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Abstract
Flexible and localized initial alignment control of liquid crystal (LC) is important to enhance the performance and functionality of LC hybrid silicon photonic devices. This work proposes an initial LC alignment control technique based on integration of a nanometer-scale groove array in the buried oxide layer near a Si waveguide. We achieved control of the initial angle of LC director around the Si waveguide by selecting the required integrated groove direction and reduced the driving voltage by introducing a vertical groove array into the phase shifter. We then used the local and flexible LC initial alignment controllability to develop a Mach-Zehnder optical switch and ring-resonator wavelength filter. This approach will be helpful when integrating LC-loaded devices with various characteristics and functionalities into optical integrated circuits.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Heterogeneous material integration into silicon photonics has been actively researched because it enables fabrication of high-performance and high-functionality devices that are difficult to realize using only silicon and silica materials. To date, many devices of this type have been developed, including III-V/Si lasers, Ge/Si photodetectors, modulators containing electro-optic polymers, isolators containing magneto-optical materials, optical attenuators with metal-insulator transition materials, and athermal waveguides using materials with negative thermal refractive index coefficients.
Nematic liquid crystals (LCs) are prospective integration materials that will provide the ability to realize a wide-range Si phase shifter in a small footprint. This is because the LC has high birefringence among its refractive index properties; thus, when we introduce this material as a cladding layer for a Si waveguide and control its director, we can vary the equivalent index of the waveguide strongly. Additionally, the LC-loaded phase shifter enables low driving power consumption because the LC director can be controlled using an externally applied electric field, as illustrated in Fig. 1. To date, various LC-loaded functional devices have been reported, including optical path and wavelength switches [1–14], modulators [15], variable optical attenuators [16] and polarization rotators [17].
Fig. 1 General operating mechanism of LC loaded Si phase shifter for transverse-electric (TE) polarized light. (a) Off state. (b) On state.
In LC-loaded Si phase shifters, the initial LC alignment condition affects the device performance strongly. Generally, the refractive index of LC material for the polarized light (which is TE mode in this work) has a nonlinear response as a function of angle of LC director. Therefore, we can reduce the driving voltage of the LC-loaded devices including optical switches by choosing the initial angle of LC director to have large slope of refractive index change. This affects the phase tunable range of the phase shifter at the same time. In addition, the LC anchoring energy at the Si waveguides is related to the initial LC alignment direction, and this affects both the driving voltage and the response speed. Furthermore, the desired initial alignment direction for an in-plane switching (IPS) driving mode [1,6,7,12–15,18], where the electrodes are placed at both sides of the Si waveguides as shown in Fig. 1, is different to the desired alignment for a vertically aligned (VA) driving mode [2–4,8,11,16,19], where the waveguide is sandwiched between the upper and lower electrodes (this will be described in Section 3). For these reasons, management of the initial LC alignment is important, and several techniques to manage the initial alignment direction have been reported to date, including a photo-alignment technique using photosensitive polymers [9], assembly of a rubbing polyimide (PI) alignment film that is tilted against the waveguide [10], and spin-coating of the PI film on the Si waveguide [8].
LC initial alignment techniques based on arrayed fine structures that are integrated near the Si waveguides are another promising approach that can realize local and flexible controllability [4,18,19]. It has been reported that LC alignment was affected by the groove structures under the Berreman’s theory [20–23]. Recently, we also proposed the introduction of a nanometer-scale groove array on the buried oxide (BOX) layer of the device, as shown in Fig. 2(a) [18,19]. LC generally tends to align along the surrounding structures, so in the device with no grooves shown in Fig. 2(b), the LC is aligned along the sidewall of the waveguide having an anchoring energy between the LC and the waveguide surface. However, when the groove arrays are integrated near the waveguide, as shown in Fig. 2(c), the LC that fills the grooves then aligns along these grooves; this therefore affects the LC alignment condition at the waveguide surface because the LC tends to form a domain with a similar director. As a result, we can control the initial LC alignment condition around the Si waveguides by simply selecting the location and direction of these grooves. As shown in Fig. 1, width of the optical mode field for Si waveguides was generally as small as about 1 µm, so only the LC director close to the sidewall of Si waveguide determines the properties of propagating light. This technique is compatible with both the prevailing Si photonics fabrication platforms and the mature LC display manufacturing methods because these grooves can be realized using alignment lithography and dry-etching processes. Furthermore, the refractive index contrast between general LC materials (NLC) and the BOX layer (NSiO2) is small (NLC and NSiO2 of ~1.45 at a wavelength of 1.55 µm), and thus the optical reflection and diffraction behavior at groove arrays that are filled with LC could be suppressed considerably when compared with a similar approach using Si gratings with high refractive indices [4].
Fig. 2 Concept for the proposed LC initial alignment management technique using groove arrays on the BOX layer.
In this work, we investigate the effectiveness of the proposed initial alignment management technique through polarized microscopic observations and transmission measurements. In addition, we introduce this technique to some representative phase-shifter devices in the form of a Mach-Zehnder (MZ) switch and a ring-resonator wavelength filter. Part of this work has been presented at previous conferences, and this paper complements the conference abstracts [18,19].
2. Management of LC initial director
In this section, we experimentally investigate the relationship between the direction of the integrated grooves and the initial angle of LC director near the Si waveguide using an IPS-mode LC-loaded Si directional coupler switch [18].
Figure 3 shows the fabrication flow for the device. First, Si waveguides were formed using a combination of electron beam lithography (EBL) and inductively coupled plasma reactive ion etching (ICP-RIE) with C4F8 and SF6 gases. Next, nanometer-scale groove arrays were formed locally near the Si waveguide using alignment EBL and ICP-RIE with the same gases. Then, the in-plane Ti/Au electrodes were formed to sandwich the grooves and waveguides together using a photo lithography and a lift-off process. After the optical circuits were covered with a 1.5-µm-thick polymer material (AZ-5214E, MicroChemicals Inc.) with the exception of the phase shifter area, a borosilicate glass plate was placed and fixed on the device chip using ultraviolet (UV) curable resin across 5-µm-diameter spacer balls. Finally, a commercially available nematic LC with positive dielectric anisotropy (LIXON-JC5143XX, JNC Corp.) was filled into the phase shifter area by capillary action. The material properties were summarized in Table 1.
Table 1. Material Properties of LIXON-JC5143XX
Figure 4 shows the scanning electron microscope (SEM) images of the fabricated device, which was integrated with 45°-tilted groove arrays. A 500-µm-long Si directional coupler was sandwiched between 350-nm-thick Ti/Au electrodes with a gap width of approximately 5 µm. The center position of the electrodes was shifted downward because of the limitations of the alignment lithography process. The Si waveguide width was as narrow as 290 nm to increase the optical evanescent field that penetrates into the LC cladding, and the gap width of the directional coupler was 250 nm. The distance between the Si waveguide and the groove array, which had a width and a depth of 200 and 100 nm, respectively, was approximately 500 nm.
We examined the LC alignment on the groove arrays through polarized microscopic observations. During these observations, the device under test (DUT) was placed between the polarizer and the analyzer, which were crossed (i.e., in a crossed-Nicol arrangement), and the transparent infrared light that passed though the DUT was detected using a charge-coupled device (CCD) camera. In this setup, when the LC is aligned parallel to the direction of the polarizer or the analyzer, the light is blocked at the analyzer (i.e., a dark field occurs). In contrast, when the LC alignment direction is tilted against the polarizer and the analyzer, the light is transmitted (i.e., a bright field occurs).
Figures 5(a) and 5(b)–5(e) show optical microscopic and polarized microscopic images of the device without and with the various directed grooves. The images in the upper row show the results obtained when the DUT was placed in parallel with the analyzer, while those in the lower row show the results obtained when the DUT was placed at a tilt angle of 45°. In the electrode area, the light was blocked from transmission through the DUT, and dark patterns were thus observed.
Fig. 5 (a) Optical and (b)–(e) polarized microscopic images of the LC-loaded Si DC switches with and without groove arrays. The device under test (DUT) was placed between the crossed polarizer and analyzer.
For the device without grooves shown in Fig. 5(b), the dark and bright fields appeared roughly around the directional coupler in the upper and lower rows, respectively, because the LC was aligned along the sidewall of the waveguide. However, there were some local alignment defects because of the relatively weak alignment force that was generated by the Si waveguide surface. For the device with grooves that is shown in Figs. 5(c)–5(e), the dark bands were only observed on the groove area of the device when the grooves were oriented parallel to the polarizer or the analyzer. Conversely, the bright bands were observed when the grooves were 45°-tilted towards them. Therefore, we confirmed that the LC was aligned along the groove directions. The LC director near the top glass plate seems to be standing against the plate surface because there was no in-plane twist in LC director through the polarized microscopic observation shown in Fig. 5. Moreover, disorganized patterns appeared in the areas with no grooves because there was no on-chip specific external binding force in that case.
Next, we measured the device characteristics. Transverse-electric (TE) polarized light with a wavelength of 1.55 µm from an amplified spontaneous emission (ASE) light source was chip-edge coupled into the device through a polarization-maintaining (PM) tip-lensed optical fiber. In the device chip, the DC switch was connected using an approximately 7.5-mm-long Si waveguide. The output light was detected using the spectrometer via a single-mode tip-lensed optical fiber. Figure 6(a) shows the transmitted spectra, which include the losses due to fiber coupling (8.2 dB), the connecting waveguide (3.2 dB) and the device itself. The solid lines represent the spectra for the LC-loaded device integrated with the vertical groove array. Relatively smooth waveforms were obtained from the bar and cross ports. This means that the reflection and diffraction effects caused by the grooves were negligibly small. In addition, the insertion loss was found to be so small as to be within the measurement error range, and the additional material loss related to the LC was shown to be quite small through comparison with a reference device that was entirely covered with the AZ polymer cladding; its results were plotted using dashed lines. Figure 6(b) shows the measured wavelength shift as a function of the applied voltage. In this measurement process, a 5 kHz oscillating wave was applied to the device to avoid buildup of an ionic charge layer within the LC cladding. At low voltages, the wavelength shift was quite small because the LC anchoring energy was higher than the applied voltage. Then, as the voltage increased, the coupling wavelength was blue-shifted in accordance with the change in LC director near the Si waveguides. The threshold voltages for the devices without grooves and those with parallel grooves were measured to be 3 V, while the threshold voltages for devices with larger angled grooves became small. This occurred because the device with larger angled grooves resulted in larger initial angle of LC director near the Si waveguide surface, and the anchoring energy was thus also small, as illustrated in Fig. 2. The path of the output light was changed when the wavelength shift amounted to half of the free spectral range (FSR) of the directional coupler. Then, the switching voltage for the device with vertical grooves was improved to be approximately 70% of that of a device without grooves.
Fig. 6 Measured device characteristics. (a) Transmitted spectra of devices with LC and polymer claddings. (b) Applied voltage dependence of wavelength shifts for devices with and without the various directed grooves.
While the vertical grooves achieved reduction of switching voltage, this device has a small wavelength tunable range compared to the other devices due to the limitation of rotatable range of LC director when the electric field perpendicular to the waveguide was applied. Therefore, the wavelength shift for the vertical grooves will be saturated in lower level compared to the other devices when further high voltage (> 8V) is applied to the device in Fig. 6(b).
We then evaluated the switching speeds of the devices with and without the vertical grooves. In these measurements, a tunable laser diode (TLD) was used as the light source. In addition, a 10 Hz square wave with an amplitude of 5.3 V was applied to both devices to act as a driving signal, albeit the switching voltage for the device without grooves was measured to be approximately 7 V, as shown in Fig. 6(b). We applied the same voltage to both devices to evaluate the effect of the anchoring energy alone because the magnitude of the voltage also affects the response speed of the LC director (mainly in terms of rise time) in addition to the effects of differences in anchoring energy that apply a binding force towards the initial alignment. Figure 7 shows the switching responses. While the device with grooves showed a sharp response at the beginning of the switch-on process, it took a long time when compared with the device without grooves until the output reached a steady state. In contrast, for the switch-off response, the device without grooves showed a completely sharp response. This behavior can be explained by the small anchoring energy of the device with vertical grooves when compared with that of the device with no grooves.
The rise times for the devices with and without grooves were measured to be 1.5 ms. In addition, the fall times for these devices were 3.9 and 1.2 ms, respectively. Here, we defined the elapse time from the moment of changing the voltage to the moment of rising up to 95% of full amplitude of the optical output as the rise time. Also, we defined the elapse time from the moment of changing the applied voltage to the moment of falling down to 5% of full amplitude of the output light as the fall time. We believe that these times can be reduced by introduction of a pre-emphasis driving scheme.
3. VA-mode Mach-Zehnder optical switch
We introduced the proposed alignment control technique into a VA-mode Si MZ optical switch that was designed for TE-polarized light, as shown in Fig. 8(a), where a vertical electric field between the ITO film assembled on the glass plate and the Si substrate was applied to the phase shifters [19]. This device takes advantage of the local and flexible controllability of the initial LC alignment provided by the proposed technique, and the mutually orthogonal grooves in this device were integrated into each arm waveguide. Under no applied voltage, a phase difference exists between the propagating lights for each arm waveguide because of the difference between the initial LC alignment directions. In contrast, when a vertical electric field is applied uniformly to both arm waveguides, the LC directors for both arm waveguides begin to stand up and the phase difference then gradually disappears, depending on the applied voltage. We can then realize optical path switching by controlling the magnitude of this phase difference. This operating mechanism allows use of a simple upper-electrode configuration without requiring segmentation of the electrode area between the arm waveguides, and it is difficult to realize this operating mechanism when using other initial alignment techniques, which generally affect the overall device [8,10].
Fig. 8 (a) Schematic of VA-mode MZ optical switch. The directions of the grooves for the arm waveguides cross each other. (b) Optical microscopic and polarized microscopic images of the device under no voltage.
The width and length of the Si phase shifters were 230 nm and 450 µm, respectively, while the waveguide width for the main circuits was 430 nm. The dimensions of the directional couplers constituting this device were 250 nm for gap width and 200 µm for coupler length. Also, the depth, width and pitch of grooves were 100, 220 and 370 nm, respectively. This device can be fabricated using the fabrication process described earlier and illustrated in Fig. 3. We assembled a glass plate that was coated with indium tin oxide (ITO) and rubbed PI films (RS-D607M1N, EHC.Co.,Ltd.) on the device chip using a UV curable adhesive resin rather than use a conventional in-plane electrode formation process. The LC layer was thicker than 1.5 µm considering thickness of AZ cladding and adhesive resin.
Figure 8(b) shows optical and polarized microscopic images of the device in the OFF state. The LC director on the right phase shifter, which was integrated with the vertical grooves, were twisted in the thickness direction, while those on the left phase shifter, which was integrated with the parallel grooves, were consistently in the same alignment direction because the integrated rubbing PI film was in the same direction (RU) as the phase shifters. Therefore, only the light that had passed through the area with the vertical grooves could be detected during the polarized microscopic observation with the crossed-Nicol arrangement.
Figure 9(a) shows the transmission spectra for each of the applied voltages in the range from 3 to 5.5 V. A 2 kHz oscillating wave was adopted as an electric signal to avoid buildup of an ionic charge layer within the LC cladding which may degrade the performance of devices. We obtained smooth spectra because of the negligibly small optical interference (in terms of both reflection and diffraction) produced by the grooves. The switching wavelength occurred at 1567.5 nm, which was determined by the design of the 3 dB directional couplers used in this device. Figure 9(b) shows the optical transmittance as a function of the applied voltage at the switching wavelength. This frequency differed from that used for the Si DC optical switch shown in Fig. 4, which was operated at 5 kHz, because the frequencies that were suitable for efficient modulation were different for the IPS and VA modes. The output ports were alternately changed at the voltages of 3, 5.5 and 8.5 V, and the minimum voltage-length product (Vπ·L) was calculated to be 1.1 V·mm. This will be improved by bringing the assembled upper electrode close to the Si phase shifters. In addition, the offset voltage of 3 V can be reduced to zero by adjusting the initial phase difference to be a multiple of π by adjusting either the groove directions or the length of the phase shifter. The path crosstalk was measured to be 22 and 19 dB for the bar and cross states, respectively, while the device insertion loss was within the measurement error range.
Fig. 9 Measured device characteristics. (a) Transmitted spectra for the different applied voltages. (b) Applied voltage dependence of the transmittance for each of the output ports at a wavelength of 1567.5 nm.
Figure 10 illustrates the switching response of this device. We applied a 10 Hz electric signal with a voltage range from 5.5 to 8.5 V and an oscillation frequency of 2 kHz, as plotted in orange from the result of Fig. 9(b). Also, the output light to the bar port was plotted with navy.
Fig. 10 Waveforms of the detected output light from bar port and the applied signal for (a) Switching from cross state to bar state and (b) Switching from bar state to cross state.
The rise time (up to 95% of the full amplitude) and the fall time (down to 5% of the full amplitude) were measured to be 1.5 and 2.5 ms, respectively; these values were comparable with the response time of the aforementioned Si DC switch. The rise time will increase when a driving voltage of between 3 and 5.5 V is selected. Moreover, a fluctuated output waveform was observed when we applied voltage of 8.5V as shown in Fig. 10(b). We have considered the reason was unstable condition for LC director affected by the 2-kHz oscillating signal albeit further investigation is necessary.
4. VA-mode ring-resonator wavelength filter
Finally, we introduced the proposed LC initial alignment control technique to the VA-mode ring resonator wavelength filter, as shown in Fig. 11(a). Ring resonator devices have been used as wavelength add-drop filters for the wavelength division multiplexing (WDM) system, and phase-shifting function is necessary to realize wavelength signal selection and post-fabrication center wavelength trimming. They generally have round-shaped phase shifters and it is difficult to offer an efficient initial alignment condition in the case where a unidirectional alignment film is introduced via the rubbing process. Although the photo-alignment technique [9] enables flexible directional alignment, it required high-precision assembly alignment. However, this technique does realize efficient modulation by simply integrating the radially directed grooves into the resonators without assembly alignment. Furthermore, the VA mode is favorable approach to apply the electric field to the device effectively in terms of electrode layout flexibility compared to the IPS-mode approach.
Fig. 11 (a) Optical microscopic and SEM images of ring resonator wavelength filter with radial groove arrays before LC filling. (b) Operating mechanisms of the resonators with various directional grooves.
In this work, we compared three types of devices, including a device without grooves and devices that were integrated with circular and radial grooves, as shown in Fig. 11(b). While the LCs in the devices without grooves and those with the circular grooves basically tended to align along the resonator, the LCs in the device with radial grooves are aligned at a large angle with respect to the resonator. Therefore, the device with the radial grooves is expected to realize effective phase shifting to TE-polarized light.
The waveguide width and the resonator radius were designed to be 310 nm and 50 µm, respectively, while the gap width of the coupling area between the bus waveguide and the resonator was 450 nm. Figure 12(a) shows the transmitted spectra for the device with the radial grooves. These spectra include the losses of a fiber coupling and connecting waveguide. Smooth spectra were obtained with an FSR of 1.8 nm at the operating wavelength of 1.55 µm, and the effective index of the resonator was thus approximately 4.1. In addition, the rounding loss of the resonator and the electric field transmission coefficient at the coupling area were estimated to be 1 dB and 0.89, respectively, from a theoretical fitting using the least-squares approach. The Q factor was calculated to be approximately 6000.
Fig. 12 Measured device characteristics. (a) Transmitted spectra of the device with radial grooves for the through and drop ports. (b) Applied voltage dependence of the wavelength shift.
Figure 12(b) shows the applied voltage dependence of the wavelength shift. While the behaviors of the devices without grooves and those with circular grooves were almost the same, the device with the radial grooves shows a low threshold voltage of approximately 2 V and an efficient wavelength shift (Δλ/ΔV) of 0.16 nm/V, which is 1.6 times higher than that of the former devices. There was no difference in the wavelength shift between the devices without grooves and with circular grooves. We have considered that this was because the LC binding force by the in-plane circular grooves was negligibly small in the vertical direction when the vertical electric field was applied.
5. Conclusions
We proposed and investigated a locally and flexibly controllable initial LC alignment technique by forming a nanometer-scale groove array on the BOX layer near the Si waveguide. We confirmed experimentally that the initial angle of the LC director around the Si waveguide could be controlled by appropriate selection of the direction of the groove array through polarized microscopic observation and transmission measurement of the device. By introducing large angled grooves to the Si waveguide, we reduced the threshold and the driving voltage of the optical switch when compared with the device without grooves.
We also introduced this technique into a VA-mode MZ optical switch and a ring-resonator wavelength filter. For the MZ optical switch, we realized a simple upper electrode configuration without segmentation of the electrode area between the two arm waveguides by introducing grooves with crossing directions into each arm waveguide, thus taking advantage of the local and flexible controllability of the initial LC alignment. The fabricated device showed smooth transmission spectra and switching efficiency with a Vπ·L of 1.1 V·mm. The response times for the rise and fall actions were 1.5 and 2.5 ms, respectively. In addition, the ring-resonator wavelength filter that was integrated with the radial grooves showed a coefficient in the wavelength shift that was 1.6 times higher than that of the device with no grooves.
This approach is compatible with the prevailing Si photonics fabrication processes and mature LC display manufacturing techniques. Furthermore, the localized and flexible controllability of this approach will be helpful in allowing the coexistence of various LC-loaded devices with their different characteristics on a device chip, although further theoretical analysis of LC alignment properties including the anchoring energy of such grooves is necessary to bring this technique to practical phase.
Funding
Japan Society for the Promotion of Science (JSPS) (16K18098).
Acknowledgments
Part of this work was conducted at the AIST Nano-Processing Facility, which is supported by the “Nanotechnology Platform Program.”
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