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Purely Nonlinear Photonic Crystals

Final Report Summary - PNPCS (Purely Nonlinear Photonic Crystals)

Project Objective:
The project explored a brand new approach to photonic crystals, namely Purely Nonlinear Photonic Crystals (PNPCs), based on structured optical nonlinearities. Its main objective was to develop an integrated nonlinear nano-photonic platform on ferroelectrics of the lithium niobate crystal family and achieve with it a few basic device demonstrators. The ultimate research aim encompasses photonic devices with applications to telecom and quantum optics and potentially also biology and sensing.

Project Overview:
The Marie Curie fellowship allowed Katia to join the group led by Prof. Laurell at KTH, where she was trained on state-of-the nanotechnologies, established a photonic platform for LiNbO3 materials (bringing in her previous expertise from 10-years research outside Sweden) and then combined these capabilities to implement novel PNPC devices. The latter rely on a new approach to material engineering, affecting the nonlinear properties (as opposed to the refractive index) of the substrate. The key technology for achieving such a purely nonlinear structuring is known as 'periodic poling'.

Main scientific achievements

Technology. The technological activity of the project addressed in the first instance the challenge of controlling the periodic poling in two-dimensions, to implement non conventional (2D) lattice topologies and achieve high aspect ratios (depth over width) in the periodic patterning. To this aim Katia explored new solutions combining conventional patterning (performed with external electrodes) and chemical methods (to create in-built space charge fields in the crystals to enhance the poling spatial resolution). This allowed the reliable fabrication of 1D1 and 2D2 PNPCs with record aspect ratios (4:1000=featurewidth/depth) as well as the achievement of sub-micron patterning (Fig.2 to be published).

Theory and experiments. With this technology toolbox, extended to new LiNbO3 and LiTaO3 substrates with enhanced optical performance (Mg-doped and stoichiometric), Katia implemented a few basic PNPC demonstrators, for frequency up- and down- conversion as well as cascaded conversion processes (Fig.1). Recent highlights include: the broadest bandwidth to date (180 THz) achieved in parametric generation; and he demonstration of twin-beam parametric generation, a scheme unique to 2D PNPCs.