Silicon Nanowire Waveguides Research Articles

Nanowires and sidewall Bragg gratings in silicon as enabling technologies for microwave photonic filters

We describe the use of various silicon photonic device technologies to implement microwave photonic filters (MPFs). We demonstrate four-wave mixing in a silicon nanowire waveguide (SNW) to increase the number of taps for MPFs based on finite impulse response filter designs. Using a 12 mm long SNW reduces the footprint by five orders of magnitude compared to silica highly nonlinear fiber while only requiring approximately two times more input power. We also demonstrate optical delays based on serial sidewall Bragg grating arrays and step-chirped sidewall Bragg gratings in silicon waveguides. We obtain up to 63 ps delay in discrete steps from 15 ps to 32 ps over a wide bandwidth range from 33 nm to at least 62 nm. These components can be integrated with other silicon-based components such as integrated spectral shapers and modulators to realize a fully integrated MPF.

Compact low loss and broadband hybrid plasmonic directional coupler

We propose a novel broadband coupler for silicon photonics using a hybrid plasmonic waveguide section. The hybrid plasmonic waveguide is used to create an asymmetric section in the middle of a silicon nanowire waveguide coupler to introduce a phase delay to allow for a 3-dB power coupling ratio over a 150 nm bandwidth around 1.55 µm. The device is very compact (<8.5 µm) and has a low insertion loss (<0.15 dB).

Ultra-Compact Birefringence-Compensated Arrayed Waveguide Grating Triplexer Based on Silicon-On-Insulator

We demonstrate an ultra-compact birefringence- compensated arrayed waveguide grating (AWG) triplexer for application in optical network units (ONUs) based on silicon nanowire waveguides. The cross-order design is employed to cover three operating wavelengths of 1310, 1490 and 1550 nm for the triplexer. The large birefringence of the silicon-on-insulator (SOI) waveguide is compensated by using angled star coupler design in combination with different diffraction orders for TE and TM modes. The device has a small footprint of only 0.18×0.12 mm2. The excess loss for TM polarization for three channels is 3 ~ 4 dB and the crosstalk is better than -18 dB. The polarization dependent wavelength shift (PDλ) is reduced from about 370 nm to less than 2.5 nm for wavelength channels at 1490-1550 nm.

A Compact 1×3 Silicon Photonic Waveguide Switch Based on Precise Investigation of Coupling Characteristics of Variable-Gap Coupler

A variable-gap silicon nanowire waveguide coupler was fabricated and its characteristics were measured for comparison with those determined by a rigorous electromagnetic simulation. The three-dimensional model of the coupler was simulated by the finite-difference time-domain method. On the basis of the results obtained, a compact 1×3 silicon nanowire coupler switch was designed and fabricated. The 1×3 switch was composed of two couplers with parallel silicon nanowire waveguides of 400 nm width and 260 nm thickness and comb-drive electrostatic actuators. The output isolation and extinction ratio of the variable coupler were measured to be 12.9 and 18.6 dB, respectively. The loss of the variable coupler was 0.21 dB. The measured coupling coefficient was explained quantitatively by the simulation. The proposed 1×3 switch was demonstrated by showing the infrared images of the switch outputs. The port isolation of the 1×3 switch was better than 11.3 dB.

Demonstration of a Silicon Waveguide Optical Circulator

A four-port optical circulator based on a Mach–Zehnder interferometer is fabricated in a silicon nanowire waveguide. A magneto-optical garnet is directly bonded to the silicon waveguide using a surface activated bonding technique. Four-port circulator operation is demonstrated with a maximum isolation of 15.3 dB at a wavelength of 1531 nm.

Design of compact bi-directional triplexer based on silicon nanowire waveguides

A compact bi-directional (BiDi) triplexer using grating-assisted multimode interference (MMI) coupler is proposed based on silicon nanowire waveguides.Because of the high index contrast between silicon and silicon dioxide, the size of the structure is greatly reduced with a footprint of 2.5 × 911 (μm). Asymmetrical ports are introduced in the MMI structure to satisfy the bandwidth requirements of the industrial standards ITU-T G.983.3-dB bandwidths of 100, 22, and 15 nm are obtained for the wavelengths of 1 310, 1 490, and 1 550 nm, respectively. The device can be readily fabricated using a commercial CMOS process.

A Compact and Low-Loss MMI Coupler Fabricated With CMOS Technology

We present the design, fabrication, and measurement of a compact and low-loss multimode interference (MMI) coupler based on the silicon nanowire waveguide. The device is carefully designed to achieve both a good performance and a compact size by using the mode matching method. The device is fabricated on silicon-on-insulator (SOI) with 0.13-μm CMOS technology. By measuring the MMI coupler with a cascaded configuration, a very low excess loss of 0.06 dB at the wavelength of 1550 nm is obtained. The device can also work well for a wide wavelength band. The present MMI coupler is very compact with a footprint of ~ 3.6 × 11.5 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for the multimode region.

A silicon nanowire factorable photon pair source

We propose a scheme for generating pairs of de-correlated photons in silicon nanowire waveguides. By properly engineering the geometrical dispersion of nanoscale waveguides we achieve both four-wave-mixing and group velocity matching. Factorable pure photon states are found that do not require spectral filtering. Highly anti-correlated two-photon states can be realized far from the Raman background of the waveguide. Our results hold promise for the realization of an integrated single photon source.

Flat-topped and low loss silicon-nanowire-type optical MUX/DeMUX employing multi-stage microring resonator assisted delayed Mach-Zehnder interferometers

We propose a novel silicon-nanowire-type multiplexer (MUX) / demultiplexer (DeMUX) based on multi-stage microring resonator assisted delayed Mach-Zehnder interferometers. It is theoretically shown that spectral flatness of DeMUX spectra can be accomplished by incorporating nonlinear phase behaviors of microring resonators into the multi-stage delayed Mach-Zehnder interferometers. We experimentally demonstrate flat-topped 400GHz-spacing 1 × 4Ch demultiplexing operation in the fabricated device with silicon-nanowire waveguides. Furthermore, by integrating the micro-heaters on the top cladding layer of the fabricated device, the DeMUX performance is upgraded in terms of excess loss (<0.8dB) and crosstalk (<-10dB) without any degradation of filter spectral flatness at each channel grid.

Introduction to the Issue on Quantum and Nanoscale Photonics

The purpose of this issue is to highlight the recent progress and trends in the development of novel nano- and quantum photonic technologies. The articles published in this issue cover a broad range of areas, including: 1) novel nanophotonic light sources-both classical (lasers) and nonclassical (single- and entangled photon sources); 2) nonlinear optics in nanophotonic structures; 3) quantum optics and quantum information processing based on nanophotonics platform; 4) efficient devices for optical interconnects and optical information processing based on novel nanophotonic structures. This issue contains 25 papers, including eight invited, authored by well-established research groups and promising scientists from all over the world. The invited articles include extended reviews on entangled photon generation using silicon nanowire waveguides; cavity quantum electrodynamics and lasing oscillation in single quantum dot photonic crystal nanocavity systems; quantum plasmonic circuits; theoretical analysis of few-photon single-atom cavity quantum electrodynamics; semiconductor quantum dot micropillar microcavities for quantum optics in solid state; nanofiber-based optical trapping of cold neutral atoms; switching and counting with atomic vapors in photonic-crystal fibers; and production of multiple diamondbased single-photon sources. The contributed articles cover a broad variety of key nanophotonics and quantum photonics research areas, including recently obtained results on bright single-photon emission from a quantum dot in a circular Bragg grating microcavity, supercontinuum generation in nanostructured silicon waveguides, electrically driven photonic crystal nanocavity devices, and silicon quantum dots.

An analytical coupling coefficient for MEMS tunable silicon nanowire waveguide coupler devices

Silicon nanowire waveguides are promising for future integration of photonic circuits with silicon electronics. Electromechanical control of waveguide is also favorable for variable silicon nanowire waveguide devices. In this study, we investigated analytically the characteristics of a silicon nanowire waveguide coupler for electromechanical waveguide devices. The electric field of the silicon nanowire waveguide was enhanced by the high-index contrast. The enhanced electric field increased the coupling coefficient by a factor of 2.7 for a silicon waveguide of 400 nm in width and 260 nm in thickness compared with the approximation on the basis of low-index-contract. The analytically derived coupling coefficient was evaluated experimentally by investigating a waveguide coupler switch with a micro-electromechanical actuator.

Single and multiple optical switches that use freestanding silicon nanowire waveguide couplers

Silicon photonic devices consisting of nanowire waveguides are a promising technology for on-chip integration in future optical telecommunication and interconnection systems based on silicon-large scale integration fabrication. However, the accommodation of variable optical components on a chip remains challenging due to the small size of microchips. In this study, we investigated the characteristics of a microelectromechanical silicon nanowire waveguide switch with a gap-variable coupler. Due to its capacitive operation, the proposed waveguide switch consumed negligible power relative to switches that use a thermo-optical effect and carrier injection. The proposed switch was characterized using analyses based on coupled-mode theory for rectangular waveguides, as well as a simulation using the finite difference time domain method. A 2×2 single switch with an improved configuration and a 2×6 multiple switch composed of the 2×2 switches was designed and fabricated by a combination of electron beam lithography, fast-atom beam etching and hydrofluoric acid vapor sacrificial etching. The properties of the switches were measured and evaluated at a wavelength of 1.55 µm. A new miniature silicon optical switch requires far less electrical power than schemes employing thermo-optic effects or carrier injection. The device, developed by Yuta Akihama and Kazuhiro Hane from Tohoku University in Japan, relies on optical coupling between two arms in a 2×2 silicon waveguide coupler. By allowing one of the arms to move and connecting it to an electrostatic comb-drive actuator, an electrical voltage can be used to vary the gap between the waveguide arms and thus control the amount of coupling. The result is an efficient switch that operates at the wavelength of 1.55 µm and has a footprint of 100 µm×100 µm. As well as demonstrating a single 2×2 switch, the researchers also cascaded several devices together to construct a 2×6 switch.

Width-modulation of Si photonic wires for quasi-phase-matching of four-wave-mixing: experimental and theoretical demonstration

We experimentally demonstrate quasi-phase-matched (QPM) four-wave-mixing (FWM) in silicon (Si) nanowire waveguides with sinusoidally modulated width. We perform discrete wavelength conversion over 250 nm, and observe 12 dB conversion efficiency (CE) enhancement for targeted wavelengths more than 100 nm away from the edge of the 3-dB conversion bandwidth. The QPM process in Si nanowires is rigorously modeled, with results explaining experimental observations. The model is further used to investigate the dependence of the CE on key device parameters, and to introduce devices that facilitate wavelength conversion between the C-band and mid-IR. Devices based on a superposition of sinusoidal gratings are investigated theoretically, and are shown to provide CE enhancement over the entire C-band. Width-modulation is further shown to be compatible with zero-dispersion-wavelength pumping for broadband wavelength conversion. The results indicate that QPM via width-modulation is an effective technique for extending the spectral domain of efficient FWM in Si waveguides.

Nanomechanically suspended low-loss silicon nanowire waveguide as in-plane displacement sensor

We have devised an air-suspended nano-optomechanical structure that is capable of precisely detecting in-plane motion up to a precision of subnanometer level. To achieve the detection, we utilized a basic silicon photonic building block (i.e., a nanowire waveguide directional coupler) in conjunction with nanoelectromechanical systems (NEMS). We numerically optimized the design and experimentally demonstrated a displacement sensitivity of 8.83×10−5 V/nm−1 with a low noise-level resolution of 0.172 nm/√Hz in a 1 Hz bandwidth centered at 950 Hz. As the waveguide coupler design does not segmentize nanowire waveguides, we eliminated the undesirable insertion losses and coupling losses irrelevant to the measurand. Furthermore the design is simple, ultracompact, and can be easily integrated with on-chip photonic systems, which may be beneficial for applications that require a compact displacement sensor with high accuracy and precision.

Thermally Tunable Filters Based on Third-Order Microring Resonators for WDM Applications

We design and fabricate a thermally tunable third-order microring resonator filter based on silicon nanowire waveguides. Box-like response with low intraband ripple (~0.65 dB) and high out-of-band rejection over 40 dB is demonstrated. The insertion loss at the center of the passband is less than 0.9 dB. The filter response is successfully tuned by one free spectral range through thermo-optic effect with a tuning efficiency of 48.4 mW/nm. The dynamic tuning measurement shows the 10%-90% rise and 90%-10% fall time of the filter are 12.63 and 6.31 μs, respectively.

Design of low-dispersion-discrepancy silicon waveguide for broadband polarization-independent wavelength conversion

We report a broadband polarization-independent wavelength conversion by reducing the dispersion discrepancy between the fundamental transverse electric (TE) and transverse magnetic (TM) modes in a silicon nanowire waveguide, through optimizing of the waveguide geometry. The size of the waveguide is optimized to 780 nm×795 nm (height×width) to ensure that the TE and TM zero-dispersion wavelengths locate at ~1550 nm, and the group velocity dispersion difference between the TE and TM modes is less than 33 ps/(nm·km) in the wavelength range of 1300–1800 nm. Based on an angled-polarization pumped four-wave mixing scheme, a 1 dB polarization-independent bandwidth of 400 nm is achieved in a 0.8 cm-long optimized waveguide when the pump is set at 1553 nm. The tolerance to the pump angle detuning is also strengthened in the optimized waveguide.

Nonlinear Optics in Silicon Wire Waveguides: Towards Integrated Long Wavelength Light Sources

ABSTRACTMost of the research on silicon-on-insulator integrated circuits has been focused on applications for telecommunication. By using the large refractive index of silicon, compact complex photonic functions have been integrated on a silicon chip. However, the transparency of silicon up to 8.5 μm enables the use of the platform for the mid infrared wavelength region, albeit limited by the absorption in silicon oxide from 4 μm on. This could lead to a whole new set of integrated photonics circuits for sensing, given the distinct absorption bands of many molecules in this wavelength region. These long wavelength integrated photonic circuits would preferably need broadband or widely tunable sources to probe these absorption bands.We propose the use of nonlinear optics in silicon wire waveguides to generate light in this wavelength range. Nonlinear interactions in just a few cm of silicon wire waveguides can be very efficient as a result of both the high nonlinear index of silicon and the high optical confinement obtained in these waveguides. We demonstrate the generation of a supercontinuum spanning from 1.53 μm up to 2.55 μm in a 2 cm dispersion engineered silicon nanowire waveguide by pumping the waveguide with strong picoseconds pulses at 2.12 μm [1]. Furthermore we demonstrate broadband nonlinear optical amplification in the mid infrared up to 50 dB [2] in these silicon waveguides. By using this broadband parametric gain a silicon-based synchronously pumped optical parametric oscillator (OPO) is constructed [3]. This OPO is tunable over 70 nm around a central wavelength of 2080 nm.Finally, we also demonstrate the use of higher order dispersion terms to get phase matching between optical signals at very different optical frequencies in silicon wire waveguides. In this way we demonstrate conversion of signals at 2.44 μm to the telecommunication band with efficiencies up to +19.5 dB [4]. One particularly attractive application of such wide conversion is the possibility of converting weak signals in the mid-IR to the telecom window after which they can be detected by a high-sensitivity telecom-band optical receiver.

Uniform Polarization-Dispersion Compensation of All Channels in Highly Birefringent Silicon Nanowire-Based Arrayed Waveguide Grating

A design method for compensating polarization dispersion in arrayed waveguide gratings (AWGs) based on highly birefringent silicon nanowire waveguides is proposed by using angled star couplers in combination with different diffraction orders for TE and TM polarizations. Polarization-dispersion compensation for all output channels is achieved with the maximal polarization-dependent wavelength shift of 0.025 nm in an eight-channel AWG with 200-GHz spacing using 380 nm × 220 nm silicon waveguide with SU-8 upper-cladding.

Numerical Analysis of Asymmetric Silicon Nanowire Waveguide as Compact Polarization Rotator

In this paper, an ultracompact design of a low-loss silicon (Si) polarization rotator based on a silicon-on-insulator platform, which contains an asymmetric strip Si nanowire waveguide core, is presented. A full-vectorial finite element method and the least squares boundary residual method are used to study the effects of the device parameters, the results of which indicate that a very high polarization conversion with a very low polarization crosstalk is possible.

Directionally anisotropic Si nanowires: on-chip nonlinear grating devices in uniform waveguides

Computational studies are used to show that the crystalline structure of Si causes the waveguide Kerr effective nonlinearity, γ, to vary by 10% for in-plane variation of the orientation of a silicon nanowire waveguide (SiNWG) fabricated on a standard silicon-on-insulator wafer. Our analysis shows that this angular dependence of γ can be employed to form a nonlinear Kerr grating in dimensionally uniform SiNWGs based on either ring resonators or cascaded waveguide bends. The magnitude of the nonlinear index variation in these gratings is found to be sufficient for phase matching in four-wave mixing and other optical parametric processes.