Gong-Ru Lin

Oxygen defect and Si nanocrystal dependent white-light and near-infrared electroluminescence of Si-implanted and plasma-enhanced chemical-vapor deposition-grown Si-rich SiO2

The mechanisms for silicon (Si) defect and nanocrystal related white and near-infrared electroluminescences (ELs) of Si-rich SiO2 films synthesized by Si-ion implantation and plasma-enhanced chemical-vapor deposition (PECVD) are investigated. The strong photoluminescence (PL) of Si-ion-implanted SiO2 (SiO2:Si+) at 415–455 nm contributed by weak-oxygen bond and neutral oxygen vacancy defects is observed after 1100 °C annealing for 180 min. The white-light EL of a reverse-biased SiO2:Si+ metal-oxide-semiconductor (MOS) diode with a turn-on voltage of 3.3 V originates from the minority-carrier tunneling and recombination in the defect states of SiO2:Si+, which exhibits maximum EL power of 120 nW at bias of 15 V with a power–current slope of 2.2μW∕A. The precipitation of nanocrystallite silicon (nc-Si) in SiO2:Si+ is less pronounced due to relatively small excess Si density. In contrast, the 4-nm nc-Si contributed to PL and EL at about 760 nm is precipitated in the PECVD-grown Si-rich SiOx film after annealin...

Using graphene nano-particle embedded in photonic crystal fiber for evanescent wave mode-locking of fiber laser

A photonic crystal fiber (PCF) with high-quality graphene nano-particles uniformly dispersed in the hole cladding are demonstrated to passively mode-lock the erbium-doped fiber laser (EDFL) by evanescent-wave interaction. The few-layer graphene nano-particles are obtained by a stabilized electrochemical exfoliation at a threshold bias. These slowly and softly exfoliated graphene nano-particle exhibits an intense 2D band and an almost disappeared D band in the Raman scattering spectrum. The saturable phenomena of the extinction coefficient β in the cladding provides a loss modulation for the intracavity photon intensity by the evanescent-wave interaction. The evanescent-wave mode-locking scheme effectively enlarges the interaction length of saturable absorption with graphene nano-particle to provide an increasing transmittance ΔT of 5% and modulation depth of 13%. By comparing the core-wave and evanescent-wave mode-locking under the same linear transmittance, the transmittance of the graphene nano-particles on the end-face of SMF only enlarges from 0.54 to 0.578 with ΔT = 3.8% and the modulation depth of 10.8%. The evanescent wave interaction is found to be better than the traditional approach which confines the graphene nano-particles at the interface of two SMF patchcords. When enlarging the intra-cavity gain by simultaneously increasing the pumping current of 980-nm and 1480-nm pumping laser diodes (LDs) to 900 mA, the passively mode-locked EDFL shortens its pulsewidth to 650 fs and broadens its spectral linewidth to 3.92 nm. An extremely low carrier amplitude jitter (CAJ) of 1.2-1.6% is observed to confirm the stable EDFL pulse-train with the cladding graphene nano-particle based evanescent-wave mode-locking.

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.

450-nm GaN laser diode enables high-speed visible light communication with 9-Gbps QAM-OFDM

A TO-38-can packaged Gallium nitride (GaN) blue laser diode (LD) based free-space visible light communication (VLC) with 64-quadrature amplitude modulation (QAM) and 32-subcarrier orthogonal frequency division multiplexing (OFDM) transmission at 9 Gbps is preliminarily demonstrated over a 5-m free-space link. The 3-dB analog modulation bandwidth of the TO-38-can packaged GaN blue LD biased at 65 mA and controlled at 25°C is only 900 MHz, which can be extended to 1.5 GHz for OFDM encoding after throughput intensity optimization. When delivering the 4-Gbps 16-QAM OFDM data within 1-GHz bandwidth, the error vector magnitude (EVM), signal-to-noise ratio (SNR) and bit-error-rate (BER) of the received data are observed as 8.4%, 22.4 dB and 3.5 × 10(-8), respectively. By increasing the encoded bandwidth to 1.5 GHz, the TO-38-can packaged GaN blue LD enlarges its transmission capacity to 6 Gbps but degrades its transmitted BER to 1.7 × 10(-3). The same transmission capacity of 6 Gbps can also be achieved with a BER of 1 × 10(-6) by encoding 64-QAM OFDM data within 1-GHz bandwidth. Using the 1.5-GHz full bandwidth of the TO-38-can packaged GaN blue LD provides the 64-QAM OFDM transmission up to 9 Gbps, which successfully delivers data with an EVM of 5.1%, an SNR of 22 dB and a BER of 3.6 × 10(-3) passed the forward error correction (FEC) criterion.

Soliton compression of the erbium-doped fiber laser weakly started mode-locking by nanoscale p-type Bi2Te3 topological insulator particles

We demonstrate the nanoscale p-type Bi2Te3 powder-based saturable absorber-induced passive mode-locking of the erbium-doped fiber laser (EDFL) with sub-picosecond pulsewidth. Such a nanoscale topological insulator powder is obtained by polishing the bulk p-type Bi2Te3 in a commercial thermoelectric cooler (TE cooler). This is then directly brushed onto the end-face of a single-mode fiber patchcord, to avoid any mis-connecting loss caused by laser beam divergence, which can result in a mode-locked pulsewidth of 436 fs in the self-amplitude modulation mode of a TE cooler. To further shorten the pulse, the soliton compression is operated by well-controlling the group delay dispersion and self-phase modulation, providing the passively mode-locked EDFL with a pulsewidth as short as 403 fs.

Nanophotonic switch: gold-in-Ga2O3 peapod nanowires.

A novel metal-insulator heterostructure made of twinned Ga2O3 nanowires embedding discrete gold particles along the twin boundary was formed through a reaction between gold, gallium, and silica at 800 degrees C during simple thermal annealing. The Au-in-Ga2O3 peapods spontaneously crystallized under phase separation induced by the formation of twin boundaries. The nanostructures were analyzed by field emission scanning (FESEM) and transmission electron microscopes (FETEM), and their photoresponse was investigated using a double-frequency Nd:YAG laser with a wavelength of 532 nm on a designed single-nanowire device. The surface plasmon resonance (SPR) effects of embedded Au nanoparticles are proposed to be responsible for the remarkable photoresponse of these novel structures.

Improving carrier transport and light emission in a silicon-nanocrystal based MOS light-emitting diode on silicon nanopillar array

A silicon-nanocrystal (nc-Si) based metal-oxide-semiconductor light-emitting diode (MOSLED) on Si nanopillar array with size, height, and density of 30nm, 350nm, and 2.8×1010cm−2, respectively, is characterized. The nanopillar roughened Si surface contributes to the improved turn-on characteristics by enhancing Fowler-Nordheim tunneling and reducing effective barrier height, providing the MOSLED a maximum optical power of 0.7μW obtained at biased current of 375μA. The optical intensity, turn-on current, and power-current slope of nc-Si MOSLED on high-aspect-ratio Si nanopillar array are 140μW∕cm2, 5μA, 2±0.8mW∕A, respectively. A maximum external quantum efficiency of 0.1% is reported.

Subwavelength Si nanowire arrays for self-cleaning antireflection coatings

Galvanic wet etching was adopted to fabricate Si nanowire arrays (NWAs) as a near-perfect subwavelength structure (SWS), which is an optically effective gradient-index antireflection (AR) surface and also exhibits super-hydrophobicity with an extremely high water contact angle (159°). Fresnel reflection and diffuse reflection over the broad spectrum can be eliminated by a Si NWA AR coating. Moreover, Si NWA SWSs show polarization-independent and omnidirectional AR properties. The wavelength-averaged specular and diffuse reflectance of Si NWA SWSs are as low as 0.06% and 2.51%, respectively. The effects of the surface profile of this biomimetic SWS on the AR and super-hydrophobic properties were investigated systematically.

Comparison on the electroluminescence of Si-rich SiNx and SiOx based light-emitting diodes

Electroluminescence (EL) of the metal-insulator-semiconductor light-emitting diodes (MISLEDs) made by Si-rich SiNx and SiOx films with buried Si nanocrystals are compared. The SiNx facilitates carrier transport and EL from MISLED with turn-on current and voltage of 4 μA and 12 V by reducing barrier heights at indium tin oxide /SiNx and SiNx/Si-nc interfaces. The SiNx MISLED exhibits larger charge loss rate of 12% within 200 s and shorter delay time of 3.86×10−4 sec than SiOx one, which limit its external EL quantum efficiency by strong carrier escaping effect due to the insufficient carrier confinement in Si nanocrystals with low interfacial barriers.

ZnO/Al2O3 core-shell nanorod arrays : growth, structural characterization, and luminescent properties

We reported the aqueous chemical method to fabricate the well-aligned ZnO/Al<inf>2</inf>O<inf>3</inf> nanocrystal (NC) core-shell nanorod arrays (NRAs). Structural characterization showed that the shell layers are composed of α-Al<inf>2</inf>O<inf>3</inf> nanocrystals. Photoluminescence measurements showed the enhancement of near-band-edge (NBE) emission of ZnO NRAs due to the presence of Al<inf>2</inf>O<inf>3</inf> NC shells. The Al<inf>2</inf>O<inf>3</inf> NC shell layer resulting in flatband effect near ZnO surface leads to a stronger overlap of the wavefunctions of electrons and holes in the ZnO core, further enhancing the NBE emission. This approach should be very useful in designing many other core-shell NRAs for creating varieties of high-efficiency optoelectronic devices.