Preparation of amorphous silicon-doped Y  O  aerogel enabling nonlinear optical features for ultrafast photonics

: Amorphous aerogels with the microscopic nano-scale three-dimensional meshes provide superb platforms for investigating unique physicochemical properties. In order to enhance the physical, thermal and mechanical performances, one efficient and common approach is integrating diverse functional materials. Herein, we report a simple strategy to fabricate the amorphous silicon doped Y 2 O 3 aerogels with the post-gelation method under the N 2 /EtOH supercritical atmosphere. The impact of Si concentration on the nonlinear optical properties is investigated for the ﬁrst time. The maximum modulation depth is 1.65 % with a saturation intensity of 0.78 MW cm − 2 with the 1-ps laser excitation at 1590 nm. Finally, we incorporated the silicon-doped Y 2 O 3 aerogel based saturable absorber (SA) into an erbium-doped ﬁber laser (EDFL) and achieved various mode-locking operations at different wavelengths in the super C band, in terms of the conventional soliton, harmonic soliton molecules pulses, and dual-wavelength soliton mode-locking. Overall, this work conﬁrms that amorphous silicon-doped Y 2 O 3 aerogels are good nonlinear


Introduction
Aerogels possess a three-dimensional mesoporous network with ultralow thermal conductivity and high specific surface area [1], [2].Owing to the unique structure, aerogels exhibit numerous properties such as high specific surface area, large porosity, ultra-low thermal conductivity, ultralow dielectric constant, the tailored density, and the manipulated refractive index [3]- [5].Therefore, aerogel materials have garnered widespread attention and been successfully applied in fields such as thermal insulation [6], biomedicine [7], impurity adsorption [8], [9], and catalyst [10].Moreover, after the special manufacturing and processing, favorable properties of aerogels can even be efficiently enhanced [11].
In fact, SiO 2 aerogels are the earliest invented [12] and commercialized materials.Although the strength-weight ratio in the aerogels is extremely high [13], SiO 2 aerogels are normally fragile [14].With the exhaustive investigation and methodology evolution, versatile metal oxide aerogels [15]- [19] have been successively developed including ZrO 2 , Al 2 O 3 , and TiO 2 .The advent of the metal oxide aerogels instills surprised electronic, mechanical, and surface/interface properties.However, the metal oxide aerogels suffer the instability especially at the higher temperatures [20], [21].In recent years, Y 2 O 3 has been poured attention in because of the high melting temperature and excellent physicochemical stability.In addition, as a third-order nonlinear optical response, the saturable absorption in Y 2 O 3 can be applied in the fiber lasers to realize the Q-switching and mode-locking operations [22]- [24].Therefore, researchers pay great attention to Y 2 O 3 materials.In recent years, the field of ultrafast photonics has been rapidly developed, and various achievements have emerged in an endless stream [25]- [29].Among them, passive mode-locking fiber lasers have been widely studied because of their self-started operation, excellent beam quality and compact structure [30]- [32].Previous studies have also shown that Y 2 O 3 is a promising material in the field of nonlinear optics.
However, for the metal oxide aerogels fabrication, owing to the hydrolysis and the condensation of the precursor, the aerogels are typically amorphous.The amorphous materials normally exhibit the distinct features owing the symmetry breaking and disorder.Compared with crystalline materials, amorphous materials do not have longrange ordered periodicity, which is conducive to the generation of localized electrons/holes in the band gap [33]- [35].The structural disorder of amorphous materials can promote charge capture in the materials through the relaxation of the local atomic environment.In fact, the intrinsic electron and hole capture in amorphous oxide semiconductors have been demonstrated by theoretical calculations and experiments [36]- [38].The charge-trapping effect of amorphous materials is much greater than that of the same kind of crystalline materials, and the quantity of electric charges trapped increases with the increase of the disorder, which leads to electrons-holes separation, thus enhancing the nonlinear optical properties of amorphous oxide semiconductors [39], [40].Thus far, many amorphous molecular materials have been paid attention to and studied, and have been applied for the supercontinuum generation [41].However, as far as we know, there are few reports about the application of amorphous materials in ultrafast lasers, which has aroused our concern.
In this paper, we fabricated the amorphous Si-doped Y 2 O 3 aerogels using the post-gelation method under the supercritical atmosphere.Under the different conditions, we synthesized the Si-doped Y 2 O 3 aerogels with different Si concentration.The impact of the Si concentration on the nonlinear optical properties was comprehensively studied.The highest modulation depth was 1.65 % with a saturation intensity of 0.78 MW/cm 2 with the 1-ps 1590-nm laser pulse excitation, demonstrating the favorable nonlinear optical properties of the Y 2 O 3 -based saturable absorber.Finally, we incorporated the Si-doped Y 2 O 3 aerogel based saturable absorber (SA) into an erbium-doped fiber laser (EDFL).By adjusting the evanescent light field interactions with the amorphous aerogel, various soliton mode-locking operations in the super C band were achieved, producing the conventional soliton pulses, harmonic soliton molecules pulses, and dual-wavelength soliton pulses.Our work indicates that amorphous Y 2 O 3 aerogel is a great potential nonlinear optical material and will expand the future applications of the amorphous materials in the ultrafast photonics.

Characterization of amorphous silicon-doped Y  O  aerogel
Based on previous studies [42]- [44], it has been established that yttrium ions exhibit a coordination number of eight with water molecules in aqueous solutions, owing to their heightened electrophilicity.In the current investigation, a precursor of yttrium chloride hexahydrate was dissolved in absolute ethanol; the yttrium ions were anticipated to undergo hydration, forming connections with six water molecules, which is the maximum possible coordination due to an insufficient quantity of available water molecules.
In this context, it was postulated that the yttrium ions, coordinated with six water molecules, would undergo hydrolysis, releasing hydrogen ions subsequent to the introduction of a propylene oxide (PO) gel initiator.The hydroxyl yttrium species would then proceed to polycondense, leading to the formation of bi-continuously connected Y 2 O 3 gel networks through a concurrent phase separation process.The entire detailed process is shown in Figure 1    To investigate the optical absorption properties of the prepared amorphous Y 2 O 3 aerogel, the sample was ground into powder, then dissolved into anhydrous ethanol solution for ultrasonic and centrifugal treatment, and the supernatant was taken for use.The optical absorption characteristics of the material were measured using a UV-VIS-IR spectrophotometer.Clearly, as shown in Figure 2(a), the amorphous Y 2 O 3 material exhibits broad absorption characteristics in the near-infrared range, suggesting the great potential for various photonics applications.Figure 2(b) represents the X-ray diffraction (XRD) analysis of the Y 2 O 3 aerogels.It can be seen that with the incorporation of Si, the crystallinity of Y 2 O 3 aerogels continues to deteriorate, and the reason is analyzed as follows: The Si 4+ ions have a significantly smaller ionic radius (R Si4+ = 0.04 nm) compared to the Y 3+ ions (R Y3+ = 0.09 nm).As a result Si doping causes stress in the Y 2 O 3 lattice, degrading its crystal structure [45].To confirm the chemical bonding in the amorphous silicondoped Y 2 O 3 aerogel, the X-ray photoelectron spectroscopy (XPS) analysis was carried out.Additionally, in order to study the effect of silicon concentration on the optical properties of Y 2 O 3 aerogels, X-ray diffraction (XRD) patterns, UV-VIS-IR absorption spectra and the Fourier-transformed infrared (FT-IR) spectra were used to analysis the Y 2 O 3 aerogels, as shown in Figure S3.Clearly, with the Si concentration increase, the aerogel was getting more and more amorphous, while the absorption peak attributed to the Si-O bond was stronger, indicating that the Si incorporation degraded the crystal structure.

Nonlinear optical properties in Y  O  aerogels
The nonlinear optical absorption measurement was carried out with an in-line twin-detector system excited with the picosecond 1590-nm fiber laser, as shown in the Supplementary Material.Obviously, the nonlinear transmission curve of the Y 2 O 3 aerogels can be fitted by [47]: here, T, ΔT, I, I sat , and  0 denote the transmission, modulation depth, on-axis pump intensity, the saturation intensity and the unsaturation loss of the prepared samples.Obviously, all samples in Figure 3 possess the typical saturable absorption effect at 1590 nm.The Y 2 O 3 -3 aerogel exhibits the largest modulation depth of 1.65 % with a relative low saturation intensity of 0.78 MW/cm 2 .The saturable absorption properties enabled the mode-locking performance to generate the ultrafast optical pulses.By tuning the polarization state and pump power, the mode-locking pulses of the EDFL near 1530 nm are realized, as shown in Figure 4. Figure 4(a) shows that the modelocked laser runs in the conventional soliton mode-locking operation when the pump power is 191.76 mW.The spectral diagram in Figure 4(a1) shows clear and symmetrical Kelly sidebands.The central spectral wavelength was 1531.6 nm with a 3-dB bandwidth (Full width at half-maximum, FWHM) of 2.47 nm.In fact, the Kelly sidebands are resulted from the constructive interference of the pulse and dispersive wave in the laser resonator [48].The pulse train in Figure 4(a2) shows the stable time interval of 133.6 ns between the adjacent pulses, coinciding well with the cavity length of 27.31 m. Figure 4(a3) shows the radio frequency (RF) spectrum with a central frequency of 7.487 MHz and a signal-to-noise ratio (SNR) of about 48.2 dB, indicating a stable mode-locking operation.The stability of the modelocking state was confirmed by the RF spectrum in the 500-MHz range, as shown in the inset of Figure 4(a3).The autocorrelation trajectory in Figure 4(a4) reveals a FWHM of 1.76 ps, corresponding to a pulse width of 1.14 ps with the squared hyperbolic secant fitting.

Mode-locking EDF laser near 1530 nm
When the pump power was 245.76 mW, harmonic conventional soliton mode-locking pulses were obtained, as shown in Figure 4(b).Figure 4(b1) shows the spectrum of the harmonic conventional soliton mode-locking operation.The central wavelength was 1532.3 nm, and the 3-dB spectral bandwidth was 2.68 nm with clear and symmetric Kelly sidebands.Figure 4(b2) shows the pulse sequence displayed on the oscilloscope, with adjacent pulses spaced by 2.3 ns. Figure 4(b3) shows the RF spectrum with the SNR of about 44.1 dB, a repetition rate of 434.2 MHz, corresponding to the 58th harmonic mode-locking operation.The stability of the mode-locking state was confirmed by the RF spectrum in the 3-GHz range, as shown in the inset of Figure 4(b3).The autocorrelation trace in Figure 4(b4) possesses a FWHM of 2.14 ps, with a corresponding pulse width of 1.39 ps with the squared hyperbolic secant fitting.The reasons for the formation of harmonic mode-locking operation are explained as follows.For passive mode-locking fiber lasers, with an increase in the pump power, the energy of a single soliton pulse exceeded its maximum saturable energy.Due to the peak power limitation effect and the soliton area theorem, a single soliton pulse was split into multiple pulses to form an unstable multiple-pulse state.In the case of the gain, loss, nonlinearity, and dispersion interaction of the ring cavity, these multiple pulses can equalize via a self-arrangement process and finally form an orderly distributed pulse sequence with uniform spacing, which is referred to as harmonic mode-locking [49], [50].
After obtaining the output of conventional soliton pulses, the pump power was adjusted to 215.76 mW to achieve the output of harmonic soliton molecules pulses, as shown in Figure 4(c).The spectral diagram reveals a central wavelength of 1532.6 nm, and the spectrum exhibits modulation with the modulation period of 1.86 nm, as shown in Figure 4(c1).The pulse sequence diagram shown by the oscilloscope in Figure 4(c2) has an interval of 5.56 ns between the neighbored pulses.In Figure 4(c3), the SNR is about 45 dB, and the repetition frequency is 179.7 MHz, corresponding to the 24th harmonic mode-locking operation.The inset of Figure 4(c3) shows the RF spectrum in the 1-GHz range.The autocorrelation trace with the multi-peak fitting curve is shown in Figure 4(c4), in which the FWHM of the highest peak is 2.17 ps, corresponding to the pulse duration of 1.41 ps and the peak interval is 3.8 ps.The time interval Δt and modulation period Δ are related as follow [51]: The time interval was calculated as 4.2 ps, almost coincident with the experimental data.The formation mechanism of the harmonic soliton molecules pulses can be explained as follows.For the fundamental frequency modelocked single soliton pulse, the peak power of the optical pulse was clamped due to the quantization effect of the soliton energy attributed to the soliton area mechanism and limited gain bandwidth of the active optical fiber.However, with the increase of pump power, owing to the accumulated nonlinear effect and peak power limiting effect of the soliton, a single soliton pulse splits into multiple pulses with low peak energy.The pulses interact with each other through repulsive and attractive forces.Finally, when the repulsive and attractive forces got a dynamical equilibrium, stable bound solitons are generated [52].Moreover, under the combined action of cavity gain, loss, dispersion, and cumulative nonlinearity, multiple soliton molecules pulses were automatically arranged to exhibit a uniform amplitude and equal time interval within one round trip.Consequently, the repetition rate of the output pulse was amplified to an integral multiple of the fundamental repetition rate, thus leading to the generation of harmonic soliton molecules pulses [49].In general, there are both short distance and long distance interactions in the formation process of harmonic soliton molecules mode-locking operation: the former leads to the formation of bound state solitons, and the latter rearranges and distributes the bound state solitons uniformly in the resonator, and finally makes the laser work in the harmonic solitons molecules mode-locking operation [53].

Synchronized dual-wavelength mode-locking near 1530 and 1560 nm
In addition, synchronized dual-wavelength mode-locking near 1530 and 1560 nm are obtained under different pump power and polarization states, as shown in Figure 5.When the pump power was adjusted to 227.76 mW, the harmonic conventional soliton mode-locked EDFL coexisting with the continuous wave can be achieved after fine tune of the polarization controller.The spectrum of the harmonic conventional soliton mode-locking operation is illustrated in Figure 5(a1), showcasing the continuous wave wavelength at 1558.7 nm and the pulsed wave wavelength at 1532.1 nm, with a 3-dB bandwidth of 2.5 nm.The distinct Kelly When the pump power was adjusted to 203.76 mW, the output of 22nd harmonic soliton molecules is obtained by tuning the polarization state, as shown in Figure 5(b1).The central wavelength of the soliton molecules was 1532.1 nm and the modulation period was 1.8 nm.There was an obvious continuous wave component in the spectrum with a central wavelength of 1557.1 nm.The adjacent pulse interval was 6 ns, corresponding to repetition rate of 164.7 MHz, and the SNR is as high as 62.6 dB.The inset of Figure 5(b3) shows the RF spectrum in the 500-MHz range.Increasing the pump power to 227.76 mW, the output of the 29th harmonic soliton molecules can be obtained, as shown in Figure 5(c).The central wavelength of the soliton molecules was 1531.5 nm and the modulation period was 1.76 nm.There was also an obvious continuous wave component with a central wavelength of 1555.7 nm.The SNR is about 56.5 dB, the repetition rate is 217.1 MHz, corresponding to an adjacent pulse interval of 4.6 ns.The inset of Figure 5(c3) shows the RF spectrum in the 1-GHz range, which demonstrates the stable mode-locking operations.
Increasing the pump power to 245.76 mW and adjusting the polarization controller, we were able to achieve conventional soliton mode-locking operation and soliton molecules mode-locking operation near 1560 nm.The conventional soliton mode-locked laser operated at 1555.9 nm with a FWHM of 1.85 nm was realized, as depicted in Figure 5(d1).The continuous wave component operated at 1532.6 nm.The pulse sequence displayed by the oscilloscope, as shown in Figure 5(d2), has a pulse interval of 133.6 ns, which matches the cavity length.In Figure 5(d3), the fundamental repetition frequency is 7.487 MHz, which is consistent with the conventional soliton mode-locking operation at the wavelength of 1531.6 nm with an SNR of about 47.6 dB.The inset shows the mode-locking pulse RF spectrum within a span of 500 MHz. Figure 5(e) shows the soliton molecules modelocking operation near 1560 nm.In Figure 5(e1), we find that the spectrum exhibits modulation with a modulation period of 1.7 nm and the central wavelength of 1557.6 nm and an obvious continuous-wave spectral component at 1531.8 nm. Figure 5(e2) shows the pulse interval is 133.6 ns.The SNR in Figure 5(e3) is about 36.1 dB, the repetition frequency is 7.487 MHz.The RF spectrum in the range from 0-500 MHz was displayed in inset of Figure 5(e3).
Finally, we achieve the synchronized dual-wavelength conventional soliton mode-locking operation near 1530 and 1560 nm.After maintaining the pump power at 245.76 mW, the polarization state was carefully tuned to achieve the dual-wavelength conventional soliton mode-locking operation.The broad gain bandwidth of Er-doped fiber enabled the laser to produce synchronized conventional soliton pulses at two different central wavelengths [54].The dualwavelength mode-locking spectrum, shown in Figure 5(f1), displays two separated spectral peaks at 1531.4 nm and 1557.3 nm, with 3-dB bandwidths of 2.2 nm and 2.0 nm, respectively.It can be observed that continuous wave is present in the spectrum.Figure 5(f2) displays the pulse sequence at this point, with a pulse interval of 133.6 ns, agreeing with the conventional soliton mode-locking operation when the pump power was 191.76 mW.In Figure 5(f3), the SNR is about 50.2 dB.The fundamental repetition frequency is 7.487 MHz, and the inset displays the RF spectrum in the range from 0-500 MHz, indicating the stable modelocking operation.Owing to the wide gain band of EDFL and the high nonlinearity of SA, the laser has an approximate gain level near 1531.4nm and 1557.3 nm, and can realize the synchronized dual-wavelength pulse output under specific pump power and polarization state.Besides, the continuous light at the top of the spectrum is the result of the interaction between the birefringence effect of the fiber and the nonlinear optical effect of SA [55].

Synthetic methods
Yttrium chloride hexahydrate (YCl 3 •6H 2 O, Aladdin, 99.5 %) and PO (Aladdin, 99.5 %) were utilized as the primary raw materials in this process.To initiate the synthesis, YCl 3 •6H 2 O was dissolved in ethanol (EtOH, Fuyu, 99.5 %) within a polyethylene mold, and the resulting solution was stirred for 30 min until a clear sol solution was obtained.Following the sol preparation, the solution was then cooled in an ice bath until reaching a temperature of 0 • C. Subsequently, a measured quantity of PO was slowly added drop by drop to the sol.In this case, the YCl 3 •6H 2 O EtOH solution had a molar concentration of 0.5 mol/L.The final sol with the PO addition, the molar ratio of Y 3+ and PO was set as 1:4.After a 5-min stirring period, the solution was allowed to undergo gelation under standard room temperature conditions (25 • C, 1 bar) and aged for 1 h.During the gelation process, which lasted approximately 10 min, the flowable sol transformed into a gel structure.Two of three wet gels were solidified by immersing in 25 vol% and 50 vol% TEOS/EtOH solution for one day, respectively.To remove any impurities and by-products, all three wet gels were thoroughly washed with EtOH, a process repeated 6 times with 12-h intervals between each washing step.Finally, the wet gel was subjected to supercritical drying at 280 • C under a pressure of 100 bar for 24 h, employing an N 2 /EtOH supercritical atmosphere.This step allowed for the effective removal of the liquid phase, resulting in the formation of the desired Y 2 O 3 aerogel.The one without treatment in TEOS/EtOH solution, consisting of no silicon, was labeled as Y 2 O 3 -1 aerogel.The aerogels treated in 25 vol% and 50 vol% TEOS/EtOH solutions were denoted as Y 2 O 3 -2 and Y 2 O 3 -3 aerogels, respectively.

Fabrication of silicon-doped Y  O  aerogel saturable absorber
Amorphous silicon-doped Y 2 O 3 aerogel (Y 2 O 3 -3) was ground into the powder and added to anhydrous ethanol to create a suspension, from which the supernatant was collected.Then, the partial coating of a single-mode fiber (SMF-28e, Corning Inc.) was removed using wire strippers.Then, we fixed the ends of the stripped coated part of the fiber and heat the exposed part of the fiber with an alcohol lamp.In the meantime, the fiber was slowly stretched from both ends to create a tapered shape.The minimum waist diameter was 9.6 μm, and the length of the tapered section was 6.0 mm using a microscope.At this time, the insertion loss was measured to be about 97 %.Finally, the Y 2 O 3 supernatant was dropped onto the tapered fiber, while the CW laser beam at 974 nm propagated in the tapered fiber to promote the optical deposition on the tapered fiber.After deposition, the insertion loss of Y 2 O 3 -based saturable absorber was measured to be 98 %.The preparation process of the other two Y 2 O 3 -based SAs is the same as described above.In order to ensure the consistency of experimental conditions as much as possible, the insertion loss of the new tapered fibers was kept consistent with that of the tapered fiber mentioned above when we prepared them, and the insertion loss was still consistent after the deposition of the material.The preparation process is detailed in Supplementary Material.

Characterization
The X-ray diffraction patterns were obtained by an X-ray diffactometer (Shimadzu XRD-6100).The X-ray photoelectron spectrum was performed on a XPS system (Thermo fisher scientific K-alpha).The morphologic images were carried out with the scanning electron microscope (FEI Quanta 250 FEG) and the transmission electron microscope (FEI Tecnai G2 F20).The UV-VIS-IR absorption and the Fourier transformed infrared absorption were implemented with a UV-VIS-NIR spectrometry (Shimadzu UV-3600) and FTIR spectrometer (Thermo Fisher Scientific Nicolet iS 5).

In-line twin-detector system
To study the optical properties of the amorphous silicon-doped Y 2 O 3 aerogel, we characterized nonlinear absorption properties using a home-made twin-balanced-detector system.A passively NPE modelocked EDFL running at 1590 nm was utilized as the excitation laser with a pulse duration of 1.1 ps.Then in order to ensure the enough output power for the nonlinear optical absorption measurement, an erbium-doped fiber amplifier (EDFA) was applied to further boost the excitation intensity.A variable optical attenuator (VOA) was used to adjust the laser power input into an optical coupler (OC).The output beam from the OC was divided into two sub-beams by a 50:50 tap coupler: One sub-beam was directly detected with a power meter as the reference and the other beam entered an identical power meter after passing through the saturable absorber.The schematic diagram of in-line balanced twin-detector system is provided in Supplementary Material.

Laser configuration
The 974-nm laser diode (LD) emitted the pump beam into the laser cavity via a 980/1550 wavelength division multiplexer (WDM).The gain medium of the cavity was a 1.48-meter-long Erbium-doped fiber (EDF, Fibercore I-25) with a group velocity dispersion (GVD) of 40 ps 2 /km.A polarization-independent isolator (PI-ISO) and polarization controller (PC) were used to maintain unidirectional transmission of the laser beam and adjust the polarization, respectively.The total fiber length of the cavity was 27.31 m, including 25.83 m of single-mode fiber with a GVD of −22.3 ps 2 /km.A 10:90 tap coupler was used to filter out 10 % of the power and the rest of beam was reflected back to the laser resonator.To analyze the output mode-locked pulses, several measurement instruments were used, including an oscilloscope (MDO4104C, Tektronix Inc.), an InGaAs photodetector (3 GHz bandwidth), a spectrum signal analyzer (FPC1000, Rohde&Schwarz Inc.), a spectral analyzer (6375D, Yokogawa Inc.), and an autocorrelator (FR-103XL, Femtochrome Inc.).

Conclusions
In this paper, amorphous silicon-doped Y 2 O 3 aerogel was prepared by sol-gel method, and the microstructure, elemental composition and optical absorption characteristics were studied.The introducing Si can efficiently distort the Y 2 O 3 crystal structure, leading to the amorphous phase.Subsequently, the amorphous silicon-doped Y 2 O 3 aerogel was optically deposited on a tapered fiber as a saturable absorber for the ultrafast mode-locking operation.The largest modulation depth was 1.65 % with a saturation intensity of 0.78 MW/cm

Figure 1 (
b), which clearly show the distribution of elements Y, O, and Si.The typical SEM images with different resolution are displayed in Figure 1(c) and (d).Obviously, SEM images uncovered the irregular pores in the prepared Y 2 O 3 aerogel with a size of ∼30 nm.The inset of Figure 1(c) and (d) shows the overall morphology of the sample at a scale of 10 μm.

Figure 1 (
Figure 1(e) shows a typical TEM image captured at a scale of 100 nm, further revealing the porous structure of the Y 2 O 3 aerogel material.The SEM and TEM images of Y 2 O 3 -1 and Y 2 O 3 -2 can be seen in Figures S1 and S2.

Figure 1 :
Figure 1: Fabrication and Characterization.(a) Graphical illustration of the formation of Y 2 O 3 -3 gel networks.(b) The energy-dispersive X-ray spectroscopy (EDS) elemental mappings of amorphous Y 2 O 3 aerogel.(c) And (d) SEM images at different scales.(e) TEM image.
Figure 2(c) shows the distinct evidence of the existence of Y 3d, O 1s, and Si 2p peaks.For the Y 3d binding spectrum, there are two prominent peaks at 158.0 and 160.1 eV in Figure 2(d), which can be assigned to the divisive Y 3d 5/2 and Y 3d 3/2 states [46].Displayed in Figure 2(e), the XPS spectra of O can be deconvoluted into three peaks at 530.2, 531.5, and 532.8 eV, respectively.The binding energy of 530.2 eV came from the O-Y bond.While the other peaks at 531.5 and 532.8 eV indicated the presence of the Y-O-Si and O-Si bonds, suggesting the incorporation of Si during the material synthesis process.Furthermore, the only peak of Si 2p spectrum in Figure 2(f) is observed at 101.4 eV, which can be attributed to the inhomogeneous broadening in the non-crystalline aerogel.
The nonlinear optical (NLO) responses of Y 2 O 3 -based SAs show that they have admirable saturable absorption characteristics.Therefore, to compare their potential in ultrafast photonics applications, they were coupled into the ring Er-doped fiber laser cavity operating in the super C band.It is worth noting that hereby we only demonstrated the mode-locking operation with the Y 2 O 3 -3 aerogel SA, owing to the large modulation depth.The detailed experimental results from the passively mode-locked EDFLs with Y 2 O 3 -1 and Y 2 O 3 -2 aerogels SAs are attached in the Supplementary Material.
2, demonstrating the favorable nonlinear optical properties of the Y 2 O 3 -based saturable absorber.Compared with previously published Y 2 O 3 thin film based saturable absorbers, Y 2 O 3 aerogel saturable absorbers enhance light-matter interactions due to their high porosity and large surface area, enabling a variety of mode-locking operations.These results show that amorphous silicon-doped Y 2 O 3 aerogel is a highly promising optical material and will be extensively studied and applied in the field of ultrafast photonics in the near future.This reminds us that in future work, by doping different elements or doping different amounts of the same element into amorphous Y 2 O 3 aerogel material, we can enhance its optical properties to make it more in line with people's expectations.This work was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 12274263, 12004213, 12174223, 52072351, 62175128, 12304466), and Shandong Provincial Science and Technology Demonstration Project (2022SFGC0401).Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.Conflict of interest: Authors state no conflicts of interest.Data availability: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.