Design of narrowband Bragg spectral filters in subwavelength grating metamaterial waveguides

Nanophotonic Bragg grating assisted Mach-Zehnder interferometers for O-band add-drop filters

Spectral filters are fundamental building blocks in integrated photonics. Bragg grating filters have been demonstrated in silicon waveguides with a wide range of spectral responses and are suitable for wavelength division multiplexing applications. However, retrieving Bragg grating reflections typically requires external components such as fiber optic circulators. In this work, we develop fully integrated add-drop filters based on cladding-modulated Bragg gratings incorporated in a Mach-Zehnder interferometer configuration. We design complex spectral filtering devices with single and dual-band flat-top responses for the specified bandwidth. Additionally, we propose a novel design methodology which aims to minimize phase errors within the filters. We experimentally demonstrate add-drop filters with single-band and two-band rejection spectra at the datacom O-band, fabricated on a 220-nm thick silicon-on-insulator platform. Our results show an insertion loss below 1 dB and a crosstalk of around -20 dB at the channel center for a 4.5-nm wavelength grid and 3-nm wide channels.

Thermally induced sideband generation in silicon-on-insulator cladding modulated Bragg notch filters

We investigate and experimentally demonstrate a cladding modulated Bragg grating superstructure as a dynamically tunable and reconfigurable multi-wavelength notch filter. A non-uniform heater element was implemented to periodically modulate the effective index of the grating. The Bragg grating bandwidth is controlled by judiciously positioning loading segments away from the waveguide core, resulting in a formation of periodically spaced reflection sidebands. The thermal modulation of a periodically configured heater elements modifies the waveguide effective index, where an applied current controls the number and intensity of the secondary peaks. The device was designed to operate in TM polarization near the central wavelength of 1550 nm and was fabricated on a 220-nm silicon-on-insulator platform, using titanium-tungsten heating elements and aluminum interconnects. We experimentally demonstrate that the Bragg grating self-coupling coefficient can be effectively controlled in a range from 7 mm^-1 to 110 mm^-1 by thermal tuning, with a measured bandgap and sideband separation of 1 nm and 3 nm, respectively. The experimental results are in excellent agreement with simulations.

Anisotropic leaky-like perturbation with subwavelength gratings enables zero crosstalk

Electromagnetic coupling via an evanescent field or radiative wave is a primary characteristic of light, allowing optical signal/power transfer in a photonic circuit but limiting integration density. A leaky mode, which combines both evanescent field and radiative wave, causes stronger coupling and is thus considered not ideal for dense integration. Here we show that a leaky oscillation with anisotropic perturbation rather can achieve completely zero crosstalk realized by subwavelength grating (SWG) metamaterials. The oscillating fields in the SWGs enable coupling coefficients in each direction to counteract each other, resulting in completely zero crosstalk. We experimentally demonstrate such an extraordinarily low coupling between closely spaced identical leaky SWG waveguides, suppressing the crosstalk by ≈40 dB compared to conventional strip waveguides, corresponding to ≈100 times longer coupling length. This leaky-SWG suppresses the crosstalk of transverse-magnetic (TM) mode, which is challenging due to its low confinement, and marks a novel approach in electromagnetic coupling applicable to other spectral regimes and generic devices.

Integrated Optical Filters with Hyperbolic Metamaterials

The growing development of nanotechnology requires the design of new devices that integrate different functionalities at a reduced scale. For on-chip applications such as optical communications or biosensing, it is necessary to selectively transmit a portion of the electromagnetic spectrum. This function is performed by the so-called band-pass filters. While several plasmonic nanostructures of complex fabrication integrated to optical waveguides have been proposed, hyperbolic metamaterials remain almost unexplored for the design of integrated band-pass filters at optical wavelengths. By making use of the effective medium theory and finite integration technique, in this contribution we numerically study an integrated device consisting of a one-dimensional hyperbolic metamaterial placed on top of a photonic waveguide. The results show that the filling fraction, period, and number of layers modify the spectral response of the device, but not for type II and effective metal metamaterials. For the proposed Au-TiO₂ multilayered system, the filter operates at a wavelength of 760 nm, spectral bandwidth of 100 nm and transmission efficiency above 40%. The designed devices open new perspectives for the development of integrated band-pass filters of small scale for on-chip integrated optics applications.

Polarization independent Bragg gratings using tilted subwavelength grating waveguide Bragg gratings

We propose and experimentally demonstrate a polarization independent subwavelength grating (SWG) waveguide Bragg grating (WBG) by using an SWG waveguide with tilted segments. By optimizing the tilting angle and other geometry parameters, such as the width and the length of the loading segments used to create the BG, we can obtain a zero birefringence tilted SWG waveguide and consequently, a polarization independent SWG WBG. In our simulations, the optimal tilting angle is ∼ 58°, whereas the optimal angle obtained in fabrication is ∼ 46°. This deviation is mainly due to fabrication errors, e.g., on the sidewall angle of the silicon segments. For the optimal tilting angle of 46°, the characterized Bragg wavelengths of the TE and TM modes are both ∼ 1517 nm. We believe that the proposed device can have applications in optical communications and interconnections.

Simulation of a High-Performance Polarization Beam Splitter Assisted by Two-Dimensional Metamaterials

It is challenging to simultaneously consider device dimension, polarization extinction ratio (PER), insertion loss (IL), and operable bandwidth (BW) to design a polarization beam splitter (PBS) that is extensively used in photonic integrated circuits. The function of a PBS is to separate polarizations of light, doubling the transmission bandwidth in optical communication systems. In this work, we report a high-performance PBS comprising two-dimensional subwavelength grating metamaterials (2D SWGMs) between slot waveguides. The 2D SWGMs exhibited biaxial permittivity by tailoring the material anisotropy. The proposed PBS showed PERs of 26.8 and 26.4 dB for TE and TM modes, respectively, and ILs of ~0.25 dB for both modes, with an unprecedented small footprint of 1.35 μm × 2.75 μm working at the wavelength λ = 1550 nm. Moreover, the present structure attained satisfactory PERs of >20 dB and ILs of <0.5 dB within an ultrabroad BW of 200 nm.

Complex spectral filters in silicon waveguides based on cladding-modulated Bragg gratings

Spectral filters are important building blocks for many applications in integrated photonics, including datacom and telecom, optical signal processing and astrophotonics. Sidewall-corrugated waveguide grating is typically the preferred option to implement spectral filters in integrated photonic devices. However, in the high-index contrast silicon-on-insulator (SOI) platform, designs with corrugation sizes of only a few tens of nanometers are often required, which hinders their fabrication. In this work, we propose a novel geometry to design complex Bragg filters with an arbitrary spectral response in silicon waveguides with laterally coupled Bragg loading segments. The waveguide core is designed to operate with a delocalized mode field, which helps reduce sensitivity to fabrication errors and increase accuracy on synthesized coupling coefficients and the corresponding spectral shape control. We present an efficient design strategy, based on the layer-peeling and layer-adding algorithms, that allows to readily synthesize an arbitrary target spectrum for our cladding-modulated Bragg gratings. The proposed filter concept and design methodology are validated by designing and experimentally demonstrating a complex spectral filter in an SOI platform, with 20 non-uniformly spaced spectral notches with a 3-dB linewidth as small as 210 pm.

Subwavelength structure enabled ultra-long waveguide grating antenna

Because of the high index contrast, current silicon photonics based optical phased arrays cannot achieve small beam divergence and large field-of-view simultaneously without increasing fabrication complexity. To resolve the dilemma, we propose an ultra-long waveguide grating antenna formed by placing subwavelength segments within the evanescent field of a conventional strip waveguide. Bound state in the continuum effect is leveraged to suppress the sidewall emission. As a proof of concept, we theoretically demonstrated a millimeter-long through-etched waveguide grating antenna with a divergence angle of 0.081° and a feature size compatible with current silicon photonics foundries.

Subwavelength grating waveguide filter based on cladding modulation with a phase-change material grating

Subwavelength engineering and utilizing phase-change materials with large contrast in their optical properties have become powerful design tools for integrated silicon photonics. Reversible phase-transition of phase-change materials such as Ge₂Sb₂Te₅ (GST) provide a new degree of freedom and open up the possibility of adding new functionalities to the designed devices. We present an optical filter based on a silicon subwavelength grating (SWG) waveguide evanescently coupled to phase-change material loading segments arranged periodically around the SWG core. The effect of the GST loading segments' geometry and their distance from the SWG core on the filter's central wavelength and bandwidth are studied with three-dimensional finite-difference time-domain simulations. The employment of GST in the structure adds a switching functionality with an extinction ratio of 28.8 dB. We also examine the possibility of using the proposed structure as a reconfigurable filter by controlling the partial crystallization of the GST offering a blueshift of more than 4 nm.

Polarization-dependent tuning of Bragg reflection enabled through tilted subwavelength grating waveguide Bragg gratings

We propose and demonstrate experimentally tilted subwavelength grating (SWG) waveguide Bragg gratings (WBGs). By tilting the SWG segments and optimizing the duty cycle, we can achieve polarization-dependent tuning of the spectral response of the SWG WBG, namely, the spectral response of the fundamental transverse electric (TE) mode shifts toward shorter wavelengths, while that for the transverse magnetic (TM) mode remains almost unchanged. In particular, for tilting angles of 5° and 30°, we can obtain a blueshift in the Bragg wavelength of 7 and 35 nm for the TE mode, while the Bragg wavelength for the TM mode remains within 0.5 nm. The proposed tilted SWG WBGs provide a novel method to manage polarization and/or obtain polarization-dependent wavelength selectivity with integrated WBG devices.

Narrowband Bragg filters based on subwavelength grating waveguides for silicon photonic sensing

Subwavelength grating (SWG) waveguides have been shown to provide enhanced light-matter interaction resulting in superior sensitivity in integrated photonics sensors. Narrowband integrated optical filters can be made by combining SWG waveguides with evanescently coupled Bragg gratings. In this paper, we assess the sensing capabilities of this novel filtering component with rigorous electromagnetic simulations. Our design is optimized for an operating wavelength of 1310 nm to benefit from lower water absorption and achieve narrower bandwidths than at the conventional wavelength of 1550 nm. Results show that the sensor achieves a sensitivity of 507 nm/RIU and a quality factor of 4.9 × 10⁴, over a large dynamic range circumventing the free spectral range limit of conventional devices. Furthermore, the intrinsic limit of detection, 5.1 × 10^-5 RIU constitutes a 10-fold enhancement compared to state-of-the-art resonant waveguide sensors.

Theoretical model of a subwavelength grating polarization beam splitter

This study presents a theoretical model of a subwavelength grating polarization beam splitter (SWGPBS) using a combination of the theory of thin-film interference and the effective medium theory for guided-mode resonance. The structural parameters of SWGPBSs at oblique incidence calculated by our theoretical models and electromagnetic wave simulation were in good agreement within the range of the predicted approximation error. Feasibility of the oblique incidence SWGPBSs was verified, and the physical limitations of the SWGPBSs were clarified. Because the design procedure of SWGPBSs was simplified with our theoretical analysis, the range of their applications can be expanded to other fields.

Efficient Mode Transfer on a Compact Silicon Chip by Encircling Moving Exceptional Points

Exceptional points (EPs) are branch point singularities of self-intersecting Riemann sheets, and they can be observed in a non-Hermitian system with complex eigenvalues. It has been revealed recently that dynamically encircling EPs by adiabatically changing the parameters of a system composed of lossy optical waveguides could lead to asymmetric (input-output) mode transfer. However, the length of the waveguides had to be considerable to ensure adiabatic evolution. Here we demonstrate that the parameters can change adiabatically along a smaller encircling loop by utilizing moving EPs, leading to significant shortening of the structures compared to fixed EPs. Meanwhile, the mode transmittance is remarkably improved and the transfer efficiency persists at ∼90%. Moving EPs are very promising for applications such as highly integrated broadband optical switches and convertors operating at telecommunication wavelengths.

Low-loss polysilicon subwavelength grating waveguides and narrowband Bragg reflectors in bulk CMOS

The performance of a photonic functional device in bulk CMOS has been limited by the high propagation loss in polysilicon strip waveguide. Based on the zero-process-change methodology, we successfully reduce the propagation loss of polysilicon waveguide from 112 dB/cm to only 38 dB/cm by solely engineering the waveguide geometry for the first time. Low propagation loss is attributed to a significantly reduced optical overlap factor of 0.09 to bulk polysilicon using subwavelength grating (SWG) waveguide design. These findings prompt us to demonstrate a narrowband SWG-based cladding-modulated Bragg reflector in bulk CMOS, which provides a full-width at half maximum (FWHM) of 1.63 nm, an extinction ratio of 24.5 dB, and a reduced temperature sensitivity of 27.3 pm/°C. Further reducing the FWHM to 0.848 nm is also achieved by decreasing the grating coupling strength. We believe the achievements made in this work validate a promising design path towards practical photonic-electronic applications in bulk CMOS.

Distributed Bragg deflector coupler for on-chip shaping of optical beams

In integrated optical circuits light typically travels in waveguides which provide both vertical and horizontal confinement, enabling efficient routing between different parts of the chip. However, for a variety of applications, including on-chip wireless communications, steerable phased arrays or free-space inspired integrated optics, optical beams that can freely propagate in the horizontal plane of a 2D slab waveguide are advantageous. Here we present a distributed Bragg deflector that enables well controlled coupling from a waveguide mode to such a 2D on-chip beam. The device consists of a channel waveguide and a slab waveguide region separated by a subwavelength metamaterial spacer to prevent uncontrolled leakage of the guided mode. A blazed grating in the waveguide sidewall is used to gradually diffract light into the slab region. We develop a computationally efficient strategy for designing gratings that generate arbitrarily shaped beams. As a proof-of-concept we design, in the silicon-on-insulator platform, a compact ×75 Gaussian beam expander and a partial beam deflector. For the latter, we also demonstrate a prototype device with experimental results showing good agreement with our theoretical predictions. We also demonstrate via a rigorous simulation that two such couplers in a back-to-back configuration efficiently couple light, suggesting that these devices can be used as highly directive antennas in the chip plane.

Label-free biosensing with a multi-box sub-wavelength phase-shifted Bragg grating waveguide

Sub-wavelength grating (SWG) metamaterials have been considered to provide promising solutions in the development of next-generation photonic integrated circuits. In recent years, increasied interest has been paid to silicon photonic planar biosensors based on SWG geometries for performance enhancement. In this work, we demonstrate a highly sensitive label-free phase-shifted Bragg grating (PSBG) sensing configuration, which consists of sub-wavelength block arrays in both propagation and transverse directions. By introducing salt serial dilutions and electrostatic polymers assays, bulk and surface sensitivities of the proposed sensor are characterized, obtaining measured results up to 579.2 nm/RIU and 1914 pm/nm, respectively. Moreover, the proposed multi-box PSBG sensor presents an improved quality factor as high as ∼ 8000 , roughly 3-fold of the microring-based counterpart, which further improves the detection limit. At last, by employing a biotin-streptavidin affinity assay, the capability for small molecule monitoring is exemplified with a minimum detectable concentration of biotin down to 2.28 × 10 - 8 M .

Bragg filter bandwidth engineering in subwavelength grating metamaterial waveguides

Bragg gratings are fundamental building blocks for integrated photonic circuits. In the high-index contrast silicon-on-insulator material platform, it is challenging to accurately control the grating strength and achieve narrow spectral bandwidths. Here we demonstrate a novel Bragg grating geometry utilizing a silicon subwavelength grating (SWG) waveguide with evanescently coupled periodic Bragg loading segments placed outside the SWG core. We report experimental 3 dB filter bandwidths in a range from 8 nm to 150 pm by adjusting the distance of the Bragg loading segments from the core and the relative phase shift of the segments on the two sides of the waveguide, with a structure that has a minimum feature size of 100 nm.

Tilted subwavelength gratings: controlling anisotropy in metamaterial nanophotonic waveguides

Subwavelength grating (SWG) structures are an essential tool in silicon photonics, enabling the synthesis of metamaterials with a controllable refractive index. Here we propose, for the first time to the best of our knowledge, tilting the grating elements to gain control over the anisotropy of the metamaterial. Rigorous finite difference time domain simulations demonstrate that a 45° tilt results in an effective index variation on the fundamental TE mode of 0.23 refractive index units, whereas the change in the TM mode is 20 times smaller. Our simulation predictions are corroborated by experimental results. We furthermore propose an accurate theoretical model for designing tilted SWG structures based on rotated uniaxial crystals that is functional over a wide wavelength range and for both the fundamental and higher order modes. The proposed control over anisotropy opens promising venues in polarization management devices and transformation optics in silicon photonics.

Subwavelength integrated photonics

In the late nineteenth century, Heinrich Hertz demonstrated that the electromagnetic properties of materials are intimately related to their structure at the subwavelength scale by using wire grids with centimetre spacing to manipulate metre-long radio waves. More recently, the availability of nanometre-scale fabrication techniques has inspired scientists to investigate subwavelength-structured metamaterials with engineered optical properties at much shorter wavelengths, in the infrared and visible regions of the spectrum. Here we review how optical metamaterials are expected to enhance the performance of the next generation of integrated photonic devices, and explore some of the challenges encountered in the transition from concept demonstration to viable technology.

Subwavelength engineering and asymmetry: two efficient tools for sub-nanometer-bandwidth silicon Bragg filters

Bragg filters stand as key building blocks of the silicon-on-insulator (SOI) photonics platform, allowing the implementation of advanced on-chip signal manipulation. However, achieving narrowband Bragg filters with large rejection levels is often hindered by fabrication constraints and imperfections. Here, we show that the combination of single-side corrugation asymmetry and subwavelength engineering provides a narrowband response with large corrugations, overcoming minimum feature size constraints of conventional Si Bragg filters. We comprehensively study the impact of the corrugation asymmetry in conventional and subwavelength single-etched SOI Bragg filters, showing their potential for bandwidth reduction. Finally, we experimentally demonstrate novel subwavelength geometry based on shifted corrugation teeth, achieving null-to-null bandwidths and rejections of 0.8 nm and 40 dB for the symmetric configuration and 0.6 nm and 15 dB for the asymmetric case.

Tunable optical-microwave filters optimized for 100 MHz resolution

New continuously tunable RF-spectrum analyzers, RF receivers, and RF signal generators are proposed and analyzed for the silicon-on-insulator integrated-photonic platform at the ~1550 nm wavelength. These RF system-on-a-chip applications are enabled by a new narrowband 2x2 Mach-Zehnder interferometer (MZI) tuned filters for reconfigurable multiplexing, demultiplexing and RF channel selection. The filter can be optimized for ~100 MHz 3-dB bandwidth (BW) by utilizing N closely coupled Bragg-grating resonators to form one effective waveguide resonator in the single-mode silicon nanowire used for each MZI arm. The number of periods M within each individual resonator is selected to engineer BW in the 0.1 to 1 GHz range. Butterworth design is employed. Continuous tuning of the 100 MHz-BW devices over 18.6 GHz has been simulated by using local micron-scale thermo-optical heater stripes on the MZI arms with a temperature rise from 0 to 48K. For the case of N = 3 and 100-nm silicon side teeth, some representative performance predictions are: insertion loss (IL) = -10.7 dB, BW = 80.5 MHz and L = 113 μm for M = 58; while IL = -0.74 dB, BW = 1210 MHz and L = 86 μm for M = 44.