Timo Lipka

Amorphous Silicon 3-D Tapers for Si Photonic Wires Fabricated With Shadow Masks

Three-dimensional taper structures made from plasma-enhanced chemical vapor-deposited hydrogenated amorphous silicon were evaluated by calculating the deposition profiles with a line-of-sight model and the corresponding mode profiles with a full-vectorial mode solver. The tapers were fabricated with shadow-masks made from KOH-etched Si wafers, and with the tapers light was coupled into 500-nm-wide photonic wires from lensed fibers.

Label-free photonic biosensors fabricated with low-loss hydrogenated amorphous silicon resonators

Abstract. The precise detection of chemicals and biomolecules is of great interest in the areas of biotechnology and medical diagnostics. Thus, there is a need for highly sensitive, small area, and low-cost sensors. We fabricated and optically characterized hydrogenated amorphous silicon photonic resonators for label-free lab-on-chip biosensors. The sensing was performed with small-footprint microdisk and microring resonators that detect a refractive-index change via the evanescent electric field. Homogeneous sensing with NaCl and surface-sensing experiments with immobilized bovine serum albumin (BSA) were carried out. A sensitivity as high as 460  nm/RIU was measured for NaCl dissolved in deionized water for the disk, whereas about 50  nm/RIU was determined for the ring resonator. The intrinsic limits of detection were calculated to be 3.3×10−4 and 3.2×10−3 at 1550-nm wavelength. We measured the binding of BSA to functionalized ring resonators and found that molecular masses can be detected down to the clinically relevant femtogram regime. The detection and quantification of related analytes with hydrogenated amorphous silicon photonic sensors can be used in medical healthcare diagnostics like point-of-care-testing and biotechnological screening.

Athermal and wavelength-trimmable photonic filters based on TiO₂-cladded amorphous-SOI.

Large-scale integrated silicon photonic circuits suffer from two inevitable issues that boost the overall power consumption. First, fabrication imperfections even on sub-nm scale result in spectral device non-uniformity that require fine-tuning during device operation. Second, the photonic devices need to be actively corrected to compensate thermal drifts. As a result significant amount of power is wasted if no athermal and wavelength-trimmable solutions are utilized. Consequently, in order to minimize the total power requirement of photonic circuits in a passive way, trimming methods are required to correct the device inhomogeneities from manufacturing and athermal solutions are essential to oppose temperature fluctuations of the passive/active components during run-time. We present an approach to fabricate CMOS backend-compatible and athermal passive photonic filters that can be corrected for fabrication inhomogeneities by UV-trimming based on low-loss amorphous-SOI waveguides with TiO2 cladding. The trimming of highly confined 10 μm ring resonators is proven over a free spectral range retaining athermal operation. The athermal functionality of 2nd-order 5 μm add/drop microrings is demonstrated over 40°C covering a broad wavelength interval of 60 nm.

Amorphous waveguides for high index photonic circuitry

Photonic wires, channel and rib waveguides as well as tapers were fabricated with amorphous silicon showing low propagation loss. Material analysis and RTA was carried out in order to tune the refractive index post deposition.

Amorphous silicon spot-size converters fabricated with a shadow mask

A rib-like spot-size converter was fabricated with a KOH etched shadow mask. The improvement in coupling and an expansion of the spot-size were evaluated with simulations and confirmed by transmission loss measurements.

Photonic integrated circuit components based on amorphous silicon-on-insulator technology

We present integrated-optic building blocks and functional photonic devices based on amorphous silicon-on-insulator technology. Efficient deep-etched fiber-to-chip grating couplers, low-loss single-mode photonic wire waveguides, and compact power splitters are presented. Based on the sub-μm photonic wires, 2×2 Mach–Zehnder interferometers and add/drop microring resonators (MRRs) with low device footprints and high finesse up to 200 were realized and studied. Compact polarization rotators and splitters with ≥10  dB polarization extinction ratio were fabricated for the polarization management on-chip. The tuning and trimming capabilities of the material platform are demonstrated with efficient microheaters and a permanent device trimming method, which enabled the realization of energy-efficient photonic circuits. Wavelength multiplexers in the form of cascaded filter banks and 4×4 routers based on MRR switches are presented. Fabrication imperfections were analyzed and permanently corrected by an accurate laser-trimming method, thus enabling eight-channel multiplexers with record low metrics of sub-mW static power consumption and ≤1°C temperature overhead. The high quality of the functional devices, the high tuning efficiency, and the excellent trimming capabilities demonstrate the potential to realize low-cost, densely integrated, and ultralow-power 3D-stacked photonic circuits on top of CMOS microelectronics.

Energy-Efficient Wavelength Multiplexers Based on Hydrogenated Amorphous Silicon Resonators

Optical multiplexers are key components of modern data transmission systems that have evolved from long-haul fiber communication applications down to the photonic interconnect level on-chip, which demand high bandwidths and low-power photonic links with small footprint. We present compact, energy-efficient, and high-bandwidth optical add/drop multiplexers that are based on complementary metal-oxide-semiconductor (CMOS) backend-compatible hydrogenated amorphous silicon microring resonators. We study the manufacturing nonuniformity of the as-fabricated devices and analyze the static power consumption that is required to actively align the multiplexers to a 100-GHz grid by using state-of-the-art microheaters. The microring filter banks are in excellent agreement with the design and satisfy a good tradeoff between concurrent properties of high-data-rate capability, low filter loss, high channel isolation, and manufacturing uniformity, which facilitates the operation with low static power consumption. In addition, we demonstrate that it is possible to permanently correct the unavoidable fabrication imperfections and to arrange the individual wavelength channels by a postfabrication trimming method so that the static power is reduced by more than an order of magnitude and allows minimization of these parts of the overall power requirements of such photonic integrated circuits down to record low metrics of a few femtojoules per bit.

Fabrication of Freestanding SiO

This work describes the design and fabrication of an electrostatically deflectable SiO2-membrane system with low-loss silicon-on-insulator (SOI) photonic wires. The photonic wires have been tuned thermo-optically and elasto-optically. A 10-mW thermo-optic switch is presented.

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Contact lithography with i-line (365 nm) or DUV (248 nm) is widely used in laboratories for prototyping. The achievable line width of 300 nm is sufficient for photonic wires, but a process with larger line width is more controllable. The sidewall roughness induced by the lithography and by the following etching steps results in high optical losses. Thermal oxidation is known to smoothen the silicon surface. The oxidation also consumes silicon, so that the photonic wire will shrink and a wider lithography linewidth can be applied. The silicon dioxide is used as a low loss cladding, which further reduces the refractive index contrast, so that the remaining roughness causes less losses. Single mode silicon nanowires with 500nm by 200nm cross section and optical losses of 2dB/cm were produced. The index contrast is still high enough for small bending radii for highly integrated photonic devices. Sharp branches used in Y-couplers can not be fabricated by this oxidation technique, due to the waveguide shrinkage. 3dB-couplers are easily realized by multimode interference (MMI)-couplers, with the output branches sufficiently apart. Using such couplers, Mach-Zehnder interferometers were fabricated. For electric contacts, the SiO2-cladding is locally removed and ZnO and Al electrodes are applied. The c-axis of sputtered ZnO grows preferentially perpendicular to the surface, which allows to utilize the electro optic effect.

-Membrane Systems for Thermo-Optic Adjusting of SOI Photonic Wires

The large-scale and low-cost fabrication of high sensitivity sensors for the real-time detection of biochemicals and molecular substances opens up new opportunities in the areas of bioanalytic screening and medical diagnostics. Planar integrated photonic resonators that can be fabricated with a low footprint, in spatial and wavelength multiplexed arrangements, and that enable integration with microfluidics on the wafer scale have emerged as a promising sensing platform for these application fields. We realized an optofluidic and label-free biosensor that is based on hydrogenated amorphous silicon microring resonators embedded in silicon/glass microfluidic channels for analyte injection and biomolecule immobilization. The optofluidic sensor merits for refractive index and biomolecule sensing are evaluated by sensitivity and detection limit simulations, whereas a proof of concept is demonstrated by real-time protein immobilization experiments of functionalized resonators.