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Janulewicz, K A,Kim, C M Published by the American Physical Society through 2010 Physical review. E, Statistical, nonlinear, and so Vol.82 No.5
<P>Soft x-ray lasers pumped in the grazing incidence geometry show strongly reduced energetic needs but hardly changed conversion efficiency between the pump energy and the output short-wavelength radiation. Numerical analysis presented in the paper concerns with performance of a Ni-like Ag soft-x-ray laser pumped by a triple-pulse structure in the grazing incidence geometry as a function of the puming conditions. It was found that a weak precursor preceding the main preforming and heating pulses by a few nanoseconds is crucial for the energy deposition. Its presence enables in different arrangements a reasonable reduction in the pump energy and relaxation of the steep density gradients as well as a control over partition of the deposited energy. As a consequence, it was concluded that a well energetically balanced three- or multipulse composition seems to be a reasonable way to achieve performance improvement.</P>
Janulewicz, K A,Kim, C M,Stiel, H Optical Society of America 2013 Optics express Vol.21 No.3
<P>It is shown that the statistics of the intensity distribution in the output beam of a collisional X-ray laser, analysed in terms of the degree of freedom or equivalently the number of the coherence modes in the beam cross-section, has non-Gaussian character. The non-Gaussian character seems to be caused by the small-scale plasma/medium fluctuations. It was assumed that these overlap the modal structure imposed by the geometry of the medium and considered as equivalent to a large-scale inhomogeneity. Thus, the fluctuations decide about the character of the output beam transverse coherence. It is also shown that the relevant to this model compound statistics of the intensity fluctuations in the output beam is well described by the m-m-distribution, a specific form of the K-distribution. The deviation from the Gaussian statistics was confirmed by the field correlation function at the laser exit plane, retrieved from the experimental data.</P>
Structural transformations in femtosecond laser-processed <i>n</i>-type 4H-SiC
Rehman, Z.U.,Janulewicz, K.A. Elsevier 2016 APPLIED SURFACE SCIENCE - Vol.385 No.-
<P><B>Abstract</B></P> <P>We present a comprehensive study of morphological modification induced on and below the surface of <I>n</I>-type 4H-SiC by irradiation with femtosecond laser pulses under tight focusing condition. Spectroscopic investigation of local electronic and structural transformations in SiC-micro/nanostructures suggested bond breaking i.e. transformation of crystalline SiC to amorphous silicon (<I>a</I>-Si) and amorphous carbon (<I>a</I>-C). These observations were augmented by investigations applying atomic force microscopy (AFM), micro-photoluminescence (PL) spectroscopy, and high resolution X-ray diffraction (HRXRD). A high-resolution cross-sectional study of laser-modified region with transmission electron microscope (TEM) showed a thin amorphous layer in the vicinity of the geometrical focus with deformations and stacking faults in the sub-surface area. Having considered the existing ablation theories, a complex interplay of fast laser heating followed by melting and rapid re-solidification as well as dynamic relaxation of the laser-induced stresses seems to be responsible for formation of the observed structural changes.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Photoluminescence of SiC surface treated with a burst of femtosecond laser pulses. </LI> <LI> Compositional 2-D mapping of the post-ablated area. </LI> <LI> HRXRD and the rocking curve of the crystalline SiC after femtosecond irradiation. </LI> <LI> Sample morphology after a high-fluence irradiation. </LI> </UL> </P>
Structural transformation of monocrystalline diamond driven by ultrashort laser pulses
Rehman, Z.U.,Janulewicz, K.A. Elsevier 2016 Diamond and related materials Vol.70 No.-
<P>We report on morphological and phase transformations in monocrystalline diamond (type-IIa) caused by tightly focused femtosecond laser pulses. Laser-induced periodic surface structure (LIPSS) of high spatial frequency was observed under multiple-pulse surface irradiation. An average spatial periodicity of 140 +/- 15nm was independent of the incidence fluence. Compositional analysis performed to evaluate the structural properties of the irradiated region showed existence of graphite and amorphous carbon. The cross-sectional analysis of the area with use of a transmission electron microscopy (TEM) revealed a thin layer of amorphous carbon with a thickness of a few tens of nanometers, capping the monocrystalline diamond. A more detailed inspection of the internal structure within the layer with a high-resolution TEM (HRTEM) highlighted presence of different nano-graphitic structures, immersed in the amorphous carbon and giving in this way diamond-like-carbon (DLC). Formation of two graphite sheets from three diamond planes (111) was revealed as the main transformation process at the interface. Generally, graphite sheets tended to have diverse curved forms. The sub-layer area consisted of defected crystalline diamond with few cracks due to mechanical force exerted at the high laser fluence. (C) 2016 Elsevier B.V. All rights reserved.</P>
Optimization of a laser plasma x-ray source for ultrafast x-ray absorption spectroscopy
Iqbal, M.,Ijaz, M.,Noh, D. Y.,Janulewicz, K. A.,Stiel, H.,Nickles, P. V. 한국물리학회 2017 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol. No.
<P>We present optimization of laser plasma x-ray experimental conditions for ultrafast x-ray absorption spectroscopy measurements on broad range of transition metal oxides. First, the x-ray flux generated from a laser plasma source was optimized with an emphasis on the Bremsstrahlung by investigating the influence of the angle of the incidence of laser beam on a Cu tape target. The x-ray flux emitted in both the front and transmitted side of the target was found to be optimal at the incident angle of 15 similar to 25 degrees. Moreover the manipulation of the Bremsstrahlung peak energy by the laser focus distribution was discussed. In addition to the source optimization, we present a scheme to find the time-delay zero position in a pump-probe experiment together with a normalization scheme for x-ray source fluctuations. As a feasibility check, we present the transmitted spectra of two materials, Ni and NiO.</P>
I Jong Kim,최일우,Karol Adam Janulewicz,Jongmin Lee 한국광학회 2009 Current Optics and Photonics Vol.13 No.1
High-order harmonic generation from a solid surface affected by the contrast of a laser pulse was studied using an oscillating mirror model. High-order harmonics generated from solid surfaces have unusual properties such as spectral redshift, and an intensity difference between even- and odd-order harmonics which is not reported for high-order harmonics generated by a gas medium. We confirmed that high-order harmonics from solid surfaces have selectivity of polarization as well as cut-off extension and the enhancement of conversion efficiency proportional to laser intensity. And the principle of operation and the characteristics of a plasma mirror system developed for achieving high contrast laser pulses to pursue the experimental realization of high-harmonic generation from solid surfaces are reported. Energy fluence on the plasma mirrors is tunable between 10 J/cm2 and 1000 J/cm2 and around 1000 shots are available before the plasma mirrors require replacement.