Due to its excellent electro-optical and nonlinear optical properties, paired with a relative easiness of fabricating waveguides, the lithium niobate (LN) crystal has its place as an ideal platform for highly efficient and compact devices for quantum and/or classical optical applications. Up to now, most of commercially available integrated optics devices are based on LN waveguides fabricated either by Ti-indiffusion or by one of the conventional liquid-phase Proton Exchange techniques. These techniques allow fabricating waveguides with preserved electro-optical and nonlinear properties but, unfortunately, exhibiting low-index contrast typically in the range of 0.01 and 0.03. These low values limit the benefit linked to the confinement of the light in a waveguide. In this context, the challenge of modern photonics based on LN is to provide integrated configurations that ideally meet all these requirements: high-index contrast, low propagation losses, undegraded optical nonlinearity and electro-optical efficiency, stability, robustness, compactness, easiness of implementation, and low power consumption.
Dr. Alicia Petronela Rambu and co-workers from Research Center on Advanced Materials and Technologies (RAMTECH), Alexandru Ioan Cuza University of Iasi in Romania in collaboration with researchers from Institut de Physique de Nice (INPHYNI), Université Côte d’Azur in France developed a way to fabricate high-index contrast waveguides up to 0.1 for tight light confinement while exhibiting low propagations losses and perfect preserved nonlinearities of LN crystal substrates, which will not only dramatically reduce the size of nonlinear devices, paving the way for scalability, but also allow unprecedented stability and control and further enable the creation of previously unfeasible optical circuits with high component density for the practical implementation of classical and/or quantum optical devices. Starting from this, it will be possible to develop critically needed integrated optics technologies on lithium niobate and create novel concepts which will leverage off these unprecedented levels of efficiency and integration. The results are published in Photonics Research, Vol. 8, Issue 1, 2020 (Alicia Petronela Rambu, Alin Marian Apetrei, Florent Doutre, Hervé Tronche, Vasile Tiron, Marc de Micheli, Sorin Tascu. Lithium niobate waveguides with high-index contrast and preserved nonlinearity fabricated by a high vacuum vapor-phase proton exchange[J]. Photonics Research, 2020, 8(1): 01000008)
Dr. Sorin Tascu from Alexandru Ioan Cuza University of Iasi in Romania (who led this research) believes that beyond its importance for nonlinear optics (the obtained results have the potential to increase by one order of magnitude the efficiency of nonlinear optical devices), tight confinement waveguides fabricated on LN crystal substrate would offer ample improvements from all single-device perspective as well. For example, electrodes of electro-optical modulators could be brought closer allowing increasing the applied electric field per volt, thereby enhancing electro-optical modulation efficiency.
Future work will be focused on achieving proof-of-principle demonstration of highly efficient nonlinear devices based on high index contrast waveguides fabricated on Periodically Poled Lithium Niobate (PPLN) substrates. The successful outcome of further works could represent a foundational milestone in the development of nonlinear applications and devices, as well as a revolutionary breakthrough for modern integrated photonics.
Far field investigation of TE polarized hybrid modes in HiVac-VPE channel waveguide at λ=633 nm. The hybrid modes are generated by the strains and stresses induced by the non-modified crystal under and on the sides of the waveguide.
