J. W. Wu

Single-beam polarization interferometry measurement of the linear electro-optic effect in poled polymer films with a reflection configuration

Measurement of the linear electro-optic effect in poled polymer thin films with two parallel electrodes is analyzed. A single-beam polarization interferometry is adopted in a reflection configuration. The dependence of the linear electro-optic modulated intensity on linear optical parameters is investigated experimentally and is compared with the theoretical analysis. Specifically, the electro-optic modulated intensity is measured as a function of optical bias, optical polarization direction, and incidence angle. In particular, the dependence of the electro-optic modulated signal on the incidence angle shows that the linear electro-optic effect in the polymer thin film induces the modulations of both the refractive angle and the linear refractive index. Also, the amount of Fabry–Perot effect influencing the modulation signal is analyzed.

Enhanced nonlinear optical effects due to the excitation of optical Tamm plasmon polaritons in one-dimensional photonic crystal structures

We suggest optimally designed one-dimensional metal/photonic crystal structures for the excitation of optical Tamm plasmon polaritons, which show strongly enhanced electromagnetic field intensities compared to those due to conventional surface plasmon excitations. We assume that the photonic crystal is made of weakly nonlinear optical materials and calculate the reflectance and the electromagnetic field distribution precisely, using the invariant imbedding method generalized to nonlinear media. We find field intensity enhancement factors as large as 3,000 at the metal/photonic crystal interface. We verify that due to this strong enhancement, nonlinear optical effects such as optical bistability can be observed for very small values of the incident wave power. Our results imply that using our structure, very strong surface enhanced Raman scattering signals can be achieved and optical switching devices can be operated in much lower threshold light intensities.

Holographically generated twisted nematic liquid crystal gratings

A reflection holographic method is introduced to fabricate an electro-optically tunable twisted nematic (TN) liquid crystal (LC) grating, forgoing the geometrical drawing. The photoisomerization process occurring on the LC alignment layers of an LC cell in the reflection holographic configuration gives a control over the twist angle, and the grating spacing is determined by the slant angle of reflection holographic configuration. The resulting diffraction grating is in a structure of a reverse TN LC, permitting a polarization-independent diffraction efficiency. The electro-optic tunability of the diffraction efficiency is also demonstrated.

Enhanced optical nonlinearity near the photonic bandgap edges of a cholesteric liquid crystal

The third-order Kerr nonlinear optical effect of a one-dimensional photonic bandgap structure of a cholesteric liquid crystal is investigated. In a femtosecond nonlinear transmission measurement, nonlinear optical changes in the bandgap edges are observed. From analysis of the dispersion relation, Kerr nonlinear coefficients of nematics, forming the cholesteric liquid crystal, are found to be enhanced by 1-2 orders of magnitude through the photonic bandgap structure.

Tuning the Direction of Intramolecular Charge Transfer and the Nature of the Fluorescent State in a T‑Shaped Molecular Dyad

Controlling photoinduced intramolecular charge transfer at the molecular scale is key to the development of molecular devices for nanooptoelectronics. Here, we describe the design, synthesis, electronic characterization, and photophysical properties of two electron donor-acceptor molecular systems that consist of tolane and BF2-containing curcuminoid chromophoric subunits connected in a T-shaped arrangement. The two π-conjugated segments intersect at the electron acceptor dioxaborine core. From steady-state electronic absorption and fluorescence emission, we find that the photophysics of the dialkylamino-substituted analogue is governed by the occurrence of two closely lying excited states. From DFT calculations, we show that excitation in either of these two states results in a distinct shift of the electron density, whether it occurs along the curcuminoid or tolane moiety. Femtosecond transient absorption spectroscopy confirmed these findings. As a consequence, the nature of the emitting state and the photophysical properties are strongly dependent on solvent polarity. Moreover, these characteristics can also be switched by protonation or complexation at the nitrogen atom of the amino group. These features set new approaches toward the construction of a three-terminal molecular system in which the lateral branch would transduce a change of electronic state and ultimately control charge transport in a molecular-scale device.

Solvent-free fluidic organic dye lasers

We report on the demonstration of liquid organic dye lasers based on 9-(2-ethylhexyl)carbazole (EHCz), so-called liquid carbazole, doped with green- and red-emitting laser dyes. Both waveguide and Fabry-Perot type microcavity fluidic organic dye lasers were prepared by capillary action under solvent-free conditions. Cascade Förster-type energy transfer processes from liquid carbazole to laser dyes were employed to achieve color-variable amplified spontaneous emission and lasing. Overall, this study provides the first step towards the development of solvent-free fluidic organic semiconducting lasers and demonstrates a new kind of optoelectronic applications for liquid organic semiconductors.

Electrogyration effect in a chiral bent-core molecular system

The linear electrogyration effect is observed in the crystallike B4 phase of a mixture of chiral and achiral bent-core mesogens. A large birefringent monodomain with a single sense of chirality was selected for experiment. Upon application of a modulating electric field to the sample cell, a strong modulation of refractive index was observed. Analysis showed that both the linear electro-optic and the linear electrogyration effect are responsible for the index modulation with the latter being dominant. The effective electro-optic coefficient was measured to be reff=21.6 pm/V at 488 nm.

Mach–Zehnder interferometer measurement of the Pockels effect in a poled polymer film with a coplanar electrode structure

Mach–Zehnder interferometry was used to measure the Pockels effect in a poled electro‐optic polymer thin film with a coplanar electrode structure. The beam at the sample arm of the Mach–Zehnder interferometer passed through a polymer thin film which had been spin‐coated on top of a clear gap between two electrodes patterned on an optical substrate. This unique optical geometry enabled the Pockels coefficients of the poled electro‐optic polymer film in the directions of the ordinary and the extraordinary optic axes to be determined independently. As an example, the tensor ratio r33/r13 for a stilbene‐dye‐doped polyimide guest/host polymer film was determined experimentally; the ratio turned out to be 4.6, which was higher than the value of 3 predicted by the thermodynamic model.

Determination of optical Kerr nonlinearity of a photonic bandgap structure by Z-scan measurement.

Through analysis of the dispersion relation in a photonic bandgap structure, the effective optical Kerr nonlinearity that determines a Z-scan profile particularly near the stop-band edges, is derived. Near and inside the stop band, the nonlinear optical phase change that originates from an off-resonant response is converted into a change in nonlinear optical intensity through Bragg reflection. The Z-scan measurement of a cholesteric liquid-crystal photonic bandgap structure confirmed that off-resonant Kerr nonlinearity is responsible for the characteristic open-aperture Z-scan profiles near the stop-band edges.

Defect modes in a one-dimensional photonic crystal with a chiral defect layer

We investigate the propagation of electromagnetic waves in a one-dimensional photonic crystal containing a defect layer made of an isotropic chiral medium. Using the invariant imbedding method, we calculate the transmission spectrum for both linearly- and circularly-polarized incident waves. In the normal incidence case, there is one defect mode, which does not depend on the chiral index and the polarization of the incident wave. When the waves are incident obliquely, however, we find that there appear double defect modes regardless of the polarization. The interval between the two defect frequencies increases monotonically as the chiral index or the incident angle increases. We argue that this phenomenon occurs due to the coupling and conversion between s and p waves inside the chiral defect layer.