To obtain a more compact and robust quadratically nonlinear resonator, we investigated the design and fabrication of monolithic resonators made of a nonlinear optical material, with a radial photonic structure of periodically inverted ferroelectric domains, which will improve the efficiency of the comb generation process. The resonator is based on the propagation of the so-called whispering gallery modes inside an optical-grade-polished disk. We have identified lithium tantalate as the nonlinear material that realizes the best compromise between optical and mechanical properties, and the possibility of efficiently engineering the photonic structures. The design aims at determining the optimal geometric parameters of the disk and of the photonic structure. We are also establishing the optimal techniques and procedures for the polishing of the disks. A first sample of optical polished disk with radial poling has been realized, in collaboration with Université Grenoble Alpes and Tel Aviv University, and optimized for wavelengths around 1550 nm (Fig. 1).

A few milliwatts of laser light at 1550 nm from a DFB lasser diode has been coupled into and out of the resonator by means of a rutile prism, and the resonator has been characterized in terms of free spectral range (6.3 GHz) and quality factor (Q = 7×10⁶). We were able to couple about 50% of the input light to one of the resonator mode. We also made a preliminary measurement of the SHG efficiency curve, exploiting a low power widely tunable diode laser source, and collecting the integrated second harmonic light with a CCD sensor (Fig. 2). The observed SHG efficiency spreads out a wider spectral interval than the typical sinc-square curve expected for the designed radial poling. Supported by numerical simulations, we explain this behaviour as due to a small offset between the geometric centre of the WGM disk and the centre of the radial poling structure, which leads to an effective adiabatically variable poling period for the light propagating along the rim of the disk. The laser light has been successively replaced by a new diode laser system, amplified up to 2 W through an Er-doped fibre amplifier, while a new WGM resonator is under manufacturing, in order to improve the homogeneity of radial poling and the optical quality factor.