Large Nonlinear Optical Response Research Articles

Giant nonlinear optical response of fullerene polymer fragments: a DFT perspective

Organic π-conjugated materials exhibit exceptional nonlinear optical (NLO) properties due to their unique electronic structures, characterized by short response times and large NLO responses. Fullerene (C60) is one of the few polymer species that possess a rich π-conjugated system, making it a promising material with significant NLO responses. In the present paper, we designed three C60 polymer fragments and employed density functional theory to estimate their molecular static first and second hyperpolarizabilities. Compared to previously reported fullerene derivatives, the designed C60 polymer fragments can exhibit notable second-order and third-order molecular NLO responses. The study shows that the hyperpolarizabilities and energy gaps of the investigated C60 polymer fragments are greatly influenced by their topological structures and bonding modes. These findings provide new insights for the design of novel NLO materials based on C60 polymers, and the realization of tunable NLO responses in C60 cluster-based molecular systems, which may have significant applications in nanophotonic devices.

Controlling Nonlinear Optical Switch of Diarylethene by Changing Sizes of Fullerene: A Theoretical Study

AbstractFullerene is a material with good electrical conductivity and thermal stability, and its size is an important factor to influence its photoelectric property. In this work, effect of fullerene size (C20, C60, and C70) on nonlinear optical (NLO) switchable molecule diarylethene (DAE) was explored. In the aspect of molecular NLO properties, it could be found that the static first hyperpolarizability βtot of Cx‐O (x=20, 60, 70) was larger than that of Cx‐C (x=20, 60, 70), and the βtot difference between open‐ring and closed‐ring structure increased from 19 to 1431 au with the size of fullerene increased. Especially for C70‐DAE, its largest contrast of βtot values between open‐ring and closed‐ring structure suggested its superiority among three groups of NLO switchable complexes. The result could attribute to C70‐O exhibits a strong dipole characteristic, whereas C70‐C displays a weak dipole characteristic. By analyzing dynamic first hyperpolarizability, it could be found large NLO response mainly depended on molecular dipole characteristics. As a result, for C70‐DAE, βHRS of open‐ring structure was still larger than that of closed‐ring structure. However, the dynamic first hyperpolarizability βHRS of open‐ring structure was smaller than that of closed‐ring structure in both C20‐DAE and C60‐DAE. The switching efficiency was reversed.

Polaron Formation in Conducting Polymers: A Novel Approach to Designing Materials with a Larger NLO Response.

Substantial efforts have been made to design and investigate new approaches for high-performance nonlinear optical (NLO) materials. Herein, we report polaron formation in conducting polymers as a new approach to designing materials with a large NLO response. A comparative study of polypyrrole and polypyrrole-based polaron (nPy+ where n = 1, 3, 5, 7, and 9) is carried out for optoelectronic and NLO properties. The studied polarons (PPy+) show excellent electronic properties and have reduced ionization potential (IP) as compared to neutral PPy, and a monotonic decrease is observed with increased chain lengths (1Py to 9Py). Interesting trends of global reactivity descriptors can be seen; the softness (S) increases with an increase in the chain length of PPy, while the hardness (η) decreases in the same fashion. The EH-L gaps for the PPy+ polaronic state are significantly lower than their corresponding neutral PPy. In the polaronic model (PPy+), radicals decisively reduce the crucial excitation energy, reminiscent of excess electrons (alkali metals). The performed TDOS spectral analysis further justifies the better conductive and electronic properties of polarons (PPy+) with increased chain lengths (conjugation). The static hyperpolarizability response (βo) is recorded up to 1.3 × 102 au for 9Py, while for polaron 9Py+, it has increased up to 3.2 × 104 au. The static hyperpolarizability of the 9Py+ polaronic state is 246 times higher than that of the corresponding neutral analogue, 9Py. It is observed that the values of βo obtained at the CAM-B3LYP/6-311+G(d,p) level of theory are comparable to those obtained at the LC-BLYP and ωB97XD functionals. The βvec values show a strong correlation with the total hyperpolarizability (βo). Furthermore, the calculated second harmonic generation (SHG) values are up to 4.0 × 106 au at 532 nm, whereas electro-optic Pockel's effect (EOPE) is much more pronounced at the smaller dispersion frequency (1064 nm). The TD-DFT study reveal the red-shifted absorption maxima (λmax) with an increased length of PPy+. A significant reduction in excitation energy (ΔE) is observed with increased length of PPy and PPy+, which also favors the improved NLO response. Hence, the studied thermally conducting polypyrrole-based polarons (PPy+) are new entries into NLO materials with better electrical and optical features.

Li+(3)Mg−: A new alkaline earthide with large nonlinear optical response

Alkali metal doping has been identified as a vital step in facilitating an increase in the first hyperpolarizability(β0). To further explore this concept, we utilized Calix (Zhang et al., 2018) [3]pyrrole (3), Calix (Ma et al., 2017) [4]pyrrole (4), and Calix (Aloraini et al., 2022) [5]pyrrole (5) as our three ligands in this study. The choice of these ligands was based on their ability to undergo alkali metal doping, which ultimately led to an enhancement in the first hyperpolarizability. By doping alkali metal lithium, three new compounds Li@3, Li@4 and Li@5 were theoretically designed. Then the alkaline earthide Mg was doped onto the Li@3, and a new alkaline earthide compound Li+(3)Mg− was designed. The geometric structure and nonlinear optical properties of the molecules were studied using the CAM-B3LYP functional. The results show that Li+(3)Mg− has a considerable first-order hyperpolarizability, β0 = 21401 au, The β0 value for the 3 system is merely 37 au, whereas for the Li@3 system, it reaches a high of 12257 au Li+(3)Mg− exhibits a β0 value that is over 500 times greater than that of 3. It is evident that the electronic structure and nonlinear optical properties are significantly affected by the presence of alkali metal and alkaline earthide atoms doped in 3. The excess electron derived from alkali metal atoms and alkaline earthide atoms are significant contributors to the sudden increase in the β0 within Li+(3)Mg−. We sincerely hope that the development and production of materials with nonlinear optical properties in this study can provide new ideas and methods for the innovation of electride.

Third Harmonic Generation in Thin NbOI2 and TaOI2.

The niobium oxide dihalides have recently been identified as a new class of van der Waals materials exhibiting exceptionally large second-order nonlinear optical responses and robust in-plane ferroelectricity. In contrast to second-order nonlinear processes, third-order optical nonlinearities can arise irrespective of whether a crystal lattice is centrosymmetric. Here, we report third harmonic generation (THG) in two-dimensional (2D) transition metal oxide iodides, namely NbOI2 and TaOI2. We observe a comparable THG intensity from both materials. By benchmarking against THG from monolayer WS2, we deduce that the third-order susceptibility is approximately on the same order. THG resonances are revealed at different excitation wavelengths, likely due to enhancement by excitonic states and band edge resonances. The THG intensity increases for material thicknesses up to 30 nm, owing to weak interlayer coupling. After this threshold, it shows saturation or a decrease, due to optical interference effects. Our results establish niobium and tantalum oxide iodides as promising 2D materials for third-order nonlinear optics, with intrinsic in-plane ferroelectricity and thickness-tunable nonlinear efficiency.

A Quantum Chemical Study on the Enormously Large Nonlinear Optical Response of Doped Endohedral Superalkali Na2F and Superhalogens MF4 (M = B, Al) with CNT (C56H16) Cage

AbstractIn the present communication, the interaction of single‐wall carbon nanotubes (SWCNTs) CNT (C56H16) with endohedral doped superhalogens (MF4:M = B, Al) placed outside of CNT cage and superalkali (Na2F) placed inside the cage has been studied by using a combination of DFT/B3LYP method and 6–311G(d, p) basis set. The geometry and stability of MF4@CNT‐Li2F have been studied using optimization parameters, highest occupied molecular orbit (HOMO), lowest unoccupied molecular orbital (LUMO), vibration frequencies, and thermodynamic parameters of absorption reactions. The quantum theory in atoms in molecules (QTAIM) analysis is used to analyze the nature of interactions between MF4 (M = B, Al) and Na2F@CNT. Several electronic parameters are computed by using HOMO–LUMO energy. The dipole moment, polarizability, hyperpolarizability, order parameters, anisotropic polarizability, and molar refractivity of MF4@CNT‐Li2F (M = Al, B) are used to calculate the nonlinear optical parameters (MR). The NBO analysis is used to calculate the transfer of charge to stabilized system. The calculated hyperpolarizability of Na2F@CNT‐BF4/Na2F@CNT‐AlF4 is nearly 81 times that of reference material urea (40 a.u.). The intramolecular charge transfer (ICT) moment of π electron cloud is responsible for the nonlinear optical behavior of the system.

High-throughput virtual screening of organic second-order nonlinear optical chromophores within the donor-π-bridge-acceptor framework.

In view of the theoretical importance and huge application potential of second-order nonlinear optical (NLO) materials, it is of great significance to conduct high-throughput virtual screening (HTVS) on a compound library to find candidate NLO chromophores. Under the donor-π-bridge-acceptor structural framework, a virtual compound library (size = 27 090) was constructed by enumeration of structural fragments. The kernel property adopted for optimization is the static first hyperpolarizability (β0). By combining machine learning and quantum chemical calculations, we have performed an HTVS procedure to sieve NLO chromophores out, and the response mechanism of the selected optimal NLO chromophores was examined. We have found: (a) The multi-layer perceptron/extended connectivity fingerprint combination with 20% selection ratio gives the highest prediction accuracy for the studied systems. (b) The two optimal donors are bis(4-diphenylaminophenyl)aminyl and bis(4-tert-butylphenyl)aminyl; the optimal π-bridges are composed of two thiophenyl, selenophenyl or furanyl units; and the two optimal acceptors are tri-s-triazinyl and 2,3-dicyanopyrazinyl. (c) The no. 1 candidate molecule can exhibit a calculated β0 equal to 8.55 × 104 a.u. (d) The difference in NLO responses of the optimal 16 molecules comes from the synergistic interaction of ES1, Δμ and f, by employing the two-level model. In addition, the sizable Δμ and f allow the studied optimal molecules to obtain a large NLO response in the meantime keeping a not-too-low excitation energy (retaining good optical transparency in the restricted range of the visible spectrum region). (e) With further modification on the acceptor, the designed DPA-π-TRZ-A' (A' = CN or NO2, π = oligo-thiophenyl or selenophenyl) systems can exhibit a rather large NLO response (maximum β0 = 3.17 × 105 a.u.), hence should have considerable potential as second-order NLO chromophores. With the above observations, we expect to provide some insight for the research community into the HTVS of organic second-order NLO chromophores.

“Three‐in‐One”: A New Hg‐Based Selenide Hg7P2Se12 Exhibiting Wide Infrared Transparency Range and Strong Nonlinear Optical Effect

AbstractThe exploration of high‐performance mid‐ and far‐infrared (IR) nonlinear optical (NLO) crystals is an urgent need. Herein, a new Hg‐based selenide Hg7P2Se12 (HPSe) is designed and synthesized by a rigid and flexible units coupled strategy. It is interesting that in the compound, three types of NLO‐active units, including linear [HgSe2], triangular [HgSe3], and tetrahedral (MSe4) (M = Hg/P) groups, simultaneously exist. It shows a wide transparency range (0.84–22.8 µm), large SHG response (1 × AGS), high laser‐induced damage threshold (≈2 × AGS), and a large birefringence (0.102 @ 2090 nm). Moreover, the compound melts congruently along with good crystal growth habits, and single crystals with a maximum size of ≈7 × 5 × 2 mm3 are obtained. Theoretical analyses confirm that the large NLO response originates from the synergistic effects of Hg–Se units, (PSe4) and nonbonding electrons. The results indicate that HPSe is a promising mid‐ and far‐IR NLO material, and inspire a path to finding new classes of IR NLO materials that are different from traditional chalcopyrite materials by grouping the linear, planar, and tetrahedral units.

Tailoring the optical limiting response of methyl orange via protonation

In order to realize effective optical limiting (OL), it is necessary to utilize materials that exhibit distinct traits, such as a large and rapid nonlinear optical response, high photothermal stability, and cost-effectiveness. In the present study, the nonlinear optical (NLO) response of methyl orange (MO) is modified by inducing conformational and structural changes through appropriate modification of pH conditions. The NLO properties of MO under two different pH conditions were investigated via the z-scan technique under 7 nanoseconds excitation in the visible region (532 nm). From z-scan results, the nonlinear absorption coefficient of pristine MO and protonated-MO (P-MO) was estimated. Compared to MO, the nonlinear absorption coefficient of P-MO increased by 3.6 times, and it can be attributed to the redistribution of energy levels of MO, which favored resonant absorption in the system. Molecular energy shift and the corresponding apparent red shift in the absorption spectrum of MO are confirmed from both experimental and theoretical analysis. The mechanism behind the nonlinear absorption (NLA) and OL activity of MO is found to be effective two-photon absorption. The closed aperture (CA) z-scan data reveal a negative nonlinear refraction response of MO and P-MO. These findings suggest an efficient way to tune the NLO response of MO through protonation, and this method readily applies to numerous nonlinear optical applications, including but not limited to OL.

(SO3CF3)−: A Non‐π‐Conjugated Motif for Nonlinear Optical Crystals Transparent into the Deep‐Ultraviolet Region

AbstractThe search for new ultraviolet (UV) or deep‐UV nonlinear optical (NLO) crystals would greatly prompt the development of laser science and technology. As the NLO performance of these crystals is mainly determined by the anionic groups, herein the (SO3CF3)− group is highlighted as a promising non‐π‐conjugated NLO‐active motif with excellent deep‐ultraviolet (DUV) transmission and good NLO properties. First‐principles studies reveal that this group possesses the largest HUMO‐LOMO gap among the (SO4)2−‐based units, as well as a relatively large NLO response. The further experiments demonstrate that the CsSO3CF3 compound containing the (SO3CF3)− motif exhibits th UV cutoff edge below 200 nm and an NLO effect ≈0.8× KH2PO4, confirming the potentials of this new motif for UV and DUV NLO applications.

Impact of photoinduced energy transfer and LSPR of Au and Ag nanoparticles on nonlinear optical response of methyl orange

In order to realize effective optical limiting (OL), it is necessary to utilize materials that exhibit distinct traits, such as a large and rapid nonlinear optical (NLO) response, high photothermal stability, and cost-effectiveness. In this work, the NLO properties of methyl orange (MO), a popular pH indicator, a potential nonlinear material, and an azo dye of great industrial importance are modulated by forming nanohybrids with Ag and Au nanoparticles. The NLO properties of the samples were investigated via the Z-scan technique using 7 ns laser pulses at a wavelength of 532 nm. A huge enhancement is observed in the nonlinear absorption (NLA) and OL performance MO-Ag and MO-Au nanohybrid compared to its constituent compounds, and it is attributable to photoinduced energy transfer between MO and metal NPs as well as the local field effect of NPs. The mechanisms responsible for the NLA and OL activity are primarily Excited State Absorption (ESA) with contributions from a weak Two-Photon Absorption (TPA). Notably, the nanohybrid system incorporating Au NPs outperforms the one with Ag NPs in terms of nonlinear optical properties under 532 nm excitation, primarily due to the better resonant energy transfer of Au compared to the Ag nanoparticle and intense local field effects of Au on exciting at 532 nm due to resonant absorption. The Closed Aperture (CA) Z-scan measurements were employed to investigate the nonlinear refraction property of the compounds, and both the pristine MO and its nanohybrid exhibited negative nonlinear refraction. These findings indicate that forming nanohybrids is an efficient way to modify the NLO response of MO. It also suggests MO has the potential for a diverse array of nonlinear optical applications, including but not limited to optical limiting.

Three-in-One Tetrahedral Functional Units Constructing Diamondlike Ag2In2SiS3.06Se2.94 with High-Performance Nonlinear-Optical Activity.

Exploration of new functional materials with enhanced performance from known ones is always an attractive strategy. A new infrared (IR) nonlinear-optical (NLO) mixed chalcogenide Ag2In2SiS3.06Se2.94 (1), was obtained through partial congener substitution originated from Ag2In2SiS6 (0). 1 crystallizes in the monoclinic space group Cc, and its three-dimensional (3D) polyanionic network is composed of {[In4Si2Se5(S/Se)11]12-}∞ helical chains sharing S/Se(5) corner atoms with cavities embedded with counterion Ag+ ions. It exhibits a much enhanced NLO response compared to that of 0, reaching 1.1 × AgGaS2. Further theoretical analysis results indicate that the large NLO response can be attributed to the synergistic effect of AgQ4 and InQ4 tetrahedral functional motifs. This work not only reports a new high-performance IR NLO material but also enriches the partial ion substitution strategy to obtain new functional materials.

Superalkalis fabricated Te-containing [8]circulenes as outstanding NLO materials; a DFT perspective

There is a growing demand for thermodynamically stable and highly polarizable materials with excess electrons in the fields of optics. In this study, a new class of theoretically designed materials (AxY@C16Te8 (A = K, Na, Li, and x = 2, 3 4)) are explored for optoelectronic and nonlinear (NLO) applications via DFT calculations. Various superalaklis (AxY (Y = O, N, F and x = 2, 3, 4)) are doped on C16Te8 nano-surface. The outcomes of DFT revealed high thermodynamic stability of these complexes. Natural bond orbital (NBO) charge analysis illustrated that charge transfer occurred from superalkali toward Te-containing [8]circulene (C16Te8). HOMO − LUMO energy gap is reduced after doping. Moreover, (AxY@C16Te8 (A = K, Na, Li, x= 2, 3 4, and Y = O, N, F) complexes exhibit remarkably large nonlinear optical (NLO) response. The static first hyperpolarizability (βo) of these complexes ranges between 3.28 × 103 – 2.60 × 105 au. The NLO response of the designed complexes is further explored by computing second hyperpolarizability (γtot), frequency-dependent hyperpolarizability, and nonlinear refractive index.

Alkali metals doping into open-cage fullerenes: Novel alkalides (M+@C50N5H5)Mʹ− with large nonlinear optical responses

Based on open-cage fullerenes C50N5H5, a novel type of compounds (M+@C50N5H5)Mʹ− (M = Li, Na; Mʹ = Li, Na and K) was designed and studied. Firstly, their alkalides’ properties are confirmed by their FMO and NPA charges analysis. In particular, the proposed alkalides exhibited considerable first hyperpolarizability (β0) up to 1.91 × 105 a.u., which indicated that they could be regarded as high-performance novel NLO molecules. These alkalides will pave the way for further exploration of electronic-stable, high-performance NLO materials.

Surface Functionalization of Sulflower with Superhalogens Doping to Achieve Robustly Large Nonlinear Optical Response: Interactive Design Computation of New NLO Materials for Modern Electro-Optic Applications

Developing highly effective nonlinear optical (NLO) materials is a cutting-edge field of study. The desirability of promising NLO materials for optoelectronic and electronic applications drove us to investigate the sulflower (C18S9) doped complexes for developing highly efficient NLO materials. A successful computational design technique for superhalogens (BeF3, MgF3, CaF3, CaCl3) doping is employed on sulflower, and eight stable isomers (BeF3@SF-5M, BeF3@SF-9M, MgF3@SF-5M, MgF3@SF-9M, CaF3@SF-5M, CaF3@SF-9M, CaCl3@ SF-5M, CaCl3@SF-9M) are proposed for NLO properties. DFT and TD-DFT simulations confirm the potential use of superhalogen-doped sulflower complexes for NLO response applications by evaluating NLO response properties, photophysical aspects, frontier molecular orbital (FMO), natural bond orbitals (NBO), non-covalent interaction (NCI), vertical ionization energies (VIE), interaction energies (E int), molecular electrostatic potential (MEP) and density of state (DOS) analysis. The Eint (−28.37 to −53.86 kcal/mol) and VIE (7.0-7.4 eV) findings suggest that superhalogens-doped sulflower complexes are thermodynamically stable and show durable interaction among sulflower and doped superhalogens. Superhalogens doping on sulflower effectively shorten the energy gap from 4.46 eV (pure sulflower) to 4.17 eV in CaCl3@SF-5M isomer. Superhalogens doping causes a change in the dipole moment from 0 D of pure sulflower to 12.57 D in the CaCl3@SF-5M isomer. Proficient intermolecular charge transfers between sulflower and doped superhalogens were established by NCI and NBO analyses. UV-Vis investigation revealed that all superhalogen-doped sulflower complexes are adequately transparent in the near-infrared and visible wavelength ranges. Augmentation in polarizability value from 333.86 a.u.−569.98 a.u and in first hyperpolarizability from 0.00 a.u to 6.6 × 104 a.u. is achieved upon superhalogens doping. A striking NLO response (β 0 = 6.6 × 104 a.u) is exhibited by CaCl3@SF-5M isomer. This report provides an efficient superhalogens doping technique for creating highly effective future NLO systems and recommends superhalogens-doped sulflower complexes as ideal entrants for future NLO applications.

Theoretical Perspective on Two-State "ON-OFF" NLO Switch of Rh(III)-Azobenzene Complex with Considerable First Hyperpolarizability.

In this paper, a light-induced Rh(III)-azobenzene (azo) complex in its two conformations (cis- and trans-form) is analyzed via density functional theory methods, focusing on the geometrical and electronic structure, linear and second-order nonlinear optical (NLO) properties, as well as UV-vis absorption spectra. The results show that trans-Rh exhibits more obvious charge separation and more extensive π-conjugation than cis-Rh. The energy gaps (Egap) between the highest occupied molecular orbital and lowest unoccupied molecular orbital of cis-Rh and trans-Rh are as small as 0.28 and 0.36 eV, respectively. The unusual small Egap facilitates electronic transition and accompanies large NLO responses. In particular, cis-Rh and trans-Rh possess considerably large second-order NLO responses with the first hyperpolarizability (βtot) up to 7.0 × 104 a.u. ("OFF" state) and 8.5 × 104 a.u. ("ON" state), respectively. trans-Rh shows good π-conjugation and obvious electric delocalization relative to cis-Rh and thus a considerable βtot value. The switchable ratio of βtot (trans-Rh)/βtot (cis-Rh) is equal to 1.2, showing switch characteristics. UV-vis analyses indicate that the ON and OFF states can finely tune the absorption range due to the strongest absorption peaks of cis-Rh (301 nm) and trans-Rh (446 nm) lying in the UV and vis region, respectively. It is expected that the studied Rh(III)-azo complexes may be applied in the field of optoelectronics.

Exploring the Bicorannulenyl Surface for Alkali Metal Doping: Towards a New NLO Material

AbstractRecent years have seen a rise in the importance of designing materials with large nonlinear optical (NLO) responses. Different compounds having large nonlinear optical responses are being extensively studied for their boundless applications in this field. Contributing to this, we are presenting here an attempt to design novel nonlinear optical materials. In this study, alkali metal‐doped bicorannulenyl (C40H38) compounds (Li@Bicora, Na@Bicora, and K@Bicora) are designed and computationally investigated by using DFT calculations. Thermodynamic stabilities of all complexes are evident from their binding energies. A prodigious first hyperpolarizability value is observed after doping the bicorannulenyl with alkali metals. A significantly enhanced hyperpolarizability value (βo=2.68×106 au) is shown by isomer VII of K@Bicora. Other analyses, such as TDOS and PDOS, NBO, QTAIM and NCI, are also performed to investigate the contributions of various systems, charge transfer, and modes of interaction. Absorption analysis has confirmed the transparency in the UV region. This theoretical insight into NLO material‘s prodigious NLO response will serve as a springboard for novel ideas in this area.

ASecond-Order Nonlinear Optical Material Consisting of Two π-Conjugated Groups.

Herewereporta second-order nonlinear optical (NLO) material [C(NH2)3]3C3N3S3(GU3TMT), consisting ofπ-conjugated planar (C3N3S3)3-and triangular [C(NH2)3]+groups. Interestingly, GU3TMT exhibits large NLO response (2.0x KH2PO4) and moderate birefringence 0.067 at550 nm, even though (C3N3S3)3-and [C(NH2)3]＋do not exhibit the most favorable arrangement in the structure of GU3TMT. First-principles calculations suggest that NLO properties mainly originate from the highlyπ-conjugated (C3N3S3)3-rings, and theπ-conjugated [C(NH2)3]＋triangles contribute much less to the overall NLO response. This work will inspire new thoughts with in-depth on the role ofπ-conjugated groups in NLO crystals.

A new infrared nonlinear optical material BaZnGeS4 with a wide band gap and large nonlinear optical response

Based on the “electronic structure engineering bucket effect”, a new AEM-containing sulfide BaZnGeS4 with a wide band gap (∼3.36 eV) and large SHG response (∼0.8× AGS) has been designed and synthesized via high-temperature solid-state reactions.

Giant saturation absorption of tungsten trioxide film prepared based on the seedless layer hydrothermal method

Nonlinear materials have gained wide interest as saturable absorbers and pulse compression for pulsed laser applications due to their unique optical properties. This work investigates the third-order nonlinear phenomenon of tungsten trioxide (WO3) thin films. The giant nonlinear absorption and nonlinear refractive index of WO3 thin films were characterized by Z-scan method at 800 nm. We experimentally observed the giant saturable absorption (SA) and nonlinear refractive index of WO3 thin films prepared by the seedless layer hydrothermal method, with SA coefficient being as high as –2.59 × 105 cm⋅GW−1. The SA coefficient is at least one order of magnitude larger than those of the conventional semiconductors. The nonlinear refractive index n 2 of WO3 film has been observed for the first time in recent studies and the corresponding coefficient can be up to 1.793 cm2⋅GW−1. The large third-order nonlinear optical (NLO) response enables WO3 thin films to be promising candidates for optoelectronic and photonic applications in the near-infrared domain.