Silicon microelectronics on unconventional substrates has led to numerous unprecedented applications. Inspired by the great success, it is a natural desire to integrate silicon photonic circuitry on unconventional substrates in the hope of extending the applicability range of silicon photonics to a multitude of novel hybrid silicon photonic devices. However, photonic devices usually have larger dimensions and more complicated morphologies. The transfer method used in electronics cannot be applied directly to transfer photonic devices. Here, we propose a low temperature transfer technique based on adhesive bonding and deep reactive ion etching. A defect-free transfer of 2 cm × 2 cm, 250 nm thick silicon nanomembrane onto a glass slide has been demonstrated. Single mode waveguides and splitters fabricated on the transferred SiNM exhibit comparable results to those fabricated on silicon-on-insulator. With a low process temperature, this method can be easily applied to transfer silicon nanomembranes onto various types of substrates.
We have investigated the feasibility of multimode polysilicon waveguides to demonstrate the suitability of polysilicon as
a candidate for multilayer photonic applications. Solid Phase Crystallization (SPC) with a maximum temperature of
1000°C is used to create polysilicon on thermally grown SiO2. We then measure the propagation losses for various
waveguide widths on both polysilicon and crystalline silicon platforms. We find that as the width increases for
polysilicon waveguides, the propagation loss decreases similar to crystalline silicon waveguides. The difference in loss
between the two platforms for a given waveguide width is due to the scattering from the polysilicon grain boundaries,
which excites higher order modes. Depending on the waveguide width, these modes either propagate as higher order
modes or are lost as radiation modes. Due to their different propagation constants, the presence of higher order modes is
confirmed using sub-wavelength grating couplers. At a waveguide width of 10μm, polysilicon and crystalline silicon
waveguides have propagation losses of 0.56dB/cm and 0.31dB/cm, respectively, indicating there is little bulk absorption
from the polysilicon. This propagation loss is the lowest for polysilicon demonstrated to date. Modal conversion in
multimode waveguides by polysilicon grain boundary scattering are investigated using a sub-wavelength grating coupler
and discussed. These results vindicate the use of polysilicon waveguides of varying widths in photonic integrated circuits.
In this paper we demonstrate a three-dimensional (3D) photonic integration scheme based on crystalline silicon. We
develop a process using SU-8 based adhesive bonding to fabricate vertically stacked, double-layer silicon
nanomembranes. A single-layer silicon photonic integrated circuit fabricated on a silicon-on-insulator (SOI) chip and a
bare SOI chip are bonded together, followed by removal of the bare SOI chip’s silicon substrate and buried oxide layer,
to form a silicon nanomembrane as a platform for additional photonic layer. We designed and fabricated subwavelength
nanostructure based fiber-to-chip grating coupler on the bonded silicon nanomembrane, and also inter-layer grating
coupler for coupling between two silicon nanomembranes. The fiber-to-chip grating coupler has a peak efficiency of -3.9
dB at 1545 nm operating wavelength with transverse-electric (TE) polarization. The inter-layer grating coupler has a
peak efficiency of -6.8 dB at 1533 nm operating wavelength with TE polarization. The demonstrated approach serves as
a potential solution for 3D photonic integration and novel 3D photonic devices.
In this paper, we present a highly compact silicon nano-membrane based optical phased array fabricated using
conventional CMOS processing on silicon-on-insulator that provides for over 10 degrees of beam steering in a silicon
slab at λ=1.55μm using transverse-electrical polarized light. A low loss 1-to-12 multi-mode interference (MMI) optical
beam splitter with high uniformity is used to provide inputs to the optical phased array. Using an unequally spaced
waveguide array permits us to relax the half-wavelength spacing requirement for large angle beam steering, thereby
avoiding the optical coupling between adjacent waveguides and reducing the side-lobe-level of the array radiation
pattern. S-bend waveguides convert the equally spaced MMI output to the unequally spaced wave guide array, while
passively equalizing the phases of each array element to compensate for the MMI output phase profile. Independently
controllable thin film metal heaters are used to achieve phase shifting using the strong thermo-optic response of silicon.
Heat-insulating air grooves minimize thermal crosstalk, while also achieving and low power consumption.
We present a design methodology for silicon-on-insulator photonic crystal waveguides to achieve wideband lowdispersion
slow light with only tuning the position of the first three inner rows. We aim to maximize the group index -
bandwidth product or the slowdown factor. Our design achieves a constant group index of 39.3 over 12 nm bandwidth
around 1550 nm, corresponding to a slow down factor of 0.3.
We experimentally demonstrate a Mach-Zehnder modulator based on electro-optic (EO) polymer (AJ-CKL1/PMMA)
infiltrated photonic crystal slot waveguide. The modulator design combines the advantage of excellent optical
confinement in silicon slot waveguide, slow light enhancement in photonic crystal waveguide, and strong electro-optical
response in EO polymer. This design allows us to achieve electro-optic modulation in active region only 352μm long.
Matching the mode profile and group velocity between strip and photonic crystal slot waveguide enables coupling into
slow light regime. The modulator shows a 22V switching voltage and an improved in-device EO coefficient of 51pm/V
as compared to recently reported value.
In this paper, we present the results of the design and fabrication of a 12 channel nano-membrane-based optical phased
array that allows for large angle beam steering operating at wavelength=1.55μm. Our device is fabricated on silicon-oninsulator
using standard CMOS process. By implementing unequally spaced waveguide array elements, we can relax the
half-wavelength spacing requirement for large angle beam steering, thereby avoiding the optical coupling between
adjacent waveguides and reducing the side-lobe-level of the array radiation pattern. 1D beam steering of tranverseelectric
polarized single mode light is designed to be achieved thermo-optically through the use of thin film metal phase
shifters.
We derive analytical formulations for the output phase profile of symmetrically excited one-to-N multimode interference
couplers. We show that the output phase increases quadratically from the middle of the MMI waveguide, which needs to
be taken into account for phase-dependent applications such as optical phased arrays.
We present a new approach to wide angle optical beam steering based on nano-membrane-based phased array structures with unequally spaced elements. In our approach, the array elements are positioned in such way that grating lobes associated with different sub-arrays do not overlap. Therefore, we reduce the side-lobe-level of the array radiation pattern and at the same time we can avoid the optical coupling between adjacent waveguides by relaxing the half-wavelength spacing requirement for large angle beam steering. By optimizing the optical waveguide structure for the maximum full-width at half-maximum of the single radiator's radiation pattern we discuss the optimum performance achievable using the Unequally-spaced Waveguide Arrays.
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