GIPO newsletter

Development of High-Index Transparent Electrodes for Optoelectronic Applications

Professor Chung-Chih Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Light out-coupling has been a long-lasting issue for organic light-emitting devices (OLEDs). Aside from light extraction schemes relying on generally complicated optical structures, the most convenient and desirable approach, however, is to directly enhance light extraction through modulating optical properties of active layers in conventional planar OLEDs. We study the adoption of a high refractive index transparent electrode as a new avenue for greatly boosting OLED light extraction efficiency. By using the Nb-doped TiO₂ transparent electrodes with a high refractive index of 2.4, along with state-of-the-art low-index organic carrier transporters and highly horizontal dipole emitters, a very high external quantum efficiency of exceeding 40% (41.5%) is realized for simple, conventional planar OLEDs. As a perspective, simulation also predicts extremely high optical out-coupling efficiency approaching or exceeding 70% with conventional planar OLEDs, if even higher-index transparent electrodes, purely horizontal dipole emitters, and even lower-index carrier transporters can be realized. The results of this work manifest simple device architectures for extremely efficient OLEDs, and are believed to provide useful guidelines regarding future development of materials and devices for extremely efficient OLEDs.

Fig. 1 High efficiency OLED adopting high-index transparent electrodes.

A Wide-angle Wideband Polarization-insensitive Metamaterial Absorber Based on UPML Mechanism

Professor Yih-Peng Chiou’s Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

1. Introduction

The absorption spectral response of conventional broadband metamaterial absorbers (MMA) is usually sensitive to the light polarization when oblique incident angle increases. Normally thickness is increased, sometimes much larger than operating wavelength, to avoid the exacebation of absorption and bandwidth under oblique incidence. We adopt the concept of uni-axial perfect match layer (UPML). Transform optics is reallized to analyze the relative permittivity and permeability tensors for a uni-axial medium to build a MMA which can achieve perfect absorption without extra thickness.

This proposed wide-angle, wideband, polarization-insensitive terahertz absorptive MMA consists of a paired slot-frequency selective surface (FSS), a vertical rod VIA and a split-ring resonator (SRR) based on the design mechanism of UPML. By tailoring the dispersion of periodic VIAs and SRRs, the components of our MMA satisfy the required macroscopic material anisotropy.

2. Theory and Structure

Fig. 1 illustrates the unit cell structure of our terahertz MM absorber consisting of a multi-layered structure with four components. The top layer is resistive chromium SRR. Details of the structure can be found in [1].

Fig. 1 The unit cell of THz MMA [1].

It is well known that the electromagnetic property of a uni-axial anisotropic medium can be described as an diagonal tensor. The relative permittivity (permeability) tensor for the uniaxial medium can be expressed as,

The SRR and VIA shown in Fig. 1 determine the optical axis directions of permittivity ε_a and permeability μ_c, respectively. Through derivation of Fresnel’s coefficient on the interface where the optical-axis is parallel to the normal vector of interface between isotropic medium and uniaxial medium, the reflectionless condition can be obtain as μ_d = ε_b, ε_a = ε_b-1, and ε_a=μ_c [1], [2].

Fig. 2 (a) The real part of ε_a, and ε_b^-1. (b) The real part of ε_a, μ_c and μ_d. The green dashed line indicates the permeability μ_d in vacuum.

Each component involving VIA-rod, SRR, and slot-FSS of MMA can be tailored to make macroscopic dispersion of each part satisfying the required UPML relation. The values of effective ε_a, ε_b^-1, and μc are closed to each other after 3 THz. Due to the lack of magnetic materials in high frequency regime, the value of μ_d for our proposed absorber is close to one without dispersion as shown in Fig. 2.

3. Full-wave Simulation

Fig. 3 Reflectance spectra of the proposed structure as function of incident angle for (a) TE polarization and (b) TM polarization. The white dash curves and black dash-dot curves indicate the gradient contours for 10% and 50% reflectance, respectively.

The MMA shown in Fig. 1 is simulated and optimized using the commercial full-wave simulation software (Ansys HFSS). The simulated reflectance spectra (Fig. 2(a) and Fig. 2(b)) show a outstand performance. The reflectance of the MM absorber is below 10% in a wide incident angles ranging from 0^o to 60^o for both TE and TM polarizations within the frequency regime of 0.9 to 10.5 THz, corresponding to 168% bandwidth to the central wavelength.

4. Summary

We demonstrated a wideband and wide-angle terahertz absorber based on the concept of UPML. The SRR, a bi-layered slot-FSS, and VIA structures were optimized to achieve equivalent electric and magnetic plasma frequencies and similar dispersion profiles to approach the effective permittivity and permeability tensor of an ideal UPML. The reflectance can achieve excellently below 10% from 0.9THz to 10.5THz, corresponding to 168% of the central frequency under 0^o to 60^o incidence, for both TE and TM waves.

Reference:

[1] S.-K. Tseng, H.-H. Hsiao, and Y.-P. Chiou, “Wide-angle wideband polarization-insensitive perfect absorber based on uniaxial anisotropic metasurfaces,” OSA Opt. Mater. Express, 10(5), 1193-1203 (2020).

[2] A. Taflove and S. C. Hagness, “Computational Electrodynamics: The FDTD method, 5th ed”, Chap. 7 (Artech House, 2007)

FDTD simulation analysis of the nano structures color appearances of biological creatures

Professor Snow H. Tseng’s Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Abstract: Here we report a simulation approach to analyze the effect of nano structure and regularity on biological color appearances. The finite-difference time-domain (FDTD) simulation technique is employed to analyze the optical characteristics of various biological species: Morpho menelaus, Euprymna scolopes, Dynastes hercules, Hoplia coerulea, and Paracheirodon innesi. Various parameters of their optical structures are analyzed, including: size, spacing and regularity of the structure and the refractive indices of the structure’s component materials. Simulation findings demonstrate that the proposed simulation is robust, yields prediction color matching the actual color appearances observed in nature. More importantly, the reported simulation provides a means to analyze and envision the color appearance for engineered structures.

Figure. Schematics of the simulation model of various species [1-5]. The simulation variable for each model: (a) Hercules beetle: the refractive index of the gap between layered nano structures. (b) Cerulean chafer beetle: the refractive index of the gap between layered nano structures. (c) Morpho butterfly: the number of stacked nano layers. (d) Neon tetra: the width of spacing and structure thickness. (e) Hawaiian bobtail squid: thickness of the nano structure layers.

Reference:

[1] S. Yoshioka, B. Matsuhana, S. Tanaka, Y. Inouye, N. Oshima, and S. Kinoshita, "Mechanism of variable structural colour in the neon tetra: quantitative evaluation of the Venetian blind model," J R Soc Interface 8, 56-66 (2011).

[2] S. R. Mouchet, M. Lobet, B. Kolaric, A. M. Kaczmarek, R. Van Deun, P. Vukusic, O. Deparis, and E. Van Hooijdonk, "Photonic scales of Hoplia coerulea beetle: any colour you like," Materials Today: Proceedings 4, 4979-4986 (2017).

[3] S. Kinoshita, S. Yoshioka, Y. Fujii, and N. Okamoto, "Photophysics of Structural Color in the Morpho Butterflies," FORMA 17, 103-121 (2002).

[4] H. E. Hinton, and G. M. Jarman, "Physiological colour change in the elytra of the hercules beetle, Dynastes hercules," Journal of Insect Physiology 19, 533-549 (1973).

[5] A. R. Tao, D. G. DeMartini, M. Izumi, A. M. Sweeney, A. L. Holt, and D. E. Morse, "The role of protein assembly in dynamically tunable bio-optical tissues," Biomaterials 31, 793-801 (2010).

Ultrawide-Angle and High-Efficiency Metalens in Hexagonal Arrangement

Professor Guo-Dung Su

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Wide-angle optical systems play a vital role in imaging applications and have been researched for many years. In traditional lenses, attaining a wide field of view (FOV) by using a single optical component is difficult because these lenses have crucial aberrations. In this study, we developed a wide-angle metalens with a numerical aperture of 0.25 that provided a diffraction-limited FOV of over 170° for a wavelength of 532 nm without the need for image stitching or multiple lenses. The designed wide-angle metalens is free of aberration and polarization, and its full width of half maximum is close to the diffraction limit at all angles. Moreover, the metalens which is designed through a hexagonal arrangement exhibits higher focusing efficiency at all angles than most-seen square arrangement. The focusing efficiencies are as high as 82% at a normal incident and 45% at an incident of 85°. Compared with traditional optical components, the proposed metalens exhibits higher FOV and provides a more satisfactory image quality because of aberration correction. Because of the advantages of the proposed metalens, which are difficult to achieve for a traditional single lens, it has the potential to be applied in camera systems and virtual and augmented reality.

Schematic of the metalens and its top view at incident angles of 0°, 30°, 60°, and 85° (a) Layout at different incident angles; (b) schematic of the wide-angle metalens with hexagonal arrangement at different angles; and (d) top view of the metalens at different angles.

Organic light-emitting diode with fluorophore sensitized triplet-triplet annihilation

Professor Jiun-Haw Lee

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

An organic light-emitting diode (OLED) based on triplet-triplet annihilation upconversion (TTAUC) was demonstrated consisting of tri-layer structure: tris-(8-hydroxyquinoline)aluminum (Alq₃),1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP), and 9,10-bis(2’-naphthyl) anthracene (ADN) which acted as sensitizer, triplet-diffusion-singlet-blocking (TDSB) layer, and emitter, respectively. Carriers recombined at the sensitizer which formed 25% singlets and 75% triplets. Triplets transferred the energy to emitter through TDSB for TTAUC emission, while TDSB blocked ADN singlet quenching by sensitizer. Intrinsic efficiency of TTAUC, defined as output triplet exciton number from TTAUC process over input triplet exciton, reached 86.1% in this OLED.

Fig. 1. Schematic diagram of energy transfer routes in the TTAUC-OLED.