Ying Y. Tsui

Detection of lead in water using laser-induced breakdown spectroscopy and laser-induced fluorescence.

Laser-induced breakdown spectroscopy (LIBS) is a well-known technique for fast, stand-off, and nondestructive analysis of the elemental composition of a sample. We have been investigating micro-LIBS for the past few years and demonstrating its application to microanalysis of surfaces. Recently, we have integrated micro-LIBS with laser-induced fluorescence (LIF), and this combination, laser ablation laser-induced fluorescence (LA-LIF), allows one to achieve much higher sensitivity than traditional LIBS. In this study, we use a 170 microJ laser pulse to ablate a liquid sample in order to measure the lead content. The plasma created was re-excited by a 10 microJ laser pulse tuned to one of the lead resonant lines. Upon optimization, the 3sigma limit of detection was found to be 35 +/- 7 ppb, which is close to the EPA standard for the level of lead allowed in drinking water.

Production of porous carbon thin films by pulsed laser deposition

Pulsed laser deposition (PLD) has been used together with the Glancing Angle Deposition (GLAD) technique [1,2] for the first time to produce highly porous structured films. A laser produced carbon plasma and vapour plume was deposited at a highly oblique incident angle onto rotating Si substrates, resulting in films exhibiting high bulk porosity and controlled columnar microstructure. By varying the substrate rotation rate, the shape of the microcolumns can be tailored. These results extend the versatility of the GLAD process to materials not readily deposited by means of traditional physical vapour deposition techniques.

Pressure dependence of emission intensity in femtosecond laser-induced breakdown spectroscopy

Femtosecond laser produced plasma emission has been characterized as a function of pressure for applications in laser induced breakdown spectroscopy (LIBS). Experiments were performed with a Ti:sapphire laser system (130 fs, 800 nm), from atmospheric pressures down to 10−3 Torr and at pulse energies on the order of 1–50 μJ, (0.1–5 J cm−2). Characteristic emission lines from Al, Mg, Si and Cu elements exhibited significant enhancement in signal intensity at a few Torr background air pressure as compared to atmospheric air pressure. Spatially and temporally resolved emission measurements indicate enhancement due to a longer lifetime of the plasma expanding to a larger size at lower background pressures. Further reduction in pressure down to 10−3 Torr resulted in a decrease in signal intensity, as a result of a reduction of collisional excitation of the emission lines which occurs when the plasma plume expands into the ambient atmosphere. It has been also observed that signal enhancement at low pressure is very much dependent on the measurement delay time and on the transition being observed. With a delay time of 200 ns the integrated intensity of the neutral Al I lines at 396 nm exhibited 67 times enhancement in signal intensity at 4 Torr of pressure with respect to atmospheric pressure, whereas signal enhancement is only 4 times when no measurement delay time was used. The best signal to noise ratio of 850 was observed at 4 Torr pressure for an 85 ns delay time. Measurements of crater size showed no pressure dependent changes in the ablated mass, indicating that little plasma shielding occurs due to the short pulse duration of the femtosecond laser pulses.

Microscope-based label-free microfluidic cytometry.

A microscope-based label-free microfluidic cytometer capable of acquiring two dimensional light scatter patterns from single cells, pattern analysis of which determines cellular information such as cell size, orientation and inner nanostructure, was developed. Finite-difference time-domain numerical simulations compared favorably with experimental scatter patterns from micrometer-sized beads and cells. The device was capable of obtaining light scattering patterns from the smallest mature blood cells (platelets) and cord blood hematopoietic stem/progenitor cells 
(CD34 + cells) and myeloid precursor cells. The potential for evaluation of cells using this label-free microfluidic cytometric technique was discussed.

Sensitive detection of metals in water using laser-induced breakdown spectroscopy on wood sample substrates

Water contaminated with toxic heavy metals can be a great risk to humans. Laser-induced breakdown spectroscopy (LIBS) is a promising candidate to monitor heavy metals in aqueous solutions on site, but the sensitivity is still a major problem. To perform sensitive analysis of analyte metals in aqueous solutions with LIBS, a thin wood sample substrate was used as a liquid absorber to transform the liquid sample analysis to a solid sample analysis. We focus on investigating the performance of this technique using different laser wavelengths (266, 532, and 1064 nm) with a low pulse energy (<5 mJ) and a different number of shots (from 10 to 1000). We demonstrate that a limit of detection of 30 ppb can be achieved using low energy pulses with a 1000 shot accumulation. This technique provides a potentially simple approach for a portable micro LIBS system to monitor water samples.

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.

Optical properties of porous nanostructured Y2O3:Eu thin films

Nanostructured photoluminescent 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 technique. GLAD makes use of controlled substrate motion during extremely oblique physical vapor deposition of a thin film resulting in a high degree of control over the nanostructure of the film. Scanning electron microscopy and x-ray diffraction were used to characterize film nanostructure, while the light emission properties of these films were characterized by photoluminescence measurements. Transmission ellipsometry measurements were used to determine the degree of selective transmission of polarized light through the samples. The polarization of the emission from chevronic and helical films was determined through the measurement of the Stokes parameters of the emitted light for each sample. Comparison of the degree of polarization during the emission experiments to the degree of selective ...

Effect of Low-Intensity Pulsed Ultrasound on Orthodontically Induced Root Resorption in Beagle Dogs

We investigated the effect of low-intensity pulsed ultrasound (LIPUS) on orthodontically induced inflammatory root resorption in vivo. Ten beagle dogs were treated with an orthodontic appliance to move the mandibular fourth premolars bodily. The orthodontic movement was carried out for 4 wk with a continuous force of 1 N/side; using a split-mouth model, LIPUS was applied daily for 20 min. Fourth premolar and surrounding periodontal tissue were evaluated with micro-computed tomography and hematoxylin and eosin and tartrate-resistant acid phosphatase staining. We calculated the number, volume and distribution of root resorption lacunae and their percentage relative to total root volume, orthodontic tooth movement and periodontal ligament space. There was no significant difference in orthodontic tooth movement between the two sides. LIPUS significantly reduced the number of orthodontically induced inflammatory root resorption initiation areas by 71%, reduced their total volume by 68% and reduced their volume relative to the affected root total volume by 70%. LIPUS induced the formation of a precementum layer, thicker cementum and reparative cellular cementum.

Permanent fine tuning of silicon microring devices by femtosecond laser surface amorphization and ablation

We demonstrate the fine tuning capability of femtosecond laser surface modification as a permanent trimming mechanism for silicon photonic components. Silicon microring resonators with a 15 µm radius were irradiated with single 400 nm wavelength laser pulses at varying fluences. Below the laser ablation threshold, surface amorphization of the crystalline silicon waveguides yielded a tuning rate of 20 ± 2 nm/J · cm(-2)with a minimum resonance wavelength shift of 0.10nm. Above that threshold, ablation yielded a minimum resonance shift of -1.7 nm. There was some increase in waveguide loss for both trimming mechanisms. We also demonstrated the application of the method by using it to permanently correct the resonance mismatch of a second-order microring filter.

Schottky barrier thin film transistors using solution-processed n-ZnO.

Solution-processed ZnO thin films are attractive as active materials in thin film transistors (TFTs) for low-cost electronic device applications. However, the lack of true enhancement mode operation, low mobility, and unreliability in transistor characteristics due to the high density of traps and other defects present challenges in using such TFTs in circuits. We demonstrate in this report that the electrical characteristics of such TFTs can be improved by source injection barriers. Asymmetrical Schottky source metal-oxide-semiconductor field-effect transistors (MOSFETs) have been fabricated by utilizing heavily doped solution-processed ZnO as the active layer. n(+)-ZnO was obtained by using triethylamine as the stabilizer in the solution process instead of the more commonly used monoethanolamine. Au was chosen for source metallization to create a Schottky contact to the ZnO and an Al ohmic contact was chosen as the drain. Voltage applied to the gate induced field emission through the Schottky barrier and allowed modulation of the drain current by varying the width of the barrier. By operating the asymmetrical MOSFET when the Schottky contact is reverse biased, effective control over the transistor characteristics was obtained.