Slotted Photonic Crystal Waveguide Research Articles

Extreme optical confinement in a slotted photonic crystal waveguide

Using Optical Coherence Tomography, we measure the attenuation of slow light modes in slotted photonic crystal waveguides. When the group index is close to 20, the attenuation is below 300 dB cm−1. Here, the optical confinement in the empty slot is very strong, corresponding to an ultra-small effective cross section of 0.02 μm2. This is nearly 10 times below the diffraction limit at λ = 1.5 μm, and it enables an effective interaction with a very small volume of functionalized matter.

Large group-index bandwidth product empty core slow light photonic crystal waveguides for hybrid silicon photonics

This paper investigates the slow light propagation in silicon on insulator wide slot photonic crystal waveguides (PCWs). Two design schemes are presented, relying on the dispersion engineering of hole lattice and slot, respectively. Mode patterns and band diagrams are calculated by 3D-plane wave expansion method. Then, coupling and slow light propagations are modeled using finite difference time domain method in a full Mach-Zehnder interferometer (MZI). Results show high amplitudes interference fringes and high coupling efficiencies. Fabrication and measurement of devices lead to slow light propagation with group indices above 50, while multiple scattering and localized modes near the band edge also observed. This study provides insights for losses in hollow core slot high group index waveguides.

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.

Contra-directional coupling into slotted photonic crystals for spectrometric applications.

We propose and demonstrate the concept of a contra-directional coupler between a W1 and a slotted photonic crystal waveguide. The bandwidth and operating wavelength of such a coupler can be controlled via its geometrical parameters, and power transfer is not periodic unlike in the more familiar codirectional case. Light of specific wavelengths can be extracted from the W1 mode into air slot modes using this design, with W1/slot coupling efficiencies of up to 99±1%, and waveguide extracted coupling efficiencies of up to 51±12% demonstrated experimentally. Combining several of these couplers in series, we demonstrate the spectral filtering functionality on-chip. The device therefore combines the well-known sensing function of the slotted waveguide geometry with the spectrometer function, thus uniting two essential biosensor functions in a monolithic device.

Slow-Light Optimization of Polymer-Infiltrated Slot Photonic Crystal Waveguide

A tunable slow light with wide bandwidth, large group index, and low dispersion is realized in polymer-infiltrated slot photonic crystal waveguide (PI-SPCW). The slow-light properties of PI-SPCW are first optimized by shifting the first and second rows of air holes adjacent to the slot. Using the three-dimensional (3-D) plane wave expansion method, the low dispersion slow light with nearly constant group indices of 55, 100, 172.5, and 222 over bandwidths of 9.0, 3.5, 1.1, and 0.8 nm are demonstrated. Then, the dynamic modulation of the optimized slow light in PI-SPCW is investigated. Thanks to the electro-optic effect of the infiltrated polymer, the central wavelength of flatband slow light could be flexibly tuned by the external driving voltage. For the slow light with high group index of 100, the modulation sensitivity up to -0.75 nm/V is observed and the effective bandwidth could be further enlarged to 11 nm. These achievements open the possibility for achieving a wide range of targeted optical functionalities in PI-SPCW that can be dynamically controlled according to the practical requirements and highlight the versatility, flexibility, and tunability offered by electro-optic effect in PI-SPCW.

Dispersion Engineering of Slow Light in Ellipse-Shaped-Hole Slotted Photonic Crystal Waveguide

Wideband slow light in an ellipse-shaped-hole slotted photonic crystal waveguide (ESPCW) has been theoretically demonstrated. By adjusting the length of major and minor axes of air holes and tuning the holes position of the second row adjacent to the slot, the group index and bandwidth of slow light can be engineered effectively at the same time. Through the optimization of ESPCW structures by the plane wave expansion (PWE) method, the nearly constant group indices at 46, 80.7, and 215.5 are obtained, and we accordingly attain the flat bandwidths of 4.81, 2.66, and 1.02 nm around the operating frequency of 1550 nm, while the variation of each group index is restricted within ±10% range. Meanwhile, the normalized delay-bandwidth product stays around 0.14, with minimal dispersion on the order of 10 $^6 $ ps $^{2}$ /km for all the cases.

High efficiency asymmetric directional coupler for slow light slot photonic crystal waveguides.

An asymmetric directional coupler scheme for the efficient injection of light into slow light slot photonic crystal waveguide modes is proposed and investigated using finite-difference time-domain simulation. Coupling wavelengths can be flexibly controlled by the geometrical parameters of a side-coupled subwavelength corrugated strip waveguide. This approach leads to a ~1dB insertion loss level up to moderately high light group indices (nG≈30) in wavelength ranges of 5-10nm. This work brings new opportunities to inject light into the slow modes of slot photonic crystal waveguides for on-chip communications using hybrid silicon photonics or sensing based on hollow core waveguides.

Integrated Photonic Electromagnetic Field Sensor Based on Broadband Bowtie Antenna Coupled Silicon Organic Hybrid Modulator

We present the design, fabrication and characterization of a compact and highly sensitive integrated photonic electromagnetic field sensor based on a silicon-organic hybrid modulator driven by a bowtie antenna. Slow-light effects in the electro-optic (EO) polymer refilled silicon slot photonic crystal waveguide (PCW), together with broadband electric field enhancement provided by the bowtie antenna, are utilized to enhance the interaction of microwaves and optical waves, enabling an ultra large effective in-device EO coefficient over 1000 pm/V and thus a high sensitivity. The EO polymer refilled slot PCW is designed for low-dispersion slow-light propagation, high poling efficiency, and high optical mode confinement inside the slot. The bowtie antenna acts not only as a receiving antenna, but also as poling electrodes during the fabrication process. A bowtie antenna integrated on doped silicon slot PCW is demonstrated to have a broad operational bandwidth, with a maximum resonance at the frequency of 10 GHz. The strongly enhanced broadband electric field is used to directly modulate the phase of the optical waves propagating through the slot PCW embedded inside the feed gap of the bowtie antenna. The phase modulation is then converted to intensity modulation using an external reference arm to form a Mach-Zehnder interferometer in our experimental setup. The sensing of electromagnetic field at 8.4 GHz is experimentally demonstrated, with a minimum detectable electromagnetic power density of 8.4 mW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , corresponding to a minimum detectable electric field of 2.5 V/m.

Dispersion optimization of slow light in slotted photonic crystal waveguide by selective air holes infiltration

This work proposed a methodology based on the liquid infiltration of slotted photonic crystal waveguide (SPCW). By choosing the refractive index that infiltrated in the first and second rows of air holes adjacent to the slot, respectively, SPCW was optimized to possess wideband slow light with large group index and low dispersion. The properties of SPCW were numerically simulated by plane wave expansion (PWE) method and finite-difference time-domain (FDTD) method. Simulation results showed that the designed SPCW could control the group index for the same SPCW with the nearly constant group index of 50, 68, 81, 150, and 200 over 7.5nm, 5.5nm, 3.1nm, 1.65nm, and 1.15nm. In addition, we demonstrated that this post-fabrication liquid infiltrated technology has the potential for realizing reconfigurable and tunable SPCW, in which the flexible wavelength range of SPCW can also be controlled by different liquid infiltration.

Wideband slow light with low dispersion in asymmetric slotted photonic crystal waveguides

A new procedure of designing slotted photonic crystal waveguides is proposed to achieve slow light with improved normalized delay-bandwidth product and low group velocity dispersion that is suitable for both the W1 defect mode and the slot mode. The lateral symmetry of the waveguide in our study is broken by shifting the air holes periodically along the slot axis. The conversion of the "flat band" from band-up slow light to band-down slow light is achieved for the W1 defect mode. The group index curves of the W1 mode change from U-like to step-like and the group indices of 47, 67 and 130 are obtained with the bandwidth over 7.2, 4.8, and 2.3nm around 1550nm, respectively. We also obtain the group indices of 42, 55, and 108 for the slot mode with the bandwidth over 6.2, 5.6, and 2.2nm, respectively. Then the low dispersion slow light propagation is numerically demonstrated by the finite-difference time-domain method.

Wide optical spectrum range, subvolt, compact modulator based on an electro-optic polymer refilled silicon slot photonic crystal waveguide

We design and demonstrate a compact and low-power, band-engineered, electro-optic (EO) polymer refilled silicon slot photonic crystal waveguide (PCW) modulator. The EO polymer is engineered for large EO activity and near-infrared transparency. A PCW step coupler is used for optimum coupling to the slow-light mode of the band-engineered PCW. The half-wave switching voltage is measured to be Vπ = 0.97 ± 0.02 V over an optical spectrum range of 8 nm, corresponding to the effective in-device r(33) of 1199 pm/V and V(π) × L = 0.291 ± 0.006 V × mm in a push-pull configuration. Excluding the slow-light effect, we estimate that the EO polymer is poled with an efficiency of 89 pm/V in the slot.

Polymer-Based Hybrid-Integrated Photonic Devices for Silicon On-Chip Modulation and Board-Level Optical Interconnects

The accelerating increase in information traffic demands the expansion of optical access network systems that require the cost reduction of optical and photonic components. Low cost, ease of fabrication, and integration capabilities of low optical-loss polymers make them attractive for photonic applications. In addition to passive wave-guiding components, electro-optic (EO) polymers consisting of a polymeric matrix doped with organic nonlinear chromophores have enabled wide-RF-bandwidth and low-power optical modulators. Beside board level passive and active optical components, compact on-chip modulators (a few 100 μm to a few millimeters) have been made possible by hybrid integration of EO polymers onto the silicon platform. This paper summarizes some of the recent progress in polymer-based optical modulators and interconnects. A highly linear, broadband directional coupler modulator for use in analog optical links and compact, and low-power silicon/polymer hybrid slot photonic crystal waveguide modulators for on chip applications are presented. Recently, cost-effective roll-to-roll fabrication of electronic and photonic systems on flexible substrates has been gaining interest. A low-cost imprinted/ink-jet-printed Mach–Zehnder modulator and board-to-board optical interconnects using microlens integrated 45° mirror couplers compatible with the roll-to-roll fabrication platforms are also presented.

A merged photonic crystal slot waveguide embedded in ALD-TiO_2

We demonstrate the concept of a merged nanoscale photonic crystal slot waveguide that acts as a bandpass filter in the near infrared region of the spectrum. The device is based on the integration of a photonic crystal cavity in a slot waveguide on a silicon on insulator substrate. The device is further embedded in amorphous titanium dioxide using atomic layer deposition, which allows to reduce two-photon absorption losses and creates the possibility to combine nonlinear guided-wave optics resulting from the strong field confinement in the slot region with slow light effects in the photonic crystal cavity. Our approach is fully compatible with complementary metal oxide semiconductor technology and opens up new perspectives for the integration of all-optical signal processing functionalities in hybrid silicon nanophotonics platforms.

Optimizing the slow light properties of slotted photonic crystal waveguide and its application in a high-sensitivity gas sensing system

To enhance the sensitivity of a gas sensing system, slotted photonic crystal waveguide (SPCW) was introduced as the gas cell, and its structure was properly optimized to obtain slow light with large bandwidth and high group index. Simulation results indicated that a relatively high group index of 110 over 2.2 nm could be achieved and the field enhancement factor would reach 12, by shifting the first and second rows of air holes adjacent to the slot. Combined with the correlation spectroscopy technology for signal processing, a detection limit of 3.15 ppm was achieved for measurement of acetylene concentration. Additionally, the active region was only 300 µm long, which could be of great advantage in the use of miniaturized absorption spectrometers.

Optimization of Slow Light in Slotted Photonic Crystal Waveguide With Liquid Infiltration

Slow light in slotted photonic crystal waveguide (SPCW) has been theoretically optimized with the post-fabrication technology of liquid infiltration. By choosing the refractive indices that infiltrated in the first and second rows of air holes adjacent to the slot, SPCW was optimized to possess wideband slow light with high group index and low dispersion. Simulation results showed that the slow light in SPCW for the same structure could be optimized with nearly constant group indices of 50, 82.5, 150, and 176 over 6.9 nm, 3.1 nm, 1.65 nm, and 1.15 nm bandwidths. Then, it was demonstrated that this method has the potential for realizing reconfigurable SPCW, in which the tunable slow light could be obtained. Besides, the capability to control the slow light properties after fabrication was interesting to compensate fabrication imperfections, and it was particularly interesting to realize temperature-stable SPCW devices. The preliminary results of this study have provided important theoretical basis for the potential application offered by the SPCW in future optical technologies.

Liquid refractive index sensor based on slow light in slotted photonic crystal waveguide

A high sensitive and compact refractive index sensor based on slotted photonic crystal waveguide (S-PhCW) is demonstrated. This design is worked on a Mach–Zehnder interferometer (MZI) configuration with S-PhCW as the measuring arm, which can be used to detect any changes in refractive index that correspond to different concentration of the measuring liquid. Combining the slow light enhancement in photonic crystal waveguide (PhCW) with the advantage of excellent optical confinement in slot waveguide, the sensitivity of this simple scheme can reach to 2.3×109nm/RIU with the active region of only 1mm long.

Multi-component gas sensing based on slotted photonic crystal waveguide with liquid infiltration

A high sensitive multi-component gas sensing system was proposed, in which the slotted photonic crystal waveguide (SPCW) was used as the gas cell and the fiber Fabry–Perot (FFP) was adopted as the wavelength filter. By choosing the refractive index that infiltrated in the first and second rows of air holes adjacent to the slot respectively, the working wavelength of SPCW could be tuned to matches with one absorption line of the measured gas, thus, the system could detect different kinds of gases selectively. Meanwhile, a very high group index of 150 with flat bandwidth could be achieved and the electric field enhancement factor would reach to 12. Combined with the wavelength modulation techniques (WMS) for signal processing, the minimum detection limit of 1.56ppm was achieved.

Slow light property in ring-shape-hole slotted photonic crystal waveguide

Slow light in photonic crystal waveguide is now being heavily investigated for applications in optical devices. A ring-shape-hole slotted PhCW (RSPhCW) is designed in the paper. Band structures and group indices are calculated through plane wave expansion method. Flat band slow light property is observed, which provides low group velocity dispersion and avoids high speed optics signal from distortion in transmission. A method is proposed by tuning the positions of the first and second rows with the ring-shape-holes adjacent to the slot, which can tune the group index and the bandwidth of the guided band at the same time. This method is used to optimize the slow light property of the RSPhCW. The bandwidths ranging from 1.8nm to 17.3nm for group indices from 15 to 67 are obtained. From the point of view of reality, this method has its advantage of fabricating than tuning the outer and inner radii of the ring-shape-holes.

Research on slow light for slotted photonic crystal waveguide with mixed dielectric constant

Slotted photonic crystal waveguide is formed by removing a row of rods and replaced by air rods in perfect photonic crystal. The dielectric constant of cylinders near slotted photonic crystal waveguide is changed and the mixed dielectric constant photonic crystal models are formed based on single line defect structure. The energy-band feature, group velocity and group velocity dispersion characteristics of slow-light mode for different mixed dielectric constant photonic crystal waveguide models are analyzed by plane wave expansion method. The simulation results show that with the increase of dielectric constant of cylinders near the waveguide, the guided frequency of defected mode decreases and the group velocity also reduces. The group velocity reduces 0.0775c compare with single line defect model while using the Fe3O4 semiconductor materials near the waveguide. Besides, the slow light transmission for models with mixed dielectric constant has flat GVD region which can ensure the efficient transmission of light.

Dispersion engineered slot photonic crystal waveguides for slow light operation

We introduce a novel design of wide Slot Photonic Crystal Waveguides (SPCW) by structuring the slot as a comb. This allows performing dispersion engineering in order to achieve very low group velocities over a few nanometers bandwidth. This kind of SPCW offers opportunities to realize devices requiring strong interactions between light and an optically nonlinear low index material by providing an ultrahigh optical density while easing the filling of the slot due to its width. We will present dispersion engineering results by the plane wave expansion method and finite difference time domain analysis, followed by experimental realization.