Zhongguo Li

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.

Dynamics of optical nonlinearities in GaN

The dynamics of optical nonlinearities in bulk GaN crystal are investigated by a modified time-resolved pump-probe system with phase object at 532 nm under picosecond pulse excitation. The samples optical nonlinearities can be determined by measuring the normalized transmittance with/without an aperture in the far field. The different nonlinear mechanisms in GaN are separated. Experimental results show that the nonlinear response of GaN is a combination of bound-electronic and free-carrier nonlinearities mechanisms. With the theory of two-photon-induced free-carrier nonlinearity, the related optical nonlinear parameters, including free-carrier nonlinearities and free-carrier life time which are difficult to be determined, are obtained.

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.

Strain dependent ultrafast carrier dynamics in EuTiO3 films

The photocarrier relaxation dynamics of EuTiO3 films have been investigated using femtosecond transient absorption spectroscopy. Two kinds of EuTiO3 films, with and without strain, have been included. In both films, the decay dynamics of 2p-3d t2g transition has a fast (∼2 ps) and slow (ns) components which are independent of the strain. Remarkably, the transient photobleaching of 4f-3d t2g transition is found to be enhanced considerably in the strained film, accompanied with a subnanosecond carrier lifetime. This behavior can be linked to the strain induced band structure modulation. Our results reveal the dynamical interactions in EuTiO3, identifying the critical roles of strain in photo induced phenomena.

Nonlinear Optical Properties Tuning in Meso-Tetraphenylporphyrin Derivatives Substituted with Donor/Acceptor Groups in Picosecond and Nanosecond Regimes

meso-Tetraphenylporphyrin (TPP) and its two substituted derivatives (meso-tetrakis(4-cyanophenyl)porphyrin [TPP(CN)4] and meso-tetrakis(4-methoxyphenyl)porphyrin [TPP(OMe)4]) were synthesized. Their nonlinear absorption and refraction properties were studied using the Z-scan technique in the picosecond (ps) and nanosecond (ns) regimes. The open aperture Z-scan results reveal that TPP and TPP(CN)4 display an identical reverse saturable absorption (RSA) character in the ps and ns regimes. While TPP(OMe)4 exhibits a transition from saturable absorption (SA) to RSA in the ps regime and a typical RSA character in the ns regime. The closed aperture Z-scan results show that TPP(CN)4 and TPP(OMe)4 have regular enhancement of the magnitude of nonlinear refraction as compared to their parent TPP in both the ps and ns regimes. In addition, the second-order molecular hyperpolarizabilities (γ) of these three porphyrins are calculated, and the γ values of TPP(CN)4 and TPP(OMe)4 are remarkable larger than that of TPP. The introduction of the electron-withdrawing group CN and the electron-donating group OMe into TPP has enhanced its nonlinear refraction and γ value, and tuned its nonlinear absorption (TPP(OMe)4), which could be useful for porphyrin-related applications based on the desired NLO properties.

Optical nonlinearities and carrier dynamics in Fe doped GaN single crystal

Optical nonlinearities and transient dynamics of Fe doped GaN (GaN:Fe) were studied by Z-scan and pump-probe with phase object techniques under picosecond and nanosecond at 532 nm. From the pump-probe results, an additional decay pathway subsequent to two photon excitation was observed due to the carrier trapping of Fe3+/Fe2+ deep acceptors. The trapping state in the band gap results in a pronounced modulation to nonlinear responses of GaN:Fe compared to that of undoped GaN. With an effective three-level model as well as carrier trapping effect we described the photo-physical dynamics in GaN:Fe unambiguously.

Intensive two-photon absorption induced decay pathway in a ZnO crystal: Impact of light-induced defect state

Using the pump-probe with phase object technique with 20 ps laser pulses at 532 nm, we investigated the carrier relaxation process subsequent to two-photon absorption (TPA) in ZnO. As a result, we found that an additional subnanosecond decay pathway is activated when the pump beam intensity surpasses 0.4 GW/cm2. We attributed this intensity-dependent pathway to a TPA induced bulk defect state and our results demonstrate that this photo induced defect state has potential applications in ZnO based optoelectronic and spintronic devices.

Ultrafast carrier dynamics in SnOx thin films

We studied the carrier dynamics in a series of SnOx thin films using femtosecond transient absorption (TA) spectroscopy. The observed carrier relaxation was found to be strongly dependent on thin film stoichiometry. The TA spectra corresponding to free carriers, trapped carriers, and state filling were observed in the picosecond time region for SnO2, SnOx, and SnO film, respectively. The TA decay kinetics of all films were best fit with a tri-exponential decay model with fast (1 ps), medium (∼10 ps), and slow (ns) components. Our results revealed the carrier relaxation and recombination processes in SnOx thin films, identifying the critical role of stoichiometry in photo-induced phenomena.

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.

Investigation of optical nonlinear properties in semiconductor GaN

The optical nonlinearities of bulk GaN single crystal are investigated by conducting Z-scan experimental measurements, using nanosecond pulses at 532nm. The material’s nonlinear absorptive and refractive optical nonlinearities are determined by measuring the normalized transmittance with and without an aperture in the far field. After simulating the experimental curves, the two-photon absorption (TPA) coefficient, especially the refractive-index change induced by two-photon-excited free carrier, are obtained. The experimental results show that GaN has reverse saturable absorption and self-defocusing effect at 532 nm, indicating this material is a good candidate for future photonics applications. The theoretical simulation fit well with experimental results.