An optical neural chip for implementing complex-valued neural network.

Complex-valued neural networks have many advantages over their real-valued counterparts. Conventional digital electronic computing platforms are incapable of executing truly complex-valued representations and operations. In contrast, optical computing platforms that encode information in both phase and magnitude can execute complex arithmetic by optical interference, offering significantly enhanced computational speed and energy efficiency. However, to date, most demonstrations of optical neural networks still only utilize conventional real-valued frameworks that are designed for digital computers, forfeiting many of the advantages of optical computing such as efficient complex-valued operations. In this article, we highlight an optical neural chip (ONC) that implements truly complex-valued neural networks. We benchmark the performance of our complex-valued ONC in four settings: simple Boolean tasks, species classification of an Iris dataset, classifying nonlinear datasets (Circle and Spiral), and handwriting recognition. Strong learning capabilities (i.e., high accuracy, fast convergence and the capability to construct nonlinear decision boundaries) are achieved by our complex-valued ONC compared to its real-valued counterpart.

Silicon photonic integrated circuits with electrically programmable non-volatile memory functions.

Conventional silicon photonic integrated circuits do not normally possess memory functions, which require on-chip power in order to maintain circuit states in tuned or field-configured switching routes. In this context, we present an electrically programmable add/drop microring resonator with a wavelength shift of 426 pm between the ON/OFF states. Electrical pulses are used to control the choice of the state. Our experimental results show a wavelength shift of 2.8 pm/ms and a light intensity variation of ~0.12 dB/ms for a fixed wavelength in the OFF state. Theoretically, our device can accommodate up to 65 states of multi-level memory functions. Such memory functions can be integrated into wavelength division mutiplexing (WDM) filters and applied to optical routers and computing architectures fulfilling large data downloading demands.

Slot waveguide ring resonators coated by an atomic layer deposited organic/inorganic nanolaminate.

In this study, slot waveguide ring resonators patterned on a silicon-on-insulator (SOI) wafer and coated with an atomic layer deposited nanolaminate consisting of alternating layers of tantalum pentoxide and polyimide were fabricated and characterized. To the best of our knowledge, this is the first demonstration of atomic layer deposition (ALD) of organic materials in waveguiding applications. In our nanolaminate ring resonators, the optical power is not only confined in the narrow central air slot but also in several parallel sub-10 nm wide vertical polyimide slots. This indicates that the mode profiles in the silicon slot waveguide can be accurately tuned by the ALD method. Our results show that ALD of organic and inorganic materials can be combined with conventional silicon waveguide fabrication techniques to create slot waveguide ring resonators with varying mode profiles. This can potentially open new possibilities for various photonic applications, such as optical sensing and all-optical signal processing.

Highly spectrum-selective ultraviolet photodetector based on p-NiO/n-IGZO thin film heterojunction structure.

Ultraviolet photodetector with p-n heterojunction is fabricated by magnetron sputtering deposition of n-type indium gallium zinc oxide (n-IGZO) and p-type nickel oxide (p-NiO) thin films on ITO glass. The performance of the photodetector is largely affected by the conductivity of the p-NiO thin film, which can be controlled by varying the oxygen partial pressure during the deposition of the p-NiO thin film. A highly spectrum-selective ultraviolet photodetector has been achieved with the p-NiO layer with a high conductivity. The results can be explained in terms of the "optically-filtering" function of the NiO layer.

Efficient silicon nitride grating coupler with distributed Bragg reflectors.

In this paper we have designed, fabricated and characterized a high efficiency Silicon nitride grating coupler at 1490 nm. Distributed Bragg reflectors as bottom mirrors are employed to improve the coupling efficiency by reflecting the downward traveling light. The peak coupling efficiency obtained is about -2.5 dB and the 1-dB bandwidth is 53 nm. The fabrication process is CMOS-compatible and is ready to be integrated with photonic circuits.

Silicon photonics packaging with lateral fiber coupling to apodized grating coupler embedded circuit.

We report a novel lateral packaging approach using laser welding technique with angle polished fiber coupling to grating coupler embedded silicon photonic circuit. Measurements show the relax alignment tolerance for fiber packaging process. The packaging excess loss of 1.2 dB is achieved. The use of angle polished fiber for lateral fiber coupling enables an alternative way for cost-effective deployment of silicon photonics packaging in telecommunication systems.

Design of an ultra-compact electro-absorption modulator comprised of a deposited TiN/HfO2/ITO/Cu stack for CMOS backend integration.

An ultra-compact electro-absorption (EA) modulator operating around 1.55-µm telecom wavelengths is proposed and theoretically investigated. The modulator is comprised of a stack of TiN/HfO2

Silicon nitride based plasmonic components for CMOS back-end-of-line integration.

Silicon nitride waveguides provide low propagation loss but weak mode confinement due to the relatively small refractive index contrast between the Si3N4 core and the SiO2 cladding. On the other hand, metal-insulator-metal (MIM) plasmonic waveguides offer strong mode confinement but large propagation loss. In this work, MIM-like plasmonic waveguides and passive devices based on horizontal Cu-Si3N4-Cu or Cu-SiO2-Si3N4-SiO2-Cu structures are integrated in the conventional Si3N4 waveguide circuits using standard CMOS backend processes, and are characterized around 1550-nm telecom wavelengths using the conventional fiber-waveguide-fiber method. The Cu-Si3N4(~100 nm)-Cu devices exhibit ~0.78-dB/µm propagation loss for straight waveguides, ~38% coupling efficiency with the conventional 1-µm-wide Si3N4 waveguide through a 2-µm-long taper coupler, ~0.2-dB bending loss for sharp 90° bends, and ~0.1-dB excess loss for ultracompact 1 × 2 and 1 × 4 power splitters. Inserting a ~10-nm SiO2 layer between the Si3N4 core and the Cu cover (i.e., the Cu-SiO2(~10 nm)-Si3N4(~100 nm)-SiO2(~10 nm)-Cu devices), the propagation loss and the coupling efficiency are improved to ~0.37 dB/µm and ~52% while the bending loss and the excess loss are degraded to ~3.2 dB and ~2.1 dB, respectively. These experimental results are roughly consistent with the numerical simulation results after taking the influence of possible imperfect fabrication into account. Ultracompact plasmonic ring resonators with 1-µm radius are demonstrated with an extinction ratio of ~18 dB and a quality factor of ~84, close to the theoretical prediction.

Theoretical investigation of ultracompact and athermal Si electro-optic modulator based on Cu-TiO2-Si hybrid plasmonic donut resonator.

An ultracompact silicon electro-optic modulator operating at 1550-nm telecom wavelengths is proposed and analyzed theoretically, which consists of a Cu-TiO(2)-Si hybrid plasmonic donut resonator evanescently coupled with a conventional Si channel waveguide. Owing to a negative thermo-optic coefficient of TiO(2) (~-1.8 × 10(-4) K(-1)), the real part of effective modal index of the curved Cu-TiO(2)-Si hybrid waveguide can be temperature-independent (i.e., athermal) if the TiO(2) interlayer and the beneath Si core have a certain thickness ratio. A voltage applied between the ring-shaped Cu cap and a cylinder metal electrode positioned at the center of the donut,--which makes Ohmic contact to Si, induces a ~1-nm-thick free-electron accumulation layer at the TiO(2)/Si interface. The optical field intensity in this thin accumulation layer is significantly enhanced if the accumulation concentration is sufficiently large (i.e., > ~6 × 10(20) cm(-3)), which in turn modulates both the resonance wavelengths and the extinction ratio of the donut resonator simultaneously. For a modulator with the total footprint inclusive electrodes of ~8.6 µm(2), 50-nm-thick TiO(2), and 160-nm-thick Si core, FDTD simulation predicts that it has an insertion loss of ~2 dB, a modulation depth of ~8 dB at a voltage swing of ~6 V, a speed-of-response of ~35 GHz, and a switching energy of ~0.45 pJ/bit, and it is athermal around room temperature. The modulator's performances can be further improved by optimization of the coupling strength between the bus waveguide and the donut resonator.

CMOS-compatible high efficiency double-etched apodized waveguide grating coupler.

We present a high efficiency double-etched apodized fiber-to-waveguide grating coupler on a silicon-on-insulator substrate, which can be fabricated using deep UV photolithography. The fabricated grating coupler yields a coupling loss of -1.5 dB with 3-dB bandwidth of 54 nm at a wavelength of 1560 nm. Measurements and simulations show that the double-etched apodized grating coupler design is robust and tolerant to fabrication process variations.

Phase modulation in horizontal metal-insulator-silicon-insulator-metal plasmonic waveguides.

An extremely compact Si phase modulator is proposed and validated, which relies on effective modulation of the real part of modal index of horizontal metal-insulator-Si-insulator-metal plasmonic waveguides by a voltage applied between the metal cover and the Si core. Proof-of-concept devices are fabricated on silicon-on-insulator substrates using standard complementary metal-oxide-semiconductor technology using copper as the metal and thermal silicon dioxide as the insulator. A modulator with a 1-µm-long phase shifter inserted in an asymmetric Si Mach-Zehnder interferometer exhibits 9-dB extinction ratio under a 6-V/10-kHz voltage swing. Numerical simulations suggest that high speed and low driving voltage could be achieved by shortening the distance between the Si core and the n(+)-contact and by using a high-κ dielectric as the insulator, respectively.

Microelectromechanical Maltese-cross metamaterial with tunable terahertz anisotropy.

Dichroic polarizers and waveplates exploiting anisotropic materials have vast applications in displays and numerous optical components, such as filters, beamsplitters and isolators. Artificial anisotropic media were recently suggested for the realization of negative refraction, cloaking, hyperlenses, and controlling luminescence. However, extending these applications into the terahertz domain is hampered by a lack of natural anisotropic media, while artificial metamaterials offer a strong engineered anisotropic response. Here we demonstrate a terahertz metamaterial with anisotropy tunable from positive to negative values. It is based on the Maltese-cross pattern, where anisotropy is induced by breaking the four-fold symmetry of the cross by displacing one of its beams. The symmetry breaking permits the excitation of a Fano mode active for one of the polarization eigenstates controlled by actuators using microelectromechanical systems. The metamaterial offers new opportunities for the development of terahertz variable waveplates, tunable filters and polarimetry.

CMOS compatible polarization splitter using hybrid plasmonic waveguide.

We design and experimentally demonstrate an ultrashort integrated polarization splitter on silicon-on-insulator (SOI) platform. Our polarization splitter uses a hybrid plasmonic waveguide as the middle waveguide in a three-core arrangement to achieve large birefringence, allowing only transverse-magnetic (TM) polarized light to directionally couple to the cross port of the directional coupler. Finite-difference time-domain (FDTD) and eigenmode expansive (EME) calculations show that the splitter can achieve an extinction ratio of greater than 15 dB with less than 0.5 dB insertion losses. The polarization splitter was fabricated on SOI platform using standard complementary metal-oxide-semiconductor (CMOS) technology and measured at telecommunications wavelengths. Extinction ratios of 12.3 dB and 13.9 dB for the transverse-electric (TE) and TM polarizations were obtained, together with insertion losses of 2.8 dB and 6.0 dB.

Effect of cladding layer and subsequent heat treatment on hydrogenated amorphous silicon waveguides.

Although intrinsic hydrogenated amorphous silicon (a-Si:H) wire waveguides clad with normal SiO(2) layers have low propagation loss of 2.7 ± 0.1 dB/cm for transverse electric (TE) mode in the 1550-nm range, the transparency degrades when interfaced with other dielectrics (e.g., air) and/or exposed to elevated temperatures due to degradation of surface passivation in the a-Si:H waveguides. The thermal stability of a-Si:H wire waveguides with various cladding layers is systematically investigated, showing that the a-Si:H wire waveguides are stable at annealing temperature lower than ~350°C, while they degrade quickly when annealed at a higher temperature. It indicates that the thermal stability is mainly determined by the annealing temperature rather than the annealing time, which may be attributed to quick evolution of weakly bonded hydrogen in the a-Si:H waveguides. A thin Si(3)N(4) intercladding layer between SiO(2) cladding and a-Si:H waveguide core may degrade transparency due to N-H bond absorption and is of no benefit to the thermal stability, thus its overall effect on the a-Si:H waveguides is detrimental.

Performance of ultracompact copper-capped silicon hybrid plasmonic waveguide-ring resonators at telecom wavelengths.

Ultracompact Cu-capped Si hybrid plasmonic waveguide-ring resonators (WRRs) with ring radii of 1.09-2.59 µm are fabricated on silicon on insulator substrates using standard complementary metal-oxide-semiconductor technology and characterized over the telecom wavelength range of 1.52-1.62 µm. The dependence of the spectral characteristics on the key structural parameters such as the Si core width, the ring radius, the separation gap between the ring and bus waveguides, and the ring configuration is systematically studied. A WRR with 2.59-µm radius and 0.250-µm nominal gap exhibits good performances such as normalized insertion loss of ~0.1 dB, extinction ratio of ~12.8 dB, free spectral range of ~47 nm, and quality factor of ~275. The resonance wavelength is redshifted by ~4.6 nm and an extinction ratio of ~7.5 dB is achieved with temperature increasing from 27 to 82°C. The corresponding effective thermo-optical coefficient (dn(g)/dT) is estimated to be ~1.6 × 10(-4) K(-1), which is contributed by the thermo-optical effect of both the Si core and the Cu cap, as revealed by numerical simulations. Combined with the compact size and the high thermal conductivity of Cu, various effective thermo-optical devices based on these Cu-capped plasmonic WRRs could be realized for seamless integration in existing Si electronic-photonic integrated circuits.

310 GHz gain-bandwidth product Ge/Si avalanche photodetector for 1550 nm light detection.

We report a normal incidence Ge/Si avalanche photodiode with separate-absorption-charge-multiplication (SACM) structure by selective epitaxial growth. By proper design of charge and multiplication layers and by optimizing the electric field distribution in the depletion region to eliminate germanium impact-ionization at high gain, a high responsivity of 12 A/W and a large gain-bandwidth product of 310 GHz have been achieved at 1550 nm.

Dependences of photoluminescence from P-implanted epitaxial Ge.

A systematic investigation has been carried out to study the influence of various annealings and implantations on the photoluminescence (PL) properties of phosphorus (P)-implanted Ge epitaxial films on Si substrate. For un-capped Ge samples, rapid thermal annealing (RTA) at 700 °C for 300 seconds yields the strongest PL emission peaked at 1550 nm. The influence of employing various capping layers (i.e., SiO(2), Si(3)N(4), and α-Si ) on the PL properties has been investigated. The capping layers are found to effectively decrease the dopant loss, leading to a significant PL enhancement. Si(3)N(4) is found to be the most efficient capping layer to prevent dopant out-diffusion and thus lead to strongest PL. Furthermore, it has been found that capping layers not only enhance the PL intensities but also make PL emission peak red- and blue- shift, depending on the stress type of the capping films. The effect of implantation dose on the PL has been also investigated.

Integrated in-band optical signal-to-noise ratio monitor implemented on SOI platform.

Based on different coherence properties of signal and noise, we measured the in-band optical signal-to-noise ratio using an integrated thermally tunable Mach-Zehnder optical delay interferometer on SOI platform. The experimental results exhibit errors smaller than 1 dB for signals with bit rate <40 Gbps over an OSNR range of 9~30 dB. The effects of the extinction ratio, noise equivalent bandwidth and arm length difference on the implementation of measurement are analyzed.

Components for silicon plasmonic nanocircuits based on horizontal Cu-SiO2-Si-SiO2-Cu nanoplasmonic waveguides.

We report systematic results on the development of horizontal Cu-SiO2-Si-SiO2-Cu nanoplasmonic waveguide components operating at 1550-nm telecom wavelengths, including straight waveguides, sharp 90° bends, power splitters, and Mach-Zehnder interferometers (MZIs). Owing to the relatively low loss for propagating (~0.3 dB/µm) and for 90° sharply bending (~0.73 dB/turn), various ultracompact power splitters and MZIs are experimentally realized on a silicon-on-insulator (SOI) platform using standard CMOS technology. The demonstrated splitters exhibit a relatively low excess loss and the MZIs exhibit good performance such as high extinction ratio of ~18 dB and low normalized insertion loss of ~1.7 dB. The experimental results of these devices agree well with those predicted from numerical simulations with suitable Cu permittivity data.

Silicon polarization independent microring resonator-based optical tunable filter circuit with fiber assembly.

We report the design, fabrication, photonic packaging and the characterization of a silicon polarization independent optical tunable filter circuit with fiber assembly. We demonstrate the polarization transparent filter characteristics with an insertion loss of ~13.4 dB, an extinction ratio of ~20 dB, and a 3 dB bandwidth of 0.2 nm. The tuning range is of ~11.72 nm, along with the tuning speed of less than 400 µs. The tuning efficiency is ~0.23 nm/mW. The use of polarization diversity scheme and the silicon photonic packaging bridges the gap between the silicon photonic circuits and the real applications.