This work addresses the need to spectrally analyze of the absorption of middle-infrared (mid-IR) radiation in single living cells, with subwavelength spatial resolution, to identify molecular groups in them. The challenge is considerable, no lens can be used, so to realize such a device, a near-field probe was developed, from an optical fiber that is transparent in the mid-IR, non soluble in water, non-toxic and mechanically suitable. Incorporation of this probe in a scanning microscope, and use on a specially contained single living cell in water, allowed to achieve subwavelength imaging. Our fiber-material of choice is silver halides, i.e. AgClxBr1-x, made in the Applied Phyics Group of Tel-Aviv University. In spite of being bulky they were mechanically adapted to scanning microscopy. Theoretical and experimental investigations into the dampening of the motion of the probe in water were performed. A grid-like holder for containing living-cells for near-field microscopy has been introduced. The operating principle of this grid is based on sinking the cells inside the holes of the grid and letting them only negligibly protrude out of the holes (compared to the height-range of motion of the tip), in air and water. The result is a demonstration of the operation of the SNIM on different types of objects, including yeast cells, in water.
The principal possibility to recognize liquid explosives and their components in various
glass and plastic containers with different transparency in visible spectral range was demonstrated.
Acetone was used as a target, as alone and mixed with traditional liquids. The advantage of gated
Raman spectroscopy over the CW was proved. It was found that using 532 nm, 6 ns laser pulses any
real target with characteristic Raman spectrum with intensity similar to those for acetone may be
detected in 100 % of glass and in 80 % of plastic containers. The mixing with different liquid makes
detection more difficult and acetone was detected in 55 % of studied cases. The main reasons for
detection difficulties are intrinsic Raman and luminescence of plastic containers and liquids relevant
to airport passengers. In case of strong luminescence the advantages of red light excitation over
green light was demonstrated.
Real time detection and identification of explosives at a standoff distance is a major issue in efforts
to develop defense against so-called Improvised Explosive Devices (IED). It is recognized that the only
technique, which is potentially capable to standoff detection of minimal amounts of explosives is laser-based
spectroscopy. LDS activity is based on a combination of laser-based spectroscopic methods with orthogonal
capabilities. Our technique belongs to trace detection, namely to its micro-particles variety. It is based on
commonly held belief that surface contamination was very difficult to avoid and could be exploited for standoff
detection. We has applied optical techniques including gated Raman and time-resolved luminescence
spectroscopy for detection of main explosive materials, both factory and homemade. We developed and tested a
Raman system for the field remote detection and identification of minimal amounts of explosives on relevant
surfaces at a distance of up to 30 meters.
Real time detection and identification of explosives at a standoff distance is a major issue in efforts
to develop defense against so-called Improvised Explosive Devices (IED). It is recognized that the only
technique, which is potentially capable to standoff detection of minimal amounts of explosives is laser-based
spectroscopy. LDS technique belongs to trace detection, namely to its micro-particles variety. We
applied gated Raman and time-resolved luminescence spectroscopy for detection of main explosive
materials, both factory and homemade. Raman system was developed and tested by LDS for field remote
detection and identification of minimal amounts of explosives on relevant surfaces at a distance of up to
30 meters.
Middle infrared laser systems for countermeasures against heat seeking missiles are currently under development. These systems, based on optical parametric oscillators, are complex, bulky and expensive. Middle-infrared fiber lasers emitting in the 3-5μm spectral region may provide an attractive alternative to the systems under development. We have investigated luminescence of silver bromide-chloride crystals and fibers doped with rare earth ions (e.g. Pr3+, Tb3+ and Nd3+) in the near and middle infrared spectral ranges. The emission, excitation, and absorption spectra, as well as the kinetic parameters, were measured over a broad temperature range. The crystal doping was produced by growing from the melt. No significant differences were found between the luminescence properties in bulk crystals and in fibers. The Judd Ofelt analysis was applied to the doped crystals, and the transition rates, branching ratios, and quantum efficiencies were calculated. Good agreement was obtained between theory and experiment. The strong middle-infrared luminescence and the kinetic parameters of these crystals make them good candidates for the fabrication of fiber lasers in the 4-5.5μm spectral range.
Middle infrared lasers for countermeasures against heat seeking missiles are currently under development. These systems, based on diode pumped solid state lasers pumping optical parametric oscillators, are complex, bulky and expensive. Middle infrared fiber lasers in the 3 to 5 μm spectral region which operate without a need for frequency conversion may provide an attractive option. We have investigated the luminescence of silver bromide-chloride crystals and fibers doped with Pr3+ ions in the near and middle infrared spectral ranges. The emission, excitation, and absorption spectra, as well as the kinetic parameters, were measured over a broad temperature range. The crystal activation was produced by growing from the melt. No differences were found between the luminescence properties in the crystals and the fibers. The Judd Ofelt analysis was applied to the Pr doped crystals, and the transition rates, branching ratios, and the quantum efficiencies were calculated. Good agreement was obtained between the theory and the experiment. The strong middle infrared luminescence and the kinetic parameters of these crystals make them good candidates for the fabrication of fiber lasers in 4 -5.5μm spectral range. Such lasers would be very useful for countermeasure devices.
The luminescence of silver bromide crystals doped with rare earth ions (Nd3+) was investigated in the visible and near IR spectral ranges. The emission, excitation, and absorption spectra, as well as the kinetic parameters, were measured over a broad temperature range. Crystal doping was produced by growing in the melt and by a diffusion method. A novel method for measuring the diffusion profile of rare earth ions in AgBr crystals, based on the luminescence distribution of the dopant, was proposed. The luminescence parameters of AgBr:Nd crystals doped by the diffusion method were compared with these for crystals doped by adding Nd3+ to the melt. The spectroscopy parameters of Pr3+ and Er3+ ions in AgBr crystals doped in the melt were also investigated. The Judd-Ofelt analysis was applied to the rare-earth doped crystals, and transition rates, branching ratios, and quantum efficiencies were calculated. Good agreement between theory and experiment was obtained.
A beam-homogenizing device consisting of a short tip of a silver-halide infrared fiber removes the effect of low-order multimode generated by bending of hollow guides. A series of experiments employing a CO2 laser shows that the solid fiber functions as a homogenizer with a small insertion loss of 0.2 dB.
Theoretical modeling of strongly pumped doped silver halide fiber amplifiers is presented. This is an analytic part of an effort to introduce new lasing materials, composed of doped silver halides, in the near or mid IR region. The analysis is based on a rate equation formalism, for which experimental measurements of various optical properties of doped silver halide and crystals were applied as input parameters. The model includes solutions for both three- and four-level systems, such as Er3+ and Nd3+. Optimal range of fiber dimensions, doping materials and concentrations, and other optical properties are suggested in order to achieve significant amplification.
The luminescence of silver bromide crystals doped with rare earth ions (Nd3+, Pr3+, and Er3+) was investigated in the visible and near infrared spectral ranges. The emission, excitation, and absorption spectra, as well as the kinetic parameter, were measured over a broad temperature range. The Judd-Ofelt analysis was applied to the rare-earth doped crystals, and transition rates, branching ratios, and quantum efficiencies were calculated. Good agreement between theory and experiment was obtained.
Lasers suitable for myringotomy are the erbium:YAG laser (2940 nm) and the carbon-dioxide laser (10600 nm). The study examines the laser-tissue interaction with tympanic membranes of guinea-pigs, horses and formalin-fixed human tympanic membranes and the effects demonstrated by light-microscopy and scanning-electron-microscopy. The minimum energy densities for a perforation with the erbium:YAG laser in guinea-pig ear drums and formalin-fixed human tympanic membranes are 8 J/cm2 and 16 J/cm2 respectively. There are no thermic side effects. With the carbon-dioxide laser thermic side effects only occur with energy transmission via silver halide polycrystalline fiber. The minimum power density for perforation is 400 W/cm2 (pulse duration 50 ms). With the microslad 719 micromanipulator (Sharplan, Israel, Tel Aviv), the minimum power densities for perforation of guinea-pig and horse eardrums and for formalin-fixed human tympanic membranes are 150 W/cm2, 300 W/cm2 and 600 W/cm2 (pulse duration: 50 ms) respectively. The minimum power density to achieve a perforation with the SwiftLaseTM 757 scanner (Sharplan, Israel, Tel Aviv), is 250 W/cm2 in guinea-pig eardrums (pulse duration: 100 ms). A prototype of a hand-held carbon-dioxide laser otoscope is suitable for performing laser myringotomies in formalin-fixed human tympanic membranes.
Laser-induced breakdown (LIB) thresholds in AgClxBr1-x crystals and fibers were studied under CO2 laser pulses and CW excitation. The value of LIB threshold Pc of the bulk crystals is about 7.2 X 108 W/cm2 for AgCl and 4 X 109 W/cm2 for AgBr under 60 ns TEA laser excitation. The LIB threshold in fibers is much smaller; about 2 X 108 W/cm2. The absorption of the crystals at 10.6 micrometers changes from 8 X 10-5 cm-1 for AgCl to 2 X 10-5 cm-1 for AgBr; for fibers with the same composition absorption is much greater. The dependence of the LIB and IR absorption on composition, and mechanical and temperature treatments suggests that the LIB in silver halide crystals and fibers is due to the avalanche electrons in a high electric field. The initial free electrons for this process are supplied by ionization of the cation vacancy - charged dislocation complexes. The relatively lower optical stability of the fibers is due to the increased concentration of defects formed in the hot extrusion of the crystal. A simple thermal annealing method for reduction of the IR absorption is proposed.
The maximal laser power transmitted through polycrystalline silver halide fibers is limited by their IR absorption and optical strength. The mechanisms of the IR absorption and laser-induced breakdown (LIB) thresholds Pc in AgClxBr1-x crystals and polycrystalline fibers were studied. Investigations, including mechanical treatments, heat treatments, and luminescence properties were performed to clarify the reason for the IR absorption and LIB in silver halide crystals and fibers. From these experiments we concluded that one of the reasons for LIB may be the avalanche of electrons in the conduction band of the crystal. These seed electrons are produced by the IR absorption of the cation vacancies in these crystals. A simple annealing procedure for reduction of the IR absorption of fibers is proposed.
Laser power transmission, luminescence, mechanical behavior and effects of various thermal treatments have been investigated on mixed sliver halide crystals. This knowledge may suggest ways of optimizing the fabrication of optical fibers from such crystals. Anti-reflection coating for high-power 10.6 micrometer light transmission was studied both on crystals and fibers. Laser power transmission and mechanical properties of the fibers also have been investigated. On the basis of these experiments the prototype of the medical catheter was produced and surgery experiments with this catheter have been performed.
Laser-induced breakdown (LIB) thresholds in AgClxBr1-x crystals and fibers were studied under CO2 pulse and cw laser excitation. The dependence of the LIB and IR absorption on composition and temperature treatment suggests, that the LIB in silver halide crystals and fibers is due to the absorption at 10.6 micrometers caused by cation vacancies bound with charged dislocation silver halides. The lower optical stability of the fibers is due to defects induced by the heating under pressure, applied during the production of the fibers by extrusion and the subsequent fast quenching to RT. A simple method for reduction of the IR absorption is proposed.
Laser power transmission experiments through silver halide discs and fibers have been performed. In these experiments cw, long pulsed and short pulsed (TEA) CO2 lasers, with power levels of 20 - 40 W or pulsed laser with energies of hundreds of mJ per pulse, have been used. The mechanical and optical properties of unclad fibers made of various alloys AgClxBr1-x with 0 < x < 1 were investigated. Compounds with x approximately equals .5 proved most suitable. They have optical losses of about .2 dB/m at (lambda) equals 10.6 micrometers and good mechanical properties.
Laser induced breakdown (LIB) thresholds of the AgClxBr1-x (0 2 lasers. Bulk and surface crystal/polycrystalline samples and fibers were investigated. The intrinsic LIB threshold of the crystals is dependent on x and are 3.8$CTR108W/cm2 for x equals 1; 8.2$CTR102W/cm2 for x equals 0 under 100 ns TEA CO2 laser excitation. The LIB threshold in fibers is roughly 2$CTR107W/cm2.
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