CMOS-compatible Silicon Research Articles

Dual slot-mode NOEM phase shifter.

Photonic system component counts are increasing rapidly, particularly in CMOS-compatible silicon photonics processes. Large numbers of cascaded active photonic devices are difficult to implement when accounting for constraints on area, power dissipation, and response time. Plasma dispersion and the thermo-optic effect, both available in CMOS-compatible silicon processes, address a subset of these criteria. With the addition of a few back-end-of-line etch processing steps, silicon photonics platforms can support nano-opto-electro-mechanical (NOEM) phase shifters. Realizing NOEM phase shifters that operate at CMOS-compatible voltages (≤ 1.2 V) and with low insertion loss remains a challenge. Here, we introduce a novel NOEM phase shifter fabricated alongside 90 nanometer transistors that imparts 5.63 radians phase shift at 1.08 volts bias over an actuation length of 25μm with an insertion loss of less than 0.04 dB and 3 dB bandwidth of 0.26 MHz.

Electrical and Label-Free Detection of T-Cell Activation Against COVID-19: A Method Towards Point-of-Care Monitoring of Immune System

Despite great progress in biotechnology, COVID19 has revealed the inadequacy of current healthcare technologies to contain an infectious outbreak, specifically in rapid infection recognition to facilitate containment. Wide deployment of testing methods such as viral RNA (antigen) PCR and antibody detection have provided some relief in this regard; however, current technologies still require many hours to provide results. Thus, there remains a critical need for fast, sensitive, low-cost point-of-care (POC) biosensing assays to report on the infection status and immunity of patients. Emerging data on the importance of T-cell responses in generating long-term immunity against COVID19 has highlighted T-cell activation detection as a potential way for real-time monitoring native and acquired immunity. However, current approaches for T-cell stimulation monitoring are complex, laborious, time-consuming, and costly.Here we provide proof-of-principle experiments which indicate that an electrical and label-free detection method based on CMOS-compatible silicon nanowire ion-selective field-effect transistors (SNW-ISFET) can act as a fast, sensitive, reliable, and low-cost approach to monitor T-cell response to pathogens such COVID-19. This technology enables direct detection of COVID-19-specific T-cells directly from patient blood, with potential utility as a POC diagnostic device. Detection of antigen-specific T-cell activation from clonal human T-cells lines utilizing CD8-restricted peptide-MHC complexes has been demonstrated using SNW-ISFET devices. The cell concentration was varied to achieve a clear understanding about the device sensitivity, detection limit, and reliability (Figure 1). SNW-ISFET structures were applied to detect T-cell activation in clonal T-cell lines and whole peripheral blood mononuclear cell (PBMC) with multiple cell concentrations (Figure 2). Reference experiments were applied to ensure the device reliability and sensing mechanism (Figure 3). These findings show that SNW-FETs platform are promising candidates for antigen-specific T-cell activation detection from whole blood. Figure 1

Sub-100-nm precision distance measurement by means of all-fiber photonic microwave mixing

The importance of dimensional metrology has gradually emerged from fundamental research to high-technology industries. In the era of the fourth industrial revolution, absolute distance measurements are required to cope with various applications, such as unmanned vehicles, intelligent robots, and positioning sensors for smart factories. In such cases, the size, weight, power, and cost (SWaP-C) should essentially be restricted. In this paper, sub-100 nm precision distance measurements based on an amplitude-modulated continuous-wave laser (AMCW) with an all-fiber photonic microwave mixing technique is proposed and realized potentially to satisfy SWaP-C requirements. Target distances of 0.879 m and 8.198 m were measured by detecting the phase delay of 15 GHz modulation frequencies. According to our measurement results, the repeatability could reach 43 nm at an average time of 1 s, a result not previously achieved by conventional AMCW laser distance measurement methods. Moreover, the performance by the proposed method in terms of Allan deviation is competitive with most frequency-comb-based absolute distance measurement methods, even with a simple configuration. Because the proposed method has a simple configuration such that it can be easily utilized and demonstrated on a chip-scale platform using CMOS-compatible silicon photonics, it is expected to herald new possibilities, leading to the practical realization of a fully integrated chip-scale LIDAR system.

Controlling optical return loss in production silicon photonic metamaterial fiber couplers.

A cost-efficient and low-complexity optical input/output (I/O) packaging solution is a substantial challenge for volume production of photonic integrated circuits. To address this, metamaterial fiber couplers are an attractive solution for integrated photonic devices especially for optical I/O, interfacing standard optical fibers to photonic chips. They offer the advantages of refractive index engineering to achieve better mode match as well as higher fabrication tolerances. Metamaterial waveguides, as a fundamental building block of these fiber couplers, have attracted tremendous attention in recent years. Here, we report on effective optical return loss control in Si metamaterial waveguide designs to achieve ultra-low reflection loss in CMOS-compatible silicon photonics implemented in a 300 mm production line. Low backscattering is a substantial consideration for a range of applications. Here, a return loss of better than -30dB is achieved.

Passive devices at 2 µm wavelength on 200 mm CMOS-compatible silicon photonics platform [Invited

As a promising spectral window for optical communication and sensing, it is of great significance to realize on-chip devices at the 2 µm waveband. The development of the 2 µm silicon photonic platform mainly depends on the performance of passive devices. In this work, the passive devices were fabricated in the silicon photonic multi-project wafer process. The designed micro-ring resonator with a 0.6 µm wide silicon ridge waveguide based on a 220 nm silicon-on-insulator platform achieves a high intrinsic quality factor of 3.0×105. The propagation loss is calculated as 1.62 dB/cm. In addition, the waveguide crossing, multimode interferometer, and Mach–Zehnder interferometer were demonstrated at 2 µm with good performances.

Silicon photonics interfaced with integrated electronics for 9 GHz measurement of squeezed light

Photonic quantum technology can be enhanced by monolithic fabrication of both the underpinning quantum hardware and the corresponding electronics for classical readout and control. Together, this enables miniaturisation and mass-manufacture of small quantum devices---such as quantum communication nodes, quantum sensors and sources of randomness---and promises the precision and scale of fabrication required to assemble useful quantum computers. Here we combine CMOS compatible silicon and germanium-on-silicon nano-photonics with silicon-germanium integrated amplification electronics to improve performance of on-chip homodyne detection of quantum light. We observe a 3 dB bandwidth of 1.7 GHz, shot-noise limited performance beyond 9 GHz and minaturise the required footprint to 0.84 mm. We use the device to observe quantum squeezed light, from 100 MHz to 9 GHz, generated in a lithium niobate waveguide. This demonstrates that an all-integrated approach yields faster homodyne detectors for quantum technology than has been achieved to-date and opens the way to full-stack integration of photonic quantum devices.

A Non-Mechanical Multi-Wavelength Integrated Photonic Beam Steering System

In this work we experimentally demonstrate a fully packaged one-dimensional beam steering system that is based on a fully CMOS compatible silicon photonics platform. This configuration enables multi-wavelength operation and is suitable for short-wavelength infrared (SWIR) applications from light detection and ranging (LIDAR) to telecommunication and sensing applications. The switching fabric in this system employs a 1x9 array of add/drop ring resonators that are fed by a common waveguide through a single etch input grating. The switching fabric re-directs the resonance wavelengths at each drop waveguide to the output facet at the edge of the chip and into free space. The divergent beams that exit the waveguides along the output facet are then collimated and magnified using an array of micro-lenses (pitch = 750 μm, Diameter = 700 μm) in addition to a two lens afocal system in order to enhance the fill factor and diminish any blind zones between adjacent beams. Each one of these drop waveguides is used to address portion of the steering aperture. Combined together they enable steering across the entire aperture. Multi-wavelength operation is obtained by placing individual titanium tungsten alloy (TiW) heaters on top of each ring resonator to enable wavelength tuning.

The design and analysis of a CMOS-compatible silicon photonic ON–OFF switch based on a mode-coupling mechanism

A directional-coupler-based 1 $$\times$$ 1 silicon-on-insulator photonic ON–OFF switch with a complementary metal–oxide–semiconductor (CMOS)-compatible driving voltage is proposed in this paper. The directional coupling of the switch is accomplished by a carrier injection method with the help of a P-i-N diode phase shifter. The advantage of using the directional coupler as a switch is its smaller layout requirement, which makes it more suitable for use in integrated photonic applications. The ON–OFF switch has potential applications in programmable photonic switch fabrics, where the OFF-state behavior is used to prevent the input from being propagated to the output. The proposed switch offers a wide range of cross power-coupling coefficient $$(\kappa ^2)$$ values, ranging from 0 to 0.3 for the ON-state and from 0.79 to 1 for the OFF-state. The finite-difference beam propagation and two-dimensional (2D) finite-difference time-domain methods are used for photonic simulations. The results are compared analytically using the coupled mode theory with the help of the MATLAB software package.

Subpicosecond light pulses induced by Fano antiresonance buildup process.

We propose a simple technique of cutting short pulses out of a sharp edge input signal. The technique is based on the Fano antiresonance buildup dynamics. The output pulse duration is inverse proportional to the coupling strength to the resonator. We show that this coupling can be effectively increased by using more than one resonator and exploiting the antiresonance coalescence phenomenon. Analytical calculations for a model of standing-wave resonators and whispering gallery mode (WGM) resonators are performed within the coupled mode theory. We show that the latter can provide better pulse compression. Analytical results for WGM resonators are verified numerically by finite difference time domain method. Ability to generate pulses as short as a few hundreds of femtoseconds at 1.55 μm wavelength has been demonstrated for a potentially CMOS compatible silicon waveguide, which does not require optical nonlinearities to operate.

Simulation study of multilayer hybrid plasmonic switch using Franz–Keldysh effect

A CMOS compatible three-port all-optical silicon switch working in 1.473 to 1.502 μm (extinction ratio ( ER ) = 5.5 dB, λC = 1.488 μm) and 1.512 to 1.5306 μm (ER = 3.079 dB, λC = 1.52 μm) bands is demonstrated in this work through numerical simulations. However, in spite of the all optical control, having null refractive index contrast between the transmitting and control waveguides of the switch causes the switching merit to deteriorate because of light leaking from the transmitting waveguide. Later, by employing Franz–Keldysh effect-induced absorption coefficient tuning of Si1 − x Gex (x = 0.85) to replace the silicon control port of the switch, 2.95-dB leakage reduction in the ON state is achieved, which is assessed in detail. Also, our numerical simulations confirmed the bandwidth of 38 GHz, which suggested a multilayer plasmonic waveguide structure.

Large-scale silicon photonics switch based on 45-nm CMOS technology

In optical networks for telecom and datacom, an unprecedented energy efficient system is demanded since the amount of data traffic is rapidly increasing while a significant improvement of the energy efficiency in electronics devices such as switch ASICs is no longer expected. To overcome the energy constraints, introduction of energy-efficient optical circuit switching (OCS) to the conventional electrical packet switching network is considered. The energy consumption of the OCS is almost independent of the bandwidth because the transparent optical path with no optical-to-electrical/electrical-to-optical signal conversion supports an ultrawide bandwidth of highly multiplexed optical signals. In the OCS, high-speed and large-port-count optical switches are essential. We have been working on optical switches based on CMOS-compatible silicon photonics that offer fast switching, compactness, low power consumption, and low cost. By using our 45-nm CMOS process, we have recently demonstrated a low loss 32 × 32 silicon photonics switch and its performance improvements of wide operation bandwidth, polarization-independent operation, and so forth. In this paper, we review the recent progress of our silicon photonics switches. We discuss the importance of the silicon photonics switches for a sustainable optical network with a comparison of other switching technologies and explain comprehensively how our numerous achievements are advanced and related to each other.

Low loss CMOS-compatible silicon nitride photonics utilizing reactive sputtered thin films.

Low temperature deposition of low loss silicon nitride (SiN) thin-films is very attractive as it opens opportunities for realization of multi-layer photonic chips and hybrid integration of optical waveguides with temperature sensitive platforms such as processed CMOS silicon electronics or lithium niobate on insulator. So far, the most common low-temperature deposition technique for SiN is plasma enhanced chemical vapor deposition (PECVD), however such SiN thin-films can suffer from significant losses at C-band wavelengths due to unwanted hydrogen bonds. In this contribution we present a back end of line (< 400°C), low loss SiN platform based on reactive sputtering for telecommunication applications. Waveguide losses of 0.8 dB/cm at 1550 nm and as low as 0.6 dB/cm at 1580 nm have been achieved for moderate confined waveguides which appear to be limited by patterning rather than material. These findings show that reactive sputtered SiN thin-films can have lower optical losses compared to PECVD SiN thin-films, and thus show promise for future hybrid integration platforms for applications such as high Q resonators, optical filters and delay lines for optical signal processing.

Perfect secrecy cryptography via mixing of chaotic waves in irreversible time-varying silicon chips

Protecting confidential data is a major worldwide challenge. Classical cryptography is fast and scalable, but is broken by quantum algorithms. Quantum cryptography is unclonable, but requires quantum installations that are more expensive, slower, and less scalable than classical optical networks. Here we show a perfect secrecy cryptography in classical optical channels. The system exploits correlated chaotic wavepackets, which are mixed in inexpensive and CMOS compatible silicon chips. The chips can generate 0.1 Tbit of different keys for every mm of length of the input channel, and require the transmission of an amount of data that can be as small as 1/1000 of the message’s length. We discuss the security of this protocol for an attacker with unlimited technological power, and who can access the system copying any of its part, including the chips. The second law of thermodynamics and the exponential sensitivity of chaos unconditionally protect this scheme against any possible attack.

Perspective: optically-pumped III–V quantum dot microcavity lasers via CMOS compatible patterned Si (001) substrates

Direct epitaxial growth III–V quantum dot (QD) structures on CMOS-compatible silicon substrates is considered as one of the most promising approaches to achieve low-cost and high-yield Si-based lasers for silicon photonic integration. However, epitaxial growth of III–V materials on Si encounters the following three major challenges: high density of threading dislocations, antiphase boundaries and thermal cracks, which significantly degrade the crystal quality and potential device performance. In this review, we will focus on some recent results related to InAs/GaAs quantum dot lasers on Si (001) substrates by III–V/IV hybrid epitaxial growth via (111)-faceted Si hollow structures. Moreover, by using the step-graded epitaxial growth process the emission wavelength of InAs QDs can be extended from O-band to C/L-band. High-performance InAs/GaAs QD micro-disk lasers with sub-milliwatts threshold on Si (001) substrates are fabricated and characterized. The above results pave a promising path towards the on-chip lasers for optical interconnect applications.

Microwave signal switching on a silicon photonic chip

Microwave photonics uses light to carry and process microwave signals over a photonic link. However, light can instead be used as a stimulus to microwave devices that directly control microwave signals. Such optically controlled amplitude and phase-shift switches are investigated for use in reconfigurable microwave systems, but they suffer from large footprint, high optical power level required for switching, lack of scalability and complex integration requirements, restricting their implementation in practical microwave systems. Here, we report Monolithic Optically Reconfigurable Integrated Microwave Switches (MORIMSs) built on a CMOS compatible silicon photonic chip that addresses all of the stringent requirements. Our scalable micrometer-scale switches provide higher switching efficiency and require optical power orders of magnitude lower than the state-of-the-art. Also, it opens a new research direction on silicon photonic platforms integrating microwave circuitry. This work has important implications in reconfigurable microwave and millimeter wave devices for future communication networks.

Broadband, low-loss silicon photonic Y-junction with an arbitrary power splitting ratio.

Photonic integrated circuits (PICs) often require broadband power splitters such as Y-junctions for signal monitoring, signal feedback, power distribution, etc., with various splitting ratios. Therefore, a parameterized Y-junction with an arbitrary power splitting ratio that can be selected in layout design is desired in a PIC library. Here, we propose an ultra-compact (1.4 μm × 2.3 μm) Y-junction on the 220-nm-thick silicon-on-insulator (SOI) platform for an arbitrary splitting ratio with a programmable design. It applies smooth curvatures for a good tolerance to fabrication errors. Rigorous 3D-FDTD simulations predict an excess loss below 0.36 dB and a splitting-ratio variation of less than 1 dB over 100 nm. Experimental results achieved using a CMOS-compatible silicon photonics process show an excess loss of lower than 0.5 dB for a splitting ratio varied from 0 to -18 dB across the entire C band. Both numerical and experimental results show that the power splitting ratio of the proposed device is weakly wavelength dependent.

A Nonuniform Sparse 2-D Large-FOV Optical Phased Array With a Low-Power PWM Drive

Integrated optical phased arrays (OPAs) capable of adaptive beamforming and beam steering enable a wide range of applications. For many of these applications, a large scale 2-D OPA with full phase control for each radiating element is essential to achieve a functional low-cost solution. However, the scalability of such OPAs has been hampered by the optical feed distribution difficulties in a planar photonics process, as well as the high power consumption associated with having a large number of phase control units. In this paper, we present a two-chip solution low-power scalable OPA with a nonuniform sparse aperture, providing radiation pattern adjustment and feed distribution feasibility in a CMOS compatible silicon photonics process. The demonstrated OPA with a 128-element aperture achieves the highest reported grating-lobe-free field-of-view (FOV)-to-beamwidth ratio of 16°/0.8°, which is equivalent to a 484-element uniform array. This translates to at least 400 resolvable spots, 30 times more than the state-of-the-art 2-D OPAs. Moreover, by utilizing compact phase shifters in a row–column power delivery grid, we reduce the number of required drivers from 144 to 37. A high-swing pulsewidth modulation (PWM) driving circuit featuring breakdown voltage multipliers and soft turn-on activation significantly reduces the power consumption of the system. The electronic driver chip and the integrated photonic chip are fabricated on a 65-nm CMOS process and a thick silicon-on-insulator (SOI) silicon photonics process, occupying 1.7 mm2 and 2.08 mm2 of active area, respectively.

O-Band and C/L-Band III-V Quantum Dot Lasers Monolithically Grown on Ge and Si Substrate

Direct epitaxial growth of III-V heterostructure on CMOS-compatible silicon wafer offers substantial manufacturing cost and scalability advantages. Quantum dot (QD) devices are less sensitive to defect and temperature, which makes epitaxially grown III-V QD lasers on Si one of the most promising technologies for achieving low-cost, scalable integration with silicon photonics. The major challenges are that heteroepitaxial growth of III-V materials on Si normally encounters high densities of mismatch dislocations, antiphase boundaries and thermal cracks, which limit the device performance and lifetime. This paper reviews some of the recent developments on hybrid InAs/GaAs QD growth on Ge substrates and highly uniform (111)-faceted hollow Si (001) substrates by molecular beam epitaxy (MBE). By implementing step-graded epitaxial growth techniques, the emission wavelength can be tuned into either an O band or C/L band. Furthermore, microcavity QD laser devices are fabricated and characterized. The epitaxially grown III-V/IV hybrid platform paves the way to provide a promising approach for future on-chip silicon photonic integration.

High speed optical modulator based on silicon slotted-rib waveguide

Abstract A high-speed electro-optic modulator based on CMOS compatible silicon waveguide is proposed. The optical waveguide in silicon is formed by creating a slot in a rib structure that results in a strong optical confinement with an acceptably low propagation loss. The proposed design of slotted-rib waveguide with a traveling-wave electrode facilitates an efficient optical modulation in silicon. Taking the advantage of velocity matched between optical and microwave modes, a high-speed operation up to 70 Gbps with an extinction ratio of ≈ 4.9 dB is reported at a peak to peak voltage of 3 V pp under a reverse bias of 4 V. An insertion loss of 3.1 dB is obtained for a 4 mm long device.

Ultralow Crosstalk and Loss CMOS Compatible Silicon Waveguide Star-Crossings with Arbitrary Included Angles

As the integration level of silicon photonics improves, using a large number of 2 × 2 crossings is seemingly inevitable. However, with the help of a multiports crossing, which has low crosstalk and loss, the number of crossings can be greatly reduced, then the integration density and routing flexibility can be enhanced. Here, we propose and demonstrate the novel 3 × 3, 4 × 4, 5 × 5 and 6 × 6 silicon waveguide star-crossings based on self-imaging. The star-crossings are fabrication compatible with the 180 nm commercial CMOS process. The fabricated devices show ultralow crosstalk and insertion loss. The crosstalk of those crossings are less than −50, −50, −49.2, and −48 dB, respectively, in C-band. The corresponding insertion loss are less than 0.067 dB, 0.075 dB, 0.081 and 0.133 dB, respectively. We also propose the high performance star-crossings with arbitrary angles, which can reduce the difficulty of the waveguide routing and further improve the integration density of photonics IC. The footprint and minimum angle of the proposed star-crossing are also discussed. The crossings have high fabrication tolerance, and will be widely used in the future integrated photonics.