1. Introduction

Microring resonators have been studied for use in numerous devices, including optical signal filters [1], laser resonator [2], and biochemical sensors [3], owing to their excellent wavelength- selecting ability. The operational mechanisms of micro-ring resonators are similar to those of Fabry–Perot filters. However, their small size and simple waveguide structure make them particularly attractive for use in complex integrated optical systems. Micro-ring resonators exhibit a sharp resonance at specific wavelengths, which can be determined by the waveguide structure design and the adopted core and cladding materials. Hence, numerous attempts have been made to develop an active microring resonator and various optical operation functions, such as optical modulation [4, 5] and optical bistability [6–8]. Some commonly used methods for modulating ring resonators are based on the electro-optical effect [9], thermo-optic effect [10], and free carrier injection [11]. However, these methods still are associated with problems, such as large power consumption and dissipation, small tuning range, and low integration density, which restrict their range of practical applications. Furthermore, all of these schemes require external sustain power to be maintained in the switched state.

Recently, the integration of microring resonators with organic nematic liquid crystal (NLC) cladding materials have been attracted attention because the NLC has a large variable refractive index, which can be easily controlled by external forces and so enables the tuning of a resonant wavelength over a wide range with a low switching power [12–15]. Cort et al. proposed wide-ranging electrical tuning of an Si ring resonator using NLC as top cladding [13]. Electrically controlling the orientation of the LC directors changes the effective refractive index of the resonator, shifting the resonant wavelength over a wide range of 31 nm in TM mode and 4.5nm in TE mode. Wang et al. also demonstrated electrically controlled NLC based SiN ring resonator [14]. Nevertheless, although the NLC-based ring resonator supports excellent optical modulation, to the best of the authors’ knowledge, no study has demonstrated an optically bistable microring resonator that is based on a liquid crystal material.

This work demonstrated, for the first time, the optical bistability in an SiN microring resonator with an azo dye-doped liquid crystal (DDLC) cladding. The DDLC can be switched between NLC and PHI states by the photo-induced isomerization of azo dyes in liquid crystal, and it remains in each state for tens of hours [16–18]. Switching between the two states provides the resonator with two cladding refractive indices and, therefore, two sets of resonance wavelengths. The tunability, stability, and high-contrast filter performance of the hybrid LC-SiN microring resonator were studied.

2. Sample fabrication and measurement

Figure 1(a) presents the structure of an SiN microring resonator with azo dye doped liquid crystal (DDLC) cladding. The developed device consists of two parts - the waveguide chip and the organic DDLC film. The chip comprises an Si substrate, a 16 µm-thick SiO₂ layer and a 0.5 µm-thick SiN layer. The SiO₂ layer, which is located between the SiN layer and the Si substrate, acts as the lower cladding layer to reduce losses by optical leakage. The photonic device with SiN as the core material is suitable for integration with liquid crystals, which have a similar refractive index, enabling tuning of the microring resonator over a wide range. The designed resonator structure, as shown in Fig. 1(b), is composed 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 0.5 µm. To integrate the chip with a DDLC upper cladding layer, a glass substrate coated with vertical alignment film (DMOAP, Aldrich) was covered on the SiO₂ layer and they were sandwiched with surrounded plastic ball spacers to form an empty cell. The polyimide-coated glass and ball spacers were used to induce the homeotropic alignment and control the thickness for liquid crystals, respectively. It is noticed that the thickness of spacer must be larger than the evanescent tail length of the optical field in the microring resonator (around 1.8 µm, calculated by commercial software, PhoeniX) to obtain good tuning effect. The spacers with the diameter of 5µm were chosen herein which was large enough and could reduce the influence of nonuniform LC thickness to optical performance of microring resonator. The organic materials were prepared by doping azo dye 4-methoxyazobenzene (4MAB, Fluka) into a nematic liquid crystal (LC) (E7, Merck). The mixing ratio of 4MAB: E7 adopted in this work was 30:70 by weight. The mixture in isotropic state was injected into the cell and then cooled to room temperature to form DDLC film. The DDLC film has two stable states, NLC and photo-induced isotropic (PHI) states, which can be switched from one to the other by applying 532 nm and 408nm light. Azo dyes comprise two molecular configurations - trans and cis. The rod-like trans isomers are aligned with LC molecules by the guest host effect in the DDLC cell. Upon UV-blue irradiation, the rod-like trans isomers are transformed into bent cis isomers via the photo isomerization process. These bent cis isomers disturb the order of the LC molecules and switch the nematic state to the PHI state [16, 17]. The PHI state can last for many hours before they relax back to the NLC state. Applying green light causes fast back-isomerization, which takes approximately a few seconds, from the PHI to the nematic state.

 

Fig. 1 (a) Schematic cross-section and (b) top view of the proposed resonator.

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3. Result and discussion

Figure 2 schematically depicts the bistable operation of the designed micro-ring resonator between the NLC and PHI states of the DDLC film. When the DDLC is in the NLC state, NLC and azo dye molecules are aligned parallel to each other, perpendicular to the glass substrate under the influence of the hometropic alignment film. Hence, if the incident light in the SiN waveguide is in the TE mode, the cladding refractive index that it sees is the ordinary refractive index (n_o) of the NLC molecules, whereas the TM-polarized light experiences extraordinary refractive index (n_e). Upon irradiation under 408nm light, the rod-like trans isomers of azo dyes are transformed into bent cis isomers by the trans-cis isomerization. Then, these bent cis isomers disturb the order of the LC molecules and change the NLC to the PHI state, which exhibits isotropic optical property. After the 408nm light has been turned off, the PHI state remains, and the refractive index of the cladding changes to n_iso, which can be represented as $n_{iso}={\left(n_e^2/3+2n_o^2/3\right)}^{1/2}.$ The change in the refractive index of the cladding modifies the effective refractive index of the microring resonator and thereby shifts the resonant wavelength as specified by the resonance equation, ${\lambda }_m=2\pi R{n}_{eff}/m,$ where m, the grating order, is a positive integer; λ_m denotes the wavelength of the m-order resonant mode; R is the radius of the resonator, and n_eff is the effective refractive index of the resonator. In the absence of 408nm light, the PHI state naturally relaxes back to the nematic state via the cis-trans isomerization process. The time of the reaction from the PHI state to the nematic state depends on the lifetime of the cis isomers [18]. The PHI state, with its long period of stability (up to several hours) can be realized by doping a high concentration of azo dye dopant, as demonstrated herein. Once the PHI state has been switched back to the NLC state by irradiating with 532nm light, the corresponding resonant wavelength quickly shifts back to its original value. Therefore, all-optical bistable switching operation can be realized by switching between NLC and PHI states by irradiation under 532 nm and 408nm light, respectively.

 

Fig. 2 Principles of two bistable switching operation of the proposed resonator at the NLC and PHI states.

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Figure 3(a) shows the structure of the resonator with DDLC cladding, which was observed under a polarization optical microscope in reflection mode (R-POM) without a polarizer. The designed structure consists of a straight waveguide and a microring, and their input and output ports were on the same side to facilitate measurement. Figures 3(b) and 3(c) display images of the ring resonator with DDLC cladding in the NLC and PHI states in a zero field. The dark images induced by the homeotropic LC alignment can be observed when we rotate the sample under the R-POM with crossed polarizers. As shown in Fin. 3 (b), minor LC directors close to ring and straight waveguides are affected by the etched edge structure, in which LC directors can be regarded as distributed with symmetric azimuth angles [13, 14] and some light leakage can be observed. When irradiated with 408nm light, the LC material entered the PHI state. Since the PHI state is an isotropic liquid state without any birefringence, the device is perfectly dark under the crossed R-POM, as presented in Fig. 3 (c).

 

Fig. 3 Photographs of proposed resonator with a DDLC cladding, observed under R-POM (a) without any polarizer and with crossed polarizers at two stable states: (b) NLC state, (c) PHI state.

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Figure 4 presents the normalized transmission spectra of the hybrid LC-SiN resonator in TE mode with NLC and PHI states. In this experiment, an infrared tunable laser was used as a light source and a power meter was utilized to measure the output intensity. Two unpolarized DPSS 408nm and 532nm lasers with an intensity of 150mW/cm² were applied to switch the bistable liquid crystals of the resonators. The diameter of the pumping spot on the waveguide was adjusted to 5 mm so as to cover all regions in the device. The spectral range was from 1530nm to 1570nm, in the infrared region, which includes the commonly used optical communication region. It included nine transmission dips that corresponded to the resonance states of the microring. The free spectral range of the resonator was approximately 4.4 nm. When the NLC state was transformed into the PHI state by irradiation using 408nm laser light, the transmission peaks shifted toward longer wavelengths because the effective refractive index of the DDLC was increased; the transmission spectra were shifted by around 1.9 nm. To compare the properties of the hybrid LC-SiN micro-ring resonator in the two states, Fig. 4(b) displays the two resonant peaks (NLC state at 1548.7 nm and PHI state at 1550.6 nm). The FWHM of the resonance in the NLC state and PHI state are 0.15 and 0.07 nm, and the corresponding Q-factors are 10324 and 22152, respectively. The Q-factor in the NLC state is lower than that in the PHI state, primarily because of the scattering loss from the nematic liquid crystal, especially close to the edge of the waveguide structure. Unlike the NLC state, PHI state with a uniform refractive index has a smaller scattering loss and a higher extinction ratio.

 

Fig. 4 (a) Transmission spectral of proposed resonator at NLC and PHI states, (b) detailed transmission spectra between 1548 and 1552nm.

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The stabilities of the NLC and PHI states were examined by first conditioning the device into either the NLC or the PHI state under irradiation with 532nm or 408nm light and monitoring the change in the resonance wavelength as a function of time in the absence of illumination. Figure 5 plots the change of resonance wavelength as a function of time for both NLC and PHI states. As expected, the resonance wavelength in the NLC state is stable because the state is stable. The resonance wavelength of the PHI state is initially almost constant; then the DDLC relaxes from the PHI state to the NLC state, causing a blue-shift toward the resonance wavelength of the NLC state. After ten hours, the orientation of the LC directors had still not returned to the most stable orientation, causing the resonance wavelength to undershoot the resonance wavelength of the NLC state. After a long relaxation period of ca. 15 hours, DDLC settles into the NLC state. Based on the above illumination-induced switching, the proposed DDLC-SiN micro-ring device can be utilized as an all-optical bistable switch, operating in the “normally on” condition. The device is conditioned to the NLC state at the resonant wavelength of the NLC. The output signal is in the low state (on resonance). Then, illumination under 408nm light switches the device into the off state (off resonance). In this case, the device remains in the off state without further illumination (sustain power) for nearly ten hours. This period can be changed by appropriately choosing the azo dye [19].

 

Fig. 5 Variations of resonance wavelength as a function of relaxation time for both NLC and PHI states.

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To further confirm the reversibility and stability of the bistable switching of the proposed resonator, switching between NLC and PHI states was repeated and the corresponding resonant wavelengths were measured. The irradiating intensity and duration of the two pumping 408nm and 532nm lasers were set to 150mW/cm² and 30s, respectively. Figure 6 plots the reversible optical bistable switching of the resonant wavelength between 1548 and 1551 nm. The resonant wavelengths of PHI or NLC were independent of the number of switching cycles; their corresponding error ranges were 0.03 and 0.06 nm, respectively. A large reversible spectral shift in the bistable switching operation of the proposed resonator is desirable. The optical bistability of DDLC exceeds that in the micro-ring resonator that exploits the thermal-optic non-linear effect or the free-carrier dispersion effect, providing such advantages as a wide spectral shift and a long memory effect; most importantly, its optical bistability is bias-free, and it does not require a constant power supply to continue to be operated in two states.

 

Fig. 6 Dependence of the resonant wavelength on the switching cycle. Observed resonant peaks between 1548 and 1551 nm

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4. Conclusion

In conclusion, this study demonstrates optical bistability in an SiN micro-ring resonator that is based on a DDLC film as a cladding material. The phase transition between the NLC and PHI states in the DDLC film changes the cladding refractive index and shifts the resonance wavelength by 1.9nm. Both NLC and PHI states can be retained stably for many hours, and one can be switched to the other by irradiation with 532 and 408nm light, respectively. Such a bistable device has many advantages, such as a wide range of wavelength shift, a long memory characteristic, and zero idling-power consumption. It therefore has great potential for practical use in optical signal processing, optical switching and even optical computing