1. 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$\lambda =np$, where p is the helical pitch and n the average refractive index. Compared to a conventional one-dimensional 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 [1–4]. Because of these properties, CLCs have attracted great interest for many applications, such as optical switch [5], mirrorless laser [6,7], bi-stable display [8], variable optical attenuator [9], 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 cannot provide continuously variable gray scale. Even though some devices with variable transmittance, such as variable optical attenuator, has been developed using CLC [9], the variation of the transmittance is based on the scattery induced by the CLCs’ focal conic structure. However, a scattery based device 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 a polarization independent variable optical attenuator (VOA) by sandwiching a λ/2 film between two CLC films with the same handedness. Different from a conventional VOA, which can only change the optical intensity either in the reflection or transmission direction, our developed VOA can change the optical intensity in both the reflection and transmission direction by applying voltage on it. This unique property will allow it to be widely used in many applications.

2. Sample Preparation and Experimental Setup

The CLC mixture was prepared by mixing 80.5 wt% E44 (Merck LC mixture) with 14 wt% of a left-handed chiral dopant ZLI-811 (also from Merck) and 5% 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 pre-tilt angle is ~3°. After the temperature was gradually cooled down to the room temperature, a CLC sample with left-handed (LH) helix was formed. Then, the sample was illuminated under a 50 mw/cm² UV light for 30 minutes. Polymer network is formed inside the sample. Using this method, we fabricated two pieces of left handed polymer-stabilized CLCs.

The schematic experimental setup is shown in Fig. 1 . A broadband light source ranging from 1200 to 1700 nm was used as the light source. The light reflected from the sample is coupled to fiber-1 and the transmitted light is coupled to fiber-2. Then they are input to an optical spectrum analyzer (OSA).

 

Fig. 1 The schematic diagram of the experimental setup.

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3. Results and Discussion

First, we characterized the transmission and reflection spectra of the polymer-stabilized CLC sample at different voltages. The results are plotted in Fig. 2 . Figure 2(a) and 2(b) are the transmission and reflection spectra of the polymer-stabilized CLC at different voltages, respectively. We can see that the CLC exhibits a reflection band from 1450 to 1650 nm. At V = 0 V, both the transmittance and reflectivity in the reflection band of the polymer-stabilized CLC are ~45%. This is because that CLCs’ reflection band is polarization sensitive, only reflecting the circularly polarized light with the same handedness as its helical structure. In addition, due to some optical losses, both the reflection and transmission is below 50%. As the applied voltage is increased, the reflectivity is decreased and the transmittance is increased. When the applied voltage is increased to 120 V, the reflectivity is decreased to < 3% and the transmittance is increased to ~80%. The loss in the transmission is due to the surface reflection. The results indicate that the polymer-stabilized CLC is a good candidate for VOA.

 

Fig. 2 Reflectance (a) and transmittance (b) spectra of the CLC sample at different applied voltages

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However, a device with a single CLC is always polarization sensitive. In order to develop a polarization insensitive device, we sandwiched a λ/2 film between two left handed polymer-stabilized CLCs, as illustrated in Fig. 3 .

 

Fig. 3 The schematic configuration of the VOA

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It is known that the left-handed CLC reflects the left circularly polarized light within its reflection band at V = 0 V. With enough applied voltage, the liquid crystal inside the CLC is tilted up. As a result, all the light will be transmitted without any obstacle. Figure 4 illustrates the schematic diagram of the light propagation inside the sample. When an unpolarized light illuminates on the sample, only the left handed circularly polarized light within the reflection band of the CLC is reflected after passing through the first left-handed CLC. The transmitted right-handed circularly polarized light within its reflection band will be converted to left-handed circularly polarized light by the λ/2 film and then reflected by the second left-handed CLC. As a result, all the left and right circularly polarized light within the CLC’s reflection band is blocked by the sample. When enough voltage is applied on the CLCs, all the left and right circularly polarized light will be transmitted without any obstacle. Obviously, the device is polarization insensitive.

 

Fig. 4 Schematic diagram of the light propagation inside the sample

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Figure 5(a) shows the reflectance spectra of the sample. At V = 0 V the reflection of the sample is ~-2dB. With the increase of the applied voltage, the reflectivity is decreased. When the applied voltage is increased to 120 V, the reflectivity is decreased to ~-16 dB. The extracted reflection as a function of the applied voltage is plotted in Fig. 5(b). Ideally, the reflection within the CLC’s reflection band should be flat and there are some sidelobe only outside the reflection band, which is due to the interference inside the CLC. From Fig. 5(a), it is seen that there are two different kinds of oscillation: the larger oscillation and the smaller oscillation. The larger one only occurs outside the CLC’s reflection band which is the intrinsic oscillation of the CLC induced by the CLC’s internal surface interference. The smaller one appears all over the reflection spectrum, which is induced by the surface interference of each component. The smaller oscillation can be avoided by using the index matching oil and anti-reflection coating. Another phenomenon in Fig. 5(a) is that the reflection outside the CLC’s reflection band is also decreased with the applied voltage. This indicates that this sample is not perfect planer CLC structure and there is some scattering inside the sample. It needs to be improved by modifying the materials and recipe in the future.

 

Fig. 5 (a) Reflectance spectra of the VOA at different applied voltages, (b) the reflectance as a function of the applied voltage.

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Figure 6(a) illustrates the transmittance spectra of the sample. At V = 0 V, the transmittance of the sample is ~-17dB. With the increase of the applied voltage, the transmittance is increased. When the applied voltage is increased to 120 V, the transmittance is increased to ~-2 dB. The extracted transmittance as a function of the applied voltage is plotted in Fig. 6(b). From Fig. 5 and 6, we can see that as the applied voltage is increased, the reflectivity is decreased while the transmittance is increased. This indicates that the VOA is actually a two-way electrically controllable VOA. Moreover, it is insensitive to the incident polarization. The rise and decay time were measured to be 2 ms and 15 ms, respectively when the applied voltage is at 120 V, as shown in Fig. 7 .

 

Fig. 6 (a) Transmittance spectra of the VOA at different applied voltages, (b) the transmittance as a function of the applied voltage.

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Fig. 7 Response time of the VOA when the applied voltage is 120 V

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In summary, we have developed a polymer-stabilized CLC that the reflection is decreased and transmittance is increased with the increase of the applied voltage. Based on this property, we demonstrated a polarization independent two-way VOA by sandwiching a λ/2 film between two left-handed polymer-stabilized CLC films. Different from a conventional VOA, the VOA based on the polymer-stabilized CLC can change the optical intensity in both the reflectivity and transmittance. 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 dB for the transmission and reflection modes, respectively. When the applied voltage is 120 V, the rise and decay time were measured to be around 2 ms and 15 ms, respectively. The performance of the VOA can be further improved by modifying the material recipe and fabrication process.