Multiphoton upconversion is a process where two or more photons are absorbed simultaneously to excite an electron to an excited state and, subsequently, the relaxation of electron gives rise to the emission of a photon with frequency greater than those of the absorbed photons. Materials possessing such property attracted attention due to applications in biological imaging, photodynamic therapy, three-dimensional optical data storage, frequency-upconverted lasing and optical power limiting. Here we report four-photon upconversion in metal–organic frameworks containing the ligand, trans, trans-9,10-bis(4-pyridylethenyl)anthracene. The ligand has a symmetrical acceptor–π–donor–π–acceptor structure and a singlet biradical electronic ground state, which boosted its multiphoton absorption cross-sections. We demonstrate that the upconversion efficiency can be enhanced by Förster resonance energy transfer within host–guest metal–organic frameworks consisting of encapsulated high quantum yielding guest molecules. Using these strategies, metal–organic framework materials, which can exhibit frequency-upconverted photoluminescence excited by simultaneous multiphoton absorption, can be rationally designed and synthesized.

We report our observation of two-photon absorption (2PA) in graphene enhanced by the excitonic Fano resonance at the saddle point with femtosecond laser pulses in the spectral range from 1.8 to 2.9 eV (or, from 435 to 700 nm). On the basis of the second-order, time-dependent perturbation theory on interband transitions among three states near the saddle point of two-dimensional systems (used in the Bassani−Hassan theoretical model), we develop a semiempirical model to take excitons in graphene into consideration. We find that the model is in agreement with the photon-energy dependence of the observed 2PA spectrum with a scaling factor of B = (1−5) × 10 2 cm/MW/eV 5 . ■ INTRODUCTIONThe interaction between graphene and femtosecond laser pulses is of relevance to many practical applications in photonics and optoelectronics. One of the successful applications is that saturation in interband transitions induced by one-photon absorption (1PA) in graphene has been utilized for mode-locking technology. 1−3 1PA saturation results from band-filling effects when graphene is exposed to intense laser pulses. These reports, however, have ignored the presence of two-photon absorption (2PA), which has been experimentally confirmed in saturable absorber mirrors made of InGaAs quantum wells. 4 Theoretical simulations show that the presence of 2PA has a significant impact onto the stability of continuouswave, mode-locking operation. 4 Interestingly, apart from 1PA saturation, two-photon-induced interband transitions in graphene also have potential for photonic applications. In ref 5, coherent control and noncontact generation of ballistic photocurrents in multilayer epitaxial graphene at wavelengths of 3.2 μm/1.6 μm and 4.8 μm/2.4 μm have been demonstrated by utilizing the quantum interference between one-photon-induced and two-photoninduced interband transitions. Utilizing such noncontact method for generating electrical currents can overcome the difficulty of making reliable contacts for future graphene-based electronic devices, which have drawn intensive research interest 6−8 because of the superior electron transport properties of graphene. 9−11 Moreover, such a method eliminates the parasitic effects due to electrical contacts on graphene in potential applications. Therefore, two-photon absorption is of importance to graphene photonics and optoelectronics.From the fundamental point of material properties, 2PA of graphene has not been systematically investigated in the visible spectral region. We have previously demonstrated, both theoretically and experimentally, that 2PA is as important as 1PA saturation in AB-stacking bilayer epitaxial graphene in the near-infrared (NIR) spectral range. 12 In the report, we considered only the optical transitions near the Dirac point (K-point) in the theoretical calculation, and no significant contributions from the optical transitions around the saddle point (M-point) of graphene was experimentally observed in this NIR spectral range. Strong optical transitions around the saddle point due t...

An ultrathin hexagonal boron nitride film is synthesized by a method of chemical vapor deposition. Irradiated by femtosecond laser pulses in the visible spectrum of 400-800 nm, it exhibits multiphoton absorption and positive nonlinear refraction properties. The two-photon and three-photon absorption coefficients are of the order of 10^-5 cm W^-1 and 10^-14 cm³ W^-2, respectively. The nonlinear refraction coefficient is as large as ∼10^-8 cm² W^-1. These nonlinear coefficients lead to figures of merit that meet the material requirements for all-optical switching devices.

We report on the existence of surface dark screening solitons at the interface between a dielectric medium (air) and a self-defocusing nonlinear material, taking advantage of photorefractive diffusion and drift nonlinearities. It is very interesting that a surface dark soliton is just like half of a dark soliton in bulk, but not a whole dark soliton propagating along surface. The excitation, propagation, and stability of this type of soliton are studied by using the beam-propagation method. Another interesting thing is that this type of dark soliton can be excited by a planar light beam without a necessary dark notch.

Surface dark solitons in photovoltaic nonlinear media are reported. Taking advantage of diffusion and photovoltaic nonlinearities we demonstrated the surface dark solitons and their behaviors near surface theoretically and experimentally in LiNbO₃ crystal. It is very interesting that surface dark soliton is just half of dark soliton in bulk. Another interesting thing is that transverse modulation instability can be perfectly suppressed by surface dark soliton in virtue of surface. In addition, surface waveguides were written successfully utilizing surface dark soliton.