Applied Physics B: Lasers and Optics
Effects of volatile coatings on the laser-induced incandescence of soot
Tue, 2013-05-21 02:00 — GuestAbstract
We have measured time-resolved laser-induced incandescence (LII) from combustion-generated mature soot extracted from a burner and (1) coated with oleic acid or (2) coated with oleic acid and then thermally denuded using a thermodenuder. The soot samples were size selected using a differential mobility analyzer and characterized with a scanning mobility particle sizer, centrifugal particle mass analyzer, and transmission electron microscope. The results demonstrate a strong influence of coatings on the magnitude and temporal evolution of the LII signal. For coated particles, higher laser fluences are required to reach signal levels comparable to those of uncoated particles. The peak LII curve is shifted to increasingly higher fluences with increasing coating thickness until this effect saturates at a coating thickness of ~75 % by mass. These effects are predominantly attributable to the additional energy needed to vaporize the coating while heating the particle.
Spatial mode cleaning in radically asymmetric strongly focused laser beams
Sun, 2013-05-19 02:00 — GuestAbstract
We demonstrate that a femtosecond laser pulse strongly focused in air can produce a highly symmetric damage pattern on glass. This damage pattern contains a series of near-perfect radial rings, with diameters much larger than the predicted focal spot diameter. These rings disappear when the experiment is conducted in vacuum, indicating atmospheric involvement. Surprisingly, the shape and size of the rings seem to be nearly independent of the shape of the generating laser beam, showing dramatic spatial mode cleaning. A “half moon” initial laser mode created by obscuring one side of the round beam produces rings of similar quality to those obtained with the unclipped beam. While spatial mode cleaning has previously been reported in filaments, this is the most dramatic demonstration of the effect that we are aware of. We argue that the effect is due primarily to ionization, in contrast to studies in longer filaments that attribute it to self-focusing.
Theoretical treatments of ultrashort pulse laser processing of transparent materials: toward understanding the volume nanograting formation and “quill” writing effect
Sun, 2013-05-19 02:00 — GuestAbstract
The dynamics of ultrashort-laser-induced generation of free electron plasma inside bulk glass is analyzed. The results of modeling are presented for typical glass modification regimes, obtained on the basis of Maxwell’s equations supplemented with the equations describing electron plasma formation and the laser-induced electric current. The model has been demonstrated to allow revealing important features of laser beam propagation in the regimes of dense electron plasma generation such as strong scattering up to complete displacing of light from the plasma region followed by beam refocusing. The geometry of the laser energy absorption zone is determined, and the glass temperature is mapped which may be foreseen at the end of electron–glass matrix relaxation. This, in turn, allows estimating the laser-induced stress levels and making conclusions on the routes of glass modification.
Influence of the 61 S 0–63 P 1 resonance on continuous Lyman-α generation in mercury
Sat, 2013-05-18 02:00 — GuestAbstract
Continuous coherent radiation in the vacuum ultraviolet at 122 nm (Lyman-α) can be generated using sum-frequency mixing of three fundamental laser beams in mercury vapor. One of the fundamental beams is at 254 nm wavelength, which is close to the 61
S
0–63
P
1 resonance in mercury. Experiments have been performed to investigate the effect of this one-photon resonance on phase-matching, absorption and the nonlinear yield. The efficiency of continuous Lyman-α generation has been improved by a factor of 4.5.
Nd:YAG laser ablation characteristics of thin CIGS solar cell films
Sat, 2013-05-18 02:00 — GuestAbstract
This work reports that the ablation characteristics of thin CuIn1−x
Ga
x
Se2 (CIGS) solar cell film differ significantly with elemental composition and laser pulse energy. From in situ shadowgraphs measured during Nd:YAG laser (1,064 nm) irradiation of CIGS films and crater morphologies, it was found that strong surface evaporation is dominant for low Ga concentration films of which band gap is well below the photon energy. As the band gap of CIGS film becomes close to or over the laser photon energy due to increased Ga content, surface absorption diminishes and at low laser energy, laser heating of the film plays an important role. It is demonstrated that for the CIGS films with Ga/(Ga + In) ratio being approximately over 0.2, the laser irradiation leads to solid phase removal of the film due to thermomechanical fracture at low laser energy but to ablative evaporation at elevated energy.
Soot sublimation studies in a premixed flat flame using laser-induced incandescence (LII) and elastic light scattering (ELS)
Sat, 2013-05-18 02:00 — GuestAbstract
Laser-induced incandescence (LII) as a diagnostic technique is based on rapid heating of soot particles to temperatures of several thousand Kelvin and subsequent detection of the thermal radiation from the laser-heated particles. At such high temperatures, soot sublimation effects must be considered when estimating uncertainties in LII measurements. In this work we have investigated the use of various laser fluences in LII using a Nd:YAG laser at 1,064 nm. Using another Nd:YAG laser at 532 nm, the elastic light scattering (ELS) signal from soot particles heated by the 1,064-nm laser was monitored. This approach makes it possible to determine at which fluence of the LII laser soot sublimation starts to become visible as a decrease in the ELS signal. By performing the measurements at various heights in a premixed sooting flat ethylene/air flame, the fluence threshold above which the ELS signal decreased was found to be higher at the lower flame heights corresponding to younger, smaller and less aggregated particles.
The concept of 2D gated imaging for particle sizing in a laminar diffusion flame
Sat, 2013-05-18 02:00 — GuestAbstract
In this work, time-resolved laser-induced incandescence (TiRe LII) has been employed to measure primary particle diameters of soot in an atmospheric laminar ethylene diffusion flame. The generated data set complements existing data determined in one single location and takes advantage of the good spatial resolution of the ICCD detection. Time resolution is achieved by shifting the camera gate along the LII decay. One key input parameter for the analysis of time-resolved LII is the local flame temperature. This was determined on a grid throughout the flame by coherent anti-Stokes Raman scattering. The accurate temperature data, in combination with other published data from this flame, are well suited for soot model validation purposes while we showed feasibility of a shifted gate approach to deduce 2D particle sizes in the chosen standard flame.
Dual-wavelength diode-pumped laser operation of N p-cut and N g-cut Tm:KLu(WO4)2 crystals
Fri, 2013-05-17 02:00 — GuestAbstract
Simultaneous continuous-wave laser oscillation at two wavelengths has been observed and studied in a diode-pumped monoclinic N
p-cut Tm:KLu(WO4)2 for different transmission of the output coupler. The maximum output power reached 1.15 W with a slope efficiency of 20.4 % with respect to the absorbed power for polarization parallel to the N
m optical axis. In an analogous N
g-cut crystal, the dual-wavelength laser operation is accompanied by polarization switching with increasing pump power and the switching point depends on the output coupling.
Electrical properties of Sb-doped epitaxial SnO2 thin films prepared using excimer-laser-assisted metal–organic deposition
Fri, 2013-05-17 02:00 — GuestAbstract
Excimer-laser-assisted metal–organic deposition (ELAMOD) was used to prepare Sb-doped epitaxial (001) SnO2 thin films on (001) TiO2 substrates at room temperature. The effects of laser fluence, the number of shots with the laser, and Sb content on the electrical properties such as resistivity, carrier concentration, and carrier mobility of the films were investigated. The resistivity of the Sb-doped epitaxial (001) SnO2 thin film prepared using an ArF laser was lower than that of the film prepared using a KrF laser. The van der Pauw method was used to measure the resistivity, carrier concentration, and carrier mobility of the Sb-doped epitaxial (001) SnO2 thin films in order to determine the effect of Sb content on the electrical resistivity of the films. The lowest resistivity obtained for the Sb-doped epitaxial (001) SnO2 thin films prepared using ELAMOD with the ArF laser and 2 % Sb content was 2.5 × 10−3 Ω cm.
Remote sensing of atmospheric gases with optical correlation spectroscopy and lidar: first experimental results on water vapor profile measurements
Fri, 2013-05-17 02:00 — GuestAbstract
In this paper, the first experimental demonstration of the optical correlation spectroscopy lidar (OCS-lidar) is proposed. It is a new active remote sensing methodology to measure range-resolved atmospheric gas concentrations, based on broadband laser spectroscopy and light amplitude modulation. As a first step, a numerical study is performed for OCS-lidar measurements to optimize the accuracy of the range-resolved gas concentration measurement. Then, we demonstrate the ability of the OCS-lidar methodology to monitor the water vapor in the planetary boundary layer using the 4ν 720-nm absorption band. In addition to this first experimental proof, two different experimental configurations are proposed. The amplitude modulation, related to the optical correlation spectroscopy, is operated either at the emission with an active amplitude modulator before the backscattering process, or with passive optical filters on the laser backscattered light. For both configurations, range-resolved gas concentration measurements, achieved with a micro-pulse ground-based OCS-lidar, are presented.
