1. Introduction

Microring resonators have attracted much attention as optical filters because of their inherent wavelength-selecting ability, which enable the exploitation of wavelength-division-multiplexing (WDM) techniques [1–3]. Microring resonators exhibit a sharp resonance at specific wavelengths that depend on the designed waveguide structure and the adopted core and/or cladding materials. Accordingly, the resonant wavelength of the microring resonator can be easily tuned through the change in the refractive index of the core and/or the cladding materials even when the structure of microring resonator is fixed. Over the last few years, several wavelength-tunable microring resonators that exploit the electro-optic effect [4], the thermo-optic effect [5], free carrier injection [6, 7], and nematic liquid crystals (NLCs) have been demonstrated [8–11]. Each driving method has advantages and disadvantages. NLC-based microring resonators have a wide range of resonant tuning wavelengths with a low switching power, owing to large change in the birefringence of the NLCs. The orientation of the LC directors can be electrically controlled to change the effective refractive index of the microring resonator, enabling a shift in the resonant wavelength. Although the NLC-based ring resonator performs various excellent optical modulation-related functions, integrated devices still suffer from such problems as large scattering loss and a slow switching time. In particular, scattering loss from the NLC cladding that is caused by the long-range orientational order severely degrades the Q-factor of the microring resonator. This phenomenon is commonly observed when a device exhibits imperfect alignment treatment of substrate surfaces and is operated in an applied field [12–14]. To solve such problems, Costache et al. replaced NLCs with isotropic liquid crystals and demonstrated sub-microsecond optical switching and routing waveguide devices. Isotropic liquid crystals with a uniform refractive index do not require any surface alignment nor do they cause scattering loss. Isotropic liquid crystals whose molecules exhibit short-range order have a large electro-optics Kerr effect and thus have a response time of submilliseconds in an electric field [15]. However, the birefringence that is induced by Kerr effect in the isotropic phase is temperature-dependent. The birefringence is highest only at the clearing point temperature; as temperature increases beyond the clearing point, the consequent drop in birefringence raises difficulties for practical applications [16].

This paper demonstrates an electrically tunable SiN microring resonator with a fast response and low optical loss, and polymer-stabilized blue phase liquid crystals (PSBPLCs) as a cladding. Some of the optical properties of PSBPLCs are similar to those of isotropic liquid crystals [17–19]. The PSBPLCs with a symmetric crystal structure are not only optically isotropic but also capable of inducing large birefringence by the Kerr effect. Most importantly, PSBPLCs can induce a large electro- optical Kerr effect over a wide spectrum of temperatures, such that the operating temperature of the device is not limited to a certain range. In an electric field, BPLCs provide the resonator with a variable cladding refractive index and the tunability of resonant wavelengths with a response time in the submillisecond range. Furthermore, the proposed device has an ultrahigh Q-factor greater than 20,000 both under and without an applied electric field. To the best of the authors’ knowledge, this device has a higher Q-factor than all microring resonators with NLC cladding that have been reported to this day.

2. Sample fabrication and measurement

Figure 1(a) depicts the device structure of the SiN microring resonator with PSBPLC cladding. The proposed device comprises two parts – a waveguide chip and an organic PSBPLC film. The chip consists of a Si substrate, a 16 µm-thick SiO2 layer and a 0.5 µm-thick SiN layer. The SiO2 layer, located between the SiN layer and the Si substrate, serves as the lower cladding layer for the SiN micro-ring. SiN is chosen as the core material of the microring resonator because its refractive index is close to that of the liquid crystal, providing a wide tuning range of the microring resonator. Figures 1(b) and 1(c) show the designed resonator structure, which consist of a straight waveguide and a microring with a radius of 40 µm. The width and thickness of both SiN waveguides are 1.2 and 0.5 µm, respectively, and the coupling gap of the resonator is 0.65 µm. An ITO glass substrate without any alignment film was placed 5 µm above the waveguides to form an empty cell. To prepare the material of the PSBPLCs, two positive nematic LCs, JC1041 and 5CB, were mixed with two UV-curable monomers, TMPTA and RM257, a chiral agent, R1011, and the photoinitiator DMPAP in a ratio of 43.6:33.5:5.4:7.1:10:0.4. The mixture was mixed to homogeneity, placed on a temperature-controlled stage and then used to fill an empty cell in the isotropic state. The sample was cooled to 36 °C (BPI) at a rate of 0.5 °C /min, and then irradiated with ultraviolet light (0.8mW/cm²) for 20min to achieve phase separation, forming PSBPLC, which existed over a wide temperature range, from 20 °C to 58 °C.

 

Fig. 1 (a) Cross-section, (b) top view of proposed resonator, and (c) photograph of proposed resonator with PSBPLC cladding, taken under R-POM without a polarizer.

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3. Result and discussion

Figure 2 schematically depicts the operation of the designed micro-ring resonator with an electrically tunable wavelength under the influence of an electric field. Herein, the BPLCs- based device is mainly designed for TM-polarized light. In the waveguide structure, the TE-polarized light owns strong light confinement in the core waveguide and only has slight influence on the variation of the cladding layer. Besides, the induced birefringence of BPLCs for TE-polarized light is much smaller than that for TM-polarized light. Therefore, only TM-polarized light is discussed in this experiment. In the V_off state, the PSBPLCs are in the BPI phase with a simple cubic (sc) structure, which is made of double-twist cylinders and disclination lines, as presented in Fig. 2(a). In a double-twisted cylinder, all central liquid crystal molecules are parallel to the z axis, while those on the outer circumference of the cylinder have a twisting angle of ± 45° with respect to z axis. Such a symmetrical crystal structure causes the PSBPLCs to be optically isotropic with an average refractive index of

 

Fig. 2 Principles of electrical switching of proposed resonator in (a)Voff and (b)Von states.

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As the applied electric field is increased, the TM-polarized light experiences the increase in the effective refractive index of the PSBPLCs. The change in the refractive index of the cladding alters the effective refractive index of the microring resonator, causing a shift in the resonant wavelength, according to the resonance equation${\lambda }_m=2\pi R{n}_{eff}/m$, where m, the grating order, is a positive integer; λ_m is the wavelength of m^th-order resonant mode; R is the radius of the resonator, and n_eff is the effective refractive index of the resonator. The isotropic-to-anisotropic transition can be regarded as a local realignment of the director of the PSBPLCs, while the anisotropic-to-anisotropic transition of NLCs is a change in the orientation of the director in the bulk of the cell. Hence, PSBPLCs exhibit a shorter response time (submillisecond) than traditional NLCs (millisecond).

To examine the tunable properties of the PSBPLC-based filter, the normalized transmission spectra of the hybrid PSBPLC SiN resonator in the TM mode were obtained in the NIR region at various applied voltages. An infrared tunable laser was used as the light source and a power meter measured the output intensity. The spectral range was in the optical telecommunications region from 1530nm to 1570nm. The free spectral range of the resonator with its radius of 40 μm is around 4.5 nm (not shown). Figure 3(a) plots the variation of the resonant wavelength (originally at 1548.98nm) of this filter with the applied voltage from 0 to 150V. As the applied voltage was increased, the resonant wavelength was red-shifted because the effective refractive index of the PSBPLCs increased. When the applied voltage was 150V, the resonant wavelength shifted by 0.45 nm from 1548.98 to 1549.43 nm. Figure 3(b) presents corresponding spectra that were obtained at voltages of 0, 50, 90,120, 150V.

 

Fig. 3 (a) Variation of resonant wavelength as a function of driving voltage, and corresponding spectra (at 1548.98nm) at V = 0, 50, 90,120, 150V.

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To compare the properties of the hybrid LC SiN micro-ring resonator in an applied electric field, Fig. 4 shows the FWHM and Q-factor of the resonant peak at 1548.98nm at V = 0, 30, 90, 120, and 150V. The FWHM of the resonance at 0, 30, 90, 120, and 150V are 0.06, 0.06, 0.064, 0.066 and 0.062 nm, and their corresponding Q-factors are 25892, 24274, 24275, 24276 and 24991, respectively. The experimental results reveal that the PSBPLC-based resonator maintains its high quality under an electric field, indicating that PSBPLCs offer a uniform change in refractive index and exhibit a much smaller scattering loss than NLCs.

 

Fig. 4 FWHM and Q-factor at V = 0, 50, 90,120, 150V.

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Figure 5(a) plots the measured response times of the PSBP-based ring resonator at various applied voltages form 40 to 150V. As the applied voltage increased, the rise time decreased and the decay time increased. Figure 5(b) plots the response time of the PSBP-based filter at applied voltage of 150V: the rise time is ~300μs and the decay time is ~930μs. Since the polymer network restrains the electrostriction of PSBPLCs, local reorientation occurs upon application of the electric field, yielding a sub-millisecond response time. However, as the applied voltage increases above 150V, the decay time increases and the corresponding response time is of the order of a few milliseconds, owing to the electrostriction or/and the phase transition of the PSBPLCs. Besides, the required voltage of the proposed device can be efficiently decreased by reducing the thickness of the SiO₂ layer in waveguide chip or choosing new blue phase materials with higher Kerr constant.

 

Fig. 5 (a) Response time versus applied voltage. (b) Response time of designed device at applied voltage of 150V.

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4. Conclusion

In conclusion, this work demonstrates an electrically tunable SiN microring resonator that has polymer-stabilized blue phase liquid crystals (PSBPLCs) as upper cladding, with a fast response and a high Q-factor. In an electric field, the birefringence that is induced by the Kerr effect in the PSBPLC film changes the cladding refractive index and yields a 0.45nm shift of the resonant wavelength. During the driving of the PSBPLC-based device, the rise and decay times are in the submillisecond range, and the Q-factor remains high at 20,000. This favorable optical performance of the LC-based ring resonator makes the device useful in practice.