Mike Taschuk

Comparative study of laser-induced plasma emission from microjoule picosecond and nanosecond KrF-laser pulses

Abstract In this paper, the emission of laser produced silicon and aluminum plasmas is investigated in the energy range from 0.1 to 100 μJ (0.5–500 J/cm 2 ) using 10 ns and 50 ps KrF laser pulses focused to a 5 μm diameter spot. For energies higher than 3 μJ, there is little difference between 50 ps and 10 ns pulses in the plasma emission both in terms of the intensity of the emission lines and in terms of lifetime of the emission. Differences become significant only at very low fluences approaching the plasma formation threshold, which is significantly lower for 50 ps pulses than for 10 ns pulses. Calculations using a plasma ablation model show that initial plasma conditions are significantly different for 50 ps and 10 ns pulses during irradiation by the laser pulses. However, the dominant process leading to plasma emission at later times is from expansion and cooling of the plasma plume in the form of a blast wave in the ambient air which is primarily dependent on the energy deposited in the plasma and not the pulse length. Calibrations have also been carried out in order to give the emission in absolute numbers of photons emitted and thus facilitate the comparison with modeling and future experiments.

Laser-induced breakdown spectroscopy for microanalysis using submillijoule UV laser pulses

This paper presents a study of laser-induced breakdown spectroscopy (LIBS) at low energies using KrF laser pulses of only 50–300 μJ. Very small focal spots with diameters of 5 to 20 μm are employed in order to achieve strong plasma emission. The focused intensities were in the range of 1.6 to 150 GW/cm2. The evolution of the micro-plasma progresses more rapidly in this energy range compared to conventional LIBS using mJ laser pulses. The optimum delay time for the detection of emission from minor constituent elements in aluminum is between 100 and 360 ns after the laser pulse hits the sample. The corresponding limits of detection are in the range of 2 to 450 ppm and are comparable to experiments that have used much higher laser energies. The amount of ablated material is significantly reduced using low laser energies and typical crater diameters are approximately 15–40 μm.

Detection and Mapping of Latent Fingerprints by Laser-Induced Breakdown Spectroscopy

Detection of latent fingerprints on a Si wafer by laser-induced breakdown spectroscopy (LIBS) is demonstrated using approximately 120 fs pulses at 400 nm with energies of 84 ± 7 μJ. The presence of a fingerprint ridge is found by observing the Na emission lines from the transferred skin oil. The presence of the thin layer of transferred oil was also found to be sufficient to suppress the LIBS signal from the Si substrate, giving an alternative method of mapping the latent fingerprint using the Si emission. A two-dimensional image of a latent fingerprint can be successfully collected using these techniques.

Spectrochemical microanalysis of aluminum alloys by laser-induced breakdown spectroscopy: identification of precipitates

Multielemental microanalysis of commercially available aluminum alloys has been performed in air by laser-induced breakdown spectroscopy (LIBS) by use of UV laser pulses with energies below 10 microJ. It is shown that the LIBS technique is capable of detecting the elemental composition of particles less than 10 microm in size, such as precipitates in an aluminum alloy matrix, by using single laser shots. Chemical mapping with a lateral resolution of approximately 10 microm of the distribution of precipitates in the surface plane of a sample was also carried out. Two main types of precipitate, namely, Mn-Fe-Cu (type I) and Mg-Cu (type II), were unambiguously distinguished in our LIBS experiments, in good agreement with x-ray microanalysis measurements. The relative standard deviations of emission of the main minor constituent elements (Cu, Mg, Mn) of the aluminum 2024 alloy range from 33% to 39% when laser shots on the precipitates are included in the analysis but decrease to a range from 5.3% to 7.4% when laser shots are taken only on the matrix material, excluding the precipitates.

Nanomilling surfaces using near-threshold femtosecond laser pulses

We have produced crater depths of less than 10 nanometers using 100-1000 pulses of a near-infrared femtosecond laser (800 nm, 125 fs) on a copper thin film surface. By determining the single-shot ablation threshold, incubation coefficient and surface reflectivity, the femtosecond laser pulse parameters for surface nanomilling are established close to the multiple-pulse ablation threshold limit for a copper thin film. Photomultiplier measurements of a copper emission line were used as a real time monitor of the nanomilling process for which photons were detected only once every several shots. The results are consistent with a model that ablation occurs in bursts every several shots after a number of intervening incubation energy storage shots.

Quantitative emission from femtosecond microplasmas for laser-induced breakdown spectroscopy

An ongoing study of the scaling of Laser-Induced Breakdown Spectroscopy (LIBS) to microjoule pulse energies is being conducted to quantify the LIBS process. The use of microplasmas for LIBS requires good understanding of the emission scaling in order to maximize the sensitivity of the LIBS technique at low energies. The quantitative scaling of emission of Al, Cu and Si microplasmas from 100 μJ down to 100 nJ is presented. The scaling of line emission from major and minor constituents in Al 5052 alloy is investigated and evaluated for analytical LIBS. Ablated crater volume scaling and emission efficiency for Si microplasmas are investigated.

Using film nanostructure to control photoluminescence angular emission profiles

Luminescent thin films are used for many applications, including light-emitting diodes, lasers and flat panel displays. Glancing angle deposition (GLAD) is a physical vapor deposition technique which relies on highly oblique flux angles to create porous thin films. When combined with real-time substrate motion control and measurement of deposition rates, it is possible to produce high quality nanostructered thin films. A rugate filter uses a sinusoidally varying index profile to produce a stop band. Using the GLAD technique, it is possible to produce a rugate filter from a single material. The central wavelength, depth and width of the stop band can be designed by adjusting the film nanostructure. In this paper, rugates composed of Y2O3:Eu are used to control the angular emission profiles of the photoluminescent thin film. Confined, annular and isotropic emission profiles film is nearly uniform for emission angles between ~ -60° and ~60°.

Performance and Modeling of a Nanostructured Relative Humidity Sensor

Capacitive humidity sensors were fabricated using interdigitated electrodes coated with amorphous nanostructured TiO2 thin films grown by glancing angle deposition. The sensor exhibited a large change in capacitance, increasing exponentially from ∼ 1 nF to ∼ 1 μF for an increase in relative humidity from 2 % to 92 %. A simple model of the capacitive response and dielectric constant of the devices has been developed and compared to the experimental results. From this comparison, it is clear that the magnitude of the device response observed cannot be explained with bulk dielectric constants or literature values.

Luminescence of nanostructured Y2O3:Eu thin films fabricated using glancing angle deposition techniques

Thin films of europium-doped yttrium oxide (Y2O3:Eu), a well-known luminescent material, were grown using electron beam evaporation, in combination with the Glancing Angle Deposition (GLAD) technique. GLAD makes use of controlled substrate motion during physical vapour deposition (PVD), resulting in a high degree of control over the nanostructure of the film. Until recently GLAD had not been used with luminescent materials. Films were deposited using pre-doped Y2O3:Eu source material, with either 4.0% (wt) Eu doping or 5.6% (wt) Eu doping. The nanostructure of these films was characterized through scanning electron microscopy, while the light emission properties of these films was characterized by photoluminescence measurements. In order to optimize the light emission properties of the films the partial pressure of oxygen during the deposition of the films was varied. Films were deposited on both silicon and sapphire substrates, in order to compare how different substrates affect the growth and light emission of the films.

Circularly polarized luminescence from chiral thin films

Photoluminescent nanostructured thin films have been fabricated using physical vapour deposition and the glancing angle deposition (GLAD) technique. Precision controlled substrate motion and oblique incidence (>75o) enable the fabrication of a variety of 3-D morphologies including vertical posts, helical (chiral) columns and chevrons. Scanning electron microscopy and X-ray diffraction were used to characterize the film nanostructure. These experiments focussed on the chiral morphology which exhibits intriguing polarization behaviour such as selective transmission of circularly polarized light and circularly polarized photoluminescence. Helical films of Y2O3:Eu and Alq3 were fabricated with thicknesses in excess of 2 μm and densities nominally 60% of bulk. Transmission spectroscopic ellipsometry measurements were used to determine the degree of selective transmission of polarized light through the samples. The degree of circular polarization for the photoluminescent light emitted from helical films was measured with the use of a quarter waveplate and linear polarizer. Polarized photoluminescence efficiencies were consistent with the observed selective transmission of circularly polarized light through the films. The use of GLAD to control the nanoscale morphology of the films allows for the spectral location and strength of these polarization effects to be easily and accurately selected.