Yicheng Lai

Demonstration of heterogeneous III–V/Si integration with a compact optical vertical interconnect access

Heterogeneous III-V/Si integration with a compact optical vertical interconnect access is fabricated and the light coupling efficiency between the III-V/Si waveguide and the silicon nanophotonic waveguide is characterized. The III-V semiconductor material is directly bonded to the silicon-on-insulator (SOI) substrate and etched to form the III-V/Si waveguide for a higher light confinement in the active region. The compact optical vertical interconnect access is formed through tapering a III-V and an SOI layer in the same direction. The measured III-V/Si waveguide has a light coupling efficiency above ~90% to the silicon photonic layer with the tapering structure. This heterogeneous and light coupling structure can provide an efficient platform for photonic systems on chip, including passive and active devices.

Continuous-Wave InP-InGaAsP Microsquare Laser—A Comparison to Microdisk Laser

We demonstrate compact optically pumped microsquare cavity laser on InP-based multiple-quantum-wells material platform. Continuous-wave operation is achieved for microsquare cavity with footprint as small as 4 × 4 μm2. Numerical mode analysis and experimental characterization of the microsquare laser are conducted, and the results are compared with the commonly used microdisk cavity laser fabricated on the same platform. The microsquare laser shows a lower lasing threshold and infers a higher differential efficiency than the microdisk counterpart. The microsquare cavity laser has also sufficiently high quality factor, and higher pumping injection efficiency due to the more evenly distributed field profile as compared to that of the microdisk. Experimental result also shows that the microsquare laser has better temperature stability than the microdisk. These results promise a potential alternative laser structure for on-chip light source applications.

High Spatial Resolution Photodetectors Based on Nanoscale Three-Dimensional Structures

A high spatial resolution photodetector is designed and realized using a newly developed two-sided nanoscale three-dimensional fabrication technique. Two types of nanophotodetectors with slab and channel structures are fabricated and characterized. By comparison, nanophotodetectors with channel structures show a 10 times lower dark current, higher detection resolution, and signal-to-noise ratio than nanophotodetectors with slab structures. The smallest nanophotodetector fabricated is as small as 100 nm wide. A responsivity of 0.19 A/W at 1310-nm wavelength and 3-V bias for a channelized nanophotodetector is registered, with a photocurrent of 135 nA over a dark current of 1.25 nA. In addition, using a modified near-field scanning optical microscopy-based photocurrent mapping system, we demonstrated a high spatial resolution photodetection of 400 nm using a nanophotodetector with channel structure fabricated.

Exact Solution of Facet Reflections for Guided Modes in High-Refractive-Index-Contrast Sub-Wavelength Waveguide Via a Fourier Analysis and Perturbative Series Summation: Derivation and Applications

Facet reflections at the waveguide-air interface for strongly-guiding waveguides with sub-wavelength scale dimensions do not follow the usual Snells law. Significant amount of reflected power can be channeled into higher order modes as well as radiation modes. This paper shows for the first time how the exact analytical solution of the facet reflection can be obtained by using a new technique based on Fourier analysis and perturbative series summation without the need for approximation or iteration. The proposed analysis enables the distribution of power reflected into various guided and radiation modes to be readily computed. Through this technique, a spectral overlapping criterion and a coupling matrix are derived that analyze effectively the power distribution among all the strongly and weakly-coupled radiation modes in an end-facet reflection. Accurate pre-determination of the number of radiation modes for efficient computation without compromising resultant accuracy is achieved. More importantly, the anomalous wave reflection behaviors at the facet of a strongly-guiding waveguide are presented. These include anomalous high radiation modes coupling as a function of cladding refractive index not reported before. This paper further includes an exemplary illustration of the analysis based on a symmetric planar nano-waveguide with high refractive index contrast for both TE and TM polarization under fundamental incident mode.

Effects of SiO 2 hard masks on si nanophotonic waveguide loss for photonic device integration

As the basic building block for photonic device integration, silicon nanophotonic waveguide requires low-loss propagation for high-performance ultra-compact photonic device. We experimentally study silicon dioxide hard masks grown by two different methods, i.e., thermal oxidation and plasma-enhanced chemical vapor deposition (PECVD) for silicon nano-waveguides fabrication and their effects on the propagation loss. It is found that the denser and smoother quality of thermally grown silicon dioxide increases the etch selectivity against silicon and reduces the line edge roughness transferred to the silicon nano-waveguide sidewalls, hence resulting in a lower loss as compared with the PECVD silicon dioxide hard mask. With thermally grown silicon dioxide as a hard mask, the silicon nano-waveguides loss can be halved for a 650-nm-wide nano-waveguide, and the loss is comparable with a waveguide fabricated with a resist etch mask.

Near-field nanoimaging by nanoscale photodetector array

We report a demonstration of near-field nanoimaging using nanoscale photodetector (NPD) array. The NPD array has detector pixels with subwavelength dimension and is capable of pixel addressing. An active-media finite-difference time-domain simulation of the NPD array shows an imaging resolution of 150 nm for 1.55 microm light. Additionally, we demonstrate the realization and characterization of the NPD array. The smallest NPD array obtained has 100-nm-wide pixels with 100 nm spacing. A responsivity of 0.28 A/W at 1.31 microm and 3.3 V bias is registered for a 2x2 NPD array pixel. The corresponding photocurrent is 735 nA with a dark current of 0.483 nA. Using near-field photocurrent microscopy, an imaging resolution of 390 nm has been demonstrated.

Fabrication of sub-200 nm AlN-GaN-AlN waveguide with cleaved end facet

The authors report the first realization of sub-200 nm wide AlN-GaN-AlN (AGA) ridge waveguide with height-to-width ratio of ∼6:1, fabricated via inductively coupled plasma (ICP) etching with Cl2/Ar gas chemistry. Reactive ion etching (RIE) power and ICP power were varied in the ranges of 100–450 W and 200–600 W, respectively. An optimized RIE power and ICP power at 100 and 400 W, respectively, reduced the density of nanorods formed in the etched trenches. Further optimization of the gas flow rate of Cl2/Ar to 40/10 sccm improved the slope of the etched waveguide. In addition, the authors also developed a simple and novel dice-and-cleave technique to achieve cleaved end facet of AGA waveguide.

Demonstration of continuous-wave microsquare lasers and comparison to microdisk laser

Optically-pumped microsquare cavity laser on InP-based multiple-quantum-wells (MQW) material platform is demonstrated. Continuous wave operation is achieved for microsquare cavity with footprint as small as 4×4μm2. Numerical mode analysis and experimental characterization of the microsquare laser are conducted, and the results are compared with the commonly used microdisk cavity laser fabricated on the same platform. The microsquare laser shows a lower lasing threshold and infers a higher differential efficiency than the microdisk counterpart. The microsquare cavity laser has sufficiently high quality factor, and higher pumping injection efficiency due to the more evenly distributed field profile as compared to that of the microdisk. Experimental result also shows that the microsquare laser has better temperature stability than the microdisk. These results promise a potential alternative laser structure for onchip light source applications.

All-optical switching with switching gain in a hybrid III-V/silicon single nano-waveguide

We report the first experimental demonstration of all-optical switching gain in a single nano-waveguide device based on a hybrid silicon photonic integration platform. The switch features 6nm broadband operation, small device footprint and low sensitivity to fabrication error.

Fabrication of high-efficiency heterogeneous Si/III-V integration with short optical vertical interconnect access

Silicon nanophotonic platform based on a silicon-on-insulator substrate enables dense photonic integration due to transparency for light propagation and ultra-high refractive index contrast for light confinement. Here, we integrate silicon together with III-V for high-efficiency heterogeneous Silicon/III-V and short vertical optical interconnect access. The fabrication involves 3 critical processes: 1) obtaining more than 80% maximum bonded areas of Si with III-V, 2) precise alignment of III-V nano-devices on top of the passive devices and 3) vertical sidewall etch profile of Si and III-V devices. The measurement results show around 90% coupling efficiency. The realization of this heterogeneous Si/III-V integration platform will open up enormous opportunities for photonic system on silicon through integrating various devices.