The University of Tokyo · Engineering
Professor Zhenzhou Cheng's research lab specializes in nanophotonics and integrated optoelectronics, focusing on the design, simulation, and fabrication of advanced photonic devices for mid-infrared and telecommunications applications. Key research directions include suspended membrane waveguides, graphene-based optoelectronic modulators and photodetectors, and subwavelength grating couplers for polarization-insensitive and broadband coupling. The lab leverages group-IV materials—especially silicon and graphene—enabling CMOS-compatible, compact, and energy-efficient on-chip optical sensors and signal processing components.
Figures are computed from collected data and may differ slightly.
Theoretical and experimental studies of mid-infrared (mid-IR) suspended membrane waveguide (SMW) and suspended membrane ring (SMR) resonator are presented. An array of periodical holes beside the rib waveguide facilitates the local removal of buried oxide to form suspended membrane devices on silicon-on-insulator (SOI). The waveguide design is optimized in terms of hole size, etch depth, and bend radius to minimize device strain and optical loss. We calculate waveguide dimension to attain the wi
Compact and smart optical gas sensors have attracted significant attention over the past few decades. Among the materials used for developing such gas sensors, group-IV materials, including silicon, germanium, carbon allotropes, and their compounds, are considered the most promising candidates. By virtue of their inherent compatibility with the CMOS fabrication process in the mature microelectronics industry, on-chip optical gas sensors based on group-IV materials have merits of appreciable sens
We present a graphene-on-silicon (GoS) suspended vertical slot waveguide. By changing the Fermi level of graphene, the variation in the effective refractive index (RI) of the waveguide is a factor of two larger than that in the traditional GoS rib waveguide. The improvement is due to the light-intensity enhancement and the poor confinement of the optical mode in the slot nanostructure. We design Mach-Zehnder interferometer (MZI) and microring modulators based on the GoS suspended vertical slot w
We demonstrated strong optical absorption in a graphene integrated silicon slot waveguide. Due to the increase in light intensity and decrease of optical mode confinement in the silicon slot, the graphene experienced an increased interaction to the in-plane light. A waveguide absorption of 0.935 dB μm(-1) was measured at 1.55 μm wavelengths. Based on the graphene-on-silicon slot waveguide, a compact and high-responsivity photodetector was demonstrated. Benefited from the lack of efficient electr
A mid-infrared (mid-IR)-focusing subwavelength grating (SWG) coupler and suspended membrane waveguide (SMW) on a silicon-on-insulator wafer are studied. For a transverse-electric mode uniform SWG, finite-difference time-domain simulation predicts 44.2% coupling efficiency with 1 dB bandwidth of about 220 nm and backreflection of 0.78% at 2.75 μm. Then the uniform SWG is curved to a focusing SWG using a phase-matching formula. The SMWs are analyzed by the finite element method and fabricated. An
We present an air-cladding apodized focusing subwavelength grating that can effectively couple two polarizations into a single waveguide. For the transverse magnetic mode, -3.2 dB maximum coupling efficiency with ∼28 nm 1 dB bandwidth is achieved. With the same grating, -4.3 dB maximum coupling efficiency with ∼58 nm 1 dB bandwidth is achieved for the transverse electric mode. The minimum difference between two polarizations' coupling peaks is demonstrated to be ∼32 nm. At the 1525 nm wavelength
We experimentally study the in-plane optical absorption and free carrier absorption (FCA) in graphene-on-silicon waveguides using a pump-probe measurement over microsecond timescales. The silicon waveguide is fabricated using complementary metal-oxide-semiconductor compatible processes, and directly covered by a graphene layer. Saturable absorption in the graphene is observed at the beginning of the pump pulse followed by an increase in absorption. The increase in absorption builds up over sever
We demonstrate an apodized focusing subwavelength grating (SWG) for suspended membrane waveguides on silicon-on-insulator. Finite-difference time-domain simulation predicts −1.7 dB coupling efficiency and a 3 dB bandwidth of ∼50 nm for the transverse-magnetic mode apodized SWG, which has 98% field overlap with propagation mode in the single mode fiber. A modified phase matching formula is proposed to design the focusing apodized SWG. Better than −3.0 dB coupling efficiency and a 3 dB optical ban
Theoretical and experimental results of mid-infrared (mid-IR) gratings for coupling between transverse-electric (TE)/transverse-magnetic (TM) mode silicon-on-sapphire (SOS) waveguides and zirconium, barium, lanthanum, aluminum, and sodium fluoride (ZBLAN) fibers are presented. The shallow-etched uniform grating, full-etched subwavelength grating, and apodized grating are analyzed theoretically. TE mode shallow-etched apodized gratings with coupling efficiency of 80.6% and TM mode full-etched apo
We propose and demonstrate broadband focusing grating couplers for suspended-membrane waveguides on silicon-on-insulator both in near-infrared (near-IR) and in mid-IR wavelength range. Finite-difference time-domain simulation predicts ∼100 nm 3 dB bandwidth with -1.7 dB coupling efficiency for an apodized grating in near-IR. -3.5 dB maximum coupling efficiency and ∼90 nm 3 dB bandwidth are realized experimentally. In mid-IR, -5.5 dB maximum coupling efficiency from a uniform focusing grating is
A fully suspended mid-infrared (FSMIR) slot waveguide is proposed and experimentally demonstrated on a silicon-on-insulator (SOI) platform for the first time. The slotted waveguide core is mechanically supported by lateral subwavelength grating claddings. The fabricated waveguides possess low propagation loss, which is measured to be 7.9 dB/cm at the wavelength of 2.25 μm. With the underlying buried oxide (BOX) removed, the FSMIR slot waveguide has a broad spectral range of transparency that is
Abstract Hyperuniform disordered solids (HUDS) waveguides, a type of emerging artificial photonic bandgap (PBG) devices, are demonstrated to possess large, complete, and isotropic PBGs, being promising for developing applications in optoelectronics, nonlinear optics, and sensing. However, optical losses of HUDS waveguides are usually limited by giant light scattering from the irregular distribution of HUDS cells. Herein, HUDS waveguide devices are demonstrated with low optical losses and large P
A transverse-electric-mode focusing subwavelength grating coupler (FSWGC) is proposed and demonstrated for dual-wavelength-band (DWB) coupling from a single-mode fiber into a suspended-membrane waveguide for the first time, to the best of our knowledge. Location and separation of the two coupling peaks can be flexibly tailored based on a proposed design methodology. As a proof of concept, two DWB FSWGCs working at (1486.0, 1594.5) nm and (1481.5, 1661.5) nm are experimentally demonstrated with c
Abstract Hyperuniform disordered solid (HUDS) structures can provide large, uniform, complete, and isotropic light confinement at the nanoscale after precise design. Based on the HUDS structures, in‐plane light confinement for developing photonic integrated circuits is also explored. To improve the performance of HUDS devices, researchers have mainly focused on cell size or cell distribution optimization in HUDS, which suffers from time‐consuming computation or moderate photonic bandgap (PBG) mo
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