Chongjun Zhao

Optical nonlinearities of space selectively precipitated Au nanoparticles inside glasses

Optical nonlinearities of Au nanoparticles precipitated inside glasses induced by irradiation of a focused femtosecond laser were investigated using Z-scan technique with 8 ns pulses at 532 nm. The transformation behaviors both from self-defocusing to self-focusing and from saturable absorption to reverse saturable absorption were observed in the glass samples, which depend drastically on the irradiation power density of the femtosecond laser used. The optical nonlinearities were analyzed in detail and proved numerically by the excited-state theory of conduction band electrons.

Metal nanoparticle precipitation in periodic arrays in Au2O-doped glass by two interfered femtosecond laser pulses

We report on the precipitation control of Au nanoparticles in periodic arrays in silicate glass. Au2O-doped glass samples were first irradiated by two 800 nm interfered femtosecond laser pulses at room temperature and then heat treated at 550 °C for the Au nanoparticle precipitation in the laser irradiation areas. One-dimensional periodic arrays of the Au nanoparticles were controlled by changing the pulse energy and the incident angle between the interfered laser pulses. The smallest dimension in the obtained arrays was a width of 300 nm. The mechanism of the metal nanoparticle precipitation by this technique was discussed. Only two pulses are required to encode these periodic microstructures, which are applicable to emerging nanostructure devices such as optical memory with ultrahigh storage density, micrograting with high diffractive efficiency and integrative micro-optical switches.

Controllable precipitation and dissolution of silver nanoparticles in ultrafast laser pulses irradiated Ag+-doped phosphate glass.

We report a controllable process of recipitation and dissolution of silver nanoparticles in ultrashort laser pulses irradiated Ag+-doped phosphate glass. Absorption spectra, transmission electron microscopy and refractive index measurement revealed that metallic silver nanoparticles were precipitated in the glass sample after irradiation by an 800-nm femtosecond laser and subsequent annealing at 300 degrees C, and dissolved after further annealing at 450 degrees C. We discuss a mechanism that combines the formation and decoloration of color centers, precipitation and dissolution of silver nanoparticles.

Photoinduced formation of colloidal Au by a near-infrared femtosecond laser

A Au colloid was prepared in a 5 mM HAuCl 4 solution through irradiation with a focused infrared femtosecond laser at 800 nm. The Au colloid was characterized by absorption spectra, transmission electron microscopy, and x-ray diffraction analysis. The appearance of absorption peak around 526 nm in the absorption spectra and the wine-red color of sample solution HAuCl 4 under focused laser irradiation verified the formation of Au colloid. The solution color changed in the order of yellow → orange → wine-red due to the local formation of Au nanoparticles near the focus. The pulse energy, focus position of laser beam, and solvent composite play important roles in formation, grain size, and stability of the Au colloid. A mechanism for the precipitate of Au nanoparticles was proposed, and a multiphoton process of femtosecond laser was involved.

Fabrication of internal diffraction gratings in calcium fluoride crystals by a focused femtosecond laser

We report the fabrication of internal diffraction gratings in calcium fluoride crystals by a focused near-IR 800 nm femtosecond laser. The diffraction efficiency and refractive index change were evaluated after femtosecond laser irradiation and subsequent annealing. The maximum refractive index change was estimated to be 3.57x10(-4). Optical absorption spectra, measured for the crystals before and after the laser irradiation and subsequent annealing, indicate that the absorbance increase after femtosecond laser irradiation and decrease with increasing annealing temperature. The mechanisms of refractive index change are proposed. The results may be useful for fabrication of three-dimensional integrated optics devices in the crystals.

Optical limiting properties in phthalocyanines with different substituent groups

The optical limiting (OL) properties of free-base phthalocyanines and nickel metallophthalocyanines with the same structure but, with different substituent groups, octa-dodecaoxyl long chains, were investigated by using nanosecond pulses at 532 nm. The experimental results show that the OL capabilities in the two kinds of phthalocyanines depend nonmonotonically on lengths of the side chains.

Preparation of aligned Ca3Co2O6 nanorods and their steplike magnetization

Well-aligned Ca3Co2O6 nanorods (∼300nm in length and ∼40nm in diameter) on Si substrates are prepared by pulsed laser deposition. The steplike magnetization feature as identified in bulk Ca3Co2O6 can be retained in the nanorods in spite of the enhanced distortion of the magnetically ordered spin chains due to the finite size effect. The out-of-plane magnetization value is slightly larger than the in-plane value, which shows the small magnetic anisotropy. An additional distinct transition at 54K associated with the spin frustration is revealed, different from the bulk samples.

One-off writing of multimicrogratings on glass by two interfered femtosecond laser pulses

Multimicrogratings are one-off written on silicate glass by two interfered femtosecond pulsed laser beams with the aid of a mask. The period and depth of the multimicrogratings are revealed by optical microscopy and atomic force microscopy. The depth is dependent on both the colliding angle between the two interfered laser beams and the laser pulse energy, but the period relies on the colliding angle only. We also observe a series of grooves formed at the middle of each bulge of the multimicrogratings and attribute it to the higher-order modulation arising from second-harmonic generation of the femtosecond laser pulse during the one-off writing processes.

Mechanisms of the refractive index change in femtosecond laser-irradiated Au3+-doped silicate glasses

We report on the refractive index change in Au3+-doped silicate glasses irradiated by a femtosecond laser and successive heat treatment. The refractive index of the irradiated area increased after the femtosecond laser irradiation and decreased with increasing annealing temperature up to 450°C and then increased again with increasing annealing temperature. Absorption spectra of the glass samples before and after femtosecond laser irradiation and after further annealing manifested the formation of color centers after laser irradiation, disappearance of color centers after annealing at 300°C, and precipitation of gold nanoparticles after annealing at high temperature. The mechanisms of the observed phenomena are discussed.

Optical transfer of periodic microstructures by interfering femtosecond laser beams.

We report on an optical interference method for transferring periodic microstructures of metal film from a supporting substrate to a receiving substrate by means of five-beam interference of femtosecond laser pulses. Scanning electron microscopy and optical microscopy revealed microstructures with micrometer-order were transferred to the receiving substrate. In the meanwhile, a negative copy of the transferred structures was induced in the metal film on the supporting substrate. The diffraction characteristics of the transferred structures were also evaluated. The present technique allows one-step realization of functional optoelectronic devices.