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Timestamp: 2019-04-18 17:05:50+00:00

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We demonstrate two non-destructive methods of studying the gradual poling of thin-film lithium niobate waveguides by the application of a sequence of high-voltage pulses, and we show the transition from under-poling to over-poling and the identification of the optimal stopping point of the poling process. The first diagnostic method is based on changes in continuous-wave light transmission through a hybrid waveguide as it is gradually poled by using a second set of monitoring electrodes fabricated alongside the principal poling electrodes. The second method is based on confocal back-reflected second-harmonic microscopy by using femtosecond optical probe pulses. The results from the two methods are in agreement with each other and may be useful as non-destructive in situ diagnostic methods for optimized poling of integrated waveguides.
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Fig. 1 (a) Schematic illustration of the poling monitoring process. Inset shows the not-to-scale cross-section of the hybrid Si-TFLN waveguide. (b) Top-view optical micrograph of the fabricated chip, which includes hybrid Si-TFLN waveguides and two sets of poling and monitoring electrodes.
Fig. 2 Numerically simulated electric field distributions (all shown in Ez component) with 400 V applied to the monitoring electrodes (separated by 150 μm, not shown here). (a) Ez distribution in the y–z plane, at a slice through the middle of the TFLN region. The white solid lines indicate the edges of three pairs of poling electrodes, and the “+”, “−” labels show the polarity of the electrodes. (b) Ez variation in the z direction along the two dashed lines shown in (a). Simulated E field intensity of the fundamental TE optical mode is overlayed in the plot, which shows the electrical field distribution seen by the optical mode. (c) Ez distribution in the x–z plane. The “+”, “−” labels show the polarity of the electrodes.
Fig. 3 (a) Not-to-scale schematic cross section of the hybrid Si-TFLN waveguide. (b) Simulated TE and TM mode Poynting vector components along the direction of propagation at 1550 nm. (c) Calculated TE and TM mode refractive index variation as a function of the applied electric field in hybrid Si-TFLN waveguides (solid line) and bulk LN (dashed line). (d) Calculated 1−cos[Γ(Ez)] as a function of the applied electric field with different poling duty cycles.
Fig. 4 Schematic illustrations of the poling and poling monitoring setup (a), measurement process (b), and the SH microscopy setup (c).
Fig. 5 (a) Typical SH microscopy scan result of a poled sample. For clarity the waveguide and electrodes are highlighted in red and yellow, respectively. (b) Sketch of the imaging and domain geometry of the scanned region in (a). Poled regions are marked in orange, while unpoled lithium niobate is colored in gray. (c) Not-to-scale cross-section of the sample highlighting the involved processes, i.e. BW SHG, FW SHG and reflections at the interfaces. (d) Simulated SH signal for a varying relative domain depth x. (e)–(f) Line scans of the nonlinear signal taken along the lines highlighted in (a). (g)–(h) Sketches of the suggested depth profile of the domains estimated from the simulation results in (d).
Fig. 6 (a) Measured poling voltage and current waveforms. (b) Recorded poling monitoring signals. (c) Second-harmonic confocal microscope images, after the indicated number of poling pulses were applied.
Fig. 7 Comparison of the predicted poling duty cycles (ξm) inferred from the recorded monitoring signals, and the calculated poling duty cycles (ξSH) based on the measured SH images.
(2) Γ = 2 π λ ( n z − n x ) L + 2 π λ E z [ ( n x 3 2 r 13 − n z 3 2 r 33 ) ( L − x ) − ( n x 3 2 r 13 − n z 3 2 r 33 ) x ] = 2 π λ ( n z − n x ) L + 2 π λ E z ( n x 3 2 r 13 − n z 3 2 r 33 ) ( L − 2 x ) .
(4) P out = A × ( 1 − cos [ Δ ϕ in ( ξ m ) + Δ ϕ ( ξ m ) + θ ] ) + C .
(5) ξ SH = A O ∩ A in A O × 100 % .

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