Xingzhi Wu

Ultrafast broadband optical limiting in simple pyrene-based molecules with high transmittance from visible to infrared regions

Pyrene is considered as one of the most promising nonlinear functional building blocks. The nonlinear optical properties of materials can be improved by modifying the main nonlinear group possession ratio occupied in frontier molecular orbitals. Accordingly, we designed and synthesized two isomeric molecules based on pyrene for ultrafast broadband optical limiters. Different pyrene possession ratios occupied in frontier molecular orbitals were calculated through quantum chemical methods between two molecules. The higher average pyrene possession ratio occupied in frontier molecular orbitals exhibited better nonlinear properties in such structures, which was consistent with the experimental data. Excellent optical limiting behaviors were observed under femtosecond laser excitations at multiple wavelengths (range from 515 to 900 nm), which resulted from two-photon absorption (TPA) and TPA induced excited-state absorption (ESA). Moreover, the sample exhibited extremely high transmittance (>91%) from visible to infrared regions (500 to 900 nm). Our results show that pyrene-based derivatives can be considered as promising candidates for broadband ultrafast optical limiters.

Anisotropy of two-photon absorption and free-carrier effect in nonpolar GaN

We reported a systematic study about the anisotropic optical nonlinearities in bulk m-plane and a-plane GaN crystals by Z-scan and pump-probe with phase object methods under picosecond at 532 nm. The two-photon absorption coefficient, which was measured as a function of polarization angle, exhibited oscillation curves with a period of π/2, indicating a highly polarized optical third-order nonlinearity in both nonpolar GaN samples. Furthermore, free-carrier absorption revealed stronger hole-related absorption for E⊥c than for E//c probe polarization. In contrast, free-carrier refraction was found almost isotropic due to electron-related refraction in the isotropic conduction bands.

Optical nonlinearities and ultrafast all-optical switching of m-plane GaN in the near-infrared

We reported a systematic investigation on the three-photon absorption (3PA) spectra and wavelength dispersion of Kerr refraction of bulk m-plane GaN crystal with both polarization E⊥c and E//c by femtosecond Z-scan technique in the near-infrared region from 760 to 1030 nm. Both 3PA spectra and Kerr refraction dispersion were in good agreement with two-band models. The calculated nonlinear figure of merit and measured ultrafast nonlinear refraction dynamics via femtosecond pump-probe with phase object method revealed that m-plane GaN would be a promising candidate for ultrafast all-optical switching and autocorrelation applications at telecommunication wavelengths.

Effect of Fe-doping on nonlinear optical responses and carrier trapping dynamics in GaN single crystals

We presented a quantitative study on the Fe-doping concentration dependence of optical nonlinearities and ultrafast carrier dynamics in Fe-doped GaN (GaN:Fe) single crystals using picosecond Z-scan and femtosecond pump-probe with phase object techniques under two-photon excitation. In contrast to the two-photon absorption that was found to be independent on the Fe-doping, the nonlinear refraction decreased with the Fe concentration due to the fast carrier trapping effect of Fe3+/Fe2+ deep acceptors, which simultaneously acted as an efficient non-radiative recombination channels for excess carriers. Remarkably, compared to that of Si-doped GaN bulk crystal, the free-carrier refraction effect in GaN:Fe crystals was found to be enhanced considerably since Fe-doping and the effective carrier lifetime (∼10 ps) could be tuned over three orders of magnitude at high Fe-doping level of 1 × 1019 cm−3.

Ultrafast carrier dynamics in a p-type GaN wafer under different carrier distributions

The dependence of the carrier distribution on photoexcited carrier dynamics in a p-type Mg-doped GaN (GaN:Mg) wafer were systematically measured by femtosecond transient absorption (TA) spectroscopy. The homogeneity of the carrier distribution was modified by tuning the wavelength of the UV pulse excitation around the band gap of GaN:Mg. The TA kinetics appeared to be biexponential for all carrier distributions, and only the slower component decayed faster as the inhomogeneity of the carrier distribution increased. It was concluded that the faster component (50–70 ps) corresponded to the trap process of holes by the Mg acceptors, and the slower component (150–600 ps) corresponded to the combination of non-radiative surface recombination and intrinsic carrier recombination via dislocations. Moreover, the slower component increased gradually with the incident fluence due to the saturation of surface states.

Template-free fabrication of Ag nanowire arrays/Al2O3 assembly with flexible collective longitudinal-mode resonance and ultrafast nonlinear optical response

We utilized a co-sputtering technique without any templates, featuring growing and etching synchronously, to delicately fabricate dense and ultrafine Ag nanowire arrays/alumina matrix composite films. Both the diameter and separation distance of the Ag nanowire arrays in the composites are not only within the scope of sub-10 nm but also tunable, which is very hard to accomplish for the conventional optical lithography- or template-based method. It is exhibited that the collective longitudinal plasmon resonance of the composite films, covering a wide range from visible to the near infrared region, is extremely sensitive to the geometrical parameters of the Ag nanowires, owing to the strong plasmonic coupling among neighboring nanowires. The experimental observations were also theoretically supported by the near-field electromagnetic numerical simulation. More interestingly, the fabricated composite films demonstrated ultrafast nonlinear optical response in the visible light region under femtosecond laser excitation, possessing a short relaxation time of 1.45 ps for the longitudinal mode (L mode) resonance. These results indicate that the proposed composite films as a building block with exotic optical properties could provide an opportunity to construct integrated nanodevices for plasmonic optical applications.

Ultrafast broadband reverse saturable absorption based on two-photon induced singlet state

The broadband reverse absorption in a multi-branched conjugated compound TPPh is investigated. Transient absorptive spectra of TPPh solution is recorded and a broadband excited-state absorption (475~780 nm) is discovered. The lifetime of this broadband excited-state absorption was measured to be about 20 ns. Transient fluorescence experiment was conducted to confirm that the long-lived broadband excited-state absorption is established on the first singlet state. Optical limiting with extremely high linear transmittance is achieved under the excitation of 532 nm, 21 ps pulses. Theoretically analysis showed that both two-photon absorption and excited-state absorption played a part in it, optical limiting capability is thus enhanced via cooperating effects of two-photon absorption and excited-state absorption. All of the findings suggested that TPPh is a good broadband nonlinear absorptive material and could be further optimized for optical limiting applications.

Investigation of surface plasmon resonance in composite nanostructure of silver film and nanowire array

We investigate the surface plasmon resonance in a new composite nanostructure (Nanowires array beneath metal film). Computational simulation results exhibit that, for both transverse electric(TE) and transverse magnetic (TM) polarization, the positions of resonance peaks is extremely sensitive to the change of center distance (Filling ratio of nanowires). When the diameter of Nanowires is 4nm and under TM polarization, the resonance angle increasing with the increase of center distance. In the case of TE polarization, the result is completely the opposite within limits. It is also shown that changes in thickness of Ag film(At the top of the Ag nanowire) has little direct effect on the resonance angle, But the characteritics of SPR intensity is influenced by the thickness of Ag film in the most degree. When the thickness of Ag film is 50 nm, In range of 10nm to 100nm, the minimum value of the reflectance is only 0.05, the result is consistent with the previous studies. Additionally, the nano composite structure material is very sensitive to the refractive index change of the lowest layer when under the TE- polarization. we have done mode analysis of the SPR structure for both simple and practical structures using comsol multiphysics, our approach is intend to show the feasibity and extend the applicability of the plasmonic nanowires, could lead to provide the basis for design the new structure of nanowires array.