Three-photon-induced four-photon absorption and nonlinear refraction in ZnO quantum dots

Intramolecular charge transfer enhanced optical limiting in novel hydrazone derivatives with a D1-D-Ai-π-A structure

The present paper investigates one of the hydrazone derivatives (BTH with a D-π-A structure) based on density functional theory. With the computation results of ground state absorption (GSA), excited-state absorption (ESA) and multi-photon absorption (MPA), the optical limiting effect observed in the experiment for the BTH molecule can be well predicted and elucidated by the MPA-ESA mechanism. The analysis of the hole-electron and the electron density differences between two transition states reveal that the main transitions involved in the GSA and ESA of BTH could be recognized as local excitation. Based on these observations, four novel hydrazone derivatives based on the BTH unit with a D1-D-Ai-π-A structure were designed to promote intramolecular charge transfer (ICT). It shows that the ICT effect is well improved by adding the D1 and Ai units. Compared with the original BTH molecule, the main bands of GSA and ESA of D1-D-Ai-π-A molecules are both red-shifted. In addition, GSA, ESA and MPA probabilities are all improved because the obvious charge transfer character results in the transition dipole moment change from localized to delocalized. Accordingly, the optical limiting effect in these hydrazone derivatives is well enhanced. These observations provide guidance for designing novel optical limiting materials based on the hydrazone derivatives.

Ultrafast and large optical nonlinearity of a TiSe2 saturable absorber in the 2 μm wavelength region

The non-equilibrium state of correlated electron materials is crucial for both scientific research and practical applications in optoelectronic and photonic devices. Because of the weak optical nonlinearity of most materials even under a dense optical excitation, it is desirable to achieve a significant nonlinear optical response with ultrafast and large optical nonlinearity utilizing a common material. Here, an ultrafast response and large optical nonlinearity induced by non-equilibrium electrons in typical transition metal dichalcogenides, TiSe2, are investigated in the 1.55-2.0 μm wavelength region. Significantly, we observe an ultrafast transient dynamics of 491 femtoseconds as well as a large optical nonlinearity with a saturable coefficient of -0.17 cm GW-1 (1.55 μm) and -0.10 cm GW-1 (2.0 μm). Upon increasing pump fluence, TiSe2 exhibits an enhanced bleaching response amplitude up to 563%. Furthermore, a stable Q-switched fiber laser in the 2.0 μm wavelength region is achieved by employing the TiSe2-saturable absorber. The findings offer the potential design to enhance the optical nonlinearity via non-equilibrium electrons for advanced photonic devices.

The structure and photoluminescence of a ZnO phosphor synthesized by the sol gel method under praseodymium doping

This work outlines some interesting results regarding the effects of Pr³⁺ substitution on the structural and optical properties of (x = 0 and 0.02) samples.

Two-photon and three-photon absorption in ZnO nanocrystals embedded in Al2O3 matrix influenced by defect states

The broadband nonlinear absorption in ZnO nanocrystals embedded in Al₂O₃ matrix was investigated by Z-scan and pump-probe techniques from 400 nm to 800 nm. The effective two-photon absorption and three-photon absorption coefficients were determined to be ∼1.1×10³  cm/GW at 400 nm and ∼1.1×10^-1  cm³/GW² at 800 nm, respectively, which are at least two orders of magnitude greater than that in ZnO bulk crystal. It may be attributed to the defect-states-mediated multiphoton absorption process, which was proofed by comparison experiments with different densities of interfacial defect states. The corresponding lifetimes for the intraband relaxation, defect-states trapping, and interband recombination processes were measured by femtosecond transient absorption measurements as τ1 ∼ 1  ps, τ2 ∼ 13  ps, and τ3 ∼ 350  ps, respectively.

Nonlinear optical and multi-photon absorption properties in graphene-ZnO nanocomposites

Graphene-ZnO (GZO) nanocomposites were synthesized by a modified solvothermal method, and characterized by transmission electron microscopy, x-ray diffraction, Raman spectra, and UV-vis absorption spectra. The controllable nonlinear optical (NLO) properties of as-prepared GZO nanocomposites were tested by an open-aperture Z-scan method with 1030 nm fs laser pulses; the tested results showed that there were five-photon absorption (5PA) at 46.8 GW cm^-2, 3PA at 28.1 GW cm^-2, 2PA at 18.7 GW cm^-2, and a vital change from saturable absorption (SA) to reverse SA (RSA) with the increase of incident intensity. This was the first time that 5PA was found in GZO nanocomposites at such a low intensity, 46.8 GW cm^-2. The tunable NLO property from SA to RSA and controllable multi-photon absorption provided a facile approach for their applications in optical, optoelectronic devices, and information storage.

Saturation of two photon emission in ZnO nanoparticles with second order nonlinearity

The nonlinear optical effects such as second harmonic generation and two-photon emission in ZnO nanocrystals have been studied using microscopic time-resolved and time-integrated emission spectroscopy.

Competition between second harmonic generation and two-photon-induced luminescence in single, double and multiple ZnO nanorods

The nonlinear optical properties of single, double and multiple ZnO nanorods (NRs) were investigated by using a focused femtosecond (fs) laser beam. The excitation wavelength of the fs laser was intentionally chosen to be 754 nm at which the energy of two photons is slightly larger than that of the exciton ground state but smaller than the bandgap energy of ZnO. Second harmonic generation (SHG) or/and two-photon-induced luminescence (TPL) were observed and their dependences on excitation density were examined. For single ZnO NRs, only SHG was observed even at the highest excitation density we used in the experiments. The situation was changed when the joint point of two ZnO NRs perpendicular to each other was excited. In this case, TPL could be detected at low excitation densities and it increased rapidly with increasing excitation density. At the highest excitation density of ~15 MW/cm(2), the intensity of the TPL became comparable to that of the SHG. For an ensemble of ZnO NRs packed closely, a rapid increase of TPL with a slope of more than 7.0 and a gradual saturation of SHG with a slope of ~0.34 were found at high excitation densities. Consequently, the nonlinear response spectrum was eventually dominated by the TPL at high excitation densities and the SHG appeared to be very weak. We interpret this phenomenon by considering both the difference in electric field distribution and the effect of heat accumulation. It is suggested that the electric field enhancement in double and multiple NRs plays a crucial role in determining the nonlinear response of the NRs. In addition, the reduction in the bandgap energy induced by the heat accumulation effect also leads to the significant change in nonlinear response. This explanation is supported by the calculation of the electric field distribution using the discrete dipole approximation method and the simulation of temperature rise in different ZnO NRs based on the finite element method.

Third order nonlinear optical property of Bi₂Se₃

The third order nonlinear optical property of Bi₂Se₃, a kind of topological insulator (TI), has been investigated under femto-second laser excitation. The open and closed aperture Z-scan measurements were used to unambiguously distinguish the real and imaginary part of the third order optical nonlinearity of the TI. When excited at 800 nm, the TI exhibits saturable absorption with a saturation intensity of 10.12 GW/cm² and a modulation depth of 61.2%, and a giant nonlinear refractive index of 10⁻¹⁴ m²/W, almost six orders of magnitude larger than that of bulk dielectrics. This finding suggests that the TI:Bi₂Se₃ is indeed a promising nonlinear optical material and thus can find potential applications from passive laser mode locker to optical Kerr effect based photonic devices.

Mixed two- and three-photon absorption in bulk rutile (TiO2) around 800 nm

We observe mixed two- and three-photon absorption in bulk rutile (TiO2) around 800 nm using the open aperture Z-scan technique. We fit the data with an extended model that includes multiphoton absorption, beam quality, and ellipticity. The extracted two- and three-photon absorption coefficients are below 1 mm/GW and 2 mm3/GW2, respectively. We observe negligible two-photon absorption for 813-nm light polarized along the extraordinary axis. We measure the nonlinear index of refraction and obtain two-photon nonlinear figures of merit greater than 1.1 at 774 nm and greater than 12 at 813 nm. Similarly, we obtain three-photon figures of merit that allow operational intensities up to 0.57 GW/mm2. We conclude that rutile is a promising material for all-optical switching applications around 800 nm.