Electrically Controllable Polarization Independent Two-Way Variable Optical Attenuator Based on Polymer-Stabilized Cholesteric Liquid Crystal

An electrically controllable two-way variable optical attenuator (VOA) has been demonstrated by sandwiching a &#61548;/4 film between two polymer-stabilized cholesteric liquid crystal films. The VOA is polarization independent and both the reflectivity and transmittance are varied with the variation of the applied voltage. With the increase of the applied voltage, the transmittance is increased and the reflectivity is decreased. The maximum extinction ratio is -17 dB and -16.5 dB for the transmittance and reflection modes, respectively and the rising and decay time is around 2 ms and 15 ms, respectively.


Introduction
Cholesteric liquid crystals (CLCs) are very unique materials that exhibit a self-organized periodic helical structure. They can be fabricated by doping chiral molecules in a liquid crystal host. Since liquid crystal (LC) is a highly birefringent medium, the periodic helical structure gives a periodic modulation of the refractive index. Consequently, a one-dimensional photonic band gap (PBG) is established with the central wavelength at np = λ , where p is the helical pitch and n the average refractive index. Compared to a conventional onedimensional photonic crystal, CLCs exhibit many unique properties including supramolecular helicoidal periodic structure (the period can be set in a wide range from 100 nm up to infinity), 100% selective reflection of a circularly polarized light, and the ability of shifting the selective reflection wavelength by external factors including electric, magnetic, acoustic fields, temperature, and light irradiation. [1][2][3][4][5] Because of these properties, CLCs have attracted great interest for many applications, such as optical switch, mirrorless laser, bi-stable display, attenuator, etc.
Normally, CLCs reflect the incident light with the same handedness as its helical structure and the wavelength within its reflection band without applied voltage and transmit the incident light as the applied voltage is above a certain value (threshold voltage). However, when the applied voltage is below the threshold voltage, the conventional CLCs exhibit focal conic structure. As a result, the incident light will be scattered. Therefore, most of the current applications based on CLCs can not provide continuously variable gray scale. Even though some devices with variable transmittance, such as variable optical attenuator, has been developed using CLC, 6 the variation of the transmittance is based on the scattery induced by the CLCs' focal conic structure. However ， a scattery based devices has many shortcomings because all the light energy is actually passed through and the light is just scattered to different directions. This dramatically limits its applications.
In this paper, we developed a polymer-stabilized CLC whose band reflection is decreased while transmittance is increased with the increase of the applied voltage.
Based on this property, we demonstrated electrically controllable two-way VOA which is polarization independent by sandwiching a λ/4 film between two CLC films.

Sample Preparation
The CLC mixture was prepared by mixing 80 wt% E44 (Merck LC mixture) with 14 wt% of a left-handed chiral dopant ZLI-811 (also from Merck) and 6% monomer RM82 (from Merck). The mixture was stirred in isotropic phase for ~4h to make the constituents uniformly mixed and then capillary filled in the isotropic phase into a 20 μm thick LC cell. Inside the cell, both the glass substrates were coated with a thin polyimide alignment layer. The anti-parallel rubbing-induced pretilt angle is ~3 o . After the temperature was gradually cooled down to room temperature, a CLC sample with left-handed (LH) helix was formed. Then, we illuminate the UV light to cure the sample. Using this method, we fabricated two pieces of Then, we sandwiched a λ/4 film between the two CLCs. The configuration of the VOA is illustrated in Fig.1  the transmitted and reflected light was coupled to a optical fiber-1 and fiber-2, respectively, and then input to an optical spectrum analyzer (OSA)

Results
First, we measured the reflection spectrum of the sample at different voltage. The results are plotted in Figure 3 (a).
We can see that the sample exhibits a reflection band from 1450-1650 nm. At V = 0 V, the reflectivity is around -1.5 dB As the applied voltage is increased, the reflectivity is decreased. When the applied voltage is increased to 360V, the reflectivity is around -16.5 dB. Fig.3 (b) shows the extracted reflectivity of the sample as a function of the applied voltage.  As shown in Fig.4 (a), the transmittance of the sample is -17 dB without applied voltage. With the increase of the applied voltage, the transmittance is increased. When the applied voltage is increased to 360 V, the transmittance is increased to -2 dB. see that as the applied voltage is increased, the reflectivity is decreased while the transmittance is increased. This indicates that the loss in reflection is transmitted through Light source CLC sample OSA Fiber-2 Fiber-1 the sample rather than scattered.
The extracted transmittance as a function of the applied voltage is plotted in Fig.4 (b). From Fig.3 and 4, we can see that as the applied voltage is increased, the reflectivity is decreased while the transmittance is increased. This indicates that the loss in reflection is transmitted through the sample rather than scattered.

Conclusion
In summary, an electrically controllable two-way variable optical attenuator (VOA) has been demonstrated by sandwiching a λ/4 film between two polymerstabilized CLC films.
The VOA is polarization independent and both the reflectivity and transmittance are varied with the variation of the applied voltage. With the increase of the applied voltage, the transmittance is increased and the reflectivity is decreased. The maximum extinction ratio is ~17 dB and ~16.5 dB for the transmittance and reflection modes, respectively. The rising and decay time is around 2 ms and 15 ms. Since the reflectivity and transmittance of the device can be varied with the applied voltage, it can be used in many fields. For practical application, the applied voltage needs to be decreased by improving the materials.