High-order harmonic generation from Xe–Ar gas mixturein the tight focusing laser

Faming Lu n, Yuanqin Xia, Sheng Zhang, Deying ChenNational Key Laboratory of Tunable Laser Technology, Institute of Opto-Electronics, Harbin Institute of Technology, 150001 Harbin, China

a r t i c l e i n f o

Article history:Received 18 June 2013Received in revised form26 September 2013Accepted 10 October 2013Available online 31 October 2013

Keywords:Femtosecond laserHigh harmonic generationGas mixture

a b s t r a c t

We report on the harmonics radiation and spectrum properties in the Xe–Ar gas mixture by using tightfocusing laser pulses. The cutoff position is extended from the harmonic H33–H37 in mixed gases. Theextended orders are attributed to the harmonic radiation from Xeþ . We observe that the harmonics aredecreased in the plateau region. The result is attributed to the destructive interference of the harmonicsgenerated from the different atoms. Our results suggest that using the Xe–Ar mixture as nonlinear canobtain the coherent XUV source.

Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

High harmonic generation (HHG) is a coherent extreme-ultraviolet(XUV) light source [1,2]. The microscopic physics process of HHG iswell understood by a three-step model [3]: ionization, acceleration,and recollision. The highest energy of the harmonic photon isIpþ3.17Up [4]. Here, Ip is the ionization potential of an atom, and Up

is the quiver energy of a free electron in an oscillating laser electricfield. The properties of HHGmake it possible to capture the atoms andmolecules' structure, or track the dynamics of electronic [5–12]. Thewide rang of the applications encourages the development of HHG.

Most of the HHG experiments were performed in the pure gasmedium. Recently, there has been an increasing interest in theharmonics generated in mixed gases. HHG from the mixed gases isan attractive technique for probing the dynamics of the nuclear orelectron wave packet [13,14], and enhancing the conversion efficiencyof harmonics [15–18]. In Refs. [15,16], Biegert et al. generated the X-rayfrom Xe firstly. Then they used this X-ray and an Infrared (IR) laser toobtain the enhanced HHG from He. Whereas, Takahashi et al. [17]firstly achieved the dramatic enhancement of harmonics by using theXe–He gas mixture target as the nonlinear medium. However, usingthe Xe–Ar gas mixture as the generating medium has not beenreported to our knowledge. Xe atom has a larger re-collision crosssection for generating the bright harmonics radiation. However, thelow ionization potential of Xe atom results in the serious electrondispersion and the ionization-induced laser defocusing. Ar is advanta-geous for clarifying the effect of propagation and phase matching. It is

relatively transparent for the HHG. Therefore, the Xe–Ar gas mixture ischosen for investigating the characteristics of HHG.

In the performed experiments, the loose focusing laser pulseswere used for reducing the harmonics absorption. However, thespectral characteristics of the nonlinear medium can be studied inthe tightly focused geometry [19–21]. In the tight focused geo-metry, the confocal parameter b is smaller than the length of thegas medium L. In Ref. [19], Huillierb et al. compared the third-harmonic in a gas cell and a pulsed gas jet. The laser intensitydependent of the harmonic radiation is different. The resultsdepended on the harmonics that were generated in the gas cellor in the gas jet. The confocal parameter in the gas cell is largerthan that of in the pulsed gas jet. The length of the ionizedmedium in the gas cell increases with the strong laser intensity. Inthe pulsed gas jet, the length of the ionized medium is limited bythe gas jet width. In addition, the tight focused geometry providesa chance to investigate the spectroscopy properties of the boundstates in the strong field [20]. The harmonics property is perturbedby the atomic resonances in Xe near 109 nm.

In this paper, we report the harmonics properties from the Xe–Argas mixture using the tight focusing laser pulses. The cutoff position ofthe harmonic in the mixed gases is extended from harmonic H33 toH37, namely high conversion efficiency. The results can be explainedas that the harmonics come from Xeþ in mixed gases. Moreover, thedestructive interference is observed in the Xe–Ar gas mixture.

2. Experimental setup

The schematic of the experimental setup is shows in Fig. 1. Theexperimental system consists of a Ti:sapphire chirped pulse

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0030-3992/$ - see front matter Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.optlastec.2013.10.011

n Corresponding author. Tel.: þ86 451 864 12753.E-mail address: lufaming2011@126.com (F. Lu).

Optics & Laser Technology 57 (2014) 145–148

amplifier laser system, a vacuum chamber and a flat field spectro-meter. The 1 kHz, 40 fs pulses centered at 800 nm has a diameterof 10 mm and an energy of 1.8 mJ per pulse. In order to generatethe high harmonics, the laser beam is focused into a 5 mm longXe–Ar gas cell. The focal length of the fused silica lens is 400 mm.The intensity at the interaction region is estimated to be1.9�1014 W/cm2. The confocal parameter is smaller than thelength of the gas cell. In the gas cell, the running pressure ismonitored by a digital vacuum gauge. The Xe atoms concentrationis 50%.

After the gas cell, the total harmonic fields are spectrallydispersed by a grazing incidence flat field spectrometer. Thespectral range of the spectrometer is 3–50 nm. Then, HHG arerecorded by a back-illuminated XUV charge-coupled device (CCD)camera (Princeton Instruments 2048�512 pixels). The CCD camerais cooled down to �10 1C for reducing the thermal noise. To avoidoverexposure of CCD camera, a 500-nm-thick Al metal foil is usedfor filtering the infrared laser beam and transmitting about 10% ofthe harmonic energy. The energy of the laser pulses, the position ofthe laser focus, which is relative to the gas cell, and the laser pulseschirps are optimized for the maximum flux of harmonics.

3. Experimental Results

Fig. 2 shows the typical harmonic images in (a) pure Xe,(b) pure Ar, and (c) Ar–Xe mixture at the pressure of 2.3 Torr.The harmonic images are recorded during an integration time of1 s, corresponding to 103 laser pulses. Comparing with Fig. 2(a) and (b), the higher orders are observed in Fig. 2(c). The resultsindicate that the cutoff position is changed. As can be seen theharmonics orders depend on the nonlinear medium. In view of theimages in Fig. 2, the harmonic spectrums are analyzed in Fig. 3.

Fig. 3 shows the comparison of harmonics spectrum generatedfrom the pure Xe, the pure Ar, and the Xe–Ar gas mixture at thepressure of 2.3 Torr. The spectrums in the Xe–Ar gas mixtureconsists of the ten harmonics from H19 to H37. The centralwavelength is harmonic H27 (29.63 nm). Note that the peakintensities of harmonics are shifted to the higher order. Theconversion efficiency in the cutoff region is better in mixed gases.In order to clarify the harmonics of the cutoff region, we amplifythe spectrums from harmonic H29 to H37 in the pure gas and thegas mixture (Fig. 3(b)). The harmonic in the cutoff position isextended.

Fig. 4 shows the harmonic spectrums at the pressure of 0.5 Torr,2.3 Torr, and 3.8 Torr. For the maximum flux of harmonics in thecutoff region, the gas pressure is optimized at the pressure of2.3 Torr in the gas mixture. There are more free electrons and laserdefocusing in the higher gas pressure. Therefore, increasing the

gas pressure leads to a lower harmonic in the cutoff region.In contrast, the harmonics radiation is reduced at lower gaspressure due to the low-density of free electrons.

Fig. 1. Experimental setup.

Fig. 2. Harmonics images recorded by XUV CCD camera at the pressure of 2.3 Torr.(a) Pure Xe, (b) pure Ar and (c) Xe–Ar gas mixture.

Fig. 3. (a) Comparison of spectrums at the pressure of 2.3 Torr in the pure Xe (blackdotted line), the pure Ar (red dashed line) and the Xe–Ar gas mixture (blue solidline). (b) Partial enlarged view of the spectrum from the harmonic H29–H37 in thepure Ar and in the Xe–Ar gas mixture. (For interpretation of the references to colorin this figure legend, the reader is referred to the web version of this article.)

F. Lu et al. / Optics & Laser Technology 57 (2014) 145–148146

4. Analysis and discussions

Using the cutoff law of single atom response [4], the cutoffposition of harmonic is the H31 in Xe and the H33 in Ar with the IRintensity of 1.9�1014 W/cm2. However, in our results, the cutofforder in Xe is harmonic H25. The disappeared orders can beattributed to the increased phase mismatch which is caused by thehigh gas dispersion from Xe. Furthermore, ionization-induceddefocusing by a great number of the free electrons from Xe isanother factor. Since the conversion efficiency of the harmonics incutoff region is sensitive to the peak laser intensities, the cutoffposition of harmonic should be influenced by a large number ofelectrons in mixed gases [22]. The strong laser defocusingdecreases the harmonic orders.

Assuming that the harmonics in the Xe–Ar gas mixture isgenerated from the Ar atoms, the disappearance of orders comesfrom the phase mismatch induced by free electrons. If the hypothesisis true, we can obtain the same cutoff orders in the pure Ar. Thus, weexamine the harmonic spectrum from the pure Ar at the pressure of1.2 Torr, 2.3 Torr (Fig. 5). Furthermore, the harmonic spectrum in theXe–Ar gas mixture at the pressure of 2.3 Torr is added for comparison.As can be seen the harmonic order remains unchanged in Ar at thepressure of 1.2 Torr. The harmonic spectrum from Ar exhibits thesimilar profile. The harmonic intensities are strong in the low orders

region and weak in the cutoff region. Therefore, another factor leads tothe harmonic generation in the Xe–Ar gas mixture.

Two factors result in the extension of the harmonic orders in theXe–Ar gas mixture. For one thing, the photon energy of harmonic H11is 17.08 eV which is very close to the 5p–6d transition energy of Xe(17.16 eV). For another, the laser electric field results in the Starkbroaden of energy level. The ionization energy of Ar and Xe atoms issimilar. In the strong peak laser intensity, both atoms in the gasmixture are ionized almost simultaneously. Then, the harmonics fromdifferent atoms are emitted. The single photon absorption of the XUVradiation significantly decreases the high ionization threshold of otheratomics. Therefore, with the help of harmonic H11, the electron fromXeþ is tunneled to the continuum state through an intermediate stateeasily. In our experiments, the cutoff position of harmonics in themixed gases is the harmonic H37. The result agrees with the predictedcutoff order generated from Xeþ . Therefore, the harmonics H19–H37contains the harmonics from Xeþ in the gas mixture.

In addition, we observe that the harmonics near the cutoffregion are enhanced and those in the plateau region are sup-pressed from the Xe–Ar gas mixture in Fig. 3. Generally, theharmonics spectrum from the pure Xe or pure Ar shows the nosuppressed harmonic orders. The minimum in HHG usually comesfrom the atomic electronic structure or the electronic dynamicsinduced by the laser field. The former is known as Cooperminimum [23]. The Cooper minimum observed in Ar and Xe isaround 50 eV and 200 eV respectively [24], which is disagreeswith our experimental results. In our experiment, the harmonicsfrom Xeþ overlaps the harmonics from Ar in mixture case.

Let us consider the harmonic from gas A, gas B, and the gasmixture AþB. In the case of gas mixture, the intensity of the totalfield is

IAþBðωÞ ¼ jdAðωqÞj2þjdBðωqÞj2þ2Re½dAðωqÞdn

BðωqÞ� ð1Þwhere dAðωqÞ and dBðωqÞ are the harmonic amplitude at the site A andB respectively. The interference term is 2Re½dAðωqÞdn

BðωqÞ�. In themixed gases, the equation can be simplified as [13]

IAþBðωqÞpρ2AjdAðωqÞj2j1þr=ð1�rÞjdBðωqÞ=dAðωqÞjeiðφBðωqÞ�φAðωqÞÞj2� IpropðωqÞ

ð2Þwhere r¼ ½1þðdBðωqÞ=dAðωqÞÞ��1, φBðωqÞ�φAðωqÞ is the relativeharmonic phase from gas A and gas B, IpropðωqÞ is the dimensionlesspropagation term in gas mixture. The interference effect is decided bythe relative phase of harmonics from different gases. If the relativephase is the ð2nþ1Þπ, the destructive interference can be achieved.In Ref. [13], the harmonics interference is observed in a mixed gas ofHe and Ne. It is attributed to the modulation of the chirped intrinsicphases of harmonics from He atoms and Ne atoms. The variedintensities are attributed to the harmonics interference generatedfrom the different medium. Therefore, in our results, we attribute thesuppressed harmonics to the destructive interference of the harmonicsfrom different atoms.

5. Conclusion

In conclusion, we have experimentally shown the harmonicradiation and its spectral properties in Xe–Ar gas mixture in tightfocusing limit. The cutoff position is extended from harmonic H33to H37. The extended orders are attributed to the harmonicradiation from Xeþ . Since the XUV radiation from Ar atoms isbeneficial to single photon absorption of Xe. In the gas mixture,the suppressed harmonics in the platform are attributed to thedestructive interference of the harmonics. Our results indicatethat the coherent XUV source may be generated in the Xe–Ar gasmixture.

Fig. 4. The harmonic spectrums at the pressure of 0.5 Torr (black dashed line),2.3 Torr (blue solid line), and 3.8 Torr (purple short dashed line) in the Xe–Ar gasmixture. The inset shows the harmonics from H33 to H37 for clarity. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

Fig. 5. The comparison of the HHG spectrums. The dark yellow dash dotted lineshows the harmonics spectra at the pressure of 1.3 Torr in the pure Ar. The reddashed line shows the harmonics spectra at the pressure of 2.3 Torr in the pure Ar.The blue solid line shows the harmonics spectra at the pressure of 2.3 Torr in theXe–Ar gas mixture. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

F. Lu et al. / Optics & Laser Technology 57 (2014) 145–148 147

Acknowledgment

Project supported by the National Natural Science Foundationof China (Grant no. 10774033).

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