Solution-derived barium titanate waveguides for integrated electro-optic modulation

TL;DR

This work demonstrates a monolithic, etchless electro-optic modulator built entirely from solution-derived BaTiO$_{3}$, patterned via soft nanoimprinting lithography to enable scalable oxide photonics. By optimizing sol-gel processing and annealing, the authors dramatically reduce optical losses (up to two orders of magnitude) and achieve measurable electro-optic modulation with moderate $V_{\pi}L$ values, aided by partial domain poling to reach full $\pi$-phase modulation. The approach yields a high-refractive-index, polycrystalline BaTiO$_{3}$ platform compatible with various substrates, offering a low-cost route toward large-scale, oxide-based integrated photonics and reconfigurable photonic processing. The combination of etchless patterning, strong Pockels nonlinearity, and scalable fabrication promises impactful applications in on-chip modulators, interconnects, and neuromorphic photonic systems.

Abstract

Metal oxides with strong nonlinear optical properties and wide transparency window are key materials for the development of compact and efficient photonic integrated circuits used for electro-optic modulators and entangled photon sources. Among them, barium titanate (BaTiO$_{3}$) is particularly attractive due to its large Pockels coefficient. However, its use has been limited by challenges in material synthesis and in nanopatterning, owing to its chemical stability and inertness. Here, we demonstrate a monolithic electro-optic modulator entirely based on solution-deposited BaTiO$_{3}$, fabricated through a bottom-up soft nanoimprinting lithography process. Fine-tuning the synthesis and nanofabrication enhances the optical properties of the polycrystalline material. By optimizing the process parameters, we achieve a reduction in propagation losses of two orders of magnitude, enabling efficient electro-optic modulation. This scalable, etch-free approach enables direct patterning of high-quality BaTiO$_{3}$ structures, establishing a new route for low-cost, large-scale integrated electro-optic devices entirely based on oxide material compatible with a wide range of substrates.

Figures (4)

• Figure 1: BTO sol-gel electro-optic modulator. Schematic of the monolithic electro-optic modulator based on sol-gel BTO. The laser output is split into two arms: one is coupled into the BTO waveguide, where an applied AC voltage induces a phase modulation between 0 and $\pi$, while the reference arm remains unmodulated. Their recombination produces an amplitude-modulated output signal.
• Figure 2: Material analysis of BTO sol-gel. (a) Fabrication process of BTO sol–gel thin films. After spin-coating two BTO sol-gel layers, a calcination step at 400$^{\circ}$C is performed to remove organic by-products. This sequence is repeated N times to achieve the desired film thickness. SEM top-view images of two BTO sol-gel samples after final high-temperature annealing at (b) 700$^{\circ}$C and (c) 800$^{\circ}$C. The lower temperature film exhibits a more uniform morphology with smaller grains. (d) XRD patterns of the 700$^{\circ}$C (green) and 800$^{\circ}$C (blue) annealed samples. Peaks marked with asterisks are assigned to the Ba$_{2}$Ti$_{5}$O$_{12}$ phase. Reference XRD peak positions correspond to BaTiO$_{3}$ (PDF no. 00-005-0626) and to Ba$_{2}$Ti$_{5}$O$_{12}$ (PDF no. 00-017-0661). (e) HR-TEM of a single domain of the polycrystalline BTO sol-gel annealed at 800$^{\circ}$C. (f) STEM dark-field image of a 800$^{\circ}$C annealed BTO film cross-section on a SiO$_{2}$/Si substrate. (g) EDX elemental analysis at the interface, black square in the STEM image, between the substrate and the BTO film. Si element is shown in blue (K$\alpha$ = 1.74 keV), while yellow represents the combined signal of Ba (L$\alpha$ = 4.46 keV) and Ti (K$\alpha$ = 4.51 keV) elements.
• Figure 3: Fabrication of BTO sol-gel waveguides. (a) Sketch of the soft nanoimprint lithography (SNIL) process used to fabricate the BTO sol-gel waveguides as described in the text. (b) Schematic of the simulated grating couplers used to optimise coupling with the BTO sol-gel waveguide. The optimized grating coupler parameters are the following: width = 470 nm, height = 350 nm and a period = 1.2 $\mu\text{m}$. (c) SEM top-view image of the fabricated grating couplers achieved by the bottom-up approach shown in (a). (d) SEM image taken at a 30° tilt showing a higher-magnification view of the grating sidewalls. (e) SEM cross-section image of the grating coupler obtained by focused ion beam milling, confirming a uniform filling of the PDMS mold by the BTO sol-gel. (f) Normalized transmission spectrum of a waveguide of length $400\,\mu\text{m}$ with input and output grating couplers.
• Figure 4: Electro-optic characterization. FEM simulations of the electric (a) and optical (b) field distributions for a BTO sol-gel waveguide on a 100 nm residual BTO layer on a SiO$_2$/Si substrate. The two top electrodes are separated by $9.7 \mu\text{m}$. The electrostatic simulation is performed with an applied voltage of 1 V. (c) SEM image of the fabricated electrodes close to the waveguide. (d) Sketch of the experimental setup used to achieve interference between the modulated light transmitted through the BTO waveguide and a reference fiber arm outside the chip. (e) Normalized transmission signal as a function of time for the 700$^{\circ}$C sample before (solid green line) and after (solid blue line) DC poling. The application of a DC bias enables domain alignment, resulting in a $\pi$-phase modulation at a driven frequency of 100 kHz. (f) Normalized transmission signal as a function of the applied voltage. The sample annealed at 700$^{\circ}$C (blue line) exhibits a lower $V_{\pi} L$ compared to the 800$^{\circ}$C-annealed sample (green line).