High-power femtosecond filaments—laser-light beams capable of kilometer-long propagation—attract interest of nonlinear-optics community due to their numerous applications in remote sensing, lightning protection, virtual antennas, and waveguiding. Specific arrangements of filaments, into waveguides or hyperbolic metamaterials, allow for efficient control and guiding of electromagnetic radiation, radar-beam manipulation, and resolution enhancement. These applications require spatially uniform distribution of densely packed filaments.
In order to address this challenge, we investigate the dynamic properties of large rectangular filament arrays propagating in air depending on four parameters: the phase difference between the neighboring beams, the size of the array, separation between the beams, and excitation power. We demonstrate that, as a result of the mutual interaction between the filaments, the arrays where the nearest neighbor beams are out-of-phase are more robust than the arrays with all the beams in phase.
Our analysis of the array stability reveals that there exist certain trade-offs between the stability of a single filament and the stability of the entire array. We show that in the design of the experiment, the input parameters have to be chosen in such a way that they ensure a sufficiently high filling fraction, but caution has to be used in order not to compromise the overall array stability.
In addition, we show the possibility of filament formation by combining multiple beams with energies below the filamentation threshold. This approach offers additional control over filament formation and allows one to avoid the surface damage of external optics used for filamentation.
Utilizing the transparency of silicon at 2 μm, we are able to ablate the backside of 500-μm thick
silicon wafers without causing any damage to the front surface using a novel nanosecond
Tm:fiber laser system. We report on our high energy/high peak power nanosecond Tm:fiber
laser and provide an initial description of the effects of laser parameters such as pulse duration
and energy density on the ablation, and compare thresholds for front and backside machining.
The ability to selectively machine the backside of silicon wafers without disturbing the front
surface may lead to new processing techniques for advanced manufacturing in solar cell and
microelectronics industries.
Optical trapping of single biological cells has become an established technique for controlling and studying
fundamental behavior of single cells with their environment without having "many-body" interference. The development
of such an instrument for optical diagnostics (including Raman and fluorescence for molecular diagnostics) via laser
spectroscopy with either the "trapping" beam or secondary beams is still in progress. This paper shows the development
of modular multi-spectral imaging optical tweezers combining Raman and Fluorescence diagnostics of biological cells.
We report the performance of an actively Q-switched Tm fiber laser system. The laser was stabilized to sub-nanometer
spectral width using each of two feedback elements: a blazed reflection grating and a volume Bragg grating. Maximum
pulse energy using the reflection grating was 325 μJ pulses at 1992 nm (< 200 pm width) with a 125 ns duration at a 20
kHz repetition rate. Maximum pulse energy using the volume Bragg grating was 225 μJ pulses at 2052 nm (<200 pm
width) with a 200 ns duration also at 20 kHz. We also report the laser's performance as an ablation source for LIBS
experiments on copper.
This study makes a comparison of LIBS emission from molecular species in plasmas produced from organic
residues on a non-metallic substrate by both a 5 ns Nd:YAG laser (1064 nm) and a 40 fs Ti:Sapphire laser (800 nm)
in air and argon atmospheres. The organic samples analyzed had varying amounts of carbon, nitrogen, hydrogen,
and oxygen in their molecular structure. The characterization was based on the atomic carbon, hydrogen, nitrogen,
and oxygen lines as well as the diatomic species CN (B2Σ+ - X2Σ+) and the C2 (d3Πg - a3Πu). Principal Component
Analysis (PCA) was used to identify similarities of the organic analyte via the emission spectra. The corresponding
Receiver Operating Characteristics (ROC) curves show the limitations of the PCA model for the nanosecond regime
in air.
Laser Induced Breakdown Spectroscopy (LIBS) by self-channeled femtosecond pulses is characterized for detection
of energetic materials. Different polymers are spin coated on silicon wafers to provide a thin organic layer with
controllable thickness ranging from 500 nm to 1 μm. Spectral analysis of atomic and molecular carbon emission
shows CN molecular signal from samples that do not contain nitrogen. This can be explained by possible molecular
recombination between native atomic carbon and atmospheric nitrogen. As a consequence, caution must be
exercised when using spectral signatures based on CN emission for explosive detection by filament-induced LIBS.
As an alternative to focusing nanosecond pulses for stand-off LIBS detection of energetic materials, we use
self-channeled femtosecond pulses from a Ti:Sapphire laser to produce filaments at 12 meters and create a plasma on
copper, graphite and polyisobutylene film. We show the possibilities of this Laser-Induced Breakdown Spectroscopy
configuration for thin organic sample detection on a surface at a distance.
We demonstrate in this paper that laser ablation allows efficient analysis of organic and biological materials. Such analysis is based on laser-induced breakdown spectroscopy (LIBS) which consists in the detection of the optical emission from the plasma induced by a high intensity laser pulse focused on the sample surface. The optimization of the ablation regime in terms of laser parameters (pulse duration, wavelength, fluence) is important to generate a plasma suitable for the analysis. We first present the results of a study of laser ablation of organic samples with different laser parameters using time-resolved shadowgraph. We correlate the early stage expansion of the plasma to its optical emission properties, which allows us to choose suitable laser parameters for an efficient analysis of organic or biological samples by LIBS. As an illustration of the analytical ability of LIBS for biological materials, we show that the emission from CN molecules can be used to distinguish between biological and inorganic samples. Native CN molecular fragment directly ablated from a biological sample are identified using time-resolved LIBS. Those due to recombination with nitrogen contained in atmospheric air can be distinguished with their specific time evolution behavior.
Laser-Induced Breakdown Spectroscopy (LIBS) has been used since 40 years on typical samples such as metals, alloys,
rocks. Detection of organic hazards or analysis of biological compounds under atmospheric pressure with LIBS
represents a new challenge. For this purpose, we need better understandings of the physico-chemical properties of the
plasma in atmosphere and their influences on the LIBS signal.
As a model sample of organic materials, Nylon 6-6 has been studied under nanosecond ablation at different
wavelengths (1064 nm and 266 nm) and energies (from 1 to 5 mJ) in order to observe the influence of these parameters.
Shadowgraph technique is used to image the plasma at its early stage of expansion (0 to 40 ns). Time-resolved LIBS
signal is recorded for longer times (50 ns to 5 μs).
In the infrared regime, the expansion of the plume is faster along the laser axis, perpendicular to the sample
surface. On the contrary, for UV ablation, the expansion of the plume is quite isotropic. We can also observe different
regimes of expansion due to Laser-Supported Detonation Waves (LSDW) above 3 mJ in the UV regime.
In particular, these observations provide us ideas to understand the kinetics of the CN emission in the LIBS
signal. In the IR regime, a formation of CN due to carbon present in the sample and nitrogen in the air via the
reaction 2C + N2 → 2CN can be observed. In the UV regime, the direct ablation of CN bonds is clearly seen but other
effects like screening and recombination due to LSDW have also been observed.
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