Phase-matched high-order harmonic generation in a gas cell with midinfrared femtosecond pulses Yuxi Fu, 1,2 Hui Xiong, 1 Han Xu, 1 Jinping Yao, 1,2 Yongli Yu, 1,2 Bin Zeng, 1,2 Wei Chu, 1,2 X. Liu, 3 J. Chen, 4,5 Ya Cheng, 1, * and Zhizhan Xu 1,† 1 State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China 2 Graduate University of Chinese Academy of Sciences, Beijing 100049, China 3 State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China 4 Center for Applied Physics and Technology, Peking University, Beijing 100084, China 5 Institute of Applied Physics and Computational Mathematics, P.O. Box 8009 (28), Beijing 100088, China Received 15 September 2008; published 6 January 2009 We experimentally investigate the generation of high-order harmonics in a 4-mm-long gas cell using mid- infrared femtosecond pulses at various wavelengths of 1240 nm, 1500 nm, and 1800 nm. It is observed that the yield and cutoff energy of the generated high-order harmonics critically depend on focal position, gas pressure, and size of the input beam which can be controlled by an aperture placed in front of the focal lens. By optimizing the experimental parameters, we achieve a cutoff energy at 190 eV with the 1500 nm driving pulses, which is the highest for the three wavelengths chosen in our experiment. DOI: 10.1103/PhysRevA.79.013802 PACS numbers: 42.65.Ky, 42.65.Re I. INTRODUCTION As a potential table-top coherent extreme ultraviolet euv light source, high-order harmonic generation HHG has been attracting significant attention since it was discovered nearly two decades ago 1. In most cases, high-order har- monics are generated with regeneratively amplified Ti:sap- phire laser systems based on chirped pulse amplification at 800 nm wavelength, which can offer multimillijoule fem- tosecond laser pulses at multi-kHz repetition rates. However, it has been realized that the use of longer wavelengths e.g., midinfrared wavelengths, 1 m  5 m will bring sev- eral important benefits for HHG such as extended cutoff en- ergy and reduced chirp on harmonics, which can both result in shorter attosecond pulses 2–4. Recently, the rapid devel- opment of ultrafast laser technology has enabled construction and commercialization of high-power femtosecond optical parametric amplifiers OPAs, which are able to offer milli- joule level pulses at midinfrared wavelengths 3–7. Previ- ously, by use of midinfrared pulses at wavelengths of 1.5 and 2 m, harmonic cutoff energies had been extended to 160 eV 3 and 220 eV 4, respectively. Even more re- cently, fine interference fringes formed in high-order har- monic spectra were also observed with midinfrared driving pulses at 1240 nm wavelength 5, indicating the existence of new physical mechanisms for HHG driven by midinfrared pulses. In this work, we carry out the HHG experiment in a 4-mm-long gas cell using midinfrared femtosecond pulses at various wavelengths of 1240 nm, 1500 nm, and 1800 nm. The use of the gas cell offers a longer interaction length in comparison with a gas nozzle, which could facilitate an en- hanced phase matching. On the other hand, as compared with a hollow waveguide 8,9, the use of the gas cell makes it possible to employ a tight focusing geometry in order to achieve sufficiently high peak intensity at a limited pulse energy e.g., 0.5 mJ of the midinfrared beam from an optical parametric amplifier OPA. We found that the harmonic yield and cutoff energy are sensitive to a series of experi- mental parameters, such as the focal position in the gas cell, the gas pressure, and the beam size of the input pulses con- trollable by adjusting the size of an annular aperture placed in front of the focal lens. By optimizing these parameters, we demonstrate that a HHG cutoff energy of 190 eV in argon can be achieved with the 1500 nm driving pulses. II. EXPERIMENT The experimental setup is shown in Fig. 1. Wavelength- tunable midinfrared laser pulses are generated by an optical parametric amplifier OPA, TOPAS-C, Light Conversion, * ycheng-45277@hotmail.com † zzxu@mail.shcnc.ac.cn FIG. 1. Color online Schematic of experimental setup for gen- eration of high-order harmonics with midinfrared driving pulses. PHYSICAL REVIEW A 79, 013802 2009 1050-2947/2009/791/0138026 ©2009 The American Physical Society 013802-1