1,354,430 research outputs found
Advanced Optical Techniques for micro-Fluid Dynamics
No full textThe impact of microfluidic technologies in the field of life sciences has dramatically increased during the last years, due to the need to develop new products for real applications. The work reported in the present dissertation is focused on the development of optical systems for the study of fluid dynamic phenomena at the micro and nano-scale. The research activities were carried out in the frame of two main topics and, consequently, the dissertation is divided into two main parts.
The first part of the dissertation is focused on the experimental study of cavitation in liquids, in which a bubble is generated by focusing a laser beam under extremely controlled conditions. The aim is addressing all different phases of the process. Cavitation is studied using a combined system of a fast camera, for the complete reconstruction of the plasma shape and bubbles dynamics, and a fiber optical hydrophone (FOHP), for the detection of the pressure shock waves in proximity of the bubble. The breakdown phenomenon and the bubble dynamics have been characterized when the optical arrangement of the focusing system is modified, e.g. using different expansion factors of the beam expander to change the focusing angle of the laser beam, or when the laser pulse energy is tuned. Data analysis shows a strong correlation between the number of plasma sites and the number of shock waves detected by the fiber hydrophone.
The second part of the dissertation is focused on the study of a particular class of surface electromagnetic waves and on their use for sensing applications. Such waves are sustained at the surface of finite one-dimensional photonic crystals (1DPC) and are generally named Bloch surface waves (BSW). The robustness of optical sensors based on BSW has been investigated experimentally and numerically. The distributions of sensor characteristics caused by the fabrication uncertainties in dielectric layer thicknesses have been analysed. It is demonstrated that the performance of the surface wave sensors is sufficiently robust with respect to the changes of the photonic crystal layer thicknesses. Layer thickness optimization of the photonic crystal, carried out to achieve low limit of detection, leads to an improvement of the robustness of the surface wave sensors that is attributed to Bloch states lying deeper in the photonic band gap.
The work reported in the dissertation demonstrates the use of an optical sensing platform based on BSW as a novel optical tool to probe in real time the fluid flow at a boundary wall of a microfluidic channel under dynamic conditions. Exploiting the properties of the BSW, we can put into evidence the temporal evolution of the interface during the injection of liquids with different refractive index (RI). We introduce ξ defined as the distance between the interface of the liquids of different RI and the 1DPC surface. Reconstructed experimental maps of ξ(z,t), as a function of time and of the position across the μ-fluidic channel, allow us to recovery the temporal evolution of the fluids interface in proximity of the wall. From the data analysis, the diffusion coefficient of a solute in water is measured and found in good agreement with the literature value.
Moreover, biosensors based on BSWs have been studied and their practical application was demonstrated by detecting a specific glycoprotein, Angiopoietin 2, that is involved in angiogenesis and inflammation processes, and to detect clinically relevant concentrations of the breast cancer biomarker ERBB2 in cell lysates. The protocol used for the label-free detection of Angiopoietin 2 is described and the results of an exemplary assay are given, confirming that an efficient detection can be achieved. The limit of detection of the biochips for Angiopoietin 2, based on the protocol used, is 1.5 pg/mm^2 in buffer solution. To detect soluble ERBB2, we develoed an optical set-up which operates in both label-free and fluorescence modes. The resolution obtained in both modes meet international guidelines and recommendations (15 ng/mL ) for ERBB2 quantification assays, providing an alternative tool to phenotype and diagnose molecular cancer subtypes.
During the last part of the doctorate period, the two research directions pursued during the first and the second part, devoted to bubble cavitation and to BSW based sensors, merged in a new innovative setup. Starting from the BSW concept, a new type of optical hydrophone based on BSW for the detection of pressure shock waves with a larger sensitivity than FOHP has been proposed and setup. Some preliminary and encouraging results are presented and discussed
Microdrilling of metals with an inexpensive and compact ultra-short-pulse fiber amplified microchip laser
No full textWe have investigated the ultra-fast microdrilling of metals using a compact and cheap fiber amplified passively Q-switched microchip laser. This laser system delivers 100-ps pulses with repetition rates higher than 100 kHz and pulse energies up to 80 μJ. The ablation process has been studied on metals with quite different thermal properties (copper, carbon steel and stainless steel). The dependence of the ablation depth per pulse on the pulse energy follows the same logarithmic scaling laws governing laser ablation with sub-picosecond pulses. Structures ablated with 100-ps laser pulses are accompanied only by a thin layer of melted material. Despite this, results with a high level of precision are obtained when using the laser trepanning technique. This simple and affordable laser system could be a valid alternative to nanosecond laser sources for micromachining applications. © 2008 Springer-Verlag
Vacuum Instability and Pair Production in an Optical Setting
No full textIn the Dirac-sea picture, the physics of pair production and instability of the quantum electrodynamics
vacuum in presence of an oscillating electric field resembles the phenomenon of interband transition of
light waves in photonic superlattices induced by a geometric curvature. We realize a binary wave guide
superlattice with a curved optical axis mimicking dynamical pair production induced by two counterpropagating
ultrastrong laser pulses. Our optical analogue enables visualization of formation of electronpositron
pair in physical space as splitting of a wave packet, originally representing an electron in the
Dirac sea
High repetition rate ultrashort pulse micromachining with fiber lasers
No full textDespite its advantages with respect to precision, ultrashort pulse micromachining often suffers from a low processing speed. We will discuss the opportunities for high repetition rate and high average power ultrafast fiber lasers to overcome these problems. © OSA / FILAS 2011
Microdrilling of metals using femtosecond laser pulses and high average powers at 515 nm and 1030 nm
No full textWe investigate the microdrilling of metals (stainless steel, copper and tungsten) for two different wavelengths, 1030 nm and 515 nm, in the regime of femtosecond laser pulses. An ytterbium-doped fibre CPA system provides high pulse energies (up to 70 μJ) and high repetition rates (up to 800 kHz), corresponding to high average powers of about 50 W, for this experimental study. © 2010 Springer-Verlag
Correspondence to Joseph E. Lowery from Carlos Tunnermann Bernheim, December 4, 1984
No full textA letter from Nicaraguan Ambassador Carlos Tunnermann Bernheim to Joseph E. Lowery inviting Lowery to the inauguration ceremony for Daniel Ortega Saavedra as President of Nicaragua. 2 pages.The Atlanta University Center Robert W. Woodruff Library acknowledges the generous support of the Joseph & Evelyn Lowery Institute for Justice and Human Rights, the Joseph Echols Lowery Irrevocable Trust, and other donors in supporting the processing and digitization of Morehouse College's Joseph Echols and Evelyn Gibson Lowery Collection
Laser drilling using a high repetition rate and high average power femtosecond fiber CPA system
No full textWe report on laser drilling experiments on copper and stainless steel samples using a novel ultrafast fiber CPA laser amplifier. Effects of particle shielding and heat accumulation at high average powers are discussed. © 2008 Optical Society of America
Femtosecond laser-written quasi-phase-matched waveguides in lithium niobate
No full textWe report on thermally stable optical waveguides written in periodically poled lithium niobate with a femtosecond laser using a two line approach. Both fundamental and second harmonic light at 1064 and 532 nm are guided with low damping losses. In a 10-mm -long sample a conversion efficiency of 58% was achieved. © 2007 American Institute of Physics
Critical performance aspects of ultrashort pulse laser materials processing at high repetition rates and average powers
No full textAn experimental investigation is presented on the ultrashort pulse laser drilling of different metals with diverse thermal properties in the high repetition rate and high average power regime. An Ytterbium-doped fiber CPA system was used, providing pulse energies and repetition rates up to 70 μJ and 1 MHz, respectively. It has been found that at a few hundred kilohertz particle shielding causes a decrease of the ablation rate, depending on the pulse energy. At higher repetition rates, the heat accumulation effect overbalances particle shielding, but significant melt ejection affects the hole quality. The influence, in this regime, of pulse duration (800 fs to 19 ps) and wavelength (1030 nm and 515 nm) on the drilling efficiency and on the achievable precision have been further experimentally studied
Evanescent coupling in arrays of type II femtosecond laser-written waveguides in bulk x -cut lithium niobate
No full textWe report on the first fabrication of evanescently coupled planar arrays of type II waveguides in x -cut lithium niobate (LiNbO3) by femtosecond laser inscription. A particular choice of writing parameters allows the evanescent field of the individual waveguide modes to extend significantly beyond the damage lines into the neighboring guides, thus realizing a coupling constant of 2.2 cm-1 at a wavelength of 633 nm. © 2008 American Institute of Physics
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