Photo-patterning micro-mirror devices using azo dye-doped cholesteric liquid crystals.

Abstract

A simple method for fabricating patternable micro-mirror devices by photo-induced alignment of dye-doped cholesteric liquid crystal (CLC) is demonstrated. The CLC texture can be changed from random distribution to nearly perfect planar by the photo-excited adsorbed dyes. This structure transformation leads to a substantial reflectivity increase. Using this photo-patterning technique, one- and two-dimensional micro-mirror arrays which function as gratings are also demonstrated.

Full text

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Photo-patterning micro-mirror devices using azo
dye-doped cholesteric liquid crystals
Tsung-Hsien Lin,1, 2 Yuhua Huang,1 Ying Zhou,1 Andy Y. G. Fuh,2and Shin-Tson Wu1
1College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816
2Institute of Electro-Optics, National Cheng Kung University, Tainan, Taiwan 701, ROC
swu@mail.ucf.edu

http:/lcd.creol.ucf.edu
Abstract: A simple method for fabricating patternable micro-mirror devices
by photo-induced alignment of dye-doped cholesteric liquid crystal (CLC)
is demonstrated. The CLC texture can be changed from random distribution
to nearly perfect planar by the photo-excited adsorbed dyes. This structure
transformation leads to a substantial reflectivity increase. Using this
photo-patterning technique, one- and two-dimensional micro-mirror arrays
which function as gratings are also demonstrated.
©2006 Optical Society of America
OCIS codes: (230.3720) Liquid-crystal devices; (050.1970) Diffractive optics
References and links
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1. Introduction
Cholesteric liquid crystal (CLC) is regarded as a one-dimensional photonic crystal because of
its self-organized chiral structure. A circularly polarized light with the same handedness as the
CLC propagating along the helical axis is selectively reflected. The central wavelength
and bandwidth
λΔ of the CLC reflection band are related to the pitch length p and the
refractive indices of the CLC layer as:
o
=<λ
the average refractive index (
2 / )(
oe
nn +
) and
Many applications of CLC devices for strong optical rotatory powers and selective reflection
of circularly polarized light [1, 2], reflective liquid crystal displays [3-7], and photonic crystal
lasers [8-13] have been proposed.
In this paper, we develop a simple photo-patterning method for controlling the reflectivity
of the dye-doped CLC micro-photonic devices. Initially, the dye-doped CLC sample is not
aligned so that it functions as a diffusive reflector. Upon laser exposure, the azo dye
molecules go through trans-cis transition and the photo-induced LC alignment takes place. As
a result, the CLC layer is reconfigured to a planar structure and its reflectivity increases
dramatically. By exposing the dye-doped CLC device with a green laser through a photomask,
both one- and two-dimensional gratings are fabricated. The device resolution is ~50 μm.
o
λ
pn
Δ
⋅
=
>
n
and
n
pn⋅Δ=
is the birefringence of the LC.
Δλ
n
, where
>< n
denotes
)(
oe
−
2. Device fabrication
A right-handed CLC sample was prepared by mixing 39 wt% CB15 chiral agent to a nematic
BL006 LC (from Merck). The herein employed azo dye is methyl red (MR, purchased from
Aldrich); its trans-state has an absorption spectrum spanning from ~400 to 540 nm. The
mixing ratio of MR dye to CLC is 1:99 wt%. Two indium-tin-oxide (ITO)-coated glass slides,
separated by 8 μm plastic spacers, were used to fabricate an empty cell without any surface
alignment treatment. The CLC and dye mixture was stirred thoroughly before being injected
to the empty cell. As depicted in Fig. 1(a), the helical axis of the dye-doped CLC is almost
randomly distributed. As a result, it acts as a diffusive reflector, i.e., the incident circularly
polarized light is diffused to a large cone.
θ θθ θθ θ

(a) (b)
Fig. 1. Schematic representation of the (a) imperfect CLC planar texture with nearly randomly
distributed helical directions, (b) photo-aligned planar CLC texture.
To reorient the CLC directors on the untreated surface to a desired direction, in principle,
we can illuminate the dye-doped CLC with a linearly polarized, continuous wave (CW) laser
whose wavelength is around the azo dye’s absorption peak. Under such a circumstance, the
azo dyes in the CLC sample undergo trans–cis isomerization, followed by molecular
reorientation, diffusion and finally adsorption onto the ITO surface. As a result, the long axes
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Received 13 March 2006; revised 2 May 2006; accepted 4 May 2006
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of the azo dyes are aligned along the untreated substrate of the CLC cell but perpendicular to
the polarization of the pumping beam. The adsorbed dyes then force the LC molecules on the
untreated surface to reorient along the direction of the azo dyes’ long axes [14-18]. The CLC
molecules on the untreated surface can be aligned along the direction of the azo dyes’ director
only when the adsorbed azo dyes establish sufficient surface anchoring force. This can be
achieved by irradiating the dye-doped CLC sample with a linearly polarized light for a long
enough time and sufficiently high intensity because the high laser dosage would induce the
adsorbed dyes to aggregate and then generate adequate anchoring force. When the front LC
molecules are forced to align parallel to the substrate, as shown by the red line in Fig. 1(b), the
CLC helical axes would be perpendicular to the substrate. However, to achieve a complete
Bragg reflection, at least 10 CLC layers need to be established [19]. A fewer planar CLC
layers would lead to a lower reflectivity.
3. Experimental
In our experiment, a linearly polarized CW laser from a diode-pumped solid state laser was
used to irradiate the dye-doped CLC sample because the laser wavelength (λ=532 nm) is
within the absorption band of the Methyl Red. The laser beam was expanded and collimated
to 2 cm diameter. After laser illumination, we used an experimental setup as depicted in Fig. 2
to measure the reflection properties of the photo-aligned dye-doped CLC sample. In Fig. 2, a
broadband white light was used as a light source. A pair of crossed polarizer and analyzer was
used to filter the surface reflections from the glass substrates. Reflection spectrum and
intensity of the CLC were measured using a spectrum meter and a photo diode, respectively.
Light
Source
Spectrum Meter/
Photo Detector Photo Detector
Aperture
PolarizerPolarizer
AnalyzerAnalyzer
CLC SampleCLC Sample
Light
Source
Spectrum Meter/
Aperture

Fig. 2. Experimental setup for measuring the reflection spectrum and intensity of the dye-doped CLC.
4. Results and discussions
Figure 3 shows a picture of the photo-aligned CLC taken by a digital camera. The green circle
(8 mm diameter) represents the region illuminated by a linearly polarized CW laser at
intensity I~50 mW/cm2 for 60 min. The surrounding is not exposed. Due to the
photo-alignment effect of the azo dyes, the excited azo dyes are adsorbed to the inner surface
of the front substrate to form a homogeneous alignment. The texture of the illuminated region
is transformed from random to nearly perfect planar, as Fig. 1 depicts. Therefore, a strong
Bragg reflection from the exposed region is observed.
8 mm8 mm

Fig. 3. A photo of dye-doped CLC cell with (green circle) and without laser illumination.
We also compared the texture of dye-doped CLC with different irradiation time. Figure 4
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presents the microscopic textures of the photo-aligned CLC which were observed under a
reflection-type polarizing optical microscope. Figure 4(a) shows the texture of unexposed
dye-doped CLC. Random structure is observed and its reflectance is fairly low. Figures 4(b),
4(c) and 4(d) represent the dye-doped CLC with 20 min, 40 min, and 60 min exposure time,
respectively. Initially, the azo dye molecules are distributed randomly in the cell. Those
dichroic dyes whose principal molecular axis parallel to the pumping laser’s polarization
would be excited more easily than the rest dyes. Thus, different molecular domains would
have different optical thresholds for the CLC reorientation to occur. As the exposure time
increases, more planar domains would appear. After 60 min of laser illumination, almost all
the exposed regions are converted to planar texture as shown in Fig. 4(d).
100 μ μm100 μ μm
(a)(a)
(b)(b)
(c)(c)
(d) (d)

Fig. 4. The microscopic textures of a photo-aligned CLC observed under a reflection optical
microscope with (a) 0 min, (b) 20 min, (c) 40 min and (d) 60 min illumination time.
0%0%
10%10%
20%20%
30%30%
40%40%
50%50%
450450 500500 550550 600600 650650 700700
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Reflectivity (%)
60min
40min
20min
10min
0min0min0min
Perfect planar Perfect planar
Reflectivity
Reflectivity (%)
60min
40min
20min
10min10min
60min
40min
20min
Reflectivity

Fig. 5. Reflection spectrum of the photo-aligned dye-doped CLC at different laser exposure time.
Figure 5 shows the measured reflection spectra of the photo-aligned dye-doped CLC with
different exposure time. The blue line represents the reflection spectrum of a perfect planar
CLC using a homogeneous cell. The peak reflectivity reaches the theoretical value, which is
~50% for a right-handed circularly polarized incident light. The bottom black line represents
the unexposed dye-doped CLC sample. Because the CLC directors are randomly distributed,
the reflectivity of the cell is nearly vanished at the reflection angle (θ~30o in this experiment).
As explained above, as the illumination time increases more planar domains are generated and
the reflectivity increases. The reflectivity of the dye-doped CLC sample (with 60 min
exposure) reaches ~80% level of a perfect planar structure. During such a long exposure time,
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the LC and dye molecules could diffuse away so that the resolution is degraded. The exposure
time could be reduced to ~5 min. if the ITO-glass is coated with a polymer layer [16].
We also measured the reflected light intensity of the photo-aligned dye-doped CLC.
Figure 6(a) plots the exposure time dependent reflectivity of the CLC sample. The reflected
light is collected at the reflection angle and its intensity normalized to that of a perfect planar
CLC. As the laser exposure time increases, more planar CLC domains appear which, in turn,
enhance the reflectivity. When the exposure time gets longer than 60 min, the reflectivity
reaches above 80% and gradually saturates. It indicates that the exposed region in the sample
has been converted to multi-domain planar structure. However, some defects between the
boundaries of multiple domains are still present, as Fig. 4(d) shows. As a result, the
reflectivity does not reach 100%.
In order to determine how fine the photo-alignment process is, we exposed the sample
through a commercially available resolution target. It has several scale groups as Fig. 6(b)
shows. We can determine the resolution of an optical storage film by illuminating a uniform
intensity beam through the resolution target which is in proximity contact with the storage
film. The smallest but distinguishable spacing between two adjacent lines of the group image
defines the resolution. The collimated CW green laser beam with ~50 mW/cm2 intensity was
incident normal to the sample through the resolution target. Figure 6(b) shows the image
observed from a reflection polarizing optical microscope. The resolution of the photo-aligned
dye-doped CLC sample is better than 50 μm. In this experiment, the resolution target was put
on top of the CLC cell. That means, the resolution target and the CLC film are separated by a
glass substrate, which is ~0.5 mm. Thus, the beam passing the resolution target will be
diffracted before reaching the CLC film [20]. To improve resolution, a thinner top glass
substrate or a more collimated exposure system can be considered.
0% 0%
20% 20%
40% 40%
60% 60%
80%80%
100%100%
00 1010 2020 303040 4050 5060 6070 70
Exposure time, minExposure time, min

ReflectivityReflectivity

(a) (b)
Fig. 6. (a) Reflectivity of the photo-aligned dye-doped CLC with different illumination time.
100% stands for the reflectivity of a perfect planar CLC. (b) Resolution of this sample observed
under a reflective polarizing optical microscope.
Based on the above-mentioned mechanism, we demonstrated two kinds of micro mirror
arrays using photo-aligned dye-doped CLC. The first type is a 1D reflection grating. To
fabricate such a grating, we irradiated the CLC sample using a green CW laser (I~50 mW/cm2)
through a grating photomask. The grating spacing is about 100 μm. After laser exposure, a
photo-aligned planar grating was formed. Figure 7(a) shows the image of the photo-aligned
CLC under a reflective polarizing optical microscope. The bright lines (high reflectance) are
the exposed regions where the nearly perfect planar textures are established. On the other
hand, the darker regions originate from the random CLC textures, as described in Fig. 1(a).
Figure 7(b) shows the diffraction patterns of this photo-aligned CLC reflective grating. Four
diffraction orders are observed clearly and the 1st order diffraction efficiency is ~10%.
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Received 13 March 2006; revised 2 May 2006; accepted 4 May 2006
15 May 2006 / Vol. 14, No. 10 / OPTICS EXPRESS 4483