Hsin-Hui Kuo

Stable mode-locked fiber laser based on CVD fabricated graphene saturable absorber.

A stable mode-locked fiber laser (MLFL) employing multi-layer graphene as saturable absorber (SA) is presented. The multi-layer graphene were grown by chemical vapor deposition (CVD) on Ni close to A-A stacking. Linear absorbance spectrum of multi-layer graphene was observed without absorption peak from 400 to 2000 nm. Optical nonlinearities of different atomic-layers (7-, 11-, 14-, and 21- layers) graphene based SA are investigated and compared. The results found that the thicker 21-layer graphene based SA exhibited a smaller modulation depth (MD) value of 2.93% due to more available density of states in the band structure of multi-layer graphene and favored SA nonlinearity. A stable MLFL of 21-layer graphene based SA showed a pulsewidth of 432.47 fs, a bandwidth of 6.16 nm, and a time-bandwidth product (TBP) of 0.323 at fundamental soliton-like operation. This study demonstrates that the atomic-layer structure of graphene from CVD process may provide a reliable graphene based SA for stable soliton-like pulse formation of the MLFL.

Development of Broadband Single-Mode Cr-Doped Silica Fibers

The fabrication of broadband single-mode Cr-doped silica fibers (SMCDSFs) using the fiber drawing-tower method with the modified rod-in-tube technique is demonstrated for the first time. A single-mode characteristic of SMCDSF was observed when the propagation wavelengths were longer than 1310 nm. The transmission loss was about 8 dB/m at 1550 nm. The successful fabrication of SMCDSFs may facilitate the possibility for utilizing the SMCDSFs as a new generation broadband fiber amplifier to cover the bandwidths in the whole 1300- to 1600-nm range of low-loss windows of silica fibers.

Role of SiNx Barrier Layer on the Performances of Polyimide Ga2O3-doped ZnO p-i-n Hydrogenated Amorphous Silicon Thin Film Solar Cells

In this study, silicon nitride (SiNx) thin films were deposited on polyimide (PI) substrates as barrier layers by a plasma enhanced chemical vapor deposition (PECVD) system. The gallium-doped zinc oxide (GZO) thin films were deposited on PI and SiNx/PI substrates at room temperature (RT), 100 and 200 °C by radio frequency (RF) magnetron sputtering. The thicknesses of the GZO and SiNx thin films were controlled at around 160 ± 12 nm and 150 ± 10 nm, respectively. The optimal deposition parameters for the SiNx thin films were a working pressure of 800 × 10−3 Torr, a deposition power of 20 W, a deposition temperature of 200 °C, and gas flowing rates of SiH4 = 20 sccm and NH3 = 210 sccm, respectively. For the GZO/PI and GZO-SiNx/PI structures we had found that the GZO thin films deposited at 100 and 200 °C had higher crystallinity, higher electron mobility, larger carrier concentration, smaller resistivity, and higher optical transmittance ratio. For that, the GZO thin films deposited at 100 and 200 °C on PI and SiNx/PI substrates with thickness of ~000 nm were used to fabricate p-i-n hydrogenated amorphous silicon (α-Si) thin film solar cells. 0.5% HCl solution was used to etch the surfaces of the GZO/PI and GZO-SiNx/PI substrates. Finally, PECVD system was used to deposit α-Si thin film onto the etched surfaces of the GZO/PI and GZO-SiNx/PI substrates to fabricate α-Si thin film solar cells, and the solar cells’ properties were also investigated. We had found that substrates to get the optimally solar cells’ efficiency were 200 °C-deposited GZO-SiNx/PI.

Fabrication of CIS Absorber Layers with Different Thicknesses Using A Non-Vacuum Spray Coating Method

In this study, a new thin-film deposition process, spray coating method (SPM), was investigated to deposit the high-densified CuInSe2 absorber layers. The spray coating method developed in this study was a non-vacuum process, based on dispersed nano-scale CuInSe2 precursor and could offer a simple, inexpensive, and alternative formation technology for CuInSe2 absorber layers. After spraying on Mo/glass substrates, the CuInSe2 thin films were annealed at 550 °C by changing the annealing time from 5 min to 30 min in a selenization furnace, using N2 as atmosphere. When the CuInSe2 thin films were annealed, without extra Se or H2Se gas used as the compensation source during the annealing process. The aim of this project was to investigate the influence of annealing time on the densification and crystallization of the CuInSe2 absorber layers to optimize the quality for cost effective solar cell production. The thickness of the CuInSe2 absorber layers could be controlled as the volume of used dispersed CuInSe2-isopropyl alcohol solution was controlled. In this work, X-ray diffraction patterns, field emission scanning electron microscopy, and Hall parameter measurements were performed in order to verify the quality of the CuInSe2 absorber layers obtained by the Spray Coating Method.

Nanographene-Based Saturable Absorbers for Ultrafast Fiber Lasers

The generation of femtosecond pulse laser in the erbium-doped fiber laser system is presented by integrating of the nanographene-based saturable absorbers (SAs). A simplified method of dispersed nanographene-based SAs side-polished fiber device with controllable polished length and depth was also developed. The dependence of geometry of a graphene-deposited side-polished fiber device on optical nonlinear characteristics and on the performance of the MLFL was screened. We found that the 10 mm polished length with 1.68 dB insertion loss had the highest modulation depth (MD) of 1.2%. A stable MLFL with graphene-based SAs employing the optimized side-polished fiber device showed a pulse width, a 3 dB bandwidth, a time-bandwidth product (TBP), a repetition rate, and pulse energy of 523 fs, 5.4 nm, 0.347, 16.7 MHz, and 0.18 nJ, respectively, at fundamental soliton-like operation. The femtosecond pulse laser is achieved by evanescent field coupling through graphene-deposited side-polished fiber devices in the laser cavity. This study demonstrates that the polished depth is the key fabrication geometric parameter affecting the overall optical performance and better results exist within the certain polished range.

Effects of Doping Ratio and Thermal Annealing on Structural and Electrical Properties of Boron-Doped ZnO Thin Films by Spray Pyrolysis

Boron-doped zinc oxide (BZO) thin films have been fabricated by spray pyrolysis on a glass substrate. The morphology and electrical properties of the thin films were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed. It was found that [B]/[Zn] ratio altered both the microstructure and concentration of the BZO thin films. The film grain size was reduced by increasing the [B]/[Zn] ratio. The highest Hall mobility was 3.65 cm2 V-1 s-1 for the undoped ZnO thin film, and the highest carrier concentration of 1.0×1019 cm-3 was achieved for the as-deposited BZO thin film with [B]/[Zn] = 1.5 at. %. Conductivity was determined at different measurement temperatures and shallow donors provided the dominate conduction mechanism for the as-deposited BZO thin films. After 600 °C annealing, shallow level reduction and donors with a high activation energy of 129±6 meV in the BZO thin films were characterized, and the shallow donors that dominate the carrier concentration for the as-deposited spray-pyrolized BZO thin film were eliminated.

Performance of Graphene Mediated Saturable Absorber on Stable Mode-Locked Fiber Lasers Employing Different Nano-Dispersants

High-performance stable mode-locked fiber lasers (MLFLs) employing nano-composite polymer-graphene saturable absorber (SA) are demonstrated. The graphene layers were dispersed by two different dispersants including fluorinated MICA clay (MICA) and poly(oxyethylene)-segmented imide (POEM). Using the SA made by graphene dispersed in MICA with thickness and concentration product (TCP) of 36 (μ m*wt%), the MLFLs exhibited pulsewidth, 3-dB spectral bandwidth, and modulation depth (MD) of 393±14 fs, 6.6 nm, and 2.57%, respectively. By contrast, the graphene dispersed in POEM provides a TCP of 38 (μm*wt%) to make the MLFLs deliver pulsewidth, 3-dB spectral bandwidth, and MD of 442±32 fs, 5.9 nm, and 1.70%, respectively. In comparison, the graphene SA dispersed by MICA performs a better MLFL pulse quality and uniformity than that dispersed by POEM. Both MLFLs with SAs fabricated by graphene/MICA and graphene/POEM reveal shortened pulsewidth and enhanced MD as the TCP increases. This observation indicates that the TCP and MD serve as key parameters for characterizing the MLFL pulsewidth. In this study, the nano-MICA dispersant is used for the first time to fabricate the graphene-polymer based SA film and has demonstrated highly stable and uniform ML pulse laser output. Judicious selection of dispersants for dispersing graphene in a homogeneous state is essential for enhancing the MLFL performance. This stable and uniform mode-locked pulse formation by employing the novel graphene-mediated SA has proven the high performance MLFLs that potentially applicable for a myriad of low-cost nano-devices.

Fabrication and characteristics of ultra-broadband singlemode Cr-doped fibers

The fabrication of ultra-broadband singlemode Cr-doped fibers (SMCDFs) using fiber drawing-tower method is demonstrated for the first time. The success in fabrication of SMCDFs may open the possibility for utilizing SMCDFs as ultra-broadband fiber amplifiers.

Few-mode Cr-doped double-clad crystalline fibers

The fabrication of few-mode Cr-doped double-clad crystalline fiber (FMCDCF) using a multiple core tuning process is demonstrated for the first time. The FMCDCF exhibited a core diameter of 2 ± 0.6 µm and a V-value of 3.77 to ensure the few-mode characteristics.

Few-Layer Graphene-Based Saturable Absorbers Employing Mica Dispersant for Fiber Lasers

Few-layer graphene-based saturable absorbers (SAs) employing the novel fluorinated mica clay (mica) dispersant for stable mode-locked fiber lasers (MLFLs) are developed. Optical linear absorbance spectrum of graphene-in-polymer film is observed without absorption peak from 400 to 2000 nm. Optical nonlinearities of 1, 2, and 3 wt% graphene-based SAs are initially screened and optimized to be 3 wt% as the highest modulation depth value of 2.46%. By utilizing the mica dispersant for graphene, the developed SA exhibits a stable MLFL showing a pulse width of 382 fs, a bandwidth of 6.80 nm, and a time-bandwidth product of 0.322 at fundamental soliton-like operation. The use of plate-like mica to uniformly disperse graphene-based SAs is crucial for producing stable, ultrafast, and high-performance MLFLs.