1. Introduction

The HWGs are important devices in optical micro-electro-mechanical-systems, since they can serve as the microfluidic channels but also the optical waveguides. In the last decade, several structures of HWGs have been proposed. Liquid-core waveguide (LCW) [1–5], nanoporous cladding waveguides [6] and liquid-liquid waveguides(L²s) [7] provide total reflection to confine the propagating light in the core of the waveguide. Bragg optical waveguides [8], anti-resonant reflecting optical waveguides [9–12] and photonic crystal fibers(PCFs) [13–17] can confine the light by the interference. By infiltrating the electro-optical materials such as LC, the optical properties of the waveguide can be tuned. In PCFs or V-groove waveguides infiltrated with LC in core, the wavelength of the light which can be confined in the waveguides can be varied by changing the temperature [13] or the output light intensity can be changed by applying the external voltage [5,14]. The waveguides with optical tunable property by infiltrating LC or liquid can serve as optical modulator and optical attenuator [5,13,14] and biosensor [18,19].

The optical switch or optical attenuator by infiltrating LC, E7, into optical channel waveguide has been proposed by D. Donisi et al [5]. By applying the external AC voltage with frequency of 1 kHz on the device with the distance of 12 μm among electrodes, the LC directors in the optical channel waveguide is re-orientated and the propagated light can be radiated out of channel waveguide. The maximum extinction ratio of the channel waveguide with length of 0.8 cm between ON-OFF in excess of 44 dB can be achieved for the driving voltage of 10 volt. The propagation loss and coupling loss of the device are 6 dB/cm and 4.5 dB, respectively. The corresponding extinction ratio per unit length and per voltage is 5.5(dB/cm/volt). However, the driving voltage applied on the device is relatively high resulting from the fact that the anchoring force provides a high driving voltage for the horizontal alignment [20,21]. Besides, in LC due to the electro-optic Kerr effect [22], the applied voltage is proportional to the distance between two electrodes. Therefore, the dimension between the electrodes is an another decisive factor to determine the driving voltage of LC filled device. The case of tunable LC filled PCFs has been proposed by Fang Du et al [14]. By applying external field on the device with the distance of 120 μm between two electrodes, the maximum attenuation of the PCFs with length of 1.2 cm between ON-OFF in excess of 30 dB can be measured for the driving voltage of 60 volt. The corresponding attenuation per unit length and per voltage is 0.42(dB/cm/volt). However, in Ref. 22, the author has mentioned that the capillary with relatively small diameter is used to decrease the driving voltage. Therefore, the LC filled PCFs without any alignment proposed by Fang Du [14] et al. has the high driving voltage due to the long distance between two electrodes. The other case of tunable PCFs using thermo-optic tuning of the LC has been proposed by T. T. Larsen [13] et al. By heating the LC filled PCFs, the maximum extinction ratio of the PCFs with the length of 2 cm between the temperature of 26.5 °C and 26.9 °C in excess of 60 dB can be measured. For the thermooptic tuning method, the temperature should be kept precisely to maintain the refractive index of liquid crystal. In Ref. 13, the author shows the 60 dB extinction ratio at the temperature interval between 0.1 and 0.4 °C. However this is difficult to maintain due to the thermal fluctuation from the background.

In this paper, we fabricate the HWGs structure on the silicon wafer and infiltrate the LC, E7, into the hollow core of waveguides. By applying the external AC voltage with frequency at 1 kHz on the LC filled waveguides, the maximum extinction ratio at the output of the waveguide with length of 0.4 cm between ON-OFF in excess of 30 dB can be measured for the driving voltage as low as 4 volt. The corresponding extinction ratio per unit length and per voltage can reach as high as 18.75(dB/cm/volt). Our structure that can be operated with relatively low driving voltage is suitable to be employed in the integrated optical circuit due to our LC filled HWGs structure without any alignment and with relatively smaller distance of 9 μm between two electrodes.

2. Simulation

In this study, we use the Beam Propagation Method (BPM) to calculate the propagation loss of the structure that we propose and we take into account two indices of guiding material in the simulation. One of two indices of the guiding material, liquid crystal E7, is ordinary index for uniform distribution of LC directors such as n_o and another one is the average index [23], $\overline{n}=\frac{2}{3}{n}_o+\frac{1}{3}{n}_e$, for non-uniform distribution of LC directors. The electrical permittivities of E7 LC are ε_// = 20 and ε_⊥ = 7. Figure 1 schematically shows the structure of HWGs for simulation. The SiO₂ cladding layer thickness, ${\text{d}}_{{\text{SiO}}_{\text{2}}}$, is a variable parameter in the simulation. The guiding material is liquid crystal E7 in the hollow core of waveguide with width, ${\text{d}}_{\text{core}}$, of 5 μm. The ITO layers with thickness, ${\text{d}}_{\text{ITO}}$, of 0.5 μm serve as the electrodes. The index of SiO₂, ${\text{n}}_{{\text{SiO}}_{\text{2}}}$, is 1.48. The index of indium tin oxide (ITO), ${\text{n}}_{\text{ITO}}$, is 1.27. The ordinary index [5] of LC, n_o, is 1.5 and the extra-ordinary index [5] of LC, n_e, is 1.68. The average index of LC, $\overline{n}$, is 1.56. All indices of the materials in this study are for the wavelength of 1550 nm.

 

Fig. 1 Schematic drawing of the cross-section of the HWGs.

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The incident light in the TE (y-direction) and the TM (x-direction) polarizations are calculated, respectively. Figure 2 shows the propagation loss versus different thicknesses of cladding layer. Figure 2(a) shows that the propagating light cannot be confined by the LC filled waveguide with the cladding layer thickness less than 3 μm. Figure 2(b) shows the propagating light can be confined by the LC filled waveguide with the cladding layer thickness larger than 2 μm. The propagation loss rapidly increases while the SiO₂ cladding thickness is less than 2 μm. If the SiO₂ thickness is defined to be 2 μm, as the voltage is applied on the sample, the index changes from $\overline{n}$ to n_o, the guided eigen-mode of the waveguide in which the core index is $\overline{n}$ of LC can be transformed to the leakage mode due to low core index. The energy of light can be radiated into the silicon substrate. By using this phenomenon, the attenuator can be fabricated.

 

Fig. 2 Propagation loss of the devices for different applying voltages and for the incident light with different polarizations. (a) the case for the direction of LC directors is parallel to the x-direction where n_o contributes to the leakage of the light. (b) the case for the direction of LC directors is non-uniform distribution.

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3. Sample fabrication

Silicon is used as the substrate of the device. The crystal orientation of silicon wafer is (100). The resistivity of silicon wafer is 1 ~10 ohm-cm. The thickness of silicon wafer is 525 μm. The silicon wafer is N-type doped. Before fabricating the HWGs structure, the silicon wafer is cleaned in turn by acetone (ACE), isopropyl alcohol (IPA) and deionizes water (DI-water). The SiO₂ layer with thickness of 2 μm is deposited on the silicon wafer as the bottom cladding layer of the waveguide by plasma-enhance chemical vapor deposition method. The negative photo-resist, SU-8, is coated as sacrificial layer with thickness of 5 μm on the SiO₂ layer and patterned by the conventional photolithography process with width of 10 μm. The SiO₂ layer with thickness of 2 μm is deposited on the sample as the top cladding layer. The photo-resist, SU-8, is removed by photo-resist remover to form the hollow core waveguide. For applying the external AC voltage on the hollow core of waveguide, the transparent conductive film, ITO, with thickness of 0.5 μm is chosen to deposit as the electrode on the structure of HWGs and the rear of silicon wafer. The sample is cleaved to obtain the facet perpendicular to sample surface as shown in Fig. 3 . Finally, for infiltrating the liquid crystal E7 into the hollow core of waveguides, the LC is heated to liquid phase at 80 °C on the hot plate. By immersing the facet of waveguide into the LC, the LC can be easily infiltrated into the hollow core of waveguide by capillary effect. No crack is observed at the top of the waveguide after the fabrication process and after the operation with external voltage indicating good mechanical stability of the device.

 

Fig. 3 SEM image of the cleaved waveguide facet.

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4. Results and discussion

To characterize LC filled waveguide, the linearly or circularly polarized light at the wavelength of 1550 nm is coupled into the core of the LC filled waveguide by a single-mode fiber. The power of the incident light is 6 dBm. The polarization state of the incident light is controlled by the polarization controller. The polarization maintaining single-mode fiber is used to maintain the polarized state of the incident light. Simultaneously, the external AC voltage with the frequency of 1 kHz is applied in the direction perpendicular to the sample surface (x-direction) on the structure as shown in Fig. 4 . The external AC voltage is varied from 0 to 10 volt. We measure the output intensity of light from the LC filled waveguide by the power meter. In our experiment, the image of polarized optical microscopy shows that the LC is randomly orientated in the waveguide without applying voltage. However, in our other study [24], as the width of LC is increased to be 50 μm, the LC at the edge of the waveguide is oriented to 45° along the waveguide direction. The result shows that the wider waveguide width may provide self-alignment effect without rubbing process.

 

Fig. 4 (a) Output field image without applying external voltage. (b) Output field image with applying external voltage at 4 V_pp.

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Figure 4(a) shows the optical field image of the waveguide output showing the fact that the propagating light is well confined in the LC filled waveguide as the applied voltage is zero. According to the simulation results shown in Fig. 2(b), the propagation loss of LC filled waveguide is slight due to the strong confinement of the waveguides with large index difference between the guiding material index of 1.56 for non-uniform distribution of LC directors and the cladding layer index of 1.48. For the applying external AC voltage at 4 V_pp, the propagating light cannot be confined in the LC filled waveguide as shown in Fig. 4(b). The direction of LC directors is re-orientated to the x-direction after applying the external AC voltage. In this case, the property of birefringence of LC, therefore, becomes progressively obvious by increasing the external field leading to the fact that the guided eigen-mode corresponding to $\overline{n}$ is transformed to the leakage mode due to the appearance of low index n_o. As the birefringence appears, since the light in the HWG is not weakly confined, the polarization cannot be maintained in the HWG. The polarized light propagated in the HWG can be depolarized by the multi-reflection in rectangular waveguide structure. It means that the polarization of the input light can be coupled to the other polarization states. As the external voltage is applied, the LC orientation is aligned along the electric field. Although the extraordinary index of the LC could provide the stronger optical confinement, the ordinary index of LC provides leaky mode to radiate the optical energy to the silicon substrate. The attenuation occurs as the light is depolarized and is coupled to the leaky mode during the propagation in the HWG. Therefore, the optical loss is polarization.

The propagation loss of the HWG is 2.26 ± 0.25 and 4.64 ± 0.18dB/mm for non-applying and applying voltage, respectively. Propagation loss can be reduced by increasing the thickness of SiO₂ layer to provide stronger confinement. The average coupling loss is 24.49 ± 0.13dB. The residual LC at the facet of HWG induces scattering of input light degrading the light coupling. The coupling loss may be reduced by removing the residual LC. The average insertion loss is 47.26 ± 1.57 dB.

Figure 5 shows the output power of the waveguide for the incident light with different polarizations including linear and circular polarization by varying the applied voltage. The θ in the inset represents the angle between the x-direction and the linear polarization direction. The linearly polarized light with the angle θ of 0, 45, 90 and 135 degrees and circularly polarized light are launched into the waveguide. There is no significant difference between the results for different polarizations. This is due to the fact that the polarized light is depolarized in the waveguide. The lowest output intensity is occurred at the external AC voltage between 4 and 5 volt. The corresponding electric field, E = V/D, is 4.4 × 10⁵ (volt/m) where V is applying external voltage and D is the height of structure (D = 2 × ${\text{d}}_{{\text{SiO}}_{\text{2}}}$ + ${\text{d}}_{\text{core}}$). This electric field is typical value to orientate the E7 LC into aligned state [25]. The intensity attenuation between the external AC voltage 0 and 5 V can be over 30 dB. As the external voltage such as 5 V is applied, the power fluctuation is less than 0.13 dBm during 10 minutes.

 

Fig. 5 Output power for different polarized lights and for different applied voltages.

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In the HWG, the randomly orientated LC is progressively aligned as the voltage is increased. This results in the light attenuation due to the fact that the index of the LC decreases from the average index towards the ordinary index. We observe that as the applied voltage larger than 7 V, the attenuation is reduced. The phenomenon reveals that the index of the LC arises as the voltage increases from 7 V. This might be due to the fact that the electrodes on the right and left sidewalls of the HWG disturb the vertical electric field provided by the top and the bottom electrodes. The orientation of the LC at the right and left edges of the HWG might be tuned to be randomly orientated as the applied voltage is larger than 7 V.

5. Conclusion

In this study, the tunable attenuators are fabricated by infiltrating the LC into the hollow core of waveguide. The LC filled waveguide can well confine the propagated light without applying external voltage. After applying the external AC voltage of 4 ~5 volt, the guiding mode can be transformed into the leakage mode leading to the fact that the propagating light radiates out of the LC filled waveguide. The optical property of mode transformation of the propagated light can be used to fabricate the optical modulators or optical attenuators. The maximum attenuation of the light in the LC filled HWG between on and off states can be over 30 dB. The orientation of the LC is tuned by the vertical electric field using parallel electrodes on and below the device. The structure of the electrode can be modified to parallel and co-planar to provide both of the vertical and horizontal electric field, respectively. The output beam direction may be tuned by the HWG. The HWG may be integrated with light sources for the application of the scanning retina display. The large attenuation and low operation voltage properties of the HWG can provide the high dynamic range of the displayed images. The ARROW waveguides may also be infiltrated with LC to modulate the output intensity due to changing the resonant wavelength.