Two laser-assisted approaches to fabricating nanoscale structures much smaller than the diffraction limit of laser wavelengths have been investigated. In the first approach, optical resonance in spherical particles was used to "focus" an incident laser beam to a size much smaller than the particle size and the laser wavelength. Spherical silica particles with a size of 1 mu m were self-assembled on Si substrates. After laser irradiation, holes with a size of 100 nm were obtained at the original positions of the particles. The mechanism for formation of sub-wavelength structures was investigated and found to be the near-field optical resonance effect induced by the particles on surfaces. In the second approach, monolayers of particles, with different diameters ranging from 0.16 to 5 mu m, were deposited on silicon substrates by self-assembly. A quartz plate was tightly placed on the surface of each sample. A KrF excimer laser beam (248 nm) was irradiated on the quartz/nanoparticles/silicon structure. The silica particles were imprinted into silicon substrates by the quartz due to the transient Si surface melting during the laser pulse. Ultrasonic cleaning and hydrofluoric acid (HF) solution were used to remove the silica particles. 2-D and 3-D photonic bandgap structures with nanoscale features were formed on Si substrate surfaces. The nanofabrication methods using laser beam in combination with self-assembled nanoparticles have potential applications in nanolithography, fabrication of nanophotonics devices, tribology and biology.

Results are presented on experimental and theoretical work performed to compare diffraction phenomena for ultrashort 10 fs pulses and continuous-wave propagation modes illuminating different-sized pinholes and slits. Results demonstrate that 10 fs pulses do not produce high-frequency diffraction like that produced with continuous-wave illumination. The diffraction through a 1 mm pinhole of temporally stretched pulses obtained by using fused silica plates whose frequency spectrum remains the same is compared with those of 10 fs pulses. The overall diffraction intensity profiles are, however, nearly identical in this case. The simulations of diffraction patterns for 100 fs, 10 fs, and 1 fs incident pulse were compared theoretically for different aperture sizes and frequencies. Calculations indicate that the lack of high-frequency diffraction for the mode-locked case is due to the broadband nature of the ultrashort laser pulses; i.e., the distribution of the frequency contained in the pulse ends up washing out when objects are illuminated with pulses of broad frequency content. The results of this work have important application in biomedical imaging and remote imaging applications, to name only a few.

Steam laser cleaning of alumina and titanium carbide nanoparticles from silicon substrates is presented. A KrF excimer laser with a wavelength of 248 nm was used to irradiate the substrates in laser cleaning. A water layer of micrometer thickness was deposited on silicon substrates to improve the cleaning process. Cleaning efficiency was measured for different laser fluences ranging from 50 to 250 mJ/cm(2) and pulse numbers from 1 to 100. Research work was carried out to address the factors governing steam laser cleaning, during which thickness of water thin film and lift-off velocities of water films from Si substrate surfaces were monitored. In addition, one-dimensional simulations were employed to estimate the temperature increase on the material surfaces upon laser irradiation. Water layer thickness was measured using Fourier Transform Infrared Spectroscopy. Monitoring of both lift-off velocities and water thin film removal time were carried out by optical probing approaches using He-Ne laser of 632.8 nm wavelength.

Multi-layered inverse opals were fabricated by laser-assisted imprinting of self-assembled silica particles into silicon substrates. A single pulse (pulse duration 23 ns) of a KrF excimer laser instantaneously melts the silicon substrate, which infiltrates and solidifies over the assembled silica particles on the substrate. By removing silica particles embedded in the silicon surface using hydrofluoric acid, inverse-opal photonic crystals were fabricated. This technique is potentially capable of controlling the photonic crystal properties by flexibly varying the silica particle size and the substrate material.

Diamond-like carbon (DLC) films were deposited on tungsten tips under KrF excimer laser irradiation in benzene solution. The deposition process was found to be highly dependent on tip sharpness. Tips with larger curvature radii and smaller aspect ratios could not be coated with DLC films under the same condition as that for sharp tips. Raman spectra showed that more sp3 tetrahedral structures were present in the DLC films on a tip with a smaller curvature radius. Simulation results showed that the tip sharpness dependent local optical enhancement played an important role in the DLC deposition process. An optical field gradient from apex to tip body was also found in the simulation. We suggest that there are two modes in the process of DLC deposition on nanotips under different laser fluences, i.e., local apex DLC deposition under low laser fluences and phase-graded DLC deposition under high laser fluences.

Fabrication of 2-D and 3-D photonic bandgap (PBG) structures on silicon substrates using laser-assisted nanoimprinting of silica particles has been investigated. Monolayers of silica particles, with different diameters ranging from 160 nm to 5 νm, were deposited on silicon substrates by self-assembly. A quartz plate, which is transparent to the laser wavelength of 248 nm, was tightly placed on the substrate surface. A KrF excimer laser beam with the wavelength of 248 nm was vertically irradiated on the quartz/nanoparticle/silicon structure. The silica particles were imprinted into silicon substrates by the quartz to form a 2-D PBG structure due to the transient Si surface melting during the laser pulse. 3-D PBG structures can be fabricated by directly imprinting multilayer self-assembled silica particles into Si substrates. They can also be fabricated by repeating a process cycle of silica nanoparticles self-assembly, amorphous Si layer deposition, and simultaneous laser melting, imprinting and recrystallization.

A monolayer of self-assembled silica particles can be imprinted into a silicon (Si) substrate by laser irradiation (KrF excimer laser, λ=248 nm). Periodical hemispherical cavities can be therefore created on the substrate surface. The influences of various particle sizes and laser fluence were investigated. In addition, preheating of the substrate significantly improves the performance. One-dimensional thermal calculation was employed to understand the thermal effects in this process. Three-dimensional optical simulation provided an accurate insight into the light intensity enhancement. Raman spectroscopy was used to examine the stress induced by the laser imprinting process resided in the cavity structures. Self-assembly of binary-size nanoparticles was achieved and its application on the fabrication of binary-size nanocavity arrays by laser-assisted nanoimprinting was investigated.

Laser-assisted nanopatterning of aluminium (Al) thin films using particle-induced near-field optical enhancement and nanoimprinting has been investigated experimentally and theoretically. It is found that nano pit arrays can be created on Al surfaces by laser irradiation (KrF excimer laser, λ = 248 nm) on an Al surface on which a monolayer of silica particles has been self-assembled. The influence of particle size and laser fluence on the structuring of Al surfaces has been examined. Particles with various diameters of 0.97, 2.34 and 5.06 µm were used in the experiment. Near-field optical enhancement and nanoimprinting were identified to explain the mechanisms for the formation of different structures on Al surfaces under different laser fluences. A high frequency structure simulator (HFSS) was used to simulate the optical field distribution in the particles attached on Al surfaces.

Cleaning efficiency dependence on various material types of nanoparticles in steam laser cleaning of silicon substrates will be presented. Nanoparticles, including alumina and titanium carbide were deposited on silicon substrates. A KrF excimer laser with a wavelength of 248 nm was used to irradiate the substrates in laser cleaning. A water steam layer of micrometer thickness was deposited on silicon substrates to improve the cleaning process. Cleaning efficiency was measured for different laser fluences ranging from 50 to 250 mJ/cm2 and pulse numbers from 1 to 100. Scanning electron micrographs of silicon samples before and after laser cleaning were used to calculate the cleaning efficiency. Ultrafast photodetector was used to monitor the water steam deposition and removal in the steam laser cleaning process.

A monolayer of self-assembled silica particles can be imprinted into a silicon substrate by laser irradiation (KrF excimer laser, λ=248nm). Periodical hemispherical cavities can be therefore created on the substrate surface. The influences of various particle sizes and laser fluence were investigated. In addition, preheating of the substrate significantly improves the performance. One-dimensional thermal calculation was employed to understand the thermal effect in this process. Three-dimensional optical simulation provided an accurate insight into the light intensity enhancement. Raman spectroscopy was used to examine the stress induced by the laser imprinting process resided in the cavity structures.

A new approach to fabricating hemispherical cavity arrays on silicon (Si) substrate using laser-assisted nanoimprinting of self-assembled particles is presented. A monolayer of silica particles, with different diameters of 0.30 and 0.97 µm, was deposited on a Si substrate by self-assembly. A quartz plate was tightly placed on the sample surface to firmly sandwich the self-assembled nanoparticle monolayer. The silica particles were imprinted into Si substrates after laser irradiation (KrF excimer laser, λ = 248 nm) on the quartz/nanoparticle/Si structure with a single pulse. Ultrasonic cleaning and hydrofluoric acid (HF) solution were used to remove the silica particles in the sample surface. Hemispherical cavities were formed on the substrate surface. The influence of laser fluence and particle size on the structuring of the surface has been investigated. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were performed to observe the dimensions of the cavities. One-dimensional thermal calculation was employed to calculate the thermal effects in this process.

A new approach to fabricating hemispherical-cavity arrays on silicon substrates using laser-assisted nanoimprinting of self-assembled particles has been developed. Monolayers of silica or polystyrene particles, with different diameters ranging from 160 nm to 5 /spl mu/m, were deposited on silicon substrates by self-assembly. A quartz plate, which is transparent to the laser wavelength of 248 nm, was tightly placed on the surface of each sample, which has been preheated. A KrF excimer laser beam with a wavelength of 248 nm was vertically irradiated on the quartz / nanoparticle / silicon structure. The silica particles were imprinted into silicon substrates by the quartz due to the transient Si surface melting during the laser irradiation. Ultrasonic cleaning and hydrofluoric-acid (HF) solution were then used to remove the silica particles. Hemispherical cavities on the silicon substrate were formed on the substrate surface. The effects of silica particle size, different laser fluence, laser pulse number and substrate temperature will be discussed in detail. It was found that polystyrene particles could not be used for the laser-assisted imprinting due to their low melting point. Raman spectroscopy will be used to analyze the material properties in the fabricated structures. Periodical multiple-layer structures (3-D structures) will also be made on silicon substrates.

A new approach to fabricate 3-D photonic bandgap structures on silicon substrates using colloidal crystals and laser-assisted nanoimprinting is presented. Self assembly was used to deposit two layers of silica micro particles with a diameter of 0.97 µm. A KrF excimer laser beam with a wavelength of 248 nm was vertically irradiated on the quartz plate placed on the silicon substrate containing two layers of silica particles. The silica particles were imprinted into silicon substrate by the quartz plate during the laser pulse irradiation. Ultrasonic cleaning and hydrofluoric-acid (HF) solution were then used to remove the silica particles. 3-D hemispherical cavities were formed on the silicon substrate surface.

Microfabrication of sub-micron holes on 30 nm thick aluminum films on fused silica was investigated using pulse durations form 300 fs to 6 ns at 400 nm wavelength. Micromachined areas were investigate using atomic force microscopy for quality and size of features produced. Ablation diameters less than 400 nm was achieved with all pulse widths. Pulses less than 5 ps removed the films cleanly and left a flat- bottomed crater with no evidence of substrate melting over a wide fluence range.