Fig. 2: Nanophotonic lithium niobate waveguides. | Nature Communications

Fig. 2: Nanophotonic lithium niobate waveguides.

From: Ultraviolet astronomical spectrograph calibration with laser frequency combs from nanophotonic lithium niobate waveguides

Fig. 2

a Geometry of a nanophotonic lithium niobate (LiNbO3) waveguide, showing the optical mode profile for the fundamental transverse electric field polarization (TE) mode and a possible poling pattern with spatially variable poling period Λ. The extraordinary crystal axis is oriented along the x-direction to access the highest electro-optic tensor element of LiNbO3. b Poling periods Λ required for quasi-phase-matching of various nonlinear optical processes for waveguides of different (top) width as a function of the fundamental comb’s wavelength. SHG: second-harmonic generation, SHG+F: sum-frequency generation of second-harmonic and fundamental comb, THG+F: sum-frequency generation of third-harmonic and fundamental comb, SHG+SHG: sum-frequency generation of the second-harmonic. Respective higher-order phase matching with three-fold period Λ is indicated (the current fabrication limit only allows for Λ > 2 μm). c Photograph of a waveguide in operation and scanning electron microscope image of the LiNbO3 on silica (SiO2). dg Examples of waveguide designs showing poling pattern (top), experimentally generated spectra for different input pulse energies provided by a 100 MHz, 80 fs mode-locked laser with a central wavelength of 1560 nm (waveguide cross-section are shown as insets and on chip-pulse energy is indicated by the color code) and pyChi64 simulation results (bottom) for a pulse energy of 50 pJ. The spurious spikes observed in the traces for 4 pJ pulse energy are manifestations of the noise floor of the optical spectrum analyzer (Yokogawa AQ6374).

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