CCPS/SiN Hybrid System for Polarization Control

• The CCPS/SiN hybrid system is an integrated photonic platform that combines multiferroic CuCrP2S6 with silicon nitride waveguides for advanced polarization management.
• It achieves TM-pass filtering with over 25 dB extinction and minimal insertion loss (0.2–0.4 dB) by engineering polarization-dependent optical mode overlap.
• Validated through simulations and experiments, the system also enables azimuth rotation up to 92.9° without significantly altering ellipticity.

The CCPS/SiN hybrid system denotes an integrated photonic platform in which the multiferroic two-dimensional material CuCrP$_2$S$_6$ (CCPS) is combined with silicon nitride (SiN) waveguides and microring resonators to realize polarization-selective filtering and polarization rotation. In the reported implementation, microring resonators incorporating CCPS exhibit transverse magnetic (TM)-pass filtering with a polarization extinction ratio exceeding 25 dB and a low insertion loss of $\sim 0.2$–$0.4$ dB at 1500–1600 nm, while CCPS-loaded straight waveguides can produce azimuth angle shifts reaching up to $92.9^\circ$ under TM-mode input. Simulations and experimental validation indicate that the dominant mechanism is polarization-dependent optical mode overlap, governed by the refractive index profile of the CCPS/SiN hybrid system and the waveguide geometry, with CCPS anisotropy providing further enhancement but playing a secondary role (Dushaq et al., 2 Sep 2025).

1. Material platform and device configuration

The demonstrated platform uses silicon nitride photonic chips featuring both microring resonators (MRRs) and straight waveguides. The baseline SiN structure is a 400 nm-thick, air-clad SiN waveguide on 2 $\mu$m buried SiO$_2$. The MRRs have 1100 nm width and 50 $\mu$m radius. Onto these devices, multilayer CCPS flakes are deterministically dry-transferred, with reported flake thicknesses of 30–90 nm and a $\sim 61$ nm typical value. For polarization-rotation experiments, CCPS flakes (120–300 $\mu$m) were integrated on 3 mm SiN straight waveguides (Dushaq et al., 2 Sep 2025).

Material characterization is integral to the system definition. Atomic force microscopy (AFM) was used to measure CCPS flake thickness, and spectroscopic ellipsometry was used to extract the real and imaginary dielectric constants of CCPS. The ellipsometry results also confirmed low absorption in the C-band (1500–1600 nm). This low-absorption characteristic is important because the observed device behavior is not reducible to simple broadband absorption by a lossy 2D overlay; rather, it is tied to the hybrid modal structure of the CCPS/SiN stack (Dushaq et al., 2 Sep 2025).

Within the SiN photonics landscape, this configuration belongs to a broader class of hybrid systems in which a low-loss passive SiN backbone is combined with dissimilar active, nonlinear, or quantum materials. In the specific CCPS/SiN realization, however, the salient function is polarization management, not lasing, single-photon generation, or Kerr enhancement (Dushaq et al., 2 Sep 2025).

2. TM-pass filtering and polarization-selective attenuation

The principal filtering function of the CCPS/SiN hybrid system is TM-pass polarization-selective filtering. Its operational principle is described as mode overlap engineering: the filtering arises from differences in how the TE and TM modes interact with the CCPS layer because of their distinct field distributions and the refractive-index profile of the heterostructure (Dushaq et al., 2 Sep 2025).

For the TE mode, the presence of CCPS redistributes the field into the CCPS layer, resulting in strong attenuation through increased scattering and optical loss, and this attenuation is magnified by cavity feedback inside the ring resonator. For the TM mode, CCPS integration causes the mode to become more tightly confined within the SiN core, thereby reducing overlap with lossy surfaces and retaining high transmission. In the reported microsimulations, the TM mode confinement in the SiN core increased from 40.5% (bare) to 65.1% (with 60 nm CCPS) (Dushaq et al., 2 Sep 2025).

Experimentally, the resulting TM-pass filters achieved a measured extinction ratio exceeding 25 dB, defined as the transmission difference between the transmitted TM and the filtered TE, together with low TM propagation loss of $_6$0–$_6$1 dB for 10 $_6$2m CCPS interaction. The device also exhibited low, wavelength-independent loss across the C-band. These figures place the system in a low-loss regime that is unusual relative to many 2D-material or metal-clad polarizers (Dushaq et al., 2 Sep 2025).

A common simplification is to attribute the TM-pass effect primarily to the intrinsic anisotropy of CCPS. The reported simulations and measurements do not support that interpretation. The paper states that the effect is fundamentally governed by polarization-dependent mode overlap, controlled by SiN geometry and the refractive-index contrast provided by CCPS, whereas CCPS’s inherent optical anisotropy—described as high $_6$3 with moderate birefringence—assists, but plays a secondary role (Dushaq et al., 2 Sep 2025).

3. Polarization rotation in CCPS-loaded straight waveguides

Beyond filtering, the CCPS/SiN hybrid system also supports substantial polarization rotation in straight waveguides. In the reported experiments, the input polarization state was established using calibrated ring resonators, and the output state was monitored with a polarimeter. After propagation through the CCPS/SiN region, the measured azimuth angle shifts reached $_6$4 for TM input and $_6$5 for TE input (Dushaq et al., 2 Sep 2025).

The reported observation is that the output polarization becomes nearly orthogonal to the input after interaction with the CCPS section. At the same time, there was no substantial change to ellipticity: the handedness and ellipticity remained largely unaltered, and the dominant change was rotation of the polarization axis. This distinguishes the effect from processes that predominantly convert linear polarization into strongly elliptical or circular states (Dushaq et al., 2 Sep 2025).

The mechanistic explanation combines boundary-induced field redistribution, mode hybridization, and phase accumulation. The abrupt transition from air/SiN to CCPS/SiN causes interface-driven redistribution of the fields. Because the TE and TM modes acquire different effective indices in the hybrid section, they accumulate different phases over propagation length $_6$6, with the phase difference written as

$_6$7

The polarization-state evolution is described with a Jones rotation matrix,

$_6$8

where the rotation angle $_6$9 depends on the accumulated phase difference and the interface coupling coefficients (Dushaq et al., 2 Sep 2025).

The reported beam-propagation simulations show periodic transfer between $\sim 0.2$0 (TE) and $\sim 0.2$1 (TM) components, interpreted as a clear signature of polarization rotation through hybridization. The study further states that, after exiting the CCPS section, the new polarization eigenmode persists, and there is no reversion to the original orientation. This persistence is a defining property of the demonstrated straight-waveguide rotator (Dushaq et al., 2 Sep 2025).

4. Simulation framework and analytical description

The CCPS/SiN hybrid system was analyzed using a combination of mode solving, beam-propagation simulation, and transfer-matrix formalism. The Lumerical Mode Solver was used with the extracted dielectric data of CCPS to analyze field profiles and quantify confinement. These simulations were used to establish the modal redistribution that underpins both TM-pass filtering and polarization rotation (Dushaq et al., 2 Sep 2025).

For the rotation dynamics, beam-propagation simulations tracked the evolution of field components along the hybrid section and showed periodically varying field components associated with energy exchange between TE and TM. The analytical model described the cumulative polarization rotation via a transfer-matrix formalism, relating the output state to the phase difference and interface coupling coefficients. In this framework, the hybrid section is not treated as a simple perturbative absorber but as a region that supports coupled polarization evolution (Dushaq et al., 2 Sep 2025).

A useful contextual point comes from earlier CCPS photonics work on hybrid silicon microring resonators. In that work, electrically driven Cu ions in multilayer CCPS were reported to tune the effective refractive index on the order of $\sim 0.2$2 RIU while preserving extinction ratios and resonance linewidth, with low optical losses of 2.7 dB/cm and modulation efficiency of 0.25 V.cm (Dushaq et al., 2023). Taken together with the later CCPS/SiN polarization study, this indicates that CCPS can participate in photonic functionality through more than one physical channel: electro-refractive tuning in hybrid silicon resonators and polarization-dependent modal engineering in SiN-based devices (Dushaq et al., 2023).

5. Comparative position among 2D-material and SiN hybrid photonic systems

The reported paper explicitly compares CCPS/SiN polarization filtering with several prior 2D-material platforms. In the comparison below, the numerical values are those given in the source summary.

Material	PER (dB)	IL (dB)
CCPS/SiN	$\sim 0.2$3	$\sim 0.2$4–$\sim 0.2$5
Graphene	$\sim 0.2$6	$\sim 0.2$7
Graphene Oxide	$\sim 0.2$8	$\sim 0.2$9
MoS$0.4$0, MoSe$0.4$1	$0.4$2–13	$0.4$3

The same comparison lists the optical bandwidth for CCPS/SiN as 1.5–1.6 $0.4$4m, for graphene as 1.23–1.61 $0.4$5m, for graphene oxide as 0.63–1.6 $0.4$6m, and for MoS$0.4$7, MoSe$0.4$8 as 0.65–1.55 $0.4$9m (Dushaq et al., 2 Sep 2025).

The immediate significance of this comparison is that the CCPS/SiN hybrid achieves a competitive extinction ratio with dramatically lower insertion loss. In the broader literature on SiN hybrid integration, other material pairings have been used to realize very different functionalities. Graphene oxide integrated with SiN nanowires was used to enhance four-wave mixing, yielding a 7.3 dB improvement in conversion efficiency for a uniformly coated device with 1 layer of GO and 9.1 dB for a patterned device with 5 layers, together with enhancement of the effective nonlinear parameter by over a factor of 100 (Qu et al., 2020). hBN single-photon emitters have been deterministically integrated onto SiN waveguides via femtosecond laser processing, preserving single-photon characteristics under on-chip excitation (Yamashita et al., 28 Apr 2025). III-V/SiN hybrids have been used for indistinguishable-photon generation (Yeung et al., 2023), octave-spanning comb generation (Briles et al., 2021), and tunable narrow-linewidth lasers for distributed sensing (Gaur et al., 24 Jun 2026, Idjadi et al., 2024).

This broader comparison suggests that SiN functions as a versatile low-loss host for disparate hybrid materials, while the CCPS/SiN implementation occupies a specific niche centered on compact polarization control, rather than gain, quantum emission, or third-order nonlinearity.

6. Significance, misconceptions, and future directions

The reported work identifies the CCPS/SiN platform as the first reported integration of a 2D ferroionic material (CCPS) with SiN photonics for advanced polarization management. The demonstrated device classes are described as CMOS-compatible, low-loss, high-selectivity, and scalable, and the cited potential applications include secure optical communications, polarization-diverse circuits, sensing, and signal processing (Dushaq et al., 2 Sep 2025).

Two interpretive points are central. First, the system is not well described as a conventional absorptive polarizer based solely on 2D-material loss; the dominant mechanism is refractive-index-engineered, polarization-dependent mode overlap. Second, the large polarization rotation is not primarily an ellipticity-conversion effect; the reported data emphasize axis rotation with largely unaltered ellipticity (Dushaq et al., 2 Sep 2025).

The broader CCPS photonics literature also frames likely future research directions. Earlier electro-optic studies of CCPS-based devices emphasized issues such as scalability of deterministic dry transfer, exploration of other ferroionic 2D materials or heterostructures, and the need to study speed, switching time, and endurance associated with ion migration (Dushaq et al., 2023). A plausible implication is that analogous questions will become important for CCPS/SiN polarization devices as they move from proof-of-principle demonstrations toward programmable photonic subsystems.

In that sense, the CCPS/SiN hybrid system is best understood as a materials-and-waveguide co-design platform. Its demonstrated functions—TM-pass filtering with $92.9^\circ$0 dB extinction ratio, $92.9^\circ$1–$92.9^\circ$2 dB insertion loss, and azimuth rotation up to $92.9^\circ$3—derive from the interplay of high-index CCPS, SiN modal confinement, and geometric control of overlap, rather than from any single material parameter in isolation (Dushaq et al., 2 Sep 2025).