GIPO newsletter

Passively mode-locked erbium-doped fiber laser with graphite and charcoal nano-particle

Professor Gong-Ru Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

It is the first demonstration that the graphite nano-particle can serve as a saturable absorber to passively mode-lock the erbium-doped fiber laser (EDFL). By directly brushing the graphite nano-particle on the end-face of a FC/APC with the assistance of polymer polyvinyl alcohol (PVA) film (see Fig. 1), such a novel approach greatly simplifies the fabrication procedure and suppresses the film-thickness induced laser-beam divergent loss to promote the pulsewidth shortening ability. This work was select as the cover story of the journal of Laser Physics Letters (see Fig. 2) and the highly cited paper. The saturable absorber with an optimized area coverage ratio of 25% provides the modulation depth of 20% to generate a passively mode-locked EDFL pulsewidth of 482 fs (Fig. 1). Later on, the free-standing graphite nano-particle is also successful on mode-locking the EDFL without the assistance of PVA film; however, the self-aggregated graphite nano-particles induce a high cavity loss to result in a broadened pulsewidth of 1.67 ps and a time-bandwidth product of 0.325 under an intra-cavity group delay dispersion of –0.017 ps² (Fig. 3). The graphite nano-particle with reduced size and sufficient coverage ratio can effectively decrease the insertion loss and lasing threshold to compete with other fast saturable absorbers such as carbon nanotube or graphene. In addition, the triturated charcoal nano-powder is preliminarily used to passively mode-lock the EDFL. These dispersant-free charcoal nano-powder with few crystalline graphene phase and highly disordered carbon structure can easily provide a passively mode-locked pulsewidth as short as 0.9-1.36 ps in a low-gain EDFL (Fig. 4). The nanoscale charcoal saturable absorber shows its potential for generating subpicosecond pulse under a relatively low cavity gain. An appropriate cavity design can compensate the defect-induced loss to overcome the limitation for obtaining a shortened pulsewidth in the near future.


Figure 1. Schematic diagram of the direct brushing process and the autocorrelation trace of the passively mode-locked EDFL, as well as the image of graphite nano-particle on the end-face of FC/APC.	Figure 2. The cover story of Laser Physics Letters in vol. 8, pp. 880-886, 2011 (SCI Highly cited paper in 2012).


Figure 3. The graphite nano-particle based passively mode-locked EDFL system with the autocorrelation trace and the optical spectrum.	Figure 4. The charcoal nano-particle based passively mode-locked EDFL system with the autocorrelation trace and the optical spectrum.

Applications of nanostructured materials for organic/nano/hybrid optoelectornics

Professor Chung-Chih Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We have been dedicated to development of nanostructured materials and their applications to various organic/nano/hybrid optoelectronic devices, for instance, nanostructured dye-sensitized solar cells (DSSCs). DSSCs offer various advantages and are one of promising solar cell technologies. A typical DSSC consists of transparent conductive substrate (anode), a porous thin-film photoelectrode consisting of TiO₂ nanoparticles, dye, electrolyte, and a counter electrode. Each part of the cell has crucial effects on overall cell efficiency. We had investigated the effects of nanostructured anodes (transparent conducting oxides, TCO) on characteristics and performances of DSSCs. TCO having varied nano-textures were studied for their morpholoical, electrical and optical properties and were subjected to device applications (Fig. 1). Highly textured and high-haze TCO can effectively scatter the incident light into larger angles in the device (Fig. 1), enhancing the propagation length and trapping of the incident light within the device and enhancing absorption/light harvesting (and thus short-circuit current, quantum efficiency, and power conversion efficiency) of the device. We also investigated the growth/formation of nano-textured/nanoporous /nanorod counter electrodes of DSSCs (Fig. 2). Counter electrodes having varied nano-textured/nanoporous/nanorod structures were characterized for their morphological and electrochemical properties (Fig. 2), and were subjected to device applications. Highly nano-textured/nanoporous /nanorod counter electrodes can effectively enhance active surface areas, electrocatalytic ability and charge exchange at the interface with the DSSC electrolyte (Fig. 2). By incorporating these effective nanostructured electrodes, highly efficient DSSCs with reduced interface impedance were achieved simply using typical dye sensitizers (Fig. 3).

Fig. 1. (a) DSSC having nano-textured TCO anode. (b) Normal-direction (red) and integrated (black) transmittance spectra of highly textured TCO anode, showing high integrated transmittance and strong optical scattering.

Fig. 2. (a) DSSC having nano-textured/nanoporous /nanorod counter electrode. (b) SEM of nano-textured/nanoporous /nanorod counter electrode. (c) Cyclic voltammetry (CV) and enhanced electro-catalytic activity of nano-textured/nanoporous /nanorod counter electrode.

Fig. 3. (a) I-V, (b) IPCE, and (c) impedance spectroscopy and reduced interface impedance of DSSC incorporating nanotextured anode and nano-textured/nanoporous /nanorod counter electrode.

The investigation of the diffusion length of cathode materials in organic light emitting devices through impedance characteristics

Professor Chih-I Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The impedance-voltage (Z-V) characteristic is introduced as a non-destructive method to analyze the diffusion length of the cathode materials in organic light emitting diodes (OLEDs). The transition voltage is the voltage where the impedance decreases before the device turns on and the hole transport layer (HTL) becomes conductive. The electrical potential of each layer inside the OLED is illustrated in Fig 1, indicating that the transition voltage is determined by the density of the accumulation charges Q and the thickness of the electron transport layer (ETL). The phase-voltage, current density-voltage (J-V), and Z-V characteristics of devices with different thicknesses of the ETLs and various cathode structures are shown in Fig 2 and Fig 3, respectively. The correlations of the transition voltage and the thickness of the ETL of devices with varied film structures are also investigated systematically in Fig 4. The results reveal that the density of the accumulation charges at the organic layers interfaces is constant in devices with the identical carrier transport layers. However, the transition voltages shift toward higher bias voltages in the devices with a lower work-function metal as the cathode (Mg) or an electron injection layers (LiF or Cs₂CO₃). This shift of the transition voltages is attributed to the diffusion of the cathode atoms, which decreases the effective ETL thickness for providing the electrical potential between two electrodes, as shown in Fig 5. Based on this assumption, the diffusion length of the cathode materials being used can be estimated by the overall decreasing of the electrical potential, and the analysis of transition voltages in the Z-V is proposed as a potential method to measure the cathode atoms diffusion length in OLED devices.

Fig 1. Illustration of the electrical potential inside the OLED devices at the transition voltage. The arrow indicates the difference between the turn-on voltage and the transition voltage.

		Fig 4. The transition voltage V₀ versus Alq₃ thickness characteristics of devices with various cathode structures.


		Fig 5. Illustration of the electrical potential inside the devices with the diffusion of the cathode materials.
Fig 2. (a) Phase-voltage and current J-V (inset) and (b) Z-V characteristics of devices with different thicknesses of the ETLs. The film structures of these devices are ITO/NPB (800Å)/Alq₃/Al.	Fig 3. (a) Phase-voltage and J-V (inset) and (b) Z-V characteristics of devices with various cathodes. The film structures of these devices are ITO/NPB(800Å)/Alq₃(800Å)/ cathode.

Efficiency improvement of organic solar cell through deposition rate control

Professor Jiun-Haw Lee

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Increase of open circuit voltage (Voc) and fill factor (FF) were achieved in a small-molecules heterojunction solar cell by varying the deposition rate of donor material subphthalocyanine chloride (SubPc). With increasing the deposition rate of SubPC, we found that: (1) energy level shifted away from the vacuum level, and (2) the molecular stacking tended to amorphous, which resulted from the increase of Voc. Besides, the mobility of subPC thin film decreased with increasing depostion rate, which affected the charge balance in such a device. With suitable treatment of anode layer, considerable improvement in both the VOC (1.02 V) and fill factor (65.37%) of the device, and a 3.96% final efficiency of the device was achieved. [published in Sol. Energy Mater. Sol. Cells. 103, 69, 2012].

Fig. 1. The J–V characteristics of OPV devices at various SubPc deposition rates under (a) dark condition (hollow symbol: experiment results, line: fitting results) and (b) 1-sun AM 1.5G solar illumination. The deposition rates of SubPc in Device D, E, and F are 0.03 nm/s, 0.15 nm/s, and 0.50 nm/s, respectively.

Effects of the spatial frequency composition of the target pattern and the number of quantization levels in diffractive beam shaper design

Professor Hoang-Yan Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

A mathematical model is derived and numerical simulation is analyzed for laser beam shaping by using multi-level phase-only diffractive optical elements. We used the simulated annealing algorithm to design the beam shapers. There is an essential effect on the diffractive pattern quality caused by the spatial frequency composition of target patterns (Fig. 1) for the same incident Gaussian beam size and target pattern area. The root mean square error between the diffractive and target patterns is smaller for the target patterns with lower spatial frequencies (Fig. 2). Moreover, the effect of spatial frequency composition can be relaxed for the cases of larger incident Gaussian beam size (Fig. 3). Besides, finer quality controlling on diffraction pattern can be obtained by increasing the number of quantization levels at the diffractive optical elements.

Fig. 1. Target patterns comprising of squares with various frequency compositions.

Fig. 2. The 4-level beam shapers designed for the target patterns (a) A1; (b) A3; and (c) A5, respectively by using R= 0.25 mm incident Gaussian beam.

Fig. 3. The diffractive patterns of the 4-level beam shapers by using (a) R= 0.25 mm; and (b) R= 0.50 mm incident Gaussian beams.

Far-field self-focusing and -defocusing radiation behaviors of the electroluminescent light sources due to negative refraction

Professor JianJang Huang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Negative refraction was observed from a semiconductor light source with embedded PhC nanohole arrays. For TE mode, when photons interact with the PhC, changes between positive and negative group indices in the dispersive band structure modulate the group velocity, resulting in the confinement of radiation light to 10°. On the other hand, TM-mode photons are less perturbed by the air hole structure. This study uses 3D far-field analysis to visualize the energy distribution radiating from the devices and EFC analysis to obtain the group-velocity dispersion in PhCs. Theoretical and experimental results show that the inward and outward propagating light collide intensely near the Brillouin zone in the TE mode. The polarization-dependent spatial distribution makes it possible to achieve self-collimated electroluminescent devices, enabling negative refraction with short optical path lengths and larger numerical apertures for devices such as lasers and LEDs.

Fig. 1. Device structure of a photonic crystal LED (PhCLED) with surrounding nanohole arrays.

Fig. 2. (a) Electroluminescent radiation profiles. (b) shows the measured TE photonic band, and (c) shows the measured TM. (d) Principle of the negative refraction and light collimation of PhC nanoholes under TE light.

Fig. 3. Far-field radiation of the PhCLED measured (a) for TE and (b) for TM at a wavelength of 454 nm.