is the
Fresnel microlens, which can be integrated with other optical
components, owing to its nearly flat surface [
8]. An elliptic-
cone-shaped microlens offers great advantages in efficient cou-
pling between high-power laser diodes and single-mode fibers
[9]. Cylind rical microlens arrays can increase the luminous cur-
rent efficiency of the OLED panel and make the spectrum of
the OLED panel more insensitive to the viewing angle [
10].
In this Letter, based on the femtosecond laser wet etching
technology [
11,12], we propose a two-step femtosecond laser
wet etching method to fabricate a concave lens-on-lens micro-
structure (LLM). The fabricated LLM has two focal lengths,
which has potential applications in optical storage [
13,14], ex-
tended depth of focus of a laser beam [
15], real- time detection
of the unconfined or fluctuating targets [
7], etc. By adjusting
laser power and etching time, we obtained LLMs with both
changeable size and focal length.
A convex LLM array was simply achieved on polydimethyl-
siloxane (PDMS) and polymethylmethacrylate (PMMA) surfa-
ces through a replica molding process. The morphology and
three-dimensional (3-D) profiles of the LLMs were measured
by a scanning electron microscope (SEM) and confocal laser
scanning microscope (CLSM). The optical performance of the
LLMs was also tested, and the results presented below show the
excellent imaging and focusing abilities of the LLMs.
The fabrication process of the LLMs, as shown in Fig.
1,
includes first femtosecond laser irradiation, first buffered HF
wet etching, second femtosecond laser irradiation, second buf-
fered HF wet etching, and replica molding. First, a square-
arranged crater array was induced on polished silica glass chips
using femtosecond laser pulses (800 nm, 50 fs, 1 KHz) with a
power (P) of 5 mW focused through an objective lens, which
had a numerical aperture of 0.5 [Fig.
1(a)]. The laser power and
irradiation time could be controlled by a variable density filter
and a fast mechanical shutter. Then, the irradiated sample
was put into a 3% hydrofluoric (HF) acid solution at room
temperature. After buffered HF wet etching, the irradiated cra-
ters were transformed into concave microlenses [Fig.
1(b)]. An
ultrasonic bath was necessary to ensure conformity and high
speed by removing the bubbles generated at a liquid–solid in-
terface during the chemical etching process. Subsequently, the
femtosecond laser with a lower power (3 mW) irradiated the
center of the bottom of the concave microlenses for the second
time [Fig.
1(c)]. Then, the sample was treated again with a 3%
HF acid solution at the same temperature [Fig. 1(d)]. Finally,
the concave LLMs were formed. Further, convex LLMs on
PDMS and PMMA surfaces could be obtained by a simple rep-
lica molding process [Figs.
1(e) and 1(f)].
To investigate the formation process of the LLMs, a series of
SEM pictures were taken at different times during the process, as
shown in Fig.
2. Figure 2(a) shows the SEM image of an
untreated crater irradiated by the femtosecond laser, which
reveals that some nanostructures exist in the crater. These
structures are considered to be Lewis bases that accelerate the
Letter
Vol. 40, No. 22 / November 15 2015 / Optics Letters 5359
0146-9592/15/225359-04$15/0$15.00 © 2015 Optical Society of America