A. A. Ilyin

Femtosecond laser-induced breakdown spectroscopy

The presented review summarizes nearly two decades of studies on femtosecond laser-induced breakdown spectrometry (fs-LIBS). When an ultra-short (<1 ps) laser pulse is used for ablation, the physics of laser-induced plasma changes dramatically in comparison with ablation by pico or nanosecond pulses. A femtosecond laser pulse interacts only with the electron subsystem, while nanosecond pulses continuously interact with different thermodynamic states of material, starting from solid through liquid into plasma. The properties of ultra-short laser radiation, the timescale of fs-laser ablation and the radiative properties of fs plasma are briefly described. We consider the advantages of fs-LIBS, namely, low ablation thresholds, high-spatial resolution, and rapid analysis of samples, which require minimal invasion and allow high-efficiency transportation of laser radiation in filamentation mode for remote analysis. Moreover, we discussed possible limitations of the technique and different approaches to overcome such constraints while retaining the unique possibilities of fs-LIBS.

Supersonic regimes of plasma expansion during optical breakdown in air

Expansion regimes of plasma (fast ionization wave, laser supported radiation wave, and laser supported detonation wave) produced by laser–induced breakdown in air have been studied. To determine a pattern of plasma expansion a numerical investigation of propagation velocity dependence on laser radiation intensity was used. Calculation results agree with the experimental data and show that the most expected high-speed mechanism is fast ionization wave.

Absorption of laser radiation by femtosecond laser-induced plasma of air and its emission characteristics

The energy absorbed by femtosecond laser plasma has nonlinear dependence on incident laser energy. The threshold power for plasma formation is 5.2 GW. Emission of nitrogen molecule, nitrogen molecule ion, atomic oxygen (unresolved triplet O I 777 nm) and nitrogen (triplet N I 742.4, 744.3 and 746.8 nm) lines is detected. Molecular emission consists of second positive and firs negative systems of nitrogen. Time-resolved spectroscopy of plasmas shows short molecular line emission (up to 1 ns) and long atomic line emission (up to 150 ns).

Application of high-power Nd:YAG lasers for environmental monitoring

The laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence (LIF) methods were applied for element and pigment concentration detection of marine water and phytoplankton samples. The spectra of plasmas generated by focusing the first harmonic λ = 1064 nm of a pulsed Nd:YAG laser on the liquid surfaces and phytoplankton samples in atmospheric pressure are described. Some of the chemical elements detecting by LIBS in the marine water (Na, Ca, Mg) and phytoplankton (Mg, Ca, Na, Fe, Si, Al) were defined. The radiation of second harmonic λ = 1064 nm of a pulsed Nd:YAG laser was used for phytoplankton pigments and dissolved organic matter detection by LIF. The results of joint application of LIBS and LIF measurements for phytoplankton communities state investigations inside Okhotsk Sea are presented.

Influence of energy and repetition rate of the femtosecond laser pulses on the spectral and temporal characteristics of plasma in laser induced breakdown spectroscopy of aqueous solutions

Experimentally determined the influence of energy and pulse repetition rate of the femtosecond Ti: sapphire laser on the intensity of the spectral lines in the optical breakdown on the surface of an aqueous solution of CaCl2 and aqueous solutions of iron. The time dependence of the intensity of the continuous and line spectra of the plasma was obtained from the laser pulse energy. It is shown that by increasing the laser pulse repetition frequency from 20 Hz to 1000 Hz is observed up to 45% reduction of the emission line intensity Ca II (393.3 nm).

Influence of laser pulse energy on emission lines intensity in the femtosecond laser-induced breakdown spectroscopy of iron in aqua solution

The influence of pulse energy on the time evolution of the intensity of the continuum and emission lines of plasma generated on the surface of aqueous solutions of iron by focused radiation Ti: sapphire laser with a wavelength of 800 nm and pulse duration of 45 fs and a range of energy 3-7 mJ was investigated. The calibration curve for iron in water and 3-ó limit of detection of iron in water was obtained.

Determination of the limits of detection of the elements in aqueous solutions by femtosecond LIBS depending on the laser pulse repetition rate

Experimentally investigated limit of detection Mg, Al, Sr, Pb. depending on the pulse repetition rate femtosecond Ti: Sa laser optical breakdown in aqueous solutions on the surface of the investigated element in the laser pulse duration 60 fs, pulse energy of 1 mJ. The study was conducted for the laser pulse repetition rate of 50 Hz, 150 Hz, 250 Hz. To choose the optimal time delay of registration relative to the laser beam for the following chemicals Na, N, Fe, Ca, Mg, Al, Pb, Mn.

Development and creation of a remote-controlled underwater laser induced breakdown spectrometer for analysis of the chemical composition of sea water and bottom sediments

The developed underwater laser induced breakdown spectrometer consists of two units: 1- remotely operated vehicle (ROV) with the next main characteristics: work deep – up to 150 meters, maximum speed of immersion 1 m/s, maximum cruise velocity - 2 m/s and 2 – spectrometer unit (SU) consist of a DPSS Nd: YAG laser excitation source (double pulse with 50 mJ energy for each pulse at wavelength 1064 nm, pulse width 12 ns and pulse repetition rate 1-15 Hz, DF251, SOL Instruments), a spectrum recording system (Maya HR4000 or 2000 Pro spectrometer, Ocean Optics) and microcomputer. These two units are connected by Ethernet network and registered spectral data are automatically processed in a MATLAB platform.

Investigation of emission lines excitation efficiency in femtosecond plasma generated on the surface of chemical elements water solution

Time-resolved laser-induced breakdown spectroscopy (LIBS) has been used as an analytical chemistry technique suitable for water analysis. Optical breakdown on the surface of water solutions was generated by the femtosecond Ti:Sa laser with the following parameters: central wavelength - 800 nm, pulse width - 60 fs, pulse energy - 1 mJ, repetition rate 50 Hz, 166 Hz, 250 Hz, 500 Hz, 1000 Hz. The limit of detection (LOD) for Mg I (285.2 nm), Mn I (403 nm, 403.3 nm), Pb I (283.3 nm), Sr I (460.7 nm) was experimentally investigated depending on the laser pulse repetition rate by the time resolved femtosecond LIBS. The best LODs for Mg I - 0.058 mg/kg, Mn I - 0.025 mg/kg, Pb I - 0.711 mg/kg, Sr I - 4.69x10-3 mg/kg water solutions were obtained for 166 Hz repetition rate.

Anomalous broadening and shift of emission lines in filaments

The temporal evolution of width and shift of N I 746.8 and O I 777.4 nm lines is investigated in filament plasma produced by tightly focused femtosecond laser pulse (0.9 mJ, 48 fs). Nitrogen line shift is determined by joint action of electron impact shift and far-off resonance AC Stark effect. Intensive (I ~ 1010 W/cm2 ) electric field of ASE and postpulses result in possible LS coupling break for O I 3p 5P level and generation of Rabi sidebands. The blue-shifted main femtosecond pulse and Rabi sideband cause the stimulated emission of N21+ system.