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The ultrafast nonlinear optical properties of bulk TlGaS2 crystal, a semiconductor with a layered structure, are studied by combining intensity dependent transmission, time-resolved transient absorption, and optical Kerr effect coupled to optical heterodyne detection. TlGaS2 demonstrates obvious two-photon absorption and electronic nonlinearities at 800 nm. The two-photon absorption coefficient and the nonlinear refractive index are determined to be of the order of 10−10 cm/W and 10−14 cm2/W, respectively. Furthermore, both the real and imaginary parts of the complex third-order susceptibility tensor elements are extracted. The large ultrafast optical nonlinearities make TlGaS2 a promising material for application in photonic techniques.

The transient optical properties of graphene on a dielectric substrate rely on both the direction of light incidence and anisotropic distribution of excited carriers in graphene. Here, we experimentally and theoretically studied the polarization dependence of transient optical reflection and transmission caused by the interplay between the above two factors. The interplay between the two factors resulted in diverse polarization dependence characteristics and could shift the converting incident angle for inverse polarization dependence in transient optical response. For large angles of incidence, transient optical transmission exhibited a stronger polarization dependence and no sign reversal, compared with transient optical reflection. Our findings can be useful for designing high-speed photonic devices with controllable transient optical response of graphene.

We used pump-probe spectroscopy to study the dependence of optical pump-induced change of graphene extinction coefficient on polarization. We find that the optical pump-induced change of graphene extinction coefficient is polarization-dependent for light with energy identical to that of the excitation light; this dependence decreases rapidly after optical excitation. However, the change of extinction coefficient is isotropic for light with energy much smaller than that of the excitation light. The spatial distribution of momenta of non-equilibrium carriers accounts for this polarization dependence. These results provide a potential way for manipulating graphene’s transient optical response characteristics for applications in ultrafast photonics.

Pump-probe differential reflection and transmission spectroscopy is a very effective tool to study the nonequilibrium carrier dynamics of graphene. The reported sign of differential reflection from graphene is not explicitly explained and not consistent. Here, we study the differential reflection and transmission signals of graphene on a dielectric substrate. The results reveal the sign of differential reflection changes with the incident direction of the probe beam with respect to the substrate. The obtained theory can be applied to predict the differential signals of other two-dimensional materials placed on various dielectric substrates.

A giant Goos-Hänchen (G-H) shift in graphene has been theoretically predicted by previous research. In this Letter, we present experimental measurements of the G-H shift in graphene, in a total internal reflection condition, using a new method we have named "the beam splitter scanning method." Our results show that a focused light source undergoes significant lateral shift when the polarization of incident light changes from transverse magnetic (TM) to transverse electric (TE) mode, indicating a large G-H shift in graphene that is polarization-dependent. We also observed that the difference in the G-H shift for TM versus TE modes (S(TM)-S(TE)) increases with increasing thickness of graphene material. A maximum difference (S(TM)-S(TE)) of 31.16 μm was observed, which is a significant result. Based on this research, the ability to engineer giant G-H shifts in graphene material has now been experimentally confirmed for the first time to the best of our knowledge. We expect that this result will lead to significant new and interesting applications of graphene in various types of optical sensors, and more.

Using a two-layer structure consisting of polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS) to support graphene grown by chemical vapor deposition (CVD), we demonstrate a flexible integrated graphene saturable absorber (SA) on microfiber for passive mode-locked soliton fiber laser. This method can optimize the light-graphene interaction by using evanescent field in the integration structure. Moreover, the fiber laser with the in-line microfiber-to-graphene SA can realize the tunabilities of both the 3dB bandwidth of output optical spectrum and the pulse width of soliton. This tunable mode-locked soliton laser has potential applications in optical communication, optical microscopy, and so on.

A versatile solid Poly-methyl-methacrylate (PMMA) composite containing porphyrin-covalently functionalized multi-walled carbon nanotubes (MWNTs-TPP) was prepared through free radical polymerization without additional dispersion stabilizer. Using nanosecond, femtosecond pulse Z-scan and degenerate femtosecond pump-probe techniques, we studied the optical limiting effect, ultrafast saturable absorption and transient differential transmission of the composite. Results show that the solid composite exhibits weaker optical limiting effects than that of the suspension at 532 nm under nanosecond pulse, due to the absence of nonlinear scattering mechanism. The composite also shows ultrafast saturable absorption with a relaxation time about 190 fs at 800 nm under femtosecond pulse due to band-filling effect, comparably to the suspension. The versatile solid composite can be the candidate for uses in applications of ultrafast optical switching and mode-locking element or optical limiter for nanosecond pulse.

The polarization characteristics of nonlinear refraction and nonlinear scattering in several solvents (toluene, o-dichlorobenzene, N, N-dimethylformamide) are studied using the polarized light Z-scan technique with femtosecond laser pulses. For nonlinear refraction, the physical mechanism of the polarization characteristics is that the nonlinear polarization component related to Re(χxyyx(3)) could be adjusted by polarization state of incident lightin isotropic media. For nonlinear scattering, the polarization characteristic is related to the self-focusing effect, which depends on the nonlinear refractive index and polarization state of incident light. In addition, the values of nonlinear refractive indices and third-order nonlinear susceptibility components are determined for these solvents at the femtosecond time scale. Both nonresonant electronic nonlinearity and fast noninstantaneous nuclear nonlinearity contribute to these values at the pulse width used in the measurements. Also, these nonlinear refractive indices and third-order nonlinear susceptibility components could be used in calculating nonlinear optical parameters of those novel materials that need to be dissolved in these solvents in nonlinear optical measurements.

We report that a modified Z-scan method by nonlinear ellipse rotation (NER) can be used to discriminate true nonlinear refraction from thermal effect in the transient regime and steady state. The combination of Z-scan and NER allows us to measure the third-order nonlinear susceptibility component without the influence of thermal-optical nonlinearity. The experimental results of pure CS(2) and CS(2) solutions of nigrosine verify that the transient thermal effect can be successfully eliminated from the NER-modified Z-scan measurements. This method is also extended to the case in which thermal-optical nonlinearities depend on a high repetition rate of femtosecond laser pulses for the N,N-dimethylmethanamide solutions of graphene oxide.

The real parts of third-order nonlinear susceptibility components of CS2 are determined by polarized lights Z-Scan technique at 800 nm, and imaginary part is verified to be negligible. The contributions to susceptibility components from electron and nuclear are separated. These susceptibility values can be used as the reference values for third-order nonlinear susceptibility measurements by degenerate four-wave-mixing, optical Kerr gate/optical Kerr effect, optical heterodyne detection of optical Kerr effect, the ellipse rotation and so on.

The normal elliptically polarized light Z-scan method is modified by adding a quarter-wave plate and an analyzer before the detector. The normalized transmittance formulas of modified elliptically polarized light Z-scan are obtained for media with negligible nonlinear absorption. Compared with normal linearly and elliptically polarized light Z-scan methods, an increase of sensitivity by a factor of larger than 4 is achieved for the real part of third-order susceptibility component's measurements using this modified elliptically polarized light Z-scan method. The analytical results are verified by studying the real part of independent susceptibility components of CS(2) liquid. Moreover, the potential application for cross-polarized wave generation is discussed.

The nonlinear optical and optical limiting properties of an oligothiophene (6THIOP) covalently functionalized graphene hybrid material (Graphene-6THIOP) were investigated by using Z-scan technique with a 5 ns Q-switched pulsed laser at 532 nm. Results show that the hybrid material of Graphene-6THIOP exhibits enhanced nonlinear optical and optical limiting properties in comparison to individual 6THIOP, graphene moiety and C60. The enhanced nonlinear optical properties of Graphene-6THIOP should be attributed to the combination of the observed nonlinear scattering with the possible photoinduced electron or energy transfer mechanism between 6THIOP moiety and graphene.

Nonlinear absorption of carbon disulfide (CS2) was studied in the wavelength region of 420-530nm by using Z-scan method with nanosecond pulses. Large nonlinear absorption was found in the short-wavelength region (420-460nm). The optical limiting properties of CS2 were also studied, and results showed that CS2 has good optical limiting capability in the 420-460nm region. The mechanism of the nonlinear absorption was analyzed by the temporal profile of pulses going through CS2. The large nonlinear absorption is believed to arise from a combination of two-photon absorption and the excited-state absorption induced by two-photon absorption.

We present a method that combines the Z-scan technique with nonlinear ellipse rotation (NER) to measure third-order nonlinear susceptibility components. The experimental details are demonstrated, and a comprehensive theoretical analysis is given. The validity of this method is verified by the measurements of the nonlinear susceptibility tensor of a well-characterized liquid, CS2.

Transmission through a single subwavelength slit surrounded by nonperiodic or approximately periodic grooves in perfect conductive film has been analyzed numerically by the finite-difference time-domain method. Results show that the periodicity of the grooves is not the necessary condition to enhance the transmission, but is sufficient to obtain high enhancement. A spectrum with low enhancement and multiple peaks is obtained through a slit with nonperiodic grooves that is due to the resonances at different wavelengths caused by nonperiodicity. The spectrum of a slit with approximately periodic grooves is almost similar to that of a slit with absolutely periodic grooves regardless of whether the grooves are symmetric with respect to the slit, especially when the deflection of the grooves is very small compared to the grooves' period.

We report on an active tuning of nonlinear absorption from reverse saturable absorption to saturable absorption in supramolecular zinc diphenylporphyrins by the combination of molecular and supramolecular levels at 532, 542 and 552 nm. Firstly, the modifications of molecular structures with bromination result in a discrete but regular change in magnitude and sign of nonlinear absorption (NLA). Furthermore, a fine tuning of NLA was obtained by the intermolecular weak interaction of pyridine and zinc porphyrins. Using an association model, we theoretically simulate the change of NLA. Compared with modifying molecular structures of conventional organic materials, the weak intermolecular interaction of supramolecular porphyrins has another advantage that it can be realized more easily and flexibly to change NLA.

Using the Gaussian decomposition (GD) method, we studied the characteristics of a Z scan for a thin nonlinear medium with a large nonlinear phase shift induced by a pulsed laser. It has been verified that the GD method is still valid for analyses of Z-scan measurements with a large nonlinear phase shift and is better than some others, i.e., Fresnel-Kirchhoff diffraction and the aberration-free approximation model. By comparing the peak-to-valley configuration of Z-scan curves for a large nonlinear phase shift induced by a pulsed laser with that by a cw laser, we found that some peak-to-valley features of Z-scan curves appear as the aperture size or the light intensity increases in the case of a large nonlinear phase shift. Meanwhile, we carried out the Z-scan experiments of pure CS2 by a picosecond pulsed laser to verify the theoretical calculations in the case of a large nonlinear phase shift. The experimental results agree well with the theoretical calculations.

The type of donor used is found to influence greatly the optical nonlinearities of iodine charge-transfer (CT) complexes in solutions for several solvents. The magnitude and sign changes of nonlinear absorption of iodine CT complexes for different solvents as donors are observed, which implies the transition from saturable absorption to reverse saturable absorption. Compared with C60 in toluene, iodine CT complexes with toluene and benzene demonstrate larger reverse saturable absorption. Meanwhile, flexible alteration of optical nonlinearities is easily obtained by adjusting the mixture ratio of different solvents, yielding a prospective means of constructing a nonlinear medium with a large nonlinear coefficient.

We present a theoretical model of optical beam propagation in a nonlinear medium for determining the nonlinear refraction and nonlinear absorption accurately and simultaneously by a single Z-scan method. The model is based on the Gaussian decomposition method and symmetric analysis. In principle, accurate nonlinear refraction and nonlinear absorption can be obtained simultaneously by exact experimental data, no matter how much they are. The treatment procedure of Z-scan experimental data is demonstrated.