Waveguide Polarizers in Silicon with MoS₂ Films

1. Introduction

The control of light polarization plays a crucial role in modern optical systems and underpins a wide range of advanced optical technologies [1,2]. Optical polarizers, which allow transmission of light in a specific polarization while suppressing the orthogonal polarization component, serve as key elements for polarization control in optical systems [3]. To date, a variety of optical polarizers have been developed based on refractive prisms [4,5], birefringent crystals [6,7], fiber components [8,9], and integrated photonic devices [10,11]. However, these polarizers based on bulk materials often struggle to achieve efficient polarization selection over wide wavelength ranges. This limitation is especially significant given the increasing demand for broadband optical polarizers driven by rapid progress in photonic technologies and systems [12,13].

Recently, due to strong anisotropy in light absorption and broadband response, two-dimensional (2D) materials with atomic-scale film thicknesses have been incorporated onto bulk optical waveguides to realize high-performance optical polarizers [14], as demonstrated by those incorporating 2D materials such as graphene [14,15,16], graphene oxide (GO) [17,18,19], and transition metal dichalcogenides (TMDCs) [20,21,22]. As a significant subgroup in the 2D material family, TMDCs exhibit a direct bandgap in their monolayer form that transitions to an indirect bandgap in few-layer or bulk forms [23,24]. This distinctive property facilitates their widespread applications for next-generation atomically thin devices such as transistors [25,26], photodetectors [27,28], and electrocatalysts [29,30]. More recently, the strong anisotropic light absorption of 2D TMDC films across broad wavelength ranges has also been explored, and their integration onto polymer and Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) waveguides for realizing optical polarizers has been successfully demonstrated [21,22].

In this work, we demonstrate the integration of 2D molybdenum disulfide (MoS₂) ‒ a representative TMDC material ‒ onto the widely used silicon photonic platform to realize high-performance optical polarizers. High-quality monolayer MoS₂ films with strong anisotropy in light absorption are synthesized via a low-pressure chemical vapor deposition (LPCVD) method, and subsequently transferred onto silicon-on-insulator (SOI) nanowire waveguides using polymer-assisted transfer process. We perform detailed measurements for the fabricated devices with different MoS₂ film coating lengths and silicon waveguide geometry, achieving a maximum polarization-dependent loss of ~21 dB and a high figure of merit of ~4.2. In addition, the hybrid waveguide polarizers exhibit broad operation bandwidth over ~100 nm and excellent power durability. Finally, we compare the performance of our device with state-of-the-art waveguide polarizers incorporating different 2D materials and find that it achieves the highest figure of merit (FOM) among all those based on the silicon photonic platform. These results reveal the strong potential of 2D TMDC films for implementing high-performance integrated polarization selective devices.

2. Device Design

As an important member of the TMDCs family that has been widely studied, MoS₂ features a hexagonal sheet of molybdenum (Mo) atoms sandwiched between two hexagonal sheets of sulfur (S) atoms [31,32]. Figure 1(a) illustrates the atomic structure of monolayer MoS₂, where Mo and S atoms are connected by strong covalent bonds. Multi-layered MoS₂ consists of vertically stacked layers that are weakly bonded through van der Waals interactions [26]. Monolayer MoS₂ has shown great potential as a semiconducting material with a direct bandgap of ~1.8 – 1.9 eV [33]. This is larger than the energy of two photons at 1550 nm (i.e., ~1.6 eV), which allows for relatively low linear light absorption as well as two-photon absorption at near infrared wavelengths. The quality of 2D MoS₂ crystals plays a crucial role in determining the material properties such as refractive index, light absorption, and optical bandgap, which correlates to the intrinsic structural defects induced by sulfur vacancies [34,35]. In this work, we choose MoS₂ to implement optical polarizers due to several compelling advantages. First, it exhibits strong anisotropic light absorption over a very broad spectral bandwidth [36,37]. Second, it possesses relatively low linear absorption in the infrared regime, with an extinction coefficient nearly 1 order of magnitude lower than that of graphene [38,39], making it particularly suitable for infrared photonic applications. Finally, we develop a simple and one-step chemical vapor deposition (CVD) method to fabricate MoS₂ films with precise control over structural defects [34], which allows us to tailor the intrinsic optical properties of MoS₂ for specific optical applications.

Figure 1(b) shows the schematic of an integrated waveguide polarizer consisting of a silicon nanowire waveguide coated with a monolayer MoS₂ film. Similar to other 2D materials such as graphene and graphene oxide (GO) [14,15,17,19], 2D MoS₂ films exhibit strong anisotropic optical absorption [40,41], with significantly higher absorption for light propagating in the in-plane direction compared to the out-of-plane direction. For the hybrid waveguide in Figure 1(b), these directions correspond to TE- and TM-polarized incident light, respectively. As a result, the hybrid waveguide can effectively operate as a TM-pass waveguide polarizer. It is worth noting that 2D MoS₂ exhibits a broad spectral range of material anisotropy spanning from visible to infrared wavelengths [36]. This wide bandwidth offers a significant advantage for MoS₂-coated integrated waveguide polarizers, which is difficult to achieve for conventional bulk silicon photonic polarizers [1,14,42].

Figure 1(c) shows a schematic of the cross section of the hybrid waveguide in Figure 1(b). The corresponding transverse electric (TE) and transverse magnetic (TM) mode profiles at 1550 nm are provided in Figure 1(d), which were simulated using commercial mode-solving software (COMSOL Multiphysics). In our simulation, the thickness of the monolayer MoS₂ film was ~0.7 nm. The refractive index (n) and extinction coefficient (k) of MoS₂ for TE polarization were n_TE = ~3.8 and k_TE = ~0.107, respectively. For TM polarization, the corresponding values were n_TM = ~3.2 and k_TM = ~0.027. These values were obtained from our measurements in the following sections. The simulated TE- and TM polarized effective indices for the hybrid waveguide were ~2.081 + 3.313 × 10^-4i and ~1.551 + 7.132 × 10^-5i, respectively. The large difference in the imaginary part highlights the polarization selectivity for the hybrid waveguide, which originates from the significant disparity between k_TE and k_TM of the 2D MoS₂ film. It should be noted that, due to the polymer-assisted transfer method used in our fabrication process (which will be discussed in Section 3), the monolayer MoS₂ film does not conformally coat on the sidewalls of the silicon nanowire waveguide, resulting in air gaps between the waveguide sidewalls and the MoS₂ film. Nevertheless, this has minimal impact on the polarization selectivity of the hybrid waveguides, as it mainly depends on the interaction between the evanescent field and the MoS₂ film on the waveguide top surface.

3. Device Fabrication and Material Characterization

Based on the device design in Section 2, we fabricated silicon photonic waveguide polarizers coated with monolayer MoS₂ films in this section. In addition, we employed a range of material characterization methods to assess the quality of the MoS₂ films on the photonic integrated chips.

We first fabricated uncoated silicon nanowire waveguides using CMOS-compatible fabrication technologies. The nanowire waveguides were fabricated on silicon-on-insulator (SOI) wafer with a 220-nm-thick top silicon layer and a 2-μm-thick silica layer. The waveguide patterns were defined using 248-nm deep ultraviolet photolithography, followed by waveguide formation through an inductively coupled plasma etching process. After this, a 1.5-μm-thick silica layer serving as an upper cladding layer was deposited on the SOI chip via plasma enhanced chemical vapor deposition (PECVD). Finally, windows of different lengths were opened on the silica upper cladding through the processes of photolithography and reactive ion etching (RIE) to enable the coating of MoS₂ films onto the silicon waveguides. All the nanowire waveguides we fabricated had the same length of ~3.0 mm, and the lengths of the opened windows (i.e., the MoS₂ film coating lengths) ranged between ~0.1 mm and ~2.2 mm.

After fabricating the uncoated silicon waveguides, we used an LPCVD method that we developed in Ref. [34] to synthesize high-quality MoS₂ films. Our method allows for precise control of the intrinsic atomic defects induced by sulfur vacancies, and supports the direct growth of high-quality and large-area 2D MoS₂ films with precise thickness control. Figure 2(a) shows a schematic of our CVD process flow using a two-temperature-zone tube furnace to synthesize MoS₂ monolayers. First, the Mo precursor was drop-casted onto an ultrasonically cleaned substrate in Zone 2 of the furnace, which was maintained at 750 °C during the CVD process. After this, S powder was vaporized at 180 °C in Zone 1, and the S vapors were carried downstream by a 70 sccm flow of argon as carrier gas into Zone 2. Finally, a surface reaction between the Mo and S species occurred, resulting in the synthesis of MoS₂ single crystals. During the growth, the process pressure was maintained at ~1 torr to maintain a low-pressure CVD conditions. Figure 2(b) shows a microscopic image of a monolayer MoS₂ film grown on a sapphire substrate, which exhibits a high film uniformity. Figure 2(c) shows an atomic force microscopy (AFM) image of a representative MoS₂ crystal synthesized by using our CVD method, which shows an average thickness of ~0.7 nm.

Following the CVD synthesis of a monolayer MoS₂ film, it was transferred onto the SOI chip with uncoated silicon waveguides using a polymer-assisted transfer process [43]. The as-grown MoS₂ film was initially spin-coated with a polystyrene (PS) support layer. Subsequently, the PS/MoS₂ stack was exfoliated from the growth substrate using a water-droplet-assisted delamination process, driven by surface energy differences between the interfaces [44]. Finally, the resulting stack was then stamped onto the SOI chip via van der Waals interactions, and the PS layer was removed by dissolving it in toluene. As shown in Figure 2(d), the transferred monolayer MoS₂ film exhibits high optical transmittance and uniform coverage across the SOI chip surface, confirming the effectiveness and quality of the transfer process.

Figure 3(a-i) and (a-ii) show the Raman spectra of the same SOI chip before and after the transfer of monolayer MoS₂ film, which were measured by using a ~514-nm excitation laser. After MoS₂ integration, the Raman spectrum exhibits two prominent peaks emerging at ~384 cm^-1 and ~404 cm^-1, corresponding to the in-plane (E¹_2g) TE and out-of-plane (A_1g) TM vibrational modes of monolayer MoS₂, respectively. Moreover, the observed frequency difference of ~20 cm^-1 is characteristic of monolayer MoS₂. An additional peak observed at ~517 cm^-1 originates from the underlying silicon substrate. These spectral features are consistent with those reported previously for CVD-grown monolayer MoS₂ [34,45,46], confirming successful and high-quality integration of 2D MoS₂ onto the SOI chip.

Figure 3(b) shows the spectra for linear optical absorption and transmittance of the synthesized MoS₂ film, which were characterized via ultraviolet–visible (UV–vis) spectrometry. It can be seen that the MoS₂ exhibited strong light absorption in the visible and infrared wavelength regions. The linear absorption spectrum exhibited a sharp increase followed by a rapid decline within the range of ~400 – 600 nm. Two strong absorption peaks were observed at ~605 cm⁻¹ and ~651 cm⁻¹, and the peak associated with van Hove singularities [47] of monolayer MoS₂ was also observed at ~430 cm⁻¹. Before a gradual decrease at wavelengths >900 nm, the linear absorption spectrum exhibited a sudden jump at ~870 nm. The transmittance of the sample had a transmittance >60% at wavelengths between ~400 nm and ~1800 nm. These results show agreement with those reported in previous studies [48,49,50] and further validate the high quality of our synthesized 2D MoS₂ films.

Figure 3(c) shows the X-ray diffraction (XRD) spectrum for the synthesized MoS₂ film. The diffraction rings can be indexed to the (100), (103), (105), and (110) reflections of hexagonal MoS₂ crystal structure, which shows an agreement with the measured XRD spectra of MoS₂ in Refs. [51,52]. Figure 3(d) shows the X-ray photoelectron spectroscopy (XPS) analysis of the synthesized MoS₂ film. In Figure 3(d-i), prominent peaks were observed at ~226.9 eV, ~229.6 eV, and ~232.7 eV, which correspond to S 2s, Mo 3d doublet of Mo⁴⁺ 3d_5/2 and Mo⁴⁺ 3d_3/2. In addition, weak peaks at ~232.3 eV and ~235.4 eV were attributed to the 3d_5/2 and 3d_3/2 components of Mo⁶⁺, indicating the presence of MoO_x species, likely originating from partial surface oxidation or physiosorbed oxygen during ambient exposure [35]. Figure 3(d-ii) displays the S 2p region, where two peaks appeared at ~162.6 eV and ~163.8 eV and corresponded to the S 2p_3/2 and S 2p_1/2, respectively, further supporting the formation of MoS₂ with the expected stoichiometry. These experimental results are consistent with previously reported XPS signatures of CVD-grown monolayer MoS₂ in Refs. [46,53,54,55].

4. Polarization-Dependent Loss Measurements

In this section, we measured the polarization-dependent loss (PDL) of the fabricated MoS₂-Si hybrid waveguides in Section 3 for input continuous-wave (CW) light with different polarization states. We performed measurements for devices with various silicon waveguide widths (W) and with monolayer MoS₂ films of different coating lengths (L_c). In our measurements, lensed fibers were employed to butt couple a CW light at ~1550 nm into and out of the fabricated devices with inverse-taper couplers at both ends. The fiber-to-chip coupling loss was ~5 dB / facet.

Figure 4(a-i) plot the measured TE- and TM-polarized insertion loss (IL) versus MoS₂ film coating length L_c for the hybrid waveguides with monolayer MoS₂ films. For comparison, all the devices had the same W = ~400 nm. During our measurements in Figure 4, the input CW power and wavelength were kept the same as P_in = ~0 dBm and λ = ~1550 nm, respectively. Unless otherwise specified, the values of P_in and IL in our following discussions refer to those after excluding the fiber-to-chip coupling loss.

In Figure 4(a-i), the data points depict the average of measurements on three duplicate devices, and the error bars reflect the variations among different devices. As can be seen, the IL increases with L_c for both TE and TM polarizations, with the former exhibiting a faster rate of increase than the latter. This reflects a higher propagation loss for TE polarization, which is associated with a larger imaginary part of its effective index, as simulated in Figure 1(d).

In Figure 4(a-ii), we further calculated the PDL (dB) by subtracting the TM-polarized IL from the TE-polarized IL in Figure 4(a-i). For the device with L_c = 2.2 mm, a maximum PDL value of ~21 dB was achieved. In contrast, the uncoated Si waveguide did not show any significant polarization-dependent IL, with a PDL below 0.5 dB. The huge difference in the PDL values highlights the polarization selectivity introduced by integrating a 2D MoS₂ film onto the silicon photonic waveguide. In Figure 4(a-i), the PDL increases with L_c, this further confirms that the exceptional polarization selectivity arises from the 2D MoS₂ film and suggests that improved PDL can be achieved by increasing the MoS₂ film coating length.

Figure 4(b-i) shows the measured TE- and TM-polarized IL versus waveguide width W for the hybrid waveguides with the same L_c = ~2.2 mm. In Figure 4(b-i), the IL increases with W for TM polarization but decreases for TE polarization. This is mainly resulting from changes in the mode overlap of TE and TM modes induced by variations in the silicon waveguide width. Figure 4(b-ii) shows the corresponding PDL calculated from the measured IL in Figure 4(b-i). The device with W = ~400 nm achieved a maximum PDL of ~21 dB, and the PDL decreased as W increased, reaching ~15 dB at W = ~600 nm. Figure 4(c) shows the polar diagrams for the measured IL of devices with the same W = ~400 nm but different L_c = ~1.0 mm and ~2.2 mm. In the polar diagrams, the variations in the IL values across different polarization angles further confirm the polarization selectivity of the MoS₂-Si hybrid waveguides.

In addition to measuring PDL at constant wavelength (λ = ~1550 nm) and power (P_in = ~0 dBm) for the input CW light in Figure 4, we also investigated the dependence of the PDL on input light wavelength and power. Figure 5(a) shows the measured PDL versus input CW wavelength λ for the hybrid waveguides with various L_c but the same W = 400 nm. During our measurements, the input CW power was maintained at P_in = ~0 dBm. For all the devices, there were no obvious changes in the PDL (< 2 dB) within the measured wavelength range of ~1500 – 1600 nm. We also notice that there was a minor increase in the PDL as λ increased, which can be attributed to a slight change in MoS₂’s mode overlap induced by dispersion. Figure 5(b) shows the measured PDL versus λ for the hybrid waveguides with various W but the same L_c = ~2.2 mm. Similarly, there were no significant variations in the PDL, with only a slight increase in the PDL as λ increased.

The results in Figure 5(a) and (b) highlight the broadband operation of the MoS₂-Si waveguide polarizers ‒ a feature that is often challenging to achieve for bulk silicon photonic polarizers [3,56]. In our measurements, the wavelength tuning range was limited by the tunable CW laser employed to scan the transmission spectra. In fact, MoS₂ films exhibit a broad bandwidth for anisotropic light absorption, which extends well beyond that demonstrated here and can span from the visible to the infrared wavelength regions [36,37].

Figure 5(c) and (d) show the measured TE- and TM-polarized IL and calculated PDL versus input CW power P_in for the hybrid waveguides with the same W = ~400 nm but different L_c = ~1.0 mm and ~2.2 mm, respectively. For comparison, the input CW wavelength was kept the same at λ = ~1550 nm. In both figures, there are no notable changes in the IL and PDL within the input power range of ~0 dBm ‒ ~28 dBm. This indicates excellent thermal stability of the 2D MoS₂ films and remarkable power durability of the hybrid devices. In contrast, 2D GO films coated on silicon waveguides are susceptible to photothermal reduction at P_in > 10 dBm, as observed in our previous measurements [57]. We did not perform measurements for P_in > ~28 dBm, as P_in = ~28 dBm was the maximum available from our experimental setup. This value accounts for a 5-dB coupling loss subtracted from the 33-dBm maximum CW power, which was achieved after amplification by an erbium-doped fiber amplifier (EDFA).

5. Discussion

In this section, we further analyze the anisotropic absorption of 2D MoS₂ films by fitting the experimental results in Section 4 with theoretical simulations. We also compare the performance of our MoS₂-Si waveguide polarizers with waveguide polarizers incorporating other 2D materials.

Figure 6(a) shows the waveguide propagation loss (PL) versus W for both TE and TM polarizations, which was extracted from the measured IL in Figure 4(b). As expected, the TE-polarized PL is much higher than the corresponding TM-polarized PL. An increase in W leads to a smaller difference between the two, mainly caused by altered mode overlap with the MoS₂ films. The excess propagation loss (EPL) induced by the MoS₂ film was further calculated by excluding the PL for the uncoated silicon waveguide, which were ~3.4 dB/cm and ~3.0 dB/cm for TE and TM polarizations, respectively. At W = 400 nm, the TE-polarized EPL was ~117 dB/cm, significantly higher than the TM-polarized EPL of ~25 dB/cm, showing agreement with those reported in Ref. [58]. We also note that the value of ~117 dB/cm is more than an order of magnitude lower than the EPL induced by monolayer graphene coated on a silicon waveguide (i.e., ~2000 dB/cm [39,59]), yet ~5 times higher than that induced by monolayer GO (i.e., ~20 dB/cm [60]).

Figure 6(b) shows the extinction coefficient of MoS₂ (k_TE, k_TM for TE and TM polarizations, respectively) obtained by fitting the results in Figure 6(a) with optical mode simulations of the hybrid waveguides (at 1550 nm). At W = ~400 nm, k_TE is ~0.107, which is about ~4 times that of k_TM. For all different W, the k values for TE polarization are significantly higher than those for TM polarization, highlighting the strong anisotropic light absorption of 2D MoS₂ films. For both polarizations, no significant variations in k were observed as W increased. This indicates that the changes in PL with W observed in Figure 6(a) are mainly caused by variations in mode overlap, rather than differences in the MoS₂ film properties, highlighting the consistency of our measurements and uniformity of the coated MoS₂ film.

In Figure 6(c), we further plot the anisotropy ratios defined as the ratios of the corresponding k values for TE and TM polarizations (k_TE / k_TM) in Figure 6(b). As can be seen, the anisotropy ratio also remained relatively consistent without any significant variations. A maximum anisotropy ratio of ~4.0 is achieved for monolayer MoS₂ films, which is close to ~4.5 reported for monolayer GO films [18].

To evaluate the performance of 2D-material-based optical polarizers, the figure of merit (FOM), defined by the following equation, is commonly used [14,17],

$$\mathit{FOM}=\mathit{PDL}/\mathit{EIL}$$

(1)

where PDL (dB) is the ratio of the maximum to minimum IL’s as we discussed in Figure 4(a-ii) and (b-ii), and EIL (dB) is the minimum insertion loss induced by the MoS₂ film over the uncoated waveguide. Note that the EIL accounts only for the IL induced by the MoS₂ film. In our case, it represents the excess MoS₂-induced IL for the TM polarization, as the TM mode exhibited a lower IL. Figure 6(d) shows the calculated FOM versus silicon waveguide width W. A higher FOM value is achieved for a lower W. A maximum FOM of ~4.2 was achieved at W = ~400 nm, which decreased to ~2.0 as W increased to ~600 nm.

In Table 1, we summarize state-of-the-art waveguide optical polarizers incorporating 2D materials and compare their performance. Here we only show the results for experimental works, and compare the key performance parameters including PDL, operation bandwidth (OBW), and FOM. Among the different polarizers, our work here marks the first demonstration of implementing optical polarizers by integrating 2D MoS₂ films onto silicon photonic devices. Although other studies have reported higher FOM values using waveguides with larger cross-sections [61], thicker 2D materials [19], or enhanced mode overlap [14], it is important to highlight that our device achieves the highest FOM among all polarizers implemented based on the silicon photonic platform, which remains the most widely used and influential integrated photonics platform [63,64,65]. It is also worth noting that there remains substantial room to improve the FOM of MoS₂-Si hybrid waveguides by tailoring waveguide geometry to optimize mode overlap. Simulations show that a FOM value of ~19.2 can be achieved for a hybrid device with a cross-section of 400 nm × 160 nm for the bare silicon waveguide.

These results confirm the effectiveness of integrating 2D GO films onto silicon photonic platforms to implement high-performance thermo-optic devices and will have implications for a wide range of devices such as optical microcombs, [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145] advanced circuits, [146,147,148,149,150,151,152,153] graphene oxide and other 2D material based devices, [154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191] and quantum optics. [192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207]

6. Concluson

In summary, we integrate 2D MoS₂ films onto SOI nanowire waveguides to implement high-performance optical polarizers. High-quality monolayer MoS₂ films exhibiting highly anisotropic light absorption are synthesized via a LPCVD method, and then transferred onto SOI nanowire waveguides using a polymer-assisted transfer process. Detailed measurements are performed for our fabricated devices with various MoS₂ film coating lengths and silicon waveguide geometry. The results show that a maximum PDL of ~21 dB and a high FOM of ~4.2 are achieved. The hybrid waveguide polarizers also demonstrate a broad operation bandwidth of over ~100 nm and excellent power durability. These results verify the effectiveness of integrating 2D MoS₂ films onto silicon photonic devices for implementing high-performance optical polarizers.