Jiwon Sohn

Tunable terahertz generation using femtosecond pulse shaping

Femtosecond pulse shaping and photomixing methods are combined to make a tunable terahertz source. Grating-pair pulse shaper selects two main frequency components to produce pulses that are modulated with a period which is inversely proportional to the frequency separation. The shaped pulse is then photomixed in semi-insulating GaAs under external bias. Terahertz (THz) emission frequency follows the modulation frequency of the excitation pulses, from 0.5 to 3 THz. Contrary to cw photomixing, this method can easily be combined with electro-optic and photoconductive sampling detection methods.

Microlensed vertical-cavity surface-emitting laser for stable single fundamental mode operation

We propose and demonstrate a vertical-cavity surface-emitting laser structure that operates predominantly in the single fundamental transverse mode over a wide operation current range. In this laser structure, a microlens is integrated on top of an otherwise ordinary surface-emitting laser so that a small portion of laser output is fed back into the cavity, forcing the lasing to occur in the fundamental mode. Model calculations reproduce the observed mode selection effect.

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).

Synthesis and properties of oligomeric poly(glycidyl ether)s with a carbazole-based multifunctional photorefractive chromophore

Abstract Novel photorefractive poly(glycidyl ether) oligomers of a carbazole-based multifunctional chromophore were synthesized via ring-opening cationic polymerization. This new class of materials showed good solubility in various organic solvents and thermal stability up to 400°C. Optical quality films of excellent transparency at the light wavelength above 430 nm were obtained by spin-coating or casting. Photoconductive sensitivity of the film was as high as 10−13 S cm−1/mW cm−2 at 532 nm. Electrooptic coefficients (r33) of 5–7 pm V−1 were easily achieved by the contact poling of spin-coated films. As evidence of photorefractivity, an asymmetric two-beam coupling gain of 3.2 cm−1 was demonstrated at an electric field of 30 V μm−1 with a low-Tg polymer blend of multifunctional poly(glycidyl ether).

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.

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.

Triphenyl amine-cored multibranched organic glasses for photonic applications

Strongly fluorescent organic nonlinear optical materials containing triphenyl amine as a core and 1-(2-ethyl-hexane-1-sulfonyl)-4-vinylbenzene or 4-[4-(2-ethyl-hexane-1-sulfonyl)-1-ethyl] styrene group as the arms which were mono-,di-, and tri-substituted at the para position of nitrogen were synthesized. Photorefractivity of these molecular glasses were measured and correlated with their structure and symmetry. Large two-photon absorption cross-section and strong up-conversion fluorescence emission were also observed in these materials.