Cluster growth in an ablation plume propagating through a buffer gas

A phenomenological mixed-propagation model that describes the expansion of an ablation plume through a buffer gas is introduced. Selected experiments including LaMnO₃ and tin ablation in oxygen, as well as tungsten ablation in argon, are analysed. For given ablation conditions the expansion parameters required to model the growth of clusters in the expanding plasma plume are deduced and the average asymptotic size of the clusters is calculated and compared (for tungsten) with the size of clusters measured by transmission electron microscopy.     

Laser nano-fabrication of large-area plasmonic structures and surface plasmon resonance tuning by thermal effect

A simple and flexible technique aimed to generate large-area periodic nano-dot array features on metal thin films by laser interference lithography (LIL) has been demonstrated. In this paper, gold nano-dot arrays with a period of ∼450 nm and a dot diameter of ∼100 nm on quartz substrates coated with a gold film of 50 nm thick were fabricated. Multiple enhanced transmission peaks were observed in this patterned film. In addition to the characteristic peak of the gold surface plasmon resonance around 500 nm, multiple shoulder peaks that range from 550 to 700 nm were also observed in the nano-chain array structures. These shoulder peaks disappeared after thermal annealing. It was found that the nano-dots became smaller and well-separated nano-balls under the high temperature annealing process. These nano-structures have potential applications in solar cell, nano-lithography and biosensing.     

Bubble creation and collapse during excimer laser ablation of weak absorbing polymers

We present evidence suggesting that XeCl laser ablation of a weakly absorbing poly-methyl-methacrylate (PMMA) polymer, done by chemical, thermal bond breaking of the polymer chain or optical breakdown of the material, which involves plasma generation, creates a cloud of small asymmetric near the surface bubbles, which subsequently expand and aggregate during the same laser pulse duration or in subsequent pulses depending on the laser pulse energy. When a critical volume is reached each bubble collapses in a high pressure and temperature central point and rebounds ejecting a hot jet of material on the non-irradiated area of the polymer and creating craters on the surface. A characteristic bipolar pressure wave corresponding to the bubble collapse, explosion and rebound is observed. The number density of the craters on the surface is a function of the laser pulse sequence number and the laser pulse energy density.     

Effect of Pulse Width and Fluence of Femtosecond Laser on Electron--Phonon Relaxation Time

The electron phonon relaxation time as functions of pulse width and fluence of femtosecond laser is studied based on the two-temperature model. The two-temperature model is solved using a finite difference method for copper target. The temperature distribution of the electron and the lattice along with space and time for a certain laser fluence is presented. The time-dependence of lattice and electron temperature of the surface for different pulse width and different laser fluence are also performed, respectively. Moreover, the variation of heat-affected zone per pulse with laser Auence is obtained. The satisfactory agreement between our numerical results and experimental data indicates that the electron-phonon relaxation time is reasonably accurate with the influences of pulse width and Auence of femtosecond laser.     

Tunable Omnidirectional Surface Plasmon Resonance in Cylindrical Plasmonic Structure

The tunable omnidirectional surface plasmon resonance in the optical range is theoretically demonstrated in a cylindrical plasmonic crystal by using rigorous coupled-wave analysis. The cylindrical plasmonic crystal consists of an infinite chain of two-dimensional cylindrical metal--dielectric-dielectric-metal structures. The dispersion relation of the cylindrical plasmonic crystal is obtained by calculating the absorptance as a function of a TM-polarized incident plane wave and its in-plane wave vector. The omnidirectional surface plasmon resonance can be tuned from UV region to visible region by adjusting the thickness of the cylindrical dielectric layers. The absorption spectrum of the infinite chain of nanocylinders is also investigated for comparison.     

The plasmon and distribution effects between incident light and active layer in PtOx-type super-resolution near-field structure

The plasmon and distribution effects of collective localized surface plasmons between incident light and active layer of PtO_x-type super resolution near-field structure (super-RENS) have been studied using finite-difference time-domain method. Four types of distribution of Pt nanoparticles, i.e., type A, B, C, and D in active layer are investigated. We find that type C and D in active layer can provide higher field intensity in a wider range of particle size when the particle sizes are varied, and the out-going filed emerging from the active layer exhibit smaller spot size than those of type A and B. Type B, C and D also provide the additional path longer than that of type A, and excite more evanescent field which located in the far edge of the bubble from the optical axis of the incident beam. Results show that the type C structure is the best choice in the view point of designing the PtO_x-type super-RENS. This study provides new information to design a super-RENS with superior resolution as well as other applications in nano photonic devices.     

Micro-humps formed in excimer laser ablation of polyimide using mask projection system

In this paper we report the results of an investigation into surface deformation caused by thermal effects during excimer laser ablation of polyimide. Obvious surface deformation around hole entrances was observed during the experiment. The surface topology and cross section of the ablated holes were analyzed using topography measurement tool and scanning electron microscopy. It was shown that a micro-hump of 17 to 150 nm in height and 1 to 3 μm in width was formed above the level of the unablated surface. The deformed surface showed rough and color-changed characteristics. An optical diffraction model was employed to explain the cause of this kind of deformation. It was found that the ablating and heating by a near- and under-threshold laser beam became a thermal effect in polyimide material ablation, which was contributed to by a diffraction effect of the optical projection system. Received: 9 October 2001 / Accepted: 17 October 2001 / Published online: 23 January 2002     

Magic numbers in vertically coupled quantum dots

A coupled quantum dot system has been studied by numerical diagonalization of the Hamiltonian. Discontinuous ground-state transitions induced by an external magnetic field have been predicted. Series of magic numbers of angular momentum which minimize the ground-state electron-electron interaction energy have been discovered. Theoretical explanations derived from the first principles have been formulated. Received: 13 July 1997 / Accepted: 7 October 1997     

Variation of attenuated total reflection due to the birefringence of liquid crystals by excitation of surface plasmons

Attenuated total reflection (ATR) is a prominent effect in examining the excitation of surface plasmons. The Kretschmann–Raether and Otto configurations are implemented to generate surface plasmons and to examine the change of the refractive index of liquid crystals (LCs) by an applied field. A mixture of nematic and cholesteric liquid crystals is sandwiched between two glass substrates coated with conducting films. The minimum ATR angle changes by only 1° as the applied voltage increases to as much as 30 V (corresponding to 6×10⁵ Vm^-1), which does not agree with the calculation based on field-induced refractive-index change. This fact suggests that the orientation of LC molecules adjacent to an electrode surface is unaffected by the external field because of the strong-anchoring effect. A Jones-matrix simulation together with a multilayered structure of a LC clearly adduces this fact. Received: 30 November 2001 / Revised version: 1 February 2002 / Published online: 14 March 2002