Engineering the passband shape of coupled-cavity filters for low loss and/or narrow bandwidth.

Emerging applications of photonic integrated circuits are calling for extremely narrowband and/or low-insertion-loss bandpass filters. Both properties are limited by cavity losses or intrinsic quality factors. However, the choice of inter-cavity and bus couplings establishes trade-offs between these two properties and the passband shape, which have been little explored. Using the widely used second-order resonant system as an example, we present new, to the best of our knowledge, classes of filter passband shapes that provide the lowest insertion loss and the narrowest bandwidth for a given loss Q. A normalized design and novel properties based on a temporal coupled-mode theory model are presented, including a design tool to apply these results. These results may benefit loss-sensitive filtering applications such as quantum-correlated photon pair sources and RF-photonic integrated circuits.

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.

Cubic-wavelength mode volume photonic crystal nanobeam cavities in a monolithic CMOS platform.

We report on the design, fabrication, and experimental characterization of photonic crystal (PhC) nanobeam cavities with the smallest footprint, largest intrinsic quality factor, and smallest mode volume to be demonstrated to date in a monolithic CMOS platform. Two types of cavities were designed, with opposite spatial mode symmetries. The opposite mode symmetry, combined with evanescent coupling, allows the nanobeam cavities to be used in reflectionless topologies, desirable in complex photonic integrated circuits (PICs). The devices were implemented and fabricated in a 45 nm monolithic electronics-photonics CMOS platform optimized for silicon photonics (GlobalFoundries 45CLO) and do not require any post-processing. Quality factors exceeding 100 000 were measured for both devices, the highest, to the best of our knowledge, among fully cladded PhC nanobeam cavities in any silicon-on-insulator (SOI) platform. Additionally, the ability of the cavities to confine light into small mode volumes, of the order of (λ/n)3, was confirmed experimentally using near-field scanning optical microscopy (NSOM). These types of cavities are an important step toward realizing ultra-low energy active devices required for the next generation of integrated optical links beyond the current microring resonator-based links and other CMOS PICs.

Polarization-insensitive 1D grating coupler based on a zero-birefringence subwavelength corelet waveguide.

Grating coupler devices provide efficient, foundry-compatible vertical fiber-to-chip coupling solutions in integrated photonic platforms. However, standard grating coupler designs are highly polarization sensitive, which hinders their adoption. We present a new, to the best of our knowledge, type of 1D polarization-insensitive grating coupler (PIGC) that is based on a zero-birefringence subwavelength "corelet" waveguide. We demonstrate a PIGC for coupling in the telecommunications O-band in a 45-nm-node monolithic silicon-on-insulator (SOI) CMOS electronic-photonic platform, with measured insertion losses of 6.7 and 6.1 dB to transverse electric and transverse magnetic polarizations, respectively, and a ±1-dB polarization dependent loss bandwidth of 73 nm.

Optical isolation using microring modulators.

Optical isolators, while commonplace in bulk and fiber optical systems, remain a key missing component in integrated photonics. Isolation using magneto-optic materials has been difficult to integrate into complementary metal-oxide-semiconductor (CMOS) fabrication platforms, motivating the use of other paths to effective non-reciprocity such as temporal modulation. We demonstrate a non-reciprocal element comprising a pair of microring modulators and a microring phase shifter in an active silicon photonic process, which, in combination with standard bandpass filters, yields an isolator on-chip. Isolation up to 13 dB is measured with a 3 dB bandwidth of 2 GHz and insertion loss of 18 dB. We also show transmission of a 4 Gbps optical data signal through the isolator while retaining a wide-open eye diagram. This compact design, in combination with increased modulation efficiency, could enable modulator-based isolators to become a standard 'black-box' component in integrated photonics CMOS foundry platform component libraries.

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.

Reflectionless dual standing-wave microcavity resonator units for photonic integrated circuits.

We propose a novel photonic circuit element configuration that emulates the through-port response of a bus coupled traveling-wave resonator using two standing-wave resonant cavities. In this "reflectionless resonator unit", the two constituent cavities, here photonic crystal (PhC) nanobeams, exhibit opposite mode symmetries and may otherwise belong to a single design family. They are coupled evanescently to the bus waveguide without mutual coupling. We show theoretically, and verify using FDTD simulations, that reflection is eliminated when the two cavities are wavelength aligned. This occurs due to symmetry-induced destructive interference at the bus coupling region in the proposed photonic circuit topology. The transmission is equivalent to that of a bus-coupled traveling-wave (e.g. microring) resonator for all coupling conditions. We experimentally demonstrate an implementation fabricated in a new 45 nm silicon-on-insulator complementary metal-oxide semiconductor (SOI CMOS) electronic-photonic process. Both PhC nanobeam cavities have a full-width half-maximum (FWHM) mode length of 4.28 µm and measured intrinsic Q's in excess of 200,000. When the resonances are tuned to degeneracy and coalesce, transmission dips of the over-coupled PhC nanobeam cavities of -16 dB and -17 dB nearly disappear showing a remaining single dip of -4.2 dB, while reflection peaks are simultaneously reduced by 10 dB, demonstrating the quasi-traveling-wave behavior. This photonic circuit topology paves the way for realizing low-energy active devices such as modulators and detectors that can be cascaded to form wavelength-division multiplexed links with smaller power consumption and footprint than traveling wave, ring resonator based implementations.

Integrated optical isolators using electrically driven acoustic waves.

We propose and investigate the performance of integrated photonic isolators based on non-reciprocal mode conversion facilitated by unidirectional, traveling acoustic waves. A triply-guided waveguide system on-chip, comprising two optical modes and an electrically-driven acoustic mode, facilitates the non-reciprocal mode conversion and is combined with spatial mode filters to create the isolator. The co-guided and co-traveling arrangement enables isolation with no additional optical loss, without magnetic-optic materials, and with low power consumption. The approach is theoretically evaluated with simulations predicting over 20 dB of isolation and 2.6 dB of insertion loss with a 370 GHz optical bandwidth and 1 cm device length. The isolator uses only 1 mW of electrical drive power, an improvement of 1-3 orders of magnitude over the state of the art. The electronic drive and lack of magneto-optic materials suggest the potential for straightforward integration with drive circuits, including in monolithic CMOS electronic-photonic platforms, enabling a fully contained 'black box' optical isolator with two optical ports and DC electrical power.

Triply resonant coupled-cavity electro-optic modulators for RF to optical signal conversion.

We propose an on-chip triply resonant electro-optic modulator architecture for RF-to-optical signal conversion and provide a detailed theoretical analysis of the optimal "circuit-level" device geometries and their performance limits. The designs maximize the RF-optical conversion efficiency through simultaneous resonant enhancement of the RF drive signal, a continuous-wave (CW) optical pump, and the generated optical sideband. The optical pump and sideband are resonantly enhanced in respective supermodes of a two-coupled-cavity optical resonator system, while the RF signal can be enhanced in addition by an LC circuit formed by capacitances of the optical resonator active regions and (integrated) matching inductors. We show that such designs can offer 15-50 dB improvement in conversion efficiency over conventional microring modulators. In the proposed configurations, the photon lifetime (resonance linewidth) limits the instantaneous RF bandwidth of the electro-optic response but does not limit its central RF frequency. The latter is set by the coupling strength between the two coupled cavities and is not subject to the photon lifetime constraint inherent to conventional singly resonant microring modulators. This feature enables efficient operation at high RF carrier frequencies without a reduction in efficiency commonly associated with the photon lifetime limit and accounts for 10-30 dB of the total improvement. Two optical configurations of the modulator are proposed: a "basic" configuration with equal Q-factors in both supermodes, most suitable for narrowband RF signals, and a "generalized" configuration with independently tailored supermode Q-factors that supports a wider instantaneous bandwidth. A second significant 5-20 dB gain in modulation efficiency is expected from RF drive signal enhancement by integrated LC resonant matching, leading to the total expected improvement of 15-50 dB. Previously studied triply-resonant modulators, with coupled longitudinal [across the free spectral range (FSR)] modes, have large resonant mode volume for typical RF frequencies, which limits the interaction between the optical and RF fields. In contrast, the proposed modulators support maximally tightly confined resonant modes, with strong coupling between the mode fields, which increases and maintains high device efficiency across a range of RF frequencies. The proposed modulator architecture is compact, efficient, capable of modulation at high RF carrier frequencies and can be applied to any cavity design or modulation mechanism. It is also well suited to moderate Q, including silicon, implementations, and may be enabling for future CMOS RF-electronic-photonic systems on chip.

Mode multiplexer for guided optical and acoustic waves.

Integrated acousto-optic (AO) devices utilize the strong overlap of acoustic and optical fields in a waveguide to facilitate efficient photon-phonon (Brillouin) interactions. For example, acoustic waves offer a lossless modulation mechanism for light. "Brillouin active" photonic platforms are currently being developed that may see optical, acoustic, and AO waveguide circuits on the same chip, where guided light and sound come together in active interaction regions. A key missing component for such a platform is a device that can multiplex modes across these two physical domains. We propose and describe a new class of optical and acoustic components, the "acoustic-optical mode multiplexer" (AOMM), a device that takes respective optical and acoustic waveguides as input ports and couples their excited guided modes into a single, joint output waveguide. We show an example suspended silicon-silicon dioxide design that combines two optical modes and a spatially separate acoustic mode into a single, co-guided output port with low insertion loss down to 0.3 dB for both optical and acoustic modes, and reflection below -20dB and -11dB, respectively. The AOMM may enable new, efficient integrated AO devices, such as isolators and circulators, where the acoustic wave generation and opto-acoustic interaction are separated.

Compact multi-million Q resonators and 100 MHz passband filter bank in a thick-SOI photonics platform.

We demonstrate ring and racetrack resonators with Qs of 3.8 to 7.5 million and 100 MHz bandwidth racetrack resonator filters, implemented in a thick silicon-on-insulator foundry platform that features a 3 µm thick device layer. We show that special racetrack resonators (with weakly guiding straight sections that transition to strongly confining bends) implemented in this platform can be preferable to rings for applications such as integrated microwave-photonic signal processing that require filters with sub-GHz bandwidth, tens of GHz of free spectral range (FSR), and a compact footprint for dense system-on-chip integration. We demonstrate ring resonators with 7.5×106 intrinsic Q, but limited FSR of 5.1 GHz and a taxing footprint of 21mm2 due to a large 2.6 mm bend-loss-limited radius. In comparison, we demonstrate two racetrack resonator designs with intrinsic Qs of 3.8×106 and 4.3×106, larger respective FSRs of 11.6 GHz and 7.9 GHz, and less than 1/20th the area of the ring resonator. Using racetrack resonators, we implemented a four-channel, 100 MHz wide passband filter bank with 4.2 to 5.4 dB insertion loss to drop ports.

Optimal design of a microring cavity optical modulator for efficient RF-to-optical conversion.

The efficiency of optical sideband generation with a microring resonator modulator as a function of modulator parameters is studied taking into account the photon dynamics inside the resonator. The best achievable modulation efficiency is determined for any choice of the resonator intrinsic quality factor, and analytic solutions for the optimum modulator parameters, namely the coupling coefficient and the detuning between the frequencies of the input laser light and the microring resonance, are provided. This analysis is carried out both for a narrowband RF signal, in which case the modulator is optimized for the center frequency of this signal, and for wideband signals, when high conversion efficiency over a wide range of RF frequencies is desired. The obtained results are expected to be useful coherent optical links, direct detection RF receivers, and optical wavelength converters.

Ring modulators with enhanced efficiency based on standing-wave operation on a field-matched, interdigitated p-n junction.

We propose ring modulators based on interdigitated p-n junctions that exploit standing rather than traveling-wave resonant modes to improve modulation efficiency, insertion loss and speed. Matching the longitudinal nodes and antinodes of a standing-wave mode with high (contacts) and low (depletion regions) carrier density regions, respectively, simultaneously lowers loss and increases sensitivity significantly. This approach permits further to relax optical constraints on contacts placement and can lead to lower device capacitance. Such structures are well-matched to fabrication in advanced microelectronics CMOS processes. Device architectures that exploit this concept are presented along with their benefits and drawbacks. A temporal coupled mode theory model is used to investigate the static and dynamic response. We show that modulation efficiencies or loss Q factors up to 2 times higher than in previous traveling-wave geometries can be achieved leading to much larger extinction ratios. Finally, we discuss more complex doping geometries that can improve carrier dynamics for higher modulation speeds in this context.

Finite-difference complex-wavevector band structure solver for analysis and design of periodic radiative microphotonic structures.

We demonstrate a finite-difference approach to complex-wavevector band structure simulation and its use as a tool for the analysis and design of periodic leaky-wave photonic devices. With the (usually real) operating frequency and unit-cell refractive index distribution as inputs, the eigenvalue problem yields the complex-wavevector eigenvalues and Bloch modes of the simulated structure. In a two-dimensional implementation for transverse-electric fields with radiation accounted for by perfectly matched layer boundaries, we validate the method and demonstrate its use in simulating the complex-wavevector band structures and modal properties of a silicon photonic crystal waveguide, an array-antenna-inspired grating coupler with unidirectional radiation, and a recently demonstrated low-loss Bloch-mode-based waveguide crossing array. Additionally, we show the first direct solution of the recently proposed open-system low-loss Bloch modes. We expect this method to be a valuable tool in photonics design, enabling the rigorous analysis and synthesis of advanced periodic and quasi-periodic photonic devices.

Four-wave mixing in silicon coupled-cavity resonators with port-selective, orthogonal supermode excitation.

We propose coupled-cavity triply-resonant systems for degenerate-pump four-wave mixing (FWM) applications that support strong nonlinear interaction between distributed pump, signal and idler modes, and allow independent coupling of the pump mode and signal/idler modes to separate ports based on nonuniform supermode profile. We demonstrate seeded FWM with wavelength conversion efficiency of -54 dB at input pump power of 3.5 dBm, and discuss applications of such orthogonal supermode coupling.

Wavelength conversion in modulated coupled-resonator systems and their design via an equivalent linear filter representation.

We propose wavelength converters based on modulated coupled resonators that achieve conversion by matching the modulation frequency to the frequency splitting of the supermodes of the unmodulated system. Using temporal coupled-mode theory, we show that these time-variant systems have an equivalent linear, time-invariant filter representation that simplifies the optimal engineering of design parameters for realistic systems. Applying our model to carrier plasma-dispersion modulators as an example implementation, we calculate conversion efficiencies between -5.4 and -1.7 dB for intrinsic quality factors of 10(4)-10(6). We show that the ratio of the resonance shift to the total linewidth is the most important parameter when determining conversion efficiency. Finally, we discuss how this model can be used to design devices such as frequency shifters, widely tunable radio frequency oscillators, and frequency combs.

Channel add-drop filter based on dual photonic crystal cavities in push-pull mode.

We demonstrate an add-drop filter based on a dual photonic crystal nanobeam cavity system that emulates the operation of a traveling wave resonator, and, thus, provides separation of the through and drop port transmission from the input port. The device is on a 3×3 mm chip fabricated in an advanced microelectronics silicon-on-insulator complementary metal-oxide semiconductor (SOI CMOS) process (IBM 45 nm SOI) without any foundry process modifications. The filter shows 1 dB of insertion loss in the drop port with a 3 dB bandwidth of 64 GHz, and 16 dB extinction in the through port. To the best of our knowledge, this is the first implementation of a port-separating, add-drop filter based on standing wave cavities coupled to conventional waveguides, and demonstrates a performance that suggests potential for photonic crystal devices within optical immersion lithography-based advanced CMOS electronics-photonics integration.

Design of triply-resonant microphotonic parametric oscillators based on Kerr nonlinearity.

We propose optimal designs for triply-resonant optical parametric oscillators (OPOs) based on degenerate four-wave mixing (FWM) in microcavities. We show that optimal designs in general call for different external coupling to pump and signal/idler resonances. We provide a number of normalized performance metrics including threshold pump power and maximum achievable conversion efficiency for OPOs with and without two-photon (TPA) and free-carrier absorption (FCA). We find that the maximum achievable conversion efficiency is bound to an upper limit by nonlinear and free-carrier losses independent of pump power, while linear losses only increase the pump power required to achieve a certain conversion efficiency. The results of this work suggest unique advantages in on-chip implementations that allow explicit engineering of resonances, mode field overlaps, dispersion, and wavelength-and mode-selective coupling. We provide universal design curves that yield optimum designs, and give example designs of microring-resonator-based OPOs in silicon at the wavelengths 1.55 µm (with TPA) and 2.3 µm (no TPA) as well as in silicon nitride (Si(3)N(4)) at 1.55 µm. For typical microcavity quality factor of 10(6), we show that the oscillation threshold in excitation bus can be well into the sub-mW regime for silicon microrings and a few mW for silicon nitride microrings. The conversion efficiency can be a few percent when pumped at 10 times of the threshold. Next, based on our results, we suggest a family of synthetic "photonic molecule"-like, coupled-cavity systems to implement optimum FWM, where structure design for control of resonant wavelengths can be separated from that of optimizing nonlinear conversion efficiency, and where furthermore pump, signal, and idler coupling to bus waveguides can be controlled independently, using interferometric cavity supermode coupling as an example. Finally, consideration of these complex geometries calls for a generalization of the nonlinear figure of merit (NFOM) as a metric for performance in nonlinear photonic systems, and shows different efficiencies for single and multi-cavity geometries, as well as for standing and traveling wave excitations.

Dark state lasers.

We propose a new type of laser resonator based on imaginary energy-level splitting (imaginary coupling or quality factor Q-splitting) in a pair of coupled microcavities. A particularly advantageous arrangement involves two microring cavities with different free-spectral ranges in a configuration wherein they are coupled by far-field interference in a shared radiation channel. A novel Vernier-like effect for laser resonators is designed in which only one longitudinal resonant mode has a lower loss than the small-signal gain and can achieve lasing while all other modes are suppressed. This configuration enables ultrawidely tunable single-frequency lasers based on either homogeneously or inhomogeneously broadened gain media. The concept is an alternative to the common external cavity configurations for achieving tunable single-mode operation in a laser. The proposed laser concept builds on a high-Q "dark state," which is established by radiative interference coupling and bears a direct analogy to parity-time symmetric Hamiltonians in optical systems. Variants of this concept should be extendable to parametric-gain-based oscillators, enabling widely tunable single-frequency light sources.