Two-photon Absorption Saturation Research Articles

Double transformation of the nonlinear absorption in silver nanoparticles.

The nonlinear absorption of 40 nm Ag nanoparticles (Ag NPs) was investigated using open aperture (OA) Z-scan technique at 532 nm. Experiments show that the nonlinear absorption behavior of Ag NPs is intensity dependent. Specifically, under low laser energy the Ag NPs shows saturable absorption (SA). At medium laser energy, the transformation of nonlinear absorption from SA to reverse saturable absorption (RSA) happens. While under stronger laser energy, double transformation (SA→RSA→SA) of nonlinear absorption occurs. The experimental results were analyzed theoretically using a model based on single-photon absorption and two-photon absorption saturation.

Third-Order Nonlinear Optical Properties and Saturation of Two-Photon Absorption in Lead-Free Double Perovskite Nanocrystals under Femtosecond Excitation

Lead halide perovskites have been widely explored in the field of photovoltaics, light-emitting diodes, and lasers due to their outstanding linear and nonlinear optical (NLO) properties. But, the p...

Comparison of Lambert W function with Adomian decomposition method for wave propagation in saturable absorption medium

We present a comparative study to examine the performances of both the Lambert W function method and Adomian decomposition method (ADM) for solving the nonlinear optical beam propagations in a saturable absorption (SA) medium for two distinct SA models. The SA-I model describes a fast saturable absorber (in which the relaxation time is much shorter than the pulse duration of the incident laser beam and no excited state absorption occurs) for a three-level system, while the SA-II model depicts a four-level system or two-photon absorption saturation. We derive the explicit analytic solutions based on the Lambert W function as well as ADM without using thin-sample approximation, and without weak saturation limit. The two different approaches to solve the nonlinear optical wave equation for two distinct SA models are compared and discussed in detail, and show excellent agreement with each other. Finally, to validate the theoretical analysis, we provide some experimental data for nonlinear optical transmission in methyl orange-doped PVA films, revealing excellent agreement with the theory of the SA-II model.

Experimental evidence of two-photon absorption and its saturation in malachite green oxalate: a femtosecond Z-scan study

Third-order nonlinear optical (NLO) properties of malachite green oxalate (MGO) dye were systematically studied at different input intensities and in a wide range of wavelengths by the Z-scan method using ∼150  fs, MHz femtosecond pulses. The sample transmittance results are explained by fitting the data with phenomenological models of linear and multi-photon absorption and their saturations. Intensity-dependent measurements demonstrated the influence of saturation on the absorption process. The data obtained suggests that by tuning the input intensity single or combination of nonlinear phenomena can be achieved in MGO. Wavelength-based measurements reveal a strong correlation between linear and NLO responses. The combinations of two- and three-photon absorption and saturation of two-photon absorption occur simultaneously in a wide wavelength range (750–900 nm). Multi-photon absorption cross sections have been calculated using the NLO coefficients and the concentration data. Two- and three-photon absorptions (including an excited state) are determined as the excitation mechanisms of nonlinear absorption in MGO.

Saturation of Two-Photon Absorption in Layered Transition Metal Dichalcogenides: Experiment and Theory

The saturation of two-photon absorption (TPA) in four types of layered transition metal dichalcogenides (TMDCs) (MoS2, WS2, MoSe2, WSe2) was systemically studied both experimentally and theoretically. It was demonstrated that the TPA coefficient is decreased when either the incident pulse intensity or the thickness of the TMDC nanofilms increases, while TPA saturation intensity has the opposite behavior, under the excitation of 1.2 eV photons with a pulse width of 350 fs. A three-level excitonic dynamics simulation indicates that the fast relaxation of the excitonic dark states, the exciton–exciton annihilation, and the depletion of electrons in the ground state contribute significantly to TPA saturation in TMDC nanofilms. Large third-order nonlinear optical responses make these layered 2D semiconductors strong candidate materials for optical modulation and other photonic applications.

Fabrication and near-infrared optical responses of 2D periodical Au/ITO nanocomposite arrays

Two-dimensional (2D) periodical Au and indium tin oxide (ITO) nanocomposite arrays have been fabricated based on a self-assembled nanosphere lithography technique. A button-shaped Au nanoparticle was formed on each hollow hemisphere-shaped ITO shell. Importantly, the underlying formation mechanism during the thermal treatment has been thoroughly explored by comparing structures resulting from different deposition conditions in detail. Compared to the Au nanoparticle arrays without ITO shells, the Au/ITO nanocomposite arrays showed a stronger localized surface plasmon resonance effect and higher absorption in the near-infrared (NIR) region, benefiting from the free-electron interaction enhancement between Au and ITO. The nonlinear optical properties were investigated using a modified femtosecond intensity-scan system, and the results demonstrated Au/ITO nanocomposite arrays with a remarkable two-photon absorption saturation effect for femtosecond pulses at 1030 nm. The versatile NIR optical responses indicate the great potential of the elaborately prepared 2D periodical Au/ITO nanocomposite arrays in many applications such as solar cells, photocatalysis, and novel nano optoelectronic devices.

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.

Occurrence of two-photon absorption saturation in Ag nanocolloids, prepared by chemical reduction method

Silver nanocolloids stabilized with polyvinyl pyrrolidone (PVP) have been prepared from (AgNO3) by a chemical reduction method, involving the intermediate preparation of (Ag2O) colloidal dispersions in the presence of sodium dodecycle sulfate as a surfactant and formaldehyde as reducing agent. The molecules of PVP play an important role in growth and agglomeration of silver nanocolloids. The formation of Ag nanocolloids was studied from the UV–vis absorption characteristics. An energy dispersive X-ray (EDX) spectrum and X-ray diffraction peak of the nanoparticles showed the highly crystalline nature of silver structure. The particle size was found to be 40nm as analyzed from Field emission scanning electron microscopy (FESEM). The nonlinear optical and optical limiting properties of these nanoparticle dispersions were studied by using the Z-scan technique at 532nm. Experimental results show that the Ag nanocolloids possess strong optical limiting effect, originated from absorption saturation followed by two-photon mechanism. The data show that Ag nanocolloids have great potential for nonlinear optical devices.

The two-photon absorption saturation process in an Au nanoparticle array

The triangular Au nanoparticle array of size 37 nm was fabricated on a quartz substrate using nanosphere lithography. By performing the Z-scan method with femtosecond laser (800 nm, 50 fs), the optical nonlinearity of the Au nanoparticle array was determined. The results showed an intensity-related two-photon absorption saturation processes. As the excitation intensity increases, the nonlinear absorption changes from two-photon absorption to saturated absorption, while the nonlinear refraction exhibits self-defocusing effect. The one- and two-photon figures of merit, W=7.5 and T=0.12, were obtained, validating a high efficiency nonlinear material for all-optical switching.

Effects of absorption saturation in colloidal quantum dots as fluorophores for multiphoton fluorescence microscopy

It is shown that colloidal semiconductor nanocrystals (quantum dots), which are promising fluorophores for multiphoton fluorescence microscopy, exhibit a two-photon absorption saturation effect at moderate powers (not exceeding 10 mW) of femtosecond pumping radiation. An analytical expression for the power of two-photon fluorescence of quantum dots as a function of the average pumping power is obtained. With this expression, the deviation of the found dependence from the quadratic law is explained by two factors, i.e., a large two-photon absorption cross section of quantum dots and their slow (compared to the typical pumping pulse-repetition period) relaxation to an unexcited state. Using an LSM 510 Carl Zeiss laser scanning microscope equipped with a Ti:Sa tunable femtosecond laser, a series of model experiments is performed to reveal the saturation effect in a solution of commercially available quantum dots. Good agreement is obtained between the measured dependence of the power of two-photon fluorescence on the average pump power and the theoretical calculation results. It is also experimentally demonstrated that, under fluorescence saturation conditions, the spatial resolution of the method of multiphoton fluorescence microscopy is lost; this effect is analyzed numerically.

Z-scan theory of two-photon absorption saturation and experimental evidence

In our article [B. Gu, Y. X. Fan, J. Wang, J. Chen, J. P. Ding, H. T. Wang, and B. Guo, Phys. Rev. A 73, 065803 (2006)] we have presented the theory of open-aperture Gaussian-beam Z-scan, based on the Adomian decomposition method, which is available only for the saturable absorption caused by single-photon absorption transition. In the present article, using the same technique (Adomian decomposition method, as a common technique), we develop an open-aperture Z-scan theory for evaluating the property of the saturable absorption originating from two-photon absorption (2PA) transition, when a spatial Gaussian beam is used as the excitation source. We find analytic polynomial expressions of the Z-scan traces for a continuous wave laser or a temporal Gaussian pulsed laser. As the experimental evidence, we investigate the saturable 2PA behaviors caused by the interband two-photon transition in the direct-gap II–VI semiconductors CdS, CdSe, ZnSe, and ZnTe, under the excitation condition of a femtosecond laser with a 140 fs pulse duration, a 1 kHz low repetition rate, and a 1.6 eV photon energy.

Observation of three-photon absorption and saturation of two-photon absorption in amorphous nanolayered Se/As2S3 thin film structures

We have studied the nonlinear optical properties of nanolayered Se/As2S3 film with a modulation period of 10 nm and a total thickness of 1.15 μm at two [1064 nm (8 ns) and 800 nm (20 ps)] wavelengths using the standard Z-scan technique. Three-photon absorption was observed at off-resonant excitation and saturation of two-photon absorption at quasiresonant excitation. The observation of the saturation of two-photon absorption is because the pulse duration is shorter than the thermalization time of the photocreated carriers in their bands and three-photon absorption is due to high excitation irradiance.

Absorption and subsequent emission saturation of two-photon excited materials: theory and experiment

The saturation of two-photon absorption and the subsequent photoluminescence of matter under ultrafast laser pulse excitation are studied. For this purpose the experimental results of using two organic conjugated polymers as model substances are discussed theoretically. One model is based on the commonly used hyperbolic approach, whereas the new theory introduced correctly describes the saturation in two-photon spectroscopy and fits the experiment closely. The new model permits microscopic analysis of the absorption saturation process and makes possible an estimate of the two-photon absorption coefficient from the saturation intensity.

Intrachain exciton quenching analysis in conjugated polymers by two-photon spectroscopy

Intrachain exciton quenching in ladder-type polyparaphenylene has been quantified for the first time. Two-phonon absorption and subsequent upconverted photoluminescence of ladder-type polyparaphenylene and polyfluorene has been measured and analyzed with a new theoretical framework for the two-photon absorption saturation. This microscopic analysis determines the number of singlet excitons as a function of the excitation intensity, which is compared to the photoluminescence intensity. The absorption saturation alone perfectly describes the photoluminescence saturation in polyfluorene, whereas in ladder-type polyparaphenylene, exciton–exciton quenching occurs. By measuring in solution, the formation of stacked polymer chains is prevented. Therefore, the quenching is due to intrachain interaction.

Nonlinear Saturation and Determination of the Two-Photon Absorption Cross Section of Green Fluorescent Protein

Two-photon absorption is a nonlinear phenomenon that is observed in materials irradiated with the combined energy of two photons, which matches the transition energy between the ground state and the excited state. Molecules exhibiting strong two-photon absorption have great potential in a wide range of applications such as three-dimensional fluorescence imaging, optical data storage, and microfabrication.9,6 Green fluorescent protein (GFP) can be excited by a two-photon mechanism and exhibits a nonlinear behavior of the effective two-photon absorption coefficient. Here we present the quantitative determination of the two-photon absorption saturation of GFP in both a homogeneously broadened and inhomogeneously broadened system. We further show that, dominated by strong nonlinear saturation, GFP has a large intrinsic two-photon absorption cross section (σTPA) value that is several orders of magnitude larger than that of chemically synthesized compounds.

Two-photon excitation and bleaching of the 400 nm luminescence band in germanium-doped-silica optical fibres

The 400 nm luminescence band of germanium-doped-silica optical fibres was excited via two-photon absorption of an argon ion laser beam. It was observed that the photoluminescence decreased by as much as 25% during the exposure of the fibres to several hundred milliwatts of optical power fora few hours. Surprisingly, the amount of bleaching is not a given proportion of the initial intensity of the luminescence band. These luminescence measurements are qualitatively related to the photosensitivity of the optical fibres. The saturation of two-photon absorption at relatively low power is also observed.