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Abstract
Slot waveguide plays an essential role in achieving high-performance on-chip photonic sensors and nonlinear devices. Ideally, slot waveguide features a large evanescent field ratio and strong electric field intensity in the slot, leading to a high waveguide sensitivity. Unfortunately, the microring resonator (MRR) based on the slot waveguide suffers the less steep spectral slope due to the low quality factor induced by the huge optical propagation loss of the slot waveguide. In this work, a novel dual mode-splitting resonator based on the slot waveguide is proposed and demonstrated to steepen the slope of lineshapes. The device is implemented by two racetrack resonators based on a slot waveguide and a feedback waveguide to introduce coherent optical mode interference, which could induce mode-splitting resonance (MR) with sharp asymmetry line shape and large extinction ratio (ER). The proposed device is fabricated by the standard complementary metal-oxide-semiconductor (CMOS) technologies on silicon-on-insulator (SOI) platform, and the characterization results show dual MRs with an ER of 45.0 dB and a slope rate (SR) of 58.3 dB/nm, exhibiting a much steeper lineshape than that of the conventional MRR with slot waveguide. And the resonance can be tuned efficiently by applying various voltages of the TiN microheater. Investigations in dual MRs devices promote many potential applications in the field of optical switching, optical modulating, and on-chip optical sensing.
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1. Introduction
Slot waveguide has attracted broad research due to its potential applications in optical sensors and nonlinear optical devices [1–5]. The slot waveguide is composed of two rails of high-index material separated by subwavelength width of a low refractive index region, which shows some distinctive characteristics including strong confinement and electric field enhancement in the low index region. Thus, low-index nonlinear materials and low-index analytes filled in the slot can interact with the light efficiently, which is promising to achieve high-performance nonlinear modulators [6,7] and on-chip optical sensors [8,9]. The slot waveguide can be applied to achieve microring resonator (MRR), which play a key role in silicon photonic technologies for a broad range of applications, including electric-optic modulators [10], optical switchings [11], and on-chip optical sensors [12], for further reinforcing the interaction between filled materials and light. However, due to the high optical propagation loss of the slot waveguide, the slot waveguide-based MRR has a limitation in achieving a high quality factor (Q-factor), which degrades the performance of functional devices, such as the detection limit of optical sensors and the power consumption of optical switchings. A lot of efforts have been put in to promote the Q-factor of MRRs to higher values [13–15], which are mainly focusing on optimizing the slot waveguide structure and the fabrication processes to reduce the optical propagation loss. The former usually adopts asymmetric slot waveguide structures to reduce the optical loss. However, these optimized structures give rise to the decrease of the relative optical power inside the slot, which will eventually degrade the performance of the slot waveguide-based devices. And the latter may require high fabrication cost and complexity. Furthermore, the typical critical dimensions of the slot waveguide structures for enhancing the light-matter interaction are beyond the fabrication capability of the current DUV lithography adopted in the mass manufacture of silicon photonic integrated chips. Thus, a feasible structure design for improving the performance of slot waveguide-based MRRs in accord with the DUV lithography fabrication capability is critically desired.
Essentially, the performance of some MRR-based devices can be enhanced by steepening the resonance slope other than boosting Q-factor [16,17]. Mode splitting is a critical phenomenon of photonic resonators, which is induced by the coherent optical mode interference and mainly contains electromagnetically induced transparency (EIT) [18], Fano resonance [19], and fast and slow light effects [20], etc., and can be utilized to achieve versatile functional devices with high performance [21]. Compared with the conventional symmetric Lorentz shape of the resonance spectrum, mode-splitting resonance (MR) presents a sharper slope rate (SR) under the same Q-factor [17,22]. Due to its distinctive characteristics, mode-splitting has attracted intensive studies in versatile filters [23], high-sensitivity sensing [24], and all-optical signal modulating [25]. Besides the single MR, the multiple MRs have gained much attention due to their superior performance in terms of biochemical sensing, optical switching, and modulating at several spectral positions simultaneously [26,27]. However, the slot waveguide-based ring resonator with enhanced SR and extinction ratio (ER) utilizing dual MRs has not been investigated in detail yet.
In this work, a novel tunable dual mode-splitting resonator based on slot waveguide was proposed and demonstrated experimentally on a CMOS-compatible 8-inch SOI photonics platform and by DUV lithography. In the proposed device, the coherent optical mode of conventional Lorentz resonances generated from two add-drop MRRs interferes with the optical mode of a feedback waveguide, leading to dual MRs and therefore a higher ER and a sharper SR. In addition, titanium nitride (TiN) micro-heaters are employed to tune the resonant wavelength, as well as tune the wavelength shift introduced by fabrication non-uniformity and temperature drift. The measurement results show the MR with a high ER of 45.0 dB and a large SR of 58.3 dB/nm, exhibiting a much higher ER and steeper lineshape of the resonance than that of the conventional MRR based on slot waveguide with a narrower gap reported before [28–30]. The proposed structure can be adopted to build high-sensitivity on-chip optical sensors and high-performance electric-optic (E-O) modulators by opening the sensing window above the MRR based on slot waveguide and filling E-O materials in the slot, respectively.
2. Working principles and device design
The schematic diagram of the proposed MRR based on slot waveguide is shown in Fig. 1, mainly consisting of four parts: two ring resonators, a feedback waveguide, grating couplers, and TiN micro-heaters. The cross-section of the slot waveguide, the top-view schematics of the strip-slot mode converter and the focusing grating coupler are shown on the bottom half of Fig. 1. The input light is coupled into the waveguide from the focusing grating coupler, and the light with the resonance wavelength is coupled into two MRRs. Then the rest of the light propagates forward in the feedback waveguide and interferes with the light coupled into the bus waveguide from ring resonators. The TiN micro-heater above two ring resonators is adopted to adjust the transmission spectrum via applying voltages. The coherent optical modes of drop ports of two ring resonators and the feedback waveguide interact with each other. As a result, the symmetric Lorentz lineshape would be transformed to an asymmetric dual mode-splitting lineshape. The connection between the different devices still employs the conventional strip waveguide for its lower optical loss, while the slot waveguide only appears in the region of functional devices. However, the optical mode in the slot waveguide is a non-Gaussian-like mode, whereas the light mode in the strip waveguide is a Gaussian-like mode, leading to an additional loss of the direct connecting [31]. Thus, a low loss strip-slot mode converter is adopted to convert the light from the strip waveguide into the slot waveguide, and such silicon mode-converters have been demonstrated to exhibit only low losses of about 0.1 dB [32].
The transfer matrix method is employed to analyze the relationships of the input and the output in the proposed device [33–35]. The feedback waveguide-coupled MRRs can be divided into two elements, single-microring resonators, and parallel transmission lines, as shown in Fig. 2. Assuming no coupling between two adjacent rings, the transfer matrix equation for the proposed device can be expressed as Eq. (1),
The terms of the numerator in Eq. (8) represent transmissions of pathways light from E1i to E1o [27]. τ11 represents discrete state and r11r12r21r22${\textrm{e}^{\textrm{j(2}{\mathrm{\varphi }_\textrm{2}}\textrm{ + }{\mathrm{\varphi }_\textrm{3}}\textrm{)}}}$ represents continuum state that light passes by MMR1 and MMR2 directly. In addition, the other terms are coupling between the optical mode of drop ports of two ring resonators and the feedback waveguide. The light of different pathways interfaces with each other to form the MR. The analysis above is focusing on the MRR based on the strip waveguide, while the proposed device structure in this work is based on the racetrack ring resonators and the slot waveguide, which is much more complex to analyze. The basic principles and device model are identical, however, the phase shift introduced by the coupling region cannot be neglected. Thus, the transmitting coefficient t, coupling coefficient k, and their conjugates in Eqs. (4), (5), (6), and (7) should be changed into $\textrm{t}{\textrm{e}^{\textrm{j}{\mathrm{\varphi }_\textrm{t}}}}\textrm{,k}{\textrm{e}^{\textrm{ - j}{\mathrm{\varphi }_\textrm{k}}}}{,\; }{\textrm{t}^{\ast }}{\textrm{e}^{\textrm{j}{\mathrm{\varphi }_\textrm{t}}}},\; \textrm{and}\; {\textrm{k}^{\ast }}{\textrm{e}^{\textrm{ - j}{\mathrm{\varphi }_\textrm{k}}}}$, respectively [37].
The transfer matrix method was also adopted to investigate the operation of the proposed device based on numerical simulation. The transmission spectral responses of the conventional slot waveguide-based ring resonator and the slot waveguide-based mode-splitting resonator with the identical coupling efficiency (k11 = k12 =0.268) and the typical optical propagation loss (10 dB/cm) are shown in Fig. 3(a) and 3(b), respectively. The transmission spectral response of the proposed device exhibits an asymmetric lineshape with sharper SR of 63.0 nm/dB and equal ER of 12.6 dB. Compared to the SR of 7.1 nm/dB of conventional the conventional slot waveguide-based MRR, the simulated results suggest the enhanced SR of 63.0 nm/dB of proposed device. The full width at half maximum (FWHM) of the slot waveguide-based MRR is 0.351 and the Q-factor is 4300, while the FWHM of the slot waveguide-based mode-splitting resonator is 0.034 nm and the Q-factor is 45000. Then, the transmission spectral response according to the change of the effective refractive index (neff) of MRRs are also investigated and numerical simulation results are shown in Fig. 4. With the increase of the neff of MRRs, the MR is tuned and redshifted. Figure 4 shows the tuning of the simulated MR of the proposed device. With the increase of Δneff of MRRs, the resonant wavelength with maximum ER shifts from MR1 to MR2. And the change of the neff of MRRs also affects the variation of the intensity of the MR1 and MR2, which is shown in Fig. 4. The ER of the resonant wavelength is significantly affected by the variation of the neff. With the increase of the neff of MRRs, the ER of the MR1 increases firstly and then decreases, and the biggest ER of the resonance shifts from left to the right, as well as the MR2. Furthermore, the variation of the length will cause the variation of the amplitude and the phase shift, leading to the change of the transmission spectrum [38].
Fig. 3. (a) Simulated Lorentz resonance of the conventional ring resonator based on slot waveguide and (b) simulated mode-splitting resonance of the proposed device.
Fig. 4. (a) Simulated mode-splitting resonance of the dual mode-splitting resonator based on slot waveguide with increases of Δneff, (b) and (c) zoomed-in view of the resonant dip.
From the numerical investigation above, the output transmission spectrum of the proposed device exhibits a higher ER and a sharper SR than that of the conventional slot waveguide-based resonator, indicating MR can be obtained and the ER and the SR can be enhanced by optimizing the parameters of the proposed device structure. The gap and rails of the slot waveguide and the radius of the MRR1 and MRR2 are 180 nm, 200 nm, R1 = 20 µm, and R2 = 20 µm, respectively. And the coupling lengths of the two racetrack rings are both 20 µm. In addition, the lengths of through waveguides L1 and L2 and the and the radius of L3 are 200 µm, 200 µm, and 20.93 µm, respectively. Furthermore, the transmission response can be tuned efficiently by adjusting the effective refractive index of the waveguide, demonstrating that the output lineshape can be optimized efficiently by changing the DC power applied to TiN micro-heaters above MRRs. The transmission spectral response is extremely sensitive to the shift of the refractive index of the microring, showing a great potential application in optical switches, electro-optic modulators, and on-chip optical biosensors.
3. Fabrication and characterization
3.1 Fabrication
The thermally tunable dual mode-splitting resonator based on the slot waveguide was designed and fabricated on the SOI wafer with 220 nm-thick top silicon and 3 µm-thick buried oxide. The proposed device was fabricated by the standard CMOS processes. The deep ultraviolet (DUV) photolithography was employed to form the waveguide and devices patterns on photoresist as a soft mask. Double Inductively coupled plasma (ICP) etching processes were applied to transfer the patterns from the photoresist layer to silicon to form waveguides and devices. 220 nm full etching silicon waveguides were defined for microrings and strip-slot mode converters, and 70 nm shallow etching waveguides were defined for grating couplers. Then 1 µm-thick SiO2 was deposited on top of the wafer by plasma-enhanced chemical vapor deposition (PECVD), which is adopted to isolate devices and TiN microheaters to avoid potential optical losses, whereas maintaining the relatively high thermo-optic efficiency. Subsequently, a 10 nm-thick Ti adhesion layer, a 20 nm-thick TiN barrier layer, an 800 nm-thick AlCu layer, and a 20 nm-thick TiN anti-reflective layer, were deposited by physical vapor deposition (PVD) and then patterned to define the electrodes. Next, a 50 nm-thick TiN layer deposition for thermo-optic effect and followed by lithography and etching process to form microheaters. Finally, a 1 µm-thick SiO2 was deposited as the top cladding layer, and followed by the bonding pad opening process.
The scanning electron microscope (SEM) images of the focusing grating, the mode converter, the coupling region, and the feedback part of the racetrack resonator are shown in Fig. 5(a)–5(d), respectively. In addition, Fig. 5(e) presents the top-view optical microscope image of the fabricated device, which monolithically integrates a slot-based dual mode-splitting racetrack resonator with a TiN microheater. In this work, the zigzag TiN heater is placed on the top of two MRRs and the coupling regions, and can be adopted to modulate each MRR independently, as shown in the Fig. 5(e). The heater was adopted to form a uniform thermal distribution, compensate the manufacturing error, and change resonant lineshapes of the MRRs efficiently for achieving enhanced sharp SR and large ER.
Fig. 5. The SEM images of (a) the grating coupler, (b) the mode converter, (c) the coupling region, and (d) the feedback part of the mode-splitting racetrack resonator; (e) The optical view of the fabricated device. The chip monolithically integrates a slot-based dual mode-splitting racetrack resonator with a TiN microheater.
3.2 Characterization
The fabricated device is characterized by using a tunable laser, a polarization controller (PC), an optical spectrum analyzer (OSA), and a tunable DC voltage source. The laser is coupled into the chip from the fiber through a focusing grating coupler with a 4.5 dB coupling loss. Then the output light is coupled into another fiber directly connecting to OSA for transmission spectrum observation. Due to the resonance mode interference between MRRs and the feedback waveguide, dual MRs are produced. The measured transmission spectral response of the fabricated device is shown in Fig. 6(a). Due to the unavoidable fabrication nonuniformity, the resonance wavelength in the MRR1 is different from that in the MRR2. Thus, lineshapes of the MR in two microrings are not identical. The maximum ER as high as 31.4 dB is at the left of MR1 and the average SR is 43.1dB/nm without applying DC voltage to the TiN microheater. The FWHM of the MR1 and MR2 is 0.33 nm and 0.24 nm, and the corresponding Q-factor is 4300 and 6400, respectively.
Fig. 6. (a) The measured transmission of the slot-based mode-splitting racetrack resonator with different tuning powers on the microheaters, (b) and (c) zoomed-in view of the resonant dip.
In order to evaluate the tunable capability of the proposed ring resonator structure, DC voltage was applied to the TiN microheater, which changes the effective index of the waveguide due to the thermo-optic effect. Figure 6 shows the measured transmission spectrum of the fabricated device with the different tuning powers. When the applied DC power is increased, the effective refractive index of the waveguide increases due to the thermo-optic effect of silicon, and therefore the transmission spectrum is tuned and the MR is redshifted, which is in accordance with the results of the numerical investigation. With the DC power increased from 0 to 19.66 mW, the MR wavelength with the maximum ER and SR redshifts from 1525.90 nm to 1534.38 nm. At 1534.38 nm wavelength, the measured ER and the average SR are up to as high as 45.0 dB and 58.3 dB/nm. The measured ER is higher than the simulated results due to the stronger coupling efficiency between the bus waveguide and the microring. The coupling efficiency is significantly influenced by the gap of between the bus waveguide and the microring, and the optical confinement of the slot waveguide. The unavoidable fabrication error of the device, including narrower gap between the bus waveguide and the microring or larger gap of the slot waveguide, lead to a stronger coupling, resulting in the higher ER.
The larger ER and the sharper SR of the proposed device provide enhanced sensitivity and detection limit in sensing applications. which is a novel configuration idea by utilizing the dual-mode splitting rather than enhancing the Q-factor of the MRR via optimizing the fabrication process. Moreover, the maximum resonance notch of the slot-based mode-splitting racetrack resonator is considerably tuned by the shift of the refractive index, which achieves a large shift of maximum resonance dip with a small change of the refractive index, showing great potential in high-performance applications like optical switches, modulator, and on-chip biosensor. The analytes or E-O material can be filled into the slot to build an on-chip biosensor with extremely high sensitivity or an E-O modulator with ultra-high bandwidth. In addition, the characteristics of the slot waveguide, such as electric field and evanescent field enhancement, make it possible to achieve multi-functional devices.
4. Conclusions
In this work, slot waveguide-based ring resonators with a feedback coupled waveguide are proposed and demonstrated to form a dual mode-splitting resonance with a sharp slope rate. In this device, a TiN microheater is adopted to tune the resonance through the thermo-optic effect. The experimental results indicate that the proposed tunable device can achieve mode-splitting resonance with a sharp lineshape. The transmission spectrum achieved a high ER of 45 dB and a sharp SR of 58.3 dB/nm. The resonance wavelength of the maximum ER of the mode-splitting resonance can be largely shifted from 1525.90 nm to 1534.38 nm with power consumption from 0 to 19.66 mW. These characteristics combined with the strong light confinement in the slot enable a high potential in on-chip sensing, switching, and modulating applications.
Funding
National Natural Science Foundation of China (61904196).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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