[Paper Review] GHz-rate optical phase shift in light matter interaction-engineered, silicon-ferroelectric nematic liquid crystals

Iman Taghavi, Omid Esmaeeli+11 more|arXiv (Cornell University)|May 14, 2024

Liquid Crystal Research AdvancementsMaterials Science3 citations

TL;DR

This paper presents a poling-free, GHz-rate electro-optic phase shifter based on silicon-ferroelectric nematic liquid crystals (Si-FN-LC), engineered via a finger-loaded, non-slotted waveguide to enhance light-matter interaction. The device achieves an EO bandwidth >4.18 GHz, a DC modulation efficiency of 0.25 V·mm, and an AC efficiency of 25.7 V·mm, with on-chip insertion loss ≈4 dB, enabling CMOS-compatible, high-speed silicon photonic integration without complex poling or fabrication steps.

ABSTRACT

Organic electro-optic materials have demonstrated promising performance in developing electro-optic phase shifters. Their integration with other silicon photonic processes, nanofabrication complexities, and durability remains to be developed. While the required poling step in electro-optic polymers limits their potential and utilization on a large scale, devices made of paraelectric nematic liquid crystals suffer from slow bandwidth. In ferroelectric nematic liquid crystals, we report an additional GHz-fast phase shift that ultimately allows for significant second-order nonlinear optical coefficients related to the Pockels effect. It avoids poling issues and can pave the way for hybrid silicon-organic systems with CMOS-foundry compatibility. We report DC and AC modulation efficiencies of $\approx$~0.25 V$\cdot$mm (from liquid crystal orientation) and $\approx$~25.7 V$\cdot$mm (from Pockels effect), respectively, an on-chip insertion loss of $\approx$~2.6 dB, and an electro-optic bandwidth of $f_ ext{-6dB}$>4.18 GHz, employing improved light-matter interaction in a waveguide architecture that calls for only one lithography step.

Motivation & Objective

• To overcome the limitations of traditional electro-optic phase shifters in silicon photonics, such as high drive voltage, low bandwidth, and high static energy consumption.
• To eliminate the need for electro-thermal poling in organic electro-optic (OEO) polymers, which hinders large-scale integration and CMOS compatibility.
• To enable high-speed, low-loss, and scalable electro-optic modulation in silicon photonic integrated circuits using a novel, poling-free OEO material platform.
• To demonstrate a practical, CMOS-compatible, and scalable solution for high-bandwidth optical phase shifting using ferroelectric nematic liquid crystals (FN-LC) in a light-matter interaction-engineered waveguide.

Proposed method

• The authors developed a finger-loaded, non-slotted waveguide structure to enhance light-matter interaction (LMI), increasing the effective electric field and overlap integral (Γ) with the FN-LC material.
• Ferroelectric nematic liquid crystals (FN-LC) were used as the electro-optic medium due to their large second-order nonlinear susceptibility (χ(2)) and spontaneous polarization, enabling a Pockels effect without poling.
• The waveguide geometry was optimized to confine the optical mode and enhance the electric field (E_x) in the FN-LC region, particularly by reducing electrode spacing and increasing field concentration in the gaps.
• A push-pull configuration was employed in Mach-Zehnder modulators (MZMs) to achieve high-speed operation, with DC bias used to set the quadrature point and RF signals applied differentially for modulation.
• The device was fabricated using standard silicon photonic processes, with FN-LC selectively applied to the EOPS sections, and optical I/O achieved via grating couplers (GC) and photonic wire waveguides (PWB).
• Device characterization was performed using a vector network analyzer (VNA) to measure S21, extract the -3 dB bandwidth, and determine the effective electro-optic coefficient (r33) and VπL product across frequency.

Figure 1: 3D representation of the modulator selectively covered by FN-LC. a 80-nm-wide fingers are used to carry the RF signals and alignment field ( $\overrightarrow{E_{\text{align}}}$ ) to an area as close as $d_{\text{f-c}}=$ 100 nm away from the $W_{\text{c}}=$ 300 nm-wide core. This design opt

Experimental results

Research questions

• RQ1 Can ferroelectric nematic liquid crystals (FN-LC) enable GHz-speed optical phase shifting without requiring electro-thermal poling, as needed in EO polymers?
• RQ2 How does light-matter interaction engineering in a finger-loaded, non-slotted waveguide improve the electro-optic efficiency and bandwidth in Si-FN-LC devices?
• RQ3 What is the achievable electro-optic bandwidth and modulation efficiency in a poling-free, silicon-organic hybrid platform based on FN-LC?
• RQ4 Can large-scale integration of FN-LC modulators be achieved on a single silicon photonic chip with low insertion loss and CMOS compatibility?

Key findings

• The device achieved an electro-optic bandwidth (f-6dB) exceeding 4.18 GHz, demonstrating GHz-rate phase shifting in a poling-free, silicon-organic hybrid platform.
• The DC modulation efficiency was measured at 0.25 V·mm, and the AC efficiency reached 25.7 V·mm, indicating strong electro-optic response and high efficiency.
• On-chip insertion loss was measured at approximately 4 dB, which is acceptable for integrated photonic applications and competitive with other OEO-based modulators.
• The light-matter interaction-engineered finger-loaded waveguide design enhanced the effective electric field and overlap integral (Γ), resulting in a ~10× improvement in E_x and ~2× higher Γ compared to conventional non-slotted waveguides.
• A fully packaged chip containing over 100 FN-LC-based Mach-Zehnder modulators was demonstrated, confirming scalability and compatibility with CMOS fabrication processes.
• The FN-LC material exhibited a large second-order nonlinear susceptibility (χ(2)) and spontaneous polarization, enabling a Pockels effect without poling, which eliminates a major fabrication bottleneck in EO polymer integration.

Figure 2: Waveguide design simulation results. a The mode evolution in each arm of the MZM demonstrated by a 10 $\upmu m$ -long adiabatic strip-to-finger mode converter (section I) coupled to a 3 $\upmu m$ -long portion of the 500 $\upmu m$ FNS waveguide (section II). Transverse electric optical fie