Yanyan Sun

Quantum confinement effect in ZnO thin films grown by pulsed laser deposition

High quality crystalline nanostructured ZnO thin films were fabricated by pulsed laser deposition on (0001) sapphire substrates. The films were investigated by x-ray diffraction, scanning electron microscopy, and optical absorption spectroscopy. The exciton peak energy from absorption spectra was used to determine the band gap energy Eg, which was found to increase systematically with the decrease in the mean grain size and ranged from 3.388to3.647eV. This is consistent with the quantum confinement model and, up until now, not observed in a convincing manner in the ZnO thin films.

Grain size dependence of electrical and optical properties in Nb-doped anatase TiO2

Anatase thin films of pure TiO2 and 6% niobium doped TiO2 (Nb:TiO2) were fabricated on LaAlO3(100) by pulsed laser deposition. The electrical properties of Nb:TiO2 films are grain-size dependent, i.e., the larger grain size, the higher conductivity, and mobility. For all TiO2 and for Nb:TiO2 with small mean grain size (d 15 nm), particularly, a blueshift in Nb:TiO2 is governed by the Burstein–Moss effect.

High performance plasmonic random laser based on nanogaps in bimetallic porous nanowires

A plasmonic random laser is fabricated using gold-silver bimetallic porous nanowires with abundant nanogaps that provide strong feedback or gain channels for coherent lasing from dye molecules. The strong confinement of the nanogaps allows the bimetallic porous nanowire-based random laser, which is pumped by ns pulses, to operate with a very low threshold and extremely low concentrations of Rhodamine 6 G (as low as 0.067 mM). This random laser can be used as a pump source for another coherent random laser based on oxazine. These results provide a basis for studies of coherent random lasing pumped by another random laser.

Cascade-pumped random lasers with coherent emission formed by Ag–Au porous nanowires

A series of sequentially cascade-pumped random lasers is reported. It consists of three random lasers in which the Ag-Au bimetallic porous nanowires play the role of scatterers, and the gain materials are coumarin 440 (C440), coumarin 153 (C153), and rhodamine 6G (R6G), respectively. The random laser with C440 is first pumped by a 355 nm pulsed laser. The emission of C440 pumps the C153, and the emission of C153 pumps the R6G sequentially. Low-threshold coherent emissions from the three random lasers are observed. The cascade-pumped random lasers can be achieved easily with low cost and can be used in applications conveniently.

Coherent plasmonic random laser pumped by nanosecond pulses far from the resonance peak of silver nanowires

Silver nanowires were used to enhance stimulated emission of Rhodamine 6G in a liquid random laser. Low-threshold coherent emission from the nanosecond-pulse-pumped random laser was achieved. Surface plasmonic resonance plays a key role in low-threshold operation of this random laser. The results demonstrate the ability of silver nanowires to enhance the stimulated emission, especially when the emission light from the dye molecules is far from the plasmonic resonance peak. Although the random laser shows different emission spectra in different directions, universal properties of strong interaction among multiple modes under different pump power densities were also demonstrated.

Two-threshold silver nanowire-based random laser with different dye concentrations

The feedback mechanisms of silver nanowire-based random lasers with different concentrations of laser dye rhodamine 6 G pumped by a nanosecond pulsed laser were demonstrated. It was shown that dye concentration greatly impacts on the optical amplification mechanism. At lower or higher dye concentrations, random lasers have a single threshold. At a proper concentration, the system shows transition from incoherent emission to coherent lasing and has two thresholds corresponding to incoherent feedback and coherent feedback, respectively. The corresponding physical mechanism was displayed. Also, the processes of fluorescence, incoherent feedback and coherent feedback were distinguished by the emission spectra in the time domain. The results will supply some guidance to clear the working mechanism of random lasers.

Size-dependent magnetic moments in ultrafine diamagnetic systems

Recently, unusual magnetic properties were found experimentally in otherwise diamagnetic nanoparticles. Our experiments and analysis revealed that a universal expression can describe the size-dependent magnetic moments in ultrafine diamagnetic systems, including Au, Pd, and HfO2 nanoparticles. A model was presented to explain the observed phenomenon. It suggested that the up and down spins in ultrafine diamagnetic systems are spatially separated and form asymmetric spin-singlet pairs. The correlation lengths of the spin-singlet pairs in Au, Pd, and HfO2 nanoparticles were estimated to be about 1.65, 13, and 4.4 nm, respectively. On the basis of this model, we explain (a) the origin and the size dependence of magnetic moments in diamagnetic nanoparticles, and (b) the origin of the extremely large anisotropy in diamagnetic nanoparticles.

Cascade pumped random lasers with coherent emission formed by Ag-Au porous nanowires

A series of sequentially cascade pumped random lasers is reported. It consists of three random lasers, in which the Ag-Au bimetallic porous nanowires play the role of scatterers, and the gain materials are coumarin 440 (C440), coumarin 153 (C153) and rhodamine 6G (R6G) respectively. The random laser with C440 is firstly pumped by 355 nm pulsed laser. The emission of C440 pumps the C153 and the emission of C153 pumps the R6G sequentially. Low threshold coherent emissions from the three random lasers are observed. The cascade pumped random lasers can be achieved easily with low cost, and can be used in applications conveniently.