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Choi S.-H.,Kyungpook National University | Sohn I.-B.,Advanced Photonics Research Institute | Lee H.,Kyungpook National University
International Journal of Precision Engineering and Manufacturing | Year: 2012

A femtosecond laser has been used to produce spatially confined micromachining on mold stainless steel STAVAX. We investigated femtosecond laser-induced line structuring on mold stainless steel STAVAX using the laser scanning mode with various scanning speeds, various fluences and two polarization configurations. The damage threshold fluence of single pulse irradiation (F th) was measured to be about 80 mJ/cm 2. The damage threshold fluence of the scanning mode was approximately identical (0.75 F th~1.25 F th) to that of the single pulse irradiation. However, a fluence of 2.50 F th was necessary to induce continuous line structures for high scanning speeds and a fluence of 1.25 F th was required to induce continuous line structures for low scanning speeds. The width of laser-machined lines increased with the irradiation fluence and decreased with the scanning speed regardless of the polarization configuration. The formation of laser-induced periodic structures (termed "ripple") was examined using various laser parameters. It was clear that the formation of the ripple was governed by the irradiation fluence and the scanning speed of the beam. We were able to produce nano-scale structures in large areas by employing multiple line irradiations with the laser beam. © KSPE and Springer 2012.

Vukovic D.,Technical University of Denmark | Vukovic D.,University of Sydney | Schroder J.,University of Sydney | Schroder J.,RMIT University | And 7 more authors.
Optics Express | Year: 2015

We experimentally demonstrate compensation of nonlinear distortion caused by the Kerr effect in a 3 × 32-Gbaud quadrature phaseshift keying (QPSK) wavelength-division multiplexing (WDM) transmission system. We use optical phase conjugation (OPC) produced by four-wave mixing (FWM) in a 7-mm long silicon nanowire. A clear improvement in Q-factor is shown after 800-km transmission with high span input power when comparing the system with and without the optical phase conjugation module. The influence of OSNR degradation introduced by the silicon nanowire is analysed by comparing transmission systems of three different lengths. This is the first demonstration of nonlinear compensation using a silicon nanowire. © 2015 Optical Society of America.

Yun H.,KAIST | Kim H.T.,Advanced Photonics Research Institute | Kim C.M.,Advanced Photonics Research Institute | Nam C.H.,KAIST | Lee J.,Advanced Photonics Research Institute
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We investigated the characteristics of molecular orientation induced by a nonresonant two-color femtosecond laser field. By analyzing the rotational dynamics of asymmetric linear molecules, we revealed that the critical parameter in characterizing the molecular orientation was the hyperpolarizability of molecules that selected the excitation paths of rotational states between parity-changing and parity-conserving transitions. Especially, in the case of molecules with small hyperpolarizability, a significant enhancement of orientation was achieved at the half-rotational period, instead of the full-rotational period. This deeper understanding of the hyperpolarizability- dependent characteristics of molecular orientation in a two-color scheme can provide an effective method to achieve significantly enhanced field-free orientation for various polar molecules. © 2011 American Physical Society.

Jeong T.M.,ASCR Institute of Physics Prague | Jeong T.M.,Advanced Photonics Research Institute | Jeong T.M.,Korea Basic Science Institute | Weber S.,ASCR Institute of Physics Prague | And 4 more authors.
Optics Express | Year: 2015

The focusing property of a focal spot of a femtosecond laser pulse is presented under tight focusing conditions (below f-number of 1). The spatial and temporal intensity distributions of a focused electric field are calculated by vector diffraction integrals and coherent superposition method. The validity of the calculation method is examined by comparing the intensity distribution obtained under a high f-number condition to that obtained with the fast Fourier transform method that assumes the scalar paraxial approximation. The spatial and temporal modifications under tight focusing conditions are described for a focused femtosecond laser pulse. The calculation results show that a peak intensity of about 2.5×1024 W/cm2 can be achievable by tightly focusing a 12-fs, 10 PW laser pulse with a f/0.5 parabolic optic. The precise information on intensity distributions of a femtosecond focal spot obtained under a tight focusing condition will be crucial in assessing a focused intensity and in describing the motion of charged particles under an extremely strong electric field in ultra-relativistic and/or relativistic laser matter-interaction studies. © 2015 Optical Society of America.

Jung G.B.,Korea University | Cho Y.J.,Korea University | Myung Y.,Korea University | Kim H.S.,Korea University | And 3 more authors.
Optics Express | Year: 2010

THz emission was observed from the vertically aligned silicon nanowire (Si NW) arrays, upon the excitation using a fs Ti-sapphire laser pulse (800 nm). The Si NWs (length = 0.3 ∼9 μm) were synthesized by the chemical etching of n-type silicon substrates. The THz emission exhibits significant length dependence; the intensity increases sharply up to a length of 3 μm and then almost saturates. Their efficient THz emission is attributed to strong local field enhancement by coherent surface plasmons, with distinctive geometry dependence. © 2010 Optical Society of America.

Afzal M.I.,Gwangju Institute of Science and Technology | Alameh K.,Edith Cowan University | Lee Y.T.,Advanced Photonics Research Institute
IEEE Journal on Selected Topics in Quantum Electronics | Year: 2014

We experimentally generate an optical frequency comb with a repetition rate of ∼43 GHz using an all-passive fiber cavity operating in varying normal dispersion regime and pumped by a multimode laser diode. Coupling of four fiber segments with different dispersion coefficients and core diameters along a ring cavity enhances the spectral broadening at a low threshold pump power. Self-induced modulation instability is experienced due to cross phase modulation that results from the interaction of co-propagating optical signals inside the cavity. Stabilization of the two fundamental modes that are phase matched with higher order modes enables the amplification of the weakly modulated stokes and antistokes modes through dissipative four wave mixing, which results in the formation of a frequency comb of constant mode spacing of 0.136 nm. Phase locked frequency modes consisting of ∼250 comb lines covering a spectral range from 966 to 1000 nm centered at 978.512 nm are synthesized and reported for the first time. © 1995-2012 IEEE.

Oh S.Y.,Advanced Photonics Research Institute | Yueh F.-Y.,Mississippi State University | Singh J.P.,Mississippi State University
Applied Optics | Year: 2010

Laser-induced breakdown spectroscopy was applied to quantitative analysis of three impurities in Sn alloy. The impurities analysis was based on the internal standard method using the Sn I 333.062-nm line as the reference line to achieve the best reproducible results. Minor-element concentrations (Ag, Cu, Pb) in the alloy were comparatively evaluated by artificial neural networks (ANNs) and calibration curves. ANN was found to effectively predict elemental concentrations with a trend of nonlinear growth due to self-absorption. The limits of detection for Ag, Cu, and Pb in Sn alloy were determined to be 29, 197, and 213 ppm, respectively. © 2010 Optical Society of America.

Yim S.-Y.,Advanced Photonics Research Institute | Kim J.H.,Advanced Photonics Research Institute | Lee J.,Advanced Photonics Research Institute
Journal of the Optical Society of Korea | Year: 2011

We demonstrate a high-spatial-resolution imaging and spectroscopy tool using a solid immersion lens (SIL), a hemispherical lens made of high refractive index glass (n ~2). Photoluminescence (PL) images of single CdSe nanocrystals confirm a numerical aperture enhancement factor of ~2, close to the refractive index of the SIL. In particular, a bare-eye observation of PL signals emitted by single nanocrystals with ~10 μm-2 densities was possible over an ~30 μm diameter region. In addition, the PL spectra of single CdSe nanocrystals were successfully measured at room temperature. Thus, this SIL microscope ensures a simple but powerful method for nanostructure spectroscopy.

Lee J.,Advanced Photonics Research Institute | Jeong T.M.,Advanced Photonics Research Institute
IEEE Photonic Society 24th Annual Meeting, PHO 2011 | Year: 2011

Ultrahigh-power laser systems are extremely useful to explore new physical phenomena through light-matter interaction processes. With the advent of Chirped-Pulse Amplification (CPA) technique, ultrashort high-power laser systems having a peak power of over TW had been quickly demonstrated, and now many groups are developing PW-class ultrashort laser systems. Recently, a 30-fs, 1 PW Ti:sapphire laser system operating at a repetition rate of 0.1-Hz is demonstrated at Advanced Photonics Research Institute (APRI) [1] and, in Europe, femtosecond 10-PW laser systems are designed and being developed for attosecond physics, secondary source generation, and laser-driven nuclear physics through Extreme Light Infrastructure (ELI) project. In this paper, we present output performances and applications for the 30-fs, PW Ti:sapphire laser system at APRI. © 2011 IEEE.

Pae K.H.,Advanced Photonics Research Institute | Choi I.W.,Advanced Photonics Research Institute | Lee J.,Advanced Photonics Research Institute
Laser and Particle Beams | Year: 2011

The characteristics of high energy protons generated from thin carbon-proton mixture targets via circularly polarized intense laser pulses are investigated using two-dimensional particle-in-cell simulations. It is found that the density ratio n between protons and carbon ions plays a key role in determining the acceleration dynamics. For low n values, the protons are mainly accelerated by the radiation pressure acceleration mechanism, resulting in a quasi-monoenergetic energy spectrum. The radiation pressure acceleration mechanism is enhanced by the directed-Coulomb-explosion of carbon ions which gives a high proton maximum energy, though a large energy spread, for high n values. From a proton acceleration point of view, the role of heavy ions is very important. The fact that the proton energy spectrum is controllable based on the target composition is especially useful in real experimental environments. © 2010 Cambridge University Press.

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