Yusuke Tamaki

Phase-matched high-order harmonic generation by guided intense femtosecond pulses

We describe the phase-matched high-order harmonic generation of femtosecond Ti:sapphire laser pulses a with two different guiding methods. Generation efficiency of the high-order harmonic was improved by phase-matched propagation in the guiding channel. More than 100-fold enhancement around the 25th harmonic (32 nm) was obtained with a 3-cm-long Ar-filled hollow fiber. The harmonics around the 49th harmonic (16 nm) were also enhanced by two orders of magnitude compared to those in the plateau with a 7-mm-long, self-guided pulse in Ne. High-harmonic conversion efficiency of 10/sup -6/ was obtained by phase-matched propagation in Ne, producing >nJ harmonics in the cutoff region around the 49th harmonic. In both experiments, the harmonics near the cutoff region were preferentially enhanced. The results are well explained by considering both the intrinsic phase based on the single-atom response and the macroscopic phase matching in the high-intensity interaction region. The flat intensity profile achieved in the guided structure is considered to clearly manifest the intrinsic phase behavior of the harmonics.

Second-harmonic generation of femtosecond high-intensity Ti:sapphire laser pulses

The second-harmonic generation (SHG) of ultrashort Ti:sapphire laser pulses in potassium dihydrogen phosphate crystal in type-I phase-matching geometry has been investigated theoretically, including the effects of cubic nonlinearity. It is found that the phase mismatch due to the broad bandwidth associated with the short pulse width limits the maximum conversion efficiency to less than 60%, and the temporal shape of the converted pulse has an intensity modulation at an incident intensity of 100 GW/cm2 for a 100 fs pulse. In order to increase the energy conversion efficiency and improve the temporal pulse shape, a new SHG geometry using two antiparallel tilted crystals is discussed.

Second-Harmonic Generation from Intense, 100-fs Ti:Sapphire Laser Pulses in Potassium Dihydrogen Phosphate, Cesium Lithium Borate and β-Barium Metaborate

Second-harmonic generation of femtosecond, intense Ti:sapphire laser pulses is described in three 1-mm-thick nonlinear crystals of KDP (KH2PO4), CLBO (CsLiB6O10) and BBO (β-BaB2O4) in type I phase-matching geometry. The phase-matching bandwidth is insufficient for BBO. Spectral modulation due to cubic nonlinearity becomes significant at 200 GW/cm2 for KDP and 30 GW/cm2 for CsLiB6O10. The energy conversion efficiencies obtained at incident intensities where the spectral modulations scarcely change the temporal profile of the second harmonic were 20% for both the crystals. Numerical calculations including the effects of cubic nonlinearity and group-velocity mismatch explained the experimental results. It is found that a 1-mm KDP crystal gives the most efficient conversion for high-power, 100 fs pulses.

Highly Coherent Soft X-Ray Generation by Macroscopic Phase Matching of High-Order Harmonics

Highly coherent high-order harmonics in the soft X-ray region are generated by macroscopic phase matching. The high fringe visibility, which indicates the degree of coherence, of 0.73 is obtained at the 27th harmonic of a Ti:sapphire laser (the corresponding wavelength is 29.6 nm) by optimizing the 10-mm phase-matched propagation in 30 Torr argon gas, while the visibility of the pump laser is 0.84.

Generation of highly efficient self-pumped phase conjugation femtosecond pulse using photorefractive BaTiO3:Co crystal

Abstract The generation of highly efficient self-pumped phase conjugate (SPPC) pulse of 180 fs, 810 nm Ti:sapphire laser at 76 MHz in a photorefractive BaTiO 3 :Co crystal is demonstrated. Brewster prisms are inserted in front of the photorefractive crystal to make wide bandwidth of the femtosecond pulse spatially separated in order to remove an influence of erasure caused by beam fanning and to avoid spectrum narrowing. A maximum reflectivity is improved from 1.5% without prisms to 6.6% with prisms. The spectral bandwidth of the phase conjugate pulse is also improved from 2.7 to 3.8 nm. The spectral bandwidth of the phase conjugate pulse with prisms measured is approaching 5.1 nm of the incident laser pulse, whereas the phase conjugate pulse without prisms is narrower. Since the refractive index grating that contributes to the generation of the SPPC pulse is only the transmission grating, therefore, the rise-time behavior of the SPPC pulse is smoother than that of continuous wave.

Highly coherent high-order harmonic generation by macroscopic phase matching

Summary form only given. A laser pulse is produced from a Ti:sapphire laser based on a chirped-pulse amplification technique. A Michelson interferometer is set before a pulse compressor, and a reflecting mirror in one arm is slanted by 80 /spl mu/rad. Laser pulses are divided into two, and then compressed to have a pulse width of 30 fs. The phase matching is achieved by controlling the medium density to 30 torr. The generated high harmonics are overlapped in the far field, forming an interference pattern.

Phase-matched high-order-harmonic generation by self-guided intense femtosecond laser pulses

We describe the phase-matched high-order-harmonic generation of femtosecond Ti:sapphire laser pulses in a self-guiding channel. Generation efficiency of the high-order harmonic was improved by phase-matched propagation in the guiding channel. The harmonics around the 49th harmonic (16 nm) were enhanced by two orders of magnitude compared to those in the plateau with a 7-mm-long, self-guided pulse in Ne. High- harmonic conversion efficiency of 10-6 was obtained, producing > nJ harmonics in the cutoff region around the 49th harmonic. The results are well explained by considering both the intrinsic phase based on the single-atom response and the macroscopic phase matching in the high-intensity interaction region.

Experimental optimization of phase-matched high-order-harmonic generation

Summary form only given. High-order-harmonic generation (HHG) has being widely studied because it is a promising method for a coherent XUV source with high peak power, short temporal width, and high brightness from a compact laser system. Such XUV sources will be used in many areas of physics and chemistry. In this study, we show the clear evidence of macroscopic phase-matching by changing a interaction length as well as an atomic density.

Phase-matched high-order-harmonic generation by guided intense femtosecond laser pulses

We present the experimental results of high-order harmonic generation by guided intense femtosecond Ti:sapphire pulses, using a rare-gas filled hollow fiber and a self-guided channel. Generation efficiency of the high-order harmonic was improved by phase-matched propagation in the guiding channel. More than 100-fold enhancement around the 25th harmonic (32 nm) was obtained with a 3-cm-long, Ar-filled hollow fiber. The harmonics around the 49th harmonic (16 nm) were also enhanced by two orders of magnitude compared to those in the plateau with a 7-mm-long, self-guided pulse in Ne. High-harmonic conversion efficiency of 10/sup -6/ was obtained by phase-matched propagation in Ne, producing > nJ harmonics in the cutoff region around the 49th harmonic. In both experiments, the harmonics near the cutoff region were preferentially enhanced. The results are well explained by considering both the intrinsic phase based on the single-atom response and the macroscopic phase matching in the high-intensity interaction region.

Phase-matched high-order-harmonic generation by guided femtosecond pulses

Summary form only given. High harmonic generation (HHG) in an ionizing medium has distinguishing characteristics among the methods of the generation of coherent extreme ultraviolet light. The shortest wavelength has reached into the water window region. The high-harmonic pulsewidth were measured to be shorter than the driving laser pulse width. Recently, a guiding technique with an Ar-filled hollow fiber has been applied to HHG in order to control the phase matching condition, resulting in 100 times enhancement around the 25th harmonic. There remains the question of whether any harmonic can be enhanced by the macroscopic phase matching in the fiber. In order to clarify these issues, we made an experiment where Ar was replaced by Ne which was expected to generate shorter wavelength. In spite of various differences of experimental condition, harmonics near the cutoff order were also enhanced by two orders of magnitude in Ne, giving the similar intensity distribution to one obtained with a Ar-filled hollow fiber. In this paper, we report the phase-matched HHG by guided intense femtosecond pulses, using a hollow fiber-guided and self-guided laser pulses. Based on the experimental results, macroscopic and microscopic phase matching of high-order harmonics are discussed.