Ningning Dong

Direct Observation of Degenerate Two-Photon Absorption and Its Saturation in WS2 and MoS2 Monolayer and Few-Layer Films

The optical nonlinearity of WS2 and MoS2 monolayer and few-layer films was investigated using the Z-scan technique with femtosecond pulses from the visible to the near-infrared range. The nonlinear absorption of few- and multilayer WS2 and MoS2 films and their dependences on excitation wavelength were studied. WS2 films with 1-3 layers exhibited a giant two-photon absorption (TPA) coefficient as high as (1.0 ± 0.8) × 10(4) cm/GW. TPA saturation was observed for the WS2 film with 1-3 layers and for the MoS2 film with 25-27 layers. The giant nonlinearity of WS2 and MoS2 films is attributed to a two-dimensional confinement, a giant exciton effect, and the band edge resonance of TPA.

Optical Limiting and Theoretical Modelling of Layered Transition Metal Dichalcogenide Nanosheets.

Nonlinear optical property of transition metal dichalcogenide (TMDC) nanosheet dispersions, including MoS2, MoSe2, WS2, and WSe2, was performed by using Z-scan technique with ns pulsed laser at 1064u2009nm and 532u2009nm. The results demonstrate that the TMDC dispersions exhibit significant optical limiting response at 1064u2009nm due to nonlinear scattering, in contrast to the combined effect of both saturable absorption and nonlinear scattering at 532u2009nm. Selenium compounds show better optical limiting performance than that of the sulfides in the near infrared. A liquid dispersion system based theoretical modelling is proposed to estimate the number density of the nanosheet dispersions, the relationship between incident laser fluence and the size of the laser generated micro-bubbles, and hence the Mie scattering-induced broadband optical limiting behavior in the TMDC dispersions.

Giant two‐photon absorption in monolayer MoS2

Strong two-photon absorption (TPA) in monolayer MoS2 is demonstrated in contrast to saturable absorption (SA) in multilayer MoS2 under the excitation of femtosecond laser pulses in the near-infrared region. MoS2 in the forms of monolayer single crystal and multilayer triangular islands are grown on either quartz or SiO2/Si by employing the seeding method through chemical vapor deposition. The nonlinear transmission measurements reveal that monolayer MoS2 possesses a nonsaturation TPA coefficient as high as ∼(7.62 ±0.15) ×103xa0cm/GW, larger than that of conventional semiconductors by a factor of 103. As a result of TPA, two-photon pumped frequency upconverted luminescence is observed directly in the monolayer MoS2. For the multilayer MoS2, the SA response is demonstrated with the ratio of the excited-state absorption cross section to ground-state cross section of ∼0.18. In addition, the laser damage threshold of the monolayer MoS2 is ∼97 GW/cm2, larger than that of the multilayer MoS2 of ∼78 GW/cm2.

Tunable effective nonlinear refractive index of graphene dispersions during the distortion of spatial self-phase modulation

Spatial self-phase modulation (SSPM) was observed directly when a focused He-Ne laser beam at 633u2009nm went through liquid-phase-exfoliated graphene dispersions. The diffraction pattern of SSPM was found to be distorted rapidly right after the incident beam horizontally passing through the dispersions, while no distortion for the vertically incident geometry. We show that the distortion is originated mainly from the non-axis-symmetrical thermal convections of the graphene nanosheets induced by laser heating, and the relative change of nonlinear refractive index can be determined by the ratio of the distortion angle to the half-cone angle. Therefore, the effective nonlinear refractive index of graphene dispersions can be tuned by changing the incident intensity and the temperature of the dispersions.

Saturable absorption behavior of free-standing graphene polymer composite films over broad wavelength and time ranges

A comparative research on saturable absorption (SA) behavior dependence on wavelength and pulse duration was performed for graphene polymer composites. Free-standing graphene-polyvinyl alcohol (PVA) composite films were fabricated by using solution cast method in combination of liquid phase exfoliation. SA responses were observed by using an open-aperture Z-scan technique for 340 fs pulses at 1030 nm and 515 nm from a mode-locked fiber laser, and 6 ns pulses at 1064 nm and 532 nm from a Q-switched Nd:YAG laser. The graphene films possess better SA property, i.e., larger SA coefficient and figure of merit (FOM), and lower saturation intensity I(s), for ns pulses than that for fs pulses at the similar near infrared (NIR) wavelength. For fs pulses, the films show better SA response at 1030 nm than that at 515 nm. By employing slow and fast SA modelling, the excited state and ground state absorption cross sections were estimated to be ~10(-17) cm(2), and the ratio was ~0.6 at NIR for both fs and ns pulses.

Controllable broadband nonlinear optical response of graphene dispersions by tuning vacuum pressure

Nonlinear scattering, originating from laser induced solvent micro-bubbles and/or micro-plasmas, is regarded as the principal mechanism for nonlinear optical (NLO) response of graphene dispersions at ns timescale. In this work, we report the significant enhancement of NLO response of graphene dispersions by decreasing the atmospheric pressure, which has strong influence on the formation and growth of micro-bubbles and/or micro-plasmas. A modified open-aperture Z-scan apparatus in combination with a vacuum system was used to study the effect of vacuum pressure on the NLO property of graphene dispersions prepared by liquid-phase exfoliation technique. We show that the atmospheric pressure can be utilized to control and tune the nonlinear responses of the graphene dispersions for ns laser pulses at both 532 nm and 1064 nm. The lower the vacuum pressure was, the larger the NLO response was. In contrast, the NLO property of fullerene was found to be independent of the pressure change, due to its nature of nonlinear absorption. This work affords a simple method to distinguish the nonlinear scattering and absorption mechanisms for NLO nanomaterials.

Covalent Modification of MoS2 with Poly(N‐vinylcarbazole) for Solid‐State Broadband Optical Limiters

New soluble MoS2 nanosheets covalently functionalized with poly(N-vinylcarbazole) (MoS2-PVK) were in situ synthesized for the first time. In contrast to MoS2 and MoS2 /PVK blends, both the solution of MoS2 -PVK in DMF and MoS2-PVK/poly(methyl methacrylate) (PMMA) film show superior nonlinear optical and optical limiting responses. The MoS2-PVK/PMMA film shows the largest nonlinear coefficients (βeff) of about 917 cm GW(-1) at λ=532 nm (cf. 100.69 cm GW(-1) for MoS2/PMMA and 125.12 cm GW(-1) for MoS2/PVK/PMMA) and about 461 cm GW(-1) at λ=1064 nm (cf. -48.92 cm GW(-1) for MoS2/PMMA and 147.56 cm GW(-1) for MoS2/PVK/PMMA). A larger optical limiting effect, with thresholds of about 0.3 GW cm(-2) at λ=532 nm and about 0.5 GW cm(-2) at λ=1064 nm, was also achieved from the MoS2-PVK/PMMA film. These values are among the highest reported for MoS2-based nonlinear optical materials. These results show that covalent functionalization of MoS2 with polymers is an effective way to improve nonlinear optical responses for efficient optical limiting devices.

Slow and fast absorption saturation of black phosphorus: experiment and modelling.

According to the excited-state decay time, a slow saturable absorber model was employed to obtain the ground-state absorption and excited-state absorption cross sections σgs and σes of black phosphorus (BP) nanosheets, which are (1.25 ± 0.07) × 10-16 cm2 and (2.97 ± 0.21) × 10-17 cm2 at 515 nm, and (5.95 ± 0.17) × 10-18 cm2 and (5.19 ± 0.71) × 10-19 cm2 at 1030 nm, respectively. In comparison, the σgs and σes of MoS2 and graphene were also obtained with the same model. The ratio σes/σgs of BP is the smallest (only 0.24 ± 0.03 at 515 nm and 0.09 ± 0.01 at 1030 nm) among the three two-dimensional layered materials both at 515 nm and 1030 nm. Relatively large ground-state transition probability and weak loss of excited-state absorption result in good saturable absorption performance. In comparison with MoS2 and graphene, the much lower saturation intensity of BP can be well explained from this perspective.

Tin diselenide as a new saturable absorber for generation of laser pulses at 1μm

The nonlinear optical responses of two-dimensional (2D) few-layer tin diselenide (SnSe2) have been investigated in this work. By applying the Z-scan technique, the saturable absorption of SnSe2 thin film has been observed at a wavelength of ~1μm, which enables SnSe2, a new saturable absorber for pulsed laser generation. A detailed exploration of the nonlinear absorption of SnSe2 film and their dependence on excitation pulse energy is carried out. Our results show that saturation intensity of the nonlinear absorption is ~23.78 GW/cm2 and corresponding nonlinear absorption coefficients are at a range from -13596 to -2967.3 cm/GW. By employing the SnSe2-coated mirror as a saturable absorber, the passively Q-switched lasing has been achieved on a crystalline waveguide platform at ~1 μm.

Host matrix effect on the near infrared saturation performance of graphene absorbers

A comparative research on the near infrared performance of three kinds of widely used graphene saturable absorbers, namely, graphene polymer composite films, neat graphene films and graphene dispersions, was performed by using Z-scan technique with 340 fs pulses at 1030 nm. The polymer films and graphene films were fabricated through solution cast method and vacuum filtration technique based on the liquid-phase exfoliated graphene dispersions, respectively. The polymer films reveal the best saturable absorption (SA) response and the lowest saturation intensity Is, in comparison with the neat films and dispersions. The graphene films show the largest SA coefficient and figure of merit due to its highest linear absorption coefficient and refractive index. By employing slow SA modeling, the excited state and ground state absorption cross sections were estimated to be ~10−17 cm2, and the ratio were 0.61, 0.57 and 0.71 for the dispersions, polymer films and neat films, respectively.