GPI RAS Laser Beam Profile Influence on LIBS: Gaussian vs Multimode Beams Vasily N. Lednev, Sergey M. Pershin, Alexey F. Bunkin Prokhorov General Physics Institute Russian Academy of Sciences Moscow, Russia September 30, 2012 Luxor, Egypt

GPI RAS 2 Prokhorov General Physics Institute A.M. Prokhorov Nobel prize winner (1964) for maser-laser principles the lasers beginning open resonator build by A.M. Prokhorov and his Ph.D. student N.G. Basov for microwave amplification (1952) from left to right: C.H. Townes, A.M. Prokhorov, N.G. Basov General Physics Institute, RAS (Moscow, Russia)

GPI RAS 3 Outline Laser beam profiles used in LIBS Better laser beam for LIBS: Gaussian vs multimode Gaussian and multimode beams characterization Plasma properties comparison Analytical signals comparison for different laser beams Analytical capabilities comparison Conclusions

GPI RAS 4 Laser beam characteristics unknown beam profile laser pulse characteristics: 1. laser type 2. wavelength 3. pulse energy 4. pulse duration 5. double or single pulse mode 6. beam profile and beam quality* unknown fluence profile unknown ablation conditions *recommendations: R. Noll, Anal. Bioanal. Chem., (2006) D.W. Hahn and N. Omenetto, Appl. Spectrosc., 64, 335A-336A (2011) "Alice: Would you tell me, please, which way I ought to go from here? The Cheshire Cat: That depends a good deal on where you want to get to" Lewis Carroll, Alice in Wonderland

GPI RAS 5 Guessing is not an option If no description of beam profile is presented: 1. specification: not always available, same laser model can have different beams profiles 2. laser type: the same laser type can have multiple beam profiles example: excimer laser (XeCl Lambda-Physik) C. Chaleard, P. Mauchien, N. Andre, J. Uebbing, J. L. Lacour, C. Geersten, J. Anal. At. Spectrom., 12, 183–188 (1997) spatial filtering beam degradation due to different faults: laser cavity alignment, broken active media, lamp deterioration, water supply temperature variations etc. direct beam measurements (CMOS or CCD) laser ablation description !

GPI RAS 6 Laser beam profiles used for LIBS Multimode beamFlat-top (top-hat) beam laser beam shaping: 1. beam shapers Gaussian Flat-top Gaussian super-Gaussian 2. adaptive optics any beam almost any profile stable resonator with multimode lasing: solid state lasers (Nd:YAG), gas laser (CO 2 ) unstable resonator: excimer lasers unstable resonator: Nd:YAG, excimer, CO 2 stable resonator with passive Q-switching: Nd:YAG super-Gaussian beam (annular-shape) stable resonator with single mode lasing: gas laser (He-Ne, Ar, etc.), solid state lasers (Nd:YAG) camomile beam (Laguerre-Gaussian) Gaussian beam

GPI RAS 7 Motivation multimode Typical laser source for LIBS is a pulsed solid state Nd:YAG laser single mode (Gaussian, TEM 00 ) Laser spectroscopy: single mode lasing (Gaussian beam) is ultimate choice LIBS: choice is not straightforward! Applications: better design of portable (or low cost) LIBS systems based on single resonator is it worthy to use single mode lasing and to loose 90% of pulse energy (and ablated mass) in order to increase fluence in laser spot and improve repeatability of laser energy Gaussian Multimode energy low high energy stability high low beam quality high low laser spot smallest larger Study goals: 1.What is laser beam profile impact on the laser ablation and plasma properties? 2.What laser beam is better for LIBS: Gaussian or multimode? ?

GPI RAS 8 Experimental setup Experiment setup: 1. Nd:YAG laser ( = 1064 nm, 6 mJ/pulse < E < 80 mJ/pulse, = 10 ns) 2. output cavity mirror, 3. flash - lamp, 4. active element rod (d = 5.9 mm), 5. diaphragm (d = 1.6 mm), 6. Q-switch, 7. cavity rear mirror, 8. oscilloscope, 9. spectrograph with ICCD, 10. computer, 11. quartz optical fiber, 12. quartz collecting lens (F = 120 mm), 13. microphone, 14. CCD camera (for beam profile study), 15. mirror, 16. focusing lens (F = 110 mm), 17. sample Two optical schemes: a) with optical fiber detection; b) with side-view detection. Diaphragm position: Gaussian (TEM 00 ) – ON Multimode – OUT

GPI RAS 9 Laser output (near field) Gaussian beam (TEM 00 )Multimode beam

GPI RAS 10 Laser output (near field): summary Pulse energy: 6 mJ 1.1 % Pulse energy: 83 mJ 1.2 % Profile SD: peak SD 11 % mean SD 5 % Profile SD: peak SD 2 % mean SD 1.4 % Differences: energy - 14-fold, profile fluctuations - 5-fold

GPI RAS 11 Beam quality measurements Beam quality measurement procedure: 1. ISO 11146:2005(E), "Lasers and laser-related equipment - Test methods for laser beam widths, divergence angles and beam propagation ratios 2. A.E. Siegman, "How to (Maybe) Measure Laser Beam Quality," in DPSS (Diode Pumped Solid State) Lasers: Applications and Issues, M. Dowley, ed., Vol. 17 of OSA Trends in Optics and Photonics (Optical Society of America, 1998), paper MQ1 M 2 also called as beam quality factor is a quantitative measure of the quality of the laser beam and according to ISO standard is defined as beam parameter product divided by the latter being the beam parameter for a diffraction-limited Gaussian beam with the same wavelength. In other words, the half-angle beam divergence is where w 0 is the beam radius at the beam waist and is the wavelength. For a pure Gaussian TEM 00 beam M 2 equals 1. For real beams, M 2 will be greater than 1, and thus the minimum beam waist will be larger by the M 2 factor.

GPI RAS 12 Laser spot (far field) Gaussian beam (TEM 00 ) Multimode beam

GPI RAS 13 Laser spot (far field): summary Stability (RSD): 1.8 / 3 % mean Stability (RSD): 5 / 10 % peak mean Dimensions ( 1/e 2 amplitude): 120x120 m Dimensions (1/e 2 amplitude): 600x550 m Differences: energy - 14-fold, peak fluence - 2-fold, fluctuations - 3-fold

GPI RAS 14 Laser beams specification GaussianMultimode Laser output (near field): Energy, mJ/pulse 6 83 Energy reproducibility (RSD), % Laser beam profile dimensions, mm Gaussian, Multipeak 1.1 x x 4.4 Fluence reproducibility: average/highest value (RSD), % 1.4 / 3 5 / 14 Beam quality, M Laser spot (far field): Spot dimensions measured by CCD (1/e 2 amplitude), μm 110 x x 500 by single shot crater, μm 120 x x 510 Fluence: peak, J/cm CCD average, J/cm crater average, J/cm Fluence reproducibility: average/highest value (RSD), % 1.8 / / fold same 5-fold 40-fold 5-fold 2-fold 20% 70% 3-fold difference

GPI RAS 15 Crater study dimensions ( m): 85 x 81 x x 450 x 4 Crater profiles and corresponding cross-sections after 100 laser pulses for Gaussian (a) and multimode (b) beams Crater diameter for Gaussian beam was 2 times smaller than expected by ablation threshold fluence. Crater diameter for multimode beam was equal to the value expected by threshold fluence samples: low-alloy steel samples

GPI RAS 16 b) time integrated spectra analyte Cr II 283.6: I(G) : I(M) : I(MeG) = 3*10 4 : 1*10 6 : 1*10 3 matrix Fe II : I(G) : I(M) : I(MeG) = 2.8*10 6 :1*10 8 :0.8*10 5 a) gated spectra analyte Cr II 283.6: I(G) : I(M) : I(MeG) = 49 : 30*10 3 : 15 matrix Fe II : I(G) : I(M) : I(MeG) = 160 : 2*10 5 : 70 Plasma spectra plasma emission duration: Gaussian (6 mJ) - 14 s Multimode (83 mJ) – 50 s Multimode (6 mJ) – 6 s G - Gaussian (6 mJ) M - Multimode (83 mJ) MeG - Multimode (6 mJ)

GPI RAS 17 Plasma temperature and electron density Plasma temperatures by Fe I lines (360 – 370 nm) Electron density by Stark broadening of Fe I line Conclusion: plasma properties differ significantly for Gaussian and multimode beams

GPI RAS 18 Signals comparison Pulse-to-pulse study for single spot sampling strategy (a) Gaussian beam (6 mJ) (b) multimode beam (83 mJ) (c) multimode beam (6 mJ) unpredictable beam profile + unpredictable crater profile self-induced instability

GPI RAS 19 Calibration curves (single spot sampling) Cu I Cr II Gaussian - better precision Multimode - better sensitivity Gaussian (6 mJ) 25 ±3 5.1 Multimode (83 mJ) 14 ± Multimode (6 mJ) 90 ± LOD, ppm RSD, % Gaussian (6 mJ) 90 ±8 6.1 Multimode (83 mJ) 60 ± Multimode (6 mJ) 170 ± LOD, ppm RSD, %

GPI RAS 20 Calibration curve (scanning sampling) Gaussian beam is the only choice for single shot analysis (i.e. moving target) Cu I Cr II Gaussian (6 mJ) 36 ± Multimode (83 mJ) ? 19.1 Multimode (6 mJ) ? 16.5 LOD, ppm RSD, % Gaussian (6 mJ) 310 ± Multimode (83 mJ) 510 ± Multimode (6 mJ) 950 ± LOD, ppm RSD, %

GPI RAS 21 Analytical figures of merit: summary drilling sampling (stationary target) scanning sampling (movable target) drilling sampling Multimode: better sensitivity, poorer precision Gaussian: comparable sensitivity, better precision AnalyteBeam type* Concentration range, mass. % LOD, ppm R2R2 RSD Cr G M MeG – Si G M MeG – Cu G M MeG 0.02 – Cr G M MeG – Si G M MeG – Cu G M MeG 0.02 – G - Gaussian (6 mJ) M - Multimode (83 mJ) MeG - Multimode (6 mJ) scanning sampling Multimode: no correlation Gaussian: poor sensitivity, better precision

GPI RAS 22 Conclusions* Laser beam profile and beam quality is a crucial parameter for laser ablation, plasma properties and LIBS analysis Plasma temperature and electron density were higher for the plasma formed by Gaussian beam compared to the plasma formed multimode beam while pulse energy was 14-times smaller Multimode laser beam sampling results in poor reproducibility of the signals due to self-induced instability of laser ablation Better beam (Gaussian vs multimode) for analysis: drilling sampling: competition between precision (Gaussian) and sensitivity (multimode) scanning sampling: Gaussian beam is the ultimate choice * V. Lednev, S. M. Pershin, A. F. Bunkin, J. Anal. At. Spectrom., 25, 1745–1757 (2010)

GPI RAS 23 Thank you

GPI RAS 24 Supplementary: optoacoustic Optoacoustic signal as a function of ablated mass

GPI RAS 25 Supplementary: gating optimization Gating optimization gate 20 s delay 2 s gate 10 s delay 5 s ratio signal/(noise+bgnd) + detector dynamical range gate 5 s delay 0.5 s