Nanoimprinting for all-polymer electro-optic waveguide devices

. We performed the design and fabrication of polymer waveguide circuits, aiming for applications as electro-optic devices. Uniform waveguides with over one centimeter of length were fabricated by soft nanoimprint lithography. These multimode waveguides present a height of 3 µm and low surface roughness (2 nm), with a thin residual layer of 600 nm. Propagation losses at 1550 nm are estimated to be around 7 dB/cm.


Introduction
Electro-optic devices are commonly fabricated in crystalline platforms such as potassium dihydrogen phosphate (KDP), beta-barium borate (BBO) and lithium niobate (LiNbO3).These materials present considerable electro-optic effect.However, the growth of crystals can be costly and time-consuming.The fabrication of micro-/nano-devices in such materials is also challenging, due to their chemical inertness [1].
In this context, polymeric materials can be interesting alternatives for the fabrication of photonic devices, since they are easy to process and can be structured by many different techniques, including Soft Nanoimprint Lithography (SNIL) [2].SNIL in polymers offers the advantage of cost-effective large-scale fabrication.Furthermore, polymeric matrices can be easily doped with different materials, including electro-optic chromophores.Advances in the design and synthesis of these molecules, with recently demonstrated electro-optic coefficients higher than 1000 pm/V, motivate their use in the fabrication of active photonic devices based solely on polymeric materials [3].
Here, we report on the design and fabrication of allpolymer waveguide circuits by SNIL, aiming for applications as electro-optic photonic devices, such as modulators and frequency combs.

Experiment and Methods
Waveguide circuits were designed by modelling the mode propagation at 1550 nm with Finite Element Analysis.The design was optimized for the optical and electrical properties of the electro-optic chromophore JRD1 dissolved in a Poly(methyl methacrylate) (PMMA) matrix (1:1) [4].The fabrication results presented herein correspond to the optimization process for the pristine PMMA matrix.
Polymer waveguides were fabricated using the SNIL technique [2].Firstly, silicon molds were fabricated using Direct Laser Writing followed by dry etching.Polydimethylsiloxane (PDMS, Sylgard 184) negative molds were then fabricated by pouring the liquid material on top of the silicon mold and curing it at 65 °C for 4 hours.
PMMA solutions were prepared by dissolving the material in cyclopentanone.Polymer films were then fabricated by spin coating the solution on Si/SiO2 substrates.A press system, that is able to apply forces above 100 N, was developed and used to stamp the PDMS mold on the film, while the sample is heated above the polymer flow temperature.Once the sample is cooled down, the PDMS mold is released, leaving the imprinted waveguide circuits on the substrate.
Optical characterization of the waveguide circuits was performed with a 1550 nm CW laser by end-fire coupling with lensed fibers.

Results
Figure 1 (left) shows the Scanning Electron Microscopy (SEM) image of the cross section of a PMMA multimode waveguide.The rough facet results from cutting the waveguide with a razor blade and can be further improved by cleaving the substrate or polishing the facet with Focused Ion Beam (FIB).From the SEM characterization we can conclude that the PDMS mold was completely filled by the imprint process, resulting in uniform cross sections of 3 µm of height (red line) and remaining film around 600 nm (green line).In Figure 1 (right), we can observe the electric field distribution, calculated via Finite Element Analysis, of the highest order transverse magnetic (TM) mode allowed in the designed JRD1 doped PMMA waveguide for excitation at 1550 nm.The simulation reveals tight confinement inside the waveguide, even with a 600 nm remaining film.The cutback method was used to estimate the propagation losses of the polymer waveguides; i. e. by measuring the transmission for different waveguide lengths, the propagation losses were found to be around 7 dB/cm at 1550 nm for the pristine PMMA waveguides.These losses are higher than the ones achieved in crystalline platforms, such as lithium niobate on insulator (1.55 dB/cm) [5].However, they can be further improved by thermal or chemical post-processing of the polymer waveguides in order to reduce defects.

Conclusions and Perspectives
We performed the design and fabrication of polymeric waveguide circuits by SNIL.Fabrication parameters were optimized for the PMMA matrix and uniform circuits with centimeters of length were produced, containing waveguides with cross section of 3 µm of height, thin remaining film of 600 nm and low surface roughness around 2 nm.The propagation losses were estimated to be around 7 dB/cm.The next steps of this work include the optimization of the fabrication parameters for the matrix doped with the electro-optic chromophore JRD1 and the integration of electrodes to the circuits, aiming for the realization of active electro-optic devices.This project has received funding from European Union's Horizon 2020 under MCSA Grant No 801459, FP-RESOMUS.

Fig. 1 .
Fig.1.Left: SEM image, obtained with the sample tilted at 30°, from the cross section of a multimode PMMA waveguide with 3 µm of height (red line) and residual film of 600 nm (green line).Right: Finite Element Analysis of the electric field distribution for the highest order transverse magnetic (TM) mode allowed in the designed chromophore-doped waveguide.

Fig. 2 (
Fig. 2 (top) shows an optical image of one waveguide circuit imprinted on PMMA, where we can observe uniform imprint over a large area, with waveguides longer than one centimeter.The inset shows an optical image of light at 635 nm being coupled to a straight waveguide by a lensed fiber.Fig. 2 (bottom) shows an Atomic Force Microscopy (AFM) image of the top surface of a waveguide, presenting low roughness of 2 nm (root mean square).

Fig. 2 .
Fig. 2. Top: Optical microscopy image of PMMA waveguides and (Inset) image of coupling tests with a 635 nm laser.Bottom: AFM image from the top surface of the waveguide.