Guohong Ma

Preparation of carbon nanoparticles with strong optical limiting properties by laser ablation in water

Carbon nanoparticle colloids were prepared by Nd:YAG laser ablation of a carbon target immersed in water. The nanoparticles were collected on substrates and analyzed with micro-Raman spectroscopy and electron microscopy. Optical limiting properties of the carbon nanoparticle colloids towards 532 nm wavelength were characterized using a nanosecond Nd:YAG laser. A strong optical limiting response was detected for the carbon nanoparticle colloids with C60 as reference at input laser fluence above 0.6 J/cm2. The effects of carbon nanoparticle concentrations upon the optical limiting behavior were also investigated. A convenient method for preparing carbon nanoparticle based optical limiters is proposed in this investigation.

Excitonic nonlinear absorption in CdS nanocrystals studied using Z-scan technique

Irradiance dependence of excitonic nonlinear absorption in cadmium sulfide (CdS) nanocrystals has been studied by using Z-scan method with nanosecond laser pulses. The wavelength dependence of nonlinear absorption has also been measured near the excitonic transition of 1S(e)–1S3/2(h). We observe the saturable absorption, which can be described by a third-order and a fifth-order nonlinear process for both 3.0-nm-sized and 2.3-nm-sized CdS nanocrystals. The experimental results show that the excitonic nonlinear absorption of CdS nanocrystals is greatly enhanced with decreasing particle size. A two-level model is utilized to explain both irradiance and wavelength dependence of the excitonic nonlinearity.

Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube films

By using femtosecond laser pulses at a wavelength range from 720 to 780nm, we have observed absorptive and refractive nonlinearities in a film of multiwalled carbon nanotubes grown mainly along the direction perpendicular to the surface of quartz substrate. The z-scans show that both absorptive and refractive nonlinearities are of negative and cubic nature in the laser irradiance range from a few to 300GW∕cm2. The magnitude of the third-order nonlinear susceptibility, χ(3), is of an order of 10−11esu. The degenerate pump–probe measurement reveals a relaxation time of ∼2ps.

Controlled surface-plasmon coupling in SiO2-coated gold nanochains for tunable nonlinear optical properties

Linear chains of SiO2-coated Au nanoparticles, exhibiting large third-order nonlinearity at multiple wavelengths ranging from visible to near infrared, were successfully synthesized and self-assembled on glass substrates using a reduction of HAuCl4 and chemical surfactant modification. The optical absorption spectra peaking at 530, 620, 715, 830, and 1100nm wavelengths were measured to the transverse and longitudinal surface plasmon resonance modes of 2, 3, 4, and 5 Au particles, respectively. Greatly enhanced third-order nonlinear optical responses have been observed at multiple wavelengths.

Novel Pb(ii) coordination frameworks: synthesis, crystal structures and unusual third-order nonlinear optical propertiesElectronic supplementary information (ESI) available: crystal packing diagram of complex 2. See http://www.rsc.org/suppdata/jm/b3/b315682f/

Three novel Pb(II) coordination complexes, [Pb(dimb)(DMF)(NO3)2]n1, [Pb(dimb)(SCN)2]n2 [dimb = 1,3-bis(imidazol-1-ylmethyl)-benzene], {[Pb(bimb)1.5(NO3)2](DMF)}n3 [bimb = 4,4′-bis(imidazol-1-methyl)-biphenyl], were synthesized and characterized by X-ray crystallography. Complex 1 exhibits a one-dimensional (1D) zigzag chain structure, which has two crystallographically independent Pb(II) atom centers. Complex 2 possesses a two-dimensional (2D) corrugated network, which contains 24-membered M2L2 metallocyclic rings. Complex 3 has a 1D infinite non-interpenetrated molecular ladder, which has very large cavities with dimensions of 17.89 × 20.34 A, and DMF molecules fill the channel formed by adjacent two ladders. The third-order nonlinear optical (NLO) properties of the three complexes were measured by a Z-scan technique in DMF solution. All three complexes possess weak absorption and strong refraction. Their third-order NLO refractive coefficients, n2, are −5.96 × 10−19 m2 W−1 for 1, −8.34 × 10−19 m2 W−1 for 2, and −7.15 × 10−19 m2 W−1 for 3. It is notable that two Pb(II) complexes 1 and 3 with 1D structure show strong self-defocusing behavior, which are different from the reported 1D coordination complexes containing d10 metal ions; also, complex 2 is the first 2D coordination compound that possesses strong self-defocusing behavior. The χ(3) values of complexes 1, 2 and 3 were calculated to be 4.46 × 10−13, 6.25 × 10−13 and 5.42 × 10−13 esu, respectively.

Observation of resonant energy transfer in Au:CdS nanocomposite

Au:CdS nanocomposite film was investigated using femtosecond pump–probe measurement at 800 nm. Time dependence of transmittance shows a two-photon absorption followed by a saturable absorption and recovery process, which clearly demonstrates that resonant energy transfer between CdS and Au nanocomposite systems occur with excitation at 800 nm. In addition, enhancement of the two-photon absorption coefficient in Au:CdS nanocomposite with a factor of more than 6 was observed compared to that of bulk CdS.

Size and dielectric dependence of the third-order nonlinear optical response of Au nanocrystals embedded in matrices

The third-order nonlinear optical response of Au nanocrystals embedded in BaTiO(3) and ZrO(2) matrices was investigated through the off-resonance femtosecond optical Kerr effect. Compared with pristine films, the nonlinearity of composite films shows an enhancement that depends on the sizes of Au particles and the refractive index of the matrices. We performed a calculation based on Lorenz-Mie scattering theory to explain the experimental results. Additionally, the nonlinearity was optimized by determination of the values of the particle size and the refractive index of the matrix.

Terahertz magnetic field induced coherent spin precession in YFeO3

We present the magnetic dipole transition at 0.299 THz excited by magnetic component of terahertz electromagnetic pulse in an antiferromagnetic YFeO3 crystal. The impulsive magnetic field of the terahertz pulse tilts the macroscopic magnetization, causing deviation from the equilibrium position, which is manifested by a sharp absorption at the frequency of the quasiferromagnetic mode of the crystal. The rotating coherent macroscopic magnetization radiates elliptically polarized emission at the frequency of the quasiferromagnetic resonance due to the dichroic absorption in the crystal.

Carrier concentration dependence of terahertz transmission on conducting ZnO films

With the dc reactive magnetron sputtering method, conducting ZnO thin films with different carrier concentrations on glass substrate were fabricated. The dielectric responses of the ZnO films are characterized with terahertz time-domain spectroscopy. Frequency-dependent conductivity, power absorption, and refractive index are obtained, and the experimental results can be well reproduced with the classic Drude model. Our results reveal that by adjusting the carrier concentration of the ZnO film, the conducting ZnO film can serve as broadband antireflection coatings for substrates and optics in the terahertz frequency range.

Size-dependent excited state properties of CdS nanocrystals

Abstract CdS nanocrystals of different particle sizes dispersed in ZrO2 films were fabricated by sol–gel and dipping method. The size dependent excited state optical properties were systematically investigated by photoluminescence, time-resolved photoluminescence and Raman spectroscopy. The experimental results show that the relaxation of the excited state is composed of two processes. The fast component that arises from band edge emission shows a weak dependence on particle size, while the slow component, which is attributed to the shallow defect level emission, exhibits strong dependence on particle sizes.