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<reference>
Isotropic fabrication of centimeter-scale, low propagation-loss periodically poled
lithium niobate nanophotonic waveguides for efficient second harmonic
generation
Guanghui Zhao,1,4, † Yixuan Yang,1,5,† Renhong Gao,2,3 Jintian Lin,1,5, § and Ya
Cheng2,3,6,7,8,9,*
1

State Key Laboratory of Ultra-intense Laser Science and Technology, Shanghai
Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai
201800, China
2
State Key Laboratory of Precision Spectroscopy, East China Normal University,
Shanghai 200062, China
3
The Extreme Optoelectromechanics Laboratory (XXL), School of Physics, East
China Normal University, Shanghai 200241, China
4
School of Physical Science and Technology, ShanghaiTech University, Shanghai
200031, China
5
Center of Materials Science and Optoelectronics Engineering, University of Chinese
Academy of Sciences, Beijing 100049, China
6
Shanghai Research Center for Quantum Sciences, Shanghai 201315, China
7
Hefei National Laboratory, Hefei 230088, China
8
Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan
030006, China
9
Collaborative Innovation Center of Light Manipulations and Applications, Shandong
Normal University, Jinan 250358, China
†
These authors contributed equally to this work
§
E-mail: jintianlin@siom.ac.cn
*E-mail: ya.cheng@siom.ac.cn
May 28, 2026 @arXiv preprint

Abstract: Periodically poled lithium niobate (PPLN) nanophotonic waveguides that
simultaneously feature low propagation-loss and uniform periodic poling are essential
for a wide range of applications ranging from classical nonlinear frequency-conversion
to scalable integrated quantum technology. However, fabrication imperfections have
frequently limited the propagation loss of fully domain-inverted PPLN nanophotonic
waveguides to a few dB/cm, primarily due to anisotropic etching issue, thereby
1

restricting the absolute conversion efficiency and scale of photonic integration. Here,
we present a fabrication approach that overcomes this challenge, yielding a 1.2-cmlong PPLN nanophotonic waveguide with low propagation loss via femtosecond-laser
photolithography-assisted chemo-mechanical etching (PLACE). By carrying out
domain inversion on a planar thin-film prior to waveguide definition, electric-field
distortion is minimized during poling, while isotropic etching of the waveguide is
achieved by PLACE with an average surface roughness of only 0.34 nm, resulting in
uniform poling of duty cycle of 50% and a record-low propagation loss of 0.042 dB/cm
in the telecom band. Under continuous-wave pumping at 1525 nm, the device
demonstrates a high normalized quasi-phase-matched SHG conversion efficiency of
2021% W−1, and an absolute conversion efficiency of 64% at a pump power of 86 mW
which represents the state of the art for single-period PPLN nanophotonic waveguides.
1. Introduction
Thin-film lithium niobate (TFLN) on insulator has emerged as a versatile platform for
photonic integration with unprecedented performance, owing to its exceptional
combination of a wide transparency window, moderate refractive-index contrast, strong
electro-optic response, and large second-order nonlinearity.[1–5] Rapid advances in
nanofabrication have enabled TFLN nanophotonic waveguides with compact crosssections and propagation loss as low as <1 dB/m in the telecom band,[6-8] thereby
providing strong optical confinement and high integration density. The potential of this
platform is evidenced by a variety of integrated photonic devices, including ultra-high
2

speed electro-optic modulators,[9,10] bi-chromatic soliton microcombs,[11-13] broadbandwidth supercontinuum generation,[14] narrow-linewidth microlasers,[15-17] and
highly efficient nonlinear frequency convertors.[18-23] In particular, periodically poled
lithium niobate (PPLN) nanophotonic waveguides on the TFLN platform dramatically
enhance nonlinear frequency conversion by simultaneously leveraging the largest
second-order nonlinear coefficient (i.e., d33~27 pm/V),[14,17,19,20,23-26] tight optical
confinement, and low propagation loss. Accordingly, low-loss PPLN nanophotonic
waveguides have been extensively employed for boosting classical nonlinear processes
such as second harmonic generation (SHG),[17,23-26] sum-frequency generation,[19] and
optical parametric oscillation,[20] substantially extending the spectral coverage of
coherent light sources. Beyond classical frequency conversion, PPLN nanophotonic
waveguides have also become key building blocks for quantum photonics, enabling
entangled photon-pair generation,[27-30] squeezed light generation,[31] and even singlephoton-level nonlinearities for quantum photonic circuits.[32–34] All these nonlinear
applications demand both low propagation loss and uniform poling—i.e., complete
domain inversion across the waveguide cross-section with an optimum 50% duty
cycle—to maximize the effective interaction length and thereby enhance the nonlinear
interaction enabled by quasi-phase matching.[35]

The fabrication of PPLN nanophotonic waveguides involves two essential steps:
waveguide etching and ferroelectric domain inversion, which can be performed in
3

either order. However, these two processes are often interdependent, leading to a
tradeoff between propagation loss and poling uniformity. If the ridge waveguide is
etched before high-voltage poling, anisotropic etching of the waveguide is significantly
suppressed, so a low propagation loss of 0.25 dB/cm can be achieved in the telecom
band.[36] However, the local electric fields become susceptible to perturbations from
sidewalls and surface topography, which can degrade domain uniformity.[37,38]
Conversely, if the ridge waveguide is fabricated after poling, the conventional dryetching and subsequent chemical cleaning processes induce anisotropic, domaindependent surface relief, period distortion, and additional scattering loss, because
oppositely poled domains exhibit different etching responses. This typically results in
a propagation loss of several dB/cm in the telecom band (e.g., 3 dB/cm in Ref. [23]).
Such imperfections are especially detrimental for centimeter-long PPLN waveguides
aimed at high absolute conversion efficiency by using quasi-phase matching (QPM)
scheme, where weak scattering and small poled-period deviations accumulate over the
entire interaction length, progressively increasing scattering loss and causing power to
flow back from the generated nonlinear signals to the pump, thereby depleting the
nonlinear output and limiting the scalability of photonic integration.

In this work, we overcome these challenges and demonstrate a 1.2-cm-long PPLN
nanophotonic waveguide featuring uniform poling and a record-low propagation loss
of 0.042 dB/cm in the telecom band, by leveraging femtosecond laser photolithography
4

assisted chemo-mechanical etching (PLACE). The key advantage of this fabrication
strategy lies in both uniform domain inversion of the planar TFLN by high-voltage
poling and patterning the pre-poled TFLN into PPLN waveguides with ultra-smooth
surfaces by chemo-mechanical polishing (CMP) rather than by a strongly anisotropic
dry-etching process, yielding ultra-efficient quasi-phase-matched SHG with an
absolute conversion efficiency as high as 64% at an on-chip pump power of 86 mW.
This work unlocks the full potential of PPLN nanophotonic waveguides for a wide
range of nonlinear photonic applications.

2. Fabrication and Characterization of Centimeter-Scale Low-Loss PPLN
Nanophotonic Waveguides
2.1 Device Fabrication and Process Strategy
The devices were fabricated on a commercial X-cut TFLN wafer composed of a 500nm-thick TFLN device layer, a 4.7-µm-thick buried SiO2 layer, and a 500-m-thick
lithium niobate handle. The fabrication flow is schematically shown in Fig. 1. First, a
200-nm-thick chromium (Cr) film was deposited on the TFLN wafer by magnetron
sputtering. Periodic microelectrodes along the Y axis of the lithium niobate crystal were
then patterned in the Cr film by spatially selective femtosecond-laser ablation with a
resolution of approximately 100 nm.[39] High-voltage poling was subsequently applied
to periodically pole the X-cut TFLN using a pre-pulse followed by a main pulse. The
pre-pulse voltage consisted of four triangular pulses with a voltage amplitude of 320 V,
5

each having a rise time and a fall time of 0.5 ms. These pre-pulses generate sufficient
domain nucleation sites near the positive electrode while suppressing excessive lateral
broadening of the domain structures. After the pre-pulses, four main pulses with a peak
voltage of 320 V and a peak duration of 6 ms were applied to drive the reversed domains
to grow through the film thickness, thereby determining the final domain width and
duty cycle. This separation of domain nucleation and domain growth facilitates highfidelity periodic poling with a duty cycle close to 50%.

After poling, the residual Cr electrodes were removed and the sample was cleaned.
Ridge waveguides were then isotropically defined in the periodically poled TFLN
regions via the PLACE technique which consisted of five steps.[8] First, a fresh Cr film
was deposited as a hard mask for waveguide fabrication. Second, stripe patterns were
written in the Cr film by spatially selective femtosecond-laser ablation. Third, the
unprotected LN (without Cr mask coverage) was removed by CMP, producing a smooth
ridge waveguide without inducing anisotropic etching. Fourth, the residual Cr mask
was stripped. Finally, a 1.5-µm-thick SiO2 upper cladding was deposited by plasmaenhanced chemical vapor deposition, and the chip facets were polished by CMP for
end-fire coupling. This fabrication sequence avoids applying the poling field to a preetched topography and employs CMP to suppress surface scattering loss.

2.2 Structural Characterization and Propagation Loss Measurement
6

The fabricated PPLN ridge waveguide features an effective poled waveguide length of
1.2 cm, a ridge top-width of approximately 1 µm, and an etch depth of around 250 nm.
The ridge waveguide is covered with a 1.5-µm-thick SiO2 cladding that symmetrizes
the optical environment, improves the stability of the phase-matching condition, and
protects the waveguide from contamination. The optical micrograph in Fig. 2(a)
confirms a straight and continuous ridge waveguide over the imaged region, and the
cross-sectional scanning electron microscope (SEM) image in Fig. 2(b) verifies that the
ridge waveguide is embedded in the SiO2/TFLN/SiO2 stack. Atomic-force microscopy
(AFM) performed on the waveguide surface before depositing the SiO2 cladding yields
a root-mean-square (RMS) roughness of only 0.34 nm (Fig. 2(c)), demonstrating that
the CMP process produces a sub-nanometer-smooth surface suitable for centimeterscale low-loss propagation. To quantify the propagation loss, a racetrack microring
resonator with a physical cavity length of 1828 m (= 2100 + 6002) fabricated on
the same wafer with the identical poling and CMP process was characterized, where
one straight section of the racetrack was periodically poled with a length of 600 m.
Fitting of the resonance with a Lorentz curve gives an intrinsic optical quality factor of
7.98×106, corresponding to an estimated propagation loss of 0.042 dB/cm (Fig. 2(d)).
This

propagation

loss

represents

an

approximately

two-order-of-magnitude

improvement over the results achieved by conventional pole-before-etch-process in
TFLN.[23,26] This result confirms that the post-poling CMP process preserves low-loss
guiding even in periodically poled regions.
7

2.3 Quasi-Phase-Matching Design and Domain Characterization
For first-order QPM in type-0 SHG, the phase-matching (i.e., momentum conservation)
condition is[35]
∆𝑘 = 𝑘2𝜔 − 2𝑘𝜔 = 2𝜋/Λ,

(1)

where Λ is the poling period. Equivalently, for a pump wavelength λω,
Λ = 𝜆𝜔 /[2(𝑛2𝜔 − 𝑛𝜔 )].

(2)

Finite-element mode simulations were employed to calculate the modal effective
indices and determine the required QPM period. As shown in Fig. 3(a), the fundamental
wave near 1525 nm and the second-harmonic wave near 762.5 nm are both TE00-like
modes, with effective indices of 1.904 and 2.091, respectively. According to Eq. (2),
first-order QPM period was designed to be 4.13 µm for TE00-to-TE00 SHG near 1525
nm. Using of the same spatial-mode order for both fields is advantageous for the mode
overlap and helps suppress conversion into higher-order modes. The influence of the
SiO2 upper cladding was then evaluated by calculating the QPM period as a function
of cladding thickness. The period varies rapidly for thin cladding but saturates when
the SiO2 thickness exceeds approximately 0.8 µm (Fig. 3(b)), indicating that the upper
boundary is sufficiently far from the optical mode. Consequently, a 1.5-μm-thick SiO2
cladding was chosen to minimize the sensitivity to deposition non-uniformity. Figure
3(c) further displays the calculated QPM period versus pump wavelength under this
cladding condition.
8

The ferroelectric domain quality was characterized using second-harmonic microscopy.
An image taken over a 100-µm-long region (Fig. 3(d)) reveals clear periodic contrast,
continuous domain inversion along the propagation direction, with a duty cycle close
to 50:50 near the waveguide path. These observations confirm that the pre-pulse/mainpulse poling waveform and the planar-film poling geometry produce sufficiently
uniform domains for the 1.2-cm-long QPM interaction.

2.4 Second-Harmonic Generation Characterization
The experimental setup for SHG characterization is illustrated in Fig. 4. A tunable
continuous-wave laser in the telecom band was used as the pump source, and its output
was amplified by an erbium-doped fiber amplifier (EDFA) when high-power
measurements were required. A fiber polarization controller was employed to adjust
the input state to transverse-electric (TE) polarization, so that the pump field could
access the largest nonlinear tensor component d33 through type-0 QPM. The pump light
was coupled into the PPLN nanophotonic waveguide by a lensed fiber, and the
transmitted pump and generated second-harmonic (SH) signals were collected by
another lensed fiber at the output facet. The output was sent to an optical spectrum
analyzer (OSA) to record the fundamental and SH spectra. A top-view microscope
imaging system was simultaneously used to monitor fiber-waveguide alignment and
visible SH emission from the waveguide, which provides a direct indication of the
9

coupling condition and frequency-conversion strength. The pump and SH powers
reported below are on-chip values after correction for the corresponding coupling losses.
Low-power pump-wavelength scans were performed to extract the device normalized
conversion efficiency while minimizing thermal drift and photorefractive effects, so
that the measured spectral response more faithfully reflects the intrinsic QPM
conversion behavior of the device. The pump power was then increased stepwise for
the absolute-efficiency measurement.

At low pump power, the normalized SHG efficiency was extracted as
𝜂𝑛𝑜𝑟𝑚 = 𝑃2𝜔 /𝑃𝜔2 ,

(3)

where Pω and P2ω are the on-chip pump and SH powers, respectively. This definition
represents the device conversion efficiency without normalization to the interaction
length squared. In the low-power measurement, the pump wavelength was scanned
around the designed QPM wavelength while the on-chip pump power was kept at
approximately 0.5 mW. The normalized conversion-efficiency spectrum reaches a
maximum of 2021% W-1 near 1525 nm (Fig. 5(a)). The measured response follows a
sinc2-shaped envelope with a full width at half maximum of approximately 1 nm. Such
a narrow and nearly ideal spectral response is consistent with coherent SHG over the
full 1.2-cm effective poled length and indicates that the QPM period, ferroelectric duty
cycle, and waveguide geometry remain sufficiently uniform along the centimeter-scale
device.
10

At higher powers, the absolute conversion efficiency was defined as
𝜂𝑎𝑏𝑠 = 𝑃2𝜔 /𝑃𝜔 .

(4)

The SH output power increased monotonically with increasing on-chip pump power, as
shown in Fig. 5(b). When the on-chip pump power was increased to 50.93 mW, the
measured SH power reached 23.10 mW, corresponding to an absolute efficiency of
45.4% (Fig. 5(c)). At an on-chip pump power of 86.09 mW, the SH output further
increased to 55.60 mW, yielding a peak absolute efficiency of 64.6% (Fig. 5(d)). To
the best of our knowledge, these values represent the highest device normalized
conversion efficiency and absolute conversion efficiency reported for a single-period
PPLN nanophotonic straight waveguide.

Because no resonant enhancement was used in these measurements, the high efficiency
originates from the combined effects of long interaction length, low propagation loss,
uniform QPM, and strong TE00-TE00 modal overlap. The simultaneous observation of
a sinc2-like wavelength response and high absolute conversion efficiency confirms that
the device operates as a uniform single-period PPLN frequency converter rather than
relying on localized enhancement or accidental phase matching.

3. Discussion and Conclusion
3.1 Discussion
11

It is worth noting that the present process is distinct from the previously reported polebefore-etch routes.[26,36,37] The key advantage lies in patterning the pre-poled TFLN by
CMP rather than by a strongly anisotropic dry-etching step. During high-voltage poling,
the unetched TFLN provides a nearly planar surface and a more uniform local electric
field; in contrast, poling performed on an already etched ridge structure would lead to
electric-field crowding near sidewalls and steps, causing nonuniform domain
nucleation, excessive domain broadening, and even duty-cycle errors. Once the domain
pattern is established, CMP defines the ridge waveguide through a nearly isotropic
material-removal process, thereby eliminating the domain-dependent etching contrast
between the inverted and non-inverted regions. This combination effectively decouples
domain formation from low-loss waveguide definition, enabling efficient quasi-phase
matching (QPM) together with sub-nanometer surface roughness in a centimeter-scale
device.

3.2 Conclusion
We have demonstrated a centimeter-scale PPLN ridge waveguide fabricated by
femtosecond-laser photolithography-assisted chemo-mechanical etching. By carrying
out domain inversion on a planar film prior to waveguide definition, the process
minimizes electric-field distortion during poling, while CMP provides a subnanometer-roughness waveguide surface. The device has a 1.2-cm effective poled
length and a designed QPM period of 4.13 µm for TE00-to-TE00 SHG near 1525 nm. It
12

exhibits an RMS roughness of 0.34 nm, an intrinsic Q factor of 7.98×106 measured
from a same-process microresonator, and a propagation loss of 0.042 dB/cm. The
device achieves a normalized SHG efficiency of 2021% W-1 and an absolute conversion
efficiency of 64% at an on-chip pump power of 86.09 mW. This fabrication strategy
provides a scalable route toward low-loss, high-efficiency TFLN nonlinear photonic
circuits.

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Deng, Y. Zhang, X. Hu, S. Zhu, “High-efficiency nonlinear frequency
15

conversion enabled by optimizing the ferroelectric domain structure in x-cut
LNOI ridge waveguide,“ Nanophotonics 13, 3477–3488 (2024).
[37] C. J. Xin, S. Lu, J. Yang, A. Shams-Ansari, B. Desiatov, L. S. Magalhães, S. S.
Ghosh, E. McGee, D. Renaud, N. Achuthan, A. Zvyagintsev, D. Barton III, N.
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Opt. Lett. 50, 4310–4313 (2025).

16

Figure 1. Fabrication process of the PPLN nanophotonic ridge waveguide. A Cr film
is deposited and patterned into periodic poling electrodes. After high-voltage poling
and electrode removal, a second Cr mask is written for waveguide definition. Chemomechanical polishing (CMP) forms the ridge waveguide by etching the unprotected
lithium niobate, followed by Cr removal and SiO2 cladding deposition.

17

Figure 2. (a) Optical micrograph of the waveguide. (b) Cross-sectional SEM image. (c)
AFM image, showing an RMS roughness of 0.34 nm. (d) Mode resonance of a racetrack
microring resonator fabricated on the TFLN wafer by the same process, showing an
intrinsic Q factor Qint of 7.98×106, corresponding to a propagation loss of only 0.042
dB/cm.

18

Figure 3. (a) Simulated TE00-like fundamental and second-harmonic modes. (b) QPM
period versus SiO2 cladding thickness. (c) QPM period versus pump wavelength for a
1.5-µm-thick cladding. (d) SH microscopy image of periodic domains over
approximately 100 µm.

19

Figure 4. Experimental setup for SHG measurement. PC, polarization controller;
EDFA, erbium-doped fiber amplifier; OSA, optical spectrum analyzer. Upper Inset:
Optical microscope image of second harmonic emission from the output port the
waveguide. Lower Inset: a photo of the waveguide under test, where PPLNOI denotes
the periodically-poled-lithium-niobate-on-insulator chip.

20

Figure 5. (a) Normalized conversion efficiency versus pump wavelength, showing
2021%/W near 1525 nm. (b) Absolute conversion efficiency versus on-chip pump
power. SH spectra at approximately (c) 50-mW and (d) 86-mW pump powers.

21
</reference>

<statements>
1. The amorphized boundary layer disrupts the non-centrosymmetric crystalline ordering necessary for optical non-centrosymmetry, locally eliminating the quadratic nonlinear tensor (d33) and reducing the effective nonlinear mode overlap
2. Localized space-charge fields alter the extraordinary refractive index, causing temporal resonance drift and phase-mismatch perturbations in quasi-phase-matched waveguides
3. Single-crystal lithium niobate possesses an extraordinary index of n_e approximately 2.14 at 1550 nm.
4. Etch + Thermal Annealing (520°C in pure O2 for 2 h) has a second-harmonic generation efficiency of 1500 - 2200 %/W·cm^2.
5. Integrated Protocol (Etch + BOE + O2 Anneal + Low-T SiO2 Clad) has a second-harmonic generation efficiency above 2500 %/W.
6. Reducing propagation loss down to 0.042 dB/cm extends the effective interaction length across multi-centimeter waveguide paths, unlocking conversion efficiencies exceeding 2000-2590 %/W and driving continuous-wave pump depletion beyond 85%.
</statements>

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