We present a technique to improve microlens arrays (MLAs) uniformity after the thermal reflow process. Traditional
photo resist thermal reflow processes cause micro lenses merge together easily due to an inexact reflow time and
temperature distribution. This results in poor uniformity and low lens height. A new MLAs fabrication method, called
the boundary-confined method, was proposed and demonstrated. By two tones of photoresist (PR), positive and negative,
only one photo mask and two photolithography steps are needed in the process. After lithography processes, the positive
PR is a slightly little smaller than the circular pattern on a photo mask and negative PR is slightly larger than it. Two
tones of PR increase tolerance to mask alignment. Fill-factor is high because of high resolution on a thin boundary. All
of flowing PR is stopped by the boundary; uniformity is improved without tight thermal dose constrains. Meanwhile,
microlenses with a large height are achievable due to "no cling" effect. The method has advantages, not only for large
area MLAs but also for a microlens that require precision diameter or positioning. Besides, we replicate MLAs with the
optical polymer to verify some optical specifications. Both the fabrication and replication are straightforward and
reliable. Our results show that the microlens is approximately a hemispherical profile. The gap between microlenses with
48 μm diameter in hexagonal arrangement is 2 μm and the height of microlens is 22 μm.
Adaptive optics (AO) is a technology which improves the performance of optical systems by reducing the effects of
rapidly changing optical distortion. Wave front active control by combining wave front sensor and MEMS deformable
mirrors which made of polyimide thin film actuated by electrostatic force is one possible solution. Wave front sensor
detects the image and aberration could be described with Zernike polynomials, and the distorted wave-front is corrected
by deformable mirror. Combining these two technologies, we fabricate a large-scale MEMS deformable mirror with a
20mm diameter circular opening and 67 hexagonal actuation electrodes in this thesis. Moreover, we use commercial
software, Ansys, to simulate the deformation behavior of the membrane with different electrodes applied and give some
device parameter tuning for versatile application. We measure the maximum stoke is 39 um as 195 volts applied to 67
electrodes. Due to the large-scale of our thin membrane, resonant frequency is around 8 Hz. Besides, we also discuss
some possible ways to improve device characteristics and we think deformable mirror has a good potential for wave
front active control based on our experiment results.
Miniaturization is the key point to design image system for portable devices. Motor-driven lens technique is the
traditional way to achieve auto-focus and zoom functions. Nevertheless, systems with moving parts are difficult to
downscale. A solution presented in this paper has more potential to downscale and maintain high optical performance
simultaneously by combining reflective optics and MEMS deformable mirror. The system we designed with MEMS
deformable mirror is a 2M pixel auto-focus image system which is only 5.4mm in thickness before packaging and
6.7mm in thickness after packaging.
Light-emitting diodes (LEDs) are very popular light sources in the market currently because of their numerous
advantages such as high efficiency, long life time, wide color gamut and cheap production costs [1]. For many
applications, such as illumination tasks, backlight modules and projector light sources, a homogeneous illumination of
the entrance pupil is desired. Because of LED's high brightness, we can combine the homogeneous illuminance areas in
the target plane together as a backlight module for LCD display to solve unreadable problems in the portable devices
under the sun. Mostly, the typical light distribution of a LED shows distinct Lambertian profile which is not suitable for
the applications. To achieve a better adapted beam profile, an optical system with beam shaping qualities can be used.
We design an optical system consisting of refractive microlens arrays and reflection-tubes that collimates and
homogenizes LED's beam [2]. The smaller angle of beams, the higher contrast ratio we will get. And the design is less
than 10mm in thickness, we can use it in many ways, such as backlight module of cell phones and head-up displays
(HUDs). We believe this technology has advantages and provides us a high-brightness display to read information easily.
In this paper, we report a technique for microlens array (MLA) fabrication based on lithography and thermal reflow
process. The boundary of MLA was defined by a thin photoresist (PR) layer. Then, the second thick PR cylinders were
patterned inside the micro-holes from the first PR layer. MLA was formed after reflowing the second think PR cylinders.
In the previous processes, the gaps between the thick PR cylinders were large due to the diffraction effect. To increase
the fill-factor, some residual PR between the cylinders makes the PR flow outward. But the PR cylinders merge together
easily due to inexact reflow time and temperature distribution. It results in small radius of curvature and low uniformity.
In our work, the first thin PR became a limiting wall so that no cylinders merge together even at over reflow time.
Therefore, both the uniformity and the radius of curvature can be well controlled. Besides, the gap is small and the fill-factor
is very high due to the diffraction effect is not significant at first thin PR layer. The results show that the gaps are
only 2 um and the radius of curvature is approximately 24 um for MLA with 50 um in pitch and hexagonal arrangement.
In this paper, we described an engineered enhancement of optical absorption in an organic photovoltaic cell via the
excitation of surface plasmon resonance (SPR) in spherical Au nanoparticles deposited on a device surface. We
deposited gold nanoparticles with 5 nm in diameter on ITO (indium tin oxide) glass substrates and then coated poly
(styrenesulfonate) / poly (2,3-dihydro-thieno-1,4-dioxin) (PEDOT) which can reduce the work function of metal atoms
and poly {2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene} (MEHPPV) with Fullerene(C60) mixtures that work
as p-type and n-type organic semiconductors, respectively. By using Mie theory simulation, we could predict SPR
resonance peak of gold nanoparticles and we assume the enhancement in electromagnetic field absorption within a
device results in increased photocurrent response in organic p-n junction diodes with gold nanoparticles. Compared with
the same organic photovoltaic structures without gold nanoparticles, the proposed device shows about 40 % power
efficiency improvement under halogen illumination. Experimental results agree well to the prediction of simulation,
which showed SPR can be applied to enhance optical absorption of organic materials.
In this paper, we report a compact 1x2 MEMS optical switch actuated by less than one voltage. Over past few years, micro-electro-mechanical systems (MEMS) have emerged as a leading candidate for achieving true all-optical multi-wavelength network. Due to the inheritance of mechanical vibration in the under damping condition, most MEMS switches might need external control circuit or mechanical stoppers to suppress the undesired jittering of optical signal. To overcome this difficulty, we redesigned a MEMS optical switch monolithically integrated with a vertical micromirror with large surface area. The micromirror is made by the low-cost anisotropic wet etching technique. The size of the micromirror is 500 μm x 1200 μm, which is large enough to accommodate the optical beam diameter of 62 μm from a ball lens fiber. Because the moving direction of the MEMS optical switch is perpendicular to the surface normal of the micromirror, the effect of the mechanical vibration is compensated by the large surface area of the micromirror. In addition, a capacitor of 100 μF is connected in parallel with the MEMS switch to serve as a low pass filter. The switching time can be improved in the 5 ms range. Finally, a calculated driving waveform derived from the square wave is used as the triggering signal, after analyzing the switching performance by Fourier transform function. Our experimental results show the rising time and fall time of the MEMS switch is 3 ms and 3.1 ms, respectively. The optical insertion loss caused by the micromirror is 0.45 dB. The actuation voltage is around 0.3 volt and the switch is actuated by electromagnetic force. The footprint of the packaged device is 18 mm x 12 mm. A low cost, small size and high performance optical switch is experimentally demonstrated.
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