Low-index Slot Region Research Articles

Highly confined dielectric guiding mode in nanoridges embedded in a conventional slot waveguide.

Plasmonic waveguides can offer a promising solution beyond the optical diffraction limit. However, the cost of shrinking mode sizes reflects in metallic ohmic losses that lead to a short propagation distance of light, hindering the practical applications of plasmonic waveguides. Herein, we tackled the practicality of a novel CMOS-compatible all-dielectric waveguide structure that exploits electromagnetic boundary conditions of both the continuous normal component of the electric displacement field and the tangential component of the electric field at a high-index-contrast interface, which allows the attainment of mode areas comparable with those of plasmonic waveguides and theoretical lossless. The proposed waveguide comprises two oppositely contacted nanoridges with semicircular tops embedded in a conventional slot waveguide. By stepping on the strong electric field in the low-index slot region of the slot waveguides, the nanoridges squeeze the mode areas further with a guiding mechanism identical to that of a surrounding slot waveguide. Through the design of the geometry parameters, the calculated mode area of the reported structure achieved an unprecedented order of 4.21 × 10-5A0, where A0 is the diffraction-limited area. The mode area dependence on fabrication imperfections and spectral response showed the robustness and broadband operation. Moreover, on the basis of extremely tight mode confinements, the present waveguide even outperformed the hybrid plasmonic waveguides in lower crosstalk. The proposed idea makes the realization of practically feasible nanoscale photonic integrated circuits without any obstructions by the limited propagation distance of light for plasmonic waveguides, thereby expanding its applications in various nanophotonic and optoelectronics devices requiring strong light-matter interaction within nanoscale regions.

Characterization of GeSbSe Based Slot Optical Waveguides

Background: Among various chalcogenides, GeSbSe shows a good transmittance in the visible, NIR and, midIR spectrum from 1-20 μm and also demonstrates excellent moldability. Objective: In current work, we have characterized GeSbSe glass for use in sensor mechanism and for adaptive polarization control. Methods: After analysing an earlier work regarding GeSbSe based Silicon on insulator optical waveguide, we implemented GeSbSe in a low refractive index slot region of SOI slot optical waveguide. Change in waveguide geometry can cause a shift in the dispersion profile, but a relatively distinct pattern has been observed. T-slot waveguide structure has also been analysed, where GeSbSe has been implemented in low refractive index slot regions with the Graphene layer beneath the horizontal slot region for enhancement in tailoring ability of the birefringence parameters. Results: Literature review led to the presence of absorption resonance wavelength in SOI slot optical waveguide with our proposed composition, which is attributed to the single average harmonic oscillator property of the chalcogenides. In the T-slot waveguide structure, it was found that a shift in Fermi energy and Mobility values can bring a change in birefringence, even with constant waveguide geometry and operating wavelength. Conclusion: Absorption resonance wavelength in GeSbSe slot optical waveguide has been exploited for proposing the refractive index dispersion sensor. Our design approach regarding T-slot waveguide may lead to the provision of automated polarization management sources for the light on chip circuits.

Ultra-high birefringence and nonlinearity photonic crystal fiber with a nanoscale core shaped by an air slot and silicon strips

A novel polarization-maintaining photonic crystal fiber (PCF) with high birefringence and nonlinearity was designed and numerically optimized. By embedding high-index silicon strips and a low-index nanoscale air slot at the core region, X- polarization mode could be well confined in the low-index slot region due to the discontinuity of its electric displacement field vector at the silicon-air interface, whereas Y-polarization mode was dominated by total internal reflection (TIR) induced by elliptical air holes of the inner cladding, which resulted in a birefringence up to 10−1 order. Moreover, according to the antiresonant reflecting guidance mechanism, when the width of silicon strips was adjusted to 0.12 μm, an ultrahigh birefringence of ~0.94 at 1.55 μm could be obtained. In addition, benefiting from the tight field confinement and highly nonlinear silicon, an ultrahigh nonlinear coefficient of up to 2.4 × 106 W−1 km−1 can be achieved at the wavelength of 1.55 μm. Furthermore, the low confinement loss can also be yielded in our proposed PCF. With these advantages, the proposed PCF will offer important potential applications in optical communications, supercontinuum generation and other fields.

Hybrid Plasmonics Slot THz Waveguide for Subwavelength Field Confinement and Crosstalk Between Two Waveguides

The slot waveguide has attracted considerable attention because of its ability to confine and guide electromagnetic energy at the subwavelength scale beyond the diffraction limit. We propose a novel terahertz slot waveguide structure to achieve a better tradeoff between propagation length and field confinement capacity, the novel waveguide consisting of a two slot structure. The performances of terahertz waveguides were investigated using the finite-element method. The results demonstrated that the hybrid slot waveguide (HSW) provides significantly enhanced field confinement in low index slot regions: more than five times that of traditional low index slot waveguides (LISWs). An optimized HSW structure was achieved by tuning the tradeoff between mode confinement and propagation length. We also showed that its integration in conventional planar waveguide circuits was greatly improved compared with the LISWs, by comparing their crosstalk. The proposed new HSW structure has great potential to enable THz production of compact integration and could lead to true semiconductor-basedTHz applications with high performance.

Ultralow Propagation Loss Slot-Waveguide in High Absorption Active Material

A slot waveguide formed by high absorption active material is proposed to reduce the propagation loss for monolithic integration. The low propagation loss is attained by concentrating the optical field inside the low-index slot region without absorption. The simulation results show that the propagation loss at 1.55 μm for the slot waveguide can be as low as 1.5 dB/cm in active material with an absorption coefficient of 3000 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , whereas the optical profile is set to be confined within 650 nm.

Dispersion modified slot optical waveguides

Advent of slot waveguide structures had opened a new era where light can be confined in low index slot guarded by high index slabs. Already in use SOI slot waveguides (contrast ratio is 2.42) have two distinct properties over the conventional waveguides, i.e. high E-field amplitude, optical power, optical intensity in low index materials, and strong E-field confinement localized to nanometer-size low index regions. We hereby propose a low refractive index contrast ratio slot waveguide structure (ratio is 1.18) comprising of commercially available glass material. Novelty lies in showing high E-field amplitude, optical power, optical intensity, and strong E-field confinement in low index slot regions despite of lowest ever reported contrast ratio. A systematic numerical study on the higher order dispersion characteristics of the widely studied SOI-based slot structure and of our proposed low refractive index contrast slot structure is carried out. It has been demonstrated that low refractive index contrast ratio slot optical waveguide GVD properties are quite different than SOI slot optical waveguide. The less normal dispersion existing in this kind of waveguide could have an impact on their applications in various nonlinear or linear applications.

Long-Range Hybrid Plasmonic Slot Waveguide

We propose and design a silicon-based long-range hybrid plasmonic slot (LRHPS) waveguide. The waveguide structure consists of a thin metal film (silver) inserted into a vertical low-index slot [filled with silicon nanocrystal (Si-nc)] between two high-index dielectrics (silicon), forming two nanoscale low-index Si-nc slot regions. The modes are confined within the two vertical regions. The designed LRHPS waveguide takes advantages of both traditional long-range surface plasmon polariton waveguide and hybrid plasmonic waveguide. The combined effects of long-range surface plasmon polaritons and discontinuity of electric field at the interface between two dielectrics with high-contrast refractive index enable a millimeter-scale propagation range together with a subwavelength mode confinement for potential high-density nanophotonic integration. The resultant quasi-TE mode properties of LRHPS waveguide, including long-range hybrid (LRH) mode and short-range hybrid (SRH) mode, are analyzed, showing a long propagation length up to 14.55 mm (corresponding to a low loss of $3\times 10^{-4} \hbox{dB}/\mu\hbox{m}$ ) with a slot size of 150 nm $\times$ 200 nm. Normalized power and intensity are also calculated, indicating tight mode confinement within subwavelength low-index slot regions. Moreover, nonlinearity and chromatic dispersion are also studied. Due to the design freedom of double-slot structure, a high nonlinearity of $7.82 \times 10^{6} \hbox{W}^{-1}\hbox{km}^{-1}$ and a low chromatic dispersion of $-$ 28.29 ps/nm/km at 1550 nm are achieved, implying possible applications in efficient nonlinear optical signal processing.

Gain enhancement in a V-shaped plasmonic slot waveguide for efficient loss compensation at the subwavelength scale

An active plasmonic slot waveguide comprising an inverted triangular metal wedge incorporated inside a V-shaped plasmonic groove with a low-index gain medium embedded between them is presented, and its guiding properties are investigated numerically at the wavelength of 1550nm. The presented waveguide is shown to be capable of supporting two fundamental plasmonic slot modes with high field localization to the V-shaped low-index slot region. Due to such strong optical confinement and significant field enhancement, the introduced gain in the slot could effectively compensate the propagation loss of the supported plasmonic modes. It is revealed that for the studied channel plasmonic slot and wedge plasmonic slot modes, notable gain enhancements are observable within a wide range of geometric parameters. For the considered structure with a 10–40nm-wide slot, the enhancements of gain can be as large as 11%–159% for the CPS mode while 43%–174% for the WPS mode. These values could be further improved by adopting even narrower slots. It is shown that, by introducing a gain medium with coefficients around hundreds of cm−1, the modal loss can be largely or even fully compensated, with a subwavelength mode area achievable simultaneously. These unique features of the studied V-shaped plasmonic slot waveguide might be useful for its potential applications in compact, active plasmonic components.

T-shaped dielectric slot waveguides for efficient control of birefringence and polarization independent directional coupling

A novel dielectric slot waveguide supporting strongly confined field in a T-shaped low-index slot region for both TE and TM polarizations is proposed and analyzed. Numerical simulations have demonstrated that quite different birefringent modal properties are achievable with tight optical confinement in the slot by tuning key geometrical parameters of the waveguide. Based on such a slot structure, the characteristics of directional couplers are investigated and the conditions for polarization independent coupling are also given. The presented T-shaped slot waveguide might be employed in integrated photonic systems as important building blocks enabling a number of potential applications.

Slot Optical Waveguide Usage in Forming Passive Optical Devices

We have reviewed the work on SOI slot optical waveguides followed by our work. In a slot waveguide structure, light can be confined in a low index slot guarded by high index slabs. Slot structures are being used in forming complex structures; such as ring resonator circuits. The increased round trip in ring resonator circuits signifies the importance of dispersion calculations. We did analytical and numerical investigations of slot structures' dispersion characteristics. Our dispersion tuned slot structures can help in reducing the dispersion effects on optical signal, which will in turn improve the efficiency of light-on-chip circuits. Since the advent of slot optical waveguides, SOI based slot optical waveguides have been under consideration. It has been found that glass based slot optical waveguide structures with relatively low refractive index contrast ratio can also play an important role in forming complex nano-size optical devices. We made use of power confined inside low index slot regions for a double slot structure. Opto-mechanical sensors have been proposed based upon: (a) variation in power confined inside low index slot region due to the movement of central high index slab under the action of external force (temperature, pressure, humidity, etc). vide Chinese Patent No. ZL 200710176770.1, 2007 (b) variation in power confined inside low refractive index slot regions due to movement of both slots under the action of external force (temperature, pressure, humidity, etc).

Broadband Enhancement of Light Harvesting in a Luminescent Solar Concentrator

Luminescent solar concentrators (LSCs) are large-area devices that absorb incident sunlight and emit luminescence photons with high quantum efficiency, which will finally be collected by a small photovoltaic (PV) system. The light-harvesting area of the PV system is much smaller than that of the LSC system, potentially reducing the cost of solar cells. Here, we present a theoretical description of the luminescent process in nanoscale LSCs where the conventional ray-optics model is no longer applicable. We demonstrate that a slot waveguide consisting of a nanometer-sized low-index slot region sandwiched by two high-index regions provides a broadband enhancement of light harvesting by the luminescent centers in the slot region. This is because the slot waveguide can 1) greatly enhance the spontaneous emission due to the Purcell effect, 2) dramatically increase the effective absorption length of luminescent centers, and 3) strongly improve the fluorescence quantum yield of luminescent centers. It is found that about 80% solar photons can be re-emitted even for a low fluorescent quantum yield of 0.5, and 80% re-emitted photons can be coupled to the slot waveguide. This LSC has the potential to be constructed in to a tandem structure which can absorb nearly full-spectrum solar photons, and also may be of special interest for building integrated nano-solar-cell applications.

Polarization properties of two-dimensional slot waveguides

We propose and study slot waveguide geometries where both quasi-TE and quasi-TM modes may propagate highly confined within the same low-index slot region. Conventional horizontal and vertical slot waveguides can only provide high slot confinement for either the quasi-TM or quasi-TE modes, respectively. Different two-dimensional slot waveguide structures are analyzed in terms of their mode characteristics, such as the effective index, the confinement factor, and the overlap of quasi-TE and -TM modes within the slot. Attention is also paid to practical manufacturability. Various waveguide structures can be tailored to have zero birefringence or equal confinement at both polarizations. Values for the confinement factors and the overlap of the two polarizations, in the slot region, can reach 0.4 to 0.5.

Demonstration of slot-waveguide structures on silicon nitride / silicon oxide platform

We report on the first demonstration of guiding light in vertical slot-waveguides on silicon nitride/silicon oxide material system. Integrated ring resonators and Fabry-Perot cavities have been fabricated and characterized in order to determine optical features of the slot-waveguides. Group index behavior evidences guiding and confinement in the low-index slot region at O-band (1260-1370nm) telecommunication wavelengths. Propagation losses of <20 dB/cm have been measured for the transverse-electric mode of the slot-waveguides.

Horizontal single and multiple slot waveguides: optical transmission at λ = 1550 nm

We experimentally demonstrate the optical transmission at 1550 nm of the fundamental slot modes (quasi-TM modes) in horizontal single and multiple slot waveguides and ring resonators consisting of deposited amorphous silicon and silicon dioxide. We demonstrate that the horizontal multiple slot configuration provides enhanced optical confinement in low index slot regions compared to a horizontal single slot structure with the same total SiO2 layer thickness by comparing their thermo-optic coefficients for the horizontal slot ring resonators. We show in these early structures that horizontal slot waveguides have low propagation loss of 6 approximately 7 dB/cm. The waveguide loss is mainly due to a-Si material absorption. The addition of a-Si/SiO(2) interfaces does not introduce significant scattering loss in a horizontal multiple slot waveguide compared to a horizontal single slot waveguide.