Jaehoon Hwang

Electrical transport through 60 base pairs of poly(dG)-poly(dC) DNA molecules

We report electrical transport through 60 base pairs of poly~dG!-poly~dC! DNA molecules. The DNA solution is dropped on two metal electrodes with the gap of 20 nm. The current‐voltage characteristics measured between the electrodes exhibits clear staircases, which are reproducible over repeated measurements. The size of the observed staircases is consistent with the energy gap obtained from a tight binding calculation.

Monolithic photorefractive molecule with excellent transparency in the visible region

9-(2-Ethyl-hexyl)-3-[2-(4-methanesulfonyl-phenyl)vinyl]-9H-carbazole (EHCS) was synthesized as a monolithic photorefractive molecule forming low Tg(∼25 °C) organic glass. It showed distinct photoconductivity due to the carbazole moiety and the optical nonlinearity by the push–pull structure. The EHCS glass showed excellent optical quality and great stability as the ethylhexyl group effectively suppressed the crystallization in the films. Also, the extremely simple structure of EHCS contributed to a very high chromophore concentration. This single-component photorefractive glass of EHCS showed large net gain (56.3 cm−1) and diffraction efficiency (12%). Photorefractivity of EHCS glass was greatly improved by doping with a small amount (<1%) of 2,4,7-trinitrofluorenone (TNF). The two-beam coupling gain of samples with TNF was 113 cm−1 and the diffraction efficiency was 29%.

Carbazole-based Photorefractive Organic Glass Showing High Diffraction Efficiency at Low External Electric Field

Photorefractive organic glass, 9-(2-ethyl-hexyl)-3-[2-(4-nitro-phenyl)-vinyl]-9H-carbazole (EHCN), showing large coupling gain and high diffraction efficiency at low external electric field was synthesized and characterized. EHCN showed the glass transition at 25°C and the good thermal stability up to 400°C. Photorefractivity of EHCN was investigated by using 100 µm thick films (S1, S2, S3) doped with 0%, 0.5%, and 1% 2,4,7-trinitrofluorenone (TNF), respectively. Two-beam coupling net gain of 50.8 cm-1 was obtained for the 0.5% TNF-doped EHCN sample (S2) at 40 V/µm field. The highest internal diffraction efficiency, obtained from the degenerate four-wave mixing at 30 V/µm field, reached 90% for the 1% TNF-doped EHCN sample (S3).

Light-emitting properties of PMMA-based new side chain polymers

Abstract A series of PMMA-based new side chain fluorescent polymers have been synthesized through radical polymerization. Carbazole containing phenylenevinylene units are attached to PMMA side chain as lumophores, and electronegative nitrile groups were selectively introduced to the double bonds of the lumophores in order to investigate the effects on photoluminescence (PL) and EL properties. The synthesized polymers were soluble in common organic solvents without evidence of gel formation. The number of average molecular weights of resulting copolymers were determined by GPC in the range of 14000–23000 depending on the polymer structure. PL peak wavelengths of the polymers were varied from 530 to 470 nm depending on the polymer structure. Single layer EL devices using the polymers were fabricated. Light-emission from the devices becomes visible between 15 and 20 V and maximum brightness of the devices has been measured to be between 10 and 20 cd/m2 depending on the device structure.

AMS Dates from Two Archaeological Sites of Korea: Blind Tests

In interpreting radiocarbon dating results, it is important that archaeologists distinguish uncertainties derived from random errors and those from systematic errors, because the two must be dealt with in different ways. One of the problems that archaeologists face in practice, however, is that when receiving dating results from laboratories, they are rarely able to critically assess whether differences between multiple 14 C dates of materials are caused by random or systematic errors. In this study, blind tests were carried out to check four possible sources of errors in dating results: repeatability of results generated under identical field and laboratory conditions, differences in results generated from the same sample given to the same laboratory submitted at different times, interlaboratory differences of results generated from the same sample, and differences in the results generated between inner and outer rings of wood. Five charred wood samples, collected from the Namgye settlement and Hongreyonbong fortress, South Korea, were divided into 80 subsamples and submitted to five internationally recognized 14 C laboratories on a blind basis twice within a 2-month interval. The results are generally in good statistical accordance and present acceptable errors at an archaeological scale. However, one laboratory showed a statistically significant variance in ages between batches for all samples and sites. Calculation of the Bayesian partial posterior predictive p value and chi-squared tests rejected the null hypothesis that the errors randomly occurred, although the source of the error is not specifically known. Our experiment suggests that it is necessary for users of 14 C dating to establish an organized strategy for dating sites before submitting samples to laboratories in order to avoid possible systematic errors.

Fine tuning of glass transition temperature in monolithic organic photorefractive material

Abstract In order to study the effect of Tg on the photorefractivity in organic monolithic system, we designed and synthesized photorefractive homopolymer (PCzBO) and organic glass (EHCzBO), consisting of the same monolithic chromophore. Photorefractive polymer composites were prepared by blending them. Tg of PCzBO and EHCzBO were 106 and 29 °C, respectively. Tg of polymer composite could be controlled precisely by the compositional ratio of PCzBO and EHCzBO in the range from 29 to 40 °C. Since both the polymer and the organic glass contained the same chromophore structure, all the samples had nearly the same chromophore concentration even though compositional ratios of them were different, as was confirmed by UV–visible spectroscopy. Frequency-dependent ellipsometric technique showed that the birefringence contribution through the orientation enhancement effect was dominant in the refractive index modulation of our low Tg composites. It was found that the reorientation of chromophore according to the modulated voltage was more limited for the higher Tg composite, which consequently reduced the birefringence contribution. The diffraction efficiency and two beam coupling coefficient were decreased with the increase of Tg.

SURFACE PLASMON RESONANCE ENHANCED SECOND-HARMONIC GENERATION IN POLED POLYMER THIN FILM

In this paper we report surface plasmon resonance (SPR)-enhanced second-harmonic generation (SHG) in a poled polymer thin film using an attenuated total reflection (ATR) configuration (Kretschmann configuration). The second-harmonic (SH) light generated in this configuration is about 7000 times larger than that in the reflection geometry. This result is in agreement with the enhancement predicted by theoretical calculation.

Charge carrier mobility and photorefractive grating buildup in bipolar organic glasses

Two-component organic glasses consisting of photoconductive molecule, N′-[9-(2-ethylhexyl)-9H-carbazol-3-ylmethylene]-N,N-diphenyl-hydrazine (EHCzHy), and multifunctional photorefractive molecules, 9-(2-ethyl-hexyl)-3-[2-(4-methanesulfonyl-phenyl)-vinyl]-9H-carbazole (EHCS) [J. Sohn et al., Appl. Phys. Lett. 77, 1422 (2000)] and 9-(2-ethylhexyl)-3-[2-(4-nitro-phenyl)-vinyl]-9H-carbazole (EHCN), were prepared, and their charge carrier mobility and photorefractive grating buildup were investigated. EHCzHy is a unipolar hole-transporting (μh=4.16×10−6 cm2/V s) molecule with no electro-optic property. EHCS and EHCN are nonlinear optical molecules with unipolar (μh=2.42×10−6 cm2/V s) and bipolar (μh=2.30×10−6 cm2/V s,μe=2.86×10−6 cm2/V s) carrier mobilities, respectively. Different behaviors of the photorefractive grating buildup in two-component organic glasses were investigated as a function of EHCN or EHCS content.

First hyperpolarizabilities of dipolar photoconductive chromophores: an approach toward monolithic molecular materials for photorefractivity

Abstract Second-order optical nonlinearities of a series of dipolar photoconductive chromophores have been investigated by semiempirical quantum chemical calculations using time-dependent Hartree–Fock formalism in conjunction with PM3 parameter and experimental evaluation of the first molecular hyperpolarizability ( β ). A linear correlation between the measured and calculated β values was observed, and the structure–property relationship discussed in terms of the structural variation. It was noted that the optical nonlinearities of the dipolar photoconductive molecules with carbazole, indole, or indoline as donor units are large enough for the electrooptic and photorefractive applications.

Population Fluctuation and the Adoption of Food Production in Prehistoric Korea: Using Radiocarbon Dates as a Proxy for Population Change

Population growth has been evoked both as a causal factor and consequence of the transition to agriculture. The use of radiocarbon ( 14 C) dates as proxies for population allows for reevaluations of population as a variable in the transition to agriculture. In Korea, numerous rescue excavations during recent decades have offered a wealth of 14 C data for this application. A summed probability distribution (SPD) of 14 C dates is investigated to reconstruct population trends preceding and following adoptions of food production in prehistoric Korea. Important cultivars were introduced to Korea in two episodes: millets during the Chulmun Period (ca. 6000–1500 BCE) and rice during the Mumun Period (ca. 1500–300 BCE). The SPD suggests that while millet production had little impact on Chulmun populations, a prominent surge in population appears to have followed the introduction of rice. The case in prehistoric Korea demonstrates that the adoption of food production does not lead inevitably towards sustained population growth. Furthermore, the data suggest that the transition towards intensive agriculture need not occur under conditions of population pressure resulting from population growth. Rather, intensive rice farming in prehistoric Korea began during a period of population stagnation.