GIPO newsletter

Plasmon-enhanced Photoluminescence of an Amorphous Silicon Quantum Dot Light-Emitting Device by Localized Surface Plasmon Polaritons in Ag/SiO_x:a-Si QDs/Ag Sandwich Nanostructures

Professor Hoang-Yan Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We investigated experimentally the plasmon-enhanced photoluminescence of the amorphous silicon quantum dots (a-Si QDs) light-emitting devices (LEDs) with the Ag/SiO_x:a-Si QDs/Ag sandwich nanostructures (Fig. 1), through the coupling between the a-Si QDs and localized surface plasmons polaritons (LSPPs) mode, by tuning a one-dimensional (1D) Ag grating on the top. The coupling of surface plasmons at the top and bottom Ag/SiO_x:a-Si QDs interfaces, resulting in the localized surface plasmon polaritons (LSPPs) confined underneath the Ag lines, which exhibit the Fabry–Pérot resonance. From the Raman spectrum, it proves that the existence of a-Si QDs embedded in Si-rich SiO_x film (SiO_x:a-Si QDs) at a low annealing temperature (300℃) to prevent the possible diffusion of Ag atoms from Ag film. The photoluminescence (PL) spectra of a-Si QDs can be precisely tuned by a 1D Ag grating with different pitches and Ag line widths (Fig. 2) were investigated. An optimized Ag grating structure, with 500 nm pitch and 125 nm Ag line width, was found to achieve up to 4.8-fold PL enhancement at 526 nm and 2.46-fold PL integrated intensity compared to the a-Si QDs LEDs without Ag grating structure, due to the strong a-Si QDs–LSPPs coupling (Fig. 3). (International Journal of Photoenergy (Volume 2015 (2015), Article ID 140617)

Fig. 1. Schematic view of a-Si QDs LEDs with a Ag/SiO_x:a-Si QDs/Ag sandwich nanostructures

Fig. 2. SEM images of a series of Ag gratings (samples B-E). Scale bar is 1 μm

Fig. 3. The plots of the PL enhancement factor of samples B-E

Direct QAM-OFDM Encoding of a Master-to-Slave Injection-Locked WRC-FPLD Pair for 28×20 Gbit/s DWDM-PON Transmission

Professor Gong-Ru Lin’s laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

This work demonstrated a master-to-slave injection-locked weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) pair, which is capable of carrying an orthogonal frequency division multiplexed 16-quadrature amplitude modulation (16-QAM OFDM) data at 20 Gbit/s/channel for 28 channels in a dense wavelength division multiplexing passive optical network (DWDM-PON), as illustrated in Fig. 1. Since the lower front-facet reflectance and longer cavity features of WRC-FPLD, it reveals broader gain spectrum with denser longitudinal modes and higher injection efficiency than a typical FPLD for supporting more DWDM channels. As a result, the optical spectra of master and slave WRC-FPLDs are shown in Fig. 2. After passing through a demultiplexer with a channel spacing of 50 GHz for the central mode of master WRC-FPLD, the filtered single-mode master with a peak power of -4 dBm was employed to injection-lock the slave WRC-FPLD, which achieves a side-mode suppression ratio (SMSR) of 50 dB. Moreover, the bit error rate (BER) performances of back-to-back and 25-km single-mode fiber (SMF) transmitted 20-Gbit/s OFDM data carried by the master-to-slave injection-locked WRC-FPLD pair at different channels are shown in Fig. 3. For the central mode injection case, the back-to-back transmitted BER of 1.6×10^-5 can be improved to 2.9×10^-7 by employing the OFDM subcarrier per-leveling technique, and it is inevitably degraded to 2.1×10^-4 due to the fiber dispersion after 25-km SMF transmission. To meet the forward error correction (FEC) the required BER of 3.8×10^-3, 28 DWDM channels can be obtained to reveal a total data-rate of 28×20=560 Gbit/s. Therein, a slightly decreased SMSR of 35 dB is also obtained for the farthest side-mode injection case. In conclusion, the master-to-slave injection-locked WRC-FPLD pair has shown its competitive performance and superior flexibility to fit multi-channel DWDM-PON transmission.

Fig. 1. The schematic diagram of the master-to-slave injection-locked WRC-FPLD pair based DWDM-PON system for direct 16-QAM OFDM modulation and transmission.

Fig. 2. The optical spectra of master and slave WRC-FPLDs.

Fig. 3. The BER performances of back-to-back and 25-km SMF transmitted 28-Gbit/s 16-QAM OFDM data.

Enhancing Light Out-Coupling of Organic Light-Emitting Devices with Nanostructured Composite Electrodes

Professor Chung-Chih Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Due to their various merits, organic light-emitting devices (OLEDs) have posed as promising candidates for the next-generation display and lighting technologies. All these applications have been imposing increasing efficiency requirements on OLEDs. While internal quantum efficiencies close to 100% have been reported with phosphorescent or other triplet-harvesting OLEDs, typical OLEDs suffer from a poor extraction efficiency of internally generated photons, resulting from a large mismatch of the refractive indices between organic layers, indium tin oxide, glass, and air. The total internal reflection at the interfaces of devices strongly traps photons inside the device, which significantly limits the external quantum efficiency (EQE) of OLEDs. It is known that ~70-80% of the generated photons are trapped in conventional OLED structures due to total internal reflection or coupling to surface plasmon polaritons (SPPs) at the metallic surface. In our studies, we have developed a novel internal extraction structure using a nanostructured composite transparent electrode (Figure 1), which not only functions as the current conductor/injector for the device but also significantly enhances out-coupling of OLED internal radiation. With such a technique, we have realized a very high externalquantum efficiency of 62% and a very high luminous efficiency of 264 lm/W in OLEDs (Figure 1).

Figure 1. Achieving very high external quantum and power efficiencies of OLEDs with nanostructured composite electrodes

Flexible Transparent Graphene-based Composite Transparent Electrode Prepared with Copper Vapor Assisted CVD and Innovative Polymer-free Transfer Method

Professor Chih-I Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The major tasks of this research project is to fabricate flexible or transparent perovskite halogenide solar cells by whole solution processes for high enough energy conversion efficiency at low sun luminance condition. The initial achievements of whole solution processes and optoelectronic characterization include one-step precursor deposition, solvent induced fast crystallization, vapor assisted solution process. Each of these processing technologies resulted in the analyses comparing their I-V curves, fill factor, open-circuit voltage, optoelectronic energy conversion efficiency. These measurements were done for different fil thickness, since the correlation between characterizations and thickness is important to evaluate the flexibility and transparency of perovskite solar cells.

Transparent graphene electrode materials have been improved by innovative copper vapor assisted C. V. D. for graphene/Cu composite thin film synthesis. Such grown composite electrode were transferred by special "Protective Frame Transfer Method" to form double layered composite electrode material that held transmittance 94.4% and low electric sheet resistance as 0.1 k ohm/sqare. 2D-band to G-band peak ratio of 1.8 measured by Raman spectroscopy, and X-ray Photoelectron Spectroscopy (X.P.S.) showed minor existence of copper on graphene, such evidence of Cu on graphene surface was supported with photograph taken using scanning electron microscopy. Those peak positions of 2D-band and G-band were found shifted to higher Raman Shift (1/cm) side. These phenomena imply that C-C bonding strength of graphene intra-plane and inter-layer van der Waal bonding could be enhanced at those portions where copper particles/islands are bonded to graphene. Comparing graphene/Cu composite and that etched off copper with nitric acid solution, their density of states analyses for valence band using Ultraviolet Photoelectron Spectroscopy showed that semi-metallic graphene’s valence band structure had no significant rising 1~2 eV near Fermi Level for this composite thin film. Thus, the enhanced electric conductivity could majorly be attributed to the bridging effect of lower resistant copper particles/islands across high resistant domain boundaries of graphene leading to increased electric current paths through low electric sheet resistance. Resulted in much higher successful film transfer ratio, this bridging effect had been supported by enhanced mechanical strength effect of graphene/Cu composite films than pure graphene films.

Fig. 1. SEM photographs: (a) Near; (b) farther from, (c) near, uniform temperature zone

Fig. 2. Raman spectroscopic plots

Fig. 3. X-ray Photoelectron Spectroscopic plots showing work function increase with graphene/copper composites

Fig. 4. Ultraviolet Photoelectron Spectroscopic plots showing no density of states increase near Fermi Level with graphene/copper composites

A Circular-Ring Miniaturized-Element Metasurface With Many Good Features for Frequency Selective Shielding Applications

Professor Yih-Peng Chiou's laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

A novel circular-ring miniaturized-element (CRME) metasurface is proposed for frequency selective electromagnetic shielding applications in this paper. The proposed structure exhibits good performance in many aspects. The CRME size can be easily reduced to 0.088_rx0.088_r, where _r is the wavelength at resonance. Such small size can be useful in applications with limited space. The newly proposed metasurface not only shows the stability to incidence angles and polarizations, but also demonstrate negligible polarization crossing. The metasurface also presents low coupling between elements, which leads to the insensitivity of periodicity fluctuation in fabrication process. Moreover, the frequency responses can be varied through the modification of the element pattern of the metasurface. It provides flexible frequency responses to use in frequency selective shielding applications. Also, this element is suitable for constructing a metasurface aligned in either square or even hexagonal lattice, which owns the best adaptability to a doubly curved surface or a spherical one. Without revising the CRME, both the squareand hexagonal-lattice metasurfaces constructed can perform competently at the same time. Finally, the metasurface prototype is fabricated and tested in a fully anechoic chamber to verify the design. The measured results well agree with the simulated ones. This work is published in IEEE Transaction on Electromagnetic Compatibility, June 2015.

Left: The fabricated prototype of the eight-turn CRME metasurface with the line width of 0.15 mm and Λ = 6.7 mm. Right: Eight-turn CRME metasurface at normal incidence. Simulated and measured results are very consistent.

Improvement of color stability in white organic light-emitting diode by phosphor dopant

Professor Jiun-Haw Lee

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

With the incorporation of thin (4 nm) and lightly doped (1%) phosphor, tris(phenylpyridine)iridium (Ir(ppy)₃), inside the emitting layer close to the electron transporting layer interface of an organic light-emitting diode (OLED), electron injection characteristic was improved. Together with the recombination zone shift under different voltage driving, a white OLED with stable electroluminescence (color coordinate shift (0.012, 0.002) from 100 to 10000 cd/m²) was demonstrated.

Fig. 1. Electroluminescence spectra of OLED (a) without and (b) with interface phosphor dopant