Takehiro Toyooka

Polarization Conversion in Surface-Plasmon-Coupled Emission from Organic Light-Emitting Diodes Using Spontaneously Formed Buckles

A surface plasmon (SP) polariton is an electromagnetic wave propagating along the interface between a dielectric and a metal, and its electromagnetic fi eld exponentially decays into the surrounding media. Because the wavevector of the SP mode is larger than that of a photon of the same frequency in vacuum, the SP mode on a fl at surface is nonradiative and its energy dissipates as heat in the metal layer. Organic light-emitting diodes (OLEDs) consisting of organic layers and metallic electrodes inevitably accompany the SP mode and the power loss to the SP mode signifi cantly limits the device effi ciency, particularly in small-moleculebased OLEDs. [ 1 , 2 ] Although wavelength-scale periodic gratings have been introduced in OLED structures to convert the dissipated energy to the SP mode into useful light, reported structures are effective only at a specifi c wavelength and angle, satisfying the Bragg diffraction condition. [ 1–6 ] In our previous study, we demonstrated that quasiperiodic buckling structures with broad distribution and directional randomness can effectively enhance the light-extraction effi ciency without introducing spectral changes and directionality by outcoupling the waveguide modes. [ 7 ] In that study, however, we could not differentiate the outcoupling of transverse electric (TE) mode from that of the SP mode (transverse magnetic (TM) mode) by buckles because of the broad periodicity of the buckling structure and the similar propagation vectors of the TE and SP modes. In this study, we report that a buckling structure is remarkably effective at outcoupling the SP mode over all emission wavelengths and angles through an OLED structure with a thin indium–tin oxide (ITO) layer, by which any other waveguide modes are suppressed and only the SP mode is excited. Interestingly, we found that the diffraction of the SP mode by buckles produces TE-polarized light in addition to TM-polarized light, which indicates polarization conversion from TM (SP) to TE mode.

Broadband Cavity-Mode Lasing from Dye-Doped Nematic Liquid Crystals Sandwiched by Broadband Cholesteric Liquid Crystal Bragg Reflectors

[*] Prof. H. Takezoe, Dr. H. Choi, Dr. F. Araoka, Prof. K. Ishikawa Department of Organic and Polymeric Materials Tokyo Institute of Technology 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552 (Japan) E-mail: takezoe.h.aa@m.titech.ac.jp Prof. J. W. Wu, J. Kim Department of Physics and Division of Nano Sciences Ewha Womans University Seoul 120-750 (Korea) E-mail: jwwu@ewha.ac.kr Dr. S. Nishimura, T. Toyooka Central Technical Research Laboratory Nippon Oil Corporation 8 Chidori-cho, Naka-ku, Yokohama 231-0815 (Japan)

Monodomain film formation and lasing in dye-doped polymer cholesteric liquid crystals

We have succeeded in fabricating monodomain polymer cholesteric liquid crystal (PCLC) films by spin coating a polymer solution onto glass substrates with polyimide coated and rubbed unidirectionally. The thickness and helical pitch were well characterized by controlling the rotational speed of the spin coater and the chiral unit content of the polymer solution, respectively. Lasing from dye-doped PCLC films utilizing the photonic structure was confirmed.

Defect-Mode Lasing from a Three-Layered Helical Cholesteric Liquid Crystal Structure

We fabricated a hybrid structure composed of a dye-doped low-molecular-weight cholesteric liquid crystal (low CLC) sandwiched between polymer cholesteric liquid crystal (PCLC) films with the same handedness as the low CLC, and studied its lasing characteristics. Defect-mode lasing was observed with a threshold reduced by a factor of 3 compared with that in a simple dye-doped low CLC cell. Experimental and simulated results for the transmittance, emission, and photonic density-of-states spectra are also discussed.

Polarization characteristics of phase retardation defect mode lasing in polymeric cholesteric liquid crystals

Abstract We have studied the lasing characteristics of a dye-doped nematic layer sandwiched by two polymeric cholesteric liquid crystal (CLC) films as photonic band gap (PBG) materials. The nematic layer acts as a defect layer, the anisotropy of which brings about the following remarkable optical characteristics: (1) reflectance in the PBG region exceeds 50% due to the retardation effect, being unpredictable from a single CLC film; (2) efficient lasing occurs either at the defect mode wavelength or at the photonic band edge; and (3) the lasing emission due to both the defect mode and the photonic band edge mode contains both right- and left-circular polarizations, while the lasing emission from a dye-doped single CLC layer with a left-handed helix is left-circularly polarized.

Determination of the Frank Elastic Constant Ratios in Nematic Liquid Crystals (nCB) by Observing Angular Dependence of Rayleigh Light Scattering Intensity

Abstract The Frank elastic constant ratios, K 12 and K 32 in a homologous series of nCB (n = 5 − 8) were determined as a function of temperature by observing the angular dependence of Rayleigh light scattering intensity. The temperature dependence of K 12 exhibits an odd-even effect. Both K l2 and K 32 obtained are clearly smaller than the reported values by means of other methods, e.g., the Frederiks transition method. However, our K 31 indirectly deduced from K 32/K 12 shows good agreement with that by others. Characteristic temperature dependences due to the appearance of the pretransitional ordering were observed in 8CB which has the smeclic A phase below the nematic phase; K 12 significantly diverges and K 12 shows a sudden descent as a smectic A is approached. On the basis of these experimental results, we claim that the Rayleigh light scattering is the most adequate method of determining the Frank constant ratios.

Lasing from Thick Anisotropic Layer Sandwiched between Polymeric Cholesteric Liquid Crystal Films

We have demonstrated a new type of lasing mode in a dye-doped 100-µm-thick nematic liquid crystal layer sandwiched between two polymeric cholesteric liquid crystal films functioning as a photonic crystal. The fabricated cell exhibits several characteristic dips in the transmittance spectrum in addition to fine fringes originating from a Fabry–Perot cavity mode. These dips are due to the phase retardation between optical eigenmodes in the birefringent medium, which is not realized in an isotropic layer. The cell shows multimode lasing at wavelengths corresponding to transmittance maxima within the stopband region when the nematic layer is doped with dyes.

Determination of Twist Elastic Constant K22 in 5CB by Four Independent Light-Scattering Techniques

The twist elastic constant K22 in 5CB was determined as a function of temperature by the following four independent methods; (1) electric field static light scattering, (2) electric field dynamic light scattering, (3) angular dependence of the scattered light intensity combined with a K11 or K33 measurement, and (4) Frederiks transition detected by forced Rayleigh scattering. The first one is a measurement of relative scattered light intensity as a function of the applied voltage in a proper optical geometry, and was used to determine K22 for the first time. Each measurement gave a K22 value within an accuracy of ±4%. The collected values that were determined by the four methods were located within ±7% from each other. It was also confirmed that they agree with other recent measurements.

Lowering the Lasing Threshold by Introducing Cholesteric Liquid Crystal Films to Dye-Doped Cholesteric Liquid Crystal Cell Surfaces

The effect of the introduction of polymer cholesteric liquid crystal (PCLC) films on the threshold of dye-doped cholesteric liquid crystal (CLC) distributed feedback (DFB) cavity lasing has been investigated. A PCLC film used to reflect a pump beam brings about the efficient use of incident energy, whereas a PCLC film used to reflect the emission contributes to amplifying the stimulated emission. As a result, the cell, in which both PCLC films are introduced, gains about a 60% reduction in the lasing threshold. It is also found that a lasing threshold exists not only for the excitation energy but also for the emission intensity. Namely, the lasing starts to occur at a certain emission level irrespective of the cell structures.

Enhancement of Laser Emission Intensity in Dye-Doped Cholesteric Liquid Crystals with Single-Output Window

Lasing from a dye-doped cholesteric liquid crystal (CLC) has been studied in cells with a single-output window. One of the glass substrates of the cells was coated with a polymeric CLC film having a high reflectance for laser emission or an aluminum layer for reflection of both pump and lasing beams. Laser emission toward one direction was observed with enhanced intensity.