Seong-Ji Kwon

Configurationally locked, phenolic polyene organic crystal 2-{3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene}malononitrile : linear and nonlinear optical properties

We show that the organic polyene crystal 2-{3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene}malononitrile (OH1) is a new material with excellent nonlinear optical and electro-optical properties. The principal refractive indices and the absorption constants have been determined over a broad wavelength range from 0.6to2.2 μm. OH1 shows a large birefringence of Δn>0.55 and a low absorption of α<1 cm−1 in a large wavelength range between 800 and 1400 nm. Furthermore, the nonlinear optical tensor elements d333=120 ± 10 pm/V, d223=13 ± 2 pm/V, and d322=8.5 ± 2 pm/V have been determined by the Maker-fringe technique at the fundamental wavelength of 1.9 μm. The electro-optic coefficients r333, r223, and r113 have been measured at the wavelengths of 633, 785, 1064, and 1319 nm, and it is shown that the dispersion can be described by a one-oscillator model. OH1 is a promising material for nonlinear optical and electro-optical applications due to the large nonlinear coefficient d333, the large electro-optic coefficient r333=109 ± 4 pm/V at 632.8 nm, the favorable crystal growth properties, as well as the orthorhombic crystal symmetry (point group mm2), which renders crystal preparation and orientation much simpler than in most organic nonlinear optical crystals.

Acentric nonlinear optical N-benzyl stilbazolium crystals with high environmental stability and enhanced molecular nonlinearity in solid state

We have developed a new cation core structure, N-benzyl stilbazolium nonlinear optical chromophore with a non-polar benzyl group to achieve acentric molecular ordering in the crystalline state. New N-benzyl stilbazolium crystal, BP3 (N,N-dimethylamino-N′-2,5-dimethylbenzyl-stilbazolium p-toluenesulfonate), exhibits an acentric crystal structure with the monoclinic P21 phase with a large macroscopic optical nonlinearity of 540 times the powder second harmonic generation efficiency of urea at the non-resonant wavelength of 1.9 µm. Compared to conventional rod-shaped N-alkyl stilbazolium crystals, an enhanced hyperpolarizability of the chromophoresβ0 in the solid state can be utilized in bent-shaped N-benzyl stilbazolium crystals. This is attributed to the decreased inter-chromophore interactions due to the larger chromophore–chromophore separation induced by the bulky and bent-shaped N-benzyl group, so-called site-isolation effect. Moreover, by introducing the non-polar dimethylbenzyl group, BP3 crystals show a high environmental stability: they exhibit almost one order of magnitude smaller solubility in water than conventional stilbazolium crystals and also do not form a hydrated centrosymmetric phase even if crystallized in water-containing solvents. We have grown good optical quality crystals large enough for optical characterization. With as-grown BP3 crystals without additional polishing and cutting procedures we have demonstrated THz generation by optical rectification using 180 fs pulses at the pump wavelength of 836 nm.

Photochemical stability of nonlinear optical chromophores in polymeric and crystalline materials

We compare the photochemical stability of the nonlinear optical chromophore configurationally locked polyene 2-{3-[2-(4-dimethylaminophenyl)vinyl]-5,5-dimethylcyclohex-2-enylidene} malononitrile (DAT2) embedded in a polymeric matrix and in a single-crystalline configuration. The results show that, under resonant light excitations, the polymeric compound degrades through an indirect process, while the DAT2 crystal follows a slow direct process. We show that chromophores in a crystalline environment exhibit three orders of magnitude better photostability as compared to guest-host polymer composites.

Electron distribution and capacitance–voltage characteristics of n-doped quantum wells

The effect of multiple quantum well (MQW) parameters on the free carrier distribution (FCD) and the apparent carrier distribution (ACD) obtained from capacitance–voltage (C–V) profiling has been investigated using a self-consistent simulation technique and the C–V profiling technique. The FCD in MQW structures reveals large outer peaks and small inner peaks even when all the parameters of each quantum wells (QWs) are the same. Such a feature becomes more apparent as the barrier layer becomes thinner or the barrier doping level becomes lower. These characteristics are found to originate from the fact that the density of carriers confined in each well is mostly determined by the depletion region formed alongside the well via the charge neutrality condition. The ACD is found to vary drastically as the thickness or the doping level of barrier changes. When the Debye averaging process is prominent, the ACD peaks are broader and smaller than the FCD peaks and are displaced toward the bottom layer side. The ACD ...

Crystal engineering by eliminating weak hydrogen bonding sites in phenolic polyene nonlinear optical crystals

A new acentric polyene crystal OH3 with a large macroscopic second-order optical nonlinearity with optimal chromophore orientation for electro-optics and THz-wave applications has been developed by crystal engineering, eliminating weak CH⋯NC hydrogen-bonding sites, but still retaining strong OH⋯NC hydrogen-bonding sites of the analogue OH1 crystals.

Spatial resolution of capacitance-voltage profiles in quantum well structures

The temperature dependence of the spatial resolution of capacitance-voltage (C−V) profiles in the compositional quantum well (CQW) is investigated. The apparent carrier distribution (ACD) peak in the In0.2Ga0.8As/GaAs single QW is observed to show a strong temperature dependence, compared to that in Si δ-doped GaAs. The ACD peak in CQW is wider (narrower) than the spatial extent of ground-state wave function at high (low) temperatures. The self-consistent numerical simulations on the carrier distribution show that the full width at half maximum of ACD peak in CQW is mainly affected by the debye averaging process at high temperatures and the change in the position expectation value of the two-dimensional electrons at low temperatures. This change in the position expectation value is found to be much smaller than the spatial extent of ground-state electron wave function.

Fabrication and phase modulation in organic single-crystalline configurationally locked, phenolic polyene OH1 waveguides

A novel and promising technique for the fabrication of electro-optically active single crystalline organic waveguides from 2-{3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene}malononitrile (OH1) is presented. OH1 is an interesting material for photonic applications due to the large electro-optic coefficients (r333 = 109+/-4 pm/V at 632.8 nm) combined with a relatively high crystal symmetry (orthorhombic with point group mm2). Due to the very favorable growth characteristics, large-area (> 150 mm(2)) single crystalline thin films with very good optical quality and thickness between 0.05-10 microm have been grown on amorphous glass substrates. We have developed and optimized optical lithography and reactive ion etching processes for the fabrication of wire optical waveguides with dimensions of w x h = 3.4 x 3.5 microm(2) and above. The technique is capable of producing low loss integrated optical waveguides having propagation losses of 2 dB/cm with a high refractive index contrast between core-cladding and core-substrate of delta n = 1.23 and 0.72, respectively at 980 nm. Electro-optic phase modulation in these waveguides has been demonstrated at 632.8 nm and 852 nm. Calculations show that with an optimized electrode configuration the half-wave voltage x length product V(pi) x L can be reduced from 8.4 Vcm, as obtained in our device, to 0.3 Vcm in the optimized case. This allows for the fabrication of sub-1 V half-wave voltage, organic electro-optic modulators with highly stable chromophore orientation.

Electron distribution and capacitance-voltage profiles of multiple quantum well structure from self-consistent simulations

Carrier profiles of multiple quantum wells are studied using self-consistent simulations. The free carrier density of the well is found to be distributed nonuniformly and symmetrically, although the doping level in barriers is uniform. The calculated apparent carrier density obtainable from the capacitance-voltage profile is found to be distributed asymmetrically. Simulation results show that, even if electrons are confined in quantum wells, the apparent electron distribution can be flattened if barrier thickness or doping level in barriers are reduced to such an extent that the Debye length is comparable to the barrier thickness.

Nonlinear optical co-crystal of analogous polyene chromophores with tailored physical properties

A new organic nonlinear optical co-crystal based on analogous configurationally locked polyene chromophores with noncentrosymmetric packing exhibits a large macroscopic second-order nonlinearity with tailored physical properties.

Influence of carrier flow on the temperature‐dependent capacitance‐voltage profiles of heterojunction structures

We suggest a model which can explain the shifting of carrier concentration peaks in the temperature‐dependent capacitance‐voltage carrier profiles of heterojunction (HJ) structures. The shift of concentration peaks, which was frequently observed in the inverted isotype HJs was previously attributed to the traps at the heterointerface. The main feature of our model is the role of band offset as a limiter to the test signal current. The model can explain the difference of the peak shift in the carrier profiles of the normal and inverted type HJs. According to this model, the peak shifts at low temperatures occur naturally for the inverted type HJs.