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ABSTRACT: We report on a systematic investigation of the influences of gas pressure, focal position and focusing geometry on high harmonic
generation by use of mid-infrared femtosecond laser pulses. We also discuss the spatial characteristics of harmonics under
different focusing conditions. By optimizing the parameters, we experimentally observed the generation of 1 kHz, low divergence
coherent X-ray beams in the water-window region.
Keywordsmid-infrared laser pulses-high harmonic generation-water window
Science China: Physics, Mechanics and Astronomy 04/2012; 53(6):1054-1059. · 0.78 Impact Factor
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Jinping Yao,
Ya Cheng,
J. Chen,
Haisu Zhang, Han Xu,
Hui Xiong,
Bin Zeng,
Wei Chu,
Jielei Ni,
X. Liu,
Zhizhan Xu
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ABSTRACT: We theoretically demonstrate the generation of wavelength-tunable, narrow-bandwidth extreme-ultraviolet (xuv) radiation by high-order harmonic generation (HHG) driven by an orthogonally polarized two-color laser field, which is composed of a 10-fs, 1500-nm laser pulse and a 40-fs, 2400-nm laser pulse. By performing classical analysis, we reveal that the rapid change of electron wave-packet dynamics at a subcycle time scale confines high-order harmonic emission to a small spectral region, leading to the generation of narrow-bandwidth xuv radiation. Furthermore, the central wavelength of the xuv radiation can be continuously tuned over a wide range by changing either the peak intensity of the driver laser or the amplitude ratio between the two laser fields at different wavelengths. It is also verified that the use of driver pulses at longer wavelengths leads to a better spectral confinement of the xuv radiation.
Phys. Rev. A. 03/2011; 83(3).
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ABSTRACT: We report on the fabrication of hollow microfluidic channels with a circular cross-sectional shape embedded in fused silica by spatiotemporally focusing the femtosecond laser beam. We demonstrate both theoretically and experimentally that the spatiotemporal focusing of femtosecond laser beam allows for the creation of a three-dimensionally symmetric spherical intensity distribution at the focal spot.
Optics Letters 04/2010; 35(7):1106-8. · 3.40 Impact Factor
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ABSTRACT: We report on an experimental study on the wavelength-scaling law of elliptical-polarization dependence of high-order harmonic generation. In the elliptical-polarization dependence measurements, we keep an identical peak intensity for pump pulses with carrier wavelength ranging from 800 nm to the mid-IR region, and the harmonic emission near the cutoff region is selected for comparison. We find that the experimental measured wavelength scaling of elliptical-polarization dependence could be well fitted with a function (epsilon(1/2) proportional to lambda(-1)) predicted by semiclassical model.
Optics Letters 02/2010; 35(4):472-4. · 3.40 Impact Factor
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Jinping Yao,
Yao Li,
Bin Zeng,
Hui Xiong, Han Xu,
Yuxi Fu,
Wei Chu,
Jielei Ni,
Xiaojun Liu,
J Chen,
Ya Cheng,
Zhizhan Xu
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ABSTRACT: We theoretically investigate the high-order harmonic generation (HHG) in helium using a two-color laser field synthesized by an intense 25-fs laser pulse at 800 nm and a relatively weak ∼ 43-fs laser pulse at 1400 nm. When the polarization between the two pulses is arranged at an angle of ∼ 73 • , supercontinuum spectra are dramatically broadened to 180 eV, which is sufficient to support an isolated ∼73-as pulse without any phase compensation. The physical mechanisms behind the phenomenon are well explained in terms of quantum and classical analyses. Furthermore, in the long-pulse regime, this method of extending the supercontinuum spectrum shows the significant advantage over previous two-color HHG schemes.
01/2010; 82.
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ABSTRACT: We experimentally demonstrate that by use of a phase plate that is placed between an adjustable aperture and a focusing lens, the length of the filament can be dramatically extended in air with a femtosecond laser pulse. In addition, the far-field beam profile captured after the filament indicates that the supercontinuum is strongly confined on axis, and a single filament appears to be attainable at relatively high input pulse power when the phase plate is used.
Optics Letters 12/2009; 34(23):3752-4. · 3.40 Impact Factor
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Hui Xiong, Han Xu,
Yuxi Fu,
Jinping Yao,
Bin Zeng,
Wei Chu,
Ya Cheng,
Zhizhan Xu,
Eiji J Takahashi,
Katsumi Midorikawa,
X Liu,
J Chen
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ABSTRACT: We demonstrate the generation of a coherent x ray in the water window region in a gas cell filled with neon gas using a wavelength-tunable mid-IR femtosecond laser operating at 1 kHz repetition rate. The cutoff energy and conversion efficiency of the water window x ray can be optimized by tuning gas pressure as well as the focal position.
Optics Letters 07/2009; 34(11):1747-9. · 3.40 Impact Factor
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ABSTRACT: We theoretically demonstrate the generation of an intense single attosecond pulse by superposing a weak sub-harmonic pulse upon a sine-waveform few-cycle driving pulse. By use of a sine-waveform few-cycle pulse instead of its traditionally used cosine waveform counterpart, we show that efficient tunnel ionization for generating electrons which can revisit their parent ion with high kinetic energy can occur only once in the few-cycle laser field, leading to an increase of efficiency by nearly two orders of magnitude in single attosecond pulse generation as compared with the use of a cosine-waveform field.
Journal of Physics B Atomic Molecular and Optical Physics 06/2009; 42(14):145604. · 1.88 Impact Factor
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ABSTRACT: A simple analytical model for intense-laser interaction with low-density plasma is used to investigate the dependence of the wake structure on the laser parameters and the background plasma density. Many of the properties observed in the existing multidimensional particle-in-cell simulations are qualitatively recovered.
Physics of Plasmas 05/2009; 16(5):053107-053107-5. · 2.15 Impact Factor
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ABSTRACT: We experimentally observe the formation of X-waves at fundamental, third harmonic, and fifth harmonic wavelengths by infrared (central wavelength at ∼ 1500 nm) femtosecond laser pulse filamentation in air. By fitting the angularly resolved spectra of the fundamental and harmonic waves using X-wave relations, we confirm that all the X-waves have nearly the same group velocity, indicating that they are locked in space and time during their propagation in filament.
Applied Physics Letters 12/2008; 93(24):241104-241104-3. · 3.84 Impact Factor
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ABSTRACT: We experimentally investigate the high-order harmonic generation in argon gas using a driving laser pulse at a center wavelength of 1240 nm. High-contrast fine interference fringes could be observed in the harmonic spectra near the propagation axis, which is attributed to the interference between long and short quantum paths. We also systematically examine the variation of the interference fringe pattern with increasing energy of the driving pulse and with different phase-matching conditions.
Phys. Rev. A. 09/2008; 78(3).
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ABSTRACT: The authors report the measurement of the angularly resolved spectrum of the third harmonic generated in a femtosecond filament in air and its evolution with increasing pump power. Pumped by a focused infrared ultrashort pulse with a carrier wavelength of 1270 nm, a pulse duration of ∼ 20 fs, and pulse energy up to 487 μJ, the generated third harmonic is composed of an on-axis emission and a conical ring emission. When the pump power is sufficiently high, angularly resolved spectra with significant X-like feature could be observed, indicating the formation of nonlinear X wave at third harmonic.
Applied Physics Letters 01/2008; 92(1):011111-011111-3. · 3.84 Impact Factor
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ABSTRACT: Ponderomotive force driven acceleration of an electron at the focus of a high-intensity short-pulse laser is considered. Accounting for the asymmetry of acceleration and deceleration due to the evolution of the Gaussian laser beam waist, the energized electron is extracted from the laser pulse by the longitudinal ponderomotive force. It is shown that an electron's energy gain in the range of MeV can be realized for laser intensities above 10(19) W microm(2)/cm(2). Final energy gain as a function of the scattering angle and the electron's initial position has also been discussed.
Physical Review E 11/2003; 68(4 Pt 2):046407. · 2.26 Impact Factor
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Yongli Yu,
Junjie Xu,
Yuxi Fu,
Hui Xiong, Han Xu,
Jinping Yao,
Bin Zeng,
Wei Chu,
J Chen,
Ya Cheng,
Zhizhan Xu
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ABSTRACT: We theoretically investigate high-order harmonic generation from the diatomic molecules in an orthogonally polarized two-color few-cycle optical field by controlling the incident angle of the recolliding electron. We show that the combination of molecules and two-color polarization gating allows us to significantly suppress undesirable satellite pulses owing to the orientation dependence of high-order harmonic generation in mol-ecules, giving rise to the generation of single attosecond pulses.
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Yuxi Fu,
Hui Xiong, Han Xu,
Jinping Yao,
Yongli Yu,
Bin Zeng,
Wei Chu,
X Liu,
J Chen,
Ya Cheng,
Zhizhan Xu
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ABSTRACT: 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.
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ABSTRACT: We experimentally investigate the evolution of an angularly resolved spectrum of third harmonic generated by infrared femtosecond laser pulse filamentation in air. We show that at low pump intensity, phase matching between the fundamental and third-harmonic waves dominates the nonlinear optical effect and induces a ring structure of the third-harmonic beam, whereas at high pump intensity, the dispersion properties of air begin to affect the angular spectrum, leading to the formation of a nonlinear X wave at third harmonic.
Phys. Rev. A. 77(4).
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Wei Chu,
Mingyan Wu,
Bin Zeng,
Jinping Yao,
Jielei Ni,
Hui Xiong, Han Xu,
Zhiyang Lin,
Huipeng Kang,
Wei Quan,
J. Chen,
X. Liu,
Ya Cheng,
Zhizhan Xu
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ABSTRACT: We present experimental and theoretical evidence on the breakdown of the simple man's model in the high-energy recollision region by examining both the energy-resolved angular distribution of photoelectrons in above-threshold ionization and the spectrum of high-order harmonics generated in elliptically polarized driver fields. With the help of a semiclassical model calculation, we show that, surprisingly, even for the high-energy recollision electrons, their wave-packet dynamics can be influenced strongly by the Coulomb field.
Phys. Rev. A. 85(2).
