Wafer-scale heterogeneous integration of thin film lithium niobate on silicon-nitride photonic integrated circuits with low loss bonding interfaces.

Silicon nitride (Si3N4) is a versatile waveguide material platform for CMOS foundry-based photonic integrated circuits (PICs) with low loss and high-power handling. The range of applications enabled by this platform is significantly expanded with the addition of a material with large electro-optic and nonlinear coefficients such as lithium niobate. This work examines the heterogeneous integration of thin-film lithium-niobate (TFLN) on silicon-nitride PICs. Bonding approaches are evaluated based on the interface used (SiO2, Al2O3 and direct) to form hybrid waveguide structures. We demonstrate low losses in chip-scale bonded ring resonators of 0.4 dB/cm (intrinsic Q = 8.19 × 105). In addition, we are able to scale the process to demonstrate bonding of full 100-mm TFLN wafers to 200-mm Si3N4 PIC wafers with high layer transfer yield. This will enable future integration with foundry processing and process design kits (PDKs) for applications such as integrated microwave photonics and quantum photonics.

Optical unmasking of spectrally overlapping RF signals.

When two signals having overlapping frequency content are received at the same time, they interfere to obstruct detection of the information carried by each individual signal. We introduce here a new nonlinear optoelectronic filtering technique that enables the ability to individually detect two concurrent and spectrally overlapping signals, even when the amplitude ratio between the signals is as high as 100,000. We demonstrate our system for application in steganography where we unveil the information carried by a hidden desired RF signal, while a dominant interferer signal is intentionally transmitted nearby and at the same frequency. Our signal recovery technique, which operates assuming no a priori knowledge of either signal, presents an additional pathway that can be used to control how information can be processed and communicated.

A nonlinear optoelectronic filter for electronic signal processing.

The conversion of electrical signals into modulated optical waves and back into electrical signals provides the capacity for low-loss radio-frequency (RF) signal transfer over optical fiber. Here, we show that the unique properties of this microwave-photonic link also enable the manipulation of RF signals beyond what is possible in conventional systems. We achieve these capabilities by realizing a novel nonlinear filter, which acts to suppress a stronger RF signal in the presence of a weaker signal independent of their separation in frequency. Using this filter, we demonstrate a relative suppression of 56 dB for a stronger signal having a 1-GHz center frequency, uncovering the presence of otherwise undetectable weaker signals located as close as 3.5 Hz away. The capabilities of the optoelectronic filter break the conventional limits of signal detection, opening up new possibilities for radar and communication systems, and for the field of precision frequency metrology.

Sub-100-nanosecond thermal reconfiguration of silicon photonic devices.

One of the limitations of thermal reconfiguration in silicon photonics is its slow response time. At the same time, there is a tradeoff between the reconfiguration speed and power consumption in conventional reconfiguration schemes that poses a challenge in improving the performance of microheaters. In this work, we theoretically and experimentally demonstrate that the high thermal conductivity of silicon can be exploited to tackle this tradeoff through direct pulsed excitation of the device silicon layer. We demonstrate 85 ns reconfiguration of 4 µm diameter microdisks, which is one order of magnitude improvement over the conventional microheaters. At the same time, 2.06 nm/mW resonance wavelength shift is achieved in these devices, which is in a par with the best microheater architectures optimized for low-power operation. We also present a system-level model that precisely describes the response of the demonstrated microheaters. A differentially addressed optical switch is also demonstrated that shows the possibility of switching in opposite directions (i.e., OFF-to-ON and ON-to-OFF) using the proposed reconfiguration scheme.

Hybrid integrated plasmonic-photonic waveguides for on-chip localized surface plasmon resonance (LSPR) sensing and spectroscopy.

We experimentally demonstrate efficient extinction spectroscopy of single plasmonic gold nanorods with exquisite fidelity (SNR > 20dB) and high efficiency light coupling (e. g., 9.7%) to individual plasmonic nanoparticles in an integrated platform. We demonstrate chip-scale integration of lithographically defined plasmonic nanoparticles on silicon nitride (Si3N4) ridge waveguides for on-chip localized surface plasmon resonance (LSPR) sensing. The integration of this hybrid plasmonic-photonic platform with microfluidic sample delivery system is also discussed for on-chip LSPR sensing of D-glucose with a large sensitivity of ∼ 250 nm/RIU. The proposed architecture provides an efficient means of interrogating individual plasmonic nanoparticles with large SNR in an integrated alignment-insensitive platform, suitable for high-density on-chip sensing and spectroscopy applications.

Low-noise RF-amplifier-free slab-coupled optical waveguide coupled optoelectronic oscillators: physics and operation.

We demonstrate a 10-GHz RF-amplifier-free slab-coupled optical waveguide coupled optoelectronic oscillator (SCOW-COEO) system operating with low phase-noise (<-115 dBc/Hz at 1 kHz offset) and large sidemode suppression (>70 dB measurement-limited). The optical pulses generated by the SCOW-COEO exhibit 26.8-ps pulse width (post compression) with a corresponding spectral bandwidth of 0.25 nm (1.8X transform-limited). We also investigate the mechanisms that limit the performance of the COEO. Our measurements indicate that degradation in the quality factor (Q) of the optical cavity significantly impacts COEO phase-noise through increases in the optical amplifier relative intensity noise (RIN).

Amplifier-free slab-coupled optical waveguide optoelectronic oscillator systems.

We demonstrate a free-running 3-GHz slab-coupled optical waveguide (SCOW) optoelectronic oscillator (OEO) with low phase-noise (<-120 dBc/Hz at 1-kHz offset) and ultra-low sidemode spurs. These sidemodes are indistinguishable from noise on a spectrum analyzer measurement (>88 dB down from carrier). The SCOW-OEO uses high-power low-noise SCOW components in a single-loop cavity employing 1.5-km delay. The noise properties of our SCOW external-cavity laser (SCOWECL) and SCOW photodiode (SCOWPD) are characterized and shown to be suitable for generation of high spectral purity microwave tones. Through comparisons made with SCOW-OEO topologies employing amplification, we observe the sidemode levels to be degraded by any amplifiers (optical or RF) introduced within the OEO cavity.

Fully reconfigurable compact RF photonic filters using high-Q silicon microdisk resonators.

We present a fully reconfigurable fourth-order RF photonic filter on SOI platform with a tunable 3-dB bandwidth of 0.9-5 GHz, more than 38 dB optical out-of-band rejection, FSR up to 650 GHz, and compact size (total area 0.25 mm(2)). The center wavelength of the filter can be tuned over a wide range with a power consumption of 10 mW/nm. The filter architecture uses a unit-cell based approach to realize the desired filter specifications. The use of high-Q resonator-based components enables a dramatic reduction in size, weight and power (SWaP) of each unit cell, with the possibility of cascading a large number of these unit cells on a single chip. Thermal reconfiguration allows for low insertion loss and therefore results in the scalability of these filters. The demonstrated filter can be used in many different applications including RF photonic front-ends and high speed optical A/D conversion.

High resolution on-chip spectroscopy based on miniaturized microdonut resonators.

We experimentally demonstrate a high resolution integrated spectrometer on silicon on insulator (SOI) substrate using a large-scale array of microdonut resonators. Through top-view imaging and processing, the measured spectral response of the spectrometer shows a linewidth of ~0.6 nm with an operating bandwidth of ~50 nm. This high resolution and bandwidth is achieved in a compact size using miniaturized microdonut resonators (radius ~2 µm) with a high quality factor, single-mode operation, and a large free spectral range. The microspectrometer is realized using silicon process compatible fabrication and has a great potential as a high-resolution, large dynamic range, light-weight, compact, high-speed, and versatile microspectrometer.

Label-free flow cytometry using multiplex coherent anti-Stokes Raman scattering (MCARS) for the analysis of biological specimens.

We present the first demonstration, to our knowledge, of a label-free flow cytometer for the analysis of biological specimens using multiplex coherent anti-Stokes Raman scattering (MCARS) and elastic scatter measurements. The MCARS system probes the Raman vibrational energy levels and the elastic scatter provides morphological information. We demonstrate these capabilities by probing a culture of Saccharomyces cerevisiae at 100 spectra/s and constructing a background-free Raman reconstruction using a Kramers-Kronig relation. A theoretical analysis shows that this system could operate at speeds of 10 kHz with appropriate hardware; thus facilitating integration into commercial flow cytometers or use as a high-speed, stand-alone device.

Silicon nanophotonic devices for integrated lab-on-a-chip sensing.

We present our recent progress in designing nanophotonic devices for integrated sensing applications. We focus on Si-based microresonators and on-chip spectrometers as the main building blocks to implement compact sensing platforms with strong light-matter interaction. The performance of these devices is discussed, and their future prospects in realizing low-power low-cost portable multi-purpose sensing systems are addressed.

Toward ultimate miniaturization of high Q silicon traveling-wave microresonators.

High Q traveling-wave resonators (TWR)s are one of the key building block components for VLSI Photonics and photonic integrated circuits (PIC). However, dense VLSI integration requires small footprint resonators. While photonic crystal resonators have shown the record in simultaneous high Q (~10(5)-10(6)) and very small mode volumes; the structural simplicity of TWRs has motivated many ongoing researches on miniaturization of these resonators with maintaining Q in the same range. In this paper, we investigate the scaling issues of silicon traveling-wave microresonators down to ultimate miniaturization levels in SOI platforms. Two main constraints that are considered during this down scaling are: 1) Preservation of the intrinsic Q of the resonator at high values, and 2) Compatibility of resonator with passive (active) integration by preserving the SiO(2) BOX layer (plus a thin Si slab layer for P-N junction fabrication). Microdisk and microdonut (an intermediate design between disk and ring shape) are considered for high Q, miniaturization, and single-mode operation over a wide wavelength range (as high as the free-spectral range). Theoretical and experimental results for miniaturized resonators are demonstrated and Q's as high as ~10(5) for resonators as small as 1.5 µm radius are achieved.

Tuning of resonance-spacing in a traveling-wave resonator device.

In this work a traveling-wave resonator device is proposed and experimentally demonstrated in silicon-on-insulator platform in which the spacing between its adjacent resonance modes can be tuned. This is achieved through the tuning of mutual coupling of two strongly coupled resonators. By incorporating metallic microheaters, tuning of the resonance-spacing in a range of 20% of the free-spectral-range (0.4nm) is experimentally demonstrated with 27mW power dissipation in the microheater. To the best of our knowledge this is the first demonstration of the tuning of resonance-spacing in an integrated traveling-wave-resonator. It is also numerically shown that these modes exhibit high field-enhancements which makes this device extremely useful for nonlinear optics and sensing applications.

Microresonators in CMOS compatible substrate.

We report our recent progress in designing and developing traveling-wave microresonators on CMOS compatible substrate, with a focus on ultra-compact Si-based microring and microdisk resonators in the visible and near infrared wavelengths with ultra-high Q and small mode volume, suitable for strong light-matter interaction. The performance of these resonators is discussed, and the design and fabrication challenges and solutions to achieve efficient coupling from the bus-waveguide to the resonator is mentioned. Coupled-resonator architectures for the design of filters are analyzed and theoretical and experimental results for the flat-band filters with wide bandwidth and large free spectral range are presented. Furthermore, by using a detailed thermal model we demonstrate optimized microresonator structures on semiconductor substrates with improved thermal conductivity that can sustain a high circulating optical intensity at the resonance wavelength, which is particularly useful for nonlinear optics and optical signal processing applications. Finally, we address some of the applications and future prospects of such microresonators in CMOS-compatible substrate technology, including portable multi-purpose bio/chemical sensing systems, reconfigurable optical signal processing modules and chip-scale optical amplifiers and lasers.

Systematic design and fabrication of high-Q single-mode pulley-coupled planar silicon nitride microdisk resonators at visible wavelengths.

High quality (Q approximately 6 x 10(5)) microdisk resonators are demonstrated in a Si(3)N(4) on SiO(2) platform at 652-660 nm with integrated in-plane wrap-around coupling waveguides to enable critical coupling to specific microdisk radial modes. Selective coupling to the first three radial modes with >20dB suppression of the other radial modes is achieved by controlling the wrap-around waveguide width. Advantages of such pulley-coupled microdisk resonators include single mode operation, ease of fabrication due to larger waveguide-resonator gaps, the possibility of resist reflow during the lithography phase to improve microdisk etched surface quality, and the ability to realize highly over-coupled microdisks suitable for low-loss delay lines and add-drop filters.

High quality planar silicon nitride microdisk resonators for integrated photonics in the visible wavelength range.

High quality factor (Q approximately 3.4 x 10(6)) microdisk resonators are demonstrated in a Si(3)N(4) on SiO(2) platform at 652-660 nm with integrated in-plane coupling waveguides. Critical coupling to several radial modes is demonstrated using a rib-like structure with a thin Si(3)N(4) layer at the air-substrate interface to improve the coupling.

Design and demonstration of compact, wide bandwidth coupled-resonator filters on a siliconon- insulator platform.

We design and fabricate a compact third-order coupledresonator filter on the silicon-on-insulator platform with focused application for on-chip optical interconnects. The filter shows a large flat bandwidth (3dB 3.3nm), large FSR (approximately 18nm), more than 18dB out-of-band rejection at the drop port and more than 12 dB extinction at the through port, as well as a negligible drop loss (<0.5dB) within a footprint of 0.0004 mm(2).

Quantitative modeling of coupling-induced resonance frequency shift in microring resonators.

We present a detailed study on the behavior of coupling-induced resonance frequency shift (CIFS) in dielectric microring resonators. CIFS is related to the phase responses of the coupling region of the resonator coupling structure, which are examined for various geometries through rigorous numerical simulations. Based on the simulation results, a model for the phase responses of the coupling structure is presented and verified to agree with the simulation results well, in which the first-order coupled mode theory (CMT) is extended to second order, and the important contributions from the inevitable bent part of practical resonators are included. This model helps increase the understanding of the CIFS behavior and makes the calculation of CIFS for practical applications without full numerical simulations possible.

Multiplex coherent anti-Stokes Raman scattering (MCARS) for chemically sensitive, label-free flow cytometry.

Flow cytometry is an ever-advancing high-throughput multivariate analysis tool that natively provides size and morphological information. To obtain molecular information, however, typically requires the addition of fluorophores, which are limited by spectral overlap, nonspecific binding, available conjugation chemistries, and cellular toxicity. A complementary or alternative, label-free approach to molecular information is through multiplex coherent anti-Stokes Raman scattering (MCARS), which is a coherent, nonlinear optical method that provides a wealth of molecular information by probing the Raman energies within a molecule. In this work, we demonstrate the unique capability of our MCARS flow cytometer to distinguish flowing particles and discuss system performance capabilities and possibilities.

Ultra-high Q planar silicon microdisk resonators for chip-scale silicon photonics.

We report the fabrication and experimental characterization of an ultra-high Q microdisk resonator in a silicon-on-insulator (SOI) platform. We examine the role of the substrate in the performance of such microdisk resonators. While substrate leakage loss has warranted the necessity of substrate undercut structures in the past, we show here that the substrate has a very useful role to play for both passive chip-scale device integration as well as active electronic device integration. Two device architectures for the disk-on-substrate are studied in order to assess the possibility of such an integration of high Q resonators and active components. Using an optimized process for fabrication of such a resonator device, we experimentally demonstrate a Q approximately 3 x 10(6), corresponding to a propagation loss approximately 0.16 dB/cm. This, to our knowledge, is the maximum Q observed for silicon microdisk cavities of this size for disk-on-substrate structures. Critical coupling for a resonance mode with an unloaded Q approximately 0.7 x 10(6) is observed. We also report a detailed comparison of the obtained experimental resonance spectrum with the theoretical and simulation analysis. The issue of waveguide-cavity coupling is investigated in detail and the conditions necessary for the existence or lack of critical coupling is elaborated.