Open Access
Add to BibSonomy
Abstract
Phase (composition) is known to play a key role in determining the electronic and optical properties of amorphous oxide semiconductors. In this work, modulating the ultrafast nonlinear optical (NLO) response of SnO2 and SnO thin films by tuning oxygen partial pressure during film sputtering is explored. Femtosecond Z-scan results demonstrate that intermediate phases have no profound impact on the two-photon absorption (TPA) response of SnO2 and SnO films. Interestingly, the magnitude of the effective nonlinear absorption coefficient (βeff) of both intermediate SnO2-x and SnOx are enhanced after the change of Sn2+/Sn4+ composition ratio, as measured by picosecond Z-scan technique. Femtosecond degenerate pump-probe measurements show that intermediate phases accelerate the carrier trapping and improve the defect-related carrier absorption in SnOx (SnO-rich) film, while intermediate phase suppress the TPA response of SnO2-x (SnO2-rich) films, therefore carrier-induced absorption dominates the NLO behavior of SnO2-x film on picosecond regime. Our results indicate a simple and effective way to modulate the NLO response of transparent conductive oxide SnO2 and SnO.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Nonlinear optical (NLO) materials with high NLO coefficient and ultrafast response speed have been proven indispensable in various fields such as optical limiting, all-optical switching, optical storage, laser micro-processing, etc [1–3]. Among the various NLO materials, inorganic semiconductors show great potential in NLO applications due to their large nonlinear absorption (NLA) response, remarkable compositional/structural flexibility, and excellent CMOS platform compatibility [4–6]. Manipulation of the NLA response of inorganic semiconductors is essential for developing practical NLO devices. Therefore, considerable effects have been devoted to modulating and enhancing the NLA performance of inorganic semiconductors via surface functional group [7], heteroatom doping [8–10], quantum confinement effect [11,12], phase transition [13,14], etc. However, these methods often require sophisticated fabrication process. As a result, it would be highly attractive to develop a simple and low-cost method that allows for the effective manipulation of NLA response.
Semiconductor tin oxide is known to exist in two stoichiometric phases, n-type SnO2 (stannic oxide) and p-type SnO (tin monoxide). Due to the multivalence nature of tin ions (Sn2+ and Sn4+), a wide range of non-stoichiometric phases SnOx (1 < x < 2) could be stabilized, which held great potential in solar cell, photocatalysis, gas sensing, lithium battery anodes, thin film transistors, etc [15]. It is well known that the NLO response of metal oxide thin films can be fine-tuned via film thickness, micro-structure, deposition conditions, etc [16–18]. Therefore, a wide range of methods have been applied to manipulate the NLO response of tin oxides, including electrical bias, defect engineering, reducing size, ultraviolet radiation, etc [19–28]. Despite these effects, however, most of the previous reports were concentrated on n-type SnO2, and the laser sources used in these previous reports were nanosecond or CW laser. The research concerning optical nonlinearity of tin oxides on femtosecond or picosecond time regimes remains rare [19–21,29]. And the physical mechanism of NLO response of tin oxides is still elusive.
Herein, we report the impact of intermediate phases on the ultrafast third-order NLA response of SnO2 and SnO thin films. The intermediate phases were introduced via tuning the oxygen partial pressure during film sputtering, which is known to be the key factor in the formation of mixed-phases SnOx (1 < x < 2) during film deposition [30,31]. The SnO and SnOx films show no NLA response under 515 nm, 190 fs laser excitation, while the mixed-phase SnO2-x is found to have a smaller two-photon absorption (TPA) coefficient than pristine SnO2 film on femtosecond timescale. Meanwhile, the nonlinear absorption coefficient is determined to be 1.7 × 10−8 m/W and 1.4 × 10−7 m/W for mixed-phase SnO2-x and SnOx films by using Z-scan measurement with 532 nm, 19 ps laser pulses, which is 1.6 and 2 times larger than pristine SnO2 and SnO film, respectively. The photo-physical mechanism of intermediate phases on nonlinear absorption dynamics are studied by femtosecond degenerate pump-probe measurements. Our result not only demonstrates that the phase engineered SnOx films as a promising candidate for NLA applications, but also indicate a new way for manipulating the NLO performance of oxide semiconductors.
2. Experimental method
The stoichiometric SnO2 and SnO thin films were prepared on quartz substrate using reactive rf magnetron sputtering method under oxygen partial pressure 12.5% and 9.9%, respectively. The non-stoichiometric mixed-phase SnO2-x (SnO2-rich) and SnOx (SnO-rich) thin films were prepared with oxygen partial pressure 11% and 8%, respectively. Films with a thickness ∼ 220 nm were obtained by a deposition time of 30 minutes. After deposition, all films were subjected to a thermal annealing at 200°C for 2 h in air. The phase composition of the sample films was characterized by X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractometer). The optical properties were analyzed by a Jasco V-570 UV/Vis/NIR spectrophotometer. The surface morphology of SnOx films are analyzed by using atomic force microscopy (AFM). Details about the fabrication procedure and film characterizations (Raman spectra, XPS, etc.) can be found in the previous report [30].
The nonlinear absorption properties of SnOx thin films were measured by using open-aperture Z-scan technique. The experimental setup can be found elsewhere [32]. The femtosecond (fs) laser pulses were delivered by a mode-locked Yb:KAG fiber laser (Light Conversion, PHAROS-SP). The output wavelength, pulse duration and repetition rate was 515 nm, 190 fs, 20 Hz, respectively. The output laser beam was focused by a convex lens of 20 cm focal length to a beam waist of ∼22 μm. The transmitted intensity from the samples as a function of the sample's position relative to the focus was measured by the power meter (Laser probe, Rj-735). The picosecond (ps) laser pulses were emitted from a mode-locked Nd:YAG solid laser (EKSPLA, PL2143B). The output wavelength, pulse duration and repetition rate was 532 nm, 19 ps, 10 Hz, respectively. The experimental setup is similar to the fs Z-scan measurement.
The ultrafast dynamics of SnOx thin films were recorded by a home-built degenerate pump-probe system. The experimental setup is similar to the one reported in the literature [32]. The fs laser source was the same used in femtosecond Z-scan measurements. The output laser beam (515 nm) was split by a 1/9 ratio into two parts, which were used as the pump and the probe beam. The delay time between the pump and probe beam was controlled by a motorized stage, and the differential transmittance signal (ΔT/T) at various time delay was detected by a Si photodetector.
3. Results and discussions
3.1 Sample characterization
Figure 1(a) shows the XRD patterns of stoichiometric SnO2, SnO and mixed-phase SnO2-x (SnO2-rich) and SnOx (SnO-rich) thin films. Clear diffraction peaks were observed at 29.89°, 33.48°, 48.31° and 57.71° in the pristine SnO film, which could be assigned to the crystal orientations (101), (110), (200) and (211) of a-SnO phase (P4/nmm, JCPDS card No. 06-0395), respectively. However, no individual crystalline peaks were observed in the other three films, indicating the SnO2, SnO2-x and SnOx films have amorphous structure. These results agree well with the previous reports [30,31]. Figure 1(b) illuminates the steady-State UV-Vis transmittance spectra of the SnOx films. Interference fringes are visible for all four films, indicating the SnOx films are highly uniform. It is found that the SnOx (SnO-rich) films have lower transmittance compared to the stoichiometric SnO films, which could be attributed to the amorphous structure or high defect concentration. The Tauc's plot (Fig. 1(c)) clearly demonstrate that the film’s bandgap gradually decrease from 3.5 eV (SnO2) to 2.7 eV (SnO), which are in agreement with the values reported in the literature [30]. The AFM images (see Fig. S1-S4 in Supplement 1) show the root mean square (rms) surface roughness values of the SnO2, SnO2-x (SnO2-rich), SnO and SnOx (SnO-rich) films are 0.19 nm, 0.20 nm, 1.28 nm and 0.93 nm, respectively. Meanwhile, the AFM images show that the surface of SnO film has small grain structures while the surface of other three films are smooth. These results agree well with the XRD data of SnO (polycrystalline) and SnO2, SnO2-x, SnOx (amorphous).
Fig. 1. (a) XRD patterns, (b) Transmittance spectra and (c) Tauc’s plot of stoichiometric SnO2, SnO and mixed-phase SnO2-x (SnO2-rich) and SnOx (SnO-rich) thin films, respectively.
3.2 Nonlinear absorption properties of SnOx thin films
To investigate the impact of mixed phase on the NLA responses of SnOx thin films, open-aperture (OA) Z-scan technique were carried out with femtosecond and picosecond laser pulses. Figure 2 presents the OA Z-scan results of SnOx films under 190 fs, 515 nm laser excitation. The incident pulse energy in the fs Z-scan measurement was 92 nJ, corresponding to the peak laser intensity of 53 GW/cm2. The Z-scan curves of SnO2 and SnO2-x films show a symmetrical valley at the focus (z = 0), indicating that the normalized transmittance decreases with the increase of incident laser intensity. Because the input photon energy (2.41 eV) is less than the bandgap of SnO2 and SnO2-x, we attribute the NLO mechanism to the two-photon absorption (TPA) effect. Meanwhile, the SnO and SnOx films show no peak or valley near the focus, meaning these two films have no optical nonlinearity with fs laser excitation at 515 nm. We did not observe any NLA response from the quartz substrates, indicating that the TPA response originated from the tin oxide films. The Z-scan measurements were repeated on multiple spots of the sample film to verify that the experimental results are repeatable. We also performed the close aperture (CA) Z-scan measurements on the four films. The normalized valley-peak signals of the SnOx films are found to be overlapped with the quartz substrate, indicating the nonlinear refraction response of the sample film can be negligible. The values of nonlinear absorption coefficients (βeff) can be derived by data fitting using the theoretical equations in Ref. [33]. In addition, the imaginary part of the third-order nonlinear susceptibility Im χ(3) and figure of merit (FOM) for the third-order optical nonlinearity can be deduced through the equation ${\mathop{\rm Im}\nolimits} {\chi ^{(3)}} = \frac{{{n^2}{c^2}}}{{240{\pi ^2}\omega }}\beta (m/W)$and $FO{M_{{\mathop{\rm Im}\nolimits} }} = |{{{{\mathop{\rm Im}\nolimits} {\chi^{(3)}}} / {{\alpha_0}}}} |$, respectively [34]. All NLO parameters of SnOx films are summarized in Table 1. The NLO parameters of other semiconductor materials are also listed in Table 1 for comparison [35,36]. Note that the results in Table 1 clearly demonstrate that the SnO2 and SnO2-x films have strong NLO response on femtosecond timescale, and intermediate phases have no pronounced effect on the TPA response of SnO2 and SnO films.
Fig. 2. Femtosecond open-aperture Z-scan data of the stoichiometric SnO2, SnO and mixed-phase SnO2-x (SnO2-rich) and SnOx (SnO-rich) thin films measured at 515 nm.
Table 1. NLO parameters of SnOx thin films and other inorganic materials on femtosecond regime
To explore the influence of laser-pulse-width on the optical nonlinearity, we examine the nonlinear absorption of the SnOx films via OA Z-scan method with 532 nm, 19 ps laser pulse excitation. The peak laser intensity in ps Z-scan measurement was kept below 5.7 GW/cm2 to exclude the laser-induced damage effects. Both SnO2 and SnO2-x films demonstrate a symmetrical valley at the focus (z = 0), as shown in Fig. 3(a) and 3(b), which are similar to the femtosecond laser excitation scenario. Interestingly, the SnO2-x film shows a large decrease in transmittance at the focus (z = 0), indicating the NLA response of SnO2-x film is stronger than SnO2 film on picosecond timescale. Meanwhile, the OA Z-scan curve of pristine SnO (Fig. 3(c)) exhibits two small peaks around a symmetrical valley at the focus (z = 0), indicating the OA signals of pristine SnO consist of both saturable absorption (SA) and reverse saturable absorption (RSA) response [10,37]:
where α(I) is the total absorption coefficient, α0 is the linear absorption coefficient, I is the input laser intensity, Is is the saturation intensity, and β is the effective TPA coefficient. Hence, Fig. 3(c) is fitted by using Eq. (1). The saturation intensity Is is estimated to be 1.69 GW/cm2, which is similar to the previous report of graphene oxide and halide perovskite under picosecond laser excitation [38,39]. The OA Z-scan results of mixed-phase SnOx film (Fig. 3(d)) clearly show a symmetrical valley at the focus, suggesting that mixed-phase SnOx film has reverse saturable absorption (RSA) on picosecond timescale [33]:Fig. 3. Picosecond open-aperture Z-scan data of the stoichiometric SnO2, SnO and mixed-phase SnO2-x (SnO2-rich) and SnOx (SnO-rich) thin films measured at 532 nm.
Figures 3(a), 3(b) and 3(d) are fitted by using Eq. (2). Close aperture (CA) Z-scan measurements were carried out on the four films, and the CA Z-scan results show that all four films have negligible nonlinear refraction response on picosecond timescale. The NLO coefficient (βeff) obtained from the fitting are summarized in Table 2 [40–43]. In our ps Z-scan measurement, the pulse repetition rate of the laser source is 10 Hz. According to the previous research on thermal nonlinearity [44,45], this low repetition rate is not sufficient to produce the accumulated thermal effects. Therefore, the thermal nonlinearity mechanism is negligible in our ps Z-scan measurements.
Table 2. NLO parameters of SnOx thin films and other NLO materials on picosecond regime
The results in Table 2 not only demonstrate that intermediate phases can enhance the NLO response of SnO2 and SnO films on picosecond time regime, but also show the SnOx films with Sn2+ valence state have stronger NLO response than the film with Sn4+ valence state. These results indicate that the NLO mechanism of SnOx films on femtosecond and picosecond timescale is different. The peak laser intensity used in our fs and ps Z-scan measurements were ∼53 GW/cm2 and <5.7 GW/cm2, respectively. The corresponding laser fluence in fs and ps Z-scan experiments were estimated to be 0.006 J/cm2 and 0.0649 J/cm2, respectively. According to the previous literatures on ultrafast laser induced phenomena in solids, the threshold fluence for laser induced structure or ablation is about 0.5-1 J/cm2 [46–49]. Therefore, our Z-scan measurements were carried out in the excitation regime below ablation threshold [16,18,37,38,43,50]. Our previous report also found that the Raman spectra of the SnOx films remain constant after multiple experiments in which the test were conducted from low optical power to high optical power then low optical power again [30]. As a result, the ireversible laser-induced structure change is negligible under our experimental condition.
3.3 Pump-probe dynamics of SnOx thin films
To reveal the NLO mechanism of single-phase and mixed-phase SnOx films, degenerate pump-probe measurement was employed to investigate the ultrafast dynamics. In our measurement, the pump fluences were kept below 100 μJ/cm2 to suppress the high-order carrier recombination processes. Figure 4 shows the pump-probe decay curves of the single-phase SnO2, SnO and mixed-phase SnO2-x (SnO2-rich), SnOx (SnO-rich) thin films, respectively. The decay curves of SnO2 and mixed-phase SnO2-x show an instantaneous dip of transmittance near zero-time-delay and a slow recovery on hundreds of ps timescale. Because the bandgaps of SnO2 and SnO2-x are larger than the pump photon energy (515 nm, 2.41 eV), the instantaneous decay is attributed to the two-photon absorption (TPA) effect. And the slow decay component is related to the TPA-induced free carrier induced absorption (FCA) [32,51]. It is evident that intermediate phases SnO2-x has smaller TPA response than the single-phase SnO2 film, which is in line with the femtosecond Z-scan results. The pump-probe decay curves of SnO2 and SnO2-x can be fitted with a tri-exponential model [52], and the fitting results are summarized in Table 3. The ultrafast, intermediate and slow decay component can be attributed to the carrier-phonon scattering, trapping into shallow trap states and inter-band recombination, respectively. These results coincide with other oxide semiconductors reported previously [52,53]. On the other hand, the decay curve of SnO manifested a photobleaching (decrease of absorption) process within 5 ps after pump excitation, then the photobleaching gradually change to a long-lived photoinduced absorption (increase of absorption) signal with lifetime of ∼ns time scale. The observed photobleaching signal could be related to the Pauli blocking of fundamental (indirect Eg∼0.7 eV) bandgap of SnO. The decay of photobleaching can be fitted with a double-exponential model, and the slow photoinduced absorption can be fitted with a single-exponential model. This result is similar with the previous report [52]. However, the decay curve of SnOx (Fig. 4(d)) shows a rapid rise of photoinduced absorption after excitation, and the lifetime of this photoinduced absorption process is rather long (on ns time scale). The rise time of photoinduced absorption in mixed-phase SnOx can be fitted to be ∼0.7 ps, which can be attributed to the carrier trapping into trap states in SnOx. Interestingly, the amplitude and decay lifetime of photoinduced absorption signal in SnOx are found to be distinctively different with that of the SnO film, indicating that trap states induced by intermediate phases play a key role in the FCA response of SnOx (SnO-rich) films.
Fig. 4. Femtosecond degenerate pump-probe decay dynamics for stoichiometric SnO2, SnO and mixed-phase SnO2-x (SnO2-rich) and SnOx (SnO-rich) thin films. The solid red curves are the fits to the experimental data. The left half shows the time window between 0 to 26 ps, and the right half shows 30-1500 ps. The green line is the instrument response function (IRF).
Table 3. Summary of fitting parameters for pump-probe decay of SnOx thin films
Previous reports show that oxygen vacancies and Sn vacancies are the dominant native defects in the SnO2 (Sn4+) and SnO (Sn2+) phase, respectively. These defect states could induce shallow levels within the bandgap, thus enhancing the absorption in the visible light region [54,55]. Our self-doping strategy induced by fine tuning the phase/stoichiometry of SnOx is advantageous compared with foreign ion dopants because (a) the cost of thin-film fabrication can be reduced and (b) the structural defects in the fabricated thin-film can be minimized. The pump-probe results demonstrate that the photo-physical mechanism of SnO and SnOx film is the slow FCA rather than the instantaneous TPA, which agrees with the absence of optical nonlinearity on fs time regime. These results indicate that the SnO2 and SnO2-x films with instantaneous TPA response is suitable for ultrafast optical devices on femtosecond time regime, while the SnO and SnOx films in which the FCA response is dominant are promising candidates for optical limiters on picosecond and nanosecond timescale.
4. Conclusion
In summary, we systematically fabricated the single-phase SnO2, SnO and mixed-phase SnO2-x (SnO2-rich), SnOx (SnO-rich) thin films with different stoichiometry by tuning oxygen partial pressure during film sputtering. The pulse width-dependent NLO properties of SnOx films were studied by using the open-aperture Z-scan method with femtosecond and picosecond laser pulses. The SnO2 and SnO2-x films exhibit good TPA response under femtosecond laser excitation (∼10−10 m/W), while SnO and SnOx films demonstrate no TPA response on fs time regime. However, all four films exhibit large RSA response under picosecond laser excitation (∼10−9 to 10−7 m/W), and intermediate phase is found to have a significant impact on the NLA properties of SnOx films on ps time regime. Femtosecond degenerate pump-probe measurements reveal that the carrier dynamics of SnO2 and SnO2-x films consists of TPA and FCA processes under 515 nm excitation, and the TPA process is suppressed in the mixed-phase SnO2-x film. The carrier dynamics of SnO film consist of SA and FCA process under 515 nm excitation, however, the dynamics changes to only FCA for SnOx film due to the defect-induced trapping and trapped carrier-induced absorption process. Our results manifested the NLO response and photo-physical mechanism of SnOx films, which shed new light on the development of NLO devices based on amorphous oxide semiconductors.
Funding
National Natural Science Foundation of China (11704048, 62174016); Qinglan Project of Jiangsu Province of China (SZ2022002); Ningbo Key Scientific and Technological Project (2021Z116); Suzhou Science and Technology Project (SZS2020313).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. O. Reshef, I. D. Leon, M. Z. Alam, and R. W. Boyd, “Nonlinear optical effects in epsilon-near-zero media,” Nat. Rev. Mater. 4(8), 535–551 (2019). [CrossRef]
2. M. Taghinejad and W. Cai, “All-Optical Control of Light in Micro- and Nanophotonics,” ACS Photonics 6(5), 1082–1093 (2019). [CrossRef]
3. X. Zhao, H. Jin, J. Liu, J. Chao, T. Liu, H. Zhang, G. Wang, W. Lyu, S. Wageh, O. A. Al-Hartomy, A. G. Al-Sehemi, B. Fu, and H. Zhang, “Integration and Applications of Nanomaterials for Ultrafast Photonics,” Laser Photonics Rev. 16(11), 2200386 (2022). [CrossRef]
4. Y. Zhou, Y. Huang, X. Xu, Z. Fan, J. B. Khurgin, and Q. Xiong, “Nonlinear optical properties of halide perovskites and their applications,” Appl. Phys. Rev. 7(4), 041313 (2020). [CrossRef]
5. M. Li, L. Zhang, L. M. Tong, and D. X. Dai, “Hybrid silicon nonlinear photonics,” Photonics Res. 6(5), B13–B22 (2018). [CrossRef]
6. X. Liu, Q. Guo, and J. Qiu, “Emerging Low-Dimensional Materials for Nonlinear Optics and Ultrafast Photonics,” Adv. Mater. 29(14), 1605886 (2017). [CrossRef]
7. H. Li, S. Chen, D. W. Boukhvalov, Z. Yu, M. G. Humphrey, Z. Huang, and C. Zhang, “Switching the Nonlinear Optical Absorption of Titanium Carbide MXene by Modulation of the Surface Terminations,” ACS Nano 16(1), 394–404 (2022). [CrossRef]
8. M. Diao, H. Li, X. Gao, R. Hou, Q. Cheng, Z. Yu, Z. Huang, and C. Zhang, “Giant Nonlinear Optical Absorption of Ion-Intercalated Tin Disulfide Associated with Abundant In-Gap Defects,” Adv. Funct. Mater. 31(49), 2106930 (2021). [CrossRef]
9. J. Liu, F. Yang, J. Lu, S. Ye, H. Guo, H. Nie, J. Zhang, J. He, B. Zhang, and Z. Ni, “High output mode-locked laser empowered by defect regulation in 2D Bi2O2Se saturable absorber,” Nat. Commun. 13(1), 3855 (2022). [CrossRef]
10. X. Zhang, S. Zhang, Y. Xie, J. Huang, L. Wang, Y. Cui, and J. Wang, “Tailoring the nonlinear optical performance of two-dimensional MoS2 nanofilms via defect engineering,” Nanoscale 10(37), 17924–17932 (2018). [CrossRef]
11. L. Wang, S. Zhang, N. McEvoy, Y. Sun, J. Huang, Y. Xie, N. Dong, X. Zhang, I. M. Kislyakov, J. M. Nunzi, L. Zhang, and J. Wang, “Nonlinear Optical Signatures of the Transition from Semiconductor to Semimetal in PtSe2,” Laser Photonics Rev. 13(8), 1900052 (2019). [CrossRef]
12. T. He, J. Li, X. Qiu, S. Xiao, C. Yin, and X. Lin, “Highly Enhanced Normalized-Volume Multiphoton Absorption in CsPbBr3 2D Nanoplates,” Adv. Optical Mater. 6(21), 1800843 (2018). [CrossRef]
13. D. Zhao, D. Zhang, Y. Yang, X. Yin, X. Liu, and J. Qiu, “Enhancement and Inversion of Absorptive Nonlinearity Induced by Topochemically Controlled Insulator-to-Metal Transition,” J. Phys. Chem. C 125(48), 27023–27031 (2021). [CrossRef]
14. Y. Pepe, Y. Tutel, E. A. Yildiz, A. Karatay, H. E. Unalan, and A. Elmali, “Thermally Induced Phase Transition and Defect-Assisted Nonlinear Absorption and Optical Limiting in Nanorod Morphology V2O5 Thin Films,” Adv. Eng. Mater. 23(10), 2100468 (2021). [CrossRef]
15. J. Shi, J. Zhang, L. Yang, M. Qu, D. C. Qi, and K. H. L. Zhang, “Wide Bandgap Oxide Semiconductors: from Materials Physics to Optoelectronic Devices,” Adv. Mater. 33(50), 2006230 (2021). [CrossRef]
16. S. Li, X. L. Zhong, G. H. Cheng, X. Liu, Y. Zhang, J. B. Wang, H. J. Song, C. B. Tan, and B. Li, “Nonlinear optical absorption tuning in Bi3.15Nd0.85Ti3O12 ferroelectric thin films by thickness,” Appl. Phys. Lett. 106(14), 142904 (2015). [CrossRef]
17. W. A. Britton, F. Sgrignuoli, and L. Dal Negro, “Structure-dependent optical nonlinearity of indium tin oxide,” Appl. Phys. Lett. 120(10), 101901 (2022). [CrossRef]
18. K. Y. Lau, Y. Yang, D. Zhao, X. Liu, and J. Qiu, “Tunable optical nonlinearity of indium tin oxide for optical switching in epsilon-near-zero region,” Nanophotonics 11(18), 4209–4219 (2022). [CrossRef]
19. R. Hou, H. Li, Y. Sun, M. Diao, Y. Liang, Z. Huang, J. Wang, M. G. Humphrey, and C. Zhang, “Electrical Tuning of the Fifth-Order Optical Nonlinearity of Antimony-Doped Tin Oxide,” Adv. Optical Mater. 9(2), 2001357 (2021). [CrossRef]
20. J. B. Han, H. J. Zhou, and Q. Q. Wang, “Conductivity and optical nonlinearity of Sb doped SnO2 films,” Mater. Lett. 60(2), 252–254 (2006). [CrossRef]
21. H. Zhu, J. Huang, J. Li, Y. He, L. Chen, J. Hu, L. Miao, Y. Xu, and C. Zhao, “Femtosecond Z-scan measurement of third-order nonlinear optical response of fluorine-doped tin oxide,” Appl. Phys. Express 15(6), 061004 (2022). [CrossRef]
22. A. Clementi, N. Chiodini, and A. Paleari, “Cubic optical nonlinearity in nanostructured SnO2,” Appl. Phys. Lett. 84(6), 960–962 (2004). [CrossRef]
23. Y. Pepe, A. Karatay, Y. O. Donar, S. Bilge, E. A. Yildiz, M. Yuksek, A. Sinag, and A. Elmali, “Effect of Cr/Sb doping and annealing on nonlinear absorption coefficients of SnO2/PMMA,” Mater. Chem. Phys. 255, 123596 (2020). [CrossRef]
24. Y. Watanabe, M. Ohnishi, M. Nakazawa, and T. Tsuchiya, “Two-photon absorption in tin dioxide single crystal at 532 nm,” Jpn. J. Appl. Phys. 35(9R), 4681–4682 (1996). [CrossRef]
25. S. P. Sheikh, V. R. Reddy, and P. Sen, “Size-dependent nonlinear optical absorption in SnO2 quantum dots,” J Opt. Soc. Am. B 37(9), 2788–2795 (2020). [CrossRef]
26. S. Yadav, S. Kumari, S. K. Ghoshal, R. Kumar, S. K. Chaudhary, and D. Mohan, “Effect of ultraviolet radiation exposure on optical nonlinearity and switching traits of SnO2 thin films deposited by thermal evaporation,” Opt. Laser Technol. 133, 106575 (2021). [CrossRef]
27. M. Shkir, M. T. Khan, V. Ganesh, I. S. Yahia, B. Ul Haq, A. Almohammedi, P. S. Patil, S. R. Maidur, and S. AlFaify, “Influence of Dy doping on key linear, nonlinear and optical limiting characteristics of SnO2 films for optoelectronic and laser applications,” Opt. Laser Technol. 108, 609–618 (2018). [CrossRef]
28. M. S. Bannur, A. Antony, K. I. Maddani, A. Ani, P. Poornesh, A. Rao, K. S. Choudhari, and S. D. Kulkarni, “Improved nonlinear absorption mechanism of tin oxide thin films: Role of strontium doping,” Opt. Mater. 94, 294–298 (2019). [CrossRef]
29. Z. G. Li, L. Y. Liang, H. T. Cao, and Y. L. Song, “Impact of stoichiometry on the linear and nonlinear optical response of SnOx thin films,” Journal of Physics: Conf. Series 844, 012017 (2017). [CrossRef]
30. H. Luo, L. Y. Liang, H. T. Cao, Z. M. Liu, and F. Zhuge, “Structural, Chemical, Optical, and Electrical Evolution of SnOx Films Deposited by Reactive rf Magnetron Sputtering,” ACS Appl. Mater. Interfaces 4(10), 5673–5677 (2012). [CrossRef]
31. J. A. Caraveo-Frescas, P. K. Nayak, H. A. Al-Jawhari, D. B. Granato, U. Schwingenschlogl, and H. N. Alshareef, “Record Mobility in Transparent p-Type Tin Monoxide Films and Devices by Phase Engineering,” ACS Nano 7(6), 5160–5167 (2013). [CrossRef]
32. Y. Sun, Z. G. Li, Y. Fang, X. Wu, W. Zhou, Z. Jia, J. Yang, and Y. Song, “Three-photon absorption and Kerr nonlinearity in pristine and doped β-Ga2O3 single crystals,” Appl. Phys. Lett. 120(3), 032101 (2022). [CrossRef]
33. M. Sheik-Bahae, A. A. Said, T.-H. Wei, D. J. Hagan, and E. W. Van Stryland, “Sensitive measurement of optical nonlinearities using a single beam,” IEEE J. Quantum Electron. 26(4), 760–769 (1990). [CrossRef]
34. N. Dong, Y. Li, S. Zhang, N. McEvoy, X. Zhang, Y. Cui, L. Zhang, G. S. Duesberg, and J. Wang, “Dispersion of nonlinear refractive index in layered WS2 and WSe2 semiconductor films induced by two-photon absorption,” Opt. Lett. 41(17), 3936–3939 (2016). [CrossRef]
35. C. Lu, D. Yang, J. Ma, M. Luo, Y. Jin, and X. Xu, “Effect of surface oxidation on nonlinear optical absorption in WS2 nanosheets,” Appl. Surf. Sci. 532, 147409 (2020). [CrossRef]
36. X. Xin, F. Liu, X. Q. Yan, W. Hui, X. Zhao, X. Gao, Z. B. Liu, and J. G. Tian, “Two-photon absorption and non-resonant electronic nonlinearities of layered semiconductor TlGaS2,” Opt. Express 26(26), 33895–33905 (2018). [CrossRef]
37. J. Zhang, H. Ouyang, X. Zheng, J. You, R. Chen, T. Zhou, Y. Sui, Y. Liu, X. Cheng, and T. Jiang, “Ultrafast saturable absorption of MoS2 nanosheets under different pulse-width excitation conditions,” Opt. Lett. 43(2), 243–246 (2018). [CrossRef]
38. H. Shi, C. Wang, Z. Sun, Y. Zhou, K. Jin, S. A. T. Redfern, and G. Yang, “Tuning the nonlinear optical absorption of reduced graphene oxide by chemical reduction,” Opt. Express 22(16), 19375–19385 (2014). [CrossRef]
39. R. Zhang, J. Fan, X. Zhang, H. Yu, H. Zhang, Y. Mai, T. Xu, J. Wang, and H. J. Snaith, “Nonlinear Optical Response of Organic-Inorganic Halide Perovskites,” ACS Photonics 3(3), 371–377 (2016). [CrossRef]
40. S. Bikorimana, P. Lama, A. Walser, R. Dorsinville, S. Anghel, A. Mitioglu, A. Micu, and L. Kulyuk, “Nonlinear optical responses in two-dimensional transition metal dichalcogenide multilayer: WS2, WSe2, MoS2 and Mo0.5W0.5S2,” Opt. Express 24(18), 20685–20695 (2016). [CrossRef]
41. Q. Zhou, C. Wang, and A. H. Shen, “Copper Nanoparticles Embedded in Natural Plagioclase Mineral Crystals: In Situ Formation and Third-Order Nonlinearity,” J. Phys. Chem. C 126(1), 387–395 (2022). [CrossRef]
42. Z. Li, C. He, J. Fan, Y. Wu, Y. Liu, J. Feng, and Y. Song, “Optical nonlinearities and excited state dynamics of self-assembled cobalt phthalocyanine multilayer films,” Mater. Lett. 221, 279–281 (2018). [CrossRef]
43. C. Torres-Torres, B. A. Can-Uc, R. Rangel-Rojo, L. Castaneda, R. Torres-Martinez, C. I. Garcia-Gil, and A. V. Khomenko, “Optical Kerr phase shift in a nanostructured nickel-doped zinc oxide thin solid film,” Opt. Express 21(18), 21357–21364 (2013). [CrossRef]
44. A. Gnoli, L. Razzari, and M. Righini, “Z-scan measurements using high repetition rate lasers: how to manage thermal effects,” Opt. Express 13(20), 7976–7981 (2005). [CrossRef]
45. N. Wickremasinghe, X. Wang, H. Schmitzer, and H. P. Wagner, “Eliminating thermal effects in z-scan measurements of thin PTCDA films,” Opt. Express 22(20), 23955–23964 (2014). [CrossRef]
46. S. K. Sundaram and E. Mazur, “Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses,” Nat. Mater. 1(4), 217–224 (2002). [CrossRef]
47. G. E. Hallum, D. Kurschner, D. Redka, D. Niethammer, W. Schulz, and H. P. Huber, “Time-resolved ultrafast laser ablation dynamics of thin film indium tin oxide,” Opt. Express 29(19), 30062–30076 (2021). [CrossRef]
48. L. Gallais and M. Commandre, “Laser-induced damage thresholds of bulk and coating optical materials at 1030 nm, 500 fs,” Appl. Opt. 53(4), A186–A196 (2014). [CrossRef]
49. Y. Kanemitsu, I. Nakada, and H. Kuroda, “Picosecond laser induced anomalous crystallization in amorphous silicon,” Appl. Phys. Lett. 47(9), 939–941 (1985). [CrossRef]
50. M. Chen, J. Shao, Y. Zhao, G. Hu, M. Zhu, Y. Chai, K. Zhang, and H. Ma, “High-sensitivity measurements of the nonlinear absorption coefficient of wide bandgap oxide thin films with the Z-scan method,” Opt. Mater. Express 12(2), 533–544 (2022). [CrossRef]
51. S. Q. Li, C. B. Yao, Y. Cai, Y. Han, K. X. Zhang, X. Wen, H. T. Yin, Q. H. Li, and W. J. Sun, “Nonlinear absorption properties and excited-state charge-transfer dynamics of Er doped ZnO films,” Opt. Mater. Express 8(11), 3262–3276 (2018). [CrossRef]
52. Z. G. Li, L. Liang, H. Cao, Z. Xiao, X. Wu, Y. Fang, J. Yang, T. H. Wei, and Y. L. Song, “Ultrafast carrier dynamics in SnOx thin films,” Appl. Phys. Lett. 106(10), 102103 (2015). [CrossRef]
53. L. Shenje, S. Larson, Y. Zhao, and S. Ullrich, “Composition Effects on Ultrafast Optical Properties of CuxOy Thin Films: A Transient Absorption Study,” J. Phys. Chem. C 124(45), 24908–24918 (2020). [CrossRef]
54. J. Wang, N. Umezawa, and H. Hosono, “Mixed Valence Tin Oxides as Novel van der Waals Materials: Theoretical Predictions and Potential Applications,” Adv. Energy Mater. 6(1), 1501190 (2016). [CrossRef]
55. D. Han, B. Jiang, J. Feng, Y. Yin, and W. Wang, “Photocatalytic Self-Doped SnO2-x Nanocrystals Drive Visible-Light Responsive Color Switching,” Angew. Chem. Int. Ed. 56(27), 7792–7796 (2017). [CrossRef]
