Microstructure and domain engineering of lithium niobate crystal films for integrated photonic applications

ABSTRACT Recently, integrated photonics has attracted considerable interest owing to its wide application in optical communication and quantum technologies. Among the numerous photonic

materials, lithium niobate film on insulator (LNOI) has become a promising photonic platform owing to its electro-optic and nonlinear optical properties along with ultralow-loss and

high-confinement nanophotonic lithium niobate waveguides fabricated by the complementary metal–oxide–semiconductor (CMOS)-compatible microstructure engineering of LNOI. Furthermore,

ferroelectric domain engineering in combination with nanophotonic waveguides on LNOI is gradually accelerating the development of integrated nonlinear photonics, which will play an important

role in quantum technologies because of its ability to be integrated with the generation, processing, and auxiliary detection of the quantum states of light. Herein, we review the recent

progress in CMOS-compatible microstructure engineering and domain engineering of LNOI for integrated lithium niobate photonics involving photonic modulation and nonlinear photonics. We

believe that the great progress in integrated photonics on LNOI will lead to a new generation of techniques. Thus, there remains an urgent need for efficient methods for the preparation of

LNOI that are suitable for large-scale and low-cost manufacturing of integrated photonic devices and systems. SIMILAR CONTENT BEING VIEWED BY OTHERS INTEGRATED ELECTRO-OPTICS ON THIN-FILM

LITHIUM NIOBATE Article 25 April 2025 HIGH DENSITY LITHIUM NIOBATE PHOTONIC INTEGRATED CIRCUITS Article Open access 10 August 2023 LITHIUM NIOBATE PHOTONIC-CRYSTAL ELECTRO-OPTIC MODULATOR

Article Open access 17 August 2020 INTRODUCTION In contemporary society, the demand for high-bandwidth optical communication, including for mobile high-definition video streaming, autonomous

vehicle applications, remote surgery, telepresence applications, and interactive 3D virtual reality gaming, is sharp increasing1,2,3,4,5. The electro-optical modulator is the key component

in optical fiber communication, which modulates the light signal for loading information through electricity. Lithium niobate (LiNbO3, LN) exhibits a high-performance electro-optic effect

and high optical transparency6,7. Therefore, LiNbO3 has been widely applied in electro-optic modulators. Generally, optical modulation is realized by a voltage-induced refractive index

change, which can be described by the change in the ellipsoid of the refraction index influenced by an external electric field as \(\Delta \beta _{ij} = \gamma _{ijk}E_k + h_{ijkl}E_kE_l +

\ldots\), where Δβ_ij_ is the variation in the dielectric impermeability under the external electric field (_E_), γ_ijk_ is the linear electro-optic coefficient or Pockels coefficient, and

_h__ijkl_ is the quadratic electro-optic coefficient or Kerr coefficient. For LN, the linear electro-optic coefficient is _γ_33 = 30.9 × 10−12 mV−1, while other photonic materials usually

have a very small or zero linear electro-optic coefficient. For example, the linear electro-optic coefficient of GaAs is _γ_41 = 1.5 × 10−12 mV−1, which is one-twentieth that of LN. An

electro-optic modulator can be constructed on LN wafers by fabricating waveguides via defect engineering, including titanium diffusion or proton exchange, which is compatible with

complementary metal–oxide–semiconductor (CMOS) technology. For example, the 50:50 Y-junction microstructure splits the input light into two LN optical waveguides that form the two arms of

the Mach–Zehnder interferometer (MZI) modulator, a traditional electric-optical modulator. As shown in Fig. 1a, the applied voltage induces a phase delay in one arm and a phase advance in

the other, which in turn changes the output intensity at the Y-combiner through interference7. The minimum voltage required to completely switch the output between on and off is defined as

the half-wave voltage (_V_π). However, the low refractive index contrast of the titanium diffusion (Δ_n_ ~0.01) or proton exchange (Δ_n_ ~0.1) waveguides resulted in weak confinement and a

large bending radius of the waveguides8. The weak optical confinement requires metal electrodes to be placed far from the optical waveguide, which in turn lowers the electro-optic

efficiency. Hence, either large voltages or long electrodes are required to sufficiently modulate traditional LN modulators (with a size of approximately several centimeters). Therefore, the

product of the half-wave voltage and electrode length, _V_π_L_, is a common figure of merit for electro-optic modulators. Nevertheless, the capacitance of the long electrodes limits the

bandwidth, which results in a trade-off between the driving voltage and bandwidth7. On the other hand, ferroelectric domain engineering of LN crystals has been extended from 1D to 2D and 3D,

which was comprehensively reviewed in reference9. Periodically poled lithium niobate (PPLN) has been widely applied in the fields of frequency conversion, nonlinear beam shaping10, and

quantum entanglement based on quasi-phase-matching (QPM) theory11,12,13,14,15. The second-order nonlinear optical coefficients (_d_33 and _d_31) of LN are 25.2 and 4.6 pm V−1 at 1064 nm,

respectively16. Because LN is only poled along the _z_-axis, PPLN is appropriate for polarizing light to parallel to the _z_-axis to exploit the largest nonlinear-optical coefficient _d_33.

As shown in Fig. 1b, a channel waveguide was fabricated on the surface of a PPLN wafer, which can confine the pump beam over the entire interaction length to greatly increase the conversion

efficiency. Moreover, considering the device size, reliability, cost, and energy consumption, photonic integrated circuit (PIC) technology has recently attracted significant interest in the

rise of the integrated photonics field17,18,19,20,21,22,23. Integrated photonics is concerned with the integration of all the key components connected by a waveguide in a single chip

(photonic platform) using a single material (monolithic integration) or multiple materials (hybrid integration). Most integrated photonic circuits have been built on four key platforms:

indium phosphide24,25, silicon-on-insulator (SOI)26,27,28, silicon nitride25,29, and LN30,31. Nevertheless, no photonic platform has yet produced an optimal overall photonics system

performance17. Generally, the selection of a material platform is based on the functionality of the optical components in the circuit, a low propagation loss, and industry-compatible

fabrication processes18. For example, the compatibility of silicon integrated circuit manufacturing is the main reason for the development of silicon photonics32. Moreover, the manufacturing

methods are suitable for the construction of SOI rib waveguides with a small cross-section of 1 μm2 with propagation losses as low as 0.1–0.5 dB cm−1 and a high index contrast between the

Si waveguide layers (_n_Si ~3.4) and SiO2 cladding layers (_n_SiO2 ~1.4), affording strong optical confinement and a small bending radius of 10 μm33. The low propagation loss and high index

contrast of photonics systems, which make SOI one of the most active photonic platforms, are important evaluation standards for photonic platforms34. However, the 3 dB electrical modulation

bandwidths of all-Si photonic devices have a theoretical limit of approximately 60 GHz35. To conclude, LN is a promising photonic matrix due to its advantageous electro-optical effect and

nonlinear optical properties. For example, on-chip generation and manipulation of entangled photons based on annealed proton exchanged waveguide circuits integrated on a z-cut PPLN crystal

was demonstrated36. Unfortunately, bulk LN waveguides have large footprints, and the fabrication technology is not compatible with CMOS technology. Above all, nanophotonic LN waveguides with

a larger refractive index contrast and a smaller optical mode size have emerged as a branch of integrated lithium niobate photonics37,38,39,40, although LN was perceived as a

difficult-to-etch material. Over the past 25 years, integrated lithium niobate photonics have relied almost exclusively on high-quality lithium niobate thin film on insulator (LNOI)

technology41 and advanced PIC technology for etching nanophotonic waveguides and microphotonic modulators. On the one hand, LNOI with a diameter of 3 in. can be fabricated by the smart cut

technique41. In this technique, a submicron single-crystalline LN film is first detached from a sacrificial wafer by crystal ion slicing and then bonded with a SiO2-deposited LN substrate or

Si substrate, forming LNOI (Fig. 2a). On the other hand, the CMOS-compatible microstructure engineering of LNOI for integrated photonic circuits, including nanophotonic waveguides and

advanced functional modulators, has advanced considerably. Briefly, the microstructure of integrated photonic circuits is transferred to LN thin films by a lithographically defined mask.

Then, the LN thin film is etched by Ar+ milling, forming thin slabs with a thickness of several hundred nanometers, which form ridge waveguides. Finally, a microscale-thick SiO2 cladding

layer is deposited on top of the etched LN thin film by plasma-enhanced chemical vapor deposition (PECVD) to form a sandwich structure (Fig. 2b). The refractive index contrast between the LN

core (_n_LN ~2.1) and SiO2 cladding (_n_SiO2 ~1.4) is approximately 0.7, which is much larger than that of traditional LN waveguides; thus, LNOI can serve as strongly guiding planar

waveguides even with a core layer of sub-micron thickness. Furthermore, domain engineering of LNOI was studied systematically for phase-matching nonlinear photonics. CMOS-compatible domain

engineering of LNOI is gradually accelerating the development of integrated nonlinear photonics, which will play an important role in integrated photonic quantum technology42. In addition,

considering the difficulty of etching LN, microstructure engineering using an easy-to-etch photonic material (e.g., SOI or silicon nitride) rib-loaded on LNOI is an attractive approach for

hybrid integration of integrated photonic circuits, which not only obviates the necessity of etching LN but also combines the scalability of silicon photonics with the excellent

electro-optic modulation of LN43. Lithium niobate, as a traditional multifunctional material, has stimulated a photonics revolution as silicon did for electronics. Herein, we review the

progress in microstructure and domain engineering of LNOI for integrated lithium niobate photonics, including photonic modulation and nonlinear photonics. Above all, ultra low-loss highly

confined LN nanophotonic waveguides are the most fundamental building block17. Furthermore, the optical modulation device is at the heart of integrated lithium niobate photonic systems, as

it encodes the RF signal. In Section “Microstructure engineering of LNOI for photonic modulation”, the progress in the microstructure engineering of LNOI for the preparation of ultra

low-loss highly confined nanophotonic waveguides and microphotonic modulators is described. Furthermore, we review the progress in on-chip nonlinear photonics involving highly confined

photonic microstructures and phase-matching techniques for LNOI in Section “Microstructure engineering of LNOI for nonlinear integrated photonics”. The focus is on the domain engineering of

LNOI for QPM nonlinear photonics. MICROSTRUCTURE ENGINEERING OF LNOI FOR PHOTONIC MODULATION Generally, the basic microstructure of integrated photonic circuits for photonic modulation

contains ridge waveguides and microphotonic structures, including micro rings, microdisks, and microracetracks. Whispering-gallery-mode (WGM) microresonators are the most compact micro

modulator with high confinement of light into small ring volumes, such as microrings, microdisks, and microracetracks44. Optical WGM microresonators have some exceptional properties, such as

a small mode volume, a high power density, and a high-quality factor _Q_ (_Q_ = _λ_/Δλ, where _λ_ is the wavelength at which resonance occurs and Δ_λ_ is the linewidth of the resonance

wavelength). The refractive index changes when an electric field is applied to LN microresonators based on the electro-optic effect, which modifies the effective optical path length of the

resonator. Thus, the resonance frequency of the microresonator shifts, thereby realizing electrical tuning. In the early stage, efforts to construct microstructures on LNOI for photonic

modulation were focused on low-loss nanophotonic waveguides and high-Q microresonators. Furthermore, MZI modulators integrated on LNOI platforms with advanced functionalities have been

reported. In this section, both complementary microstructure engineering approaches for photonic circuits, i.e., direct etching of LNOI (monolithic integration) and using other photonic

materials rib-loaded on LNOI (hybrid integration), are reviewed. MICROSTRUCTURE ENGINEERING BY DIRECT ETCHING OF LNOI Z-CUT LN FILMS FOR TM POLARIZATION Generally, the development of WGM

microresonators for integrated lithium niobate photonics is accompanied by the improvement of the LNOI preparation technique. In the early stage, z-cut LN films were generally selected

because of the limitations of lithographic techniques. Metal electrodes can be deposited between the substrate and the waveguide so that an electric field can be applied along the z-axis to

exploit the largest electro-optic coefficient, _r_33, of LN, with light polarization along the _z_-axis (the electric field direction is mainly perpendicular to the LN film, which is called

transverse magnetic (TM) polarization) (Fig. 3a). To the best of our knowledge, a slab waveguide on LNOI was prepared for the first time by crystal ion slicing and wafer bonding (called the

smart cut method) in 200445. The electro-optic coefficients and refractive index of the LN film were found to be comparable to those of the bulk LN crystal. Ridge waveguides on LNOI were

first fabricated by photolithography and Ar+ beam milling46. However, the waveguide loss was not measured accurately and was estimated to be largely owing to the rough sidewalls caused by

the etching process. It must be noted that the large mismatch in the thermal expansions of LN and SiO2/Si substrates can generate strong thermal stress during the bonding and layer transfer

processes. Park et al.47 introduced laser irradiation for the layer transfer process to promote localized layer exfoliation on an LN donor substrate and obtained a 6 mm2 LNOI sample with a

microdisk structure. Later, Guarino et al.48 introduced benzocyclobutene (BCB) as an adhesive layer between the metal electrode and LN film to reduce thermal stress and successfully prepared

a centimeter-sized LNOI sample49. More importantly, an electro-optically tuneable WGM microresonator was introduced for the time on an LNOI platform. The transmission spectrum was tuned by

the electro-optic effect, with a frequency tunability of 0.14 GHz V−1. However, BCB did not allow high-temperature annealing to repair the implantation-induced defects, which resulted in an

electro-optical activity that was approximately 50% that of the bulk LN crystal and a large propagation loss of approximately 17 dB cm−1. Nevertheless, the study validated the viability of

integrated lithium niobate photonics and stimulated further research on ultralow-loss sub-micron waveguides. Hu and co-workers prepared directly bonded LNOI without a BCB adhesive layer,

which allowed high-temperature annealing to minimize ion-implantation-induced defects. Based on this, LN photonic wires with small cross-section dimensions of 1 × 0.73 μm2 were fabricated on

an LNOI substrate (Fig. 4a) etched by inductively coupled plasma (ICP) Ar+ milling50. The measured propagation loss was 9.9 dB cm−1 at a 1.55 μm wavelength. Bo and co-workers fabricated LN

microdisks with an undercut structure on an LNOI platform by UV photolithography, Ar+ plasma etching, and HF etching51. The microdisk resonator coupled with a tapered fiber exhibited a

quality factor of 1.19 × 106, which was two orders of magnitude higher than that of the reference microring resonator48. X-CUT LN FILMS FOR TE POLARIZATION However, z-cut LN films only work

for TM polarization and are not suitable for on-chip integration. Therefore, x- or y-cut LN films were adopted, with light polarization along the _z_-axis (the electric field direction is

mainly parallel to the LN film, which is called transverse electric (TE) polarization) to exploit the electro-optic coefficient _r_33. For this, metal electrodes must be deposited on both

sidewalls of the waveguide channel, and an electric voltage should be applied along the z-axis of LN (Fig. 3b). As mentioned above, traditional LN modulators have weak optical confinement

due to the low refractive index contrast. Therefore, metal electrodes are generally placed far from the waveguide to reduce the propagation loss due to metal absorption. Importantly, the

much stronger optical confinement of channel waveguides in LNOI allows the electrodes to be placed close (micron-scale distance) to the sidewall of the waveguides, which results in a good

overlap between the optical and microwave fields as well as strong phase modulation. Bernal and co-workers improved the etching process to prepare a ridge waveguide with a smooth sidewall

and reduced the propagation loss to 5 dB cm−1 on an x-cut LNOI platform52. Wang and co-workers used an etched amorphous silicon (a-Si) mask as a resist to transfer the pattern to x-cut LN

layers followed by Ar+ milling. A uniform waveguide was prepared with a propagation loss of 3.0 ± 0.2 dB cm−1 53. More excitedly, an array of LN waveguides with slightly different widths was

successfully patterned on an LNOI platform with minimal surface roughness to obtain a manageable scattering loss, as shown in Fig. 4b. Later, microring resonators, racetrack resonators, and

MZI modulators were integrated on the x-cut LNOI platform by directly shaping the LN thin films into sub-wavelength waveguide channels (Fig. 4d)54. For racetrack resonators, the electric

fields applied to the two racetrack arms were in the same direction to double the phase shifts in the two arms. The electro-optic efficiency was measured as 7.0 pm V−1, which was similar to

that of the ring resonator. For the MZI modulator, the half-wave voltage was 9 V with 2 mm-long electrodes. Thus, the product of the switching voltage and electrode length was 1.8 V cm,

which is nearly an order of magnitude lower than that of traditional MZI modulators7. However, the propagation loss of the sub-wavelength waveguides of 3 dB cm−1 limits the application of

the LNOI photonic platform, which arises because of the sidewall roughness caused by the etch mask. By optimizing the ICP reactive-ion etching process for an LNOI platform, Zhang and

co-workers fabricated sub-wavelength waveguides with a width of 1.2 μm and a low propagation loss of 2.7 dB m−1. Furthermore, they demonstrated a nearly critically coupled microring

resonator with an ultrahigh Q of 5 × 106 (Fig. 4b)55. This important finding expanded the applicability of the LNOI photonic platform. Using a similar etching process, an integrated MZI

modulator with a half-wave voltage of 1.4 V was fabricated on an LNOI platform, which is compatible with CMOS drive voltages56. Furthermore, several MZI modulators with various microwave

signal line widths and device lengths were successfully integrated on an LNOI platform. The integrated MZI modulator with a small size of several millimeters could overcome the

voltage-bandwidth trade-off57, exhibiting very high bandwidths of up to 100 GHz. Moreover, an integrated MZI modulator on an LNOI platform demonstrated an unprecedented high electro-optic

response of up to 500 GHz58. In summary, the smart cut method for the preparation of wafer-scale LNOI samples (NanoLN, Jinan Jingzheng Electronics Co., Ltd.) greatly promoted the development

of integrated lithium niobate photonics. On the one hand, the direct bonding between LN films and substrates without an adhesive layer allowed high-temperature annealing of the LNOI sample

to minimize ion-implantation-induced defects. Then, the propagation loss of the nanophotonic LN waveguide was progressively reduced by optimizing the ICP reactive-ion etching process. On the

other hand, the substitution of a z-cut LN film by an x-cut LN film greatly promoted the integration level of photonic circuits. The channel patterns with electrodes were easily transferred

to an x-cut LN film, while the bottom electrode generally occupied the whole plane for a z-cut LN film. Importantly, the gold electrodes can be placed very close to the edge of a resonator

on an x-cut LN film, which results in strong phase modulation without affecting the Q factor. MICROSTRUCTURE ENGINEERING USING OTHER PHOTONIC MATERIALS RIB-LOADED ON LNOI As mentioned above,

no existing photonic platform can deliver an optimal overall MWP system performance. There are strict requirements for the laboratory equipment for the direct etching of LN. Thus,

microstructure engineering for integrated photonic circuits is often carried out using other photonic materials rib-loaded on LNOI. Generally, this involves two-hybrid approaches: LN thin

films are bonded onto silicon photonic circuits as a top cladding, and other photonic materials are rib-loaded on LNOI as a waveguide core. The electro-optic modulation under hybrid

integration results from the overlap of the evanescent tail of the guided mode in the LN region. LN FILMS AS TOP CLADDING In 2011, Lee et al.59 demonstrated hybrid Si-LiNbO3

electro-optically tunable ring resonators with free-standing z-cut LN thin films directly bonded to a Si microring resonator (Fig. 5a). The free-standing LN thin film was bonded onto the Si

microring resonator as a top cladding, and an effective electro-optic coefficient of 1.7 pm V−1 was obtained for the TE mode, which is approximately one-fifth _γ_13 (8.6 pm V−1).

Unfortunately, the largest electro-optic coefficient, _γ_33, was not exploited. Chen and Reano60 used BCB as an intermediate bonding layer to fabricate a hybrid Si–LiNbO3 microring

resonator. Because of the absence of an integrated electrode, the voltage applied at both ends of the device was much larger, as the SiO2 layer consumed most of the voltage. Later, a hybrid

Si–LiNbO3 microring resonator (z-cut LN thin film) with an integrated electrode was reported, which offered low-voltage tunability61. The microresonator showed a quality factor of 11,500 and

exhibited a resonance tuning of 12.5 pm V−1 for the TM mode, which used the largest electro-optic coefficient of LN, _γ_33. Because of the limitation of the annealing temperature due to

BCB, a pre-annealing process was employed to repair the crystal lattice and improve the electro-optic properties of the LN film. Here, the LN thin film was transferred onto a Si wafer and

annealed at 1000 °C before bonding to the Si microring resonators. Thus, an electro-optic microresonator on a hybrid Si–LiNbO3 platform with a bandwidth of up to 5 GHz was demonstrated for

the first time62. However, the refractive index of Si (~3.4) is much higher than that of LN (~2.2), which dramatically decreases the confinement factor in the LN region. It was found by

simulation that the fraction of the optical mode power in LiNbO3 is only 11% for the TE mode. Weigel et al.63 introduced a thin SiO2 layer (150 nm) between an x-cut LN thin film and a Si

waveguide. As shown in the cross-section schematic of a hybrid MZI modulator (Fig. 5b), the LN thin film was oxide-bonded to the patterned and planarized silicon photonic circuits with a

SiO2 interlayer. It was found that the thickness of the SiO2 layer has a significant impact on the bandwidth and _V_π_L_ of MZI modulators. For this configuration, the fraction of the

optical mode power in the LN region increased to 81%. The hybrid Si–LiNbO3 MZI modulator with a length of 0.5 cm achieved a 3 dB electrical modulation bandwidth of 106 GHz and a _V_π_L_ of

6.7 V cm. Compared with Si, Si3N4 has a slightly smaller refractive index (~1.98), a lower material loss, and a broader transparency window64. Chang et al.65 designed a hybrid Si3N4–LN

waveguide by bonding LN thin films onto a Si3N4 waveguide layer on a Si substrate. As shown in Fig. 5c, it was found by simulation that the confinement factors of the two cores are sensitive

to the Si3N4 ridge width, which is attributed to the approximate refractive index contrast between Si3N4 and LN. This configuration may be promising for a wide range of chip-level photonic

applications. However, the direct bonding of LN onto a predefined Si3N4 or Si waveguide resulted in a significant mode transition loss at the interface due to the substantial disparity in

effective indices and mode profiles. Thus, a mode converter structure must be designed to achieve a low transition loss from the waveguide cores to the hybrid core-cladding waveguide region.

As shown in Fig. 5d, an LN film with a terrace shape was etched and then bonded onto the Si3N4 waveguide on an SOI substrate. It was found by simulation that the transition loss was reduced

from 2.67 to 0.81 dB at the interface66. Furthermore, Cai and co-workers designed vertical adiabatic couplers (VACs), which transferred the optical powerfully, rather than partially,

between the two layers43. For this configuration, a nanophotonic LN waveguide was designed on the top LN film, serving as a phase modulator, as shown in the cross-section schematic of the

hybrid Si–LiNbO3 waveguide. The interfaces between the bottom silicon inverse tapers and top superimposed LN waveguides couple light up and down between the two layers, which are named VACs.

After optimizing the parameters of the hybrid MZI modulator, it exhibited an insertion loss of 2.5 dB, a _V_π_L_ of 2.2 V cm, an electro-optic bandwidth of at least 70 GHz, and modulation

rates up to 112 Gbit s−1. Therefore, it had significantly increased modulation efficiency compared with the directly bonded waveguide. LNOI AS A SUBSTRATE Considering the difficulties in the

direct etching of LN, LNOI can be used as a substrate with a rib-loaded material having a refractive index that forms a hybrid waveguide. Rabiei et al.67 designed a single-mode sub-micron

ridge composite waveguide with a tantalum pentoxide (Ta2O5) rib region loaded onto an LNOI platform (Fig. 6a). The Ta2O5 rib region was prepared by selective oxidation of Ta68. Based on the

composite waveguide, a microring resonator and an MZI modulator were prepared on a y-cut LNOI platform with a quality factor of approximately 7.2 × 104 and a _V_π_L_ of 4 V cm. Another

index-matched material, Ge23Sb7S70 chalcogenide, was also used to fabricate a composite waveguide, which simplified the fabrication process and reduced the propagation loss to 1.2 dB cm−1

69. Nevertheless, Ta2O5 and Ge23Sb7S70 chalcogenides are not conventional photonic materials and are incompatible with CMOS processing. Hybrid Si–LN waveguides were fabricated by lithography

on Si–LN platforms70, exhibiting a propagation loss of 2.5 dB cm−1 in the mid-infrared range. The _V_π_L_ of the integrated MZI modulator was high at 27 V cm because of the large gap

between the electrodes and the waveguide. In addition, silicon nitride was used to fabricate a composite waveguide by rib deposition on LNOI71. First, the silicon nitride layer was deposited

on an LN film by PECVD. Then, ridge optical waveguides were patterned by standard lithography. Rao et al.57 prepared compact MZI modulators with a hybrid SiN–LNOI composite waveguide using

a 2 μm-thick BCB layer as the top cladding. The high-performance devices showed a _V_π_L_ of 3.1 V cm under DC and of less than 6.5 V cm with a bandwidth of up to 50 GHz. Furthermore, a

tunable hybrid Si3N4–LN microring resonator (Fig. 6b)72 and a racetrack resonator (Fig. 6c)73 were lithographically deposited on an x-cut LNOI platform with a resonance tunability of up to

1.78 and 2.9 pm V−1, respectively. Both microresonators employed an air top cladding and exhibited a quality factor of 105 for TE polarization. In conclusion, the hybrid integration of

photonic materials with an LN film is an effective approach for integrated photonics that can avoid the etching of LN. However, regardless of whether the LN film works as a top cladding or a

substrate, the transition loss between the two layers is still a problem for this hybrid waveguide. Although the design of VACs nearly resolved the transition loss problem, the nanophotonic

LN waveguide was still adopted, and the waveguide parameters must be precisely controlled for high modulation efficiency and low optical loss. This does not fully reflect the strengths of

hybrid integration, which avoids the etching of LN. Therefore, the hybrid integration approach still needs further improvement. MICROSTRUCTURE ENGINEERING OF LNOI FOR NONLINEAR INTEGRATED

PHOTONICS High-efficiency, compact, and integration-compatible wavelength converters using optical waveguides involve nonlinear integrated photonics74,75,76. In nonlinear optics, the

polarization intensity of the dielectric is related to the intensity of the incident light wave as \({\mathrm{P}} = \varepsilon _0\left( {\chi ^{\left( 1 \right)}E + \chi ^{\left( 2

\right)}EE + \chi ^{\left( 3 \right)}EEE + \ldots\! } \right)\). The first term represents the first-order linear component, while the nonlinear optics is related to the higher-order

components represented typically by the second- and third-order nonlinear optical susceptibilities (_χ_(2) and _χ_(3), respectively). The higher-order components are generally ignored.

However, for nonlinear integrated photonics on semiconductor waveguides, third-order nonlinear optical interactions have been widely exploited because of some common photonic materials, such

as SiO2 and Si, lack _χ_(2), the second-order nonlinear optical susceptibility75,77. In addition, second-order nonlinear optical processes were investigated for some photonic materials,

such as GaAs and AlGaAs, as _χ_(2) is typically stronger than _χ_(3) 78. In the case of nonlinear integrated photonics on LNOI, second-order nonlinear frequency conversion has been widely

researched because _χ_(2) (linear electro-optic coefficient _r_33 = 3.09 pm V−1) is much larger than _χ_(3) (the nonlinear refractive index is ~10−15 cm2 W−1 at 1064 nm) for LN79. Generally,

photonic microstructures used for frequency conversion are designed based on strong mode confinement because the nonlinear effect can be significantly enhanced inside a small modal volume

due to the increased field strength and temporal confinement of the interacting modes80. Thus, high-Q microresonators with light confined in a cavity for a long time serve as a promising

avenue for nonlinear frequency conversion. Considering that the conversion efficiency of high-confinement resonators mainly depends on the modal overlap between the fundamental modes and

higher harmonics81,82, strict phase matching is not required. However, obvious phase mismatch between the interacting waves often results from material dispersion. Therefore, several

phase-matching techniques, as well as the associated waveguide microstructure engineering approaches, have been explored for second harmonic generation (SHG) processes, including modal phase

matching (MPM), mode-shape modulation, and domain engineering78. MICROSTRUCTURE ENGINEERING OF LNOI FOR PHASE-MATCHING-FREE NONLINEAR OPTICS MICRORING CAVITIES FOR FREQUENCY CONVERSION WGM

microresonators as optical microcavities have been exploited for many different applications, including lasing on a chip, electro-modulation, nonlinear frequency conversion, and frequency

comb generation83,84,85. Wang et al.86 demonstrated an on-chip integrated nonlinear frequency conversion process using microdisk resonators on an LNOI platform. The microdisk was undercut by

wet etching using HF followed by electron beam lithography patterning and Ar+ milling. With the aid of a silica fiber to couple light into and out of the microresonators, on-chip SHG was

obtained with a conversion efficiency of 0.109 W−1. In particular, spontaneous parametric down-conversion was achieved using microdisk resonators over a bandwidth of 400 nm87. A quasi-TM

visible pump photon at a wavelength of 774.66 nm produced a pair of 1549.32 nm photons with orthogonal quasi-TM and quasi-TE polarizations, which is the operating principle of an entangled

source. Nevertheless, microresonators with specific structures need to be designed to achieve highly efficient nonlinear frequency conversion80. Moreover, cascaded Raman scattering and

frequency-doubled emission of Raman lines were observed at the microring resonators with a _Q_ value of up to 106 88,89. An innovative tuneable coupling scheme to optimize the coupling was

demonstrated for nonlinear optical processes by changing the distance between the coupling waveguide and the WGR in the z-direction. MICRORING CAVITIES HAVE BEEN USED TO REALIZE OPTICAL

FREQUENCY COMBS Microring cavities have been used to realize optical frequency combs90,91,92, whose spectra consist of numerous discrete, equidistant laser lines, which was suggested to

revolutionize wavelength division multiplexing in optical telecommunication. The currently available optical frequency combs are based on femtosecond lasers93. Nevertheless, further

increasing the repetition rate into the frequency range above 10 GHz was an attractive challenge94. Recently, combs with wide spectra have been generated by the third-order Kerr nonlinearity

(_χ_(3)) from a monolithic microresonator in some photonic materials95,96,97. However, the electro-optic modulator for tuneable filtering cannot be monolithically integrated into a single

chip because of the lack of _χ_(2). Wang et al.98 demonstrated frequency comb generation (_χ_(3) microring resonator) and electro-optic manipulation (filtering and electro-optic modulation

via a _χ_(2) resonator) on a single integrated LNOI chip. The monolithic LNOI photonic circuits were patterned by directly shaping the LN thin films into a sub-wavelength waveguide channel

by a similar standard process. A TE-polarized comb was generated from 1400 to 2100 nm with a line spacing of approximately 250 GHz. Meanwhile, different target comb lines can be selected by

the _χ_(2) microring resonator by applying different bias voltages. However, strong phase noise and stability are the most significant problems for Kerr frequency combs because of the

complex nature of the third-order Kerr effect. An electro-optic modulation is an alternative approach for producing optical frequency combs in a resonator with excellent stability and

controllability99. This principle exploits the second-order nonlinearity involving a microwave field that modulates the optical wave within the nonlinear crystal. The resonance frequencies

in the optical resonator are separated by the free spectral range (FSR), which must be designed to match the frequency of the microwave field, enabling sum- and difference-frequency

generation of sidebands100. Rueda et al.101 demonstrated electro-optic comb generation based on an LN WGM resonator embedded in a copper microwave cavity. The WGM resonator was a

convex-shaped disk with a radius of 2.45 mm and a thickness of 0.4 mm, resulting in an FSR of approximately 8.9 GHz at a pump wavelength of 1549 nm. With pump light of 320 μW and a low-power

microwave modulation field of 20 dB at 8.9 GHz coupled into the WGM resonator, the generated comb spanned 11 nm with 180 lines in the C-band. It was demonstrated theoretically that strong

phase modulation and high Q are crucial for microresonators to generate flat and broad electro-optic frequency combs102. Zhang et al.102 realized a broader electro-optic comb generator using

high-Q microracetrack resonators integrated on a single LNOI photonic platform. The broad electro-optic frequency comb spanned 80 nm over part of the telecommunication C-band, the entire

L-band, and part of the U-band with over 900 lines spaced by 10.453 GHz, which was generated by the resonator with a size of 200 μm × 6.2 mm modulated by a microwave field at a frequency

near the resonator FSR of 10 GHz. METASURFACES Metasurfaces consisting of nanoantennas are often used to realize enhanced optical nonlinearities103,104,105,106, as nonlinear plasmonic

effects can arise from the coherent oscillations of conduction electrons near the surface of the metal structures81. A nanophotonic LN waveguide patterned with gradient metasurfaces was

fabricated to achieve a monotonic increase in SHG power107. As shown in Fig. 7a, a gradient metasurface consisting of a number of identical phased antenna arrays was patterned along the top

surface of the LN waveguide. Each array consisted of 35 amorphous silicon nanoantennas with a range of lengths. Generally, once optical power couples from the fundamental mode at the pump

frequency, TE00(_ω_), to the fundamental mode at the SH frequency, TE00(2_ω_), it immediately starts to be converted into higher-order waveguide modes at the SH frequency, TEmn(2_ω_) and

TMmn(2_ω_), by the gradient metasurface. Theoretically, the unidirectional wavevector provided by the gradient metasurface prevents optical power coupling from TEmn(2_ω_) and TMmn(2_ω_)

modes back to the TE00(2_ω_) mode. Furthermore, the optical power carried by TEmn(2_ω_) and TMmn(2_ω_) modes cannot be coupled back to the TE00(_ω_) mode yet. Therefore, the gradient

metasurfaces break the symmetry of the coupling between the pump and SH signals, resulting in high-efficiency SHG of approximately 1000% W−1 cm−2, which is three orders of magnitude more

efficient than the bare LN waveguide. MICROSTRUCTURE ENGINEERING OF LNOI FOR PHASE-MATCHING NONLINEAR OPTICS In the most generic second-order nonlinear optical process, which is taken as an

example here, three-wave mixing must satisfy energy conservation and momentum conservation. $$\omega _3 = \omega _1 + \omega _2\,{\mathrm{and}}\,k_3 = k_1 + k_2$$ where \(k_i = n_i\omega

_i/c\) Thus, momentum conservation becomes $$n_3\omega _3 = n_1\omega _1 + n_2\omega _2$$ The three waves travel at different velocities due to the dispersion of the medium, corresponding to

the different refractive indices _n_1, _n_2, and _n_3. As a result, the refractive indices for the three waves need to be precisely controlled for phase matching. MODAL PHASE MATCHING MPM

is a simple technique for phase velocity synchronism in nonlinear frequency conversion processes108. Because longer wavelengths have lower effective indices and the higher-order mode of the

wavelengths has a lower effective index in a multimode waveguide, the fundamental modes of one or two of the longer wavelengths are phase-matched with a higher-order mode of shorter

wavelength78. Generally, the type-0 configuration is employed to achieve a high conversion efficiency. Wang and co-workers demonstrated the dependence of the effective mode indices (_n_eff)

of both the fundamental mode at the incident wavelength and higher-order modes at the SH wavelength on the top width (_w_t) of the LN waveguide with fixed initial thickness and sidewall

angle. To exploit the large nonlinear coefficient _d_33, phase matching was achieved between the first-order TE modes at the fundamental wavelength and the third-order TE modes at the SH

wavelength, with normalized conversion efficiencies as high as 41% W−1 cm−2 at _w_t = 630 nm53. In addition, MPM was demonstrated between the fundamental TM mode at 1550 nm and the

second-order TM modes at the SH wavelength on monolithic z-cut LNOI by changing the top width of the LN waveguide to approximately 590 nm109. SHG of the type-I configuration was also found

for a nanophotonic LN waveguide between the fundamental quasi-transverse-electric mode (TE00) and the higher-order quasi-transverse-magnetic SH mode (TMmn). This waveguide inevitably

exhibited a low conversion efficiency due to the significantly weaker nonlinearity (_d_31) than in the type-0 configuration (_d_33). On a sub-micron LN waveguide with a cross-section of 1.2 

μW × 0.53 μW, the phase-matched SHG process occurs between the fundamental mode (TE00) and the second-order SH mode (TM20) at a wavelength of 1413 nm110. This SHG process generates an SH

power of approximately 305 pW with an incident power of 737 μW, achieving a normalized SHG efficiency of approximately 6.9% W−1 cm−2. Even so, the type-I configuration was often employed for

wavelength tunability of SHG because LN exhibits a significant thermo-optic birefringence with a value of approximately 4 × 10−5 K−1 at room temperature. Thus, a temperature change of the

device would result in a considerable variation in the material birefringence, which in turn shifts the phase-matched wavelength of the type-I SHG process. Based on the thermo-optic

birefringence of LN, highly tuneable efficient SHG with a tuning slope of 0.84 nm K−1 was demonstrated in a z-cut LN nanophotonic waveguide by phase-matching the fundamental quasi-TE mode in

the telecom band with the third-order quasi-TM mode at the SH wavelength (Fig. 7b)111. The waveguide exhibited a theoretical normalized conversion efficiency of 22.2% W−1 cm−2 and an

experimentally determined SHG efficiency of 4.7% W−1 for type-I intermodal phase matching. MODE-SHAPE MODULATION is an alternative poling-free implementation for integrated QPM with periodic

gratings112. Waveguides with periodically grooved structures were also fabricated on an x-cut LNOI platform, which was adopted to realize phase-matched SHG (Fig. 7c)53. The periodic

perturbation in the periodically grooved waveguide generates space harmonics with new propagation constants to compensate for the phase mismatch113. Moreover, a hybrid SiN_x_–LNOI waveguide

in which the width of the SiN rib was periodically modulated was fabricated to achieve QPM SHG114. DOMAIN ENGINEERING OF LNOI FOR PHASE-MATCHING NONLINEAR OPTICS In QPM theory, the phase

variation is \(\Delta k_Q = k_3 - k_2 - k_1 - k_{{\Lambda }}\), where the grating vector is \(k_{{\Lambda }} = 2\pi /{{\Lambda }}\) for grating period \({{\Lambda }}\)analogous to wave

vectors. The nonlinear coefficient is modulated with a period twice the coherence length (_l__c_) of the interaction to offset the accumulated phase mismatch. The grating period is

determined from the relevant radiation frequency and the working temperature according to the equation \({{\Lambda }} = 2{\mathrm{l}}_c = \frac{{2\pi }}{{k_3 - k_2 - k_1}}\). Owing to the

advantage of LNOI in integrated photonics, domain engineering of LNOI has attracted much interest for integrated frequency-conversion devices. The recently developed integrated

frequency-conversion devices on PPLN films also involve hybrid and monoclinic approaches. For the hybrid approach, another photonic material was integrated on the PPLN film to guide the

light, while frequency conversion was realized in the PPLN region. For the monoclinic approach, the ridge waveguide was etched in the PPLN region to form nanophotonic PPLN waveguides. PPLN

WAVEGUIDES ON LNOI PLATFORMS PPLN waveguides on LNOI platforms are crucial for realizing highly efficient, compact, and integration-compatible wavelength converters. Generally, PPLN is

prepared by applying an external electric field using periodic electrodes. Gainutdinov and co-workers realized periodic poling of z-cut LN thin films on insulators using an atomic force

microscope115, which is favorable only for the TM polarization mode to exploit the large nonlinear coefficient _d_33. Hence, the x- or y-cut geometry is preferred because of the possibility

of ease of processing on top of the thin film sample. Mackwitz et al.116 first fabricated periodically poled domain patterns in x-cut LNOI. Rao et al.117 deposited poling electrodes on the

same surface containing LN by filling the spaces of the pre-etched LN thin film, which can reduce the consumption of the electric field component parallel to the _x_-axis. In the early

stages, the approach of hybrid integration was adopted for photonic materials rib-deposited onto PPLN films. Hybrid SiN_x_–PPLN waveguides were formed by rib-loaded channels of SiN_x_ in the

periodic domain region of LN thin films, with a final deposition of a SiO2 top cladding, as shown in Fig. 8a. It was found by simulation that the SiN_x_ ribs with a cross-section of 2000 nm

 × 400 nm resulted in a poling period of approximately 5 μm for TE-polarized pump light at 1580 nm phase matched with the SH wavelength. Chang et al.118 also achieved QPM SHG with an output

power of 80 nW at a 1530 nm pump power of 0.5 mW using hybrid SiN_x_–PPLN waveguides. The SHG exhibited a peak-normalized efficiency of 160% W−1 cm−2. The duty ratio of the periodically

poled domain patterns in the x-cut LN thin film still needs to be improved119. Although the hybrid waveguide obviated the necessity of LN etching, it was difficult to enhance the nonlinear

conversion efficiency. Monolithic nanophotonic PPLN waveguides propagating along the _y_-axis were successfully prepared by standard lithography on a periodically poled x-cut Mg-doped LN

film (Fig. 8b)120. The nanophotonic PPLN waveguides with a cross-section of 1400 nm × 600 nm exhibited a domain-inverted period of ~4 nm, which indicated that the fundamental TE modes at

1550 nm were phase matched with the fundamental TE modes at the SH wavelength. Moreover, SHG of 117 mW at 775 nm was achieved using a pump power of 220 mW with an absolute conversion

efficiency of 53% in a 4 mm-long device, corresponding to the normalized conversion efficiency of 2600% W−1 cm−2. With an optically monitored iterative poling, depoling, and repoling

sequence to improve the domain-inverted structure of nanophotonic LN waveguides, conversion efficiency of up to 4600% W−1 cm−2 was achieved for SHG by pumping at approximately 1540 nm121.

PERIODICALLY POLED MICRORESONATORS In addition, a PPLN thin film was exfoliated from a bulky PPLN wafer by the crystal ion slicing method122,123. It may have potential applications in

nonlinear integrated photonics, such as micro rings and microdisks with periodic domain structures demonstrating QPM SHG124,125,126,127. A disk was fabricated by diamond polishing from a

periodically poled LN wafer, which exhibited a maximum output power of 12.3 mW at a wavelength of 775 nm, as observed by a frequency-doubling experiment. The pump power at the input of the

cavity with a wavelength of 1.55 μm was 25 mW, with a maximum SHG conversion efficiency of approximately 0.5127. In addition, the z-cut LN microring can be periodically polled by an external

electric field between the bottom aluminum plate electrode and the top radial nickel electrodes (Fig. 8c)128. The results for the periodically poled microring with a period of 7.46 μm

indicated that the fundamental TE mode was quasi-phase-matched with the SH TM mode to exploit the nonlinear coefficient _d_31. By coupling with a pulley bus waveguide, the periodically poled

microring resonator was pumped at approximately 1617 nm, which yielded a QPM SHG conversion efficiency of up to 250,000% W−1 and absolute conversion efficiency of 15%. The higher SHG

efficiency is attributed to the stronger mode confinement of the microring due to the larger modal overlap as well as to the phase-matching due to the periodic domain structure. In

conclusion, LNOI is a promising platform for nonlinear integrated photonics. Generally, the phase-matching condition is a critical factor in nonlinear frequency conversion processes.

However, phase-matching-free SHG has been achieved with high-efficiency conversion by a gradient metasurface on an on-chip integrated nonlinear photonic device. This finding will promote the

development of nonlinear integrated photonics on LNOI. Of course, many integrated nonlinear photonic devices were fabricated by following phase-matching SHG, especially

quasi-phase-matching. The preparation of PPLN films greatly promoted the nonlinear integrated photonics on LNOI, especially the nanophotonic PPLN waveguides, which realized a conversion

efficiency of up to 4600% W−1 cm−2. In the future, nanophotonic PPLN waveguides will be applied in quantum technology to develop integrated quantum technology. SUMMARY AND PROSPECTS The

recent progress in the microstructure and domain engineering of LNOI for integrated photonics was reviewed. It is feasible that microstructures can be constructed on LNOI platforms for

photonic circuits, which are also compatible with CMOS technology. Ultralow-loss nanophotonic LN waveguides and high-Q WGM microresonators have been prepared on the LNOI platform, which

enables the sophisticated manipulation of light signals. Compared with other photonic materials, LiNbO3 exhibits a high-performance electro-optic effect and nonlinear optical properties. On

the one hand, the high-bandwidth and low-drive-voltage electro-optical modulator is a critical component in optical fiber communication, which can achieve high-speed signal conversion

between electricity and light waves with low power consumption. The integrated modulator on LNOI has been realized experimentally with a 100 GHz bandwidth at a drive voltage of approximately

1 V, while the conventional LN modulators consume high power with a drive voltage of 3–5 V. This exciting progress indicates that LN modulators are likely to be manufactured in the mid to

long term. On the other hand, PPLN waveguides have been widely applied in nonlinear optics, while domain engineering of LNOI has emerged as a new branch of nonlinear integrated photonics.

Nonlinear photonics plays a very important role in quantum technologies, which comprise an emerging class of devices capable of controlling the superposition and entanglement of quantum

states of light, to realize fundamental performance advantages over ordinary classical devices. The entanglement photon-pair source and single-photon detector are very important components

of quantum technologies, which can be achieved through nonlinear integrated photonics on LNOI. Considering the low-loss propagation waveguide and high-bandwidth integrated modulators

achieved on LNOI, LNOI will be a promising matrix for integrated quantum technologies, with the entanglement photon-pair source and photon detector integrated on an optical chip (Fig. 9a,

b)129,130,131,132,133. Thus, we predict that integrated quantum photonics will enable the generation, processing, and detection of quantum states of light at increasing scale and level of

complexity. Integrated quantum photonics can provide programmable devices approaching 1000 components occupying only millimeter-scale footprints with the integrated generation of multiphoton

states. For instance, quantum computers are suggested to be the most remarkable and powerful future quantum technology134. The nonlinear integrated photonics approaches for single-photon

sources and detectors in optical quantum computers will be determined by their efficacy and practicality131. Quantum communication aims to realize quantum key distribution (QKD) transmitters

and receivers based on the laws of quantum mechanics for security, which is inaccessible by traditional communications135,136. On-chip integration with necessary components, such as tunable

lasers, attenuators, and electro-optic phase modulators, can lead to the realization of QKD transmitters, while on-chip integration with single-photon detectors and phase shifters can lead

to the realization of QKD receivers. Last, topological photonics may become an important application branch in integrated lithium niobate photonics. The topology, an indispensable degree of

freedom of photonic systems that characterize the quantized global behavior of wavefunctions, can be optimized137,138. Topological effects can be realized in photonic crystals, coupled

resonators, metamaterials and quasicrystals. Silicon photonics has been widely used to study topological features, whereas arrays of coupled optical resonators provide a toolbox to observe

the wavefunction139,140. A “photonic molecule” with two distinct energy levels has been demonstrated using coupled microring resonators on LNOI, which can be controlled by external microwave

excitation141. We predict that dynamically controlled multilevel photonic systems will promote research on topological photonics. Therefore, topological photonics on LNOI will open a new

avenue for designing and controlling the behavior of light and possibly provide revolutionary applications. The high quality and homogeneity of LN films are undoubtedly the most fundamental

requirements for any photonic application. At present, the smart cut method is the main approach to prepare a submicron-thickness LN film. This method cannot meet the demands of large-scale

manufacturing yet owing to the ion-implantation-induced defects and production efficiency. Therefore, the preparation approach of LNOI still needs to be improved and explored. The machine

thinning method was also reported for the preparation of LN films142. The LN wafer can be polished to micron-scale and even submicron-scale films after bonding to Si substrates. It was

reported that a micron-scale LN film was fabricated by this machine thinning method. An LN ridge waveguide with a cross-section of 1.3 μm × 4 μm was fabricated on this film143. With an

electric field applied along the z-axis, electro-optic modulation was demonstrated with a _V_π_L_ of 7.1 V cm, which is less than half that of a bulk LN modulator142. An LN modulator

integrated onto a Si substrate using a CMOS-compatible process exhibited a frequency response of up to 110 GHz144. First, the thinner the machined LN wafer is, the higher the confinement

achieved by the LN ridge waveguides. Second, the machine thinning method does not involve ion-implantation-induced defects. Thus, the machine thinning method should be improved, and

submicron LN films will be fabricated through this method. Thus, the machine thinning method may be an alternative approach for fabricating high-quality LN films. Until now, LN films have

generally been exfoliated from nondoped congruent LN crystals. The congruent LN crystals have a large coercive field (21 kV mm−1) and a low optical damage threshold because of intrinsic

defects145. An electric field of up to 40 kV mm−1 is needed to invert the ferroelectric domain in an x-cut LN film for QPM nonlinear integrated photonics. Therefore, an Mg-doped LN film was

adopted in integrated photonics, which reduced the poling electric field to 7.6 kV mm−1. The waveguide showed no photorefractive damage at a high SH optical intensity of ∼10 MW cm−2 after

many hours of optical pumping120. Furthermore, stoichiometric LN crystals have a smaller coercive field of ~3.5 kV mm−1 and a completely crystalline structure146,147,148,149,150,151,152.

Therefore, stoichiometric LN films should be more suitable for ultralow-loss nanophotonic waveguides153 and can be poled with a smaller electric field. Over the last decade, integrated

lithium niobate photonics has been rapidly developed and proven invaluable in the development of future optical communication and quantum technologies. The large-scale and low-cost

manufacturing of integrated photonic devices and systems by mature manufacturing processes will enable new revolutionary applications in optical communication and quantum technologies.

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funding from the National Natural Science Foundation of China (Grant nos. 51802113 and 51802116), High Technology Research and Development Program of Shandong Province (Grant no.

2018****0209), and the Natural Science Foundation of Shandong Province (Grant nos. ZR2018BEM015 and ZR2019LLZ003). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Collaborative Innovation

Center of Technology and Equipment for Biological Diagnosis and Therapy in Universities of Shandong, Institute for Advanced Interdisciplinary Research (iAIR), University of Jinan, Jinan,

250022, China Dehui Sun, Yunwu Zhang, Xiaoyan Liu, Jinbo Pang & Hong Liu * Jinan Institute of Quantum Technology, Jinan, 250101, China Dongzhou Wang & Hong Liu * CETC Deqing Huaying

Electronics Co., Ltd., Huzhou, 313200, China Wei Song * Crystrong Photoelectric Technology Co., Ltd., Jinan, 250100, China Deqiang Geng * State Key Laboratory of Crystal Materials, Shandong

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Y., Wang, D. _et al._ Microstructure and domain engineering of lithium niobate crystal films for integrated photonic applications. _Light Sci Appl_ 9, 197 (2020).

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