Laser-induced Optical Breakdown Research Articles

Laser-induced optical breakdown spectroscopy of polymer materials based on evaluation of molecular emission bands

Laser-induced breakdown spectroscopy (LIBS) for composition analysis of polymer materials results in optical spectra containing atomic and ionic emission lines as well as molecular emission bands. In the present work, the molecular bands are analyzed to obtain spectroscopic information about the plasma state in an effort to quantify the content of different elements in the polymers. Polyethylene (PE) and a rubber material from tire production are investigated employing 157nmF2 laser and 532nm Nd:YAG laser ablation in nitrogen and argon gas background or in air. The optical detection reaches from ultraviolet (UV) over the visible (VIS) to the near infrared (NIR) spectral range. In the UV/VIS range, intense molecular emissions, C2 Swan and CN violet bands, are measured with an Echelle spectrometer equipped with an intensified CCD camera. The measured molecular emission spectra can be fitted by vibrational-rotational transitions by open access programs and data sets with good agreement between measured and fitted spectra. The fits allow determining vibrational-rotational temperatures. A comparison to electronic temperatures Te derived earlier from atomic carbon vacuum-UV (VUV) emission lines show differences, which can be related to different locations of the atomic and molecular species in the expanding plasma plume. In the NIR spectral region, we also observe the CN red bands with a conventional CDD Czerny Turner spectrometer. The emission of the three strong atomic sulfur lines between 920 and 925nm is overlapped by these bands. Fitting of the CN red bands allows a separation of both spectral contributions. This makes a quantitative evaluation of sulfur contents in the start material in the order of 1wt% feasible.

Influence of absorption induced thermal initiation pathway on irradiance threshold for laser induced breakdown.

We investigated the influence of thermal initiation pathway on the irradiance threshold for laser induced breakdown in transparent, absorbing and scattering phantoms. We observed a transition from laser-induced optical breakdown to laser-induced thermal breakdown as the absorption coefficient of the medium is increased. We found that the irradiance threshold after correction for the path length dependent absorption and scattering losses in the medium is lower due to the thermal pathway for the generation of seed electrons compared to the laser-induced optical breakdown. Furthermore, irradiance threshold gradually decreases with the increase in the absorption properties of the medium. Creating breakdown with lower irradiance threshold that is specific at the target chromophore can provide intrinsic target selectivity and improve safety and efficacy of skin treatment methods that use laser induced breakdown.

Effect of multiphoton ionization on performance of crystalline lens.

This Letter presents a model for propagation of a laser pulse in a human crystalline lens. The model contains a transverse beam diffraction effect, laser-induced optical breakdown for the creation of plasma via a multiphoton ionization process, and the gradient index (GRIN) structure. Plasma introduces the nonlinearity in the crystalline lens which affects the propagation of the beam. The multiphoton ionization process generates plasma that changes the refractive index and hence leads to the defocusing of the laser beam. The Letter also points out the relevance of the present investigation to cavitation bubble formation for restoring the elasticity of the eyes.

Errata

An omission regretfully occurred in the caption for the first figure of the paper entitled, “Laser-Induced Plasma Spectroscopy of Hydrogen Balmer Series in Laboratory Air”, by Lauren D. Swafford and Christian G. Parigger [ Applied Spectroscopy, vol. 68(9): 1016-1020. doi: 10.1366/13-07388]. It should instead read as the following: Fig. 1. Schematic representation of the apparatus used during this experiment, which is a typical LIBS arrangement: (1) beam splitter, (2) mirrors, (3) focusing lens, (4) plasma, (5) focusing lenses. [Reprinted with permission from C.G. Parigger, A.C Woods, M.J. Witte, L.D. Swafford, D.M. Surmick. “Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown”. Journal of Visualized Experiments. 2014. 84: e51250. doi:10.3791/51250.]

Hydrogen Alpha Self-Absorption Effects in Laser-Induced Air Plasma

Time-resolved spectroscopy measurements of the hydrogen alpha Balmer series line following laser-induced optical breakdown in laboratory air are designed to investigate in detail the determination of electron density from Stark-broadened spectral line shapes. Comparisons of results obtained from Hβ and Hγ lines indicate higher electron density inferred from Hα early in the plasma decay, suggesting self-absorption occurs. However, detailed comparisons for time delays of 300 and 400 ns after optical breakdown reveal the minute extent of self-absorption in air breakdown experiments from (i) differences of electron density determined from the N+ lines and the Hα line, and/or from (ii) differences in recorded data sets with/without the mirror for the various time delays in the experiments.

Laser-Induced Plasma Spectroscopy of Hydrogen Balmer Series in Laboratory Air

Stark-broadened emission profiles for the hydrogen alpha and beta Balmer series lines in plasma are measured to characterize electron density and temperature. Plasma is generated using a typical laser-induced breakdown spectroscopy (LIBS) arrangement that employs a focused Q-switched neodymium-doped yttrium aluminum garnet (Nd : YAG) laser, operating at the fundamental wavelength of 1064 nm. The temporal evolution of the hydrogen Balmer series lines is explored using LIBS. Spectra from the plasma are measured following laser-induced optical breakdown in laboratory air. The electron density is primarily inferred from the Stark-broadened experimental data collected at various time delays. Due to the presence of nitrogen and oxygen in air, the hydrogen alpha and beta lines become clearly discernible from background radiation for time delays of 0.4 and 1.4 μs, respectively.

Asymmetric hydrogen beta electron density diagnostics of laser-induced plasma

The hydrogen beta line has been widely used in determining plasma parameters such as electron density. In conjunction with other Balmer series lines, electron temperature can be inferred. The asymmetric appearance of the hydrogen beta line, due to quadrupole interactions, can be utilized as well for the determination of electron density. Laser-induced optical breakdown is generated in laboratory air, and particularly for electron densities in the range of 0.3 to 1.0×1017cm−3 the use of the asymmetry parameter is elaborated for electron density diagnostics. Also included are results of analysis of the hydrogen beta profiles for which the asymmetry indicates an electron density on the order of 2.0×1018cm−3, which is significantly higher than 6.3 to 6.8×1017cm−3 maximum that was measured previously from the Stark-broadened hydrogen beta width following laser-induced optical breakdown.

Effects of cavitation bubble interaction with temporally separated fs-laser pulses.

We present a time-resolved photographic analysis of the pulse-to-pulse interaction. In particular, we studied the influence of the cavitation bubble induced by a fs-pulse on the optical focusing of the consecutive pulse and its cavitation bubble dynamics in dependence on temporal pulse separation in water. As a first result, by decreasing the temporal separation of laser pulses, there is a diminishment of the laser-induced optical breakdown (LIOB) efficiency in terms of energy conversion, caused by disturbed focusing into persisting gas bubbles at the focal volume. A LIOB at the focal spot is finally suppressed by impinging the expanding or collapsing cavitation bubble of the preceding pulse. These results could be additionally confirmed in porcine gelatin solution with various concentrations. Hence, the interaction between the laser and transparent ophthalmic tissue may be accompanied by a raised central laser energy transmission, which could be observed in case of a temporal pulse overlap. In conclusion, our experimental results are of particular importance for the optimization of the prospective ophthalmic surgical process with future generation fs-lasers.

Carbon Swan Spectra Measurements following Breakdown of Nitro Compound Explosive Simulants

Our measurements of micro-plasma following laser-induced optical breakdown of nitro compound explosive simulants, here 3-nitrobenzoic acid, show well-developed molecular spectra during the first several hundreds of nanoseconds. Analysis of recorded carbon spectra is accomplished using accurate line strengths for the diatomic molecular Swan system. Presence of hydrogen-beta allows us to infer electron density in the plasma evolution. Computational challenges include accounting for background variation and appropriate modeling of hydrogen embedded in molecular spectra. Recorded and computed spectra agree nicely for time delays on the order of 1.6 μs from optical breakdown when using a single temperature for local thermodynamic equilibrium plasma.

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C<sub>2</sub>, CN, and TiO Spectra Following Laser-induced Optical Breakdown

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement is used. Here we operate a Nd:YAG laser at a frequency of 10 Hz at the fundamental wavelength of 1,064 nm. The 14 nsec pulses with anenergy of 190 mJ/pulse are focused to a 50 µm spot size to generate a plasma from optical breakdown or laser ablation in air. The microplasma is imaged onto the entrance slit of a 0.6 m spectrometer, and spectra are recorded using an 1,800 grooves/mm grating an intensified linear diode array and optical multichannel analyzer (OMA) or an ICCD. Of interest are Stark-broadened atomic lines of the hydrogen Balmer series to infer electron density. We also elaborate on temperature measurements from diatomic emission spectra of aluminum monoxide (AlO), carbon (C2), cyanogen (CN), and titanium monoxide (TiO). The experimental procedures include wavelength and sensitivity calibrations. Analysis of the recorded molecular spectra is accomplished by the fitting of data with tabulated line strengths. Furthermore, Monte-Carlo type simulations are performed to estimate the error margins. Time-resolved measurements are essential for the transient plasma commonly encountered in LIBS.

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C<sub>2</sub>, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C<sub>2</sub>, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Acoustic radiation force on gas bubbles and soft elastic scatterers in tissue

Acoustic radiation force on a scatterer in tissue depends on the compressibility and shear modulus of both the tissue and the scatterer. This force is related to the monopole and dipole scattering coefficients. The finite shear modulus of the tissue decreases the radiation force in comparison with the force exerted on the same scatterer surrounded by liquid. Shear moduli for soft tissue range from several kilopascals (breast, liver) to tens of kilopascals and higher for cornea, cartilage, and cancerous tissue. As reported previously, the radiation force on a bubble in tissue having 100 kPa shear modulus is 50% less than if the bubble is in water. This difference decreases for scatterers with finite shear moduli, examples of which are reported here. Additionally, displacement of a scatterer due to radiation force is inversely proportional to the shear modulus of the tissue, which permits measurement of the latter. Experiments demonstrating this technique are reviewed. In these experiments, the radiation force is applied to a gas microbubble produced by laser-induced optical breakdown, while displacement of the microbubble is measured by high-frequency ultrasound as a function of time. Results are reported for tissue-mimicking phantoms and animal crystalline lenses in vitro.

水下光击穿的能量分布研究

水下光击穿的能量分布研究

Atomic Hydrogen and Molecular Carbon Emissions in Laser-Induced Breakdown Spectroscopy

This article addresses hydrogen Balmer series measurements following laser-induced optical breakdown. Electron density on the order of 1 × 1025 m−3 can be inferred using Hα Stark width and shift for plasma generated in 1 to 1.3 × 105 Pa, gaseous hydrogen. The Hβ line can be utilized for electron density up to 7 × 1023 m−3. Laser ablation of aluminium reveals limits of application of the Balmer series. Electron excitation temperature is inferred utilizing Boltzmann plot techniques that include Hα, Hβ and Hγ atomic lines. Hβ and Hγ lines show presence of molecular carbon in a 2.7 and 6.5 × 105 Pa, expanding methane flow. Occurrence of superposition spectra in the plasma decay due to recombination or due to onset of chemical reactions necessitates consideration of both atomic and molecular emissions following laser-induced optical breakdown.

BiotecVisions 2012, November

BiotecVisions 2012, November

Efficacy of minimally invasive nonthermal laser-induced optical breakdown technology for skin rejuvenation

We demonstrate the efficacy of a novel minimally invasive nonthermal skin rejuvenation technique for wrinkle and fine-line reduction based on laser-induced optical breakdown. The optical breakdown caused by tightly focused near-infrared laser pulses creates a grid of intradermal lesions without affecting the epidermis, leading to skin rejuvenation. The pilot in vivo efficacy test performed on five subjects successfully demonstrates wrinkle and fine‐line reduction, and improvement of other skin features without pain or any other unpleasant sensations or any social downtime associated with the treatment. The efficacy is evaluated objectively and subjectively by assessing the improvement of wrinkles and/or fine lines or skin texture after the treatment. The treatment is safe without side effects or social downtime, and all test subjects reported that the treatment is “perceptible but not painful.” Four out of the five subjects who participated in this pilot study were assessed to have “minor” to “significant” improvements of wrinkles and fine lines by the professional panels. The results of this clinical study are expected to bring a paradigm shift in the present laser- and light-based skin rejuvenation methods by introducing a safe treatment procedure without damaging the epidermis, with no or little social downtime and with an efficacy that might be comparable to ablative techniques.

Comparison of retina damage thresholds simulating the femtosecond-laser in situ keratomileusis (fs-LASIK) process with two laser systems in the CW- and fs-regime

The femtosecond-laser in situ keratomileusis procedure affords the opportunity to correct ametropia by cutting transparent corneal tissue with ultra-short laser pulses. Thereby the tissue cut is generated by a laser-induced optical breakdown in the cornea with ultra-short laser pulses in the near-infrared range. Compared to standard procedures such as photorefractive keratectomy and laser in-situ keratomileusis with the excimer laser, where the risk potential for the eye is low due to the complete absorption of ultraviolet irradiation from corneal tissue, only a certain amount of the pulse energy is deposited in the cornea during the fs-LASIK process. The remaining energy propagates through the eye and interacts with the retina and the strong absorbing tissue layers behind. The objective of the presented study was to determine and compare the retina damage thresholds during the fs-LASIK process simulated with two various laser systems in the CW- and fs-regime.

BiotecVisions 2012, February

BiotecVisions 2012, February

Computation of AlO B 2Σ + → X 2Σ + emission spectra

Abstract Application of molecular spectroscopy to analytical chemistry usually requires accurate description of the particular transition of interest. In this communication we describe the creation of a list of spectral lines. Following the introduction and definition of the line strength, we present a recipe for computation of diatomic-line-strengths, including the Hönl–London factor and electric dipole line strength for each spectral line. The diatomic eigenfunction is discussed including Hund’s case basis functions. In our data tables we prefer use of Hund’s case (a) basis, and we apply the usual Born–Oppenheimer approximation for the electronic-vibrational strengths. This allows us to generate the table of line strengths that we frequently apply for spectroscopic temperature determination. Using these line-strength tables, we present theoretical AlO emission spectra for the B–X system of AlO. These emission spectra are computed for temperatures of 3000 and 6000 K and for typical spectroscopic resolution used in laser-induced optical breakdown studies.

Synthesis of high-pressure phases of silica by laser-induced optical breakdown

We report on synthesis of compressed silica phase formed inside a α-quartz host crystal using powerful shock-wave and thermal transients generated via local optical breakdown induced by low-energy, tightly focused femtosecond laser pulses. Structural characterization of irradiated areas by the X-ray diffraction technique reveals signatures of a new high-pressure phase of silica, which is theoretically expected to form at a pressure of 120 GPa.