Title: Electro-Optic Modulation Based on Lossy Mode Resonance
Research proposal No: 1.1.1.9/LZP/1/24/117
Duration: 01.06.2025.-31.05.2028.
Project Leader: Ph.D. Edvīns Ļetko, Institute of Solid State Physics University of Latvia (ISSP UL)
Total budget: 184 140 EUR
European Regional Development Fund (ERDF) funding: 156 519 EUR
State budget funding: 18 414 EUR
ISSP UL budget: 9 207 EUR
Project description:
This interdisciplinary postdoctoral project introduces a novel mechanism for integrated modulation of wavelength, intensity, and polarization in photonic chips with the goal of surpassing current technologies. By leveraging the previously unexplored application of the lossy mode resonance phenomenon in photonics modulation, the project focuses on three main innovations: broad-spectrum signal modulation enabled by electrically tunable lossy mode resonance, ultra-fast modulation achieved through short-range electron migration at the lossy coating interface, and the expansion of lossy mode resonance applications beyond sensing to include optical modulation. The approach combines expertise from materials science, physics, chemistry, microfabrication, photonics, and optical computing, promising new functionalities and improved performance in integrated photonic systems.
PROJECT PROGRESS
Time period: 01.10.2025. – 31.03.2026.
Continuing the theoretical development of the device, the next stage involved optimization of the coating thicknesses. Simulations showed that the thickness of the indium tin oxide (ITO) electrodes should be as small as possible in order to minimize their influence on the LMR signal. However, the literature indicates that an electrode thickness of at least 20 nm is required to ensure a uniform external electric field. Therefore, 20 nm thick ITO electrodes were selected. It was also concluded that the aluminum oxide insulating layer should be made as thin as possible, since it effectively attenuates the applied electric field. A literature review showed that a 2 nm thick Al₂O₃ layer is sufficient to provide the required insulating properties. Meanwhile, analysis of different TiO₂ layer thicknesses revealed that the highest electro-optical sensitivity, approximately 4 nm/V, is achieved with a 60 nm thick TiO₂ layer. Consequently, the currently developed device architecture consists of 20 nm thick indium tin oxide electrodes, 2 nm thick aluminum oxide insulating coatings, and a 60 nm thick titanium dioxide modulating layer.
The obtained simulation results also enabled a more in-depth evaluation of the potential of the LMR phenomenon in active integrated photonic devices. It was demonstrated that LMR can be effectively used in electro-optically tunable filters, and the developed device achieved a high electro-optical sensitivity of approximately 4 nm/V, exceeding that of most electro-optical filters described in the literature. It was found that thinner electrodes and TiO₂ layers provide higher efficiency, whereas thicker TiO₂ layers promote a narrower LMR resonance. The obtained results confirm the potential of this approach for applications in sensing, telecommunications, and RF photonics. Based on these results, a scientific article was prepared and published in a Q2-level Scopus-indexed journal: https://doi.org/10.3390/photonics12111086
Based on the optimized coating recipes, multilayer electrode–insulator–semiconductor–insulator–electrode structures were fabricated, where ITO served as the electrode, Al₂O₃ as the insulator, and TiO₂ as the semiconductor. These structures were analyzed using an electrical probing station to verify whether leakage current forms when an external electric field is applied between the electrodes. Measurements showed that TiO₂ grown by the atomic layer deposition method begins to exhibit leakage current at voltages above 1.5 V. This is a significant result, since simulations predicted device operation at voltages up to 10 V. Additional ellipsometry measurements were also carried out while simultaneously applying voltages up to 1.5 V; however, this method did not prove sufficiently sensitive for detecting electro-optical changes. Therefore, it was concluded that a more suitable material for further studies could be TiO₂ grown by magnetron sputtering, which may possess more pronounced insulating properties. A decision has currently been made to fabricate future samples using this method, although they have not yet been experimentally tested.
At the same time, an alternative approach to achieving electro-optical modulation is also being investigated using printed electrolytes. The literature has shown that under the influence of an external electric field, ions in the electrolyte accumulate near the electrode surface and, due to their size, cannot diffuse through the material, making this approach promising for electro-optical modulation. Based on this principle, in collaboration with Swedish partners, a solid electrolyte was printed onto planar waveguides coated with ITO for LMR generation. In these experiments, a successful electro-optical LMR shift was observed, confirming the potential of this alternative approach for further device development.
Time period: 01.05.2025. – 30.09.2025.
Project implementation began with device modeling in the COMSOL Multiphysics environment. Since the operation of the proposed device is based on electro-optical effects arising from the redistribution of free charge carriers, it was necessary to master new built-in modules of this simulation tool. I already had prior experience with the Optic Wave module, so it was clear how to use it for modeling optical effects within the device. However, to understand the kinetics of free carriers, it was necessary to learn the Semiconductor module from the ground up. Mastering this module made it possible to determine which materials would be most suitable for the device design.
First, it was found that the LMR signal shift under the influence of an external electric field can be observed only in the near-infrared spectral range. This phenomenon can be explained by the Drude oscillator model, which describes the variation of optical properties depending on the concentration of free charge carriers specifically within this range. Based on the obtained results, the commercially available photoresist OrmoCore was selected as the waveguide material, characterized by losses of 0.2 dB/cm at a wavelength of 1310 nm, which are lower than those of other photoresists of a similar class.
Secondly, the analysis showed that the optimal waveguide dimensions for the designed device are 40 × 40 µm. The device follows an electrode–insulator–semiconductor–insulator–electrode architectural principle. The insulating material chosen was Al₂O₃ due to its excellent dielectric properties and widespread use in optoelectronics. The analysis also indicated that the insulating layer should be as thin as possible to minimize its impact on the LMR signal. Such thin films can be deposited using an Atomic Layer Deposition system available at ISSP UL, confirming the material’s suitability.
The electrode material selected was indium tin oxide (ITO), as it combines optical transparency with electrical conductivity, thereby preventing light losses that would occur if metallic electrodes were used. The modulating material chosen was TiO₂, whose ability to generate the LMR effect is confirmed by literature data. Moreover, the analysis showed that this material possesses a high dielectric permittivity, which increases the Debye length and thus enhances the effectiveness of the external electric field.
During the same project phase, alongside the theoretical development of the device, experimental studies were carried out to optimize thin-film deposition processes. All films were successfully deposited using atomic layer deposition and magnetron sputtering, and their optical properties were characterized to provide data for further device simulations.