High-efficiency self-focusing metamaterial grating coupler in silicon nitride with amorphous silicon overlay.

Efficient fiber-chip coupling interfaces are critically important for integrated photonics. Since surface gratings diffract optical signals vertically out of the chip, these couplers can be placed anywhere in the circuit allowing for wafer-scale testing. While state-of-the-art grating couplers have been developed for silicon-on-insulator (SOI) waveguides, the moderate index contrast of silicon nitride (SiN) presents an outstanding challenge for implementing efficient surface grating couplers on this platform. Due to the reduced grating strength, a longer structure is required to radiate the light from the chip which produces a diffracted field that is too wide to couple into the fiber. In this work, we present a novel grating coupler architecture for silicon nitride photonic integrated circuits that utilizes an amorphous silicon (α-Si) overlay. The high refractive index of the α-Si overlay breaks the coupler's vertical symmetry which increases the directionality. We implement subwavelength metamaterial apodization to optimize the overlap of the diffracted field with the optical fiber Gaussian mode profile. Furthermore, the phase of the diffracted beam is engineered to focalize the field into an SMF-28 optical fiber placed 55 µm above the surface of the chip. The coupler was designed using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations supported by genetic algorithm optimization. Our grating coupler has a footprint of 26.8 × 32.7 µm2 and operates in the O-band centered at 1.31 µm. It achieves a high directionality of 85% and a field overlap of 90% with a target fiber mode size of 9.2 µm at the focal plane. Our simulations predict a peak coupling efficiency of - 1.3 dB with a 1-dB bandwidth of 31 nm. The α-Si/SiN grating architecture presented in this work enables the development of compact and efficient optical interfaces for SiN integrated photonics circuits with applications including optical communications, sensing, and quantum photonics.

High-Efficiency Metamaterial-Engineered Grating Couplers for Silicon Nitride Photonics.

Silicon nitride (Si3N4) is an ideal candidate for the development of low-loss photonic integrated circuits. However, efficient light coupling between standard optical fibers and Si3N4 chips remains a significant challenge. For vertical grating couplers, the lower index contrast yields a weak grating strength, which translates to long diffractive structures, limiting the coupling performance. In response to the rise of hybrid photonic platforms, the adoption of multi-layer grating arrangements has emerged as a promising strategy to enhance the performance of Si3N4 couplers. In this work, we present the design of high-efficiency surface grating couplers for the Si3N4 platform with an amorphous silicon (α-Si) overlay. The surface grating, fully formed in an α-Si waveguide layer, utilizes subwavelength grating (SWG)-engineered metamaterials, enabling simple realization through single-step patterning. This not only provides an extra degree of freedom for controlling the fiber-chip coupling but also facilitates portability to existing foundry fabrication processes. Using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations, a metamaterial-engineered grating coupler is designed with a coupling efficiency of -1.7 dB at an operating wavelength of 1.31 µm, with a 1 dB bandwidth of 31 nm. Our proposed design presents a novel approach to developing high-efficiency fiber-chip interfaces for the silicon nitride integration platform for a wide range of applications, including datacom and quantum photonics.

Dirac gratings.

We propose the concept of a Dirac grating, where periodic permittivity perturbations approach a train of Dirac functions. We show that Dirac gratings can yield identical spectral characteristics for higher-order gratings compared to first-order gratings of the same length. Using an inverse Fourier transform technique, we design different types of Dirac gratings, including structures operating at the exceptional point where parity-time symmetry breaks down, producing unidirectional reflectance. We employ analytical and numerical techniques to validate our theory by modelling practical examples of Dirac gratings implemented in dielectric stacks and silicon nanophotonic waveguides.

Low-loss grating coupler based on inter-layer mode interference in a hybrid silicon nitride platform.

Surface grating couplers are an important component for interfacing photonic integrated circuits with optical fibers. However, conventional coupler designs typically provide limited performance due to low directionality and poor fiber-to-grating field overlap. The efficiency can be improved by using non-uniform grating structures at the expense of small critical dimensions complicating the fabrication process. While uniform gratings can alleviate this constraint, they produce an exponentially decaying near-field with the Gaussian fiber mode overlap limited to a theoretical maximum of 80%. In this work, we propose a uniform grating coupler that circumvents this field overlap limitation. This is achieved by leveraging inter-layer mode interference through a virtual directional coupler effect in a hybrid amorphous-silicon (α-Si) on silicon nitride (Si3N4) platform. By optimizing the inter-layer gap and grating geometry, a near-Gaussian profile of the out-radiated beam is achieved, resulting in an unprecedented grating-to-fiber overlap of 96%. The full three-dimensional (3D) finite-difference time-domain (FDTD) simulations show a high directionality of 84% and a record coupling loss of -1.27 dB with a 1-dB bandwidth of 20 nm for the uniform grating coupler design. Our device is designed for a wavelength of 950 nm aimed for use in hybrid quantum photonic integrated circuits using III-V quantum dot single photon sources.

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.

Highly efficient ultra-broad beam silicon nanophotonic antenna based on near-field phase engineering.

Optical antennas are a fundamental element in optical phased arrays (OPA) and free-space optical interconnects. An outstanding challenge in optical antenna design lies in achieving high radiation efficiency, ultra-compact footprint and broad radiation angle simultaneously, as required for dense 2D OPAs with a broad steering range. Here, we demonstrate a fundamentally new concept of a nanophotonic antenna based on near-field phase-engineering. By introducing a specific near-field phase factor in the Fraunhofer transformation, the far-field beam is widened beyond the diffraction limit for a given aperture size. We use transversally interleaved subwavelength grating nanostructures to control the near-field phase. A Bragg reflector is used at the end of the grating to increase both the efficiency and the far-field beam width. The antenna has a compact footprint of 3.1 µm × 1.75 µm and an ultra-broad far-field beam width of 52° and 62° in the longitudinal and transversal direction, respectively, while the radiation efficiency reaches 82% after incorporating a bottom reflector to further improve the directionality. This unprecedented design performance is achieved with a single-etch grating nanostructure in a 300-nm SOI platform.

Athermal echelle grating and tunable echelle grating demultiplexers using a Mach-Zehnder interferometer launch structure.

We present a comparative experimental study of three silicon photonic echelle grating demultiplexers that are integrated with a Mach-Zehnder interferometer (MZI) launch structure. By appropriate choice of the MZI configuration, the temperature induced shift of the demultiplexer channel wavelengths can be suppressed (athermal) or enhanced (super-thermal) or be controlled by an on-chip micro-heater. The latter two configurations allow the channel wavelengths to be actively tuned using lower power than possible by temperature tuning a conventional echelle demultiplexer. In the athermal configuration, the measured channel spectral shift is reduced to less than 10 pm/°C, compared to the 83 pm/°C shift for an unmodified echelle device. In super-thermal operation an enhanced channel temperature tuning rate of 170 pm/°C is achieved. Finally, by modulating the MZI phase with an on-chip heater, the demultiplexer channels can be actively tuned to correct for ambient temperature fluctuations up to 20 °C, using a drive current of less than 20 mA.

Fiber Fabry-Perot astrophotonic correlation spectroscopy for remote gas identification and radial velocity measurements.

We present a novel, to the best of our knowledge, remote gas detection and identification technique based on correlation spectroscopy with a piezoelectric tunable fiber-optic Fabry-Perot filter. We show that the spectral correlation amplitude between the filter transmission window and gas absorption features is related to the gas absorption optical depth, and that different gases can be distinguished from one another using their correlation signal phase. Using a previously captured telluric-corrected high-resolution near-infrared spectrum of Venus, we show that the radial velocity of Venus can be extracted from the phase of higher order harmonic lock-in signals. This correlation spectroscopy technique has applications in the detection and radial velocity determination of weak spectral features in astronomy and remote sensing. We experimentally demonstrate a remote CO2 detection system using a lock-in amplifier, fiber-optic Fabry-Perot filter, and single channel avalanche photodiode.

Low-loss off-axis curved waveguide grating demultiplexer.

Current optical communication systems rely on the use of wavelength division multiplexing (WDM) to keep up with the increasing data rate requirements. The wavelength demultiplexer is the key component to implement WDM systems. In this Letter, we design and experimentally demonstrate a demultiplexer based on a curved grating waveguide geometry that separates eight channels with a spacing of 10 nm (1249 GHz) around the central wavelength of 1550 nm. The fabricated device shows very low insertion loss (∼1dB) and a crosstalk (XT) below -25dB. This device leverages metamaterial index engineering to implement the lateral cladding on one side of the waveguide. This makes it possible to design a waveguide grating with highly directional lateral emission by operating in a regime where diffraction into the silica upper cladding is frustrated, thus suppressing losses due to off-chip radiation.

Efficient Bloch mode calculation of periodic systems with arbitrary geometry and open boundary conditions in the complex wavevector domain.

We show how existing iterative methods can be used to efficiently and accurately calculate Bloch periodic solutions of Maxwell's equations in arbitrary geometries. This is carried out in the complex-wavevector domain using a commercial frequency-domain finite-element solver that is available to the general user. The method is capable of dealing with leaky Bloch mode solutions, and is extremely efficient even for 3D geometries with non-trivial material distributions. We perform independent finite-difference time-domain simulations of Maxwell's equations to confirm our results. This comparison demonstrates that the iterative mode finder is more accurate, since it provides the true solutions in the complex-wavevector domain and removes the need for additional signal processing and fitting. Due to its efficiency, generality and reliability, this technique is well suited for complex and novel design tasks in integrated photonics, and also for a wider range of photonics problems.

Millimeter-long metamaterial surface-emitting antenna in the silicon photonics platform.

Integrated optical antennas are key components for on-chip light detection and ranging technology (LIDAR). In order to achieve a highly collimated far field with reduced beam divergence, antenna lengths on the order of several millimeters are required. In the high-index contrast silicon photonics platform, achieving such long antennas typically demands weakly modulated gratings with lithographic minimum feature sizes below 10 nm. Here, we experimentally demonstrate a new, to the best of our knowledge, strategy to make long antennas in silicon waveguides using a metamaterial subwavelength grating (SWG) waveguide core loaded with a lateral periodic array of radiative elements. The mode field confinement is controlled by the SWG duty cycle, and the delocalized propagating mode overlaps with the periodic perturbations. With this arrangement, weak antenna radiation strength can be achieved while maintaining a minimum feature size as large as 80 nm. Using this strategy, we experimentally demonstrate a 2-millimeter-long, single-etched subwavelength-engineered optical antenna on a conventional 220 nm SOI platform, presenting a measured far-field beam divergence of 0.1° and a wavelength scanning sensitivity of 0.13°/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.

High-efficiency conversion from waveguide mode to an on-chip beam using a metamaterial engineered Bragg deflector.

Diffraction gratings that redirect light propagating in a channel waveguide to an on-chip slab are emerging as important building blocks in integrated photonics. Such distributed Bragg deflectors enable precise shaping of slab confined beams for a variety of applications, including wavelength multiplexing, optical phased array feeding, and coupling interfaces for on-chip point-to-point communications. However, these deflectors suffer from significant losses caused by off-chip radiation. In this Letter, we show, for the first time, to the best of our knowledge, that off-chip radiation can be dramatically reduced by using the single-beam phase matching condition and subwavelength metamaterial refractive index engineering. We present a deflector design with losses below 0.3 dB, opening a path toward new applications of distributed Bragg deflectors in integrated photonics.

Compact and highly-efficient broadband surface grating antenna on a silicon platform.

We present a compact silicon-based surface grating antenna design with a high diffraction efficiency of 89% (-0.5 dB) and directionality of 0.94. The antenna is designed with subwavelength-based L-shaped radiating elements in a 300-nm silicon core, maintaining high efficiency with a compact footprint of 7.6 µm × 4.5 µm. The reflectivity remains below -10 dB over the S, C and L optical communication bands. A broad 1-dB bandwidth of 230 nm in diffraction efficiency is achieved with a central wavelength of 1550 nm.

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.

Highly efficient optical antenna with small beam divergence in silicon waveguides.

Optical antennas are key components in optical phased arrays for light detection and ranging technology requiring long sensing range and high scanning resolution. To achieve a narrow beam width in the far-field region, antenna lengths of several millimeters or more are required. To date, such long antennas have been impossible to achieve in silicon waveguides because currently demonstrated technologies do not allow accurate control of grating strength. Here, we report on a new type of surface-emitting silicon waveguide with a dramatically increased antenna length of L=3.65mm. This is achieved by using a subwavelength metamaterial waveguide core evanescently coupled with radiative segments laterally separated from the core. This results in a far-field diffracted beam width of 0.025°, which is a record small beam divergence for a silicon photonics surface-emitting device. We also demonstrate that by using a design with L-shaped surface-emitting segments, the radiation efficiency of the antenna can be substantially increased compared to a conventional design, with an efficiency of 72% at the wavelength of 1550 nm.

Spectrum-free integrated photonic remote molecular identification and sensing.

Absorption spectroscopy is widely used in sensing and astronomy to understand remote molecular compositions. However, dispersive techniques require multichannel detection, reducing detection sensitivity while increasing instrument cost when compared to spectrophotometric methods. We present a novel non-dispersive infrared molecular detection and identification scheme that performs spectral correlation optically using a specially tailored integrated silicon ring resonator. We show experimentally that the correlation amplitude is proportional to the number of overlapping ring resonances and gas lines, and that molecular specificity can be achieved from the phase of the correlation signal. This strategy can enable on-chip detection of extremely faint remote spectral signatures.

Photonic temperature and wavelength metrology by spectral pattern recognition.

Spectral pattern recognition is used to measure temperature and generate calibrated wavelength/frequency combs using a single silicon waveguide ring resonator. The ring generates two incommensurate interleaving TE and TM spectral combs that shift independently with temperature to create a spectral pattern that is unique at every temperature. Following an initial calibration, the ring temperature can be determined by recognizing the spectral resonance pattern, and as a consequence, the wavelength of every resonance is also known. Two methods of pattern-based temperature retrieval are presented. In the first method, the ring is locked to a previously determined temperature set-point defined by the coincidence of only two specific TE and TM cavity modes. Based on a prior calibration at the set-point, the ring temperature and hence all resonance wavelengths are then known and the resulting comb can be used as a wavelength calibration reference. In this configuration, all reference comb wavelengths have been reproduced within a 5 pm accuracy across an 80 nm range by using an on-chip micro-heater to tune the ring. For more general photonic thermometry, a spectral correlation algorithm is developed to recognize a resonance pattern across a 30 nm wide spectral window and thereby determine ring temperature continuously to 50 mK accuracy. The correlation method is extended to simultaneously determine temperature and to identify and correct for wavelength calibration errors in the interrogating light source. The temperature and comb wavelength accuracy is limited primarily by the linewidth of the ring resonances, with accuracy and resolution scaling with the ring quality factor.

Subwavelength grating metamaterial waveguides functionalized with tellurium oxide cladding.

We report on the design, fabrication and characterization of subwavelength grating metamaterial waveguides coated with tellurium oxide. The structures are first fabricated using a standard CMOS compatible process on a silicon-on-insulator platform. Amorphous tellurium oxide top cladding material is then deposited via post-process RF magnetron sputtering. The photonic bandstructure is controlled by adjustment of the device geometry, opening a wide range of operating regimes, including subwavelength propagation, slow light and the photonic bandgap, for various wavelength bands within the 1550 nm telecommunications window. Propagation loss of 1.0 ± 0.1 dB/mm is reported for the tellurium oxide-cladded device, compared to 1.5 ± 0.1 dB/mm propagation loss reported for the silicon dioxide-cladded reference structure. This is the first time that a high-index (n > 2) oxide cladding has been demonstrated for subwavelength grating metamaterial waveguides, thus introducing a new material platform for on-chip integrated optics.

Perfectly vertical surface grating couplers using subwavelength engineering for increased feature sizes.

We present perfectly vertical grating couplers for the 220 nm silicon-on-insulator platform incorporating subwavelength metamaterials to increase the minimum feature sizes and achieve broadband low back-reflection. Our study reveals that devices with high coupling efficiencies are distributed over a wide region of the design space with varied back-reflections, while still maintaining minimum feature sizes larger than 100 nm and even 130 nm. Using 3D-finite-difference time-domain simulations, we demonstrate devices with broadband low back-reflection of less than -20dB over more than 100 nm bandwidth centered around the C-band. Coupling efficiencies of 72% and 67% are achieved for minimum feature sizes of 106 nm and 130 nm, respectively. These gratings are also more fabrication tolerant compared to similar designs not using metamaterials.