No abstract

Manipulation of light is in strong demand in information technologies. Among the wide range of linear and nonlinear optical devices that have been used, growing attention has been paid to photonic crystals that possess a periodic modulation of dielectric function. Among many photonic bandgap (PBG) structures, liquid crystals with periodic structures are very attractive as self-assembled photonic crystals, leading to optical devices such as dye lasers. Here we report a new hetero-PBG structure consisting of an anisotropic nematic layer sandwiched between two cholesteric liquid-crystal layers with different helical pitches. We optically visualized the dispersion relation of this structure, displaying the optical diode performance: that is, the non-reciprocal transmission of circular polarized light at the photonic-bandgap regions. Transmittance spectra with circularly polarized light also reveal the diode performance, which is well simulated in calculations that include an electro-tunable diode effect. Lasing action was also confirmed to show the diode effect with a particular directionality.

Lasing via a phase retardation defect mode is reported for the first time. The Figure shows the far‐field pattern of laser emission realized by inserting an anisotropic nematic liquid crystal (LC) defect layer between polymer cholesteric LC layers. The system mimics the cuticle of Plusiotis resplendens, a beetle, using the birefringence of the anisotropic layer to achieve reflectance greater than the 50 % provided by simple cholesteric LC photonic bandgaps.

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][2][3][4][5][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.The existence of waveguide modes in OLEDs is strongly dependent on the thickness and refractive indices of device layers. In an OLED structure illustrated in Figure 1 a, we used a thin ITO layer of 40-nm thickness to minimize the TE waveguide mode and excite only one TM 0 waveguide mode that is a surface plasmon (SP) mode at the interface between the organic and cathode layers. The waveguide modes were characterized by calculating their in-plane propagation vectors for the emission wavelength using a transfer-matrix method. One weak TE 0 mode (blue line) that is leaky to resin or glass mode above ∼ 500 nm (below a dotted line in Figure 1 b), and one strong SP mode (red line) existing over entire emission wavelength from 450-750 nm were found in the OLED structure (Figure 1 b). The SP mode with a large in-plane propagation wave vector k SP can fall into the escape cone by the diffraction grating with a wave vector k G ,where k 0 denotes the absolute value (wavenumber) of the wavevector in free space, k | | its in-plane component, θ ...

Cobalt and chromium substituted lithium-excess manganese oxides, Li[Li0.2CoxCr0.4-xMn0.4]O2 (x = 0, 0.2, and 0.4), are synthesized by a simple combustion method. Crystal structures of the samples are examined by an X-ray diffraction (XRD) method. The samples crystallize into a rhombohedral layered phase with in-plane cation ordering between lithium and transition metal ions. Li[Li0.2Co0.2Cr0.2Mn0.4]O2 can deliver more than 200 mAh g−1 of discharge capacity above 3.0 V vs. Li at a rate of 10 mA g−1. Discharge capacity further increases to 300 mAh g−1 when the Li cell is discharged to 1.5 V with appearance of a voltage plateau at 1.7 V. A combined study of synchrotron XRD and X-ray absorption spectroscopy (XAS) reveals that originally trivalent chromium ions are oxidized to hexavalent state, resulting in the migration of chromium ions into face-shared tetrahedral sites in lithium layers. Although a part of hexavalent chromium ions are immobilized at the tetrahedral sites, the reduction process to 1.5 V allows the chromium ions to migrate to octahedral sites associated with the reduction process to trivalent state. The chromium substitution has a beneficial effect on stabilization of oxide ion framework structures of the lithium-excess manganese system by suppressing oxygen loss upon charge to a high voltage region, leading to good cyclability as electrode materials.