CMOS-compatible Platform Research Articles

Subwavelength pixelated CMOS color sensors based on anti-Hermitian metasurface

The demand for essential pixel components with ever-decreasing size and enhanced performance is central to current optoelectronic applications, including imaging, sensing, photovoltaics and communications. The size of the pixels, however, are severely limited by the fundamental constraints of lightwave diffraction. Current development using transmissive filters and planar absorbing layers can shrink the pixel size, yet there are two major issues, optical and electrical crosstalk, that need to be addressed when the pixel dimension approaches wavelength scale. All these fundamental constraints preclude the continual reduction of pixel dimensions and enhanced performance. Here we demonstrate subwavelength scale color pixels in a CMOS compatible platform based on anti-Hermitian metasurfaces. In stark contrast to conventional pixels, spectral filtering is achieved through structural color rather than transmissive filters leading to simultaneously high color purity and quantum efficiency. As a result, this subwavelength anti-Hermitian metasurface sensor, over 28,000 pixels, is able to sort three colors over a 100 nm bandwidth in the visible regime, independently of the polarization of normally-incident light. Furthermore, the quantum yield approaches that of commercial silicon photodiodes, with a responsivity exceeding 0.25 A/W for each channel. Our demonstration opens a new door to sub-wavelength pixelated CMOS sensors and promises future high-performance optoelectronic systems.

Energy-Efficient Stochastic Computing with Superparamagnetic Tunnel Junctions

Superparamagnetic tunnel junctions (SMTJs) have emerged as a competitive, realistic nanotechnology to support novel forms of stochastic computation in CMOS-compatible platforms. One of their applications is to generate random bitstreams suitable for use in stochastic computing implementations. We describe a method for digitally programmable bitstream generation based on pre-charge sense amplifiers. This generator is significantly more energy efficient than SMTJ-based bitstream generators that tune probabilities with spin currents and a factor of two more efficient than related CMOS-based implementations. The true randomness of this bitstream generator allows us to use them as the fundamental units of a novel neural network architecture. To take advantage of the potential savings, we codesign the algorithm with the circuit, rather than directly transcribing a classical neural network into hardware. The flexibility of the neural network mathematics allows us to adapt the network to the explicitly energy efficient choices we make at the device level. The result is a convolutional neural network design operating at $\approx$ 150 nJ per inference with 97 % performance on MNIST -- a factor of 1.4 to 7.7 improvement in energy efficiency over comparable proposals in the recent literature.

InGaAs photo field-effect-transistors (PhotoFETs) on half-inch Si wafer using layer transfer technology

InGaAs photo field-effect transistors (PhotoFETs) integrated on a half-inch Si wafer have been demonstrated by means of layer transfer technology. According to the Raman measurement and scanning transmission electron microscopy, the InGaAs layer was transferred onto the half-inch Si wafer without degradation of crystal quality or introduction of strain. The optical performance of InGaAs photoFETs on Si, operating in shortwave infrared (SWIR) region was also investigated. The photoFETs on Si wafer show a responsivity of ∼3 A W−1 at SWIR of 1550 nm with investigated range of bias. These results demonstrate InGaAs layer transfer technology is suitable for SWIR detectors on CMOS compatible platforms for monolithic integration.

Review of Recent Progress on Silicon Nitride-Based Photonic Integrated Circuits

Silicon photonic devices used in the photonics industry over the past three decades have helped in realizing large-scale photonic integrated circuits. Silicon nitride (Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) is another CMOS-compatible platform that provides several advantages such as low loss, high optical power tolerance, and broad spectral operation band from visible to infrared wavelengths. Recently, the combination of Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> waveguide technology with silicon photonics and III-V materials has opened up new areas in on-chip applications. Researchers in the field are primarily focusing on its applications such as on-chip gas sensing, nonlinear optical signal processing, and label-free biosensors based on photonic integrated circuits. In this review paper, we discuss Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> material-based platforms for a variety of applications with devices ranging from passive to active and hybrid photonic devices.

Solid-State MWIR Beam Steering Using Optical Phased Array on Germanium-Silicon Photonic Platform

We demonstrate a chip-scale germanium-silicon optical phased array (OPA) fabricated on a CMOS-compatible platform capable of 2D beam steering in the mid-infrared wavelength range. The OPA included a specially designed grating emitter waveguide array with uniform emission intensity along the mm -length waveguide propagation to realize very sharp instantaneous field-of-view (IFOV) and wide beam-steering total-field-of-view (TFOV). The experimental results indicated lateral beam-steering TFOV up to 12.7° by phase-tuning the waveguide array and longitudinal TFOV up to 12° by wavelength tuning. The 3-dB beam divergence is 3.08° × 0.18°. The demonstrated OPA architecture can employ wafer-scale fabrication and integration while supporting sensing and imaging applications in the mid-infrared spectral range.

Thermal Resistance in ALD Grown Dielectric Thin Films

The need for increased control of layer thickness and uniformity as device dimensions shrink has spurred increased use of atomic layer deposition (ALD) for thin film growth. The ability to deposit high dielectric constant (high-k) films via ALD has allowed for their widespread use in a swath of optical, optoelectronic, and electronic devices, including integration into CMOS compatible platforms. As the thickness of these films is reduced, the interfacial thermal resistance can dictate the overall thermal resistance of the material stack compared to the resistance due to the finite dielectric layer thickness. In this study, time domain thermoreflectance is used to interrogate the thermal conductivity and the thermal boundary resistance of routinely used high-k dielectrics, aluminum oxide, hafnium oxide, and titanium oxide films ALD grown on silicon, as well as films of hafnium zirconium oxide, which has demonstrated potential for applications in energy storage devices as well as electronic memory devices and architectures.

Integrated silicon nitride electro-optic modulators with atomic layer deposited overlays.

Silicon nitride (SiN) is currently the most prominent CMOS-compatible platform for photonics at wavelengths <1 μm. However, realizing fast electro-optic (EO) modulators, the key components of any integrated optics platform, remains challenging in SiN. Modulators based on the plasma dispersion effect, as in silicon, are not available. Despite the fact that significant second-harmonic generation has been reported for silicon-rich SiN, no efficient Pockels effect-based modulators have been demonstrated. Here we report the back-end CMOS-compatible atomic layer deposition (ALD) of conventional second-order nonlinear crystals, zinc oxide, and zinc sulfide, on existing SiN waveguide circuits. Using these ALD overlays, we demonstrate EO modulation in ring resonators.

Efficient, broadband third-harmonic generation in silicon nanophotonic waveguides spectrally shaped by nonlinear propagation.

Optical emission from silicon is most practically accessible through nonlinear optical wave mixing, due to the indirect bandgap of the material. Although silicon is a material that absorbs visible light, third-harmonic generation driven by infrared signals can be used to generate visible light in silicon structures. In this work, we present a comprehensive investigation into third-harmonic generation in silicon-on-insulator waveguides. We demonstrate that few-micrometer length waveguides can be used to up-convert ultrafast 1550nm laser pulses to their third-harmonic with efficiencies up to ηTHG=2.8×10-5, the highest third-harmonic generation conversion efficiency reported to date in a silicon-based structure. Nonlinear propagation through 200μm long waveguides produces self-compressing temporal solitons, which dramatically broaden and blue shift the observed third-harmonic spectrum. Such devices are envisioned to provide a method for generating coherent visible signals within an integrated, CMOS compatible platform.

Hypersonic Surface Phononic Bandgap Demonstration in a CMOS-Compatible Pillar-Based Piezoelectric Structure on Silicon

The promise of surface phononic crystals (PnCs) for $e.g.$ rf signal processing in wireless communication (like your mobile phone) has not been realized, due to the complexity of acoustic wave propagation in such structures, plus the lack of a low-loss, CMOS-compatible platform. The authors identify a wide, hypersonic surface phononic band gap in a pillar-based surface PnC. The significance of their platform lies in the reduced phononic material loss in dielectric pillars, and the use of CMOS-compatible AlN. This allows dense integration of low-loss hypersonic devices with electronics on the same die, enabling many exciting applications of surface PnCs.

Near-Field-Focusing Integrated Optical Phased Arrays

Integrated optical phased arrays that focus radiated light to a tightly-confined spot in the near field are demonstrated for the first time. The element phase distributions necessary for generating and steering focused beams using phased arrays are developed. Discussion and simulations detailing the effects of the array aperture and focal length on the focal spot are included. Two focusing phased array architectures are presented: (1) a one-dimensional splitter-tree-based architecture with novel focusing antennas and inline phase bumps and (2) a two-dimensional large-scale pixel-based architecture. Both architectures are used to experimentally demonstrate a variety of aperture size and focal length passive focusing arrays in a CMOS-compatible platform, including (i) a 512-antenna splitter-tree-based array with a wavelength-steerable 7 µm spot at a 5 mm focal length, (ii) a 1024-antenna pixel-based array with a 21 µm spot at a 5 mm focal length, and (iii) a 10,000-antenna pixel-based array with a 21 µm spot at a 10 mm focal length. The demonstrated focusing phased arrays present a new functional modality compared to standard optical phased arrays that generate diffracting beams in the far field of the array. This new focusing modality has the potential to advance a number of important application areas at the cutting edge of lightwave technology, as it will enable future integrated optical phased array implementations for a wide range of applications that require focusing operation. Such areas include optical trapping for biological characterization, laser-based three-dimensional printing, short-range light detection and ranging, and short-range optical data communications.

Aluminum plasmonic waveguides co-integrated with Si3N4 photonics using CMOS processes

Co-integrating CMOS plasmonics and photonics became the “sweet spot” to hit in order to combine their benefits and allow for volume manufacturing of plasmo-photonic integrated circuits. Plasmonics can naturally interface photonics with electronics while offering strong mode confinement, enabling in this way on-chip data interconnects when tailored to single-mode waveguides, as well as high-sensitivity biosensors when exposing Surface-Plasmon-Polariton (SPP) modes in aqueous environment. Their synergy with low-loss photonics can tolerate the high plasmonic propagation losses in interconnect applications, offering at the same time a powerful portfolio of passive photonic functions towards avoiding the use of bulk optics for SPP excitation and facilitating compact biosensor setups. The co-integration roadmap has to proceed, however, over the utilization of fully CMOS compatible material platforms and manufacturing processes in order to allow for a practical deployment route. Herein, we demonstrate for the first time Aluminum plasmonic waveguides co-integrated with Si3N4 photonics using CMOS manufacturing processes. We validate the data carrying credentials of CMOS plasmonics with 25 Gb/s data traffic and we confirm successful plasmonic propagation in both air and water-cladded waveguide configurations. This platform can potentially fuel the deployment of co-integrated plasmonic and photonic structures using CMOS processes for biosensing and on-chip interconnect applications.

Broadband transparent and CMOS-compatible flat optics with silicon nitride metasurfaces [Invited

Metasurface optics is a promising candidate for realizing the next generation of miniaturized optical components. Unlike refractive optics, these devices modify light over a wavelength-scale thickness, changing the phase, amplitude, and polarization. This review details recent developments and state-of-the-art metasurfaces realized using silicon nitride. We emphasize this material as to date it has the lowest refractive index with which metasurfaces have been experimentally demonstrated. The wide band gap of silicon nitride enables reduced absorption over a broad wavelength range relative to its higher index counterparts, providing a CMOS-compatible platform for producing a variety of high efficiency metasurface elements and systems.

Gas-Phase Microresonator-Based Comb Spectroscopy without an External Pump Laser

We present a novel approach to realize microresonator-comb-based high resolution spectroscopy that combines a fiber-laser cavity with a microresonator. Although the spectral resolution of a chip-based comb source is typically limited by the free spectral range (FSR) of the microresonator, we overcome this limit by tuning the 200-GHz repetition-rate comb over one FSR via control of an integrated heater. Our dual-cavity scheme allows for self-starting comb generation without the need for conventional pump-cavity detuning while achieving a spectral resolution equal to the comb linewidth. We measure broadband molecular absorption spectra of acetylene by interleaving 800 spectra taken at 250-MHz per spectral step using a 60-GHz-coarse-resolution spectrometer and exploits advances of integrated heater which can locally and rapidly change the refractive index of a microresonator with low electrical consumption (0.9 GHz/mW), which is orders of magnitude lower than a fiber-based comb. This approach offers a path towards a simple, robust and low-power consumption CMOS-compatible platform capable of remote sensing.

Thermal conductivity and thermal boundary resistance of atomic layer deposited high-k dielectric aluminum oxide, hafnium oxide, and titanium oxide thin films on silicon

The need for increased control of layer thickness and uniformity as device dimensions shrink has spurred increased use of atomic layer deposition (ALD) for thin film growth. The ability to deposit high dielectric constant (high-k) films via ALD has allowed for their widespread use in a swath of optical, optoelectronic, and electronic devices, including integration into CMOS compatible platforms. As the thickness of these dielectric layers is reduced, the interfacial thermal resistance can dictate the overall thermal resistance of the material stack compared to the resistance due to the finite dielectric layer thickness. Time domain thermoreflectance is used to interrogate both the thermal conductivity and the thermal boundary resistance of aluminum oxide, hafnium oxide, and titanium oxide films on silicon. We calculate a representative design map of effective thermal resistances, including those of the dielectric layers and boundary resistances, as a function of dielectric layer thickness, which will be of great importance in predicting the thermal resistances of current and future devices.

Analysis of Kerr nonlinearity enhancement in silicon nitride waveguides using one-dimensional slow light structure

A corrugated based slow-light silicon nitride waveguide is studied numerically to enhance Kerr nonlinearity by increasing the group index of the waveguide at the band edge. The structure is optimized taking into account the limitations existing in CMOS-compatible platforms. Bandwidth limitations of the structure are investigated at different wavelengths for various slow-down factors. Finite size realizations of the structure are studied using transfer matrix method to obtain the desired transmission and group velocity at the band edge. Optical loss corresponding to slow-light effect is modeled, and the limitations imposed by the slow-light loss on the nonlinear phase shift saturation are considered. It is shown that by employing slow-down factor of 3, the nonlinear phase shift is improved by a factor of 5. Also, in comparison with a straight waveguide, shorter corrugated waveguides (up to nine times) can provide the same accumulated nonlinear phase shift.

Optical pumped InGaAs/GaAs nano-ridge laser epitaxially grown on a standard 300-mm Si wafer

Fully exploiting the potential of silicon photonics requires high-performance active devices such as lasers, which can be monolithically integrated in a scalable way. However, direct bandgap III–V semiconductors exhibit a large lattice mismatch and/or strongly differing thermal expansion coefficient with silicon. This makes monolithic integration on silicon without introducing excessive defects in the material extremely difficult. The majority of the methods proposed thus far either are not compatible with further low-cost integration or rely on a special substrate. Here we demonstrate monolithic InGaAs/GaAs single-mode nano-ridge lasers directly grown on a standard (001) 300-mm Si wafer. Exploiting the aspect ratio defect trapping technique, unwanted defects are confined to a narrow trench defined in the silicon substrate. The nano-ridge structures subsequently grown out of these trenches are of high crystalline quality as shown by high-resolution transmission electron microscopy analysis and a strong photoluminescence response. They can be controlled in shape by optimizing the growth conditions, which allows us to minimize substrate leakage and maximize confinement in the InGaAs quantum wells providing optical gain. Distributed feedback lasers were fabricated by defining a first-order grating in the nano-ridge. Under pulsed optical pumping, single-mode lasing with side mode suppression over 28 dB was shown, and precise control of the emission wavelength over 60 nm was achieved. This demonstration proves the high quality of the material and provides a credible road towards a CMOS-compatible platform for high-volume manufacturing of silicon photonic integrated circuits, including laser and amplifier devices.

Methane absorption spectroscopy on a silicon photonic chip

Infrared tunable diode-laser absorption spectroscopy (IR-TDLAS) is an enabling technology for trace-gas detection, with applications ranging from air-quality monitoring to medical diagnostics. However, such sensors typically utilize discrete optical components that pose practical cost limits for large-scale network deployments. Here, we leverage silicon photonics technology to demonstrate IR-TDLAS on an integrated CMOS-compatible platform for methane (CH4) spectroscopy. Using near-IR (1650 nm) light from a distributed-feedback laser and an uncooled InGaAs detector, the evanescent optical field of a high-index contrast nanoscale silicon waveguide is used to probe ambient CH4, yielding Gaussian-noise-limited sub-100 parts-per-million by volume detection limits. Our results demonstrate the feasibility of chip-scale photonic integration for realizing compact, cost-effective, and versatile gas sensors capable of tackling diverse energy and environmental challenges, such as natural-gas leak quantification and localization for fugitive-emissions monitoring.

High repetition rate correlated photon pair generation in integrated silicon nanowires.

Integrated single-photon sources are a key component for photonic quantum technology but are generally limited to low single-photon rates. For sources based on photon pair generation by four-wave mixing, increasing the repetition rate of pump laser pulses is a straightforward way to enhance the single-photon rate, but the benefits and practical limitations have not yet been demonstrated and analyzed in a CMOS-compatible platform. In this work, we demonstrate correlated photon pair generation in integrated silicon nanowires and systematically analyze the count rate and coincidence to accidental ratio as the pump rate is varied between 156.25MHz and 10GHz. We show that the highest useful pump rate is limited by the timing resolution of the single-photon detection system, and that in this regime, the nonlinear loss of the silicon nanowire does not have a significant effect on the single-photon generation.

Coherent solid-state LIDAR with silicon photonic optical phased arrays.

We present, to the best of our knowledge, the first demonstration of coherent solid-state light detection and ranging (LIDAR) using optical phased arrays in a silicon photonics platform. An integrated transmitting and receiving frequency-modulated continuous-wave circuit was initially developed and tested to confirm on-chip ranging. Simultaneous distance and velocity measurements were performed using triangular frequency modulation. Transmitting and receiving optical phased arrays were added to the system for on-chip beam collimation, and solid-state beam steering and ranging measurements using this system are shown. A cascaded optical phase shifter architecture with multiple groups was used to simplify system control and allow for a compact packaged device. This system was fabricated within a 300mm wafer CMOS-compatible platform and paves the way for disruptive low-cost and compact LIDAR on-chip technology.

Integrated optical phased arrays for quasi-Bessel-beam generation.

Integrated optical phased arrays for generating quasi-Bessel beams are proposed and experimentally demonstrated in a CMOS-compatible platform. Owing to their elongated central beams, Bessel beams have applications in a range of fields, including multiparticle trapping and laser lithography. In this Letter, continuous Bessel theory is manipulated to formulate the phase and amplitude conditions necessary for generating free-space-propagating Bessel-Gauss beams using on-chip optical phased arrays. Discussion of the effects of select phased array parameters on the generated beam's figures of merit is included. A one-dimensional splitter-tree-based phased array architecture is modified to enable arbitrary passive control of the array's element phase and amplitude distributions. This architecture is used to experimentally demonstrate on-chip quasi-Bessel-beam generation with a ∼14 mm Bessel length and ∼30 μm power full width at half maximum.