Single-slot Waveguide Research Articles

On-Chip Chemical Sensing Using Double-Slot Silicon Waveguide

In this article, we present refractive index measurement using a double-slot silicon waveguide-based Mach–Zehnder interferometer (MZI). We present a double-slot waveguide that offers the best sensitivity and limit of detection (LOD) compared to wire and single-slot waveguides. We demonstrate ultralow loss coupling between a single-mode waveguide and a double-slot waveguide and experimental proof for double-slot excitation. The double-slot waveguide is used to demonstrate a highly sensitive concentration sensor. An unbalanced Mach–Zehnder is used as the sensor device to demonstrate concentrations of potassium chloride (KCl) in deionized water. A sensitivity of 700 nm/RIU and a LOD of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7.142\times 10^{-{6}}$ </tex-math></inline-formula> RIU are achieved experimentally. To the best of our knowledge, the demonstrated sensitivity is the highest for an ON-chip-guided waveguide sensing scheme. Index Double-slot waveguide, evanescent field, Mach–Zehnder interferometer (MZI), ON-chip, refractive index, sensitivity.

High-frequency electric field intensity sensor based on double-slot waveguides filled with electro-optical polymer

We consider the structure of a high-frequency external electric field sensor based on microring resonators using waveguides with two horizontal or vertical slots filled with an electro-optical polymer. The method of lines is used to calculate the propagation constants and mode field distributions of such waveguides. It is shown that double-slot waveguides filled with an electro-optical polymer exhibit higher optical radiation intensity in the slot region compared to single-slot waveguides, which makes it possible to increase the sensor sensitivity. The distances between the slots and the values of their widths at which a maximum sensor sensitivity is achieved are determined. The sensor allows alternating electric fields with frequencies up to 10 GHz to be measured in the range of 100−16 × 106 V m−1 with an accuracy of 150 V m−1.

Mechanically Reconfigurable Slot Array Using Accordion-Like Microactuators

A novel mechanically reconfigurable 8×11 slot array using accordion-like microactuators as an alternative to conventional electronically steered arrays is reported. The array scans 0° to –30° in elevation by simultaneously varying the inter-element spacing between sliding slot elements. To maintain low slidelobe levels, the radiating slots are designed to have nonuniform width. The array is fed by a single slotted waveguide from one side. The actuator provides continuous motion with a precision of 30 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m (λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /625) and is controlled via a printed circuit board (PCB) circuit. The accordion-like structure provides a uniform yet tunable spacing between each slot column, eliminating the need for costly and complex electronic scanning. This high precision approach reduces the system footprint, ensures high power handling capability, and ensures ease of integration in future applications. The structure, operating at 16 GHz, is fabricated using a laser-etching process and in-house additive manufacturing. Measurement results are congruent with simulations.

Slab waveguide theory for general multi-slot waveguide

Optical devices based on slot waveguide are of considerable interest in numerous applications due to the distinct feature of strong electric field confinement in a low-refractive index region. A theoretical model based on multi-slab waveguide theory is used to reveal the physical mechanism of the slot waveguide. The calculation results derived from the basic Helmholtz equation for the conventional single-slot waveguide with a ~2% validation of the effective refractive index are compared to the former experiment results by the Cornell University group. Moreover, we extend the theoretical model to a general multi-slot waveguide. Its electric field distribution and key properties such as optical power confinement factor and enhancement factor in slot are deduced theoretically and fully discussed.

Spontaneous emission rate and optical amplification of Er3+ in double slot waveguide

The spontaneous emission (SE) of Er3+ embedded in a double slot dielectric structure was studied by a quantum-electrodynamical formalism. The study shows that the slot width and the position of Er3+ in slot structure have a significant effect on the SE. The double slot waveguides were fabricated by embedding two low-index Er/Yb silicate material layers into high-index silicon. The radiative efficiency of Er3+ in the double slot waveguides is found to be higher than that of the single slot waveguide, which is consistent with the theory simulation. The 0.67 dB signal enhancement at 1.53 µm in a 4.6-mm-long slot waveguide was observed pumped by 1476 nm laser. These results show the relevance of our model to study the SE processes in multilayer structures and are important for future realization of silicon-compatible active optical devices.

Increased bandwidth with flattened and low dispersion in a horizontal double-slot silicon waveguide

We propose a horizontal double-slot silicon waveguide to achieve flattened and low dispersion from −26 ps/(km·nm) to 21 ps/(km·nm) over an 878 nm wavelength range, which is much larger than that obtained from a single-slot waveguide. The mechanism of using this double-slot structure to increase the bandwidth is studied. Our study shows that the mode transition process from one strip mode to two slot modes helps extend the bandwidth of the flattened and low dispersion to longer wavelengths compared with a single-slot waveguide. Furthermore, we show by simulation the supercontinuum generation with a 3 dB bandwidth of 188 nm in an all-normal-dispersion profile with an input pulse of 50 fs.

Optimization of multiple-slot waveguides for biochemical sensing.

In this work, we analyze and optimize an optical biochemical sensor using silicon multiple-slot waveguides. The rigorous optimization procedure considers parameters such as ridge width, slot width, the number of slots, and the effect of residual silicon left at the bottom of the slot region. These parameters are then optimized using a figure of merit to achieve the highest possible sensitivity to bulk and surface changes in the upper cladding of the sensor. The multiple-slot structure is then studied in a bend configuration in order to construct ring-resonator-based sensors. A bulk sensitivity of 912nm/refractive index unit is achieved for a change in bulk refractive index, which is three times better than single-slot waveguides.

Simultaneous Optical Half-Adder and Half-Subtracter Using a Single-Slot Waveguide

We propose a simple approach for simultaneous optical half-adder and half-subtracter using a single-slot waveguide that offers tight light confinement and enhanced nonlinearity. By exploiting the parametric depletion effect of twin degenerate four-wave mixing processes in the slot waveguide, we simulate 160-Gbit/s half-adder (A + B) and half-subtracter (A - B, B - A) and characterize the operation performance in terms of quality Q factor, extinction ratio ER, and eye opening EO. Two different nonlinear materials are considered as the slot region for comparison. One is silicon-nanocrystal (Si-nc) with Kerr nonlinear index of refraction (n2) of 4.8 ×10-17 m2/W and two-photon absorption (TPA) coefficient (βTPA) of 7 ×10-11 m/W at 1550 nm, and the other is a specific polydiacetylene, poly-[2,4 hexadiyne -1,6 diol-bis-( p -toluene sulfonate)] (PTS) with n2 = 6.25 ×10-17 m2/W and βTPA ≈ 0 at 1550 nm. PTS slot waveguide features superior performance of half-adder and half-subtracter compared with Si-nc slot waveguide under the same waveguide length and input signal power. Moreover, we study the performance dependence on the waveguide length and input power, providing an easy-to-follow theoretical basis for achieving the desired performance of half-adder and half-subtracter.

Analytical study of optimal design and gain parameters of double-slot plasmonic waveguides

We theoretically analyze guided modes in optically active and passive double-slot plasmonic waveguides. We show that for one of the two different mode symmetries supported by the waveguide, a most productive guiding condition can be realized by adjusting the thicknesses of the layers to optimal values. We also derive approximate analytic expressions to calculate the optimal geometrical parameters of the waveguide. Interestingly, our analysis shows that the propagation losses associated with the inverse mode symmetry of the double-slot waveguide are comparatively low, regardless of the dimensions of the waveguide. We further show that the propagation losses become the smallest in the limiting case of a single-slot (metal–dielectric–metal (MDM)) waveguide. For both double- and single-slot waveguides, we show that the gain required to overcome the losses can be reduced by choosing a dielectric with a low refractive index. We also derive accurate analytical expressions to readily estimate the critical gain and modal gain of the waveguides.

Broadband and highly efficient quadratic interactions in double-slot lithium niobate waveguides through phase matching

We propose and study in detail a phase-matched quadratic optical interaction in a realizable 2D double-slot lithium niobate (LN) waveguide. As opposed to the single-slot waveguide, the field can be well confined in the nanometric nonlinear material, and it is also more flexible for birefringence and dispersion design. The proposed compact double-slot structure could not only achieve form birefringence phase matching but also effectively enhance the modal overlap integral and expand the working wavelength. The calculated results on second harmonic generation show an extremely large bandwidth of ~40 nm. The modal overlap integral up to 0.035 W(3/2)/V can be realized by optimizing the waveguide geometry, and it is much better than previous results on single-slot waveguides. Its temperature dependence is low--around 25 °C. The geometry is practical considering the current micromachining technique of LN.

Bending Efficiency of Bent Multiple-Slot Waveguides

The bending efficiency of three-dimensional bent multiple-slot waveguides is studied by applying a combined method of effective-index and modified transfer-matrix methods. The effects of asymmetric structure, asymmetric slots, and asymmetric middle strips on the bending efficiency are investigated. We show that the bending efficiency can be improved by the use of asymmetric structures and asymmetric middle strips. The bending efficiency of different slot waveguides (up to quintuple-slot structure) is compared. It is revealed that although the single-slot waveguide in general provides the lowest bending loss for the same waveguide parameters, it is possible that the multiple-slot waveguide can present a lower bending loss than the single-slot one.

Design of a Compact Polarization Splitter in Horizontal Multiple-Slotted Waveguide Structures

A compact polarization splitter based on the directional coupler in horizontal multiple-slotted waveguide (HMSW) structures is proposed and designed by using a modified three-dimensional full-vectorial beam propagation method with perfectly matched layer absorbing boundary conditions. The results show that the birefringence of the HMSW is stronger than that of the single-slotted waveguide, leading to a reduction in length for the present device. Moreover, the HMSW can also enhance the optical confinement inside the slot regions. The coupling length of the directional coupler composed of two-parallel HMSWs as functions of the waveguide parameters are analyzed, and a compact device of 238.0 µm in length is achieved with the crosstalks of -27.9 and -35.6 dB for quasi-transverse-electric and quasi-transverse-magnetic modes, respectively. In addition, the evolution of the injected field along the propagation distance through the polarization splitter is also demonstrated.

Simulation Demonstration and Experimental Fabrication of a Multiple-Slot Waveguide

A multiple-slot waveguide is presented and demonstrated through theoretical simulation. Taking a double-slot waveguide as an example, the results show a nearly 30% enhancement of optical intensity compared with the traditional single-slot waveguide with the same slot gap width. Therefore, the E-field intensity of the slot can be increased by adding another smaller slot. A double-slot waveguide with oxide and air as low index slot materials is realized experimentally and the formation processes of the slots are analyzed.