David W. Doerr

Investigation of femtosecond laser assisted nano and microscale modifications in lithium niobate

A study of the physicochemical modifications at micro and nano scales as a result of femtosecond laser processing is essential to explore the viability of this process to write surface and subsurface structures in transparent media. To this end, scanning probe and transmission electron microscopy and spectroscopy techniques were used to study these modifications in lithium niobate. A variable power Ti:Sapphire system (800nm,300fs) was used to determine the ablation threshold of (110) lithium niobate, and to write these structures in the substrate for subsequent analysis. Higher processing energies were used to amplify the laser-induced effects for a clear understanding. Evidences of a number of simultaneously occurring mechanisms such as melting, ablation, and shockwave propagation are observed in the scanning electron microscope (SEM) micrographs. X-ray diffraction (XRD), Auger and electron dispersive spectroscopy (EDS) studies indicate loss of lithium and oxygen from the immediate surface of the process...

Nanoscale surface and subsurface defects induced in lithium niobate by a femtosecond laser

In this letter, electron and ion microscopy techniques have been used to characterize the changes that result when single crystals of lithium niobate are processed using a focused femtosecond laser. The prevailing observation is that of competing processes—ablation and partial redeposition, thermal shock, and extreme quenching, as well as effects associated with shock wave propagation, resulting in both amorphization and heavily defective regions at the focal point of the laser pulse. The observed microstructural defects have a direct implication in optical memory or waveguide writing, where the goal is to realize consistent structural features with uniform optical properties.

Femtosecond laser production of metal surfaces having unique surface structures that are broadband absorbers

Femtosecond laser induced modification of surface structure and the optical properties of metals are studied. A Ti:sapphire femtosecond laser is used for ablating bulk aluminum 2024 T3 alloy and highly pure neutron activation quality gold metal samples at different laser irradiation parameters. The ablation parameters are optimized for the two metals to form a low reflection surface in the wavelength range of 0.3–50 μm. “Black aluminum” and “black gold” metal surfaces are formed upon laser ablation, having a unique surface structure with micro- and nanosized features. Sample analysis is performed by several characterization techniques like scanning electron microscope, energy dispersive x ray, profilometry, and integrating sphere reflectance measurements. Correlation is drawn between the laser ablation parameters and the formed surface structure and its properties. The reflection of light is found to depend primarily on the surface structure (roughness) of the highly modified metal surfaces.

Laser-assisted nanoscale deposition of diamond-like carbon films on tungsten tips

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.

Parametric investigation of laser nanoimprinting of hemispherical cavity arrays

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.

Laser-assisted nanopatterning of aluminium using particle-induced near-field optical enhancement and nanoimprinting

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, ? = 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??m 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.

Propagation of ultrashort laser pulses through water

In this paper, propagation of ultrashort pulses through a long 3.5 meter water channel was studied. Of particular interest was the attenuation of the beam at various lengths along the variable path length and to find an explanation of why the attenuation deviates from typical Beer Lambert law around 3 meters for ultrashort laser pulse transmission. Laser pulses of 10 fs at 75 MHz, 100 fs at 80 MHz and 300 fs at 1 KHz were employed to investigate the effects of pulse duration, spectrum and repetition rate on the attenuation after propagating through water up to 3 meters. Stretched pulse attenuation measurements produced from 10 fs at a frequency of 75 MHz were compared with the 10 fs attenuation measurements. Results indicate that the broad spectrum of the ultrashort pulse is the dominant reason for the observed decrease in attenuation after 3 meters of travel in a long water channel. The repetition rate is found not to play a significant role at least for the long pulse scenario in this reported attenuation studies.

Diffraction characteristics of a Fresnel zone plate illuminated by 10 fs laser pulses

Results of experimental and theoretical work performed to compare diffraction patterns and focal distributions of a Fresnel zone plate illuminated by ultrashort 10 fs pulsed and cw laser beams are presented. It is shown that the foci intensities of 10 fs pulses are considerably lower than those of cw beams while the focal widths in the axial and radial directions are broadened. Calculations also indicate the spectral modulation along the center of the diffraction patterns. These phenomena are explained by the coherent superposition of the composing frequency content.

Diffraction characteristics of 10 fs laser pulses passing through an aperture.

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.

Femtosecond pulse stretching in microscope objectives used for micro/nanomachining

Femtosecond laser pulses are stretched when they pass through an optical microscope objective used for laser machining. The question that must be answered is how much does the pulse stretch passing through the objective. Stretching of pulses would be more prominent in the case of ultrashort laser pulses (∼9 fs). In this article, a study was performed to measure the final pulse duration for a 9 fs laser pulse passing through a microscope objective (Edmund Achromatic Objective Micro 60× DIN, 0.85 numerical aperture). Results indicate that a 9 fs pulse is stretched to 62 fs (i.e., a factor of 6) after passing through the microscope objective.