Yung-Jr Hung

Deep and tapered silicon photonic crystals for achieving anti-reflection and enhanced absorption

Tapered silicon photonic crystals (PhCs) with smooth sidewalls are realized using a novel single-step deep reactive ion etching. The PhCs can significantly reduce the surface reflection over the wavelength range between the ultra-violet and near-infrared regions. From the measurements using a spectrophotometer and an angle-variable spectroscopic ellipsometer, the sub-wavelength periodic structure can provide a broad and angular-independent antireflective window in the visible region for the TE-polarized light. The PhCs with tapered rods can further reduce the reflection due to a gradually changed effective index. On the other hand, strong optical resonances for TM-mode can be found in this structure, which is mainly due to the existence of full photonic bandgaps inside the material. Such resonance can enhance the optical absorption inside the silicon PhCs due to its increased optical paths. With the help of both antireflective and absorption-enhanced characteristics in this structure, the PhCs can be used for various applications.

Antireflective silicon surface with vertical-aligned silicon nanowires realized by simple wet chemical etching processes

Silicon antireflection is realized with vertical-aligned SiNWs by using improved metal-induced etching technique. The spectral responses of the transmission, reflection, and absorption characteristics for the SiNWs of different lengths are investigated. In order to realize short SiNWs to provide sufficiently low reflection, a post chemical etching process is developed to make the nanowires have a larger length fluctuation and/or tapered structure. The use of short SiNWs can allow a faster process time and avoid the sub-bandgap absorption that frequently occurs in long nanowires. Short SiNWs can also provide more compatible material structure and fabrication procedures than long ones can for applying to make optoelectronic devices. Taking the applications to solar cells as examples, the SiNWs fabricated by the proposed technique can provide 92% of solar weighted absorption with about 720 nm long wires because of the resultant effective graded index and enhanced multiple optical scattering from the random SiNW lengths and tapered wires after KOH etching.

Fabrication of Highly Ordered Silicon Nanowire Arrays With Controllable Sidewall Profiles for Achieving Low-Surface Reflection

A novel and simple approach is demonstrated for fabricating silicon nanowire arrays (SNWAs) with controllable sidewall profiles. A single-step deep-reactive-ion etching (SDRIE) is used to transfer the holography patterned photoresist template to silicon or silicon-on-insulator substrates. With the SDRIE etching process, scalloping of the sidewalls can be avoided while reserving the high-mask selectivity over resist and high-etching rate. The sidewall angle of resultant patterns can be adjusted by tuning the composition of the gas mixture of the process. A modified-SDRIE process with a linearly changed gas flow is further developed to extend its capability. A post-high-energy argon plasma treatment is used to create sharp tips on the top of SNWAs and to increase the filling factor. Broadband antireflective (AR) window with a low reflectivity can be realized from tall SNWAs with high-filling factor. Depositing silicon dioxide over SNWAs can further enhance the AR performance. The position and bandwidth of the AR window can be controlled by tuning the SNWA parameters.

Monolithically Integrated Gain-Flattened Ring Mode-Locked Laser for Comb-Line Generation

We demonstrate broadband comb-line generation from an integrated multiple quantum well InGaAsP/InP passively mode-locked laser (MLL) with a gain flattening filter (GFF) based on an asymmetric Mach-Zehnder interferometer. The intracavity filter flattens the nonuniform gain profile of the semiconductor material providing a more uniform net cavity gain. The GFF MLL has a -10 dB comb span of 15 nm (1.88 THz), the widest spectral width yet demonstrated for an integrated QW MLL at 1.55 μm . The measured optical linewidth at the center of the comb is 29 MHz, the -20 dB RF linewidth 500 KHz, while the output spectrum is phase-locked to produce 900 fs pulses at a repetition rate of 30 GHz with 4.6 ps integrated jitter from 100 Hz to 30 MHz.

Realization and Characterization of Aligned Silicon Nanowire Array With Thin Silver Film

Wafer-scale fabrication of aligned and uniform silicon nanowire (SiNW) arrays is achieved with good controllability and reproducibility by depositing a thin silver film on a silicon surface prior to wet etching. Fast SiNW formation with a rate of 1.4 μm/min is achieved with optimized process condition, while lower etching rate enables finer SiNW formation in a small open area. Realized SiNWs are demonstrated to have good material and optical properties. With the help of aligned SiNWs, we demonstrate the fabrication of a black nonreflecting silicon surface with a surface reflectivity of around 2-4% uniformly over a 4-in wafer area. This material is expected to be promising as a building block for various applications due to its low-cost and mass-producible fabrication and excellent characteristics.

Holographic realization of hexagonal two dimensional photonic crystal structures with elliptical geometry

A complete investigation of holographic photonic crystal structures has been conducted. From both theoretical and experimental results, profiles of resultant patterns under different process conditions can be estimated and controlled. The use of antireflection layers is crucial for realizing submicron photonic crystals with good uniformity over a large area. We successfully realize submicron-scale photonic crystal templates on silicon substrates with an aspect ratio of 2.5 and good quality by a laser holography technique. The samples are highly uniform in an area of >2×2 cm2 and present good reproducibility. A lift-off process is performed to transfer inversed pillar patterns into a chromium hard mask for the following dry etching into silicon substrates. A single-step deep reactive ion etching with controlled mixture of Ar/SF6/C4F8 gases is used to directly transfer pillar patterns into silicon. Transferred patterns with a high aspect ratio and vertical sidewalls (no scalloping) are demonstrated over a l...

InP/InGaAsP Flattened Ring Lasers With Low-Loss Etched Beam Splitters

Compact flattened InP/InGaAsP multiquantum-well (MQW) resonators based on etched beam splitters (EBS) with 300- to 800-nm gaps and circumferences of 30-300 μm are demonstrated. Comparison of the EBS coupler reflection and transmission to 3-D finite-difference time-domain (FDTD) simulations shows good agreement in the wider EBS gap devices. Lasing is observed in 90-, 150-, and 300-μm length rings at threshold currents of 15, 14, and 29 mA, respectively.

Integrated 30GHz passive ring mode-locked laser with gain flattening filter

We demonstrated a 30GHz integrated InGaAsP/InP ring mode-locked laser with a gain flattening filter that doubles the locking bandwidth and decreases the pulse width from 840fs to 620fs.

Trend and Applications of Tunable Semiconductor Lasers

Tunable semiconductor lasers have been under intense research interests for the past decades due to their vast applications in optical networks, optical characterization, and optical sensing. The required device characteristics can be very different for applying the tunable lasers to various areas. We classify the tunable lasers in terms of their tuning characteristics and switching speed. Four kinds of tunable lasers are described in this paper to manifest the application-dependent device structures and performance. The applications include the use of sampled grating based lasers to form multi-wavelength laser arrays, cascaded distributed-feedback lasers for multi-gas sensors, wavelength-selectable laser arrays for fast wavelength switching sources, and short cavity lasers for fault monitoring in passive optical networks.

Realization of silicon nanopillar arrays with controllable sidewall profiles by holography lithography and a novel single-step deep reactive ion etching

A simple and efficient approach for fabricating silicon nanopillar arrays with a high aspect ratio and controllable sidewall profiles has been developed by using holographic lithography and a novel single-step deep reactive ion etching. During the etching process, scalloping of the sidewalls can be avoided while reserving the high mask selectivity and high etching rate. Besides, the sidewall angle of resultant patterns can be adjusted by tuning the composition of the gas mixture of single-step DRIE process. We further fabricate a tapered silicon nanopillar array and observe its photonic bandgap property. We believe that the good optical performance of this tapered silicon nanopillar array realized by the proposed approach shows the promising of this process for various applications.