Low-power swept-source Raman spectroscopy.

'Molecular fingerprinting' with Raman spectroscopy can address important problems-from ensuring our food safety, detecting dangerous substances, to supporting disease diagnosis and management. However, the broad adoption of Raman spectroscopy demands low-cost, portable instruments that are sensitive and use lasers that are safe for human eye and skin. This is currently not possible with existing Raman spectroscopy approaches. Portability has been achieved with dispersive Raman spectrometers, however, fundamental entropic limits to light collection both limits sensitivity and demands high-power lasers and cooled expensive detectors. Here, we demonstrate a swept-source Raman spectrometer that improves light collection efficiency by up to 1000× compared to portable dispersive spectrometers. We demonstrate high detection sensitivity with only 1.5 mW average excitation power and an uncooled amplified silicon photodiode. The low optical power requirement allowed us to utilize miniature chip-scale MEMS-tunable lasers with close to eye-safe optical powers for excitation. We characterize the dynamic range and spectral characteristics of this Raman spectrometer in detail, and use it for fingerprinting of different molecular species consumed everyday including analgesic tablets, nutrients in vegetables, and contaminated alcohol. By moving the complexity of Raman spectroscopy from bulky spectrometers to chip-scale light sources, and by replacing expensive cooled detectors with low-cost uncooled alternatives, this swept-source Raman spectroscopy technique could make molecular fingerprinting more accessible.

Photonic Readout of Superconducting Nanowire Single Photon Counting Detectors.

Scalable, low power, high speed data transfer between cryogenic (0.1-4 K) and room temperature environments is essential for the realization of practical, large-scale systems based on superconducting technologies. A promising approach to overcome the limitations of conventional wire-based readout is the use of optical fiber communication. Optical fiber presents a 100-1,000x lower heat load than conventional electrical wiring, relaxing the requirements for thermal anchoring, and is also immune to electromagnetic interference, which allows routing of sensitive signals with improved robustness to noise and crosstalk. Most importantly, optical fibers allow for very high bandwidth densities (in the Tbps/mm2 range) by carrying multiple signals through the same physical fiber (Wavelength Division Multiplexing, WDM). Here, we demonstrate for the first time optical readout of a superconducting nanowire single-photon detector (SNSPD) directly coupled to a CMOS photonic modulator, without the need for an interfacing device. By operating the modulator in the forward bias regime at a temperature of 3.6 K, we achieve very high modulation efficiency (1,000-10,000 pm/V) and a low input impedance of 500 Ω with a low power dissipation of 40 µW. This allows us to obtain optical modulation with the low, millivolt-level signal generated by the SNSPD.

Power handling of silicon microring modulators.

Silicon photonic wavelength division multiplexing (WDM) transceivers promise to achieve multi-Tbps data rates for next-generation short-reach optical interconnects. In these systems, microring resonators are important because of their low power consumption and small footprint, two critical factors for large-scale WDM systems. However, their resonant nature and silicon's strong optical nonlinearity give rise to nonlinear effects that can deteriorate the system's performance with optical powers on the order of milliwatts, which can be reached on the transmitter side where a laser is directly coupled into resonant modulators. Here, a theoretical time-domain nonlinear model for the dynamics of optical power in silicon resonant modulators is derived, accounting for two-photon absorption, free-carrier absorption and thermal and dispersion effects. This model is used to study the effects of high input optical powers over modulation quality, and experimental data in good agreement with the model is presented. Two major consequences are identified: the importance of a correct initialization of the resonance wavelength with respect to the laser due to the system's bistability; and the existence of an optimal input optical power beyond which the modulation quality degrades.

Compact and high-precision wavemeters using the Talbot effect and signal processing.

Precise knowledge of a laser's wavelength is crucial for applications from spectroscopy to telecommunications. Here, we present a wavemeter that operates on the Talbot effect. Tone parameter extraction algorithms are used to retrieve the frequency of the periodic signal obtained in the sampled Talbot interferogram. Theoretical performance analysis based on the Cramér-Rao lower bound as well as experimental results are presented and discussed. With this scheme, we experimentally demonstrate a compact and high-precision wavemeter with below 10 pm single-shot estimation uncertainty under the 3-σ criterion around 780 nm.

Publisher Correction: Integrating photonics with silicon nanoelectronics for the next generation of systems on a chip.

In this Letter, owing to an error during the production process, the author affiliations were listed incorrectly. Affiliation number 5 (Colleges of Nanoscale Science and Engineering, State University of New York (SUNY)) was repeated, and affiliation numbers 6-8 were incorrect. In addition, the phrase "two oxide thickness variants" should have been "two gate oxide thickness variants". These errors have all been corrected online.

Integrating photonics with silicon nanoelectronics for the next generation of systems on a chip.

Electronic and photonic technologies have transformed our lives-from computing and mobile devices, to information technology and the internet. Our future demands in these fields require innovation in each technology separately, but also depend on our ability to harness their complementary physics through integrated solutions1,2. This goal is hindered by the fact that most silicon nanotechnologies-which enable our processors, computer memory, communications chips and image sensors-rely on bulk silicon substrates, a cost-effective solution with an abundant supply chain, but with substantial limitations for the integration of photonic functions. Here we introduce photonics into bulk silicon complementary metal-oxide-semiconductor (CMOS) chips using a layer of polycrystalline silicon deposited on silicon oxide (glass) islands fabricated alongside transistors. We use this single deposited layer to realize optical waveguides and resonators, high-speed optical modulators and sensitive avalanche photodetectors. We integrated this photonic platform with a 65-nanometre-transistor bulk CMOS process technology inside a 300-millimetre-diameter-wafer microelectronics foundry. We then implemented integrated high-speed optical transceivers in this platform that operate at ten gigabits per second, composed of millions of transistors, and arrayed on a single optical bus for wavelength division multiplexing, to address the demand for high-bandwidth optical interconnects in data centres and high-performance computing3,4. By decoupling the formation of photonic devices from that of transistors, this integration approach can achieve many of the goals of multi-chip solutions 5 , but with the performance, complexity and scalability of 'systems on a chip'1,6-8. As transistors smaller than ten nanometres across become commercially available 9 , and as new nanotechnologies emerge10,11, this approach could provide a way to integrate photonics with state-of-the-art nanoelectronics.

Miniature, sub-nanometer resolution Talbot spectrometer.

Miniaturization of optical spectrometers has a significant practical value as it can enable compact, affordable spectroscopic systems for chemical and biological sensing applications. For many applications, the spectrometer must gather light from sources that span a wide range of emission angles and wavelengths. Here, we report a lens-free spectrometer that is simultaneously compact (<0.6 cm3), of high resolution (<1 nm), and has a clear aperture (of 10×10 mm). The wavelength-scale pattern in the dispersive element strongly diffracts the input light to produce non-paraxial mid-field diffraction patterns that are then recorded using an optimally matched image sensor and processed to reconstruct the spectrum.

Single-chip microprocessor that communicates directly using light.

Data transport across short electrical wires is limited by both bandwidth and power density, which creates a performance bottleneck for semiconductor microchips in modern computer systems--from mobile phones to large-scale data centres. These limitations can be overcome by using optical communications based on chip-scale electronic-photonic systems enabled by silicon-based nanophotonic devices. However, combining electronics and photonics on the same chip has proved challenging, owing to microchip manufacturing conflicts between electronics and photonics. Consequently, current electronic-photonic chips are limited to niche manufacturing processes and include only a few optical devices alongside simple circuits. Here we report an electronic-photonic system on a single chip integrating over 70 million transistors and 850 photonic components that work together to provide logic, memory, and interconnect functions. This system is a realization of a microprocessor that uses on-chip photonic devices to directly communicate with other chips using light. To integrate electronics and photonics at the scale of a microprocessor chip, we adopt a 'zero-change' approach to the integration of photonics. Instead of developing a custom process to enable the fabrication of photonics, which would complicate or eliminate the possibility of integration with state-of-the-art transistors at large scale and at high yield, we design optical devices using a standard microelectronics foundry process that is used for modern microprocessors. This demonstration could represent the beginning of an era of chip-scale electronic-photonic systems with the potential to transform computing system architectures, enabling more powerful computers, from network infrastructure to data centres and supercomputers.

High-quality silicon on silicon nitride integrated optical platform with an octave-spanning adiabatic interlayer coupler.

Hybrid nanophotonic platforms based on three-dimensional integration of different photonic materials are emerging as promising ecosystems for the optoelectronic device fabrication. In order to benefit from key features of both silicon (Si) and silicon nitride (SiN) on a single chip, we have developed a wafer-scale hybrid photonic platform based on the integration of a thin crystalline Si layer on top of a thin SiN layer with an ultra-thin oxide buffer layer. A complete optical path in the hybrid platform is demonstrated by coupling light back and forth between nanophotonic devices in Si and SiN layers. Using an adiabatic tapered coupling method, a record-low interlayer coupling-loss of 0.02 dB is achieved at 1550 nm telecommunication wavelength window. We also demonstrate high-Q resonators on the hybrid material platform with intrinsic Q's as high as 3 × 10(6) for a 60 µm-radius microring resonator, which is (to the best of our knowledge) the highest Q observed for a micro-resonator on a hybrid Si/SiN platform.

Robust postfabrication trimming of ultracompact resonators on silicon on insulator with relaxed requirements on resolution and alignment.

One of the main drawbacks of the high-index-contrast silicon-on-insulator platform in integrated photonics is the high sensitivity of the resonance wavelength of resonators to dimensional variations caused by fabrication imperfection. In this work, we experimentally demonstrate an accurate postfabrication trimming technique for compensating the fabrication-induced variations in the resonance properties of nanophotonic devices. Using this technique, we reduce the variation of the resonance wavelength of 4 µm diameter microdonut resonators by more than 1 order of magnitude to about 25 pm, which is adequate for most interconnect, optical signal processing, and sensing applications. In addition, our proposed technique has improved misalignment toleration and throughput compared to previous reports.

Field-programmable optical devices based on resonance elimination.

Optical switches are among the essential building blocks in optical networks due to their unique role in routing data. In this Letter, for the first time to our knowledge, we have exploited a high-quality factor (Q) optical microresonator combined with the well-known irreversible dielectric breakdown phenomenon to introduce a simple field-programmable on/off optical switch. This simple unit can be thought of as a building block for more complex optical systems with different functionalities. By using this simple unit we have demonstrated an optical field-programmable 2×2 switch. After the device is programmed by the user, no external electrical signal is needed to maintain the state of the device. The same approach can readily be adopted to design a field-programmable arbitrary N×N optical switch.

Thermally reconfigurable device for adaptive reflection suppression on a silicon-on-insulator platform.

We demonstrate a compact, thermally reconfigurable reflection suppressor on a silicon-on-insulator (SOI) platform, without reliance on nonreciprocal mechanisms. A reflection suppression ratio of 40 dB is achieved with a footprint of 105 µm in length. The insertion loss of the device is below 0.15 dB, and its total power consumption stays below 20 mW. The operation bandwidth depends on the frequency dependence of the back reflection going into the suppressor, which is predominantly determined by the distance between the device and the source of reflection. In this work, a 20 dB reflection suppression bandwidth of 20.7 GHz was achieved.

Vertical integration of high-Q silicon nitride microresonators into silicon-on-insulator platform.

We demonstrate a vertical integration of high-Q silicon nitride microresonators into the silicon-on-insulator platform for applications at the telecommunication wavelengths. Low-loss silicon nitride films with a thickness of 400 nm are successfully grown, enabling compact silicon nitride microresonators with ultra-high intrinsic Qs (~ 6 × 10(6) for 60 µm radius and ~ 2 × 10(7) for 240 µm radius). The coupling between the silicon nitride microresonator and the underneath silicon waveguide is based on evanescent coupling with silicon dioxide as buffer. Selective coupling to a desired radial mode of the silicon nitride microresonator is also achievable using a pulley coupling scheme. In this work, a 60-µm-radius silicon nitride microresonator has been successfully integrated into the silicon-on-insulator platform, showing a single-mode operation with an intrinsic Q of 2 × 10(6).

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.

Accurate post-fabrication trimming of ultra-compact resonators on silicon.

One of the challenges of the high refractive index contrast of silicon photonics platform is the high sensitivity of the resonance wavelength of resonators to dimensional variations caused by fabrication process variations. In this work, we have experimentally demonstrated an accurate post-fabrication trimming technique for optical devices that is robust to process variations. Using this technique, we have reduced the random variation of the resonance wavelength of 4 µm diameter resonators by a factor of 6 to below 50 pm. The level of accuracy achieved in this work is adequate for most of the RF-photonic, interconnect, and optical signal processing applications. We also discuss the throughput of this technique and its viability for wafer-scale post-fabrication trimming of silicon photonic chips.

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.

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.

Enhancing the guiding bandwidth of photonic crystal waveguides on silicon-on-insulator.

In this work we present a systematic approach to increase the low-loss guiding bandwidth of PCWs by reducing the interaction of low-group-velocity modes with the surrounding photonic crystal. By this method the low-loss bandwidth of a W1 PCW is increased from 2.5 nm to 12 nm. We also present a detailed analysis of the transmission properties of W1 PCWs and elaborate on the coupling to TM-like guided modes present in the low-loss transmission bandwidth of this device.