We synthesize a vertical heterostructure (HS) between tin sulfide (SnS) and molybdenum sulfide (MoS2) by chemical vapor deposition based on the preferential adsorption of sulfur on MoS2. Most of the SnS grains nucleate on MoS2 nanosheets, formatting partially stacked HS with large overlapping regions. Photoluminescence quenching of MoS2 is observed and illustrates effective charge separation in HS. The HS shows increased reverse saturable absorption relative to MoS2 and SnS. The preferential adsorption of sulfur powders on MoS2 and HS growth reported herein will provide a promising approach to the synthesis of other two-dimensional HS.

The knowledge concerning the influence of defects on the nonlinear optical response of materials remains scarce so far. In this work, we have successfully introduced defects into SnS 2 nanosheets by plasma treatment and shown that a defect generation is an effective approach to significantly improve the reverse saturable absorption of SnS 2 . The SnS 2 nanosheets treated with Ar plasma for 40 s exhibit a nonlinear absorption coefficient (β 0 ) as large as (2.9 ± 0.12) × 10 4 cm GW −1 , which is nearly 9 times that of the untreated sample. The influence of Ar-plasma-treatment time, defect type, and defect number on the nonlinear absorption of SnS 2 nanosheets are also studied. Structure and spectroscopy characterization confirms the introduction of S and Sn vacancies with Ar-plasma etching. Surface photovoltage spectroscopy and density functional theory calculation indicate that S vacancies can induce in-gap states in the band gap. These in-gap states act as intermediate states for the successive absorption of photons during femtosecond laser excitation (namely, excited-state absorption). In contrast, Sn defects cannot lead to in-gap states and have a limited contribution to nonlinear absorption. Our result would provide a promising way to improve optical nonlinearities.

The electrical tuning of the nonlinear absorption of materials has promising application potential, while studies remain rare. In this work, we show that the third-order nonlinear absorption of poly(3,4-ethylenedioxythiophene) chemically doped with poly(styrene sulfonic acid) [PEDOT:PSS] can be effectively modulated by external voltage. The 2 nonlinear absorption of the film can be varied between reverse saturable absorption (RSA) and saturable absorption (SA) via voltage control with laser excitation at 800 nm, and corresponding nonlinear absorption coefficient can be tuned in the range -1590 to 518 cm GW -1 . The doping level and energy structure of PEDOT are modulated with different voltages. The undoped film affords two-photon absorption and accordingly the RSA response. A moderately doped sample has two polaron levels, and Pauli blocking associated with these two polaron levels results in SA. The bipolaron level in heavily doped PEDOT leads to excited state absorption and therefore RSA behavior.The approach reported here can be applied to other semiconductors, being a convenient, effective, and promising method for the electrical tuning of the optical nonlinearity.

Herein it is reported that electrochemical ion-intercalation is a convenient and effective strategy toward materials with giant nonlinear optical (NLO) absorption. Alkali-metal ions (i.e., Li + , Na + , K + ) are electrochemically intercalated into SnS 2 nanosheets. All ion-intercalated samples exhibit remarkably enhanced optical nonlinearity compared with an untreated sample, and Liintercalated SnS 2 (Li 0.952 Sn II 0.398 Sn IV 0.563 S 2 ) possesses optimized strong NLO performance. Li 0.952 Sn II 0.398 Sn IV 0.563 S 2 exhibits strong saturable absorption, and the corresponding nonlinear absorption coefficient (β eff ) is -1.7 × 10 4 cm GW -1 for the laser excitation at 515 nm. Li 0.952 Sn II 0.398 Sn IV 0.563 S 2shows prominent reverse saturable absorption with the laser excitation at 800 nm (β eff : 2.8 × 10 4 cm GW -1 ) and 1030 nm (β eff : 1.4 × 10 4 cm GW -1 ). All β eff values are larger than most of the reported inorganic NLO materials at corresponding wavelengths. The optical limiting threshold of Li 0.952 Sn II 0.398 Sn IV 0.563 S 2 is 8 × 10 -4 J cm -2 , two orders of magnitude smaller (better) than the benchmark composite (e.g., SWNT-NH-TPP). Ion intercalation introduces abundant in-gap defects. The excitation of electrons in in-gap states to conduction band intensifies the Pauli-blocking effect and therefore promotes the saturable absorption under the 515 nm laser excitation, while the in-gap defect states acting as effective excitation pathway facilitate excited-state absorption for 800 and 1030 nm laser.

In this work, the electrical tuning of the fifth‐order nonlinear absorption of antimony‐doped tin oxide (ATO) by ionic liquid gating is demonstrated. The pristine ATO film exhibits two‐photon‐induced excited‐state absorption (2PA‐ESA) with laser excitation at 1030 nm. The fifth‐order nonlinear absorption coefficient (γeff) of the ATO film can be monotonically modified in the range of 0.51 to 3.46 cm3 GW−2 by varying the sample bias, with a maximum enhancement factor of 6.8. The fundamental processes occurring during electrical tuning are revealed. The electrostatic and electrochemical capacitance is responsible for the modification in the number of free carriers in the conduction band of ATO. The electrical modulation of the nonlinear absorption of the ATO is ascribed to the voltage‐dependent diameter of the undepleted core of ATO. A smaller voltage results in the charging of ATO and a larger undepleted core, and consequently the size of the active component for 2PA‐ESA is larger. The electrochemical capacitance results from the chemisorption of H+ and OH− on the surface of the ATO, the larger water content in the ionic liquid affording a larger modulation range for the number of free carriers and the γeff.