Bum Ku Rhee

Direct observation of self-focusing near the diffraction limit in polycrystalline silicon film

We present direct observation of self-focusing near the diffraction limit by measuring the beam-spot size with a scanning fiber probe tip. We have used the polycrystalline silicon film, which exhibits a reverse-saturation (Im χ(3)≈8×10−3 esu) and self-focusing (Re χ(3)≈2×10−2 esu), as measured by the conventional z-scan method with He–Ne laser. It is observed that the beam radius of about its wavelength becomes smaller as the input laser intensity is increased, which indicates that the self-focusing effect dominates over the reverse saturation in the 300-nm-thick sample.

Nonlinear absorption of Cr4+:YAG studied with lasers of different pulsewidths

A saturable absorber Cr4+:YAG, used for a passive Q switching of Nd:YAG laser was studied in order to understand the physical mechanism of nonlinear absorption (photobleaching). An optical bleaching experiment was carried out using the two pulsed lasers with duration of pico- and nanoseconds at 1.064 μm wavelength. Experimental results were compared with numerical analysis of theoretical rate equations with all relevant energy levels. The inclusion of intersystem crossing between singlet- and triplet-excited states with reasonable rate of transition kisc was necessary to explain the difference in the saturation behaviors for nanosecond and picosecond laser pulses.

Simultaneous determination of thickness and optical constants of polymer thin film by analyzing transmittance

We present a novel method for determining both the thickness and the optical constants of a weakly absorbing thin film upon a nearly transparent substrate through analysis of transmittance measured at various incident angles with coherent light. We demonstrate this method for a polymer thin film. The refractive indices and extinction coefficients of poly(DRI-anthranilic acid) at wavelengths of 1,064, 632.8, and 532 nm were determined for the first time to our knowledge. We also confirmed the validity of our method with a polystyrene thin film whose optical constant was known. It was found that a thickness of a few hundred nanometers can easily be measured and that this method offers simplicity as well as the capability of in situ measurement.

Ultraviolet photodissociation at 266 nm of phosphorylated peptide cations

Ultraviolet (UV) photodissociation (PD) experiments using 266 nm light were performed for a series of phosphopeptide cations in a Fourier transform mass spectrometer. The objective of the experiments was to determine whether 266 nm UV irradiation on the phosphopeptide cations would induce unique peptide backbone dissociation. In addition, the general behavior of the phosphate loss (-80 or -98 Da) was monitored, particularly for those phosphopeptides with a phosphotyrosine residue that itself is a UV chromophore. For phosphopeptides with a UV chromophore, their photodissociation behavior was very similar to that of low-energy sustained off-resonance irradiation collisionally activated dissociation (SORI-CAD), with a few exceptions. For example, b- and y-type peptide backbone fragments were prevalent, and their dephosphorylation behavior was consistent with that of the SORI-CAD results. For phosphoserine peptides, the loss of a phosphate group was always observed. On the other hand, for phosphotyrosine peptides, the phosphate loss was found to be dependent on the presence of a basic amino group in the sequence and the charge state of the precursor ions, in agreement with the CAD results in the literature. However, hydrogen atom loss or aromatic side chain loss, which is known to be the excited state specific fragmentation pathway, was rarely observed in our 266 nm UV PD experiments, in contrast to the previous UV PD literature (particularly at 220 nm). The mechanism for these observations is described in terms of dominant internal conversion followed by intramolecular vibrational energy redistribution (IVR).

Novel Y-type polyimide with highly enhanced thermal stability of second harmonic generation

Abstract3,4-Bis-(3,4-dicarboxyphenylcarboxyethoxy)-4′-nitrostilbene dianhydride was prepared and reacted with 4,4′-(hexafluoroisopropylidene)dianiline to yield a novel Y-type polyimide containing the 3,4-dioxynitrostilbenyl group as an NLO-chromophore, which constituted part of the polymer backbone. The resulting polyimide was soluble in polar solvents such as acetone andN,N-dimethylformamide. The polymer exhibited good thermal stability up to 370 °C in the thermogravimetric analysis. The glass-transition temperature (Tg) obtained from the differential scanning calorimetry thermogram was near to 153 °C. The second harmonic generation (SHG) coefficient (d33) of the poled polymer film at the fundamental wavelength of 1064 cm−1 was around 2.15 × 10−8 esu (9.01 pm/V). The dipole alignment exhibited exceptionally high thermal stability even at a temperature 30 °C above theTg, and there was no SHG decay below 180 °C because of the partial main chain character of the polymer structure.

Rigorous coupled-wave analysis of antireflective surface-relief gratings

Rigorous coupled-wave analysis (RCWA) with a simplified eigenvalue problem is used to investigate the an-tireflective property of one-dimensional surface-relief gratings such as binary gratings, triangular gratings and gratings with triangle-like surface profiles. The convergence of RCWA is investigated by varying the number of layers and the number of space-harmonics used in the computation. For unpolarized light normally incident on a medium of refractive index 1.64 from vaccum, a triangle-like grating shows the reflectivity of

Determination of photovoltaic constant and photoconductivity in LiNbO3:Fe using Maker fringes

1.6 {\times} 10^{-4}

Ultra-broadband optical parametric generation and simultaneous RGB generation in periodically poled lithium niobate

in contrast to a minimum reflectivity of

Mono- and Multi-layer Langmuir-Blodgett Films of Maleimide Polymers Possessing Nonlinear Optical-Active Side Chains

3.8 {\times} 10^{-3}

Picosecond pump-probe measurement of bandgap changes in SiO2/TiO2 one-dimensional photonic bandgap structures

for a binary grating. We also study the dependence of reflectivity on the wavelength, and on the angle of incidence for a groove shape and depth which result in minimum reflectivity.