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Temporal behavior of air lasing by molecular nitrogen ions
Optics Communications 456 (2020) 124573
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Temporal behavior of air lasing by molecular nitrogen ions N.G. Ivanov, I.A. Zyatikov β, V.F. Losev, V.E. Prokopβev Institute of High Current Electronics, Siberian Branch, Russian Academy of Sciences, 2/3 Akademichesky Ave., Tomsk, 634055, Russia
ARTICLE Keywords: Laser plasma Lasing Femtosecond duration Spectrum Molecular ions
INFO
ABSTRACT The experimental results of studying the temporal and spectral characteristics of air lasing of a first negative system of molecular nitrogen ions in laser plasma are presented here. Pumping was carried out by femtosecond laser pulse with a wavelength of 950 nm. By using streak camera, the pulse duration of lasing at π = 428 nm (1.15 ps, FWHM) was measured. It was close to transform-limited (1 ps). It is shown that there is a delay (5.6 ps) of lasing pulse maximum with respect to the pumping. Lasing has higher directionality and shorter duration compared with spontaneous emission.
1. Introduction For the first time lasing in filament plasma was obtained in 2011 on the first negative system B2 π΄ + π’ (v β² ) β π 2 π΄ + π (v β²β² ) of a molecular nitrogen ion emitting at wavelengths of 391.4 nm (transition between vibrational levels with quantum numbers v β² = 0 and v β²β² = 0) and 427.8 nm (transition π£β² = 0 β π£β²β² = 1) in a forward direction [1]. Pumping was carried out by intensive ultrashort laser pulse at midIR wavelengths (π = 1.2 β 2.9 ΞΌm). For the occurrence of lasing at the above wavelengths, an additional radiation seed was used at the third or fifth pump harmonics, whose wavelength coincided with the corresponding transition of molecular nitrogen ion. Later, lasing on these transitions was observed during pumping by radiation with different wavelengths, namely 800 nm [2β7], 400 nm [5], 950 nm [8,9], 1500 nm [10]. In this case lasing occurred both in the presence of an external seed at lasing wavelengths [2,3,5] and without it (selfseeding) [4β6]. In the latter case it is believed, that supercontinuum radiation acts as an external seed. This supercontinuum appears in laser plasma simultaneously with the emission of molecular nitrogen ions. A second harmonic radiation with a central wavelength of 400 nm was used as an external seed in [7]. It was shown in [11] that the intensity of lasing lines at wavelengths of 391 and 428 nm depends on the presence or absence of chirp in the pump pulse. In addition, in [12] it was shown that the polarization of pump radiation affects the amplification degree of N2 + radiation in the laser plasma. Despite the large number of works devoted to the study of properties and conditions of lasing emergence, the nature of its appearance remains unknown. Nowadays, there are several interpretations of optical amplification in laser plasma [12β15]. In one of them [12] it is believed that the appearance of the B2 π΄ + π’ state occurs as a result of electron recollision with the parent ion. However, at the same time, it is difficult to explain the presence of the emission maximum delay relative to the
moment of exposure. In another interpretation [13] it is assumed that after multiphoton ionization the N2 + ion goes into the B2 π΄ + π’ excited state from the π 2 π΄ + π ground state due to the absorption of three pump photons. However, there is no sufficient experimental confirmation of this interpretation. In the third interpretation [14] it is believed, that population inversion between the B2 π΄ + π’ and π 2 π΄ + π states arises due to the effective population of the π΄2 π±π’ state from the π 2 π΄ + π state when the ion interacts with pump photons. However, in this case, there should be A2 π±π’ β π 2 π΄ + π radiation, which is not observed in practice. There are other interpretations of the gain realization at the B2 π΄ + π’ β π 2 π΄ + π transition but they are difficult to prove. In order to understand the nature of lasing occurrence, it is important to fully know its characteristics and lasing conditions. The most common methods for measuring the lasing temporal profile with subpicosecond resolution, such as pump-probe or cross-correlation technology, do not allow the shape of amplified radiation pulse to be correctly measured. In pump-probe method, the amplified radiation with short duration passes through the laser plasma at different moments of time [15]. The total picture of the gain in time reflects the temporal behavior of only population inversion at the B2 π΄ + π’ β π 2 π΄ + π transition. In the cross-correlation method the signal of sum frequency is recorded when the pump beam and lasing are added in a nonlinear crystal [12,16]. In this case it is well known that the process of sum frequency lasing is nonlinear. As a result, the time profile of sum frequency will differ from the lasing pulse shape. Characteristic times are units and tens of picoseconds, measured by these methods. The actual temporal shape of the radiation pulse can only be measured using a streak camera with a high temporal resolution. The maximum time resolution was demonstrated in [17] while recording the emission of nitrogen ions, where the streak camera with time resolution of 30 ps was used. However, this resolution is not sufficient to correctly measure the duration of the lasing pulse.
β Corresponding author. E-mail address: [email protected] (I.A. Zyatikov).
https://doi.org/10.1016/j.optcom.2019.124573 Received 17 July 2019; Received in revised form 19 August 2019; Accepted 12 September 2019 Available online 15 September 2019 0030-4018/Β© 2019 Elsevier B.V. All rights reserved.
N.G. Ivanov, I.A. Zyatikov, V.F. Losev et al.
Optics Communications 456 (2020) 124573
Fig. 2. Photographs of plasma emission in the absence (a) and presence (b) of astigmatism in a focused beam. The pumping radiation propagates from right to left.
Fig. 1. Experimental scheme. M1 and M2 are pivoting mirrors, L1 and L2 are positive lenses with πΉ = 15 cm.
This paper presents the results of studies of temporal and spectral lasing characteristics by N2 + ions using the streak camera with subpicosecond resolution without an external seed on the corresponding ion transition. 2. Equipment and experimental techniques In order to form laser plasma, the radiation generated by the solid-state start complex ββStart-480ββ was used. This complex is part of the multiterawatt laser system THL-100 [18]. The laser complex includes Ti:Sapphire master oscillator with continuous pump laser, grating stretcher, regenerative and two multipass amplifiers, and grating compressor. This complex formed the transform-limited radiation at a central wavelength of 950 nm and pulse duration of 60 fs (FWHM). The laser beam had energy up to 15 mJ, 1.5 cm diameter (1/e2 intensity level), and Gaussian intensity distribution. The experimental scheme is shown in Fig. 1. The laser plasma was formed in ambient air when the radiation was focused by L1 lens with focal length πΉ = 15 cm. The lens was slightly tilted to provide a certain astigmatism on the wave front of the beam. In some experiments, a small part of radiation was transformed into the second harmonic (SH) in KDP crystal. SH radiation served as a reference point in time and to determine the instrument function of the streak camera. The spectral composition of plasma emission was measured both in the direction of pump radiation propagation and across it. The streak camera (Hamamatsu, Universal Streak Camera C10910) combined with a spectrometer (Acton SpectraPro SP-2300) was used for measuring the spectral and temporal characteristics. This camera had two time sweeps: fast (minimum sweep is 100 ps) and slow (minimum sweep is 1 ns). The maximum temporal resolution of streak camera was 0.644 ps in a single-shot mode. The temporal resolution for 300 pulses increased to 2 ps in accumulation pulses mode due to the presence of jitter in the response of the streak camera. To reduce it, the camera was started by a signal from a pin-diode, installed near the spectrometer. In spectrometer there was one grating with 1200 lines/mm with resolution of 0.23 nm. To compensate the delay time of the streak camera sweep block, the laser radiation was delayed and only then it was focused to create laser plasma. The lasing spectrum was recorded by Acton SpectraPro SP-2300 spectrometer. The spontaneous emission (SE) spectrum was recorded by Shamrock SR-500i-D1 spectrometer with resolution of 0.07 nm and iStar 334T Series (Andorβ’) Camera with temporal resolution of 3 ns. For measurements close to instrument function of the device, the following formula was used to calculate the resulting pulse duration and spectral width: β (1) πΆ = π΄2 β π΅ 2 ,
Fig. 3. Normalized spectra of SE and lasing.
3. Results and discussion As mentioned above, some astigmatism was set to the wave front of the focused beam. In this case, the length of laser plasma increased in the region of the focal waist and the intensity of its emission decreased. Lasing on the molecular nitrogen ions was observed in the direction of pump propagation at a certain tilt of the lens. Fig. 2 depicts the photographs of plasma emission in the waist area in the presence (a) and absence (b) of aberration. The longitudinal size of plasma emission is 3 mm and its diameter is 0.3 mm. With astigmatism introduced by the tilt of the lens, the presence of lasing can be explained. Indeed, it was shown in [19] that there were no molecular lines in laser plasma emission in the event of tight focusing, and the lines of atoms and their ions dominated. With an increasing focal lens, on the contrary, the atomic lines in the spectrum disappeared, and only the lines of molecular nitrogen and its ion remained. The explanation of this dependence is as follows. During short focusing, plasma concentration reaches extremely high values of about 1018 cmβ3 . As a result, the quenching rate of molecular nitrogen ions becomes so high that they do not have time to emit. The creation of astigmatism in a focused beam reduces the electron concentration in plasma and increases the length of the active medium. In experiments, lasing at different wavelengths of molecular nitrogen ions was observed: 391.4 (0-0), 427.8 (0-1), 358.2 (1-0), 388.4 (1-1) and 470.9 (0-2) nm. In this work the emission characteristics at the B2 π΄ + π’ β π 2 π΄ + π (0-1) transition were studied. Lasing was observed at the distance of several meters from plasma and had the form of spot. SE was not observed at 0.5 m distance. Thus it can be concluded, that the lasing had significantly greater power and higher directionality compared with SE. Fig. 3 shows the measured spectra of SE and lasing at a wavelength of 427.8 nm. Half-width spectra of lasing and SE are 0.35 nm. Taking into account the resolution of the spectrometers (according to formula (1)), the half-width of lasing spectrum is 0.26 nm and 0.34 nm for SE spectrum. During time-resolved emission measurements lasing was recorded in a single-shot mode and SE was recorded in accumulation mode across the laser plasma. In the latter case, the pulse selection mode was used to reduce the temporal spread. Fig. 4 depicts the combined in time radiation pulses of SE, lasing, and SH. In order to measure correctly,
where C is the resulting pulse duration or spectral width, A is measured pulse duration or spectral width, and B is the resolution of the device. 2
N.G. Ivanov, I.A. Zyatikov, V.F. Losev et al.
Optics Communications 456 (2020) 124573
lasing speaks in favor of the fact that it originates from the seeding of supercontinuum. 4. Conclusion Thus, in the present study the lasing pulse duration at the B2 π΄ + π’ (0) β π 2 π΄ + π (1) transition of molecular nitrogen ions was measured. It was 1.15 ps (FWHM). This pulse duration is close to transform-limited (1 ps), according to the measurements of spectrum width (0.26 nm). It is shown that there is a 5.6 ps delay of the lasing pulse maximum relative to the pumping. This delay is probably due to the fact that the lasing emission is produced by a narrow-line transition. The lasing on molecular nitrogen ions occurs only under astigmatism conditions, at which a reduced concentration of laser plasma is realized. Based on the form of lasing recorded in the near field, the seeding for its appearance is the emission of supercontinuum arising in laser plasma. Fig. 4. Normalized time-resolved emission of SH, lasing, and SE. The inset shows the spatial distribution of lasing in the near field.
Acknowledgment The reported study was funded by Russian Foundation for Basic Research (RFBR) (19-48-700016, 18-08-00383).
the device was calibrated by registration of the radiation of the second and third harmonics, as well as white light [20]. The corrections found in this way were taken into account during measurements and results processing. SH pulse reflects the measured resolution of the device (0.8 ps), the beginning of growth of its intensity (zero) corresponds to the moment of pump exposure. The delay of the lasing pulse (5.6 ps) with respect to the pumping is probably due to the fact, that the emission is produced by a narrow-line transition. According to the uncertainty principle, the duration of the lasing pulse is inversely proportional to the linewidth. The much longer duration of the lasing pulse, appearing after the pumping, is perceived as a delayed lasing pulse. Similar delay was observed in [3]. The decrease of SE intensity after 12 ps is due to the quenching of B2 π΄ + π’ state by electrons [21]. The presence of additional maxima on the lasing pulse, separated by 4 ps from each other, is most likely due to the half-period of ion rotation, which has also been noted in other works [14,21,22]. Taking into account the measured resolution of the device (0.8 ps) and formula (1), lasing pulse duration is 1.15 ps. According to the uncertainty principle, pulse duration is 1 ps for 0.26 nm spectrum width. Thus, our measurements of spectral linewidth and pulse duration showed that the duration of lasing is close to transform-limited. These resulting pulse duration and spectral width are consistent with other studies [5]. The inset of Fig. 4 depicts the spatial distribution of lasing emission with π = 427.8 nm in the near field. It can be observed that the radiation propagates in the form of hollow cone. It is well known that supercontinuum propagates in the form of a cone after laser plasma during short focusing of the pumping beam. Therefore, this type of
References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
3
J. Yao, B. Zeng, H. Xu, et al., Phys. Rev. A. 84 (2011) 051802. J. Ni, W. Chu, C. Jing, et al., Opt. Express 21 (2013) 8746. J. Yao, G. Li, C. Jing, et al., New J. Phys. 15 (2013) 023046. Y. Liu, Y. Brelet, G. Point, et al., Opt. Express 21 (2013) 22791. T.J. Wang, J. Ju, J.F. Daigle, et al., Laser Phys. Lett. 10 (2013) 125401. T.J. Wang, J.F. Daigle, J.Ju, et al., Phys. Rev. A. 88 (2013) 053429. H. Li, H. Zhang, Y. Su, et al., J. Opt. 19 (2017) 124006. V.E. Prokopiev, N.G. Ivanov, D.A. Krivonosenko, V.F. Losev, in: Conference The interaction of highly concentrated flows of energy materials in advanced technology and medicine (2013). V.E. Prokopev, N.G. Ivanov, D.A. Krivonosenko, V.F. Losev, Russ. Phys. J. 56 (2013) 1274. A. Azarm, P. Corkum, P. Polynkin, Conference on Lasers and Electro-Optics, Optical Society of America, 2016, p. JTh4B.9. C. Jing, H. Xie, G. Li, et al., Laser Phys. Lett. 12 (2015) 015301. Y. Liu, P. Ding, G. Lambert, et al., Phys. Rev. Lett. 115 (2015) 133203. S.L. Chin, H. Xu, Y. Cheng, et al., Chin. Opt. Lett. 11 (2013) 013201. J. Yao, W. Chu, Z. Liu, et al., Appl. Phys. B 124 (2018) 73. H.L. Xu, A. Azarm, J. Bernhardt, et al., Chem. Phys. 360 (2009) 171. X. Zhong, Z. Miao, L. Zhang, et al., Phys. Rev. A. 96 (2017) 043422. M. Lei, C. Wu, Q. Liang, et al., J. Phys. B: At. Mol. Opt. Phys. 50 (2017) 145101. S.V. Alekseev, A.I. Aristov, N.G. Ivanov, et al., Las. Part. Beams 31 (2013) 17. N.G. Ivanov, V.F. Losev, V.E. Prokopβev, et al., Opt. Commun. 431 (2019) 120. N.G. Ivanov, V.F. Losev, V.E. Prokopβev, K.A. Sitnik, Opt. Commun. 387 (2017) 322. J. Yao, G. Li, C. Jing, et al., New J. Phys. 15 (2013) 023046. X. Zhong, Z. Miao, L. Zhang, et al., Phys. Rev. A 96 (2017) 043422.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Temporal behavior of air lasing by molecular nitrogen ions N.G. Ivanov, I.A. Zyatikov β, V.F. Losev, V.E. Prokopβev Institute of High Current Electronics, Siberian Branch, Russian Academy of Sciences, 2/3 Akademichesky Ave., Tomsk, 634055, Russia
ARTICLE Keywords: Laser plasma Lasing Femtosecond duration Spectrum Molecular ions
INFO
ABSTRACT The experimental results of studying the temporal and spectral characteristics of air lasing of a first negative system of molecular nitrogen ions in laser plasma are presented here. Pumping was carried out by femtosecond laser pulse with a wavelength of 950 nm. By using streak camera, the pulse duration of lasing at π = 428 nm (1.15 ps, FWHM) was measured. It was close to transform-limited (1 ps). It is shown that there is a delay (5.6 ps) of lasing pulse maximum with respect to the pumping. Lasing has higher directionality and shorter duration compared with spontaneous emission.
1. Introduction For the first time lasing in filament plasma was obtained in 2011 on the first negative system B2 π΄ + π’ (v β² ) β π 2 π΄ + π (v β²β² ) of a molecular nitrogen ion emitting at wavelengths of 391.4 nm (transition between vibrational levels with quantum numbers v β² = 0 and v β²β² = 0) and 427.8 nm (transition π£β² = 0 β π£β²β² = 1) in a forward direction [1]. Pumping was carried out by intensive ultrashort laser pulse at midIR wavelengths (π = 1.2 β 2.9 ΞΌm). For the occurrence of lasing at the above wavelengths, an additional radiation seed was used at the third or fifth pump harmonics, whose wavelength coincided with the corresponding transition of molecular nitrogen ion. Later, lasing on these transitions was observed during pumping by radiation with different wavelengths, namely 800 nm [2β7], 400 nm [5], 950 nm [8,9], 1500 nm [10]. In this case lasing occurred both in the presence of an external seed at lasing wavelengths [2,3,5] and without it (selfseeding) [4β6]. In the latter case it is believed, that supercontinuum radiation acts as an external seed. This supercontinuum appears in laser plasma simultaneously with the emission of molecular nitrogen ions. A second harmonic radiation with a central wavelength of 400 nm was used as an external seed in [7]. It was shown in [11] that the intensity of lasing lines at wavelengths of 391 and 428 nm depends on the presence or absence of chirp in the pump pulse. In addition, in [12] it was shown that the polarization of pump radiation affects the amplification degree of N2 + radiation in the laser plasma. Despite the large number of works devoted to the study of properties and conditions of lasing emergence, the nature of its appearance remains unknown. Nowadays, there are several interpretations of optical amplification in laser plasma [12β15]. In one of them [12] it is believed that the appearance of the B2 π΄ + π’ state occurs as a result of electron recollision with the parent ion. However, at the same time, it is difficult to explain the presence of the emission maximum delay relative to the
moment of exposure. In another interpretation [13] it is assumed that after multiphoton ionization the N2 + ion goes into the B2 π΄ + π’ excited state from the π 2 π΄ + π ground state due to the absorption of three pump photons. However, there is no sufficient experimental confirmation of this interpretation. In the third interpretation [14] it is believed, that population inversion between the B2 π΄ + π’ and π 2 π΄ + π states arises due to the effective population of the π΄2 π±π’ state from the π 2 π΄ + π state when the ion interacts with pump photons. However, in this case, there should be A2 π±π’ β π 2 π΄ + π radiation, which is not observed in practice. There are other interpretations of the gain realization at the B2 π΄ + π’ β π 2 π΄ + π transition but they are difficult to prove. In order to understand the nature of lasing occurrence, it is important to fully know its characteristics and lasing conditions. The most common methods for measuring the lasing temporal profile with subpicosecond resolution, such as pump-probe or cross-correlation technology, do not allow the shape of amplified radiation pulse to be correctly measured. In pump-probe method, the amplified radiation with short duration passes through the laser plasma at different moments of time [15]. The total picture of the gain in time reflects the temporal behavior of only population inversion at the B2 π΄ + π’ β π 2 π΄ + π transition. In the cross-correlation method the signal of sum frequency is recorded when the pump beam and lasing are added in a nonlinear crystal [12,16]. In this case it is well known that the process of sum frequency lasing is nonlinear. As a result, the time profile of sum frequency will differ from the lasing pulse shape. Characteristic times are units and tens of picoseconds, measured by these methods. The actual temporal shape of the radiation pulse can only be measured using a streak camera with a high temporal resolution. The maximum time resolution was demonstrated in [17] while recording the emission of nitrogen ions, where the streak camera with time resolution of 30 ps was used. However, this resolution is not sufficient to correctly measure the duration of the lasing pulse.
β Corresponding author. E-mail address: [email protected] (I.A. Zyatikov).
https://doi.org/10.1016/j.optcom.2019.124573 Received 17 July 2019; Received in revised form 19 August 2019; Accepted 12 September 2019 Available online 15 September 2019 0030-4018/Β© 2019 Elsevier B.V. All rights reserved.
N.G. Ivanov, I.A. Zyatikov, V.F. Losev et al.
Optics Communications 456 (2020) 124573
Fig. 2. Photographs of plasma emission in the absence (a) and presence (b) of astigmatism in a focused beam. The pumping radiation propagates from right to left.
Fig. 1. Experimental scheme. M1 and M2 are pivoting mirrors, L1 and L2 are positive lenses with πΉ = 15 cm.
This paper presents the results of studies of temporal and spectral lasing characteristics by N2 + ions using the streak camera with subpicosecond resolution without an external seed on the corresponding ion transition. 2. Equipment and experimental techniques In order to form laser plasma, the radiation generated by the solid-state start complex ββStart-480ββ was used. This complex is part of the multiterawatt laser system THL-100 [18]. The laser complex includes Ti:Sapphire master oscillator with continuous pump laser, grating stretcher, regenerative and two multipass amplifiers, and grating compressor. This complex formed the transform-limited radiation at a central wavelength of 950 nm and pulse duration of 60 fs (FWHM). The laser beam had energy up to 15 mJ, 1.5 cm diameter (1/e2 intensity level), and Gaussian intensity distribution. The experimental scheme is shown in Fig. 1. The laser plasma was formed in ambient air when the radiation was focused by L1 lens with focal length πΉ = 15 cm. The lens was slightly tilted to provide a certain astigmatism on the wave front of the beam. In some experiments, a small part of radiation was transformed into the second harmonic (SH) in KDP crystal. SH radiation served as a reference point in time and to determine the instrument function of the streak camera. The spectral composition of plasma emission was measured both in the direction of pump radiation propagation and across it. The streak camera (Hamamatsu, Universal Streak Camera C10910) combined with a spectrometer (Acton SpectraPro SP-2300) was used for measuring the spectral and temporal characteristics. This camera had two time sweeps: fast (minimum sweep is 100 ps) and slow (minimum sweep is 1 ns). The maximum temporal resolution of streak camera was 0.644 ps in a single-shot mode. The temporal resolution for 300 pulses increased to 2 ps in accumulation pulses mode due to the presence of jitter in the response of the streak camera. To reduce it, the camera was started by a signal from a pin-diode, installed near the spectrometer. In spectrometer there was one grating with 1200 lines/mm with resolution of 0.23 nm. To compensate the delay time of the streak camera sweep block, the laser radiation was delayed and only then it was focused to create laser plasma. The lasing spectrum was recorded by Acton SpectraPro SP-2300 spectrometer. The spontaneous emission (SE) spectrum was recorded by Shamrock SR-500i-D1 spectrometer with resolution of 0.07 nm and iStar 334T Series (Andorβ’) Camera with temporal resolution of 3 ns. For measurements close to instrument function of the device, the following formula was used to calculate the resulting pulse duration and spectral width: β (1) πΆ = π΄2 β π΅ 2 ,
Fig. 3. Normalized spectra of SE and lasing.
3. Results and discussion As mentioned above, some astigmatism was set to the wave front of the focused beam. In this case, the length of laser plasma increased in the region of the focal waist and the intensity of its emission decreased. Lasing on the molecular nitrogen ions was observed in the direction of pump propagation at a certain tilt of the lens. Fig. 2 depicts the photographs of plasma emission in the waist area in the presence (a) and absence (b) of aberration. The longitudinal size of plasma emission is 3 mm and its diameter is 0.3 mm. With astigmatism introduced by the tilt of the lens, the presence of lasing can be explained. Indeed, it was shown in [19] that there were no molecular lines in laser plasma emission in the event of tight focusing, and the lines of atoms and their ions dominated. With an increasing focal lens, on the contrary, the atomic lines in the spectrum disappeared, and only the lines of molecular nitrogen and its ion remained. The explanation of this dependence is as follows. During short focusing, plasma concentration reaches extremely high values of about 1018 cmβ3 . As a result, the quenching rate of molecular nitrogen ions becomes so high that they do not have time to emit. The creation of astigmatism in a focused beam reduces the electron concentration in plasma and increases the length of the active medium. In experiments, lasing at different wavelengths of molecular nitrogen ions was observed: 391.4 (0-0), 427.8 (0-1), 358.2 (1-0), 388.4 (1-1) and 470.9 (0-2) nm. In this work the emission characteristics at the B2 π΄ + π’ β π 2 π΄ + π (0-1) transition were studied. Lasing was observed at the distance of several meters from plasma and had the form of spot. SE was not observed at 0.5 m distance. Thus it can be concluded, that the lasing had significantly greater power and higher directionality compared with SE. Fig. 3 shows the measured spectra of SE and lasing at a wavelength of 427.8 nm. Half-width spectra of lasing and SE are 0.35 nm. Taking into account the resolution of the spectrometers (according to formula (1)), the half-width of lasing spectrum is 0.26 nm and 0.34 nm for SE spectrum. During time-resolved emission measurements lasing was recorded in a single-shot mode and SE was recorded in accumulation mode across the laser plasma. In the latter case, the pulse selection mode was used to reduce the temporal spread. Fig. 4 depicts the combined in time radiation pulses of SE, lasing, and SH. In order to measure correctly,
where C is the resulting pulse duration or spectral width, A is measured pulse duration or spectral width, and B is the resolution of the device. 2
N.G. Ivanov, I.A. Zyatikov, V.F. Losev et al.
Optics Communications 456 (2020) 124573
lasing speaks in favor of the fact that it originates from the seeding of supercontinuum. 4. Conclusion Thus, in the present study the lasing pulse duration at the B2 π΄ + π’ (0) β π 2 π΄ + π (1) transition of molecular nitrogen ions was measured. It was 1.15 ps (FWHM). This pulse duration is close to transform-limited (1 ps), according to the measurements of spectrum width (0.26 nm). It is shown that there is a 5.6 ps delay of the lasing pulse maximum relative to the pumping. This delay is probably due to the fact that the lasing emission is produced by a narrow-line transition. The lasing on molecular nitrogen ions occurs only under astigmatism conditions, at which a reduced concentration of laser plasma is realized. Based on the form of lasing recorded in the near field, the seeding for its appearance is the emission of supercontinuum arising in laser plasma. Fig. 4. Normalized time-resolved emission of SH, lasing, and SE. The inset shows the spatial distribution of lasing in the near field.
Acknowledgment The reported study was funded by Russian Foundation for Basic Research (RFBR) (19-48-700016, 18-08-00383).
the device was calibrated by registration of the radiation of the second and third harmonics, as well as white light [20]. The corrections found in this way were taken into account during measurements and results processing. SH pulse reflects the measured resolution of the device (0.8 ps), the beginning of growth of its intensity (zero) corresponds to the moment of pump exposure. The delay of the lasing pulse (5.6 ps) with respect to the pumping is probably due to the fact, that the emission is produced by a narrow-line transition. According to the uncertainty principle, the duration of the lasing pulse is inversely proportional to the linewidth. The much longer duration of the lasing pulse, appearing after the pumping, is perceived as a delayed lasing pulse. Similar delay was observed in [3]. The decrease of SE intensity after 12 ps is due to the quenching of B2 π΄ + π’ state by electrons [21]. The presence of additional maxima on the lasing pulse, separated by 4 ps from each other, is most likely due to the half-period of ion rotation, which has also been noted in other works [14,21,22]. Taking into account the measured resolution of the device (0.8 ps) and formula (1), lasing pulse duration is 1.15 ps. According to the uncertainty principle, pulse duration is 1 ps for 0.26 nm spectrum width. Thus, our measurements of spectral linewidth and pulse duration showed that the duration of lasing is close to transform-limited. These resulting pulse duration and spectral width are consistent with other studies [5]. The inset of Fig. 4 depicts the spatial distribution of lasing emission with π = 427.8 nm in the near field. It can be observed that the radiation propagates in the form of hollow cone. It is well known that supercontinuum propagates in the form of a cone after laser plasma during short focusing of the pumping beam. Therefore, this type of
References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
3
J. Yao, B. Zeng, H. Xu, et al., Phys. Rev. A. 84 (2011) 051802. J. Ni, W. Chu, C. Jing, et al., Opt. Express 21 (2013) 8746. J. Yao, G. Li, C. Jing, et al., New J. Phys. 15 (2013) 023046. Y. Liu, Y. Brelet, G. Point, et al., Opt. Express 21 (2013) 22791. T.J. Wang, J. Ju, J.F. Daigle, et al., Laser Phys. Lett. 10 (2013) 125401. T.J. Wang, J.F. Daigle, J.Ju, et al., Phys. Rev. A. 88 (2013) 053429. H. Li, H. Zhang, Y. Su, et al., J. Opt. 19 (2017) 124006. V.E. Prokopiev, N.G. Ivanov, D.A. Krivonosenko, V.F. Losev, in: Conference The interaction of highly concentrated flows of energy materials in advanced technology and medicine (2013). V.E. Prokopev, N.G. Ivanov, D.A. Krivonosenko, V.F. Losev, Russ. Phys. J. 56 (2013) 1274. A. Azarm, P. Corkum, P. Polynkin, Conference on Lasers and Electro-Optics, Optical Society of America, 2016, p. JTh4B.9. C. Jing, H. Xie, G. Li, et al., Laser Phys. Lett. 12 (2015) 015301. Y. Liu, P. Ding, G. Lambert, et al., Phys. Rev. Lett. 115 (2015) 133203. S.L. Chin, H. Xu, Y. Cheng, et al., Chin. Opt. Lett. 11 (2013) 013201. J. Yao, W. Chu, Z. Liu, et al., Appl. Phys. B 124 (2018) 73. H.L. Xu, A. Azarm, J. Bernhardt, et al., Chem. Phys. 360 (2009) 171. X. Zhong, Z. Miao, L. Zhang, et al., Phys. Rev. A. 96 (2017) 043422. M. Lei, C. Wu, Q. Liang, et al., J. Phys. B: At. Mol. Opt. Phys. 50 (2017) 145101. S.V. Alekseev, A.I. Aristov, N.G. Ivanov, et al., Las. Part. Beams 31 (2013) 17. N.G. Ivanov, V.F. Losev, V.E. Prokopβev, et al., Opt. Commun. 431 (2019) 120. N.G. Ivanov, V.F. Losev, V.E. Prokopβev, K.A. Sitnik, Opt. Commun. 387 (2017) 322. J. Yao, G. Li, C. Jing, et al., New J. Phys. 15 (2013) 023046. X. Zhong, Z. Miao, L. Zhang, et al., Phys. Rev. A 96 (2017) 043422.