We analyze the spectral engineering capabilities for broadband second harmonic and difference frequency generation (SHG and DFG) in the telecom band afforded by dispersion-engineered waveguides in x-cut lithium-niobate-on-insulator (LNOI). Considering both silica- and air-clad 5 mm-long rib photonic wires in 5 mol% MgO-doped 600nm-thick LNOI, we identify in both cases working points for 258-433 nm-wide DFG acceptance bandwidths. Furthermore, air-clad waveguides are found to afford simultaneous broadband SHG (103 nm) and DFG (433 nm) conversion with expected efficiencies of ∼4700 % W⁻¹cm⁻², a particularly appealing feature for prospective χ(2) cascading devices. Finally, we study the tolerances of such working points to waveguide and poling parameters, identifying the most critical parameter to be the waveguide sidewall angle, affecting the SHG bandwidth (central wavelength) with a sensitivity of 95 nm (200 nm) for 1° deviation around the optimal working point.

Counterpropagating (CP) χ⁽²⁾ interactions have since the inception of nonlinear optics garnered significant interest due to the unique features of their wave-dynamics and spectral response, stemming from their inherent feedback. The extremely short periods required for their quasi-phase matching (QPM) imply significant technology challenges, that are nevertheless being overcome by advances in periodically poled (PP) materials, with most prominent results achieved in bulk PPKTP under pulsed excitations [1]. Very recently, continuous-wave (CW) operation in ultralow-footprint devices has been achieved on the emerging periodically poled thin film LiNbO₃ (PPTFLN) platform, with symmetric second harmonic generation (SHG) in Z-cut waveguides [2]. Here we present the first results on CP frequency conversion in X-cut PPTFLN, the most widely used cut for photonic integrated circuits in LN [3], which has however proven harder to pole with short periods and high uniformity. At difference to the work of [2], we extensively explore the spectral response in sum frequency generation (SFG) experiments, considering nondegenerate and asymmetric regimes, which provide efficient unidirectional emission.

The development of a reliable technology for domain engineering in thin film lithium niobate is crucial to leveraging its disruptive potential for integrated nonlinear optics. However, thin film formats present outstanding challenges for traditional poling techniques with specific concern to non-polar cuts and short periods. Here, a novel approach is developed for the periodic poling of x-cut ≈500nm-thick lithium niobate on insulator (LNOI), relying on electron beams. High-quality ferroelectric gratings with periods in the 3.5–0.37 µm range are successfully fabricated, and a comprehensive analysis of their properties by piezoresponse force microscopy is presented, providing evidence for poling in highly non-equilibrium regimes, yielding regular domain gratings that remain stable over several years. Moreover, seamless integration with undoped and 5 mol% MgO-doped LNOI photonic nanowires is demonstrated, together with their nonlinear optical functionality in both co- and counter-propagating waveguide experiments. This novel poling technology appears ideally suited for submicrometric domain patterning of the most widely used cut for LNOI photonic integrated circuits and holds promise for unlocking their full potential for the realization of ultralow-footprint all-optical signal processing chips exploiting engineerable nonlinearities for classical and quantum applications.

We study second harmonic generation in LiNbO3 nanowaveguides, operated with sub-pJ pulses at 1424 nm in the picosecond regime generating 0.5 μW at 712 nm. Spectra recorded at both wavelengths provide evidence for enhanced χ(2) cascading effects.

Dispersion engineering of waveguides in photonic integrated circuits (PIC) is a crucial design tool for tailoring nonlinear functionalities in the rapidly emerging thin film LiNbO3 (TFLN) platform, exemplified by frequency comb generation, soliton formation and broadband frequency conversion [1]. Experimentally, the dispersion of PIC structures is typically measured in terms of free-spectral range (FSR) variation in ring cavity configurations or propagation time delays in long waveguides under short pulse excitation [2], methods with their own limitations when it comes to measurements in straight and relatively short (< cm) waveguides, such as TFLN ones.

Polarization control in photonic integrated circuit (PIC) waveguides is receiving broad attention for application in quantum systems and telecommunication [1]. Thin film lithium niobate (TFLN) is an ideal platform for polarization control applications due to its birefringence and electro-optic properties [2]. We observe polarization coupling between fundamental TE and TM modes in TFLN waveguides.

Nano-waveguide platforms have been developed over the past decade providing strong field enhancement and allowing precise control of modal dispersion [1] . Such nano-waveguides produced from materials with strong quadratic (χ (2) ) nonlinearity make excitation of χ (2) temporal solitons feasible [2] . Compared with Kerr solitons, χ (2) solitons have tighter dispersion criteria making nano-waveguides the only practical method for their observation [2] , [3] . As seen in previous work in photonic crystal fibres [4] , mode hybridisation, which occurs generally in waveguides with rectangular (or close to rectangular) cross-section, provides conditions to study novel types of soliton and their dynamics.

The development of advanced nano-structuring capabilities on LNOI is paving the way towards low-footprint photonic circuits leveraging appealing functionalities of LiNbO3 for ultrafast signal processing and wavelength conversion. In the talk we shall present ultra-narrow bandpass and multiresonance filters, implemented with phase-shifted Bragg gratings in LNOI photonic wires. We shall also discuss designs of dispersion engineered waveguides for broadband second harmonic generation, appealing for wavelength multicasting, ultrashort pulse frequency doubling and enhanced quadratic cascading in the telecom band.