Jungjae Park

Wave-front phase measurements of high-order harmonic beams by use of point-diffraction interferometry

We report the wave-front phase measurement of high-order harmonics employing point-diffraction interferometry. The high-order harmonics generated in a gas-filled hollow tube showed excellent spatial coherence over nearly the whole cross section of the harmonic beams. Using this coherent harmonic source in the extreme-ultraviolet-soft-x-ray region, we have demonstrated the operation of a point-diffraction interferometer and measured the wave-front phase of high-order harmonics.

Vibration-desensitized interferometer by continuous phase shifting with high-speed fringe capturing

Optical interferometers for surface metrology require high immunity to vibration when they are to be used outside the laboratory environment. Here we describe a continuous phase-shifting method as a means of vibration suppression, which implements phase shifting uninterruptedly while capturing resulting interference fringes at a high rate. The continuous phase shifting allows the interference initial phase under measurement to be locked to a single temporal frequency in captured fringes, thereby being recovered free from vibration by Fourier-based postprocessing. This continuous phase-shifting method can be realized with any phase-shifting interferometers by incorporating a high-speed digital camera with good intensity-to-voltage conversion linearity.

Double Lloyd’s mirror: versatile instrument for XUV surface interferometry and interferometric microscopy

We have developed a double Lloyds mirror wavefront-splitting interferometer, constituting a compact device for surface probing in the XUV and soft X-ray spectral domain. The device consists of two independently adjustable superpolished flat surfaces, operated under grazing incidence angle to reflect a diverging or parallel beam. When the mirrors are appropriately inclined to each other, the structure produces interference fringes at the required distance and with tuneable fringe period. The double Lloyds mirror may be used alone for surface topography with nanometric altitude resolution, or in conjunction with an imaging element for interferometric XUV surface microscopy. In the latter case, resolution in the plane of the probed surface is about micron, which is given by the quality of the imaging element and/or by the detector pixel size. Here, we present results obtained using the double Lloyds mirror in two separate X-ray laser and high harmonics generation (HHG) application projects. The first experiment was aimed at understanding microscopic nature of the effects involved in laserinduced optical damage of thin pellicles, exposed to sub-ns laser pulses (438 nm) producing fluence of up to 10 Jcm-2. The probing source in this case was a QSS neon-like zinc soft X-ray laser, proving a few mJ at 21.2 nm in ~100-ps pulses. The second experiment was carried out using a narrowly collimated HHG beam near 30 nm, employed to topographically probe the surface of a semiconductor chip.

A point-diffraction interferometer with vibration-desensitizing capability

We present a new type of point-diffraction interferometer specially designed for industrial use with high immunity to external vibration encountered in the course of measurement process. The proposed interferometer uses thermally-expanded fibers instead of conventional pinholes as the point-diffraction source to obtain a high quality reference wave with an additional advantage of relatively easy alignment of optical components. Vibration desensitization is realized through a common-path configuration that allows the influence of vibration to affect both the reference and measurement waves identically so that it is subsequently cancelled out during the interference of the two waves. A spatial phase shifter is added to capture four phase-shifted interferograms simultaneously without time delay using a single camera to avoid vibration effects. Experimental results demonstrate that the proposed interferometer is capable of providing stable measurements with a level of fringe stabilization of less than 1 nanometer in a typical workshop environment equipped with no excessive ground isolation for anti-vibration.

Enhanced X‐ray Emission from Nitrogen Clusters Ionized by Intense, Ultrashort Laser Pulses

We report soft X‐ray spectra from nitrogen clusters irradiated by 28‐fs laser pulses at an intensity of about 7 × 1016 W/cm2. While the spectrum obtained at room temperature showed entirely transitions from N4+ ions, new lines originating from N5+ and N6+ charge states appeared with cooling. By lowering the pre‐expansion gas temperature we have observed strong, nonlinear increase of X‐ray emission on lines of N4+, N5−, and N6+. The generation of highly charged ions of N5+ and N6+ is explained by collisional processes in near‐solid‐density clusters. A preliminary experiment on the feasibility of a charge‐exchange‐pumped X‐ray laser has been performed in longitudinal geometry.

Point-diffraction fiber interferometer for vibration desensitization

We present a new type of point-diffraction interferometer specially designed for industrial use to obtain high immunity to external vibration encountered in the course of measurement process. The proposed interferometer uses thermally- expanded fibers instead of conventional pinholes as the point-diffraction source to obtain a high quality reference wave with an additional advantage of relatively easy alignment of interferometric optical setup. Vibration desensitization is realized through a common-path configuration that allows the influence of vibration to identically affect both the reference wave and the measurement wave and be subsequently cancelled out during the interference of the two waves. A new spatial phase shifter is also added to capture four phase-shifted interferograms simultaneously without time delay using a single camera to avoid vibration effect. Experimental results for a spherical concave mirror prove that the proposed interferometer is capable of providing stable measurements with a level of fringe stabilization of less than 1 nanometer in a typical workshop environment equipped with no excessive ground isolation for anti-vibration. Also, we verify that the proposed interferometer using a short coherence source is applicable to the surface metrology for defect inspection of transparent substrates such as liquid crystal display panels.

Autofocus of Infinity-Corrected Optical Microscopes by Confocal Principle and Fiber Source Modulation Technique

The autofocus is one of the important processes in the automated vision inspection or measurements using optical microscopes, because it influences the measuring accuracy. In this paper, we used the confocal microscope configuration based on not a pinhole but a single-mode optical fiber. A single mode fiber has the functions of source and detector by applying the reciprocal scheme. As a result, we acquired a simple system configuration and easy alignment of the optical axis. Also, we embodied a fast autofocus system by acquiring the focus error signal through a source modulation technique. The source modulation technique can effectively reduce physical disturbances compared with objective lens modulation, and it is easily applicable to general optical microscopes. The focus error signal was measured with respect to the modulation amplitude, reflectance of the specimen and inclination angle of the measuring surface. The performance of the proposed autofocus system was verified through autofocusing flat mirror surface. In addition, we confirmed that source modulation rarely degrades the depth resolution by the comparison between the FWHMs of axial response curves.

Investigation of X‐ray Amplification in Neon Clusters

We have experimentally investigated the feasibility of X‐ray lasing in Li‐like Ne ions produced by the interaction of intense, 28‐fs laser pulses with gaseous clusters of neon. The idea of the proposed recombination (n=3–4) X‐ray laser is based on adiabatic cooling of fast expanding small‐sized Ne clusters. The driving laser beam was focused into a cryogenically cooled gas jet of Ne, and time‐integrated spectra were simultaneously measured in transverse and longitudinal direction with respect to the laser axis. The transverse spectrum from Ne at the temperature of about −137 °C showed dominant emission from highly placed levels (n=4) in Ne7− ions whilst the resonance lines (n=2–3) at 8.8, 9.8, and 10.3 nm remained very weak suggesting enhanced recombination from Ne8−. In the longitudinal spectrum, the lasing line candidate corresponding to the 3d‐4f transition of Ne7+ at 29.24 nm has been observed as a distinct, but not dominant.