Ryohei Takei

Pattern-effect-free all-optical wavelength conversion using a hydrogenated amorphous silicon waveguide with ultra-fast carrier decay

Ultra-fast carrier decay, recently discovered in a hydrogenated amorphous silicon waveguide, can be exploited for pattern-effect-free all-optical signal processing based on optical Kerr nonlinearity. In this study, we utilized a 10 Gbit/s RZ-OOK data stream as a pump for degenerate four-wave mixing in a low-loss hydrogenated amorphous silicon waveguide. The propagation loss of the waveguide used was 1.0±0.2 dB/cm at 1550 nm. Unlike crystalline silicon waveguides, no noticeable difference was observed in the BER characteristics between the cases of PRBS 2(7)-1 and 2(31)-1.

Sub-1 dB/cm submicrometer-scale amorphous silicon waveguide for backend on-chip optical interconnect

We demonstrate a submicrometer-scale hydrogenated amorphous silicon (a-Si:H) waveguide with a record low propagation loss of 0.60 ± 0.02 dB/cm because of the very low infrared optical absorption of our low defect a-Si:H film, the optimized waveguide structure and the fabrication process. The waveguide has a core with a thickness of 440 nm and a width of 780 nm that underlies a 100-nm-thick ridge structure, and is fabricated by low-cost i-line stepper photolithography and with low-temperature processing at less than 350°C, making it compatible with the backend process of complementary metal oxide semiconductor (CMOS) fabrication.

Ultranarrow Silicon Inverse Taper Waveguide Fabricated with Double-Patterning Photolithography for Low-Loss Spot-Size Converter

Using a double-patterning process of i-line photolithography that twice performs a pair of photoresist patterning and dry etching processes, we were able to form an ultranarrow silicon inverse taper waveguide with a tip end width that was much narrower than the resolution limit of photolithography. We fabricated a spot-size converter (SSC) consisting of a 50-µm-long silicon taper waveguide with gradually decreasing width from 400 to 50 nm and a polyimide second core. The insertion loss of the SSC was 0.55 dB for the transverse electric-like mode, which was the lowest value for an SSC fabricated using photolithography.

Vertical silicon waveguide coupler bent by ion implantation

We propose and demonstrate that vertically curved waveguides (VCWs) enable vertical coupling between silicon wire waveguides and optical fibers with low wavelength dependence and polarization dependence for wide telecommunication wavelength band light. To bend these VCWs, we implanted silicon ions into silicon wire cantilevers from the vertical direction. The internal stress distribution that was induced by ion implantation drove the bending force, and we achieved vertical bending of the waveguides, with curvature radii ranging from 3 to 25 μm. At a radius of curvature of 6 μm, we obtained a coupling loss of 3 dB using a lens fiber.

Low-loss and low wavelength-dependence vertical interlayer transition for 3D silicon photonics.

This paper presents optimized design and measurement results for a low-loss broadband vertical interlayer transition (VIT) device located between lower and upper Si nano-photonic waveguides. The device comprises the lower c-Si taper, the upper a-Si:H taper, and a wide and thin SiON secondary core with a 0.6-μm-thick SiO₂ interlayer. The device structure facilitates the low loss VIT, giving insertion losses of 0.87 and 0.79 dB for quasi-TE and TM modes, respectively, at 1550 nm. Also, the evanescent coupling nature of the VIT device renders it wavelength- and polarization-insensitive, leading to loss variation of within 0.5 dB in the C-band.

Spot-size converter with a SiO(2) spacer layer between tapered Si and SiON waveguides for fiber-to-chip coupling.

We experimentally demonstrate low-loss and polarization-insensitive fiber-to-chip coupling spot-size converters (SSCs) comprised of a three dimensionally tapered Si wire waveguide, a SiON secondary waveguide, and a SiO(2) spacer inserted between them. Fabricated SSCs with the SiO(2) spacer exhibit fiber-to-chip coupling loss of 1.5 dB/facet for both the quasi-TE and TM modes and a small wavelength dependence in the C- and L-band regions. The SiON secondary waveguide is present only around the SSC region, which significantly suppresses the influence of the well-known N-H absorption of plasma-deposited SiON at around 1510 nm.

S-shape spring sensor: Sensing specific low-frequency vibration by energy harvesting

We have developed a Si-based microelectromechanical systems sensor with high sensitivity for specific low-frequency vibration-sensing and energy-harvesting applications. The low-frequency vibration sensor contains a disk proof mass attached to two or three lead zirconate titanate (PZT) S-shape spring flexures. To obtain a faster and less expensive prototype, the design and optimization of the sensor structure are studied via finite-element method analysis. To validate the sensor structure to detect low-frequency vibration, the effects of geometrical dimensions, including the width and diameter of the S-shape spring of the proof mass, were analyzed and measured. The functional features, including the mechanical property and electrical performance of the vibration sensor, were evaluated. The results demonstrated that a very low resonant frequency of <11 Hz and a reasonably high voltage output of 7.5 mV at acceleration of >0.2g can be typically achieved. Given a low-frequency vibration sensor with ideal performance and mass fabrication, many advanced civilian and industrial applications can be possibly realized.

Transmission Characteristics of Hydrogenated Microcrystalline Silicon Wire Waveguide at a Wavelength of 1.55 µm

We propose hydrogenated microcrystalline silicon (µc-Si:H) as an alternative to hydrogenated amorphous silicon for optical waveguide applications with higher stability against photoinduced degradation. A 180-nm-thick µc-Si:H film containing silicon nanocrystallites with a grain size of ~20 nm was deposited by plasma-enhanced chemical vapor deposition at 250 °C. The transmission losses of µc-Si:H wire waveguides with widths of 420, 470, and 570 nm were 10.1, 6.7, and 6.5 dB/cm for the transverse electric mode, and 9.6, 5.2, and 4.7 dB/cm for the transverse magnetic mode, respectively. From the measured losses, we estimated material-induced scattering loss of less than ~7 dB/cm.

Design of piezoelectric MEMS cantilever for low-frequency vibration energy harvester

We report the design of piezoelectric MEMS cantilevers formed on a silicon-on-insulator wafer to efficiently harvest electrical power from harmonic vibration with a frequency of approximately 30 Hz. Numerical simulation indicates that a >4-µm-thick top silicon layer and >3-µm-thick piezoelectric film are preferable to maximize the output electrical power. An in-plane structure of the cantilever is also designed retaining the footprint of the cantilever. The simulation results indicate that the output power is maximized when the length ratio of the proof mass to the cantilever beam is 1.5. To ensure the accuracy of the simulation, we fabricated and characterized cantilevers with a 10-µm-thick top silicon layer and a 1.8-µm-thick piezoelectric film, resulting in 0.21 µW at a vibration of 0.5 m/s2 and 25.1 Hz. The measured output power is in agreement with the simulated value, meaning that the design is significantly reliable for low-frequency vibration energy harvesters.

Basic Study of Coupling on Three-Dimensional Crossing of Si Photonic Wire Waveguide for Optical Interconnection on Inter or Inner Chip

For speeding up of transmission and energy saving on inter or inner chips of semiconductors, hydrogenated amorphous silicon (a-Si:H), unlike conventional crystal Si (c-Si), promises optical multilevel wiring on a Si IC. For three-dimensional (3D) crossing of Si/a-Si:H photonic wire waveguides, the loss and crosstalk as S-parameters of the two propagation modes are evaluated by numerical analysis at the C-band when the waveguide core is 200 ×400 nm2. Whether the crosstalk can be suppressed to -50 dB or less is to be a criterion. Even at the crossing angle of 30°, when the distance between the waveguides of the crossing is 1 µm or less, the crosstalk is suppressed sufficiently, while the radiation loss is also small if a TE-like mode propagates. These quantitative results are derived for the first time and show that the photonic 3D crossing can rival the present electric multilevel wiring from the viewpoint of device height. An important index for the 3D waveguide crossing fabrication is obtained.