Inter-layer light transition in hybrid III-V/Si waveguides integrated by µ-transfer printing.

We demonstrate low-loss and broadband light transition from III-V functional layers to a Si platform via two-stage adiabatic-crossing coupler waveguides. A 900-µm-long and 2.7-µm-thick III-V film waveguide consisting of a GaInAsP core and InP cladding layers is transferred onto an air-cladding Si photonic chip by the µ-transfer printing (µ-TP) method. An average optical coupling loss per joint of 1.26 dB is obtained in C + L telecommunication bands (1530-1635 nm). The correlation between alignment offset and measured optical coupling loss is discussed with the frequency distribution of µ-TP samples. We also performed a photoluminescence measurement to investigate the material properties in the GaInAsP layer to see if they are distorted by the strong bending stress produced during the pick-up and print steps of the µ-TP process. The peak intensity reduction of 80-90% and a wavelength shift of 0-5 nm (blue shift) were observed after the process. The series of fundamental studies presented here, which combine multiple analyses, contribute to improving our understanding of III-V/Si photonic integration by µ-TP.

Low-loss polysilicon subwavelength grating waveguides and narrowband Bragg reflectors in bulk CMOS.

The performance of a photonic functional device in bulk CMOS has been limited by the high propagation loss in polysilicon strip waveguide. Based on the zero-process-change methodology, we successfully reduce the propagation loss of polysilicon waveguide from 112 dB/cm to only 38 dB/cm by solely engineering the waveguide geometry for the first time. Low propagation loss is attributed to a significantly reduced optical overlap factor of 0.09 to bulk polysilicon using subwavelength grating (SWG) waveguide design. These findings prompt us to demonstrate a narrowband SWG-based cladding-modulated Bragg reflector in bulk CMOS, which provides a full-width at half maximum (FWHM) of 1.63 nm, an extinction ratio of 24.5 dB, and a reduced temperature sensitivity of 27.3 pm/°C. Further reducing the FWHM to 0.848 nm is also achieved by decreasing the grating coupling strength. We believe the achievements made in this work validate a promising design path towards practical photonic-electronic applications in bulk CMOS.

Design of Wearable Headset with Steady State Visually Evoked Potential-Based Brain Computer Interface.

Brain-computer interface (BCI) is a system that allows people to communicate directly with external machines via recognizing brain activities without manual operation. However, for most current BCI systems, conventional electroencephalography (EEG) machines and computers are usually required to acquire EEG signal and translate them into control commands, respectively. The sizes of the above machines are usually large, and this increases the limitation for daily applications. Moreover, conventional EEG electrodes also require conductive gels to improve the EEG signal quality. This causes discomfort and inconvenience of use, while the conductive gels may also encounter the problem of drying out during prolonged measurements. In order to improve the above issues, a wearable headset with steady-state visually evoked potential (SSVEP)-based BCI is proposed in this study. Active dry electrodes were designed and implemented to acquire a good EEG signal quality without conductive gels from the hairy site. The SSVEP BCI algorithm was also implemented into the designed field-programmable gate array (FPGA)-based BCI module to translate SSVEP signals into control commands in real time. Moreover, a commercial tablet was used as the visual stimulus device to provide graphic control icons. The whole system was designed as a wearable device to improve convenience of use in daily life, and it could acquire and translate EEG signal directly in the front-end headset. Finally, the performance of the proposed system was validated, and the results showed that it had excellent performance (information transfer rate = 36.08 bits/min).

Mode-evolution-based silicon-on-insulator 3 dB coupler using fast quasiadiabatic dynamics.

We report a 2×2 broadband and fabrication tolerant mode-evolution-based 3 dB coupler based on silicon-on-insulator rib waveguides. The operating principle of the coupler is based on the adiabatic evolution of local eigenmodes. The key element of the device is an adiabatically tapered mode evolution region, which converts two dissimilar waveguides into two identical waveguides. Contrary to conventional designs using a linear taper function where the device adiabaticity is uneven during evolution, we use the fast quasiadiabatic approach to homogenize the adiabaticity of the device, leading to a shortcut to adiabaticity. Devices with an optimized taper region of 26.3 µm are designed and fabricated in a complementary metal-oxide-semiconductor compatible process with 193 nm deep ultraviolet lithography. The measured devices exhibit a broadband 3 dB±0.5 dB splitting within a bandwidth of 100 nm, uniformly across a 200-mm wafer, showing good tolerance against fabrication variations.

Narrowband silicon waveguide Bragg reflector achieved by highly ordered graphene oxide gratings.

Graphene oxide (GO) ultrathin film can be wafer-scale deposited by spin coating, can be patterned by laser interference lithography and oxygen plasma etching, can be thinned atomically (0.26 nm/min) and oxidized by ozone treatment, and is a relatively transparent and low-refractive-index material compared to pristine graphene. All those unique properties prompt us to realize a low-loss (∼5 dB/cm), high-extinction-ratio (19 dB), and narrowband (0.425 nm) GO/silicon hybrid waveguide Bragg reflector by transferring 7-nm-thick GO gratings (n=1.58) atop a silicon strip waveguide. Unlike a sidewall-corrugated strip waveguide Bragg reflector that generally exhibits distorted corrugation profiles and is sensitive to fabrication errors, the as-realized GO-grating-covered strip waveguide Bragg reflector exhibits a stable reflecting wavelength and controllable reflection bandwidth that can be well predicted by numerical simulations.