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Lasing effects in a laser-induced plasma plume
Optics Communications ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Lasing effects in a laser-induced plasma plume Lev Nagli n, Michael Gaft Laser Distance Spectrometry Ltd., 11 Granit Street, Petach Tikva, Israel
art ic l e i nf o
a b s t r a c t
Article history: Received 1 April 2015 Received in revised form 4 June 2015 Accepted 6 June 2015
We have studied coherent emission from optically pumped preliminarily created laser induced plasma and demonstrate the possibility to create laser sources based on laser plasma as an active medium. The effect was studied in detail with Al plasma, and preliminary but promising results were also obtained with other atoms from the 13th and 14th groups of the periodic table. These lasers may be used as coherent light sources in a variety of optical applications. & 2015 Elsevier B.V. All rights reserved.
Keywords: Laser induced plasma Al plasma laser Optically pumped gaseous lasers
1. Introduction In the gaseous phase, under strong laser excitation, varieties of nonlinear optical effects are exhibited such as harmonics generation [1–3], superradiance [4], superfluorescence [5], and yoked superfluorescence [6]. Stimulated emission (SE) and lasing in metal vapors based on photodissociation of metal halides were extensively investigated in the 1980s [7]. This investigation was continued with studies of photodissociation of oxygen molecules with subsequent oxygen atoms lasing in a flame [8] and in ambient air [9–11]. These papers are closest to our research. Coherent emission from ambient air and efficient generation of laser-like beams was studied. Optically pumped alkali metal vapors lasers were also extensively studied in more than 40 years [12]. In present work, we demonstrate a new class of lasers with a laser-induced plasma (LIP) plume as the lasing medium created in ambient air. Our experimental example is Al plasma, but a lasing effect was found in other elements of the 13th group in the periodic table and also in elements of the 14th group.
was experimentally found) the plasma was pumped by an optical parametric oscillator (OPO) (E E1 mJ, tunable in the spectral range 250–270 nm, 4 ns duration). The OPO beam was directed to the plasma plume by a dichroic mirror that fully reflects the pumping UV light and transmits longer-wavelength emission light. The pumping beam was parallel to the sample surface and focused on the plasma plume at a 300 μm distance above the sample surface by a lens (f ¼25 cm) placed at distance L¼28 cm from plasma plume center. The estimated beam waist diameter along the plasma plume is about 600 μm. The sample was placed within an optical resonator consisting of two precisely aligned Al flat mirrors: a 100% reflectance mirror placed at the forward side (the opposite side of the plume from the OPO pump laser) and a 99% reflecting mirror placed behind dielectric mirror. In LIBS and non-lasing SE experiments, the forward mirror was blocked by a non-reflecting shutter, in which case the backward mirror was used simply as an attenuating filter. The plasma emission spectrum was measured by a spectrometer combined with a fast ICCD detector. Spectral and time resolutions of the system were 0.1 nm and 1 ns, respectively.
2. Experimental Fig. 1 shows the experimental setup. A plasma plume was created by a laser pulse (λ ¼ 1064 nm, E¼ 50 mJ, 7 ns duration, and beam waist diameter of about 600 μm) focused onan Al sample, normally incident to its surface. After a certain matrix-dependent delay time, needed for the formation of neutral atoms in the cooling plasma plume (the optimal value of four μs for Al plasma n
Corresponding author. E-mail address: [email protected] (L. Nagli).
3. Experimental results and discussions Fig. 2 shows a partial Grotrian diagram for aluminum atoms. Configurations and energies of levels are taken from [13,14]. Aluminum plasma, 4 μs after its creation, exhibited ordinary emission spectra well known in laser-induced breakdown spectroscopy (LIBS); see for example [13]. Pumping this plasma plume with 256.8 nm pulses (transitions 3p22P1/2-4d2D3/2) led to a strong, well-collimated beam at the 394.4 nm emission line (transitions 4s2S1/2-3p22P1/2) in both the forward and backward
http://dx.doi.org/10.1016/j.optcom.2015.06.012 0030-4018/& 2015 Elsevier B.V. All rights reserved.
Please cite this article as: L. Nagli, M. Gaft, Optics Communications (2015), http://dx.doi.org/10.1016/j.optcom.2015.06.012i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103
L. Nagli, M. Gaft / Optics Communications ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
Laser 1060 nm Al- UV enhanced 100% mirror Dielectric mirror 1% transmitting
Lenses F=20cm H=12cm;
Plasma Optical plume shutter
F=25cm L=28cm
Al mirror
Fiber OPO
Sample
Spectrometer +ICCD
100% reflecting Al mirror Delay generator Fig. 1. Experimental setup.
Configuration
E eV
(3s24d) 2D5/2; 2D3/2
4.83
(3s25s)2S1/2
266.0 nm
396.2nm
394.4 nm
(3s23p) 2P3/2
265.3 nm
257.5 nm
4.67
256.8 nm
(3s24s)2S1/2
3.14
0.014
(3s23p) 2P1/2
0.0
-3
2.0x10
-3
1.0x10
Plasma Emission
Al I 396.2nm
Al I 394.4nm
Fig. 2. Partial Grotrian diagram for aluminum atoms.
0.0 2
Stimulated Emission
1.2x10
ex 257.5nm ex 256.8nm
1
8.0x10
1
spectra measured in the backward direction. Fig. 3(a) shows the traditional plasma emission (LIBS). Fig. 3(b) shows the plasma emissions pumped by 257.5 nm and by 256.8 nm. Fig. 3(c) is the excited plasma emissions with the sample placed inside an optical resonator, All results were measured with the pumping pulse applied 4 μs after plasma plume creation and followed by a 0 ns signal acquisition delay relatively to OPO pulse maxima and then a signal integration time of 1 μs. Spectra were measured under the same geometrical conditions and therefore the intensity units, while arbitrary, may be used to compare intensities between the three spectra. With optical pumping of Al LIP, emission lines are up to five orders of magnitude stronger than corresponding ordinary LIBS emission lines and up to six orders of magnitude stronger with optical pumping within an optical resonator. The LIBS plasma emission lines' full widths at half maximum (FWHM) (Fig. 2A) are about 0.25 nm and the FWHMs of the optically pumped emission lines were measured to be about 0.13 nm. (In fact the FWHM of the optically pumped line may be much narrower because measurement was primarily governed by our optical system's spectral resolution of 0.1 nm). The strong collimated emissions lasted only during the pumping OPO pulse, about 4 ns, that are significantly shorter than estimated spontaneous emission decay time (about 10 ns). Time dependences of the 396.2 nm line emission intensity and of the 257.5 nm OPO pumping pulse intensity were measured using the ICCD and a kinetic series method. Method is based on measuring series of spectrum where each subsequent spectrum is measured with 1 ns delay relatively to previous measurement. In all these measurements series gate width was 1 ns. Results are shown in Fig. 4. Measuring the pumped emission with an acquisition delay longer than about 5 ns with respect to the pumping pulse maxima leads to the total disappearance of the effect, and only the ordinary weak LIBS remains. The same strong and collimated emission lines at 394.4 and 396.2 nm in an Al plasma plume were observed under pumping by 265.3 (transitions 3p22P1/2-5s 2S1/2) and 266.0 nm (transitions 3p22P3/2-5s 2S1/2). The optically pumped Al plasma emission lines at 394.4 and 396.2 were strongly polarized, with their electric vectors parallel to that of the pumping light, while plasma emission (ordinary LIBS) or laser induced fluorescence (LIF) measured 10 ns after the pumping pulse were not polarized at all.
4.0x10
OPO 257.5 nm norm 396.2nm line
0.0
1.0 3
Lasing
1.5x10
ex 257.5nm ex 256.8nm
3
1.0x10
2
5.0x10
0.0 394.0
394.5
395.0
395.5
396.0
396.5
Wavelength (nm) Fig. 3. Al plasma emission spectra measured from the backward direction: (a) is the ordinary LIBS spectrum (dashed curve); (b) shows the emission spectra under 256.8 nm excitation (solid curve) and under 257.5 nm excitation (dotted curve); and (c) shows the emission spectra under excitation by 256.8 nm and 257.5 nm measured with the sample in an optical resonator. The measurements are timed with a 4 μs delay after plasma creation, followed by an emission acquisition delay of 0 ns and then a signal integration time of 1 μs.
Normalized intensities
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
0.8
0.6
0.4 FWHM =4ns
0.2
0.0 directions. Pumping the plasma plume with 257.5 nm pulses (transitions 3p2 2P3/2-4d2D5/2) led to generation of such a beam at the 396.2 nm emission line (transitions 4s2S1/2-3p2 2P3/2), also directed forward and backward, that was stronger than the 394.4 nm emission. Fig. 3 shows examples of Al plasma emission
7
8
9
10
11
12
13
14
15
16
17
Delay time (ns) Fig. 4. Time dependences of the 396.2 nm emission intensity (square data points, solid curve) and 257.5 nm OPO pumping pulse intensity (circular data points, dashed curve).
Please cite this article as: L. Nagli, M. Gaft, Optics Communications (2015), http://dx.doi.org/10.1016/j.optcom.2015.06.012i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
L. Nagli, M. Gaft / Optics Communications ∎ (∎∎∎∎) ∎∎∎–∎∎∎
0
10
-1
10
Intensity (J)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
-2
10
Slope =2.1 -3
10
-4
10
1E-3
0.01
0.1
1
Pamping energy (mJ) Fig. 5. 396.2 nm laser output energy dependence on pumping 266.0 nm pulse energy in an Al plasma plume. Dashed lines show the experimental deviations of the measurements.
The experimental results clearly demonstrate that Al LIP pumped at 256.8 and 257.5 or at 265.3 and 266 nm emits forward and backward collimated SE. Furthermore, placing the plasma plume inside of an optical resonator leads to an additional manifold increase of the backward beam intensity. This increase demonstrates that by placing a LIP plasma plume in an optical resonator, we created a laser whose active medium was optically pumped plasma. The measured laser beam pulse energy at 396.2 nm was about 0.8 μJ under pumping by a 266.0 nm pulse with energy of 0.1 mJ, thus in our experimental conditions Al plasma plume generation efficiency was about 8 10 3. Plasma plume lasing line energy dependence on pumping pulse intensity, shown in Fig. 5, demonstrates characteristic threshold behavior with a 1 μJ threshold (dependence was measured using OPHIR pyroelectric sensor PE9-C). With a pumping energy of 0.1 mJ the experimentally estimated plasma laser beam divergence is about 1.6 mrad, that is comparable to the diffraction-limited lasing divergence (0.6 mrad) from the cross section of the pumping beam's waist for 266 nm OPO pumping laser beam. The transition scheme shown in Fig. 2 and very low splitting of the ground energy state indicates that the Al LIP laser has a threelevel generation scheme. In our experimental conditions estimated Al atoms concentration in plasma plume is about 1016 cm 3 [15]. This laser involves high optical transition probabilities (about 108 s 1) and possess high optical gain α., that lead to strong backward SE when no resonator is used.
4. Conclusion We demonstrate a new class of optically pumped lasers in which a laser-induced plasma plume created in ambient air is the lasing medium. Large optical gain of these media lead to extremely low threshold of lasing (about 1 μJ) and demonstrates that an Al LIP
3
67 can operate as a stimulated emission medium and no optical 68 cavity is needed for its detection. 69 Lasers based on LIP may be used as coherent light sources in a 70 variety of optical applications. Intense collimated backward SE in a 71 LIP plume may strongly enhance selectivity and sensitivity of the 72 inductively coupled plasma optical emission spectrometry (ICP73 OES), and LIBS, used for elemental analyses in different substances. 74 It may be particularly important in remote sensing specifically of 75 minerals and explosives. 76 We started investigating SE and lasing effects in plasma from other elements in the 13th (In, Tl) and 14th (Ge, Sn, and Pb) groups Q3 77 78 of the periodic table, using appropriate excitation wavelengths. We 79 received promising preliminary results that will be published 80 soon. Some of the laser transitions in these elements possess four81 level laser systems for which plasma population inversion may be 82 created more easily than in Al plasma. 83 LIP lasing effects have to be checked on the different isotopes 84 and in case of success, it will be possible to separate them. 85 In present work, we used longitudinal laser pumping scheme 86 and we intend to check also possibility to use transverse pumping 87 scheme that may be more effective for stronger lasers creation. 88 89 90 References 91 92 [1] M. López-Arias, M. Oujja, M. Sanz, R.A. Ganeev, G.S. Boltaev, N. Kh Satlikov, R. I. Tugushev, T. Usmanov, M. Castillejo, Low-order harmonic generation in 93 metal ablation plasmas in nanosecond and picoseconds laser regimes, J. Appl. 94 Phys. 111 (2012) 04311-1. 95 [2] M. López-Arias, M. Oujja, M. Sanz, R. de Nalda, R.A. Ganeev, M. Castillejo, Generation of low-order harmonics in laser ablation plasmas, Mol. Phys. 110 96 (2012) 1651–1657. 97 [3] A. Fedotov, A. Naumov, V. Silin, S. Uryupin, A. Zheltikov, Third-harmonic 98 generation in a laser-pre-excited gas: the role of excited-state neutrals, Phys. Lett. A 271 (2000) 407–412. 99 [4] M. Scullyand, A. Svidzinsky, The super of superradiance, Science 325 (2009) 100 1510–1511. 101 [5] R. Bonifacio, L.A. Lugiato, Cooperative radiation processes in two-level systems: superfluorescence, Phys. Rev. A 12 (1975) 597–598. 102 [6] J. Brownell, X. Lu, S. Hartmann, Yoked superfluorescence, Phys. Rev. Lett. 75 103 (1995) 3265–3268. 104 [7] D. Ehrlich, R. Osgood, Metal-atom resonance-line lasers, IEEE J. Quantum Electron. 16 (1980) 257–267. 105 [8] M. Aldén, J.E.M. Goldsmith, U. Westblom, Two-photon-excited stimulated 106 emission from atomic oxygen in flames and cold gases, Opt. Lett. 14 (1989) 107 305–307. 108 [9] A. Dogariu, J. Michael, M. Scully, R. Miles, High-gain backward lasing in air, Science 331 (2011) 442–444. 109 [10] A. Traverso, R. Sanchez-Gonzalez, L. Yuan, K. Wang, D. Voronine, A. Zheltikov, 110 Y. Rostovtsev, V. Sautenkov, A. Sokolov, S. North, M. Scully, Coherence brigh111 tened laser source for atmospheric remote sensing, PNAS 119 (2012) 15185–15190. 112 [11] L. Yuan, B. Hokr, A. Traverso, D. Voronine, Y. Rostovtsev, A. Sokolov, M. Scully, 113 Theoretical analysis of the coherence-brightened laser in air, Phys. Rev. A 87 114 (2013) 023826-1–023826-9. [12] B. Zhdanov, R. Knize, Review of alkali laser research and development, Opt. 115 Eng. 52 (2013) 021010-1–021010-8. 116 [13] NIST Atomic Spectra Database Lines. 〈http://physics.nist.gov/PhysRefData/ASD/ 117 lines_form.html〉. [14] P. Smith, C. Heise, J. Esmond, R. Kurucz. 〈http://www.pmp.uni-hannover.de/ 118 cgi-bin/ssi/test/kurucz/sekur.html〉. 119 [15] L. Nagli, M. Gaft, I. Gornushkin, Fraunhofer-type absorption lines in double120 pulse laser-induced plasma, Appl. Opt. 51 (2012) B201–B212. 121 122 123 124 125 126 127 128 129 130 131 132
Please cite this article as: L. Nagli, M. Gaft, Optics Communications (2015), http://dx.doi.org/10.1016/j.optcom.2015.06.012i
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Lasing effects in a laser-induced plasma plume Lev Nagli n, Michael Gaft Laser Distance Spectrometry Ltd., 11 Granit Street, Petach Tikva, Israel
art ic l e i nf o
a b s t r a c t
Article history: Received 1 April 2015 Received in revised form 4 June 2015 Accepted 6 June 2015
We have studied coherent emission from optically pumped preliminarily created laser induced plasma and demonstrate the possibility to create laser sources based on laser plasma as an active medium. The effect was studied in detail with Al plasma, and preliminary but promising results were also obtained with other atoms from the 13th and 14th groups of the periodic table. These lasers may be used as coherent light sources in a variety of optical applications. & 2015 Elsevier B.V. All rights reserved.
Keywords: Laser induced plasma Al plasma laser Optically pumped gaseous lasers
1. Introduction In the gaseous phase, under strong laser excitation, varieties of nonlinear optical effects are exhibited such as harmonics generation [1–3], superradiance [4], superfluorescence [5], and yoked superfluorescence [6]. Stimulated emission (SE) and lasing in metal vapors based on photodissociation of metal halides were extensively investigated in the 1980s [7]. This investigation was continued with studies of photodissociation of oxygen molecules with subsequent oxygen atoms lasing in a flame [8] and in ambient air [9–11]. These papers are closest to our research. Coherent emission from ambient air and efficient generation of laser-like beams was studied. Optically pumped alkali metal vapors lasers were also extensively studied in more than 40 years [12]. In present work, we demonstrate a new class of lasers with a laser-induced plasma (LIP) plume as the lasing medium created in ambient air. Our experimental example is Al plasma, but a lasing effect was found in other elements of the 13th group in the periodic table and also in elements of the 14th group.
was experimentally found) the plasma was pumped by an optical parametric oscillator (OPO) (E E1 mJ, tunable in the spectral range 250–270 nm, 4 ns duration). The OPO beam was directed to the plasma plume by a dichroic mirror that fully reflects the pumping UV light and transmits longer-wavelength emission light. The pumping beam was parallel to the sample surface and focused on the plasma plume at a 300 μm distance above the sample surface by a lens (f ¼25 cm) placed at distance L¼28 cm from plasma plume center. The estimated beam waist diameter along the plasma plume is about 600 μm. The sample was placed within an optical resonator consisting of two precisely aligned Al flat mirrors: a 100% reflectance mirror placed at the forward side (the opposite side of the plume from the OPO pump laser) and a 99% reflecting mirror placed behind dielectric mirror. In LIBS and non-lasing SE experiments, the forward mirror was blocked by a non-reflecting shutter, in which case the backward mirror was used simply as an attenuating filter. The plasma emission spectrum was measured by a spectrometer combined with a fast ICCD detector. Spectral and time resolutions of the system were 0.1 nm and 1 ns, respectively.
2. Experimental Fig. 1 shows the experimental setup. A plasma plume was created by a laser pulse (λ ¼ 1064 nm, E¼ 50 mJ, 7 ns duration, and beam waist diameter of about 600 μm) focused onan Al sample, normally incident to its surface. After a certain matrix-dependent delay time, needed for the formation of neutral atoms in the cooling plasma plume (the optimal value of four μs for Al plasma n
Corresponding author. E-mail address: [email protected] (L. Nagli).
3. Experimental results and discussions Fig. 2 shows a partial Grotrian diagram for aluminum atoms. Configurations and energies of levels are taken from [13,14]. Aluminum plasma, 4 μs after its creation, exhibited ordinary emission spectra well known in laser-induced breakdown spectroscopy (LIBS); see for example [13]. Pumping this plasma plume with 256.8 nm pulses (transitions 3p22P1/2-4d2D3/2) led to a strong, well-collimated beam at the 394.4 nm emission line (transitions 4s2S1/2-3p22P1/2) in both the forward and backward
http://dx.doi.org/10.1016/j.optcom.2015.06.012 0030-4018/& 2015 Elsevier B.V. All rights reserved.
Please cite this article as: L. Nagli, M. Gaft, Optics Communications (2015), http://dx.doi.org/10.1016/j.optcom.2015.06.012i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103
L. Nagli, M. Gaft / Optics Communications ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
Laser 1060 nm Al- UV enhanced 100% mirror Dielectric mirror 1% transmitting
Lenses F=20cm H=12cm;
Plasma Optical plume shutter
F=25cm L=28cm
Al mirror
Fiber OPO
Sample
Spectrometer +ICCD
100% reflecting Al mirror Delay generator Fig. 1. Experimental setup.
Configuration
E eV
(3s24d) 2D5/2; 2D3/2
4.83
(3s25s)2S1/2
266.0 nm
396.2nm
394.4 nm
(3s23p) 2P3/2
265.3 nm
257.5 nm
4.67
256.8 nm
(3s24s)2S1/2
3.14
0.014
(3s23p) 2P1/2
0.0
-3
2.0x10
-3
1.0x10
Plasma Emission
Al I 396.2nm
Al I 394.4nm
Fig. 2. Partial Grotrian diagram for aluminum atoms.
0.0 2
Stimulated Emission
1.2x10
ex 257.5nm ex 256.8nm
1
8.0x10
1
spectra measured in the backward direction. Fig. 3(a) shows the traditional plasma emission (LIBS). Fig. 3(b) shows the plasma emissions pumped by 257.5 nm and by 256.8 nm. Fig. 3(c) is the excited plasma emissions with the sample placed inside an optical resonator, All results were measured with the pumping pulse applied 4 μs after plasma plume creation and followed by a 0 ns signal acquisition delay relatively to OPO pulse maxima and then a signal integration time of 1 μs. Spectra were measured under the same geometrical conditions and therefore the intensity units, while arbitrary, may be used to compare intensities between the three spectra. With optical pumping of Al LIP, emission lines are up to five orders of magnitude stronger than corresponding ordinary LIBS emission lines and up to six orders of magnitude stronger with optical pumping within an optical resonator. The LIBS plasma emission lines' full widths at half maximum (FWHM) (Fig. 2A) are about 0.25 nm and the FWHMs of the optically pumped emission lines were measured to be about 0.13 nm. (In fact the FWHM of the optically pumped line may be much narrower because measurement was primarily governed by our optical system's spectral resolution of 0.1 nm). The strong collimated emissions lasted only during the pumping OPO pulse, about 4 ns, that are significantly shorter than estimated spontaneous emission decay time (about 10 ns). Time dependences of the 396.2 nm line emission intensity and of the 257.5 nm OPO pumping pulse intensity were measured using the ICCD and a kinetic series method. Method is based on measuring series of spectrum where each subsequent spectrum is measured with 1 ns delay relatively to previous measurement. In all these measurements series gate width was 1 ns. Results are shown in Fig. 4. Measuring the pumped emission with an acquisition delay longer than about 5 ns with respect to the pumping pulse maxima leads to the total disappearance of the effect, and only the ordinary weak LIBS remains. The same strong and collimated emission lines at 394.4 and 396.2 nm in an Al plasma plume were observed under pumping by 265.3 (transitions 3p22P1/2-5s 2S1/2) and 266.0 nm (transitions 3p22P3/2-5s 2S1/2). The optically pumped Al plasma emission lines at 394.4 and 396.2 were strongly polarized, with their electric vectors parallel to that of the pumping light, while plasma emission (ordinary LIBS) or laser induced fluorescence (LIF) measured 10 ns after the pumping pulse were not polarized at all.
4.0x10
OPO 257.5 nm norm 396.2nm line
0.0
1.0 3
Lasing
1.5x10
ex 257.5nm ex 256.8nm
3
1.0x10
2
5.0x10
0.0 394.0
394.5
395.0
395.5
396.0
396.5
Wavelength (nm) Fig. 3. Al plasma emission spectra measured from the backward direction: (a) is the ordinary LIBS spectrum (dashed curve); (b) shows the emission spectra under 256.8 nm excitation (solid curve) and under 257.5 nm excitation (dotted curve); and (c) shows the emission spectra under excitation by 256.8 nm and 257.5 nm measured with the sample in an optical resonator. The measurements are timed with a 4 μs delay after plasma creation, followed by an emission acquisition delay of 0 ns and then a signal integration time of 1 μs.
Normalized intensities
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
0.8
0.6
0.4 FWHM =4ns
0.2
0.0 directions. Pumping the plasma plume with 257.5 nm pulses (transitions 3p2 2P3/2-4d2D5/2) led to generation of such a beam at the 396.2 nm emission line (transitions 4s2S1/2-3p2 2P3/2), also directed forward and backward, that was stronger than the 394.4 nm emission. Fig. 3 shows examples of Al plasma emission
7
8
9
10
11
12
13
14
15
16
17
Delay time (ns) Fig. 4. Time dependences of the 396.2 nm emission intensity (square data points, solid curve) and 257.5 nm OPO pumping pulse intensity (circular data points, dashed curve).
Please cite this article as: L. Nagli, M. Gaft, Optics Communications (2015), http://dx.doi.org/10.1016/j.optcom.2015.06.012i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
L. Nagli, M. Gaft / Optics Communications ∎ (∎∎∎∎) ∎∎∎–∎∎∎
0
10
-1
10
Intensity (J)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
-2
10
Slope =2.1 -3
10
-4
10
1E-3
0.01
0.1
1
Pamping energy (mJ) Fig. 5. 396.2 nm laser output energy dependence on pumping 266.0 nm pulse energy in an Al plasma plume. Dashed lines show the experimental deviations of the measurements.
The experimental results clearly demonstrate that Al LIP pumped at 256.8 and 257.5 or at 265.3 and 266 nm emits forward and backward collimated SE. Furthermore, placing the plasma plume inside of an optical resonator leads to an additional manifold increase of the backward beam intensity. This increase demonstrates that by placing a LIP plasma plume in an optical resonator, we created a laser whose active medium was optically pumped plasma. The measured laser beam pulse energy at 396.2 nm was about 0.8 μJ under pumping by a 266.0 nm pulse with energy of 0.1 mJ, thus in our experimental conditions Al plasma plume generation efficiency was about 8 10 3. Plasma plume lasing line energy dependence on pumping pulse intensity, shown in Fig. 5, demonstrates characteristic threshold behavior with a 1 μJ threshold (dependence was measured using OPHIR pyroelectric sensor PE9-C). With a pumping energy of 0.1 mJ the experimentally estimated plasma laser beam divergence is about 1.6 mrad, that is comparable to the diffraction-limited lasing divergence (0.6 mrad) from the cross section of the pumping beam's waist for 266 nm OPO pumping laser beam. The transition scheme shown in Fig. 2 and very low splitting of the ground energy state indicates that the Al LIP laser has a threelevel generation scheme. In our experimental conditions estimated Al atoms concentration in plasma plume is about 1016 cm 3 [15]. This laser involves high optical transition probabilities (about 108 s 1) and possess high optical gain α., that lead to strong backward SE when no resonator is used.
4. Conclusion We demonstrate a new class of optically pumped lasers in which a laser-induced plasma plume created in ambient air is the lasing medium. Large optical gain of these media lead to extremely low threshold of lasing (about 1 μJ) and demonstrates that an Al LIP
3
67 can operate as a stimulated emission medium and no optical 68 cavity is needed for its detection. 69 Lasers based on LIP may be used as coherent light sources in a 70 variety of optical applications. Intense collimated backward SE in a 71 LIP plume may strongly enhance selectivity and sensitivity of the 72 inductively coupled plasma optical emission spectrometry (ICP73 OES), and LIBS, used for elemental analyses in different substances. 74 It may be particularly important in remote sensing specifically of 75 minerals and explosives. 76 We started investigating SE and lasing effects in plasma from other elements in the 13th (In, Tl) and 14th (Ge, Sn, and Pb) groups Q3 77 78 of the periodic table, using appropriate excitation wavelengths. We 79 received promising preliminary results that will be published 80 soon. Some of the laser transitions in these elements possess four81 level laser systems for which plasma population inversion may be 82 created more easily than in Al plasma. 83 LIP lasing effects have to be checked on the different isotopes 84 and in case of success, it will be possible to separate them. 85 In present work, we used longitudinal laser pumping scheme 86 and we intend to check also possibility to use transverse pumping 87 scheme that may be more effective for stronger lasers creation. 88 89 90 References 91 92 [1] M. López-Arias, M. Oujja, M. Sanz, R.A. Ganeev, G.S. Boltaev, N. Kh Satlikov, R. I. Tugushev, T. Usmanov, M. Castillejo, Low-order harmonic generation in 93 metal ablation plasmas in nanosecond and picoseconds laser regimes, J. Appl. 94 Phys. 111 (2012) 04311-1. 95 [2] M. López-Arias, M. Oujja, M. Sanz, R. de Nalda, R.A. Ganeev, M. Castillejo, Generation of low-order harmonics in laser ablation plasmas, Mol. Phys. 110 96 (2012) 1651–1657. 97 [3] A. Fedotov, A. Naumov, V. Silin, S. Uryupin, A. Zheltikov, Third-harmonic 98 generation in a laser-pre-excited gas: the role of excited-state neutrals, Phys. Lett. A 271 (2000) 407–412. 99 [4] M. Scullyand, A. Svidzinsky, The super of superradiance, Science 325 (2009) 100 1510–1511. 101 [5] R. Bonifacio, L.A. Lugiato, Cooperative radiation processes in two-level systems: superfluorescence, Phys. Rev. A 12 (1975) 597–598. 102 [6] J. Brownell, X. Lu, S. Hartmann, Yoked superfluorescence, Phys. Rev. Lett. 75 103 (1995) 3265–3268. 104 [7] D. Ehrlich, R. Osgood, Metal-atom resonance-line lasers, IEEE J. Quantum Electron. 16 (1980) 257–267. 105 [8] M. Aldén, J.E.M. Goldsmith, U. Westblom, Two-photon-excited stimulated 106 emission from atomic oxygen in flames and cold gases, Opt. Lett. 14 (1989) 107 305–307. 108 [9] A. Dogariu, J. Michael, M. Scully, R. Miles, High-gain backward lasing in air, Science 331 (2011) 442–444. 109 [10] A. Traverso, R. Sanchez-Gonzalez, L. Yuan, K. Wang, D. Voronine, A. Zheltikov, 110 Y. Rostovtsev, V. Sautenkov, A. Sokolov, S. North, M. Scully, Coherence brigh111 tened laser source for atmospheric remote sensing, PNAS 119 (2012) 15185–15190. 112 [11] L. Yuan, B. Hokr, A. Traverso, D. Voronine, Y. Rostovtsev, A. Sokolov, M. Scully, 113 Theoretical analysis of the coherence-brightened laser in air, Phys. Rev. A 87 114 (2013) 023826-1–023826-9. [12] B. Zhdanov, R. Knize, Review of alkali laser research and development, Opt. 115 Eng. 52 (2013) 021010-1–021010-8. 116 [13] NIST Atomic Spectra Database Lines. 〈http://physics.nist.gov/PhysRefData/ASD/ 117 lines_form.html〉. [14] P. Smith, C. Heise, J. Esmond, R. Kurucz. 〈http://www.pmp.uni-hannover.de/ 118 cgi-bin/ssi/test/kurucz/sekur.html〉. 119 [15] L. Nagli, M. Gaft, I. Gornushkin, Fraunhofer-type absorption lines in double120 pulse laser-induced plasma, Appl. Opt. 51 (2012) B201–B212. 121 122 123 124 125 126 127 128 129 130 131 132
Please cite this article as: L. Nagli, M. Gaft, Optics Communications (2015), http://dx.doi.org/10.1016/j.optcom.2015.06.012i