M. H. Hong

Characteristics of Excimer Laser Induced Plasma from an Aluminum Target by Spectroscopic Study

Optical emission spectrum of Al plasma induced by a KrF excimer laser (248 nm) in a vacuum chamber is investigated using an optical multichannel analyzer (OMA). It is observed that different air pressures and delay times after the laser pulse play important roles in the characterization of emission spectrum from Al surface. The optimal conditions for spectroscopic study of Al plasma are estimated. A spectrum self-absorption effect can be observed clearly from the calibrated spectral line profiles at the initial stage of plasma expansion and high fluence. Assuming local thermodynamic equilibrium (LTE) conditions, the plasma temperature and density are estimated from two spectral lines (Al I, 309.2 and 396.1 nm) as a function of air pressure, delay time and laser fluence. The plasma temperature is derived to be in the range of 5500 to 10000 K using the relative intensities of the spectral lines. The electron density is estimated from Stark broadening profiles of the spectral lines, and found to range from 0.7×1018 to 3.5×1018 cm-3. The behaviors of the plasma temperature and density reveal a 1-D plasma expansion at a delay time shorter than 200 ns and the plasma shielding effect at high fluence.

Ultraviolet and visible Raman spectroscopy characterization of chemical vapor deposition diamond films

Abstract Polycrystalline diamond films with different grain sizes (10 nm, 100 nm and 5 μm) were prepared by hot-filament chemical vapor deposition (HFCVD). The diamond films have been studied using ultraviolet (UV, 244 nm), visible (Vis, 514 nm) and (Vis, 633 nm), near-infrared, micro-Raman scattering. The fluorescence in the excited diamond film is found to change considerably with the incident photon energy. The scattering intensity of amorphous sp 2 -bonded carbon compared to the strength of the 1331 cm −1 Raman line from sp 3 -bonded diamond is found to vary considerably as functions of the grain size of diamond film and the incident photon energy. Possible models for the structures of amorphous sp 2 -bonded carbon and sp 3 -bonded diamond phases are discussed on the basis of the present Raman data. It is shown that UV Raman spectroscopy has provided significantly greater information than visible Raman spectroscopy in characterizing diamond phases.

Laser surface cleaning and potential applications in disk drive industry

Laser cleaning has emerged as an effective cleaning technique for removing contaminants from solid surfaces. Dry laser cleaning and steam laser cleaning have been developed recently, relying on pulsed laser heating of the surface without and with the presence of a thin liquid coating. A cleaning model was established for removal of particles from solid surfaces by taking Van der Waals force, capillary force and cleaning force into account. The model can not only explain the influence of laser fluence on cleaning efficiency, but also predict the cleaning thresholds. Laser-induced removal of film-type contaminants from solid surfaces has been studied. Laser cleaning mechanisms and its applications in disk drive industry will be discussed in this article.

Tunable resonance enhancement of multi-layer terahertz metamaterials fabricated by parallel laser micro-lens array lithography on flexible substrates

Large-area split ring resonators (SRRs) array is fabricated by laser micro-lens array (MLA) lithography on flexible Polyethylene Naphthalate (PEN) substrates. Multi-layer metamaterials are formed by stacking and bonding several layers of the laser fabricated metamaterials together. The resonance of the multi-layer metamaterials is enhanced significantly as compared to the single-layer metamaterials. The roll-off value of the half-wavelength resonant dip, which reflects the strength of resonance, increases significantly from 4.9 to 11.2 as the layer number increases from 1 to 5. A logarithm relationship between the amplitude of the resonant dip and the layer number is also studied, which indicates a flexible method to tune the strength of resonance by changing the layer number. The multi-layer metamaterials with the enhanced resonance can be used to make narrow band terahertz filters.

Magnetic-electric interference in metal-dielectric-metal oligomers: generation of magneto-electric Fano resonance

The existence of magnetic resonance in designed Metal-Dielectric-Metal (MDM) oligomers is investigated. Via angling the incident light it is found that in the MDM oligomers, not only the E-component of incident field drives plasmon oscillations, but the H-component also plays an important role to excite magnetic plasmons. These magnetic plasmons give rise to a magnetic resonance in addition to classical Fano Resonance (FR). Importantly, unlike regular MDM structures which exhibit separate magnetic and electric resonances, the MDM oligomers possess the capability to exhibit both magnetic and electric resonances in the same wavelength window with proper metallic and dielectric thicknesses. It leads to the appearance of an additional FR as a result of interference between magnetic-electric plasmonic resonances rather than electric-electric resonances with a clear proof of remarkable absorption enhancement. The unique capability of MDM oligomers exhibiting both electric and magneto-electric FRs can realize many potential applications of FR.

Removal of submicron particles from nickel-phosphorus surfaces by pulsed laser irradiation

Abstract Pulsed laser cleaning was demonstrated to be an efficient way for removing submicron particles from the nickel-phosphorus (NiP) surface both experimentally and theoretically. Experimentally, it is found that using KrF excimer laser with a pulse width of 23 ns the cleaning threshold is about 20 mJ / cm 2 for removing quartz particles from the NiP surface and laser cleaning efficiency increases rapidly with increasing laser fluence. The theoretical analysis shows that the peak cleaning force (per unit area) is larger than the adhesion force (per unit area) for submicron quartz particles on the NiP surface when it is irradiated by excimer laser with a fluence above 10 mJ / cm 2 . Therefore, it is possible to remove submicron quartz particles from NiP surfaces by laser irradiation. The difference between the cleaning force (per unit area) and the adhesion force (per unit area) increases with increasing laser fluence, leading to a higher cleaning efficency for quartz particles on the NiP surface.

LASER PLASMA INTERACTION AT AN EARLY STAGE OF LASER ABLATION

Laser scattering and its interaction with plasma during KrF excimer laser ablation of silicon are investigated by ultrafast phototube detection. There are two peaks in an optical signal with the first peak attributed to laser scattering and the second one to plasma generation. For laser fluence above 5.8 J/cm2, the second peak rises earlier to overlap with the first one. The optical signal is fitted by a pulse distribution for the scattered laser light and a drifted Maxwell–Boltzmann distribution with a center-of-mass velocity for the plasma. Peak amplitude and its arrival time, full width at half maximum (FWHM), starting time, and termination time of the profiles are studied for different laser fluences and detection angles. Laser pulse is scattered from both the substrate and the plasma with the latter part as a dominant factor during the laser ablation. Peak amplitude of the scattered laser signal increases but its FWHM decreases with the laser fluence. Angular distribution of the peak amplitude can be...

Sub-50 nm nanopatterning of metallic layers by green pulsed laser combined with atomic force microscopy

Pulsed-laser assisted nanopatterning of metallic layers on silicon substrates under an atomic force microscope (AFM) tip has been investigated. A 532 nm Nd:YAG pulsed laser with a pulse duration of 7 ns was used. Boron doped silicon tips were used in contact mode. This technique enables the processing of structures with a lateral resolution down to 10 nm on gold and copper layers. Nanopatterns such as pit array and multilines with lateral dimensions between 10 and 50 nm and depths between 2.5 and 21 nm have been created. The experimental results and mechanism of the nanostructure formation are discussed. The created features were characterized by AFM, scanning electron microscope, and Auger electron spectroscopy. The apparent depth of the created pit has been studied as a function of laser intensity or laser pulse numbers. The dependence of nanoprocessing on the geometry parameters of the tip and on the optical and thermal properties of the processed sample has also been investigated. Thermal expansion of...

Optical limiting phenomena of carbon nanoparticles prepared by laser ablation in liquids

We report optical limiting properties of carbon nanoparticles, which were made in liquids by laser ablation of a bulk carbon target. The carbon nanoparticles were analyzed with micro-Raman spectroscopy, UV-Vis spectroscopy and Electron microscopy. Optical limiting responses towards 532-nm wavelength were measured with a 7-ns Nd:YAG laser. Nanoparticle size and laser pulse repetition rate effects on optical limiting behaviour were studied. A model was proposed to explain the physical origin of this nonlinear optical process. This work can provide useful information for designing carbon nanoparticle based optical limiters.

Tunable optical nonlinearity of silicon nanoparticles in solid state organic matrix

In this paper, a novel approach to fabricate a hybrid solid state system with both tunable nonlinearity and self-repairing property is studied. The optical nonlinear properties of a silicon nanoparticles system based on gel wax matrix were experimentally investigated. Tunable optical nonlinearities from optical limiting to saturable absorption were achieved by simply changing the concentration of nanoparticles inside the matrix. This approach opens a route for a low cost, one-step-synthesis nonlinear system being highly compatible with silicon optoelectronic circuits. This hybrid system also demonstrates the self-repairing property after excess exposure to laser irradiation.