In this paper we demonstrate the development and optimization of an 800 nm-thick Plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiN) photonic platform on a 300-mm silicon wafer. The implementation of ArF immersion lithography contributes to superior manufacturing processes, as it provides excellent critical dimension (CD) uniformity inter- and intra-wafers, make it an optimal platform of production of integrated circuits and nanoscale devices.
On-chip spectrometer operating in the mid-infrared (MIR) regime (λ = 2 – 14 μm) enables the miniaturization of a chemical sensing platform that identifies compounds based on their unique molecular fingerprints. Germanium-on-Silicon (Ge-on- Si) material system is a suitable candidate for its transparency in the MIR spectrum and compatibility with silicon processing. As chemical sensing is conducted by having the mode evanescent field interacting with the analyte, the design of Ge-on-Si waveguide for a compact footprint (small bending radius) and large evanescent field coverage is necessary. However, the bending radius of the Ge-on-Si waveguide is limited to hundreds of micrometers due to the low refractive index contrast between germanium and silicon. In this work, we demonstrate a 3 μm thick Ge-on-Si waveguide, with ~89° sidewall angles and a high gap aspect ratio of 10 (resolvable gaps of 300 nm). Different types of Ge-on-Si devices are fabricated including in-plane distributed Bragg grating (DBR) structures, cascaded Fabry-Perot resonators, and polarization splitters. We show that over-etching the Si lower cladding is able to reduce bending loss by ~10x, allowing us to decrease the bending radius to ~50 μm. Designs of 32 waveguide geometries for single mode propagation from 5.5 μm to 11 μm are presented, each of which is integrated with grating couplers operating at specific peak wavelengths. Our measurements show high consistency between the simulated and measured peak wavelengths of the grating couplers, with an inter-chip standard deviation of σλ ⁄ λpeak <1%
Aluminum nitride (AlN) is a promising photonics material contributed by its wide transparency window and remarkable nonlinear optical property. Moreover, its nonlinear effect can be further enhanced by doping Scandium (Sc). Such nonlinear optical property brings potential for high efficiency in nonlinear optical generation processes, such as 2nd harmonic generation and frequency comb generation. Although the nonlinear optical property of Sc-doped AlN looks promising, its waveguide is facing challenge on loss reduction. In this work, we report Sc-doped AlN photonic integrated circuit with reduced waveguide loss of 6 dB/cm around 1550 nm. The waveguide has Sc doping concentration of 10%. Its etching process is tailored through a design of experiment (DoE) approach to achieve smooth surface. An annealing process is also applied to patterned waveguide for optical loss reduction. A loaded Q of 1.41×104 has also been reported from microring resonator on the same wafer. The reported result paves the way towards low-loss Sc-doped AlN for photonic integrated circuits.
SignificanceLight-sheet fluorescence microscopy is widely used for high-speed, high-contrast, volumetric imaging. Application of this technique to in vivo brain imaging in non-transparent organisms has been limited by the geometric constraints of conventional light-sheet microscopes, which require orthogonal fluorescence excitation and collection objectives. We have recently demonstrated implantable photonic neural probes that emit addressable light sheets at depth in brain tissue, miniaturizing the excitation optics. Here, we propose a microendoscope consisting of a light-sheet neural probe packaged together with miniaturized fluorescence collection optics based on an image fiber bundle for lensless, light-field, computational fluorescence imaging.AimFoundry-fabricated, silicon-based, light-sheet neural probes can be packaged together with commercially available image fiber bundles to form microendoscopes for light-sheet light-field fluorescence imaging at depth in brain tissue.ApproachPrototype microendoscopes were developed using light-sheet neural probes with five addressable sheets and image fiber bundles. Fluorescence imaging with the microendoscopes was tested with fluorescent beads suspended in agarose and fixed mouse brain tissue.ResultsVolumetric light-sheet light-field fluorescence imaging was demonstrated using the microendoscopes. Increased imaging depth and enhanced reconstruction accuracy were observed relative to epi-illumination light-field imaging using only a fiber bundle.ConclusionsOur work offers a solution toward volumetric fluorescence imaging of brain tissue with a compact size and high contrast. The proof-of-concept demonstrations herein illustrate the operating principles and methods of the imaging approach, providing a foundation for future investigations of photonic neural probe enabled microendoscopes for deep-brain fluorescence imaging in vivo.
Fu-Der Chen, Homeira Moradi-Chameh, Prajay Shah, Ilan Felts Almog, Youngho Jung, Ting Hu, Junho Jeong, Andres Lozano, Taufik Valiante, Laurent Moreaux, Joyce Poon, Michael Roukes, Wesley Sacher, Xianshu Luo, Anton Fomenko, Thomas Lordello, Xinyu Liu, John Straguzzi, Trevor Fowler, Patrick Lo
Significance: Light-sheet fluorescence microscopy (LSFM) is a powerful technique for highspeed volumetric functional imaging. However, in typical light-sheet microscopes, the illumination and collection optics impose significant constraints upon the imaging of non-transparent brain tissues. We demonstrate that these constraints can be surmounted using a new class of implantable photonic neural probes.
Aim: Mass manufacturable, silicon-based light-sheet photonic neural probes can generate planar patterned illumination at arbitrary depths in brain tissues without any additional micro-optic components.
Approach: We develop implantable photonic neural probes that generate light sheets in tissue. The probes were fabricated in a photonics foundry on 200-mm-diameter silicon wafers. The light sheets were characterized in fluorescein and in free space. The probe-enabled imaging approach was tested in fixed, in vitro, and in vivo mouse brain tissues. Imaging tests were also performed using fluorescent beads suspended in agarose.
Results: The probes had 5 to 10 addressable sheets and average sheet thicknesses <16 μm for propagation distances up to 300 μm in free space. Imaging areas were as large as ≈240 μm × 490 μm in brain tissue. Image contrast was enhanced relative to epifluorescence microscopy.
Conclusions: The neural probes can lead to new variants of LSFM for deep brain imaging and experiments in freely moving animals.
Silicon photonics using microdisk and microring resonators are finding technologically important applications from
telecommunications and on-chip optical interconnects to optofluidics and biosensing. Silicon-based microresonators that
partially confine light by total internal reflection are versatile device structures which are highly wavelength-selective,
reconfigurable via various refractive index tuning mechanisms, micrometer-scale footprint, and readily in/out-coupled
with integrated waveguides. In this paper, we will highlight our latest progress in silicon photonics using microdisk and
microring resonators for on-chip optical interconnects, optofluidics and biosensing applications including the
experimental demonstrations of: (i) optical time delay and advance using silicon microring resonators integrated with pi-
n diodes; (ii) photocurrent spectroscopy of microdisk resonators using two-photon-absorption induced photocarriers;
(iii) optical trapping and transporting of microparticles using a water-clad silicon nitride microring resonator; and (iv)
coupled microdisk resonator optical waveguide-based refractive index sensors.
We review our recent work on silicon photonic devices for on-chip optical interconnects and optofluidics. On the optical
interconnects front, we demonstrate coupled-resonator optical waveguides with gapless inter-cavity coupling for on-chip
wide-bandwidth high-order optical channel filters and optical delay lines. We propose a 5×5 matrix switch comprising
two-dimensionally cascaded microring resonator-based electrooptic switches for network-on-chip applications and
demonstrate a 2×2 matrix switch as a proof-of-concept. We demonstrate cavity-enhanced photocurrent generation in a
p-i-n diode embedded microring resonator for wavelength-selective photodetection and monitoring on-chip optical
networks. We also investigate a serial-cascaded double-microring-based silicon photonic circuit for high-speed on-chip
clock-recovery applications. On the optofluidics front, we study silicon nitride based waveguides with integrated
microfluidic channels for optical manipulation of microparticles.
We propose a design of an optical switch on a silicon chip comprising a 5 × 5 array of cascaded waveguide-crossing-coupled microring resonator-based switches for photonic networks-on-chip applications. We adopt our recently demonstrated design of multimode-interference (MMI)-based wire waveguide crossings, instead of conventional plain waveguide crossings, for the merits of low loss and low crosstalk. The microring resonator is integrated with a lateral p-i-n diode for carrier-injection-based GHz-speed on-off switching. All 25 microring
resonators are assumed to be identical within a relatively wide resonance line width. The optical circuit switch can employ a single wavelength channel or multiple wavelength channels that are spaced by the microring resonator free spectral range. We analyze the potential performance of the proposed photonic network in terms of (i) light path cross-connections loss budget, and (ii) DC on-off power consumption for establishing a light path. As a proof-of-concept, our initial experiments on cascaded passive silicon MMI-crossing-coupled microring resonators demonstrate 3.6-Gbit/s non-return-to-zero data transmissions at on- and off-resonance wavelengths.
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