Nonlinear Absorption Mechanism Research Articles

Tetrahydroxyphthalocyanine as a potential nonlinear optical material

We have synthesized a new zinc(II)-phthalocyanine (ZnPc) incorporating hydroxyl groups at the peripheral positions. This new zinc(II)-tetra-hydroxyphthalocyanine (1) has been characterized by high-resolution MALDI TOF MS, UV-Vis, FT-IR (ATR), and 1H- and 13C-NMR techniques, and further by the electrochemical measurements. DFT and TD-DFT computations were performed to model plausible structure and also analyze the electronic structure and optical properties of 1. The effects of the polarity and hydrogen bonding ability of the hydroxyl substituents on the peripheral positions of 1 on the optical and electrochemical properties are reported in this study. The redox behavior of 1 in dimethyl sulfoxide (DMSO) was determined by voltammetry and colorimetry supported in situ spectroelectrochemistry. The nonlinear absorption and optical limiting (OL) properties of 1 were measured in tetrahydrofuran (THF) by utilizing the open aperture Z-scan technique with nanosecond pulses at 532 nm. The sample demonstrated good reverse saturable absorption and OL behaviors. The ultrafast pump-probe experiments revealed the intersystem crossing (ISC) mechanism (triplet-triplet transition) and the nonlinear absorption mechanism of the compound. The results indicate that this new phthalocyanine (1) can be considered a potential candidate for low-power OL applications.

Interfacial charge transfers and ultrafast nonlinear optical response via constructing electronic structure-induced MoS2/ZnO heterostructure

The molybdenum disulfide (MoS 2 ) and ZnO composite system have reported as micro-nano photonic crystals in integrated optics. Based on interface engineering, their tunable nonlinear absorption (NLA) and photoelectric characteristics offer a flexible design strategies for saturation absorber and optical limiters. Nevertheless, the mechanism leading to the tunable NLA behavior has not been clearly elucidated, which were attribute to complex charge transfers and carrier regulation characteristics. In this article, MoS 2 /ZnO heterostructures with excellent nonlinear properties composed of ZnO nanorods (NRs) wrapped with MoS 2 nanosheets were prepared by magnetron sputtering (MS) synthesis of MoS 2 on highly stacked ZnO NRs. The MoS 2 /ZnO heterostructures exhibit an interesting brush-like morphology with abundant heterointerfaces base on optimizing sputtering time and temperature, which was beneficial to provide efficient charge transfer pathways. Due to the uneven distribution of the lowest nucleation free energy, MoS 2 nanosheets possess a hybrid mode of parallel (C // ) and perpendicular orientations (C ┴ ) on surface of ZnO NRs, which tends to form the bending and vertical growth of layered MoS 2 nanosheets. From electron-photon coupling effect, MoS 2 nanosheets, ZnO NRs and MoS 2 /ZnO heterostructures possess favorable saturation absorption (SA), reverse saturation absorption (RSA) and coexisting absorption behavior, respectively. In addition, a changeover from RSA to SA could be realized in MoS 2 /ZnO heterostructures by increasing the sputtering time. The NLA coefficient of MoS 2 /ZnO heterostructures are in the order of 10 −10 cm/W, which are about 2 times larger than those of the ZnO NRs. Thus, the MoS 2 /ZnO heterostructures with excellent NLA properties will be promising candidates in optoelectronic devices. • The interfacial charge transfers and carrier regulation characteristics of MoS 2 /ZnO composite have been studied. • The ultrafast nonlinear optical properties of MoS 2 /ZnO composite were analyzed. • The mechanism of growth and nonlinear absorption for MoS 2 /ZnO composite is analyzed.

Mechanisms of nonlinear optical absorption in (GeO2)1-x–(PbF2)x (0.1 ≤ x ≤ 0.3) modified germanate glasses

The structural, optical, and nonlinear absorption properties of (GeO2)1-x–(PbF2)x (0.1 ≤ x ≤ 0.3) glasses synthesized by conventional melting and quenching technique were investigated. Bandgap energies and Urbach energies were calculated by evaluating the linear absorption spectra. The increase in the PbF2 content resulted in 0.51 eV increase in bandgap energy and 0.28 eV decrease in Urbach energy. Z-scan experiment were performed to investigate nonlinear absorption properties, and a theoretical model including the contribution of one photon absorption, two photon absorption, excited state absorption, and saturable absorption to nonlinear absorption was considered in the evaluation of the experimental data. Although the highest effective nonlinear absorption coefficient was found with the highest PbF2 content due to the smallest bandgap energy, the highest downtrend in effective nonlinear absorption coefficients was also found for highest PbF2 content due to the smallest Urbach energy.

Controlled sputtering time growth, ultrafast nonlinear absorption and carrier dynamics properties of vertically WSe2 films under photoexcitation

In this work, the vertically-grown-WSe2-films were successfully prepared using the RF magnetron sputtering. Their structure, morphology, growth mechanism and nonlinear optical properties were investigated by adjusting the growth time. The special "worm-like" structure was characterized by scanning electron microscopy, attributing to the two growth orientations in C-axis type I (C//) and type II (C⊥). The X-ray diffraction results confirmed the preferential growing on (002) and (008). The nonlinear optical properties were studied by femtosecond Z-scan to reveal the nonlinear absorption mechanism. WSe2 films exhibited the nonlinear optical process from saturable absorption to reverse saturable absorption with the growing time. The NA coefficient for WSe2 films is ∼10−9 m/W. The carrier relaxation time was determined by femtosecond transient absorption spectroscopy. This proposed WSe2 films will be an excellent candidate for developing optoelectronic devices due to the excellent nonlinear optical properties.

Defect assisted optical limiting performance of hexagonal boron nitride nanosheets in aqueous suspension and PMMA nanocomposite films

Defect-assisted nonlinear absorption (NLA) and optical limiting (OL) performance of hexagonal boron nitride nanosheets (h-BNNS) in aqueous suspension and in polymethyl methacrylate matrix (PMMA) as nanocomposite films were studied using open-aperture Z-scan method. To evaluate the transmission in open-aperture Z -scan data, a theoretical model accounting one photon absorption (OPA), two photon absorption (TPA), free carrier absorption (FCA) and saturation of each process was considered. Defect-assisted NLA coefficients and saturation intensity thresholds were extracted from the fitting of the experimental results for 532 and 1064 nm pulse wavelengths. Strong defect-assisted NLA response of h-BNNS was observed while NLA at 532 nm was considerably stronger. This is attributed to the excitation of a greater number of defect states over a wider energy range. Our findings showed that h-BNNS/PMMA nanocomposite films feature highly required properties in OL applications and can function in OL applications in a wide spectral range (∼200–1064 nm). • Nonlinear absorption (NLA) mechanisms of hexagonal boron nitride nanosheets (h-BNNS). • A theoretical model accounting for one photon absorption, two photon absorption and free carrier absorption. • Defect states residing within the bandgap addressed as the main source of NLA of h-BNNS. • Optical limiting performance of h-BNNS via defect assisted NLA.

Nonlinear absorption of laser radiation by aluminium particles in a potassium bromide matrix

Optical properties of a model pressed composite (potassium bromide matrix with incorporated ALEX aluminium powder) are experimentally investigated. It is shown that the spectral dependences of total transmittance and diffuse reflectance can be described with allowance for the presence of both individual aluminium nanoparticles and their aggregates with a characteristic radius of 133 nm in a sample. It is found that irradiation of a composite sample by a 1070-nm cw laser initiates nonlinear absorption of radiation by aluminium particles, which is detected by a change in the sample optical density. A model is proposed to describe the effect, which is based on the sample heating and temperature dependence of the optical characteristics of aluminium particles. The calculation results are in qualitative agreement with the observed changes in optical density; hence, the thermal mechanism of nonlinear absorption of electromagnetic radiation by metal particles is applicable to this model system.

Ultrafast laser selective phase removal for surface modification of nanocomposite materials.

Ultrafast laser processing of zirconia/alumina nanocomposite ceramics, the current gold standard material for ceramic bearing components in orthopedics, was investigated. Instead of considering the substrate as a homogeneous material, as commonly assumed in laser micromachining, the damage behavior of different phases around the laser ablation threshold upon ultrafast laser irradiation was investigated. Under appropriate experimental conditions, the zirconia phase was selectively ablated while the alumina phase remained intact. The origin of this selective ablation behavior and its relationship with the material band gaps were discussed. Due to the nonlinear absorption mechanisms under ultrafast laser irradiation, the zirconia phase, with its band gap of 5.8 eV, can absorb more laser energy than the alumina phase which has a larger band gap of 8.8 eV. The negligible heat diffusion length ensures that the absorbed laser energy remains confined in the individual phases, leading to the selective ablation of zirconia phase under the given laser fluence. Based on this observation, an ultrafast laser selective phase removal method which can be used to modify the surface composition of nanocomposite materials consisting of phases with different band gaps was proposed.

Transient thermal model of nanosecond pulsed laser ablation: Effect of heat accumulation during processing of semi-transparent ceramics

Nanosecond ablation of semi-transparent ceramics represents a particularly interesting scenario where non-linear absorption mechanism allows absorption of laser energy for photon energy below the band gap of the material. Yttria Stabilized Zirconia (YSZ) ablated by infrared radiation represents as an example of such scenario. Ablation threshold experiments reveal a stark difference between the behaviour observed during ablation of single pulses compared to multiple pulses. During single pulses, a statistical occurrence of ablation is observed. For laser intensity below the dielectric breakdown threshold, this phenomenon has been explained on the basis of a defect-mediated thermal runaway which causes an exponential increase of the bulk material absorption coefficient as function of temperature. To investigate this phenomenon, and its effect during multi-pulse ablation, a 1D modelling framework has been developed and validated. The framework focuses on simulation of ablation of trenches (i.e. partially overlapped pulses) considered as convolution of single pulses, and account for the governing equation of heat diffusion, vaporization, plasma shielding, volumetric absorption and temperature-dependent absorption coefficient by use of a simplified defect mediated absorption model which represents the main novelty of the paper.The application of the model for various laser processing parameters reveals the importance of heat accumulation on the process. A positive feedback mechanism is established over successive pulses which leads to the material becoming progressively more absorbing, and eventually results in material removal through ablation. This phenomenon explains the sharp ablation threshold observed experimentally during ablation of trenches. The modelling framework also reveals interesting dynamics caused by the temperature dependence of the material absorption behaviour, such as a delay in the starting time for material removal, and drastic changes in the energy distribution within the sample throughout the duration of a laser pulse.Ultimately, the effect of heat conservation, which is often neglected, has been shown to represent a key dynamic during multi-pulse ablation, and should be accounted for during simulations of real-world laser processes, in particular for low diffusivity semi-transparent materials.

Optical nonlinearities in chemically synthesized and femtosecond laser fabricated gold nanoparticle colloidal solutions

Gold nanoparticles prepared by two different methodologies, wet-chemical- citrate reduction synthesis and femtosecond laser ablation, are studied for optical nonlinearities. Femtosecond laser pulses at 800 nm wavelength are employed to study the optical nonlinear refraction and absorption through the Z-scan technique, up to 23.37 GW/cm2 laser intensities. Here we demonstrate the distinct crossovers of nonlinear absorption, from varieties of nonlinear absorption mechanisms, saturation of linear absorption, reverse saturation of absorption. The reverse saturation of absorption is primarily contributed by an initial two-photon absorption followed by three-photon absorption process. We could observe that in laser-ablated gold nanoparticles, the transition from saturable to reverse saturable absorption takes place at a lower excitation threshold compared to that in chemically produced samples. The nonlinear refraction data depicted a pronounced asymmetry for chemically synthesized samples, which could possibly be due to the dominance of absorption phenomenon. We observed that nonlinear refractive index of chemically produced samples is higher by one order of magnitude compared to those produced by laser ablation. An appealing application in optical limiting is studied, and we see that the laser-produced samples, due to their strong nonlinear absorption, serve as a suitable choice for the optical limiting application. This study provides an insight into the selection of synthesis method towards their optimized utilization in nonlinear optical domain given that we achieve particle size tunability in the two methods.

Preparation, modification and nonlinear optical properties of semiconducting MoS2 and MoS2/ZnO composite film

In this work, ZnO, MoS2-power, MoS2-time and MoS2/ZnO composite films with excellent linear-nonlinear absorption properties were obtained by magnetron sputtering (MS) method. The structure, nucleation process and optical properties of MoS2 and MoS2/ZnO composite films were studied. The scanning electron microscopy (SEM) images showed that the morphology of the MoS2 film transferred from granular to worm-like. This phenomenon can be attributed to that the energy of Mo and S particles increases with increasing of sputtering power. The UV–Visible absorption spectroscopy and Z-scan technique were used to study the linear-nonlinear absorption characteristics of the samples. Interestingly, it is found that nonlinear absorption behavior of the samples depends on the input energy of laser. When the input energy of laser was increased, the nonlinear absorption mechanism of the MoS2/ZnO composite film changes from saturated absorption (SA) to reverse saturated absorption (RSA). In addition, the results showed that the NLA coefficient of the MoS2/ZnO composite film was 2–5 times higher than that of the ZnO film, which means that the MoS2 film significantly promotes the NLA characteristics of the ZnO. The MoS2/ZnO composite film with controllable morphology and excellent RSA characteristics may be used to optical limiting devices which in the field of nonlinear optoelectronic functional materials.

Composition-dependent optical limiting behavior of all-inorganic halide perovskite quantum dots

Abstract Nonlinear optical nanostructured materials are gaining interest as optical limiters for various applications. Here, all-inorganic halide perovskite quantum dots (PQDs, SiO2@CsPbX3 (X = Cl, Br, I) QDs) with tunable composition were prepared via ligand assisted reprecipitation, using 3-aminopropyltriethoxysilane as a ligand. The formation of the SiO2@CsPbX3 QDs was confirmed by transmission electron microscopy, X-ray diffraction, infrared spectroscopy, ultraviolet–visible absorption spectroscopy and photoluminescence spectroscopy. The composition-dependent optical limiting (OL) behavior of the SiO2@CsPbX3 QDs was observed using nanosecond and picosecond laser pulses at a wavelength of 532 nm. The SiO2@CsPbBr3 QDs exhibit favorable OL properties. Their OL threshold for the nanosecond laser pulse is 1.68 J/cm2, which is comparable to that of carbon nanotube suspension (a benchmark optical limiter). The presence of chloride or iodide in the SiO2@CsPbX3 QDs leads to weaker OL performance. The OL behavior of the SiO2@CsPbBr3 QDs is attributed to the narrow band gap, large average size, abundant charge carriers under laser excitation and high stability. The OL mechanism was investigated using the open-aperture and closed-aperture techniques. The OL behavior of the SiO2@CsPbX3 QDs is largely attributed to the combined mechanisms of nonlinear absorption and nonlinear refraction. These results reveal the physical processes of the composition-dependent OL properties of the CsPbX3 QDs. These findings provide a means to tailor the OL response of PQDs by controlling the halide ion ratio.

Improved nonlinear optical absorption mechanism and susceptibility (χ(3)) of CdS nanostructured thin films: Role of zinc doping

Thin films of Cd1-xZnxS (CZS) nanostructures were prepared by chemical spray pyrolysis technique with different Zn-doping levels [(Cd1-xZnxS) in ‘x’ ratios of 0.00, 0.01, 0.03, 0.05 and 0.1 for 0%, 1%, 3%, 5% and 10 wt% of Zn respectively] on heated glass substrates at 350 °C. The effect of the Zn dopant on surface morphology and its effect on nonlinear optical (NLO) properties were investigated using atomic force microscopy (AFM) and Z-scan technique by Diode Pumped Solid State (DPSS) continuous-wave laser at 532 nm respectively. The surface roughness was found to be decreased with the increased Zn-doping on CdS. A significant red shift in the band edge and an increase in the transmittance of the films in the visible and NIR regions were also observed. The energy gap (Eg) of the prepared films was estimated and found to be enhanced by 3% i.e. in the range from 2.48 to 2.60 eV. The determined third-order NLO components such as nonlinear absorption coefficient (β), nonlinear refractive index (n2) and the susceptibility χ(3) were found to be in the range from 2.52 × 10−3 to 7.25 × 10−3 (cmW−1), 2.01 × 10−8 to 3.92 × 10−8 (cm2 W−1) and 1.30 × 10−6 to 2.67 × 10−6 (esu) respectively. The results indicate that NLO properties and photoluminescence of the prepared thin films can be tuned with surface morphology. This inspiring NLO results of Cd1-xZnxS thin film is a promising and capable material for optoelectronic device applications.

Enhancement of nonlinear optical response of methylene blue and azure a during association with colloidal CdS quantum dots

The nonlinear optical response of thiazine dyes during association with colloidal CdS quantum dots (QDs) applying the z-scan technique at wavelength of 532 nm with use 10 ns pulses of Nd:YAG laser second harmonic were studied. Increasing in nonlinear absorption intensity in hybrid associates in comparison with the initial components were found. Moreover, UV–vis absorption spectrum shows the effective formation of dyes H-aggregates during association with colloidal QDs. Two-photon mechanism of observed nonlinear absorption in hybrid associates is proposed, based on a sharper dependence of the normalized transmittance on the incident intensity (distance between the sample and focal plane of focusing lens). And for pure solutions, the reverse saturated absorption is preferred nonlinear absorption mechanism. The nonlinear absorption coefficients for MB and AzA in the hybrid associates were found. They are β ≈ 8.4 × 10−9 cm W-1 и ≈ 1.34 × 10-8 cm W-1, respectively. It was shown that it is possible to control the parameters of nonlinear absorption of thiazine dyes via the hybrid association.

Improving Sound Absorption Through Nonlinear Active Electroacoustic Resonators

Absorbing airborne noise at frequencies below 300 Hz is a particularly vexing problem due to the absence of natural sound absorbing materials at these frequencies. The prevailing solution for low-frequency sound absorption is the use of passive narrow-band resonators, whose absorption level and bandwidth can be further enhanced using nonlinear effects. However, these effects are typically triggered at high intensity levels, without much control over the form of the nonlinear absorption mechanism. In this study, we propose, implement, and experimentally demonstrate a nonlinear active control framework on an electroacoustic resonator prototype, allowing for unprecedented control over the form of non-linearity, and arbitrarily low sound intensity thresholds. More specifically, the proposed architecture combines a linear feedforward control on the front pressure through a first microphone located at the front face of the loudspeaker, and a nonlinear feedback on the membrane displacement estimated through the measurement of the pressure inside the back cavity with a second microphone located in the enclosure. It is experimentally shown that even at a weak excitation level, it is possible to observe and control the nonlinear behaviour of the system. Taking the cubic nonlinearity as an example, we demonstrate numerically and experimentally that in the low frequency range ([50 Hz, 500 Hz]), the nonlinear control law allows improving the sound absorption performance, i.e. enlarging the bandwidth of optimal sound absorption while increasing the maximal absorption coefficient value, and producing only a negligible amount of nonlinear distortion. The reported experimental methodology can be extended to implement various types of hybrid linear and/or nonlinear controls, thus opening new avenues for managing wave nonlinearity and achieving non-trivial wave phenomena.

Polarization-dependent nonlinear optical response in GeSe<sub>2</sub>

Germanium diselenide (GeSe<sub>2</sub>), a layered IV-VI semiconductor, has an in-plane anisotropic structure and a wide band gap, exhibiting unique optical, electrical, and thermal properties. In this paper, polarization axis Raman spectrum and linear absorption spectrum are used to characterize the crystal axis orientation and energy band characteristics of GeSe<sub>2</sub> flake, respectively. Based on the results, a micro-domain I scan system is used to study the optical nonlinear absorption mechanism of GeSe<sub>2</sub> near the resonance band. The results show that the nonlinear absorption mechanism in GeSe<sub>2</sub> is a superposition of saturation absorption and excited state absorption, and is strongly dependent on the polarization and wavelength of incident light. Under near-resonance excitation (450 nm), the excited state absorption is more greatly dependent on polarization. With different polarizations of incident light, the modulation depth can be changed from 4.6% to 9.9%; for non-resonant excitation (400 nm), the modulation depth only changes from 7.0% to 9.7%. At the same time, compared with saturation absorption, the polarization-dependent excited state absorption is greatly affected by the distance away from the resonance excitation wavelength.

Key signal contributions in photothermal deflection spectroscopy

We report on key signal contributions in photothermal deflection spectroscopy (PDS) of semiconductors at photon energies below the bandgap energy and show how to extract the actual absorption properties from the measurement data. To this end, we establish a rigorous computation scheme for the deflection signal including semianalytic ray tracing to analyze the underlying physical effects. The computation takes into account linear and nonlinear absorption processes affecting the refractive index, thus leading to a deflection of the probe beam. We find that besides the linear mirage effect, nonlinear absorption mechanisms make a substantial contribution to the signal for strongly focused pump beams and sample materials with high two-photon absorption coefficients. For example, the measured quadratic absorption contribution exceeds 5% at a pump beam intensity of about 1.3×105W/cm2 in Si and at 5×104W/cm2 in GaAs. In addition, our method also includes thermal expansion effects as well as spatial gradients of the attenuation properties. We demonstrate that these effects result in an additional deflection contribution that substantially depends on the distance of the photodetector from the readout point. This distance dependent contribution enhances the surface related PDS signal up to two orders of magnitude and may be misinterpreted as surface absorption if not corrected in the analysis of the measurement data. We verify these findings by PDS measurements on crystalline silicon at a wavelength of 1550 nm and provide guidelines on how to extract the actual attenuation coefficient from the PDS signal.

Improved nonlinear absorption mechanism of tin oxide thin films: Role of strontium doping

We report the effect of strontium doping on third-order nonlinear optical properties of tin oxide (SnO2) thin films grown via chemical spray pyrolysis technique. The X-ray diffraction (XRD) results reveal that films exhibit a polycrystalline phase with a tetragonal crystal structure. The broadening and shifting of diffraction peak indicate that the dopant Sr2+ ion successfully replaced the Sn sites of SnO2 lattice. The reduction of crystalline size from 23.92 to 12.96 nm indicates the assimilation of point defects such as interstitial and oxygen vacancies in the SnO2 lattice upon doping. The surface morphological analysis via atomic force microscopy indicates the shattering of grains into small particles upon Sr doping. The energy band gap of films shows a substantial drop from 3.97eV to 3.03eV due to the formation of sub-band levels in the forbidden gap. Photoluminescence (PL) spectra show intense, broad asymmetric emission bands centred around 390 nm and extending up to 500 nm. The third order nonlinear absorption mechanism observed in Sr: SnO2 films were attributed to free carrier absorption (FCA) induced two-photon absorption (TPA). The third order nonlinear absorption co-efficient βeff shows a one order enhancement (0.14 × 10−1 cm/W to 3. 91 × 10−1 cm/W) which indicates that competency of grown films in nonlinear optical device applications.

Defects-induced nonlinear saturable absorption mechanism in europium-doped ZnO nanoparticles synthesized by facile hydrothermal method

In the present work, the effect of europium (Eu) dopant on the third-order nonlinear optical (NLO) responses of ZnO nanoparticles (NPs) is investigated. The NPs are synthesized by facile hydrothermal technique using saponin as a capping agent. The samples are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), Energy-dispersive spectroscopy (EDS), UV–visible spectroscopy and Room-temperature photoluminescence spectroscopy (RTPL). The synthesized NPs exhibit wurtzite crystal structure with good crystallinity. Williamson–Hall analysis of XRD pattern revealed the presence of dopant induced compressive strain in the crystal lattice of Eu doped ZnO NPs. The capping of saponin on the surface of NPs is confirmed by FTIR study. A significant decrease in the optical band gap of Eu-doped ZnO NPs attributed to the presence of extended defect states below the conduction band. An enhancement in the blue luminescence is observed for the highest doping level (5 at. wt% of Eu), and this is attributed to defects-related emission in the NPs. The third-order NLO properties of the NPs are explored by a single beam Z-scan technique by employing continuous-wave laser of wavelength 532 nm. The open aperture Z-scan curves showed a remarkable switching from reverse saturable absorption to saturable absorption in 5 at. wt% Eu-doped ZnO NPs is caused due to the ground-state bleaching effect. Further, the closed aperture Z-scan curves exhibit the positive nonlinear refractive index in the 5 at. wt% Eu-doped ZnO NPs attributed to the thermo-optical effects.

Tunable nonlinear optical absorption in amorphous and crystalline Sb2Se3 thin films

The nonlinear optical absorption properties of as-deposited and crystalline Sb2Se3 films were systematically studied by using open-aperture Z-scan technique with femtosecond laser pulses at the wavelength of 800 nm. It is interesting to find that the crystallization leads nonlinear optical absorption to switch from reverse saturable absorption to saturable absorption. A tunable behavior is attributed to the competition between ground state excitation and excited state absorption, which depends critically on the facts of band gap energy and the density of localized defect states in the Sb2Se3 film. A three-level approximate model is proposed to explain the probable electronic transition and its rationality is further confirmed by second harmonic experiments under the same conditions. In particular, it is found that there are two different nonlinear optical absorption mechanisms coexisting in one kind of material with identical chemical composition, and their switching behavior is regulated merely by phase-change after thermal treatment. These special properties of Sb2Se3 thin film imply huge potential applications in the field of innovative nonlinear optical devices.

Unlocking the action mechanisms of molecular nonlinear optical absorption for optical conjugated polymers under aggregation states

Controlling the molecular microstructure and the molecular aggregation state under different conditions to improve the MNOA performance of OCPs.