Wire Waveguides Research Articles

Ultra-compact III‒V-on-Si photonic crystal memory for flip-flop operation at 5 Gb/s.

We report on a photonic crystal (PhC) nanolaser based on the heterogeneous integration of a III-V PhC nanocavity on SOI, configured to operate as a Set-Reset Flip-Flop (SR-FF). The active layer is a nanobeam cavity made of a 650 nm × 285 nm InP-based wire waveguide evanescently coupled to 500 nm × 220 nm SOI wire waveguides, demonstrating a record-low footprint of only 6.2 μm2. Injection locking enables optical bistability allowing for memory operation with only 6.4 fJ/bit switching energies and <50 ps response times. Bit-level SR-FF memory operation was evaluated at 5 Gb/s with PRBS-resembling data patterns, revealing error free operation with a negative power penalty.

Waveguide design parameters impact on absorption in graphene coated silicon photonic integrated circuits

In this paper, we propose a new way of estimating the absorption in graphene coated silicon wire waveguides based on a self-developed, modified 2D Finite Difference Method, and use it to obtain a detailed absorption dependency of the waveguide design. For the first time, we observe peaks in the TM mode absorption curves, as well as the reversals of the dominantly absorbed mode with waveguide design variation, both of which have not been predicted previously theoretically, but have been implied through experimental results. We also provide a qualitative explanation of our novel numerical results, and explain how these results can be utilized in optimization of various graphene based integrated devices like optical modulators, photodetectors and optical polarizers.

Observation of an optical event horizon in a silicon-on-insulator photonic wire waveguide.

We report on the first experimental observation of an optical analogue of an event horizon in integrated nanophotonic waveguides, through the reflection of a continuous wave on an intense pulse. The experiment is performed in a dispersion-engineered silicon-on-insulator waveguide. In this medium, solitons do not suffer from Raman induced self-frequency shift as in silica fibers, a feature that is interesting for potential applications of optical event horizons. As shown by simulations, this also allows the observation of multiple reflections at the same time on fundamental solitons ejected by soliton fission.

Theoretical Analysis of Spectral Correlations Between Photon Pairs Generated in Nanoscale Silicon Waveguides**Supported by the State Key Program for Basic Research of China under Grant No. 2012CB921802, the National Natural Science Foundation of China under Grant Nos. 91321312, 91121001, 11321063, 11174121, and 61475099, and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and

Spontaneous four wave mixing in nonlinear waveguide is one of the excellent technique for generating photon pairs in well-defined guided modes. Here we present a comprehensive study of the frequency characteristic of correlated photon pairs generated in telecom C-band from a dispersion-engineered silicon wire waveguide. We have demonstrated that the waveguide configuration, shape of pump pulse, two-photon absorption as well as linear losses have significant influences on the biphoton spectral characteristics and the amount of frequency entanglement generated. The superior performance as well as the structural compactness and CMOS compatibility makes the silicon wire waveguide an ideal integrated platform for the implementation of on-chip quantum technologies.

Wideband Circularly Polarized Vortex Surface Modes on Helically Grooved Metal Wires

We report the characteristics of a right-handed helically grooved metal wire waveguide. For the helically grooved waveguide, the normal degenerate HE1 spoof surface plasmon polariton (SSPP) mode could be decomposed into two opposite circularly polarized modes, i.e., HE−1 and HE+1 SSPP modes, due to the chirality of helical grooves. The dispersion characteristics of the nondegenerate $\mathbf{HE}\pm\mathbf{1}$ SSPP modes were numerically obtained, and their spiral electromagnetic distributions were also presented. It was found that the $\mathbf{HE}\pm\mathbf{1}$ modes were circularly polarized; they were also vortex modes and possessed an orbit angular monument. In addition, the focusing characteristics of such waves near the waveguide end were investigated. The distinct advantages, such as a relatively long focusing length and the circular polarization of the chiral $\mathbf{HE}+\mathbf{1}$ SSPP modes, make the helically grooved metal wire waveguide a potential for near-field circular dichroism detection.

Compact high extinction ratio asymmetric polarization beam splitter of periodic rods waveguide.

A compact high extinction ratio polarization beam splitter based on an asymmetric directional coupler was proposed and theoretically investigated. The asymmetric directional coupler consists of a silicon wire waveguide and a 1D periodic silicon rods waveguide, which results in an ultracompact polarization splitting length. By using the plane wave expansion method, a minimum coupling length of 3.43 μm was obtained, and the length was then confirmed by finite-difference time-domain simulation. Moreover, for 1550 nm wavelength, high extinction ratios of about 28 and 18 dB were also observed for TE and TM polarizations, respectively. The ultrahigh extinction ratio for TE polarization is mainly arising from the appearance of TM bandgap in the periodic rods waveguide. In addition, for both polarizations, the extinction ratios are all above 10 dB covering a 180 nm bandwidth, and it was also demonstrated that the device has a high transmission for TM polarization.

Analysis of silicon-on-insulator slot waveguide ring resonators targeting high Q-factors.

Vertical slot waveguide micro-ring resonators in silicon photonics have already been demonstrated in previous works and applied to several schemes, including sensing and hybrid nonlinear optics. Their performances, first quantified by the reachable Q-factors, are still perceived to be restrained by larger intrinsic propagation losses than those suffered by simple Si wire waveguides. In this Letter, the optical loss mechanisms of slot waveguide micro-ring resonators are thoroughly investigated with a special focus on the coupler loss contribution that turns out to be the key obstacle to achieving high Q-factors. By engineering the coupler design, slotted ring resonators with a 50 μm radius are experienced with a loaded Q-factor up to 10 times improvement from Q=3,000 to Q=30,600. The intrinsic losses due to the light propagation in the bent slot ring itself are proved to be as low as 1.32±0.87 dB/cm at λ=1,550 nm. These investigations of slot ring resonators open high performance potentials for on-chip nonlinear optical processing or sensing in hybrid silicon photonics.

Vertical silicon waveguide coupler bent by ion implantation.

We propose and demonstrate that vertically curved waveguides (VCWs) enable vertical coupling between silicon wire waveguides and optical fibers with low wavelength dependence and polarization dependence for wide telecommunication wavelength band light. To bend these VCWs, we implanted silicon ions into silicon wire cantilevers from the vertical direction. The internal stress distribution that was induced by ion implantation drove the bending force, and we achieved vertical bending of the waveguides, with curvature radii ranging from 3 to 25 μm. At a radius of curvature of 6 μm, we obtained a coupling loss of 3 dB using a lens fiber.

Monolithic Integration of InP Wire and $\mbox{SiO}_x$ Waveguides on Si Platform

We report a low-loss InP wire waveguide monolithically integrated with an SiO x waveguide on an Si platform. By means of directly bonding InP to SiO 2, we realized a submicrometer-scale InP wire waveguide with high optical confinement. In addition, a several-micrometer-scale SiO x waveguide with a refractive index difference of ~3% is integrated on the InP wire waveguide via a spot-size converter (SSC). Experimental results indicate that the InP wire waveguide has a propagation loss of 5 dB/cm and that the InP- SiO x SSC has an insertion loss of 0.7 dB and a reflectance of less than -50 dB. By using the SSC, a low coupling loss of 0.9 dB is achieved between the optical fiber and the InP wire waveguide.

Effective index-based matrix method for silicon waveguides in SOI platform

The work makes an attempt to design and analyze silicon waveguides in silicon on insulator (SOI) platform with the aid of effective index-based matrix method (EIMM). Effective index method is used for wave confinement in depth direction of the waveguides, thus effective indices of the guided modes are computed. A transfer matrix method is applied on the resulted one-dimensional lateral index profile to compute the guided mode propagation constants. Both wire and rib waveguides are designed from its fabrication parameters for single-mode wave propagation at a transmitting wavelength of 1.55μm. Bending losses of bent wires are estimated by a suitable conformal transformation along with EIMM. From our computed results, it is seen that the bending radii exceeding 1μm have negligibly small bending losses. The mode profiles of the silicon wires within the waveguides are also computed for both transverse electric (TE) and transverse magnetic (TM) polarizations. Analysis of some special structures, such as wire waveguide with slanted etched walls, and directional coupler made up of straight and/or curved coupled waveguides are also presented. Some of the computed results of this semi-analytical EIMM are validated with commercially available 2D-FDTD method.

Metal wire waveguide based all plasmonic refractive index sensor for terahertz frequencies

A terahertz refractive index sensor for liquid analytes consisting of a copper wire waveguide and co-axial layers of PMMA and PVDF is established numerically. The dispersion behavior of the fundamental core and surface mode and their phase matching has been demonstrated that clearly explains the working mechanism of the sensor. A core mode amplitude sensitivity of 59.9 for unit change in refractive index at 269.8μm is obtained for the proposed sensor. It has a large evanescent field penetration depth of 48μm at the phase matching point that can facilitate sensing of analyte refractive index further away from the interface. A definition for group velocity sensitivity is proposed which is then evaluated for the proposed sensor. The near dispersion free behavior of the sensor core mode except around the phase matching point gives it an excellent group velocity sensitivity of 128.9 for unit change in refractive index.

Broadband polarization independent nanophotonic coupler for silicon waveguides with ultra-high efficiency.

Coupling of light to and from integrated optical circuits has been recognized as a major practical challenge since the early years of photonics. The coupling is particularly difficult for high index contrast waveguides such as silicon-on-insulator, since the cross-sectional area of silicon wire waveguides is more than two orders of magnitude smaller than that of a standard single-mode fiber. Here, we experimentally demonstrate unprecedented control over the light coupling between the optical fiber and silicon chip by constructing the nanophotonic coupler with ultra-high coupling efficiency simultaneously for both transverse electric and transverse magnetic polarizations. We specifically demonstrate a subwavelength refractive index engineered nanostructure to mitigate loss and wavelength resonances by suppressing diffraction effects, enabling a coupling efficiency over 92% (0.32 dB) and polarization independent operation for a broad spectral range exceeding 100 nm.

Spot-size converter with a SiO(2) spacer layer between tapered Si and SiON waveguides for fiber-to-chip coupling.

We experimentally demonstrate low-loss and polarization-insensitive fiber-to-chip coupling spot-size converters (SSCs) comprised of a three dimensionally tapered Si wire waveguide, a SiON secondary waveguide, and a SiO(2) spacer inserted between them. Fabricated SSCs with the SiO(2) spacer exhibit fiber-to-chip coupling loss of 1.5 dB/facet for both the quasi-TE and TM modes and a small wavelength dependence in the C- and L-band regions. The SiON secondary waveguide is present only around the SSC region, which significantly suppresses the influence of the well-known N-H absorption of plasma-deposited SiON at around 1510 nm.

Design and evaluation of a high sensitivity spiral TDR scour sensor

Bridge scour accounts for more than half of the reported bridge failures in the United States. Scour monitoring technology based on time domain reflectometry (TDR) features the advantages of being automatic and inexpensive. The senior author’s team has developed a few generations of a TDR bridge scour monitoring system, which have succeeded in both laboratory and field evaluations. In this study, an innovative spiral TDR sensor is proposed to further improve the sensitivity of the TDR sensor in scour detection. The spiral TDR sensor is made of a parallel copper wire waveguide wrapped around a mounting rod. By using a spiral path for the waveguide, the TDR sensor achieves higher sensitivity than the traditional straight TDR probes due to longer travel distance of the electromagnetic (EM) wave per unit length in the spiral probe versus traditional probe. The performance of the new TDR spiral scour sensor is validated by calibration with liquids with known dielectric constant and wet soils. Laboratory simulated scour-refilling experiments are performed to evaluate the performance of the new spiral probe in detecting the sediment–water interface and therefore the scour-refill process. The tests results indicate that scour depth variation of less than 2 cm can be easily detected by this new spiral sensor. A theory is developed based on the dielectric mixing model to simplify the TDR signal analyses for scour depth detection. The sediment layer thickness (directly related to scour depth) varies linearly with the square root of the bulk dielectric constant of the water–sediment mixture measured by the spiral TDR probe, which matches the results of theoretical prediction. The estimated sediment layer thickness and therefore scour depth from the spiral TDR sensor agrees very well with that by direct physical measurement. The spiral TDR sensor is four times more sensitive than a traditional straight TDR probe.

A two-stage photonic crystal fiber / silicon photonic wire short-wave infrared wavelength converter/amplifier based on a 1064 nm pump source.

We demonstrate a two-stage wavelength converter that uses compact near-infrared sources to amplify and convert short-wave infrared signals. The first stage consists of a photonic crystal fiber wavelength converter pumped by a Q-switched 1064 nm pump source, while the second stage consists of a silicon photonic wire waveguide wavelength converter. The system enables on-chip amplification and conversion of up to 30 dB . We demonstrate amplification in a broad wavelength range around 2344 nm using temporally long pulses (>300ps).

Ultracompact tapered coupler for the Si/III-V heterogeneous integration.

An ultracompact tapered coupler, which is suitable for mode transformation between a 220 nm high silicon wire waveguide and a Si/III-V hybrid waveguide, is proposed for Si/III-V heterogeneous integration. The tapered coupler is composed of three sections. Since the tapered coupler avoids exciting the unwanted high-order modes in the III-V waveguide, the length of the tapered coupler can be dramatically shortened. In the proposed structure, the total length of the trisectional tapered coupler can be as short as 8 μm with a fundamental mode-coupling efficiency of over 95% in a bandwidth of over 100 nm. The alignment tolerance of the proposed structure is also analyzed.

An octave-spanning mid-infrared frequency comb generated in a silicon nanophotonic wire waveguide.

Laser frequency combs, sources with a spectrum consisting of hundred thousands evenly spaced narrow lines, have an exhilarating potential for new approaches to molecular spectroscopy and sensing in the mid-infrared region. The generation of such broadband coherent sources is presently under active exploration. Technical challenges have slowed down such developments. Identifying a versatile highly nonlinear medium for significantly broadening a mid-infrared comb spectrum remains challenging. Here we take a different approach to spectral broadening of mid-infrared frequency combs and investigate CMOS-compatible highly nonlinear dispersion-engineered silicon nanophotonic waveguides on a silicon-on-insulator chip. We record octave-spanning (1,500–3,300 nm) spectra with a coupled input pulse energy as low as 16 pJ. We demonstrate phase-coherent comb spectra broadened on a room-temperature-operating CMOS-compatible chip.

Nonlinear properties of dispersion engineered InGaP photonic wire waveguides in the telecommunication wavelength range.

We propose high index contrast InGaP photonic wires as a platform for the integration of nonlinear optical functions in the telecom wavelength window. We characterize the linear and nonlinear properties of these waveguide structures. Waveguides with a linear loss of 12 dB/cm and which are coupled to a single mode fiber through gratings with a -7.5 dB coupling loss are realized. From four wave mixing experiments, we extract the real part of the nonlinear parameter γ to be 475 ± 50 W(-1)m(-1) and from nonlinear transmission measurements we infer the absence of two-photon absorption and measure a three-photon absorption coefficient of (2.5 ± 0.5) x 10(-2) cm(3)GW(-2).

Strong sub-terahertz surface waves generated on a metal wire by high-intensity laser pulses.

Terahertz pulses trapped as surface waves on a wire waveguide can be flexibly transmitted and focused to sub-wavelength dimensions by using, for example, a tapered tip. This is particularly useful for applications that require high-field pulses. However, the generation of strong terahertz surface waves on a wire waveguide remains a challenge. Here, ultrafast field propagation along a metal wire driven by a femtosecond laser pulse with an intensity of 1018 W/cm2 is characterized by femtosecond electron deflectometry. From experimental and numerical results, we conclude that the field propagating at the speed of light is a half-cycle transverse-magnetic surface wave excited on the wire and a considerable portion of the kinetic energy of laser-produced fast electrons can be transferred to the sub-surface wave. The peak electric field strength of the surface wave and the pulse duration are estimated to be 200 MV/m and 7 ps, respectively.

Robust Ultrasonic Waveguide Based Distributed Temperature Sensing

This is a novel technique for distributed temperature measurements, using single robust ultrasonic wire or strip-like waveguides, special embodiments in the form of Helical or Spiral configurations that can cover large area/volume in enclosed regions. Such distributed temperature sensing has low cost applications in the long term monitoring critical enclosures such as containment vessels, flue gas stacks, furnaces, underground storage tanks, buildings for fire, etc. The range of temperatures that can be measured are from very low to elevated temperatures. The transduction is performed using Piezo-electric crystals that are bonded to one end of the waveguide which acts as both transmitter and receivers. The wires will have periodic reflector embodiments (bends, gratings, etc.) that allow reflections of an input ultrasonic wave, in a pulse echo mode, back to the crystal. Using the time of fight (TOF) variations at the multiple predefined reflector locations, the measured temperatures are mapped with multiple thermocouples. Using either the L(0,1) or the T(0,1)modes, or simultaneously, measurements other than temperature may also be included. This paper will describe the demonstration of this technology using a 0.5MHz longitudinal piezo-crystal for transmitting and receiving the L (0, 1) mode through the special form of waveguide at various temperatures zones.