Journal of X-Ray Science and Technology 22 (2014) 105–111 DOI 10.3233/XST-130412 IOS Press
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Coherent soft X-ray high-order harmonics using tight-focusing laser pulses in the gas mixture Faming Lu, Yuanqin Xia∗ , Sheng Zhang, Deying Chen, Yang Zhao and Bin Liu National Key Laboratory of Science and Technology on Tunable Laser, Institute of Opto-Electronics, Harbin Institute of Technology, Harbin, Heilongjiang, China Received 1 May 2013 Revised 18 October 2013 Accepted 23 October 2013 Abstract. We experimentally study the harmonics from a Xe-He gas mixture using tight-focusing femtosecond laser pulses. The spectrum in the mixed gases exhibits an extended cutoff region from the harmonic H21 to H27. The potential explanation is that the harmonics photons from Xe contribute the electrons of He atoms to transmit into the excited-state. Therefore, the harmonics are emitted from He atoms easily. Furthermore, we show that there are the suppressed harmonics H15 and H17 in the mixed gases. The underlying mechanism is the destructive interference between harmonics generated from different atoms. Our results indicate that HHG from Xe-He gas mixture is an efficient method of obtaining the coherent soft X-ray source. Keywords: Femtosecond laser pulses, soft X-ray, high harmonic generation, gas mixture
1. Introduction In nearly two decades, the technology of high harmonic generation (HHG) has become an important means to generate the coherent extreme-ultraviolet (XUV) and the soft X-ray radiation [1,2]. A semi-classical three-step model can be used to interpret the mechanism of HHG: the electronic tunnel ionization, the electron motion in response to the laser electric field, and the recombining of electron with the parent ion [3,4]. In this process, the details of the nuclear or electronic dynamics are encoded in the HHG spectrum. Therefore, several application of HHG, such as the image of molecules structure [5, 6], the track of the ultrafast electron dynamics [7–9], can be realized. Generally, high-order harmonics are generated by the femtosecond laser pulses interacting with the pure atomic or molecular gases [10,11]. In recent years, HHG from the mixed gases has attracted wide interest. It has an ability to enhance the harmonic signals or to capture nuclear dynamics [12–17]. Takahashi et al. reported a dramatic enhancement of the harmonics in the mixed gases [14]. Kanai et al. ∗
Corresponding author: Yuanqin Xia, National Key Laboratory of Tunable Laser Technology; Institute of Opto-Electronics, Room 204, Building 2A, Science Park of Harbin Institute of Technology, 2 Yi Kuang Street, Harbin 150080, Heilongjiang, China. Tel.: +86 45186412753; Fax: +86 45186412753; E-mail: [email protected]. c 2014 – IOS Press and the authors. All rights reserved 0895-3996/14/$27.50
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Fig. 1. Experimental setup for high harmonic generation in the gas mixture. (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/XST-130412)
reported destructive interference and constructive interference of harmonics in a gas mixture [15]. Moreover, they observed the structure of molecules by measuring the phase of harmonics [16]. In addition, HHG from the mixed gases can be used to probe the attosecond dynamics of the nuclear wave-packet in the molecules [17]. The previous experiments have been carried out by using the loose focusing laser beam to avoid the absorption of harmonics. There are few reports on HHG from the tight focusing regime in the gas mixture. Compared with the loose focusing laser, the tight focusing laser provides an opportunity to study the spectral characteristics of the nonlinear medium [18–20]. The confocal parameter is smaller than the medium length in the tight focused condition. Huillierb et al. investigated the third-harmonic in a gas cell and a pulsed gas jet [18]. The length of the ionized medium in the gas cell is larger than the confocal parameter. The results strongly depended on the geometry of gas apparatus. Furthermore, the spectral characteristics of the bound states of the atom can be investigated in the tight focused laser pulses [19]. In the present letter, we report on harmonic emission and its spectral properties driven by tight focusing laser pulses in a Xe-He gas mixture. We show that the position of the cutoff in the mixed gases is extended from harmonic H21 to H27. The superposition between XUV pulses and femtosecond pulses reduce the ionization energy of He atoms, and promotes harmonics generation from the He atoms. Moreover, the destructive interference is observed in the Xe-He mixture gases. We show that the gas mixture is an excellent nonlinear medium for obtaining coherent soft X-ray source.
2. Experimental setup The schematic of the experimental setup is shown in Fig. 1. The HHG experiments are performed with a 1-kHz Ti:sapphire laser system which providing 40-fs pulses at 800 nm. In order to generate harmonic spectrum, the linearly polarized laser is focused by a 400 mm focal-length lens into a 5 mm gas cell. The intensity at the interaction region is estimated to be 1.0 × 1014 W/cm2 . The confocal parameter is smaller than the length of the gas cell. For studying the harmonics spectrum from the gas mixture, the gas cell is filled with the Xe-He mixed gases. The Xe partial pressure is ∼ 10%. The running pressures in the gas cell can be monitored by a digital vacuum gauge.
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Fig. 2. Harmonic images in (a) the pure Xe and (b) the Xe-He gas mixture.
On the output end of the gas cell, the generated harmonics are spectrally dispersed with a gold-coated grazing-incidence flat-field XUV spectrometer. The spectrum images are recorded by a back-illuminated X-ray charge-coupled device (CCD) camera. The spectral image is integrated over the spatial dimension to acquire the harmonic profile. The CCD camera chip is cooled down to −10◦ C for reducing the thermal noise. In order to avoid saturating and damaging the CCD camera, a 500-nm-thick Al filter is inserted in front of the XUV spectrometer to block the infrared laser and transmit the harmonics from H11 to H45 (17–70 eV). 3. Experimental results In the experiments, we choose the Xe-He gas mixture as the nonlinear medium. He is a small atom with a low recollision cross section. The effective harmonics emission is weak. In contrast, Xe has a larger recollision cross section which results in the brighter harmonics emission. However, more free electrons can be generated from Xe easily in the intense laser field. These electrons also result in the electron dispersion and the ionization-induced laser defocusing easily. The higher-order harmonics are generated difficulty. Therefore, we expect to obtain the soft X-ray source with high brightness and short wavelength simultaneously in the mixed gases. Figures 2 (a) and (b) show the typical harmonic images in the pure Xe and the Xe-He gas mixture respectively. The righter position in the figures, there is the higher-order harmonic. From Fig. 2, we can find that the harmonic spectra in the pure Xe and the Xe-He gas mixture are different. The intensity of harmonic H17 is strongest. No harmonic signal is observed to be higher than harmonic H21 in Fig. 2(a). In contrast, the significantly clear appearance of the new harmonics from H23 to H27 is observed in Fig. 2(b). The intensity modulation also appears in the Xe-He gas mixture. As shown in Fig. 2(b), harmonics H15 and H17 show the weak intensities. The amounts and intensities of harmonic field depend on the different nonlinear medium. For the CCD images in Fig. 2, the harmonics profiles are described in Fig. 3. The harmonic spectra in the pure He is also shown in Fig. 3. In this experiment, no harmonic signal is observed in the pure He. The harmonics from H15 to H21 are obtained in the pure Xe. In the Xe-He gas mixture, the cutoff position of harmonics is H27. The result indicates that the cutoff position is extended in the mixed gases. The intensities of harmonic H15 and H17 in the Xe-He gas mixture are weaker than that in the pure Xe.
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Fig. 3. Harmonics spectrum at the pressure of 5.1 Torr in the pure Xe (black dashed line), the pure He (olive dotted line), and at the pressure of 5.1 Torr in the Xe-He mixture (red solid line). (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/XST-130412)
The lower orders of harmonic in mixed gases are suppressed. Furthermore, we note that there are the double peak structure and the broad spectrum distribution of harmonic H15 in the pure Xe. 4. Analysis and discussion In Fig. 3, the harmonic spectra are generated in the pure Xe and the Xe-He gas mixture with an intensity of 1.0 × 1014 W/cm2 . The photon energy of the cutoff harmonic equals to Ip + 3.17Up, where Ip is the atom ionization potential and Up = 9.33 × 10−14 × I (W/cm2 ) × λ2 (μm) is the ponderomotive energy. In the current experimental conditions, the predicted cutoff harmonic in Xe is H21, and it is consistent with the experimental results. However, we can not observe the harmonic signal from the pure He in Fig. 3. This is because of the high ionization energy and the low recollision cross section of He atoms. Furthermore, in the tight focusing limited conditions, the absorption of nonlinear media to HHG is another factor. In the Xe-He mixture, HHG displays the extended orders and the suppressed orders respectively. We now focus on why the HHG in the Xe-He gas mixture is significant different from that in the pure Xe and the pure He. Firstly, we assume that the extended orders in the mixed gases are from the pure Xe. Under this assumption, the role of He atoms is to decrease the density of Xe atoms and reduce the plasma defocusing. If the assumption is true, we can observe the same extended orders by means of changing the Xe pressure. Therefore, we compared the harmonics emission from the pure Xe at the pressure of 0.5 Torr and 5.1 Torr respectively. The results can be seen in Fig. 4. The two harmonics spectra show the same orders in the different gas pressures. Even if we decrease or increase the pressure of Xe, the cutoff harmonic will not reach H27. Thus, there are other factors leading to the increase of harmonics orders in the mixed gases. We note that there are two factors resulting in the variation of harmonic orders in the mixed gases. First, the photon energy of harmonic H15 is 23.29 eV which is close to the 1s2 -1s3p transition energy
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Fig. 4. Comparison of harmonic spectrums achieved at the pressure of 5.1 Torr (black dashed line) and 1.3 Torr (blue solid line) in the pure Xe. (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/XST-130412)
(23.13 eV) of He atom. The other, the field-induced Stark shift and the broadening of the 1s3p energy level promote the orbital resonance absorption. The shift of the excited-state is larger than that of the ground state. As a result, the electron from the ground state of He atoms absorb the harmonic photon from Xe. Then, the electron arrive the excited-state of He atoms. The electrons in the excited-state are ionized into the continuum sate in the intense laser field. After the oscillation movement and the recollision, harmonics photons from He are emitted. In our experiments, the cutoff harmonic from the Xe-He gas mixture is H27 which nearly agree with the predicted harmonic H29 from He atoms. Given these considerations, together with the experimental cutoff position of harmonic in Xe-He gas mixture, we conclude that the harmonics from H23 to H27 in the mixed gases are attributed to He atoms. It is confirmed that the absorption of the XUV emission generated from Xe atoms can decrease the ionization threshold of He significantly, and boost harmonics emission from He atoms. Furthermore, as can be seen from Fig. 3, an obvious spectrum feature in the gas mixture is the suppressed intensities of the harmonics H15 and H17. In Fig. 4, the harmonics spectrum from Xe shows the normal feature of the harmonics. The harmonics in the plateau exhibit the strong signal. With increasing or decreasing the laser intensity, the harmonics from Xe do not exhibit the suppressed harmonic spectrum. Harmonics H15 and H17 obtained from the gas mixture are not observed in the pure Xe regime. Generally, the HHG minimum originates from the electronic structure of the atom [21,22] or the electronic dynamics induced by the laser field in the continuum [15]. The former is known as the Cooper minimum [23]. In fact, Ar, Kr, and Xe can show the Cooper minimum, but Ne and He can’t. In addition, the Cooper minimum in Xe is near 200 eV [24]. In our experiments, the HHG minimum is inconsistent with the prediction in Xe. In our results, the lower harmonics in the Xe-He gas mixture also contain harmonics generated from Xe atoms and He atoms. The most likely explanation for the suppression of harmonics H15 and H17 in the Xe-He gas mixture is the destructive interference of harmonics from the different atoms. Such destructive interference effects could be interpreted by the modulation of the chirped intrinsic phases of the harmonics generated from the different atoms. Our experimental results
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are similar to the Ref. [15]. And our results are somewhat different. In this paper, we obtain the extension of harmonic orders and observe the destruction interference. In Figs 3 and 4, we observe a double peak structure from harmonic H15 and a broad spectrum distribution in pure Xe. The results can be attributed to the saturated conversion of the harmonics [25]. In addition, the redshift of harmonics in Xe at the high gas pressures is observed in Fig. 4. The observed redshift toward the high gas densities can be explained that the electron is ionized on the falling edge of the laser field. The low ionization energy of Xe atoms leads to a large number of free electrons in the intense laser field. The laser defocusing reduces the laser intensity. The electron is ionized on the falling edge of the laser field. The electron acquires the less energy. Therefore, a redshift in the HHG spectrum can be observed.
5. Conclusion In summary, we have shown the high harmonic generation and its spectral characteristics in the Xe-He gas mixture by using the tight focusing laser pulses. The cutoff position of harmonic is extended from the harmonic H21 to H27. The results can be explained that the harmonics are generated from the excited state of He atoms with the help of XUV emission from Xe atoms. The suppressed harmonics H15 and H17 are observed in the mixed gases. The result is attributed to the destructive interference of harmonics from the different atoms. Our results indicate that the mixed gases can be used to generate the coherent soft X-ray radiation.
Acknowledgement We acknowledge the financial support of National Natural Science Foundation of China (Grant Nos. 10774033).
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
J. Zhou, J. Peatross, M.M. Murnane and H.C. Kapteyn, Enhanced high-harmonic generation using 25 fs laser pulses, Physical Review Letters 76 (1996), 752–755. T. Popmintchev, M.-C. Chen, A. Bahabad, M. Gerrity, P. Sidorenko, O. Cohen, I.P. Christov, M.M. Murnane and H.C. Kapteyn, Phase matching of high harmonic generation in the soft and hard X-ray regions of the spectrum, Proceeding of the National Academy of Sciences of the United States of America 106 (2009), 10516–10521. P.B. Corkum, Plasma perspective on strong-field multiphoton ionization, Physical Review Letters 71 (1993), 1994–1997. K.J. Schafer, B. Yang, L.F. DiMauro and K.C. Kulander, Above threshold ionization beyond the high harmonic cutoff, Physical Review Letters 70 (1993), 1599–1602. J. Itatani, J. Levesque, D. Zeidler, H. Niikura, H. Pépin, J.C. Kieffer, P.B. Corkum and D.M. Villeneuve, Tomographic imaging of molecular orbitals, Nature (Landon) 432 (2004), 867–871. B.K. McFarland, J.P. Farrell, P.H. Bucksbaum and M. Gühr, Hihg harmonic generation from multiple orbitals in N2 , Science 322 (2008), 1232–1235. W. Li, X.B. Zhou, R. Lock, S. Patchkovskii, A. Stolow, H.C. Kapteyn and M.M. Murnane, Time-resolved dynamics in N2 O4 probed using high harmonic generation, Science 322 (2008), 1207–1211. O. Smirnova, Y. Mairesse. S. Patchkovskii, N. Dudovich, D. Villeneuve, P. Corkum and M.Yu. lvanov, High harmonic interferometry of multi-electron dynamics in molecules, Nature (Landon) 460 (2009), 972–977. Y. Mairesse, J. Higuet, N. Dudovich, D. Shafir, B. Fabre, E. Mével, E. Constant, S. Patchkovskii, Z. Walters, M.Yu. Ivanov and O. Smirnova, High harmonic spectroscopy of multichannel dynamics in strong-field ionization, Physical Review Letters 104 (2010), 213601.
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J.J. Macklin, J.D. Kmetec and C.L. Gordon III, High-order harmonic generation using intense femtosecond pulses, Physical Review Letters 70 (1993), 766–769. [11] J. Itatani, D. Zeidler, J. Levesque, M. Spanner, D.M. Villeneuve and P.B. Corkum, Controlling high harmonic generation with molecular wave packets, Physical Review Letters 94 (2005), 123902. [12] J. Biegert, A. Heinrich, C.P. Hauri, W. Kornelis, P. Schlup, M. Anscombe, K.J. Schafer, M.B. Gaarde and U. Keller, Enhancement of high-order harmonic emission using attosecond pulse trains, Laser Physics 15 (2005), 899–902. [13] A. Heinrich, W. Kornelis, M.P. Anscombe, C.P. Hauri, P. Schlup, J. Biegert and U. Keller, Enhanced VUV-assisted high harmonic generation, Journal of Physics B: Atomic Molecular and Optical Physics 39 (2006), S275–S281. [14] E.J. Takahashi, T. Kanai, K.L. Ishikawa, Y. Nabekawa and K. Midorikawa, Dramatic enhancement of high-order harmonic generation, Physical Review Letters 99 (2007), 053904. [15] T. Kanai, E.J. Takahashi, Y. Nabekawa and K. Midorikawa, Destructive interference during high harmonic generation in mixed gases, Physical Review Letters 98 (2007), 153904. [16] T. Kanai, E.J. Takahashi, Y. Nabekawa and K. Midorikawa, Observing molecular structures by using high-order harmonic generation in mixed gases, Physical Review A 77 (2008), 041402. [17] T. Kanai, E.J. Takahashi, Y. Nabekawa and K. Midorikawa, Observing the attosecond dynamics of nuclear wavepackets in molecules by using high harmonic generation in mixed gases, New Journal of Physics 10 (2008), 025036. [18] A. L’Huillier, L.A. Lompré, M. Ferray, X.F. Li, G. Mainfray and C. Manus, Third-harmonic generaiton in Xenon in a pulsed jet and a gas cell, Europhysics Letters 5 (1988), 601–605. [19] D.M. Mittleman, D.C. Douglass, Z. Henis, O.R. Wood II, R.R. Freeman and T.J. McIlrath, High-field harmonic generation in the tight-focusing limit, Journal of the Optical Society of America B 13 (1996), 170–179. [20] Q. Lin, S. Li and W. Becker, High-order harmonic generation in a tightly focused laser beam, Optics Letters 31 (2006), 2163–2165. [21] S. Minemoto, T. Umegaki, Y. Oguchi, T. Morishita, A-Thu Le, S. Watanabe and H. Sakai, Retrieving photorecombination cross section of atoms from high-order harmonic spectra, Physical Review A 78 (2008), 061402. [22] H.J. Wörmer, H. Niikura, J.B. Bertrand, P.B. Corkum and D.M. Villeneuve, Observation of electronic structure minima in high-harmonic generation, Physical Review Letters 102 (2009), 103901. [23] T.A. Carlson, A. Fahlman, M.O. Krause, T.A. Whitley and F.A. Grimm, Systematic investigation of the Cooper minimum for the hydrogen halides, Journal Chemical Physics 81 (1984), 5389–5394. [24] J.A.R. Samson and W.C. Stolte, Precision measurements of the total photoionization cross-section of He, Ne, Ar, Kr, and Xe, Journal of Electron Spectroscopy and Related Phenomena 123 (2002), 265–276. [25] K. Midorikawa, Y. Tamaki, J. Itatani, Y. Nagata and M. Obara, Phase-matched high-order harmonic generation by guided intense femtosecond pulses, IEEE Journal of Selected Topics in Quantum Electronics 5 (1999), 1475–1485.
Copyright of Journal of X-Ray Science & Technology is the property of IOS Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
Copyright of Journal of X-Ray Science & Technology is the property of IOS Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
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Coherent soft X-ray high-order harmonics using tight-focusing laser pulses in the gas mixture Faming Lu, Yuanqin Xia∗ , Sheng Zhang, Deying Chen, Yang Zhao and Bin Liu National Key Laboratory of Science and Technology on Tunable Laser, Institute of Opto-Electronics, Harbin Institute of Technology, Harbin, Heilongjiang, China Received 1 May 2013 Revised 18 October 2013 Accepted 23 October 2013 Abstract. We experimentally study the harmonics from a Xe-He gas mixture using tight-focusing femtosecond laser pulses. The spectrum in the mixed gases exhibits an extended cutoff region from the harmonic H21 to H27. The potential explanation is that the harmonics photons from Xe contribute the electrons of He atoms to transmit into the excited-state. Therefore, the harmonics are emitted from He atoms easily. Furthermore, we show that there are the suppressed harmonics H15 and H17 in the mixed gases. The underlying mechanism is the destructive interference between harmonics generated from different atoms. Our results indicate that HHG from Xe-He gas mixture is an efficient method of obtaining the coherent soft X-ray source. Keywords: Femtosecond laser pulses, soft X-ray, high harmonic generation, gas mixture
1. Introduction In nearly two decades, the technology of high harmonic generation (HHG) has become an important means to generate the coherent extreme-ultraviolet (XUV) and the soft X-ray radiation [1,2]. A semi-classical three-step model can be used to interpret the mechanism of HHG: the electronic tunnel ionization, the electron motion in response to the laser electric field, and the recombining of electron with the parent ion [3,4]. In this process, the details of the nuclear or electronic dynamics are encoded in the HHG spectrum. Therefore, several application of HHG, such as the image of molecules structure [5, 6], the track of the ultrafast electron dynamics [7–9], can be realized. Generally, high-order harmonics are generated by the femtosecond laser pulses interacting with the pure atomic or molecular gases [10,11]. In recent years, HHG from the mixed gases has attracted wide interest. It has an ability to enhance the harmonic signals or to capture nuclear dynamics [12–17]. Takahashi et al. reported a dramatic enhancement of the harmonics in the mixed gases [14]. Kanai et al. ∗
Corresponding author: Yuanqin Xia, National Key Laboratory of Tunable Laser Technology; Institute of Opto-Electronics, Room 204, Building 2A, Science Park of Harbin Institute of Technology, 2 Yi Kuang Street, Harbin 150080, Heilongjiang, China. Tel.: +86 45186412753; Fax: +86 45186412753; E-mail: [email protected]. c 2014 – IOS Press and the authors. All rights reserved 0895-3996/14/$27.50
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Fig. 1. Experimental setup for high harmonic generation in the gas mixture. (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/XST-130412)
reported destructive interference and constructive interference of harmonics in a gas mixture [15]. Moreover, they observed the structure of molecules by measuring the phase of harmonics [16]. In addition, HHG from the mixed gases can be used to probe the attosecond dynamics of the nuclear wave-packet in the molecules [17]. The previous experiments have been carried out by using the loose focusing laser beam to avoid the absorption of harmonics. There are few reports on HHG from the tight focusing regime in the gas mixture. Compared with the loose focusing laser, the tight focusing laser provides an opportunity to study the spectral characteristics of the nonlinear medium [18–20]. The confocal parameter is smaller than the medium length in the tight focused condition. Huillierb et al. investigated the third-harmonic in a gas cell and a pulsed gas jet [18]. The length of the ionized medium in the gas cell is larger than the confocal parameter. The results strongly depended on the geometry of gas apparatus. Furthermore, the spectral characteristics of the bound states of the atom can be investigated in the tight focused laser pulses [19]. In the present letter, we report on harmonic emission and its spectral properties driven by tight focusing laser pulses in a Xe-He gas mixture. We show that the position of the cutoff in the mixed gases is extended from harmonic H21 to H27. The superposition between XUV pulses and femtosecond pulses reduce the ionization energy of He atoms, and promotes harmonics generation from the He atoms. Moreover, the destructive interference is observed in the Xe-He mixture gases. We show that the gas mixture is an excellent nonlinear medium for obtaining coherent soft X-ray source.
2. Experimental setup The schematic of the experimental setup is shown in Fig. 1. The HHG experiments are performed with a 1-kHz Ti:sapphire laser system which providing 40-fs pulses at 800 nm. In order to generate harmonic spectrum, the linearly polarized laser is focused by a 400 mm focal-length lens into a 5 mm gas cell. The intensity at the interaction region is estimated to be 1.0 × 1014 W/cm2 . The confocal parameter is smaller than the length of the gas cell. For studying the harmonics spectrum from the gas mixture, the gas cell is filled with the Xe-He mixed gases. The Xe partial pressure is ∼ 10%. The running pressures in the gas cell can be monitored by a digital vacuum gauge.
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Fig. 2. Harmonic images in (a) the pure Xe and (b) the Xe-He gas mixture.
On the output end of the gas cell, the generated harmonics are spectrally dispersed with a gold-coated grazing-incidence flat-field XUV spectrometer. The spectrum images are recorded by a back-illuminated X-ray charge-coupled device (CCD) camera. The spectral image is integrated over the spatial dimension to acquire the harmonic profile. The CCD camera chip is cooled down to −10◦ C for reducing the thermal noise. In order to avoid saturating and damaging the CCD camera, a 500-nm-thick Al filter is inserted in front of the XUV spectrometer to block the infrared laser and transmit the harmonics from H11 to H45 (17–70 eV). 3. Experimental results In the experiments, we choose the Xe-He gas mixture as the nonlinear medium. He is a small atom with a low recollision cross section. The effective harmonics emission is weak. In contrast, Xe has a larger recollision cross section which results in the brighter harmonics emission. However, more free electrons can be generated from Xe easily in the intense laser field. These electrons also result in the electron dispersion and the ionization-induced laser defocusing easily. The higher-order harmonics are generated difficulty. Therefore, we expect to obtain the soft X-ray source with high brightness and short wavelength simultaneously in the mixed gases. Figures 2 (a) and (b) show the typical harmonic images in the pure Xe and the Xe-He gas mixture respectively. The righter position in the figures, there is the higher-order harmonic. From Fig. 2, we can find that the harmonic spectra in the pure Xe and the Xe-He gas mixture are different. The intensity of harmonic H17 is strongest. No harmonic signal is observed to be higher than harmonic H21 in Fig. 2(a). In contrast, the significantly clear appearance of the new harmonics from H23 to H27 is observed in Fig. 2(b). The intensity modulation also appears in the Xe-He gas mixture. As shown in Fig. 2(b), harmonics H15 and H17 show the weak intensities. The amounts and intensities of harmonic field depend on the different nonlinear medium. For the CCD images in Fig. 2, the harmonics profiles are described in Fig. 3. The harmonic spectra in the pure He is also shown in Fig. 3. In this experiment, no harmonic signal is observed in the pure He. The harmonics from H15 to H21 are obtained in the pure Xe. In the Xe-He gas mixture, the cutoff position of harmonics is H27. The result indicates that the cutoff position is extended in the mixed gases. The intensities of harmonic H15 and H17 in the Xe-He gas mixture are weaker than that in the pure Xe.
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Fig. 3. Harmonics spectrum at the pressure of 5.1 Torr in the pure Xe (black dashed line), the pure He (olive dotted line), and at the pressure of 5.1 Torr in the Xe-He mixture (red solid line). (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/XST-130412)
The lower orders of harmonic in mixed gases are suppressed. Furthermore, we note that there are the double peak structure and the broad spectrum distribution of harmonic H15 in the pure Xe. 4. Analysis and discussion In Fig. 3, the harmonic spectra are generated in the pure Xe and the Xe-He gas mixture with an intensity of 1.0 × 1014 W/cm2 . The photon energy of the cutoff harmonic equals to Ip + 3.17Up, where Ip is the atom ionization potential and Up = 9.33 × 10−14 × I (W/cm2 ) × λ2 (μm) is the ponderomotive energy. In the current experimental conditions, the predicted cutoff harmonic in Xe is H21, and it is consistent with the experimental results. However, we can not observe the harmonic signal from the pure He in Fig. 3. This is because of the high ionization energy and the low recollision cross section of He atoms. Furthermore, in the tight focusing limited conditions, the absorption of nonlinear media to HHG is another factor. In the Xe-He mixture, HHG displays the extended orders and the suppressed orders respectively. We now focus on why the HHG in the Xe-He gas mixture is significant different from that in the pure Xe and the pure He. Firstly, we assume that the extended orders in the mixed gases are from the pure Xe. Under this assumption, the role of He atoms is to decrease the density of Xe atoms and reduce the plasma defocusing. If the assumption is true, we can observe the same extended orders by means of changing the Xe pressure. Therefore, we compared the harmonics emission from the pure Xe at the pressure of 0.5 Torr and 5.1 Torr respectively. The results can be seen in Fig. 4. The two harmonics spectra show the same orders in the different gas pressures. Even if we decrease or increase the pressure of Xe, the cutoff harmonic will not reach H27. Thus, there are other factors leading to the increase of harmonics orders in the mixed gases. We note that there are two factors resulting in the variation of harmonic orders in the mixed gases. First, the photon energy of harmonic H15 is 23.29 eV which is close to the 1s2 -1s3p transition energy
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Fig. 4. Comparison of harmonic spectrums achieved at the pressure of 5.1 Torr (black dashed line) and 1.3 Torr (blue solid line) in the pure Xe. (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/XST-130412)
(23.13 eV) of He atom. The other, the field-induced Stark shift and the broadening of the 1s3p energy level promote the orbital resonance absorption. The shift of the excited-state is larger than that of the ground state. As a result, the electron from the ground state of He atoms absorb the harmonic photon from Xe. Then, the electron arrive the excited-state of He atoms. The electrons in the excited-state are ionized into the continuum sate in the intense laser field. After the oscillation movement and the recollision, harmonics photons from He are emitted. In our experiments, the cutoff harmonic from the Xe-He gas mixture is H27 which nearly agree with the predicted harmonic H29 from He atoms. Given these considerations, together with the experimental cutoff position of harmonic in Xe-He gas mixture, we conclude that the harmonics from H23 to H27 in the mixed gases are attributed to He atoms. It is confirmed that the absorption of the XUV emission generated from Xe atoms can decrease the ionization threshold of He significantly, and boost harmonics emission from He atoms. Furthermore, as can be seen from Fig. 3, an obvious spectrum feature in the gas mixture is the suppressed intensities of the harmonics H15 and H17. In Fig. 4, the harmonics spectrum from Xe shows the normal feature of the harmonics. The harmonics in the plateau exhibit the strong signal. With increasing or decreasing the laser intensity, the harmonics from Xe do not exhibit the suppressed harmonic spectrum. Harmonics H15 and H17 obtained from the gas mixture are not observed in the pure Xe regime. Generally, the HHG minimum originates from the electronic structure of the atom [21,22] or the electronic dynamics induced by the laser field in the continuum [15]. The former is known as the Cooper minimum [23]. In fact, Ar, Kr, and Xe can show the Cooper minimum, but Ne and He can’t. In addition, the Cooper minimum in Xe is near 200 eV [24]. In our experiments, the HHG minimum is inconsistent with the prediction in Xe. In our results, the lower harmonics in the Xe-He gas mixture also contain harmonics generated from Xe atoms and He atoms. The most likely explanation for the suppression of harmonics H15 and H17 in the Xe-He gas mixture is the destructive interference of harmonics from the different atoms. Such destructive interference effects could be interpreted by the modulation of the chirped intrinsic phases of the harmonics generated from the different atoms. Our experimental results
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are similar to the Ref. [15]. And our results are somewhat different. In this paper, we obtain the extension of harmonic orders and observe the destruction interference. In Figs 3 and 4, we observe a double peak structure from harmonic H15 and a broad spectrum distribution in pure Xe. The results can be attributed to the saturated conversion of the harmonics [25]. In addition, the redshift of harmonics in Xe at the high gas pressures is observed in Fig. 4. The observed redshift toward the high gas densities can be explained that the electron is ionized on the falling edge of the laser field. The low ionization energy of Xe atoms leads to a large number of free electrons in the intense laser field. The laser defocusing reduces the laser intensity. The electron is ionized on the falling edge of the laser field. The electron acquires the less energy. Therefore, a redshift in the HHG spectrum can be observed.
5. Conclusion In summary, we have shown the high harmonic generation and its spectral characteristics in the Xe-He gas mixture by using the tight focusing laser pulses. The cutoff position of harmonic is extended from the harmonic H21 to H27. The results can be explained that the harmonics are generated from the excited state of He atoms with the help of XUV emission from Xe atoms. The suppressed harmonics H15 and H17 are observed in the mixed gases. The result is attributed to the destructive interference of harmonics from the different atoms. Our results indicate that the mixed gases can be used to generate the coherent soft X-ray radiation.
Acknowledgement We acknowledge the financial support of National Natural Science Foundation of China (Grant Nos. 10774033).
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Copyright of Journal of X-Ray Science & Technology is the property of IOS Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
