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Lithium niobate waveguides with high-index contrast and preserved nonlinearity fabricated by a high vacuum vapor-phase proton exchange

Alicia Petronela Rambu, Alin Marian Apetrei, Florent Doutre, Hervé Tronche, Vasile Tiron, Marc de Micheli, and Sorin Tascu

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Abstract

Highly confining waveguides (Δne>0.1) without a degraded nonlinear coefficient and low propagation losses have been fabricated in lithium niobate (LN) by a new process that we called high vacuum vapor-phase proton exchange (HiVac-VPE). Index contrast, index profile, nonlinearity, and crystallographic phases are carefully investigated. Original analysis of index profiles indicates that the waveguides contain sub-layers whose depths depend on the exchange durations. Propagation behavior, propagation losses, and second-harmonic generation response of HiVac-VPE channel waveguides are investigated at telecom wavelength. The results recommend HiVac-VPE as a very promising technique for fabricating efficient nonlinear photonic integrated circuits in LN crystals.

© 2019 Chinese Laser Press

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Figures (8)
Fig. 1.
Fig. 1. X-ray rocking curves from (00.12) reflection of Z-cut HiVac-VPE planar waveguides: (a) logarithmic-type representation of vertical scale and (b) linear-type representation of vertical scale. The waveguides were fabricated with different exchange periods t(h) at T=350°C.

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Fig. 2.
Fig. 2. Extraordinary refractive index profile at λ=633 nm reconstructed by IWKB for planar waveguides fabricated in Z-cut LN by HiVac-VPE at 350°C with different exchange durations. The symbols represent the measured Neff of the propagating modes, except those on the ordinate that represent the raw surface indices calculated by IWKB. Dashed lines are guides for the eye.

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Fig. 3.
Fig. 3. Corrected refractive index profiles at λ=633 nm for planar waveguides fabricated in Z-cut LN by HiVac-VPE at 350°C for different durations. For t=24 h is the raw index profile. The symbols represent the measured Neff of the propagating modes, except those on the ordinate that represent the corrected surface indices. Solid lines are guides for the eye.

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Fig. 4.
Fig. 4. Top: index profiles of Z-cut HiVac-VPE waveguides fabricated for different exchange durations. The symbols represent the measured Neff of the propagating modes, except the IWKB corrected surface indices on the ordinate. The solid lines are the fits obtained by using Eq. (1). Bottom: derivative of the fits. Inset: sub-layers structures of the waveguides. The intensity of the red color suggests the refractive index value in the waveguides.

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Fig. 5.
Fig. 5. SHG profiles and reflected fundamental signal of Z-cut HiVac-VPE waveguides superimposed with index profiles (region in gray color) for (a) t=1 h and (b) t=5 h.

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Fig. 6.
Fig. 6. Near-field imaging of the modes at the output of channel waveguides fabricated at T=350°C for 1 h through the silica mask with openings of (a) 1 μm and (b) 1.5 μm width, respectively.

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Fig. 7.
Fig. 7. Near-field imaging of the modes at the output of channel waveguides fabricated at T=350°C for 1 h through the silica mask with an opening of 2 μm width. (a) Fundamental mode and (b) superior mode + fundamental.

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Fig. 8.
Fig. 8. Far-field picture on a screen of TE-polarized hybrid modes in HiVac-VPE channel waveguide at λ=633 nm for (a) 1 μm width and (b) 1.5 μm width, respectively.

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Tables (1)

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Table 1. Index Contrast Δne at λ=633 nm of Planar Waveguides Fabricated by the HiVac-VPE Processa

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Equations (1)

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n(d)=ne+A1 exp[−(d/w1)a1]+A2 exp[−(d/w2)a2],
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