DC-to-GHz Modulation In Microring Modulators Using Ferroelectric Nematic Liquid Crystal-on-Silicon in a Foundry Photonic Process

Abstract

Silicon-organic hybrid (SOH) platforms exhibit exceptional electro-optic (EO) properties, including high-speed operation, low energy consumption, and compact footprints. However, the absence of a scalable poling method for EO polymers, combined with the slow switching speeds characteristic of liquid crystals, has impeded the integration and compatibility of these materials with commercial silicon photonic foundries. On the other hand, the realization of very-large-scale photonic integrated circuits (PICs) in the native silicon photonics platform itself is impeded by the complexities associated with the wavelength and thermal stabilization for microring modulators (MRMs). This study establishes the foundation for a poling-free, CMOS-compatible SOH MRM platform by exploiting simultaneous AC phase shifts in ferroelectric nematic liquid crystals (FN-LCs). We present the first demonstration of an MRM coated with FN-LC, with both the RF signal and the DC bias applied to the same electrodes, taking advantage of the dual phase shift uniquely available in FN-LC. An EO bandwidth of f$_{-6dB}\approx$ 7.8 GHz is achieved \textendash~ the highest reported value for an SOH MRM to date. As a proof of concept, we demonstrate an approximately linear resonance shift across the full width at half maximum ($\approx$ 150 pm/V), with a static power efficiency of $\approx$ 4.5 nW/$π$ for an MRM occupying a total footprint of $\approx$ 0.084 mm$^2$ and exhibiting an on-chip optical insertion loss of $\approx$ 0.78 dB. Successful infiltration of FN-LC, selectively patterned on top of phase shifters, along with optical input/output channels established using free-form photonic wire bonds, is demonstrated in the proposed PIC.

Figures (6)

• Figure 1: Concept of a photonic integrated circuit (PIC) based on a ferroelectric nematic liquid crystal hybrid (FN-LC). (a) The PIC includes stand-alone and nested ring modulators (MRM) and Mach-Zehnder interferometers that can operate as modulators, weight banks, filters, interconnects, and switches. The material is applied selectively to active components, leaving other surface features unaffected. Subwavelength grating couplers connect individual devices to establish device-level optical I/O. An n-channel fiber array co-packaged with the PIC connects light via photonic wire bonds (PWBs) produced using two-photon lithography. (b) An MRM example with a clad opening for FN-LC infiltration into the light-matter interaction area space of the waveguide. The metal-to-silicon distance ($\textit{d}_{Si-M}$), as well as the waveguide's width ($\textit{W}_\text{wg}$) and height ($\textit{h}_\text{wg}$), can all be tuned to make a trade-off between drive voltage and electro-optic bandwidth. (c) A zoomed-in view of the material's donor-bridge-acceptor structure ("D-$\uppi$-A") rotated in response to an external field, causing the molecular director to get aligned along the spontaneously produced polarization.
• Figure 2: Device structure and design. (a) The layer stack-up of the microring modulator (MRM) (all dimensions are in $\mu m$) overlapped with the electric field ($\textbf{E}_e$). The partial etch layer (Si-N with n-type dopant implanted at a dose of 3$\times10^{17}~cm^{-3}$) is attached to the full-etch waveguide (Si undoped) on one side and to the highly doped interface (Si-N$^{++}$ with n-type dopant implanted at a dose of 1$\times10^{20}~cm^{-3}$) for ohmic contact on the other side. The single-sided rib waveguide reduced the effective distance between the anode and cathode, increasing the device factor ($\Gamma/\textit{d}$) and detuning efficiency compared to an equivalent non-slotted waveguide. $\textbf{E}_e$ and transverse electric optical field ($\textbf{E}_x$) overlap each other from the left and top-left. (b) $\textbf{E}_e$ and (c) normalized $\textbf{E}_x$ in y-slice view. (d) A false-colored scanning electron microscopy (SEM) image of the constructed MRM demonstrating the two-phase shifter regions with the shortest distance between the inner (signal or "S") and outer (ground or "G") electrodes, where $\Gamma/\textit{d}$ is maximized. SEM image of (e) one of the phase shifter regions shows the oxide open area for ferroelectric nematic liquid crystal infiltration, and (f) the coupling region between the waveguide bus and microring running under a metallic bridge carrying the RF signal.
• Figure 3: FN-LC alignment and DC characterization. (a) Optical output power as a function of externally applied electric field (E$_{ext}$). The insertion loss (IL) improves by $\approx$ 7 dB at a saturation field of $\textit{E}_{sat} \approx 1.2 V/\mu$m, corresponding to a DC voltage of $\textit{V}_{sat} \approx 2$ V, which is sufficient to achieve complete dipole alignment. (b) Polarization switching leakage current ($\textit{I}_{align}$) measured during the alignment process under V = V$_{sat}$. The current saturates for $\textit{t} \geq \textit{t}_{stat} = 50$ s, indicating that full alignment has been reached. A steady-state current of $\textit{I}_{sat} \approx 0.36$ nA is observed at $\textit{V}_{sat}$. Upon removal of $\textit{E}_{ext}$, a polarity reversal in $\textit{I}_{align}$ is detected, producing a modest back-switching current after $\textit{t}_{stat}$. (c) Normalized transmission spectrum at the through port for three different voltages ($\textit{V}_{MRM}$), demonstrating the ability to switch between on- and off-resonance states. A modulation efficiency of $\eta_{MRM} \approx 250$ pm/V (equivalent to approximately 1.67 FWHM/V), a quality factor of $\textit{Q}_{MRM} \approx 15340$, and an insertion loss of $\approx0.78$ dB are reported, where FWHM represents the full width at half maximum. (d) Two-dimensional heatmap of the transmission spectrum as a function of $\textit{V}_{MRM}$. Across the voltage range V = -5 to 5 V, a cumulative resonance shift of approximately 1.5 nm ($\approx 6.5 \times \lambda_{off-res}$) is achieved. It corresponds to an estimated $\pi$-phase voltage of $V_{\pi} \approx 13.25$ V, required for a full $\pi$ phase shift equivalent to FSR/2, where FSR is the free spectral range. (e) Approximately-linear blue and red shift tunability measures a total wavelength detuning of $\approx 1.5\times$ FWHM/V over the range V = -5 to 5 V. (f) Extinction ratio (ER) over $\textit{V}_{MRM}$ versus optical power injected to the MRM ($\textit{P}_{in,opt}$) shows minimal evidence of photo-oxidation at higher optical powers, confirming material stability under continuous optical excitation.
• Figure 4: AC characterization. (a) Schematic of the experimental setup for high-frequency characterization. A vector network analyzer (VNA) is employed for S-parameter measurements. Each microring modulator (MRM) is fed an optical carrier via either grating couplers (GCs) or photonic wire bonds. For time-domain analysis, the VNA is replaced by an arbitrary waveform generator (AWG) and an oscilloscope. (b) The proposed device architecture requires only a single dopant type and two implantation levels, in contrast to (c) a conventional pn-junction-based MRM, which utilizes three implantation doses for both P- and N-type dopants. The birefringence-induced phase shift in ferroelectric nematic liquid crystal (FN-LC) replaces the power-intensive thermo-optic heater, thereby enhancing the extinction ratio. The RF field (purple arrows) is applied parallel to the pre-set molecular alignment (green arrows). (d) A photonic integrated circuit comprising active devices coated with 10$\sim$50 nm thick FN-LC during device-level testing. (e) Measured $\textit{S}_{21}$ and (f) $\textit{S}_{11}$ parameters of the MRM under test. Two distinct phase-shift mechanisms are identified: a slow yet strong response and a faster yet weaker one, with corresponding cutoff frequencies ($\textit{f}_{-6dB}$) of 119 kHz and 7.8 GHz, respectively. These are differentiated from the previously observed substantial DC phase shift. The amplitude difference between the two response trends ($|\Delta S_{21}|\approx30~\mathrm{dB}$) suggests two separate phase modulation processes characterized by differing $\textit{r}_{33}$ values across frequency bands. (g) Time-domain response of the MRM to a 25 MHz square wave, exhibiting minimal distortion and thereby validating the presence of the Pockels effect in the FN-LC material.
• Figure 5: Implemented photonic integrated circuit (PIC) prototype incorporating ferroelectric nematic liquid crystal (FN-LC). (a) Microscopic top-down image of the fabricated microring modulator coated with FN-LC, exhibiting an overall footprint of $\approx8400~\mu m^2$. The image highlights the individual electrical pads and RF routing paths used for both device-level and chip-level characterization. (b) Close-up view of the PIC, showcasing a variety of integrated components within a compact 24 $mm^2$ area. A 16-channel, zero-angle fiber array (FA) delivers the optical carrier via photonic wire bonds (PWBs). FN-LC is selectively deposited on active regions, thereby mitigating unintentional changes in the refractive index in other areas exposed through the oxide open window. (c) Scanning electron microscopy (SEM) image of the PIC prior to FN-LC deposition, with an intentional metallic coating applied to prevent charge accumulation during imaging. (d) Magnified SEM image of two representative PWBs, illustrating bend radii. The fiber tips in the FA V-groove are aligned and bonded to the corresponding on-chip edge couplers via PWBs.