RESEARCH ARTICLE
www.advmat.de
3D Laser Writing of Low-Loss Cross-Section-Variable Type-I
Optical Waveguide Passive/Active Integrated Devices in
Single Crystals
Daoyuan Chen, Zhi Chen,* Yi Yang, Yuying Wang, Xuhu Han, Kuen Yao Lau, Zhemin Wu,
Chen Zou, Yu Zhang, Beibei Xu, Xiaofeng Liu, Zhijun Ma,* Guoping Dong,
Giuseppe Barillaro, Lijing Zhong,* and Jianrong Qiu*
Optical waveguides fabricated in single crystals offer crucial passive/active
optical components for photonic integrated circuits. Single crystals possess
inherent advantages over their amorphous counterpart, such as lower optical
losses in visible-to-mid-infrared band, larger peak emission cross-section,
higher doping concentration. However, the writing of Type-I positive refractive
index modified waveguides in single crystals using femtosecond laser
technology presents significant challenges. Herein, this work introduces a
novel femtosecond laser direct writing technique that combines slit-shaping
with an immersion oil objective to fabricate low-loss Type-I waveguides in
single crystals. This approach allows for precise control of waveguide shape,
size, mode-field, and refractive index distribution, with a spatial resolution as
high as 700 nm and a high positive refractive index variation on the order of
10−2 , introducing new degrees of freedom to design and fabricate
passive/active optical waveguide devices. As a proof-of-concept, this work
successfully produces a 7 mm-long circular-shaped gain waveguide (≈10 μm
in diameter) in an Er3+ -doped YAG single crystal, exhibiting a propagation
loss as low as 0.23 dB cm−1 , a net gain of ≈3 dB and a polarization-insensitive
character. The newly-developed technique is theoretically applicable to
arbitrary single crystals, holding promising potential for various applications
in integrated optics, optical communication, and photonic quantum circuits.
D. Chen, Y. Yang, Y. Wang, X. Han, B. Xu, J. Qiu
State Key Laboratory of Extreme Photonics and Instrumentation
College of Optical Science and Engineering
Zhejiang University
Hangzhou 310027, China
E-mail: qjr@zju.edu.cn
Z. Chen, Y. Zhang, Z. Ma
Zhejiang Lab
Hangzhou 311100, China
E-mail: zhiaimengling1314@gmail.com; zhijma@zhejianglab.com
Z. Chen
College of Materials Science and Engineering
Key Laboratory of Advanced Materials of Yunnan Province
Kunming University of Science and Technology
Kunming, Yunnan 650093, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202404493
1. Introduction
Optical waveguides (WGs) play an essential role in photonic integrated circuits
(PICs),[1–6] where they are generally employed as basic passive/active optical
components, in particular of high-gain
active devices, such as WG amplifiers and
lasers.[7,8] Among the various optical gain
media, single crystals activated with rareearth (RE) ions have garnered significant
attention as gain materials for WGs in
integrated optics, due to their advantages,
including low optical losses in visiblenear-infrared-mid-infrared band, excellent
thermal conductivity and thermal mechanical performance, large damage threshold, large peak emission cross-section,
and high doping concentrations.[9–11]
Traditional methods for fabricating
WGs in bulk crystals encompass techniques such as metal-ion diffusion,[12]
ion exchange,[13] ion implantation,[14]
and
photolithography.[15,16]
However,
these
methods
exhibit
limitations,
including
complex
processes,
low
K. Y. Lau
Key Lab of Advanced Optical Manufacturing Technologies of Jiangsu
Province & Key Lab of Modern Optical Technologies of Education
Ministry of China
School of Optoelectronic Science and Engineering & Collaborative
Innovation Center of Suzhou Nano Science and Technology
Soochow University
Suzhou 215006, China
Z. Wu, C. Zou
School of Material Science and Engineering
Centre of Electron Microscopy and State Key Laboratory of Silicon
Materials
Zhejiang University
Hangzhou 310027, China
X. Liu
School of Materials Science and Engineering
Zhejiang University
Hangzhou 310027, China
DOI: 10.1002/adma.202404493
Adv. Mater. 2024, 36, 2404493
2404493 (1 of 10)
© 2024 Wiley-VCH GmbH
www.advmat.de
efficiency, high cost, stringent requirements for a complex vacuum environment, and inability to create 3D WG devices.
In recent years, femtosecond laser direct-writing (FLDW) technique has been extensively used for the fabrication of optical
WG devices in transparent single crystals.[17–20] This technique
provides a fully no-touching processing strategy inside transparent materials, eliminating the need for additional technological
steps.[21–23] This flexibly allows for the change of the spatial distribution of refractive index (RI) enabling the fabrication of devices with 3D and complex structures.[24–27] For instance, highquality (high-Q) WG devices were fabricated by fs laser-assisted
chemo-mechanical etching of thin-film LiNbO3 crystals.[28–30]
Nevertheless, this technique cannot avoid drawbacks associated
with the enduring complex steps in traditional lithography processes, i.e., high cost, low manufacturing efficiency, and its suitability is limited to the fabrication of only 2D WG devices. Alternatively, FLDW of WG devices based on Type-II modification in single crystals is a more popular approach.[31,32] However, this technique is typically associated with the modification
of large areas and severe damages of the lattice, resulting in a
coarse track morphology with microcracks. Besides, the impact of
spherical aberration is not effectively eliminated with the FLDW
method. The Type-II modification, which elongates in the longitudinal direction with irregular morphology, poses challenges
in achieving well-designed track engineering. This is especially
apparent in the microstructural design and fabrication of visiblenear-infrared single-mode WG devices (see details in Table S1,
Supporting Information). Therefore, high-quality Type-I WGs is
highly desirable. An open question that has been debated for a
long time is whether it is possible to fabricate Type-I WGs in single crystals by using fs laser, while maintaining precise control of
WG shape, size, mode-field and RI distribution.
In this work, we propose a new FLDW technique combining slit-shaping with an immersion oil objective to fabricate lowloss Type-I WGs in single crystals, under precise control of WG
shape, size, mode-field and RI distribution, with a spatial resolution as high as 700 nm and a high positive RI contrast on the
order of 10−2 , via multi-overlapping scanning strategy. As a proofof-concept, we fabricate a 7 mm-long circular-shaped gain WG
(≈10 μm in diameter) in an Er3+ -doped YAG single crystal leveraging the high positive RI variation, which is fully compatible
G. Dong
State Key Laboratory of Luminescent Materials and Devices
and Guangdong Provincial Key Laboratory of Fiber Laser Materials and
Applied Techniques
Guangdong Engineering Technology Research and Development Center
of Special Optical Fiber Materials and Devices
School of Materials Science and Engineering
South China University of Technology
Guangzhou 510641, China
G. Barillaro
Dipartimento di Ingegneria dell’Informazione
Università di Pisa
via G. Caruso 16, Pisa 56126, Italy
L. Zhong, J. Qiu
Institute of Light+X Science and Technology
College of Information Science and Engineering
Ningbo University
Ningbo 315211, China
E-mail: zhonglijing@nbu.edu.cn
Adv. Mater. 2024, 36, 2404493
with commercial single-mode fiber (SMF-28, Corning). Surprisingly, we achieve a net gain of ≈3 dB and a background WG propagation loss as low as 0.23 dB cm−1 .
Additionally, we succeeded in writing high-quality WGs in
Yb3+ -doped YAG, Er3+ -doped LuAG, and undoped LiNbO3 single crystals. This approach is theoretically applicable to a wide
range of single crystals. Furthermore, the circular-shaped WGs
produced by this technique exhibit polarization insensitivity, indicating significant potential for various applications. Examples
include addressing polarization mode dispersion in optical communication systems and facilitating polarization encoding and
entanglement in quantum PICs.[33–36] Importantly, this technique allows for precise control of WG cross-sectional shapes, including circular-, triangular-, rectangular-, doughnut-, and highorder (LP11 ) mode-like WG shape. This expands the degrees of
freedom for the design and fabrication of passive/active optical
WG devices in integrated optics.
2. Results and Discussion
2.1. Mechanism of FLDW Low-Loss Type-I WGs in Single Crystals
In the conventional approach, the FLDW technique induces complicated RI variations in single crystals, encompassing both regions of positive and negative RI changes (refer to Figure 1,
inset, left). Despite the use of an immersion oil objective to
mitigate spherical aberration, the FLDW method generates RImodified regions with large aspect ratios, posing challenges
in the fabrication of high-quality WG with precise control of
shape, size, mode-field distribution, and RI variation along the
FLDW direction leveraging a multi-scan stacking. In contrast, the
FLDW technique proposed here, utilizing laser beam slit-shaping
strategy[37,38] in conjunction with an immersion oil objective, effectively addresses the aforementioned limitations. As shown in
the inset (right) in Figure 1, the simulated field intensity distribution of the incident laser beam matches well with the measured
cross-sectional shape of a single scan with slit-shaping. A positive RI-modified quasi-circular region in YAG single crystal with
a diameter of ≈1.0 μm results from Type-I modification with slitshaping. Surprisingly, this new FLDW technique allows for a positive RI modification with a spatial resolution as high as 700 nm
in LiNbO3 single crystal (see details in Note S5, Supporting Information). Notably, there is no occurrence of negative RI variation, and the modification region does not extend along the z
axis in the direction of the fs laser propagation. It is worth noting
that, the proposed FLDW technique enables the precise control
of WG shape, size, mode-field and RI distribution with a resolution finer that sub-μm. This level of resolution that has never
been previously attained in single crystals, offering new possibilities for designing and fabricating low-loss passive/active optical
WGs devices with arbitrary cross-sectional shape.
2.2. Circular-Shaped Gain WGs
The proposed FLDW technique enables the design and fabrication of low-loss circular-shaped gain WGs through a multiscanning strategy, which is especially beneficial for the reduction of coupling loss arising from the mismatching of mode
2404493 (2 of 10)
© 2024 Wiley-VCH GmbH
15214095, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202404493 by Université du Littoral-Côte-d'Opale, Wiley Online Library on [08/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
www.advancedsciencenews.com
www.advmat.de
Figure 1. Schematic of Type-I WG cross-sectional shape and size controlled by the FLDW technique combining slit-shaping with an immersion oil
objective in RE3+ -doped/undoped single crystals. Insets are the simulated field intensity profiles of the laser beam and the measured cross-sectional
shapes of the FLDW processing region before (left) and after (right) slit-shaping.
field between the WG and commercial single-mode fiber (SMF).
Figure 2b sketches the structure of the WGs designed based
on the FLDW multi-scanning strategy. Specifically, we fabricated
each WG in RE3+ -doped YAG single crystals using 37 scans,
where the adjacent spacing varied from 1.1 to 1.4 μm. The FLDW
parameters included a scanning speed of 1 mm s−1 , pulse energy
of 123 nJ, and the fs laser polarization direction was parallel to
x axis. As shown in Figure 2a, all the WGs have a quasi-circular
cross-sectional shape with diameter of 9.7, 9.9, 10.8„ and 11.2 μm
(Figure 2a, insets). The WG has mode ellipticity as high as 94%
and low insertion loss (IL) of 0.69 dB at 1310 nm and 0.78 dB at
1550 nm. In addition to adopt a slit and an immersion oil objective to obtain a quasi-circular WG, the translation distance along z
direction was divided by a factor 𝛽, where 𝛽 = nCrystal / nOil = ≈1.2,
to further eliminate the RI mismatch between the oil and YAG
single crystal. The intensity profile of the propagation mode field
at 1550 nm was measured in the WGs, revealing a high ellipticity, as depicted in Figure 2c. The experimentally measured mode
profile closely aligns with the simulated result (Figure 2f). Furthermore, a top-down view of the WG was captured (Figure 2d),
showing good continuity and smooth sidewalls, crucial for minimizing propagation loss.
The RI variation was retrieved by calculating the RI distribution profile of the WG from the measured propagation near-field
intensity.[39] As shown in Figure 2e, there exists a large RI contrast (Δn) on the order of 10−2 in the FLDW region. To further
verify the RI distribution profile, we simulated the mode profile
of the fabricated WG (Figure 2f): Δni = 0 in region i, which is
Adv. Mater. 2024, 36, 2404493
far from the processing region; in region iii, which is the light
propagation region with a diameter of 9.9 μm, Δniii = +0.005;
and, Δnii = −0.08 in region ii, which is a ring-shaped transition
region surrounding region iii with a thickness set to 1.55 μm.
Figure 2g shows the measured intensity distribution along the y
and z center lines and the simulated field intensity distribution
along a center line. The mode field diameter (MFD) of the WG
is ≈7 μm. The measurement and simulation results are strongly
consistent, indicating that the WG indeed achieves a large Δn on
the order of 10−2 . According to numerical calculation, the high
RI contrast of Δn = 0.085 between core and cladding (Figure 2e)
gives rise to a small bend loss at a small radius of curvature, i.e., a
1.0 dB cm−1 cutoff occurs at bends with a radius of 1.0 mm, which
demonstrates the significant advantages of the developed FLDW
technique in the fabrication of compact 3D photonic devices in
integrated circuits.[40,41]
The IL of the circular-shaped gain WG was further investigated. Figure 2h shows the emission spectrum of the Er3+ -doped
gain WG from 1450 to 1650 nm. By pumping the ground state
4
I15/2 of Er3+ to the excited state 4 I11/2 with a 980 nm laser, the
emission band peaking at 1532 nm is produced after the nonradiative transition from 4 I11/2 to spontaneous emission state 4 I13/2 .
Figure 2i shows the dependence of the IL of the circular-shaped
gain WG on the wavelength over the range from 1500 to 1630 nm.
As it is well known, the IL of a WG includes coupling loss, Fresnel loss, background WG propagation loss, and Er3+ ions absorption loss. The optimum coupling efficiency between the circularshaped gain WG and SMF was estimated to be 98.09%, based
2404493 (3 of 10)
© 2024 Wiley-VCH GmbH
15214095, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202404493 by Université du Littoral-Côte-d'Opale, Wiley Online Library on [08/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
www.advancedsciencenews.com
www.advmat.de
Figure 2. Characterizations of circular-shaped gain WGs. a) Ellipticity and IL of the WGs written with various adjacent spacings. Insets show the crosssectional optical microscope images of the corresponding WGs. b) Design of multi-scanning arrangement for the circular-shaped WGs. Red dots represent the locations of single scan. c) Intensity profile of the propagation mode field of the WG measured at 1550 nm. d) Top-down view optical microscope
image of the WG. e) Calculated RI distribution profile of the WG. f) Simulated field intensity profile of the WG. Region i: basal region, Δni = 0; Region
ii: ring transition region, Δnii = −0.08; Region iii: light propagation region, Δniii = +0.005. g) Measured and simulated intensity line distribution. Exp.y
and exp.z are the measured intensity distribution along y and z center lines. Sim. is the simulated field intensity distribution along any center line. MFD
line is 1/e2 of the peak intensity. h) Fluorescence emission spectrum of the WG pumped by a 980 nm laser. Inset is a three-level energy level diagram,
showing processes of stimulated transitions, spontaneous decay and spontaneous emission. i) Insertion loss and j) Er3+ absorption loss of the WG
measured from 1500 to 1630 nm. k) Relative transmittance versus polarization angle of the incident light measured in the circular-shaped WG from 0°
to 360°.
on the overlap integral of the SMF mode field and the WG mode
field,[42] indicating a coupling loss of ≈0.084 dB. The total Fresnel
reflection loss is calculated using the Fresnel Equation (see Note
S6, Supporting Information) and is ≈0.443 dB. Considering that
Er3+ ion has no absorption at 1310 nm, the background WG propagation loss of the gain WG is ≈0.23 dB cm−1 . Assuming that
the IL of 0.69 dB measured at 1310 nm (see Figure 2a) remains
constant across the measurement wavelengths, any additional
IL observed at other wavelengths compared to that measured at
1310 nm can be attributed to Er3+ ions absorption loss. Figure 2j
shows the Er3+ ions absorption loss per unit length of the gain
WG, with an absorption loss of ≈0.24 dB cm−1 at 1550 nm. Consequently, the IL at 1550 nm can be calculated as 0.85 dB, in
good accordance with the value obtained through experimen-
Adv. Mater. 2024, 36, 2404493
tal measurement. The low-loss WG guarantees the achievement
of high optical gain WG amplification. For the fabrication of
FLDW gain WGs, we adopted symmetrical multi-scanning with
a precise circular arrangement. This approach aimed to eliminate the inhomogeneity of stress and RI distribution resulting from uneven stress accumulation. The goals were to obtain
polarization-independent gain WGs. A home-made experimental
setup (Figure S2a, Supporting Information) was employed to verify the polarization-correlative characteristics of the gain WGs. As
shown in Figure 2k, the relative transmittance fluctuates between
97.74% and 100% across all polarization angles, indicating that
the circular-shaped gain WG possesses excellent polarizationindependent character. The circular-shaped gain WG has symmetrical and uniform retardance of birefringence (Figure S2c,d,
2404493 (4 of 10)
© 2024 Wiley-VCH GmbH
15214095, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202404493 by Université du Littoral-Côte-d'Opale, Wiley Online Library on [08/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
www.advancedsciencenews.com
www.advmat.de
Figure 3. Fine control for the cross-sectional shape, size, and mode-field distribution of WGs by the multi-scanning and dense stacking technique. The
pulse energy is fixed at 123 nJ. The scanning number and corresponding adjacent spacing were 3 times and 2 μm, 4 times and 1.7 μm, 9 times and
1.1 μm, 32 times and 1.9 μm, and 42 times and 1.5 μm, respectively. a) Design of the multi-scanning arrangements for the corresponding WGs. Orange
dots represent the locations of single scan. b) Optical microscope images of the corresponding WGs fabricated. c) Simulated field intensity profiles at
650 nm. Black circles mark the designed distribution of scans. d) Intensity profiles of the propagation mode field measured at 650 nm.
Supporting Information), indicating that there are uniform distribution of stress and RI in the WG region.
2.3. Fine Control of the WGs with Tailorable Cross-Sectional
Shape and Size
By adjusting the spacing between adjacent tracks during the
implementation of multi-scanning, we observed a significant
transformation of the positive RI-modified region, shifting from
closely packed to separated configurations (Figure S3, Supporting Information). Hence, leveraging multi-scanning and densestacking techniques, we can achieve fine control over the WG
shape, size, mode-field and RI distribution. For instance, using
the same pulse energy of 123 nJ, we fabricated WGs through 3, 4,
9, and 32 scans, each one having adjacent spacing of 2, 1.7, 1.1,
and 1.9 μm, respectively. Specific design of the multi-scanning
arrangements for the corresponding WGs are demonstrated in
Figure 3a. The fs laser-processed region based on Type-I modification with positive RI variation can be clearly observed in the optical microscope images shown in Figure 3b1–b3. When the adja-
Adv. Mater. 2024, 36, 2404493
cent spacing was set to 2 μm that is close to the diameter of the circular modified region induced by a single scan with the pulse energy of 123 nJ, there is a slight separation between the three tracks
with positive RI change (Figure 3b1), leading to a triangularlike profile of the propagation mode field (Figure 3d1). When
the adjacent spacings were set to 1.7 and 1.1 μm for slight overlap (4 scans) and excessive overlap (9 scans), respectively, complete positive RI-modified regions with different areas formed
the WGs with basic mode profiles (Figure 3b2,b3). Additionally, we designed and fabricated a LP11 -mode WG (Figure 3b4)
and a doughnut-shaped WG (Figure 3b5). The calculated profiles of the propagation mode field at 650 nm were consistent
with the measured profiles, as shown in Figure 3c,d, indicating that we can accurately design and fabricate low-loss WGs
with a high processing resolution of sub-μm. These WGs can
support not only basic mode (small size) but also high-order
mode (large size) in the visible-near-infrared bands, showcasing great potential for precise mode control in passive/active
WG devices.[43] Theoretically, the cross-sectional shapes will not
affect the propagation loss of the waveguides, but it will impact the coupling loss due to mismatch of mode field between
2404493 (5 of 10)
© 2024 Wiley-VCH GmbH
15214095, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202404493 by Université du Littoral-Côte-d'Opale, Wiley Online Library on [08/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
www.advancedsciencenews.com
www.advmat.de
Figure 4. Microstructure evolution of the WG after fs laser irradiation. a, b) HRTEM images captured in the WG region. The lower inset in a) shows the
Fourier filtered image, in which red lines marks stacking fault and blue symbol “T” marks edge dislocation. The upper inset in a) shows corresponding
fast fourier transform (FFT) pattern of the HRTEM image in (a, left). Three pairs of lines marked in b) indicate three different orientations of lattice
fringes. c) Cross-sectional optical microscope image of the WGs fabricated with different adjacent spacings. d) Raman spectra collected from the nonirradiated bulk region and fs laser induced track region, excited by a 532 nm laser. Inset shows the schematic of the tetrahedral [AlO4 ] associated with
the A1g mode. e) Mapping of Raman peak position shift at A11g (369 cm−1 ).
the waveguides and coupling fibre. This novel FLDW technique
allows fabricating 3D waveguide structures to achieve modefield matching with different optical platforms or to achieve
the conversion from basic mode to complicated mode by designing cross-sectional specific shapes, offering increased flexibility and compatibility than traditional on-chip silicon optical
waveguides.
2.4. Microstructure Characterizations
The internal microstructure of YAG single crystals underwent rearrangement upon interaction with the focused light field. Clear
modifications are evident, characterized by mutations, with the
threshold pulse energy for mutation observed to be ≈119 μJ. Following a single scan at this mutation threshold, a circular-shaped
track with a substantial positive RI change becomes apparent
(Figure 1). Plastic deformation was proposed as the mechanism
for ultrafast laser-induced modification in YAG single crystals,[44]
which typically involves the generation of lattice point defects,
dislocations, stacking faults, and twins.[45,46] The high resolution
transmission electron microscope (HRTEM) images captured in
the WG region provide a compelling proof for this hypothesis,
highlighting edge dislocation and stacking fault in the Fourierfiltered image of the HRTEM image (Figure 4a) and three differ-
Adv. Mater. 2024, 36, 2404493
ent orientations of lattice fringes (Figure 4b). Irradiated by strong
light field, the lattice will experience rapid contraction followed
by a rebound.[47] If the rebound stress is lower than the extrusion
stress exerted by the surrounding lattice, it results in a minor contraction stress; conversely, it leads to a slight expansion stress.
When the total stress exceeds the tolerance threshold of the lattice, it tends to generate lattice damage. Clearly, lattice damages
primarily appear in the forms of dislocation, stacking fault, and
changes in the orientation of the lattice fringes in YAG single
crystals. The state of the lattice varies with the energy gradient of
the focused laser, resulting in the observed RI distribution. It has
been proved that compressive stress in single crystals can compress the lattice, reducing the unit cell volume and inducing densification, which results in positive RI change.[47,48] Therefore,
both contraction stress and expansion stress compress the lattice,
forming a circular-shaped region with a positive RI change. The
thermal and localized properties of FLDW contribute to rapid
cooling of the lattice, ensuring effective preservation of the modifications.
A confocal μ-Raman microscope was employed to analyze the
impact of the fs laser on the crystalline lattice. Four WGs written through 9 scans with a 3 × 3 rectangular arrangement (adjacent spacing from 1.1 to 1.4 μm) were fabricated for μ-Raman
measurements (Figure 4c). Figure 4d shows the Raman spectra measured at non-irradiated bulk region and WG region.
2404493 (6 of 10)
© 2024 Wiley-VCH GmbH
15214095, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202404493 by Université du Littoral-Côte-d'Opale, Wiley Online Library on [08/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
www.advancedsciencenews.com
www.advmat.de
Figure 5. Optical amplification characterization of the 7 mm-long Er3+ -doped YAG gain WG. a) Home-made optical gain WG measurement setup via a
dual-end pump strategy. b) The upper shows the emission spectrum and the on-off gain under a 980 nm laser pumping. The lower shows the propagation
spectra across the gain WGs with the pump laser off and on. The measured wavelengths are 1510, 1515, 1528, 1529, 1532, 1540, and 1550 nm. c) Net
gain at 1532 nm as a function of pump power via the dual-end pumping with two 980 nm lasers that the input power of the signal are −25.6 and
−52.3 dBm, respectively. The input pump power from each side is set to be equal.
Among all the Raman active vibrational modes, the three A1g
modes are related to tetrahedral [AlO4 ] (Inset in Figure 4d) in1
(369 cm−1 )
ternal vibrations.[15] Hence, the Raman peak at A1g
was selected for mapping analysis of the peak position shift,
as depicted in Figure 4e. The increase in phonon energy (blueshift of the Raman peak position) indicates the distortion of
the tetrahedral [AlO4 ] units due to an increase in their stretching vibration energy.[49] Figure 4e demonstrates a significant
blue-shift of the peak in the WG core region, indicating the
bond-length variation of the oxygen tetrahedron induced by contraction or expansion of lattice. The mapping analysis of WGs
with different adjacent spacing reveals no significant differences.
The results above provide a clear explanation for the observed
increment of RI shown in Figure 1. Moreover, the basic lattice structure of YAG single crystals in the fs laser induced
Adv. Mater. 2024, 36, 2404493
track is well preserved, indicating that there is no significant
influence on the coordination environment of doped RE ions,
which are substituted for the Y3+ lattice and located in the L
lattice position of a dodecahedron coordinated by eight oxygen
atoms.
2.5. Optical Gain WG Amplifier
A home-made setup was employed for the measurement of optical gain WG amplification, as shown in Figure 5a. A dual-end
pumping method was adopted and the input pump power from
each side was set to be equal. To characterize the small signal
amplification properties of the circular-shaped Er3+ -doped YAG
gain WGs, we tuned the wavelengths and laser power of the
2404493 (7 of 10)
© 2024 Wiley-VCH GmbH
15214095, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202404493 by Université du Littoral-Côte-d'Opale, Wiley Online Library on [08/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
www.advancedsciencenews.com
www.advmat.de
tunable seed laser and analyzed the propagation spectra across
the WGs using an optical spectrum analyzer (OSA). The fluorescence emission power is considerably weaker than the incident
signal power (Figure 5b), meaning that transmitted powers remain significantly above the level of amplified spontaneous emission propagating in the WGs. Hence, the on-off gain can be simply defined as Gainon-off = Pon – Poff (dBm), where Pon and Poff
are the measured output powers with the pump laser on and off,
respectively, while the seed laser is always on. A maximum on-off
gain of 5.53 dB at 1532 nm is obtained in a 7 mm-long Er3+ -doped
YAG gain WG, as shown in Figure 5b, which corresponds to a net
gain per unit length of ≈5 dB cm−1 .
To achieve net gain, the measured on-off gain must overcome
the Er3+ absorption loss and inherent losses (including coupling
loss, Fresnel loss, and background WG propagation loss) associated with the gain WG. Figure 5c plots the dependence of the
net gain at 1532 nm and the pump power of the 980 nm laser
when the input power of the signal are −25.6 and −52.3 dBm,
respectively. The measured saturated total gains are ≈3 dB for
both of the two different input signal powers. However, there is a
difference in the trend to achieve saturation between the various
input powers. Compared to the input signal power of −52.3 dBm,
the input signal power of −25.6 dBm requires more particles
pumped to population inversion for the gain saturation, raising
demands for larger pump power. Ii is worth mentioning that the
input pump power from the coupling fiber is directly measured
using a free-space photodetector. Although the maximum input
pump power was measured to be 600 mW (300 mW on each endfacet), not all the input power can be launched into the WGs due
to the fiber-waveguide coupling loss. Hence, the saturated pump
power is actually lower than the results from the experimental
measurements. Moreover, the 976 nm pump light is also transmitted in the waveguide in a single mode (Figure S7, Supporting
Information), meaning that the polarization insensitivity of the
circular-shaped waveguide is also applicable to the pump light
and the spatial modal overlap factor between the pump light and
the signal light is as high as 94.84% (see Note S7, Supporting
Information).
We also measured the optical amplification characteristics
of gain WGs fabricated in Yb3+ -doped YAG single crystals
(Figures S4 and S5, Supporting Information). Remarkably, an
optical amplification is achieved for both 1030 nm narrowband
continuous laser and broadband pulse laser using a 2-mm-long
gain WG directly written in Yb3+ -doped YAG crystals through the
proposed FLDW technique. The net gains are 2.7 and 2.4 dB,
respectively. Moreover, we also successfully fabricated circularshaped single-mode WGs based on Type-I modification in Er3+ doped LuAG and undoped LiNbO3 single crystals (Figure S6,
Supporting Information). Therefore, the proposed FLDW technique offers a versatile solution for designing and writing lowloss and high-gain WGs with Type-I modification in various
RE3+ -doped/undoped gain single crystals. This opens new avenue for the development of high-quality integrated light sources
in future integrated optics and photonic chips.
3. Conclusion
In conclusion, we introduced a novel FLDW technique that combines slit-shaping with an immersion oil objective to fabricate
Adv. Mater. 2024, 36, 2404493
low-loss Type-I WGs in single crystals. We found that one track of
fs laser induces a positive RI-modified quasi-circular region with
a sub-μm resolution, as high as 700 nm, through Type-I modification in single crystals after slit-shaping. Furthermore, we successfully achieved fine control of the WG shape, size, mode-field
and RI distribution via a multi-overlapping scanning strategy. As
a proof-of-concept, we fabricated a 7 mm-long SMF-compatible
circular-shaped gain WG in an Er3+ -doped YAG single crystal
based on positive RI variation on the order of 10−2 . This resulted
in a net gain of ≈3.0 dB and a background WG propagation loss of
as low as 0.23 dB cm−1 . Moreover, we succeeded in writing highquality WGs in Yb3+ -doped YAG, Er3+ -doped LuAG, and undoped
LiNbO3 single crystals, showcasing its general applicability for
other single crystals. Notably, the circular-shaped WGs written
by this technique are polarization-insensitive, which is a key character in optical communication system and quantum PICs. Importantly, with the ability to precisely control the cross-sectional
shape, size, mode-field and RI distribution of arbitrary WG, and
leveraging the advantage of RE3+ with high doping concentration
in single crystals, this FLDW technique introduces new degrees
of freedom for the design and fabrication of highly-integrated
passive/active optical WG devices in many fields, such as integrated optics, optical communication, and quantum PICs.[50–52]
4. Experimental Section
Materials: YAG single crystals (<111>, 5 × 5 × 0.5 mm3 ) were procured from Shanghai Yulei Optoelectronic Technology Co., Ltd. The Er3+ doped YAG single crystals (<111>, 8 × 8 × 1 mm3 , 2.5 at.% doping) and
Er3+ -LuAG single crystals (<111>, 8 × 8 × 1 mm3 , 2.5 at.% doping) were
provided by Shandong University. The Yb3+ -doped YAG single crystals
(<111>, 5 × 5 × 0.23 mm3 , 10 at.% doping) were procured from Chengdu
Alfa Metal Material Co., Ltd. The LiNbO3 single crystal wafers (z-cut) were
procured from Jinan Jingzheng Electronics Co., Ltd and cut to 4.5 × 4.5 ×
0.4 mm3 . All facets of the single crystal samples used in this work were
optically polished for FLDW.
Laser Direct Writing System: Figure 1 shows the experiment setup for
the FLDW WGs. A diode-pumped Yb:KGW laser (PHAROS PH2, Light
Conversion, Lithuania) provided 213-fs pulse duration at a center wavelength of 1030 nm with a high repetition rate of 1087 kHz. A high-precision
xyz translation platform (A3200, Aerotech) was used to control the 3D motion of sample according to the set speed and path. To reduce the spherical
aberration effect of the focused beam along z direction caused by the RI
difference between the YAG single crystal (nCrystal = ≈1.82 at 1550 nm) and
air, an immersion oil objective lens (Plan 100x/1.25NA Oil, Nikon, Japan),
immersed in RI matching oil (nOil = 1.5105–1.5175), was used to tightly
focus the laser beam underneath the sample surface. A slit of 0.5 mm in
width was used for the beam shaping. The fs laser was incident normal
to the x-y plane while the sample was translated along x direction, parallel
to the polarization direction of fs laser and the long axis of the slit image
reflected by the dichroic mirror. After the FLDW WGs, the two end facets
perpendicular to x axis were optically polished again for further material
and optical characterizations.
Material Characterizations: An optical microscope (BX53MTRF-S,
Olympus, Japan) was used to observe the cross-sectional and top-down
morphology of the FLDW WGs in the single crystals. A confocal microphotoluminescence (μ-PL) microscope was used to analyze the fluorescence properties in fs laser induced tracks. A confocal micro-Raman (μRaman) microscope (InVia, Renishaw, UK) was used to characterize the
regions modified by the fs laser. A spherical aberration corrected scanning
transmission electron microscope (STEM, FEI Titan G2 60–300, Netherlands) was used to analyze crystalline lattice and microscope atomic evolution in fs laser induced tracks, after focus ion beam (FIB) thinning
2404493 (8 of 10)
© 2024 Wiley-VCH GmbH
15214095, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202404493 by Université du Littoral-Côte-d'Opale, Wiley Online Library on [08/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
www.advancedsciencenews.com
www.advmat.de
(Helios 5 CX, Thermo Fisher Scientific, USA) the sample down to ≈50 nm
in thickness.
Optical Measurements for the Gain WG Amplifiers: Home-made singleend and dual-end pumping experimental setups were utilized for gain
measurements of the optical gain by the WG amplifiers, as shown in
Figures 5a and S4, Supporting Information. In detail, the seed light source
from the tunable laser (TSL-550, 1500–1630 nm, Santec, Japan) and the
pump light source from the 980 nm diode laser were coupled by a WDM
of 980/1550, and then launched into the gain WG using a bare-fiber-to-chip
method. A long-focal distance lens (M Plan Apo NIR 50x/0.42NA, Mitutoyo, Japan) at the output end was used to obtain the near-field images of
the gain WGs after adjusting the lens to a suitable position. A collimator
was placed at the image plane of the long-focal lens to collect output light
signals, which was connected to an OSA (Yokogawa AQ6370D, Japan).
Fluorescence emission spectra of the gain WGs were measured by the
OSA under pumping with the 980 nm diode laser. A beam profiler (CMOS1201-IR, CINOGY, Germany) was placed at the collimator position to measure intensity profile of propagation mode field at 1550 and 1030 nm. An
optical power meter was placed at the collimator position to measure the
output powers with and without gain WG for the IL analysis. By switching
on the seed laser, we analyzed the output spectra collected by the OSA
before and after the 980 nm diode laser pumping for the on-off gain amplification measurements.
A home-built polarization-correlative transmittance measurement system is given in Figure S2a, Supporting Information. The linear-polarization
signal light was obtained after regulated by the first polarization controller
(PC) and divided into two beams by the polarization beam splitter (PBS).
The second PC was used to eliminate the phase difference generated by the
system after the space-to-fiber collimator. By rotating the half-wave plate
(HWP) between two collimators and recording the power measured by the
two optical power meters (OPM), the relative transmittance curve of the
incident linear-polarization light with different polarization direction can
be plotted.
Theoretical Calculations and Numerical Simulations: Simulation of the
3D field intensity distribution of the incident laser (Figure S1, Supporting Information) and RI profile of the WG were performed by MATLAB
(Figure S1, Supporting Information). COMSOL Multiphysics 5.6 was utilized for the design and modeling of the arbitrary cross-sectional shape
variable gain WGs. RSOFT FEMsim based on the finite element method
was used to numerically calculate the bending loss. Fourier filtering was
used to detect the presence of dislocations in the noisiness HRTEM image of fs laser-irradiated region. Specific steps of Fourier filtering are as
following: performing a fast Fourier transform (FFT, Figure 4a, upper of
insets), masking the reflections of interest (marked by green circles) and
inverting the reflections of interest to release most of noise. The resulting
Fourier filtered image visualizes the dislocations for selected lattice fringes
(Figure 4a, lower of insets).
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
The authors are very grateful for supporting from National Key R&D
Program of China (No. 2021YFB2802000), National Natural Science
Foundation of China (Nos. U20A20211, 62375246, 62105297), “Pioneer” and “Leading Goose” R&D Program of Zhejiang (2023C03089),
Zhejiang Provincial Natural Science Foundation (Nos. LZ23F050002,
LQ22F050022).
Conflict of Interest
The authors declare no conflict of interest.
Adv. Mater. 2024, 36, 2404493
Author Contributions
D.C. and Z.C. contributed equally to this work. Z.C. and L.Z. conceived
the idea, developed the novel FLDW technique, and conducted theoretical
analysis. J.Q. and Z.M. supervised the whole project. D.C. designed, fabricated, and characterized the WGs. Y.Y. and Y.W. assisted in the design and
fabrication of the WGs. D.C., Y.Y., X.H., and L.Z. carried out the theoretical calculations and numerical simulation. D.C., Y.Y., and K.Y.L. developed
the optical measurement methods and built the optical measurement systems. Z.W. and C.Z. conducted the HRTEM characterization. D.C. and Z.C.
written and revised the manuscript. Z.C., Y.Z., B.X., X.L., Z.M., G.D., G.B.,
L.Z., and J.Q. revised and polished the manuscript. All the authors participated in the discussion of the results.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Keywords
3D laser writing, passive/active waveguide devices, single crystal, sub-μm
resolution, type-I optical waveguide
Received: March 27, 2024
Revised: May 1, 2024
Published online: June 12, 2024
[1] W. Bogaerts, D. Pérez, J. Capmany, D. A. B. Miller, J. Poon, D.
Englund, F. Morichetti, A. Melloni, Nature 2020, 586, 207.
[2] V. Snigirev, A. Riedhauser, G. Lihachev, M. Churaev, J.
Riemensberger, R. N. Wang, A. Siddharth, G. Huang, C. Mohl,
Y. Popoff, U. Drechsler, D. Caimi, S. Honl, J. Liu, P. Seidler, T. J.
Kippenberg, Nature 2023, 615, 411.
[3] X. Y. Xu, G. H. Ren, T. Feleppa, X. M. Liu, A. Boes, A. Mitchell, A. J.
Lowery, Nat. Photonics 2022, 16, 595.
[4] Y. B. Wang, J. A. Holguín-Lerma, M. Vezzoli, Y. Guo, H. X. Tang, Nat.
Photonics 2023, 17, 338.
[5] H. Tang, X. F. Lin, Z. Feng, J. Y. Chen, J. Gao, K. Sun, C. Y. Wang, P. C.
Lai, X. Y. Xu, Y. Wang, L. F. Qiao, A. L. Yang, X. M. Jin, Sci. Adv. 2018,
4, aat3174.
[6] J. Zhou, B. Huang, Z. Yan, J. G. Bunzli, Light Sci. Appl. 2019, 8, 84.
[7] X. Sun, Z. Chen, L. Zhang, D. Chen, Y. Wang, B. Xu, Z. Ma, G. Barillaro,
L. Zhong, J. Qiu, Opt. Lett. 2024, 49, 33.
[8] X. Sun, Y. Wang, L. Zhong, D. Chen, B. Xu, Z. Ma, X. Liu, G. Barillaro,
Z. Chen, J. Qiu, Opt. Express 2023, 31, 5812.
[9] A. A. Kaminskii, Laser Crystals, Springer, Berlin, Heidelberg 1990.
[10] S. G. Pietro Ferraro, P. D. Natale, Ferroelectric Crystals for Photonic Applications, Springer, Berlin, Heidelberg 2014.
[11] A. A. Kaminskii, Laser Photonics Rev. 2007, 1, 93.
[12] R. C. Alferness, Science 1986, 234, 825.
[13] P. Mutter, C. C. Kores, M. Widarsson, A. Zukauskas, F. Laurell, C.
Canalias, Opt. Express 2020, 28, 38822.
[14] Y. Liu, Z. Qiu, X. Ji, A. Lukashchuk, J. He, J. Riemensberger, M.
Hafermann, R. N. Wang, J. Liu, C. Ronning, T. J. Kippenberg, Science
2022, 376, 1309.
[15] J. Li, J. Yan, L. Jiang, J. Yu, H. Guo, L. Qu, Light Sci. Appl. 2023, 12,
164.
[16] A. Ródenas, M. Gu, G. Corrielli, P. Paiè, S. John, A. K. Kar, R. Osellame,
Nat. Photonics 2018, 13, 105.
[17] C. Grivas, R. Ismaeel, C. Corbari, C. C. Huang, D. W. Hewak, P.
Lagoudakis, G. Brambilla, Laser Photonics Rev. 2018, 12, 1800167.
2404493 (9 of 10)
© 2024 Wiley-VCH GmbH
15214095, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202404493 by Université du Littoral-Côte-d'Opale, Wiley Online Library on [08/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
www.advancedsciencenews.com
www.advmat.de
[18] Q. X. Yang, H. L. Liu, S. He, Q. Y. Tian, B. Xu, P. F. Wu, Opt.-Electron.
Adv. 2021, 4, 200005.
[19] J. E. Bae, T. Calmano, C. Kränkel, F. Rotermund, Laser Photonics Rev.
2022, 16, 2100501.
[20] K. Liu, Y. Dong, Z. Zhang, X. Duan, R. Guo, Z. Zhai, J. Wang, Nanophotonics 2024, 13, 9.
[21] J. Lu, J. Tian, B. Poumellec, E. Garcia-Caurel, R. Ossikovski, X. Zeng,
M. Lancry, Light Sci. Appl. 2023, 12, 46.
[22] K. Sun, D. Tan, X. Fang, X. Xia, D. Lin, J. Song, Y. Lin, Z. Liu, M. Gu,
Y. Yue, J. Qiu, Science 2022, 375, 307.
[23] M. Lancry, B. Poumellec, J. Canning, K. Cook, J. C. Poulin, F. Brisset,
Laser Photonics Rev. 2013, 7, 953.
[24] Y. Jia, S. Wang, F. Chen, Opt.-Electron. Adv. 2020, 3, 190042.
[25] D. Tan, Z. Wang, B. Xu, J. Qiu, Adv. Photonics 2021, 3, 024002.
[26] L. Li, W. Kong, F. Chen, Adv. Photonics 2022, 4, 024002.
[27] B. Zhang, Z. Wang, D. Tan, J. Qiu, PhotoniX 2023, 4, 24.
[28] J. Lin, N. Yao, Z. Hao, J. Zhang, W. Mao, M. Wang, W. Chu, R. Wu,
Z. Fang, L. Qiao, W. Fang, F. Bo, Y. Cheng, Phys. Rev. Lett. 2019, 122,
173903.
[29] J. X. Zhou, Y. T. Liang, Z. X. Liu, W. Chu, H. S. Zhang, D. F. Yin, Z. W.
Fang, R. B. Wu, J. H. Zhang, W. Chen, Z. Wang, Y. Zhou, M. Wang, Y.
Cheng, Laser Photonics Rev. 2021, 15, 2100030.
[30] J. T. Lin, S. Farajollahi, Z. W. Fang, N. Yao, R. H. Gao, J. L. Guan, L.
Deng, T. Lu, M. Wang, H. S. Zhang, W. Fang, L. L. Qiao, Y. Cheng, Adv.
Photonics 2022, 4, 036001.
[31] Q. Zhang, M. Li, J. Xu, Z. J. Lin, H. F. Yu, M. Wang, Z. W. Fang, Y.
Cheng, Q. H. Gong, Y. Li, Photonics Res. 2019, 7, 503.
[32] J. E. Bae, X. Mateos, M. Aguilo, F. Diaz, J. G. Ajates, C. Romero, F.
Rotermund, Photonics Res. 2022, 10, 2584.
[33] L. Sansoni, F. Sciarrino, G. Vallone, P. Mataloni, A. Crespi,
R. Ramponi, R. Osellame, Phys. Rev. Lett. 2012, 108,
010502.
[34] U. Tegin, B. Rahmani, E. Kakkava, D. Psaltis, C. Moser, Adv. Photonics
2020, 2, 056005.
[35] D. Mao, Y. Zheng, C. Zeng, H. Lu, C. Wang, H. Zhang, W. Zhang, T.
Mei, J. Zhao, Adv. Photonics 2021, 3, 014002.
Adv. Mater. 2024, 36, 2404493
[36] Y. Zheng, C. Zhai, D. Liu, J. Mao, X. Chen, T. Dai, J. Huang, J. Bao,
Z. Fu, Y. Tong, X. Zhou, Y. Yang, B. Tang, Z. Li, Y. Li, Q. Gong, H. K.
Tsang, D. Dai, J. Wang, Science 2023, 381, 221.
[37] Y. Cheng, K. Sugioka, K. Midorikawa, M. Masuda, K. Toyoda, M.
Kawachi, K. Shihoyama, Opt. Lett. 2003, 28, 55.
[38] L. Zhong, Y. Wang, D. Tan, J. Qiu, Laser Photonics Rev. 2023, 17,
2200767.
[39] I. Mansour, F. Caccavale, J. Lightwave Technol. 1996, 14, 423.
[40] J. Lapointe, J. P. Berube, Y. Ledemi, A. Dupont, V. Fortin, Y.
Messaddeq, R. Vallee, Light Sci. Appl. 2020, 9, 64.
[41] A. J. Ross-Adams, T. T. Fernandez, M. J. Withford, S. Gross, Light: Adv.
Manuf. 2024, 5, 9.
[42] Y. Yang, L. Zhong, Y. Cui, Y. Wang, D. Chen, K. Y. Lau, X. Liu, Z. Ma,
G. Barillaro, Z. Chen, J. Qiu, Nanophotonics 2023, 12, 3069.
[43] B. Sun, F. Morozko, P. S. Salter, S. Moser, Z. Pong, R. B. Patel, I. A.
Walmsley, M. Wang, A. Hazan, N. Barre, A. Jesacher, J. Fells, C. He, A.
Katiyi, Z. N. Tian, A. Karabchevsky, M. J. Booth, Light Sci. Appl. 2022,
11, 214.
[44] S. S. Fedotov, L. N. Butvina, A. G. Okhrimchuk, Sci. Rep. 2020, 10,
19385.
[45] B. Jakobsen, H. F. Poulsen, U. Lienert, J. Almer, S. D. Shastri, H. O.
Sorensen, C. Gundlach, W. Pantleon, Science 2006, 312, 889.
[46] Y. X. Ye, Y. Y. Feng, Z. C. Lian, Y. Q. Hua, Appl. Surf. Sci. 2014, 309,
240.
[47] B. Wu, Q. Yang, B. Zhang, L. Wang, Y. Y. Ren, S. Meng, F. Chen, Adv.
Opt. Mater. 2023, 12, 2302106.
[48] B. C. Xiong, B. Zhang, Q. M. Lu, Y. Y. Ren, L. Wang, F. Chen, Opt.
Mater. 2020, 107, 110103.
[49] A. Rodenas, A. K. Kar, Opt. Express 2011, 19, 17820.
[50] Y. Wang, B. Y. Xie, Y. H. Lu, Y. J. Chang, H. F. Wang, J. Gao, Z. Q. Jiao,
Z. Feng, X. Y. Xu, F. Mei, S. Jia, M. H. Lu, X. M. Jin, Light Sci. Appl.
2021, 10, 173.
[51] X.-Y. Xu, X.-W. Wang, D.-Y. Chen, C. M. Smith, X.-M. Jin, Nat. Photonics
2021, 15, 703.
[52] Y. Shang, J. Zhou, Y. Cai, F. Wang, A. Fernandez-Bravo, C. Yang, L.
Jiang, D. Jin, Nat. Commun. 2020, 11, 6156.
2404493 (10 of 10)
© 2024 Wiley-VCH GmbH
15214095, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202404493 by Université du Littoral-Côte-d'Opale, Wiley Online Library on [08/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
www.advancedsciencenews.com