Dual-use on-chip polarizer based on straight silicon nitride platform

As an alternative to the high-index silicon platform in photonic integrated chips, the silicon nitride (Si₃N₄) platform has gained considerable interest due to its low light propagation loss^1–3 and compatibility with the complementary metal–oxide–semiconductor (CMOS) technology.^4–6 Si₃N₄ based photonic components can be found in many integrated photonic devices, including frequency comb generators,^7–10 optical gyroscopes,¹¹ and radio-frequency filtering.¹² However, the refractive index difference between Si₃N₄ and the silicon dioxide substrate results in strong birefringence, leading to polarization-sensitive performance in integrated optics devices.¹³ In fact, on-chip optical elements for polarization control, such as polarizers,^14–17 polarizing rotators,^18,19 and polarizing beam splitters,^20–22 play important roles in regulating the polarization state of integrated photonic chips.

On-chip polarization filters are widely used for purifying the polarization and minimizing the polarization crosstalk. A commonly used method to achieve broadband on-chip polarizers (OCPs) with a high polarization extinction ratio (PER) is to design a waveguide structure with a strong polarization dependent loss, say, by absorbing, reflecting, or leaking the unwanted polarization. The earlier on-chip polarizers (OCPs) using a shallow-etched ridge waveguide are long (∼1 mm)²³ and are thus not suitable for high-density integrated photonic chips. Compact silicon-based polarizers with hybrid plasmonic structures were proposed to reduce the device length, but the insertion loss (IL) is increased due to intrinsic metal absorption.^24,25 Low loss polarizers can be achieved with 2D graphene-based materials. However, the latter is incompatible with CMOS technology in large-scale fabrication.²⁶ Sub-wavelength grating, shallow etched waveguide, and slot waveguide structures have been proposed to realize polarizers with a high PER, low IL, and a wide operating bandwidth, but these require complex processing technology.^15–17 OCPs based on grating structures can achieve a high PER and a wide working bandwidth, but these structures suffer from back reflection.^27–29 

Normally, an OCP transmits a given polarization mode in a waveguide within the integrated device. It is difficult to temporally control the transmitting mode polarization. However, adjustable polarization control devices are often needed in polarization-sensitive integrated photonic chips,^30–32 such that their performance can be modulated in response to the transmitting polarization states. Traditionally, polarization management is achieved by mechanically rotating the wave plates or utilizing the fiber-squeezing birefringence effect. These methods cannot achieve single-polarized light with a high polarization extinction ratio. Some on-chip polarization control devices can allow polarization modulation by utilizing thermo-optic,³³ carrier dispersion,³⁴ and electro-optic³⁵ effects, but these devices are difficult to design and prepare, and the polarization modulation properties are unstable. In this paper, we propose an OCP that can also be switched to an optical waveguide; by covering the polarizer with a layer of high refractive index material (HRIM), it becomes a normal optical waveguide that can transmit any polarization state with low loss. This flexible OCP device also has a simple structure and stable performance. Based on the working principle of this dual-use OCP, the real-time polarization control of TM polarization can be achieved without affecting the TE polarization state by covering a phase change material with an alterable refractive index, which provides a potential application value and solution for the realization of reconfigurable optical logic gate devices with large bandwidths.

Most OCPs use the silicon-on-insulator optical platform, and polarizers based on the silicon nitride optical platform are rarely reported.³⁶ The dual-use all-Si₃N₄ TE-pass integrated OCP proposed and demonstrated here has a wide working bandwidth and a high polarization extinction ratio, as well as a simple straight ridge waveguide structure. Simulation results show that this OCP has a PER as high as 60 dB, an IL below 0.3 dB at around 1550 nm, and a bandwidth of 150 nm for the PER >40 dB. In fact, the fabricated polarizer has an ultrahigh PER of more than 40 dB over a wide bandwidth of 80 nm, and the measured IL is <1 dB in a wide wavelength range of 1540–1640 nm. Moreover, by covering the polarizer with a photoresist layer of refractive index 1.57 and thickness >1 μm, the OCP can guide two orthogonal polarization states through the waveguide structure, and the measured IL is <1 dB for both polarization modes. That is, the polarization states of light transmitting through this OCP are controllable.

The proposed polarizer on Si₃N₄ is based on a simple straight ridge waveguide structure for single mode propagation. The principle of this polarization control device is as follows: the TE₀ mode can pass through the waveguide with low loss and the TM₀ mode would leak out through the waveguide into the substrate. However, by covering the polarizer with a HRIM layer, both the TE₀ and the TM₀ modes can be transmitted in the waveguide with low loss, as shown in Fig. 1.

Si₃N₄ ridge waveguide structures can transmit TE₀ and TM₀ modes of different effective refractive indices due to the asymmetric cross section of the waveguide. With an appropriate aspect ratio of the rectangular waveguide structure, the effective refractive index of the TE₀ mode can be larger than that of the TM₀ mode. A finite difference eigenmode scheme was used to simulate the effective refractive index of the TE₀ and TM₀ modes transmitting in the waveguide. Figure 2(a) shows that if the width of the waveguide is 800 nm, the effective refractive index of both the TE₀ and TM₀ modes decreases together with the waveguide height. When the height of the waveguide is less than 380 nm, the effective refractive index of the transmitted TM₀ mode is close to that of the SiO₂ substrate, indicating that the TM₀ mode can more easily leak into the latter. Thus, working as an OCP, a Si₃N₄ ridge waveguide structure of width 800 nm and height 360 nm can transmit only the TE₀ mode with low loss, and the TM₀ mode will leak out with high loss. At an operating wavelength of 1550 nm, the effective refractive indices of the TE₀ and the TM₀ modes in this tailored waveguide are about 1.534 and 1.44, respectively, thereby guaranteeing good performance of the OCP. The performance was also analyzed using the 3D finite-difference time domain method, with the refractive indices of Si₃N₄ and SiO₂ set at 1.99 and 1.444, respectively, for 1550 nm light. As shown in Figs. 2(c) and 2(d), the TE₀ mode can propagate in the waveguide but the TM₀ mode cannot. The effective refractive index of the TE₀ and TM₀ modes can be increased by covering the ridge waveguide with a layer of HRIM. Figure 2(b) shows the refractive index vs the height of the waveguide. Although the effective refractive index of both the TE₀ and TM₀ modes still decreases together with the waveguide height, when the waveguide height is 360 nm, the effective refractive index of the TM₀ mode is about 1.53, which is larger than that of the SiO₂ substrate. Thus, both the TE₀ and the TM₀ modes can now propagate in the waveguide since the HRIM layer has significantly enhanced the confinement of the TM₀ mode in the direction perpendicular to the sample surface. Figures 2(e) and 2(f) for the simulated TE₀ and TM₀ optical fields also show that the polarizer function of the ridge waveguide is inhibited if it is covered with HRIM.

The fields of 1500, 1600, and 1700 nm TE₀ and TM₀ modes propagating in our Si₃N₄ ridge waveguide were also simulated and shown in Fig. 3. We see that the TE₀ modes can freely propagate through the device with low loss, but the TM₀ modes are dissipated. Moreover, the IL increases with the wavelength.

The transmitted light spectrum of the Si₃N₄ polarizer for 1500–1700 nm TE and TM modes are shown in Figs. 4(a) and 4(b), respectively. The PER [as defined by 10 log₁₀(T_TE/T_TM), where T_TE and T_TM are the transmittance of the TM₀ and TE₀ modes, respectively] in the wavelength range from 1540 to 1700 nm of the TM₀ mode exceeds 40 dB, and the transmission loss of the TE₀ is less than 1 dB. Figures 4(c) and 4(d) show the transmission properties of the TE₀ and TM₀ modes in the waveguide covered by a HRIM layer. According to the simulation results, both the TE₀ and the TM₀ polarization modes can be transmitted in the waveguide with low loss. In fact, the transmission loss of the TE₀ mode is slightly less than that of the TM₀ mode. Compared with the transmission loss of ∼60 dB for the TM₀ mode in the uncovered waveguide, the transmission loss in the covered waveguide is <0.6 dB, i.e., the polarizer has become a normal waveguide for both TE₀ and TM₀ modes.

We have also considered the processing tolerance of the proposed device. The transmitted spectra of the TE₀ and the TM₀ modes in straight Si₃N₄ ridge waveguides with slightly different (±10 nm) heights and widths were simulated and shown in Figs. 5(a) and 5(b). The low IL and high PER of these cases confirm the large fabrication tolerance of the polarizer structure. Figures 5(c) and 5(d) show that after the covering, both the TE and the TM polarization modes can be transmitted in all these waveguides with IL < 1 dB. Thus, our OCP has a good processing tolerance.

According to the simulation results, the on-chip polarizer based on a straight ridge waveguide structure was fabricated on the Si₃N₄ thin film using a self-aligned fabrication process, as discussed below. Using the plasma enhanced chemical vapor deposition method (PECVD), a 360 nm-thick Si₃N₄ layer was deposited onto a SiO₂ substrate. Then, a layer of negative photoresist SU-8 was spin-coated on top of the Si₃N₄ wafer, and an E-beam lithography process was used for photoresist patterning. After developing and baking the patterned photoresist, a dry etching process was used to transfer the photoresist pattern to the top-Si₃N₄ layer. The polarizer consisting of a straight ridge waveguide with a width of 800 nm and a height of 260 nm was fabricated on the Si₃N₄ thin film, as shown in Fig. 6.

To measure the transmission properties of the polarized light in our Si₃N₄ polarizer, a tunable (1510–1640 nm) laser and a three-paddle fiber polarization controller were used to produce the TE₀ and the TM₀ polarized light, which was then coupled to the Si₃N₄ OCP using a lens fiber with the help of an alignment platform. The transmission spectrum powers were measured using an optical powermeter. The schematic diagram of the chip testing platform is presented in Fig. 7(a). The transmission loss of the TE₀ and the TM₀ modes in the 0.5 and 1 mm long waveguides was measured. Because the measured coupling loss of the waveguides is almost the same, with the offset less than 0.2 dB, by subtracting the loss of the 0.5 mm waveguide from the total loss of the 1 mm waveguide, the coupling loss can be removed and the transmission loss of the waveguide was obtained. For 1550 nm light, the measured transmission losses of the TE₀ and the TM₀ modes in the OCP are 0.34 and 43 dB/mm, respectively.

The normalization transmission spectra of the fabricated Si₃N₄ polarizer were measured and presented in Figs. 7(b) and 7(c). The polarizer can achieve a bandwidth of 100 nm with a PER larger than 25 dB for the TM₀ mode, larger than 40 dB in the 1560–1640 nm regime, and a peak value of 53 dB at 1595 nm. The measured working bandwidth of the polarizer was limited by the test laser’s operation wavelength range (1510–1640 nm). However, according to the test data and simulation results, the actual working wavelength of the OCP can be up to 1700 nm, i.e., the bandwidth of the OCP is larger than 100 nm. The measured IL of the polarizer was less than 1 dB for the TE₀ mode. The transmission spectrum of the waveguide covered with a photoresist layer (HRIM) (SU8 photoresist with a refractive index of 1.57) was also measured and shown in Fig. 7(c). The transmission loss of the TM₀ mode decreased dramatically, and the IL of both the TE₀ and TM₀ modes was less than 1 dB. Therefore, after the polarizer is covered with the thin HRIM layer, it loses its polarizer function and becomes a normal waveguide. The testing results are basically consistent with the simulation results, and the inconsistency can be attributed to some factors, such as the fabrication errors, scattering loss from the rough sidewalls of the ridge waveguides, miss-alignment of the waveguide, and non-uniformity of the deposited Si₃N₄ layer.

For completeness, a comparison of the performance of our polarizer with that of the existing polarizers is given in Table I. Most reported works on OCPs are based on silicon-on-insulator platforms. One can see from the table that the performance of our Si₃N₄ polarizer based on a simple straight ridge waveguide structure is comparable with that of the existing polarizers. In fact, it possesses a high PER, low IL, a wide bandwidth, simple configuration, and large fabrication tolerance. Moreover, the length of the device can be further shortened by reducing the thickness of the waveguide or using a curved structure. These advantages make the Si₃N₄ polarizer practical for integrated photonics applications.

In addition, the HRIM cover-layer feature of this polarizer makes the polarization-sensitive integrated Si₃N₄ photonic devices multi-functional. For example, if the refractive index of the cover material is optically or otherwise controllable, the present polarizer can act as a grating device in modern optical circuits. According to our simulation results, by covering the ridge waveguide with a phase change material layer (VO₂) of refractive index 3.2 in rutile phase with thickness <1 μm, the OCP can guide two orthogonal polarization states just as a normal optical waveguide, and the measured IL is <1 dB for both polarization modes. However, the OCP function is recovered through appropriate thermal treatments to change the rutile VO₂ into metallic phase with a lower refractive index of 1.9, resulting in poor confinement of the TM polarization mode. The simulation results are presented below in Fig. 8. Therefore, the real-time adjustable OCP device can be achieved, and we will fulfill this research in the future.

In this paper, a dual-use all-Si₃N₄ TE-pass integrated polarizer with a working bandwidth of 1540–1640 nm and a high PER was proposed, fabricated, and demonstrated. Due to the asymmetric cross section of the ridge waveguide structure, the TE₀ mode can be well-confined in the polarizer and can propagate with low loss, but the TM₀ mode would leak out with a large transmission loss, so that the function of filtering the TM polarization is realized in this simple straight ridge waveguide. Moreover, by covering the waveguide with a HRIM layer, both the TE₀ and TM₀ modes can be readily transmitted through the resulting waveguide with a very low loss.

According to the simulation results, the polarizer could achieve a PER as high as 60 dB and IL below 1 dB for wavelengths from 1600 to 1700 nm, and the calculated bandwidth for the PER >40 dB is 150 nm (1550–1700 nm). The fabricated polarizer achieved an ultrahigh PER of more than 40 dB over a bandwidth of 80 nm (1560–1640 nm), and the corresponding measured IL is <1 dB. After covering the waveguide with a 1 μm thick HRIM photoresist layer, both the TE₀ and the TM₀ modes can freely propagate with an IL <1 dB.

Thus, the present OCP provides more flexible real-time polarization modulation in integrated photonic devices and can be a guide for further exploration on silicon nitride polarizers.

This work was supported by the University-Enterprise Cooperation Projects of SZTU (Grant Nos. 2023010802001 and 20243108010008), the Post-doctoral Research Project of SZTU (Grant No. 20180305), the National Natural Science Foundation of China (Grant Nos. 61935014, 61975094, 12105190, and 20211062020035), the Shenzhen Science and Technology Program (Grant No. ZDSYS 20200811143600001), the Guangdong Province Key Construction Discipline Scientific Research Capacity Improvement Project (Grant No. 2021ZDJS107), and the National Key Research and Development Program of China (Grant No. 2022YFB3605800).

The authors have no conflicts to disclose.

Chenxi Zhao and Xinzhi Zheng contributed equally to this work.

Chenxi Zhao: Formal analysis (equal); Investigation (equal); Methodology (equal); Software (equal). Xinzhi Zheng: Data curation (equal); Formal analysis (equal); Software (equal); Visualization (equal); Writing – original draft (equal). Shilong Zhao: Formal analysis (equal); Investigation (equal); Software (equal). Jinman Lv: Validation (equal); Visualization (equal). Mingyang Yu: Writing – review & editing (equal). Bingxi Xiang: Methodology (equal); Validation (equal). Fei Lu: Supervision (equal); Validation (equal). Cangtao Zhou: Resources (equal); Supervision (equal). Yujie Ma: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal). Shuangchen Ruan: Resources (equal); Supervision (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.