Silicon Slot Waveguide Research Articles

Integratable all-optical spectral quantization scheme based on chalcogenide–silicon slot waveguide

Abstract In this paper, we propose an integratable all-optical spectral quantization scheme based on soliton self-frequency shift (SSFS) effect in a chalcogenide–silicon slot waveguide (CSSW) with a high nonlinear coefficient of 139.47 W −1 /m at 1550 nm and the wide anomalous dispersion regime of 870 nm. With the input peak power of 25 W, the large frequency shift up to 388 nm is achieved in the 5 cm-long CSSW. The quantization results show that the effective number of bit (ENOB) is 2.68-bit with only 0.32-bit difference from the ideal 3-bit resolution. It is believed that the proposed spectral quantizer is CMOS-compatible, and can find important application in the photonic integratable all-optical analog-to-digital converter (AOADC).

Digitally Controlled Phase Shifter Using an SOI Slot Waveguide With Liquid Crystal Infiltration

We demonstrate a phase shifter using a silicon slot waveguide infiltrated with liquid crystal. For a 1-mm long device we achieve $73\pi $ phase shift with a 5 V voltage, with a voltage-length product of 0.0224 $\text{V}\cdot $ mm $\sim 1$ V. We drive the phase shifter with a digital 1 V, duobinary pulsewidth-modulated signal, or a 1 V frequency-modulated signal. This enables direct digital CMOS control of an analog optical phase shifter.

Free-carrier electro-refraction modulation based on a silicon slot waveguide with ITO.

Recently, silicon-waveguide-based hybrid modulators with high-performance electro-optic materials have been proposed to overcome the intrinsic limitations of silicon materials. Indium-tin-oxide (ITO) is one of the important candidates for such applications due to its unique features including the ENZ effect and electrically tunable permittivity. In this paper, we propose an ultra-compact integrated phase modulator which consists of a silicon slot waveguide with a thin ITO film in the slot region. In the near-infrared regime, bias-voltage-dependent free-carrier accumulation at the dielectric-ITO interface induces an epsilon-near-zero (ENZ) effect, and contributes to the strong phase modulation of the guided electromagnetic wave. With a voltage swing of 2 V, the device experiences a large variation of the effective modal index, resulting in a π radian phase shift within the device length of <5 μm at 210 THz according to our computer simulations. A high modulation efficiency of V(π)L(π)~0.0071 V·cm and a large device bandwidth of ~70 GHz suggest a potential for an ultra-compact optoelectronic component in the integrated silicon photonics platform.

Influence of BaTiO3 ferroelectric orientation for electro-optic modulation on silicon.

The influence of BaTiO(3) ferroelectric domain orientations for high efficiency electro-optic modulation has been thoroughly analyzed. The Mach-Zehnder modulator structure is based on a CMOS compatible silicon/BaTiO(3)/silicon slot waveguide that supports both TE and TM polarizations whereas the Pockels effect is exploited by the application of a horizontal electric field with lateral electrodes placed on top of the BaTiO(3) layer. The influence of the waveguide parameters has been optimized for each configuration and the lowest V(π) voltage combined with low losses has been determined. A V(π)L as low as 0.27 V·cm has been obtained for a-axis oriented BaTiO(3) and TE polarization by rotating the waveguide structure to an optimum angle.

Extraordinary Optical Confinement in a Silicon Slot Waveguide with Metallic Gratings**Supported by the Key Grant Project of the Ministry of Education of China under Grant No 313007.

We present a silicon slot waveguide with metallic gratings embedded on the silicon surface in the slot region. The dependence of the optical coupling between two silicon wires on the width of the metal gap and the slot size are studied in detail. The results show that the optical field in the slot region with metallic gratings is significantly enhanced compared with the traditional slot waveguide due to the surface plasmon polaritons coupling on metallic gratings. The extraordinary optical confinement is attributed to the low effective dielectric constant of metallic gratings. The effective dielectric constant decreases with the increasing wavelength, and reaches the minimum when the width of the metal gap is about 0.01 times the wavelength.

Ultra-compact TE-pass polarizer with graphene multilayer embedded in a silicon slot waveguide.

A novel silicon slot waveguide TE-pass polarizer with graphene multilayer embedded in the slot is proposed and demonstrated by utilization of the fact that the variation of the modal characteristics for the TM mode is more than that for the TE mode. The designed polarizer is shown to have the ability to significantly suppress the transmission of the TM mode, while well guiding the TE mode. We numerically demonstrate a 7-μm-long polarizer has an ultra-high insertion loss of 31.5dB for the TM mode and as little insertion loss as 0.2dB for the TE mode at 1.55μm. The presented polarizer offers the performance merits including high extinction ratio, ultra-low insertion loss, ultra-compactness, and easy integration with silicon slot waveguides without using any taper.

Nonlinear optimization of slot Si waveguides: TPA minimization with FOM(TPA) up to 4.25.

The χ(3) nonlinear properties of slotted crystalline silicon photonic waveguides filled with third-order nonlinear materials (NM) are studied by calculating the effective nonlinear susceptibilities associated to the silicon and cladding material, respectively. The adopted approach circumvents the assumptions that the introduced NM dominates the nonlinear behavior of the slotted waveguide and that strong light confinement in the slot allows neglecting the two-photon absorption (TPA) process in the silicon rails. Optimization of the geometry of silicon-slotted waveguides is performed on the basis of the nonlinear figure of merit (FOM(TPA)) of the guided mode, which is related to the balance between the Kerr and the TPA effects, allowing to reach a FOM(TPA)=4.25. The obtained results reveal the importance of properly choosing the waveguide width of the silicon rails in order to minimize the TPA effect even by tolerating a reduced overall nonlinearity.

Ultracompact Si slot waveguide‐based polarization rotators

ABSTRACTA novel silicon asymmetric tilted slot waveguide‐based polarization rotator is presented. A large birefringence in the silicon slot waveguide facilitated a compact polarization rotator of subwavelength scale. A 1287 nm long TM to TE and TE to TM polarization rotator was achieved with an extinction ratio of 10 dB and an insertion loss of less than 5 dB at an operating wavelength of 1500 nm. Further performance improvement is possible for polarization rotator designed to work on TM to TE (or TE to TM) conversion: a 711 nm long TM to TE polarization rotator with 9.25 dB extinction ratio and 2.2dB insertion loss was also numerically demonstrated. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:779–785, 2015

Design of an efficient terahertz wave source from a GaP waveguide embedded in a silicon slot waveguide

Here, we propose an efficient scheme for terahertz (THz) wave generation on the basis of difference frequency mixing (DFM) using a GaP ridge waveguide embedded in a silicon slot waveguide. Phase matching in the DFM process, between the nonlinear polarisation wave induced by two near-infrared pumps and the generated THz wave in the low-refractive-index slot waveguide, was achieved by utilizing the modal birefringence of the fundamental transverse electric- and transverse magnetic-like modes at telecom wavelengths in the GaP ridge waveguide. The effective cross-sectional area of the THz wave in the waveguide was small, 220 μm2 at 2.26 THz, resulting in a photon conversion efficiency of 5.7 × 10-2%. The THz output power approached the multi-μW level using the proposed waveguide structure.

Highly efficient mode converter for coupling light into wide slot photonic crystal waveguide.

We design, fabricate and experimentally demonstrate a highly efficient adiabatic mode converter for coupling light into a silicon slot waveguide with a slot width as large as 320 nm. This strip-to-slot mode converter is optimized to provide a measured insertion loss as low as 0.08 dB. Our mode converter provides 0.1 dB lower loss compared to a conventional V-shape mode converter. This mode converter is used to couple light into and out of a 320 nm slot photonic crystal waveguide, and it is experimentally shown to improve the coupling efficiency up to 3.5 dB compared to the V-shape mode converter, over the slow-light wavelength region.

Potential for large optical gain improvement of erbium-doped slot waveguide amplifiers in silicon photonics

The potential for on-chip optical amplification at λs=1530 nm in silicon photonics using erbium-doped rich polymers integrated in silicon-slot waveguides is investigated by using a four-level spectroscopic model and a pumping wavelength of 1.48 μm. It is shown that the key parameter driving the whole amplification efficiency is the slot waveguide linear loss level, while optimization of the hollow core waveguide cross section leads to a slot width around 130 nm. Our investigations show that an on-chip optical gain of about 10 dB can be obtained with 7 dB/cm loss slot waveguides. Such a propagation loss is directly sustained by experimental results obtained for photonic structures fabricated using silicon technology. Due to the likelihood of improved technological processes in the near future, the slot waveguide loss level was swept in the 3–10 dB/cm range, showing that on-chip optical gain up to 30 dB can be seriously envisaged for slot waveguides with optimized optical losses around 3 dB/cm.

Athermal Silicon Slot Waveguide With an Ormocomp Polymer Overlayer

We have demonstrated a proof-of-concept for an athermal silicon slot waveguide using Ormocomp as a top cladding. Preliminarily theoretical and experimental results show that the slot waveguide geometry can completely cancel out its thermo-optical coefficient, by tailoring the optical mode overlap with silicon, silicon dioxide, and Ormocomp.

Experimental demonstration of silicon slot waveguide with low transmission loss at 1064 nm

Silicon slot waveguides that can operate at the wavelength of high silicon absorption are experimentally demonstrated on SOI wafer. The measured transmission loss coefficient could be as low as 2.28±0.03dB/mm at the wavelength of 1064nm (the slot width of 100nm), which is much lower than the absorption loss of silicon (5dB/mm at 1064nm). According to the simulation, such value is dominated by the surface roughness of sidewalls. The transmission loss is potentially to be reduced to <1dB/mm if the sidewall roughness could be reduced to ~5nm. We believe that this work could pave the way to achieving all silicon photonic integrated circuits, which are attractive for future optical interconnect and chemical/biological analysis.

Excitation of Confined Modes in Silicon Slotted Waveguides and Microring Resonators for Sensing Purposes

An important class of biochemical sensors are based on slotted waveguides having an increased sensitivity due to the confinement of the optical field in the slot region. The most sensitive devices of such a type contain microrings, and in these devices, the resonant frequency shift due to presence of the analyte in the microring slot is converted to the sensing signal. However, to the best of our knowledge, in none of the publications dealing with slotted microring sensors, there is no numerical or experimental analysis if the light is actually confined in the ring slot region. Here, we investigate numerically the conditions for the excitation of the modes confined in the slot of the ring. The 3-D finite element method (FEM) is used to calculate the optical power transmission coefficients of all-pass configuration (slotted ring +bus waveguide) in wide spectral range, while the eigenfrequency spectrum of slotted rings and its dependence on the top cladding refractive index is analyzed by means of 2-D axisymmetric FEM approach. It is shown that for the sensors based on slotted microrings sensitivity attains much higher value, up to four times, when the mode is confined in the slot of the ring is excited. This can be important for the design of different biochemical sensors.

Rigorous characterization of acoustic-optical interactions in silicon slot waveguides by full-vectorial finite element method.

For the first time detailed interactions between optical and acoustic modes in a silicon slot waveguide are presented. A new computer code has been developed by using a full-vectorial formulation to study the acoustic modes in optical waveguides. The results have shown that the acoustic modes in an optical slot waveguide are not purely longitudinal or transverse but fully hybrid in nature. The model allows the effects of Stimulated Brillouin Scattering and the associated frequency shift due to the interaction of these hybrid acoustic modes with the fully hybrid optical mode also to be presented.

Broadband Compact Silicon Wire to Silicon Slot Waveguide Orthogonal Bend

An ultra-compact 90° bend between silicon (Si) wire and silicon slot waveguide (SSW) with a footprint of ~725 nm × 500 nm is experimentally demonstrated on silicon-on-insulator substrate. By achieving momentum matching of the waveguides through an orthogonal junction placement, and maximizing modal overlap through an angled facet, coupling efficiency of ~70% and 3-dB bandwidth of over 500 nm has been achieved. The nominal experimental transmission through cascaded input (Si wire to SSW) and output (SSW to Si wire) orthogonal junctions match closely those obtained from simulations, both in the range from 1270 to 1360 nm and from 1480 to 1590 nm. For slot widths ranging from 30 to 230 nm, our Si wire-SSW bend can achieve coupling efficiency comparable to that of a direct butt-coupler over a 400 nm bandwidth. This compact and wideband waveguide bend serves as an important component to enable dense integration between conventional Si wire and SSW.

Analysis of enhanced stimulated Brillouin scattering in silicon slot waveguides

Stimulated Brillouin scattering has attracted renewed interest with the promise of highly tailorable integration into the silicon photonics platform. However, significant Brillouin amplification in silicon waveguides has yet to be shown. In an effort to engineer a structure with large photon-phonon coupling, we analyzed both forward and backward Brillouin scattering in high-index-contrast silicon slot waveguides. The calculations predict that gradient forces enhance the Brillouin gain in narrow slots. We estimate a currently feasible gain of about 10(5) W(-1) m(-1) in horizontal slot waveguides, which is an order of magnitude larger than in a stand-alone silicon wire. Such efficient coupling could enable a host of Brillouin technologies on a mass-producible silicon chip.

Highly sensitive compact refractive index sensor based on phase-shifted sidewall Bragg gratings in slot waveguide

The geometrical and physical parameters of phase-shifted sidewall Bragg gratings in a silicon slot waveguide are optimized to possess performance characteristics desirable for integrated optical sensors. By tailoring the spectral response of such phase-shifted sidewall gratings, highly sensitive compact refractive index sensors detecting the resonance wavelength shift or the variation of light intensity are designed with the transfer matrix method. Both refractive index sensors have a minimum detection limit on the order of 10(-6), and a linear response and a compact structure dimension as small as 11.7 μm, offering the capabilities for sensor array and lab-on-a-chip integration. The resonance-shift sensor has a much wider detection range of 1.32 refractive index units than the intensity-measurement sensor. The performance parameters are compared with other refractive index sensors, including Mach-Zehnder interferometers, ring resonators, surface gratings, and phase-shifted gratings in silicon nanowire.

Semianalytical method to study silicon slot waveguides for optical sensing application

High-index contrast slab and slot optical waveguides have a high index variation both along the lateral and vertical interfaces and are usually analyzed numerically, requiring large computer memory and time. In this article, their analysis is done semianalytically using an effective-index based matrix method. This method, which is computationally very fast, was earlier used successfully for low-index profile waveguide structures only and is now suitably modified for use in high-index contrast structures. The electric field profile of the waveguide structures is plotted and the effective refractive index at different wavelengths is calculated. The results are compared with results obtained from numerical techniques like finite element method, finite-difference time-domain, and beam propagation method and they match very well. The dependence of their different optical characteristics with the waveguide parameters is also studied. These studies will help in obtaining improved sensitivity of slot waveguides for sensing applications.

Transmission-Efficient Structures of Bent and Crossing Silicon Slot Waveguides

We present transmission-efficient bent and crossing silicon slot waveguides for compact silicon slot waveguide-based circuits. To begin with, using the 3-D vector finite-element method, we investigate the transmission characteristics of a bent silicon slot waveguide with an asymmetric slot that is connected to straight waveguides. Numerical results show that the transmission efficiency can be improved by optimizing a taper structure that is introduced between the bend of an asymmetric slot waveguide and its straight input/output waveguides. Next, we investigate the transmission characteristics of a 3-D crossing silicon slot waveguide that has a vertical coupler. Numerical results show that our crossing structure can achieve a high transmission efficiency compared with conventional structures.