We demonstrated the intensity enhancement of a single high-order harmonic at a wavelength of 37.67 nm using the lowly ionized antimony laser-ablation plume. The conversion efficiency of this harmonic was 2.5×10-5 and the output energy was 0.3 μJ. Such an enhancement of single-harmonic was caused by the multiphoton resonance with the strong radiative transition of the Sb II ions. The cutoff energy of the harmonics generated in Sb plasma was 86 eV (55th harmonic).
© 2007 Optical Society of America
The emission of the high-order harmonics generated during the interaction between the intense laser pulse and nonlinear medium possessed both a good beam quality and ultrashort pulse duration radiation source in the extreme ultraviolet (XUV) and soft x-ray regions. During last fifteen years the high-order harmonic generation (HHG) process [1, 2],improvement of the conversion efficiency , and extension of cutoff energy  have been achieved.
The demonstration of the HHG applications such as the control of electron dynamic in molecules , nonlinear optical phenomena in the XUV region , the photoelectron spectroscopy  and ultrafast dynamics of multiphoton-induced photoelectron emission has recently been reported.
It is obvious that, for the application of the HHG emission, it is important to increase the conversion efficiency of this process. In the past, the conversion efficiency enhancement of the HHG has been demonstrated by controlling the phase-matching condition in a gas-filled capillary  or gas cell . The strongest output energies of harmonics of 7 μJ for the 11th harmonic at the wavelength of 72.7 nm, 4.7μJ for the 13th harmonic at the wavelength of 62.3 nm, and 1μJ for the 15th harmonic at the wavelength of 54 nm were achieved using the xenon gas-cell . Recently, it has been demonstrated that the orthogonal polarized two-color field (fundamental and second harmonic) in helium gas also gives a rise of the conversion efficiency .
An alternative approach for the enhancement of the conversion efficiency is the resonance enhancement of the HHG. The resonance enhancement of the 13th and 15th harmonics has been demonstrated using the argon gas medium . However the harmonics spectrum have not been directly measured using the x-ray spectrometer in previous research. Since the demonstration of the 63rd harmonic generation using the lowly excited, lowly ionized boron plasma at the wavelength of 12.6 nm , the single HHG enhancement at the wavelength of 61.2 nm have been observed in the indium laser-ablation plume for the first time . Then the single HHG enhancement at the wavelength of 46.76 nm has been observed using the tin laser-ablation plume . The conversion efficiencies of these harmonics were estimated to be about 10-4. Such high conversion efficiencies of harmonics were attributed to the multiphoton resonance with strong radiative transitions of In II and Sn II in the laser-ablation plume. Further achievements in single harmonic enhancement were recently demonstrated by controlling the chirp of the pumping laser pulse in Refs.  and , where the 29th, 27th, and 21st harmonics generated from the GaAs, Cr, and InSb plumes considerably exceeded the neighboring harmonics.
In this paper, we present the first observation, to our best knowledge, of a strong enhancement of the single harmonic at the wavelength of 37.67 nm using the antimony laser-ablation plume. The intensity of the 21st harmonic at the wavelength of 37.67 nm was 10 times higher than that of the 23rd and the 19th harmonics. The output energy of this harmonic is measured to be 0.3 μJ. The origin of this enhancement is attributed to the resonance with the strong radiative transitions of the Sb II ions. The maximum cutoff wavelength of the harmonics generated from the antimony plasma was 14.45 nm (photon energy: 85.8 eV, cutoff order: 55th).
2. Experimental setup
The schematic of the experimental setup is shown in Fig. 1. The pump laser was a commercial, chirped pulse amplification laser system (Spectra Physics: TAS-10F), whose output was further amplified using a homemade three-pass amplifier at a 10 Hz repetition. The pre-pulse was split from a portion of amplified laser beam by a beam splitter before the pulse compressor. The prepulse energy was 12 mJ with pulse duration of 210 ps. The main pump pulse output at a center wavelength of 795 nm was 12 mJ with pulse duration of 150 fs. The prepulse was focused in a vacuum chamber onto a target surface by a cylindrical lens and produced a laser-ablation plume consisting of the neutrals and lowly charged ions. The line focus size on the target surface was 100 μm × 3 mm and the intensity of pre-pulse was varied between 0.95×1010 W cm-2 to 1.6×1010 W cm-2. Antimony and silver were used as the targets in this experiment. The main pulse was focused into the ablation plume by a spherical 200 mm focal length lens 100 ns after the prepulse irradiation. The intensity of the main pulse inside the plasma plume was 2.5×1015 W cm-2. The generated high-order harmonics were measured by a grazing incidence spectrometer with a gold-coated Hitach 1200 grooves/mm grating. A gold-coated grazing incidence cylindrical mirror was used for the image translation from the target surface to the detector plane. The XUV spectrum was detected using a multichannel plate with a phosphor screen (Hamamatsu, model F2813-22P), and the optical output from the phosphor screen was recorded using a CCD camera (Hamamatsu model C4880).
3. Results and discussion
Figure 2 shows the typical HHG spectra from the antimony and silver laser-ablation plumes at the wavelength of 10 - 30 nm. The spectrum of the femtosecond radiation propagated through the antimony plasma shows high-order harmonics up to the 55th order at the cutoff wavelength of 14.45 nm. The conversion efficiency of the 55th harmonic was measured to be 2×10-7, while the harmonic efficiency in the range of 15th to 27th harmonics was estimated to be 1.2×10-6.The details of the absolute conversion efficiency calibration of the spectrometer were described in Ref. .Using the silver laser-ablation plasma, we observed the same (55th) cutoff harmonic.
Figure 3 shows the HHG spectrum from the antimony laser-ablation plume at the wavelengths of 33 - 60 nm. A strong 21st harmonic at the wavelength of 37.67 nm was obtained. The intensity of the 21st harmonic was 10 times higher than those of the 23rd and 19th harmonics. The conversion efficiency of the 21st harmonic was measured to be 2.5×10-5, and thus the pulse energy of the 37.67 nm radiation of 0.3 μJ was obtained from the pump laser energy of 12 mJ. By changing the pump laser polarization from the linear polarization to the circular one using a quarter-wave plate, the 37.67 nm radiation completely disappeared. This tendency is consistent with that of HHG, which allows concluding that the strong emission at the wavelength of 37.67 nm is generated through the HHG.
In our previous works, the single harmonic enhancement using the indium and tin plumes has been observed. Using the indium laser-ablation, the enhancement of the 13th harmonic at the wavelength of 61.26 nm has been obtained . The 4d105s2 1S0-4d95s25p (2D) 1P1 transition of In II at the wavelength of 62.2 nm, which have an oscillator strength (gf value) of 1.11, can be driven into the resonance with the 13th harmonic by the AC-Stark shift. The gf value of this transition is 10 times higher compared with the other transitions of In II in this spectral range. By changing the laser wavelength from 795 nm to 778 nm, the wavelength of the 13th harmonic was shifted away from this transition, and the intensity of the 13th harmonic considerably decreased. Therefore the enhancement of the 13th harmonic of the 795 nm radiation was explained as being in the resonance with this transition driven by the AC Stark shift. For tin laser-ablation plume, we attributed the intensity enhancement of the 17th harmonic at the wavelength of 46.76 nm due to the resonance with the 4d105s25p2P3/2-4d95s25p2 (1D) 2D5/2 transition at the wavelength of 47.256 nm .
To investigate the mechanism of the 21st harmonic enhancement using antimony plume, the central wavelength of the laser pulse was tuned in the range from 797 nm to 783 nm. Figure 4 shows the intensities of the 19th, the 21st, and the 23rd harmonics as the functions of the pump laser wavelength. By changing the laser wavelength from the longer wavelength side (797 nm) to the shorter one, the intensity of the 21st harmonic initially gradually increased and then abruptly decreased. The highest intensity of the 21st harmonics was observed at 791 nm. At the same time, the intensities of the 19th and the 23rd harmonics remained almost the same at the wavelength of 795-783 nm. In the past work, the strong Sb II transitions of 4d105s22p3P2- 4d95s25p3(2D)3D3 and 4d105s22p1D2-4d95s25p3(2D)3F3 at the wavelengths of 37.82 nm and 37.55 nm, respectively, have been reported and analyzed . The gf values of 4d105s22p3P2- 4d95s25p3(2D)3D3 and 4d105s22p1D2-4d95s25p3(2D)3F3 transitions have been calculated to be 1.36 and 1.63, respectively, which was 6 to 7 times higher than those of the neighbor transitions. The enhancement of the 21st harmonic of the 791 nm radiation was highest in our studies, although the 21st harmonic (λ = 37.67 nm) was slightly away from the 4d105s22p3P2-4d95s25p3(2D)3D3 transition (λ = 37.82 nm). In this case the enhancement of 21st harmonic was due to the resonance with the 4d105s22p3P2-4d95s25p3(2D)3D3 transition driven by the AC Stark shift. By changing the pump laser wavelength from 791 to 788 nm, the intensity of the 21st harmonic gradually decreased because this harmonic becomes away from the 4d105s22p3P2-4d95s25p3(2D)3D3 transition. However the enhancement of the 21st harmonic of the 785 nm radiation was higher than that of 788 nm radiation. The reason of this intensity enhancement was attributed to the resonance with the 4d105s22p1D2-4d95s25p3(2D)3F3 transition driven by the AC Stark shift. Further decrease of the 21st harmonic wavelength led to the mismatching with the above transitions. As a result, the intensity of the 21st harmonic pumped by the wavelength of 783 nm was considerably lower.
One of the options for varying the harmonic distribution in the plateau region is the tuning of the spectrum of driving laser radiation, which was demonstrated for harmonic enhancement in this work, as well as our previous work [13, 14]. Another approach to tune the harmonic wavelength without changing the driving laser spectrum is a control of the chirp of the fundamental radiation, which was varied by adjusting the distance between the gratings in the pulse compressor [19, 20]. This technique allows to carry out an express analysis of the availability of the resonance conditions between the harmonic wavelength and the appropriate ionic transitions of the media under investigation. Using such an approach, a strong 21st harmonic radiation (37.8 nm) for chirp-free driving radiation was observed in the case of the InSb plume . It was found that, in the case of positively chirped 140 fs pulses, the intensity of the 21st harmonic of the 793 nm laser radiation exceeded that of the neighboring ones by a factor of 20. This enhancement of the single harmonic belonging to the mid-plateau region considerably diminished in the case of negatively chirped pulses.
In present work, the enhancement in intensity of the single harmonic belonging to the mid-plateau region was smaller compared with the enhancement in the intensity of the 13th harmonic generated from indium plume reported in our previous study . The latter harmonic belongs to the beginning of the plateau distribution in the case of harmonic generation from indium plasma. The intensity enhancement of the 13th harmonic (×200) generated from indium plasma considerably exceeded that for the 21st (×10) harmonic generated from the antimony plasma. The reason of such a difference is perhaps related with the difference in oscillator strengths of the transitions involved
In conclusion, we observed the enhancement of the 21st harmonic by using the antimony laserablation plume as a nonlinear medium. The energy of this harmonic was measured to be 0.3 μJ, and the conversion efficiency was 2.5×10-5. The intensity of this harmonic 10 times exceeded those of the 23rd and the 19th harmonics. The maximum cutoff wavelength of the HHG using antimony laser-ablation plume was 14.45 nm (photon energy: 86 eV). Such an approach can pave the way toward a considerable enhancement of a single harmonic in the short-wavelength range using the appropriate target materials.
H. Kuroda and co-authors gratefully acknowledge support from the Grant-in-Aid for Creative Scientific Research (14GS0206) of Japan Society for the Promotion of Science (JSPS). T. Ozaki acknowledges the support from the Research Foundation for Opto-Science and Technology.
References and links
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