Keywords: Electrochemical Doping Nonlinear Absorption Optics Saturable Absorption Tin Disulfide
Note: This article uses the Sine Scientific Instruments OE1022D dual-channel lock-in amplifier, optical chopper OE3001 to to make measurements.
[Overview]
In September 2021, the research team of Prof. Zhipeng Huang and Academician Qi Zhang from the School of Chemical Science and Engineering of Tongji University published a research paper titled "Giant Nonlinear Optical Absorption of Ion-Intercalated Tin Disulfide Associated with Abundant In-Gap Defects" in Advanced Functional Materials, an important journal in materials science. Giant Nonlinear Optical Absorption of Ion-Intercalated Tin Disulfide Associated with Abundant In-Gap Defects" in Advanced Functional Materials, a leading materials science journal. In this study, an electrochemical ion-intercalation strategy is proposed for the first time to conveniently and efficiently modulate the nonlinear absorption properties of two-dimensional materials. The research team electrochemically intercalated a series of alkali metal ions (Li+, Na+, K+) onto layered SnS2 nanosheets, and found that the ion-intercalated products all exhibit significantly enhanced nonlinear absorption properties.
Nonlinear optics is a key fundamental optical process for laser pulse generation, nonlinear optical imaging, optical switching and optical limiting, and other ultra-intense laser-related applications, and the preparation of high-performance nonlinear optical absorbing materials has important scientific value and practical application prospects.
[Measurement methods and some experimental results]
The research team first synthesized SnS2 nanosheets in the environment by chemical bath deposition, where SnCl2-2H2O and thioacetamide were dissolved in ethanol to form a solution, and then a fluorine-doped tin oxide (FTO) substrate was immersed into the solution and deposited for 3 hours at 48°C, followed by rinsing with deionized water and ethanol and drying in a vacuum at 60°C, to obtain a uniformly covered FTO substrate with an ionic intercalated SnS2 nanosheets.
In order to investigate the nonlinear optical properties of ionically intercalated SnS2 nanosheets under different laser excitations, the team designed a pump-probe device that measures the nonlinear absorption response of the material to laser light by using an 800 nm laser as the pump beam and the probe beam. The angle between the pump beam and the probe beam was 5 degrees. A chopper OE3001 (Sine Scientific Instruments) was inserted in the pump beam to produce regular variations in light intensity. The intensity of the probe beam passing through the sample was captured by a photodiode connected to a lock-in amplifier OE1022D (Sine Scientific Instruments). As the probe light passes through the sample, its intensity changes due to the pump light, and the lock-in amplifier detects the weak changes in the probe light and compares them to the known modulation signal generated by the light cutter, detecting very small nonlinear optical effects. Figure 1 illustrates the principle of the pump-probe experiment and a diagram of the experimental structure.
Fig. 1 (a) Schematic of the pump-detect principle (b) Schematic of asynchronous optical sampling (c) Schematic of the pump-detect optical path using a lock-in amplifier.
The conclusions of the pump-probe experiments are shown in Fig. 2. Comparison of c1 and c2 reveals that for the pristine SnS2 sample, the decay curves are symmetric, indicating that the resaturation absorption (RSA) is an instantaneous process, i.e., purely a two-photon absorption process, in which the electrons are excited from the valence band to the conduction band directly under the action of the laser pulse, without any intermediate step involving the excited state. For Li0.952SnII0.398SnIV0.563S2 (lithium-ion intercalated SnS2), the decay curves are asymmetric, suggesting that the optical nonlinearity should be attributed to a continuous process, i.e., excited state absorption (ESA), where electrons may be excited first to a defect state in the bandgap under the laser excitation, and then further excited into the conduction band, involving the excited state as an intermediate step. Therefore, it can be determined by pump-probe experiments that the RSA properties of SnS2 and its ion intercalation materials are caused by excited state absorption.
The team further analyzed the mechanism by which the defects modulate the nonlinear absorption process. Figure 2(d1) depicts the nonlinear optical response of SnS2 under different laser excitations. The response of pristine SnS2 is associated with a direct jump of the valence band to the conduction band involving two- or three-photon absorption. Fig. 2(d2) The process of electron jump from valence to conduction band under laser excitation of ion-inserted SnS2 and the role of bandgap defect states in excited-state absorption. These defect states enhance the Pauli blocking effect under laser excitation, leading to their enhanced reverse saturation absorption at 800 nm and 1030 nm lasing.
Fig. 2 (c1,c2) Pump detection curves, (d1,d2) Mechanism of defect modulation of nonlinear absorption processes.
[Summary]
In the article, Li0.952SnII0.398SnIV0.563S2 materials were synthesized by electrochemical ion intercalation strategy, and the RSA properties of SnS2 and its ion intercalated materials can be determined by excited state absorption through pump-probe experiments. This study not only shows that ion-inserted materials can be used as new high-performance nonlinear optical materials, but also provides an effective strategy to construct high-performance nonlinear optical materials.
[References]
