Structure and Fabrication Method for Efficient Second-Harmonic Generation in Lithium Niobate Waveguides

Abstract

The present invention discloses a structure and a preparation method for efficiently generating second harmonic waves in a lithium niobate waveguide structure. An x-cut thin-film lithium niobate substrate is selected, and by adjusting the waveguide width and height, the effective refractive indices of two specific modes are made equal to achieve mode phase matching. Then, during the preparation process, the nonlinear distribution of the waveguide structure is artificially regulated to make the nonlinearity in a specific region opposite to that before. In the non-decaying region of the coherent frequency doubling process, the theoretical conversion efficiency of the second harmonic wave is determined by the effective nonlinear susceptibility and the mode overlap factor. The waveguide structure proposed in the present invention uses the maximum nonlinear susceptibility d 33 = 27 pm / V of lithium niobate and has a large mode overlap factor of 0.8019, and its theoretical normalized conversion efficiency of the second harmonic wave is as high as 10872%W ‑1 cm ‑2 .

Description

Technical Field

[0001] The present invention belongs to the field of nonlinear technologies, and particularly relates to a structure and manufacturing method of a lithium niobate waveguide for efficiently generating second harmonic waves. Background Art

[0002] Second-order nonlinearity is the basis for many interesting phenomena in modern optics, including second harmonic generation, sum frequency generation, and difference frequency generation. Wavelength conversion in nonlinear materials plays an important role in classical and quantum optics, including optical parametric amplification, quantum frequency conversion, entangled photon pair generation, all-optical signal processing, and supercontinuum generation, etc. Compact and efficient waveguide structures are the key components for this series of applications, and lithium niobate crystals are usually used to achieve them.

[0003] Lithium niobate (LiNbO 3 ) is an excellent optical crystal. It is transparent in a wide spectral range, has good acousto-optic and piezoelectric effects, and has a large second-order nonlinear coefficient (d 33 ≈27 pm / V). In 2012, the advent of thin-film lithium niobate provided an excellent platform for on-chip integration of various devices with excellent performance. Many schemes have been proposed for efficient nanophotonic waveguides for wavelength conversion on LNOI, such as SiN-loaded waveguides through mode-phase modulation, ridge waveguides realized by grating-induced technology, and periodically designed ferroelectric domains of crystals for quasi-phase matching (QPM) to obtain PPLN waveguides. Among them, the second harmonic conversion efficiency of PPLN waveguides has been able to reach 10 3 % W -1 cm -2 .

[0004] In nanophotonic waveguides, different modes can be selected to achieve phase matching between the three waves for interaction, that is, mode phase matching. However, the significant mismatch of different modal profiles hinders the interaction between the fundamental wave and the second harmonic wave, resulting in limited frequency doubling conversion efficiency. Summary of the Invention

[0005] The purpose of the present invention is to provide a lithium niobate waveguide structure capable of efficiently generating second harmonic waves and its preparation method to solve the problems proposed in the above background art.

[0006] To achieve the above purpose, the inventors of the present invention manipulate the χ (2) spatial distribution of the waveguide structure, so that the χ (2) in the left and right 1 / 3 regions of the waveguide structure changes from positive to negative, which can overcome the significant mismatch of different modal profiles, thereby greatly improving the frequency doubling conversion efficiency.

[0007] The principle utilized by the present invention is mode phase matching. By designing the waveguide structure, the TE of the 1550 nm fundamental frequency light00 The effective refractive index of the mode is equal to that of the TE mode of the 775 nm frequency-doubled light. 20

[0008] The substrate used for the ridge waveguide designed in the present invention is lithium niobate on 700 nm thick x-cut silicon, the top width of the waveguide is 860 nm, the ridge height is 550 nm, and the angle of the ridge is 75°.

[0009] The lithium niobate waveguide designed in the present invention has a very small effective mode area Aeff = 0.79138 um 2 .

[0010] The lithium niobate waveguide designed in the present invention has a very large mode field overlap factor ζ = 0.8019.

[0011] The lithium niobate waveguide designed in the present invention has an extremely high theoretical normalized second harmonic conversion efficiency η = 10872% W -1 cm -2 .

[0012] The method for preparing the lithium niobate waveguide for efficiently generating second harmonics includes the following steps:

[0013] 1) Uniformly coat a layer of photoresist on 700 nm thick lithium niobate on x-cut silicon (LNOI), and use extreme ultraviolet lithography technology to expose the substrate to make a waveguide lithography topography with a width of 0.86 um.

[0014] 2) Use vacuum evaporation technology to deposit a 200 nm Cr film on LNOI, and then perform a lift-off process so that only the waveguide-shaped Cr mask remains on the substrate.

[0015] 3) Use reactive ion etching technology to etch the substrate, and optimize the etching time so that the etching depth reaches 550 nm.

[0016] 4) Use Cr etching solution to remove the remaining Cr on the substrate, clean and spin-coat the photoresist again.

[0017] 5) Use alignment technology to expose the substrate, and expose the lithography topography of the planar electrodes 1 um away from both sides of the waveguide.

[0018] 6) Perform the Cr film deposition and lift-off process again.

[0019] 7) Apply an electric current to the Cr electrode, and domain inversion will occur in lithium niobate under a certain voltage, and its polarity changes from + to -. Control the voltage and pulse of the applied electric current so that domain inversion occurs at about 1 / 3 on both the left and right sides of the 0.86 um waveguide.

[0020] 8) Use Cr etching solution to remove Cr again, and the preparation of the waveguide for efficiently generating second harmonics is completed.​

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] The present invention uses an excellent nonlinear material - lithium niobate, and fully utilizes its very high nonlinear polarizability (d 33 ≈27 pm / V). By artificially regulating the nonlinear distribution of the waveguide cross-section, the mode matching is overcome, and the second harmonic conversion efficiency is greatly enhanced. The designed lithium niobate waveguide has a very small effective mode area, a very large mode field overlap factor, and an extremely high theoretical normalized second harmonic conversion efficiency. The proposed working principle is also applicable to other second-order nonlinear platforms and has great application potential in nonlinear optics. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a schematic diagram of the waveguide simulation structure of the present invention;

[0024] Figure 2 is a schematic diagram of mode phase matching simulated by the finite element analysis method of the present invention;

[0025] Figure 3 is a polarity distribution diagram of two matching modes of the present invention;

[0026] Figure 4 is a schematic diagram showing the change of the effective refractive indices of two modes of the waveguide of the present invention with the waveguide width;

[0027] Figure 5 is a preparation flow chart of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] As Figure 1 shown is a sectional view of the lithium niobate waveguide structure for efficiently generating second harmonics. The used LNOI substrate has a tangential direction of x-cut, and the LN thickness is 700 nm. The waveguide width w t is 860 nm, the ridge height h 1 is 550 nm, and the ridge angle θ is 75°.

[0029] As Figure 2 shown is a schematic diagram of mode phase matching simulated by the finite element analysis method of the present invention. The left figure shows the TE 00 mode corresponding to the 1550 nm fundamental frequency light, and the right figure shows the TE 20 mode corresponding to the 775 nm second harmonic light. The ordinate is the E x component of the mode. It can be seen that the transverse distribution of the E x of the second harmonic light has positive and negative values. Figure 3 is a polarity distribution diagram of two matching modes of the present invention. It can be seen that in regions I and III of the transverse distribution of the waveguide, the polarities of the two modes are opposite, and in region II, the polarities are the same. AsFigure 4 The figure shows a schematic diagram of the effective refractive indices of two modes of the waveguide of the present invention changing with the waveguide width. The result shows that mode phase matching is achieved at 860 nm. As Figure 5 The figure shows a preparation flow chart of the present invention. It mainly goes through steps such as spin-coating photoresist on the waveguide, coating film, lift-off, etching, removing Cr, spin-coating photoresist on the electrode, coating film again, lift-off again, applying voltage, and removing Cr again. The waveguide structure of the present invention makes the polarities in the left and right 1 / 3 regions of the waveguide reverse by applying voltage to cause domain inversion, thereby greatly increasing its mode overlap factor and achieving an extremely high second harmonic conversion efficiency.

[0030] The method for preparing a lithium niobate waveguide for efficiently generating second harmonic of the present invention includes the following steps:

[0031] 1) Uniformly coat a layer of photoresist on 700-nm-thick x-cut silicon-based lithium niobate (LNOI), and use extreme ultraviolet exposure technology to expose the substrate to make a waveguide photolithography topography with a width of 0.86 μm;

[0032] 2) Use vacuum evaporation technology to deposit a 200-nm Cr film on LNOI, and then perform a lift-off process so that only a waveguide-shaped Cr mask remains on the substrate;

[0033] 3) Use reactive ion etching technology to etch the substrate, and optimize the etching time so that the etching depth reaches 550 nm;

[0034] 4) Use Cr etchant to remove the remaining Cr on the substrate, clean and spin-coat photoresist again;

[0035] 5) Use alignment technology to expose the substrate and expose the photolithography topography of the planar electrode 1 μm away from both sides of the waveguide;

[0036] 6) Perform Cr film deposition and lift-off processes again;

[0037] 7) Apply current to the Cr electrode. At a certain voltage, domain inversion will occur in lithium niobate, and its polarity changes from + to -. Control the applied voltage and pulse so that domain inversion occurs at the left and right 1 / 3 of the 0.86-μm waveguide;

[0038] 8) Use Cr etchant to remove Cr again, and the preparation of the waveguide for efficiently generating second harmonic is completed.

[0039] This scheme proposes a new mode phase matching scheme. By modifying the χ (2) distribution, a large mode overlap factor of 0.8019 can be obtained, as well as a second harmonic conversion efficiency as high as 10872% W -1 cm -2The theoretical normalized SHG conversion efficiency. The proposed working principle is also applicable to other second-order nonlinear platforms and has great application potential in nonlinear optics.

Claims

1. Structure for efficiently generating second harmonic in a lithium niobate ridge waveguide, characterized in that the selected material substrate is lithium niobate on x-cut silicon, the thickness of the material substrate is 700 nm, the width of the top of the waveguide is 0.86 μm, the ridge height is 550 nm, the ridge waveguide angle is 75°, and domain inversion occurs in the lithium niobate in the left and right 1 / 3 regions of the waveguide respectively.

2. Method for determining the structure according to claim 1, characterized in that the steps are as follows: 1): By designing the structural parameters of the waveguide, the TE mode of the 1550 nm fundamental light 00 is made equal to the refractive index of the TE mode of the 775 nm second harmonic light 20 to achieve mode phase matching; 2): The effective refractive indices of two modes are obtained by simulation calculation using the finite element method; 3): For a lossless waveguide without pump loss, the normalized efficiency of SHG is given by the following expression: where Δk = 2ω(n 1 - n 2 ), when the effective refractive index n 1 of the fundamental optical mode is equal to the effective refractive index n 2 of the second harmonic optical mode, Δk = 0, and the highest theoretical normalized conversion efficiency is achieved at this time: Where P 1 and P 2 are the powers of the input fundamental frequency mode and the generated second harmonic mode, respectively, L is the waveguide length, d eff is the effective nonlinear coefficient, λ is the fundamental frequency optical pump wavelength, ε 0 and c are the permittivity of free space and the speed of light, respectively; A eff is the effective mode area, and ξ is the spatial mode overlap factor between the fundamental frequency mode and the second harmonic mode.

3. Method according to claim 2, characterized in that Numerical simulation shows that this structure has a small effective mode area, A eff = 0.79138 μm 2 .

4. Method according to claim 2, characterized in that Numerical simulation shows that this structure has a large mode overlap factor, ξ = 0.8019.

5. Method according to claim 2, characterized in that: In step 3), numerical simulation shows that this structure has an extremely high normalized conversion efficiency, η = 10872% W -1 cm -2 .

6. Method for fabricating the structure according to claim 1, characterized in that it includes the following steps: 1) Uniformly coat a layer of photoresist on lithium niobate on x-cut silicon with a thickness of 700 nm, and expose the substrate using extreme ultraviolet exposure technology to make a waveguide photolithography topography with a width of 0.86 μm; 2) Deposit a 200-nm Cr film on lithium niobate on x-cut silicon using vacuum evaporation technology, and then perform a lift-off process so that only a waveguide-shaped Cr mask remains on the substrate; 3) Etch the substrate using reactive ion etching technology, and optimize the etching time to make the etching depth reach 550 nm; 4) Remove the residual Cr on the substrate using Cr etchant, clean and spin-coat the photoresist again; 5) Expose the substrate using alignment technology to expose the photolithography topography of the planar electrodes 1 μm away from both sides of the waveguide; 6) Perform the Cr film deposition and lift-off process again; 7) Apply an electric current to the Cr electrode, and domain inversion will occur in the lithium niobate at a certain voltage, and its polarity changes from + to -, and control the voltage pulse applied to make the domain inversion occur in the left and right 1 / 3 regions of the waveguide respectively; 8) Use Cr etchant to remove Cr again, and the waveguide for efficiently generating second harmonic is fabricated.

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