Fabrication Imperfections Research Articles

On-chip Fourier-transform spectrometers and machine learning: a new route to smart photonic sensors.

Miniaturized silicon photonics spectrometers capable of detecting specific absorption features have great potential for mass market applications in medicine, environmental monitoring, and hazard detection. However, state-of-the-art silicon spectrometers are limited by fabrication imperfections and environmental conditions, especially temperature variations, since uncontrolled temperature drifts of only 0.1°C distort the retrieved spectrum precluding the detection and classification of the absorption features. Here we present a new strategy that exploits the robustness of machine learning algorithms to signal imperfections, enabling recognition of specific absorption features in a wide range of environmental conditions. We combine on-chip spatial heterodyne Fourier-transform spectrometers and supervised learning to classify different input spectra in the presence of fabrication errors, without temperature stabilization or monitoring. We experimentally show the differentiation of four different input spectra under an uncontrolled 10°C range of temperatures, about $ 100\times $100× increase in operational range, with a success rate up to 82.5% using state-of-the-art support vector machines and artificial neural networks.

Dimensional hierarchy of higher-order topology in three-dimensional sonic crystals

Wave trapping and manipulation are at the heart of modern integrated photonics and acoustics. Grand challenges emerge on increasing the integration density and reducing the wave leakage/noises due to fabrication imperfections, especially for waveguides and cavities at subwavelength scales. The rising of robust wave dynamics based on topological mechanisms offers possible solutions. Ideally, in a three-dimensional (3D) topological integrated chip, there are coexisting robust two-dimensional (2D) interfaces, one-dimensional (1D) waveguides and zero-dimensional (0D) cavities. Here, we report the experimental discovery of such a dimensional hierarchy of the topologically-protected 2D surface states, 1D hinge states and 0D corner states in a single 3D system. Such an unprecedented phenomenon is triggered by the higher-order topology in simple-cubic sonic crystals and protected by the space group {P}_{mbar{3}m}. Our study opens up a new regime for multidimensional wave trapping and manipulation at subwavelength scales, which may inspire future technology for integrated acoustics and photonics.

A Novel High-Symmetry Cobweb-Like Disk Resonator Gyroscope

A novel cobweb-like disk resonator gyroscope (CDRG) with a very small frequency split between two degenerate modes has been designed and implemented. In comparison with the ring-like DRG (RDRG), the CDRG has fewer fabrication imperfections owing to its all-linear structure. In addition, the CDRG displays a superior nonlinear effect compared with the RDRG, and internal differential electrode excitation and detection were used to further reduce the nonlinear effect. The CDRGs and RDRGs were placed side by side on the same wafer, and the characteristics of these as-fabricated resonators were then tested. The results showed that the resonant frequencies and Q-factors of the CDRGs and RDRGs were approximately 18 kHz and nearly 100k, respectively. The smallest as-fabricated relative frequency split (29.9 ppm) of the CDRGs was about 10.8 times smaller than that (322.5 ppm) of an RDRG built side by side on the same wafer, which had the lowest relative frequency split among all current disk resonator gyros. Finally, the preliminary results for the performance of the CDRG and RDRG in the force-to-rebalance mode and mode matching are given. Under the same conditions, the maximum vibration displacement, scale factor, angular random walk, and bias instability of the CDRG were improved by 62.8%, 112%, 700%, and 314%, respectively, compared with the RDRG.

Static and Dynamic Properties of Coherently-Coupled Photonic-Crystal Vertical-Cavity Surface-Emitting Laser Arrays

We report progress on phased 850 nm vertical-cavity surface-emitting laser arrays that can be designed to operate reliably through control of individual injection current to each laser element. In this manner, nearly all fabricated arrays can be biased to be mutually phase locked. This paper attempts to visualize and characterize the locking region between two neighboring lasers by measuring far-field visibility to create a two-dimensional visibility map. Experimental data show that the locking region can be shifted or changed by ambient temperature, asymmetric array design, and fabrication imperfections. Additionally, the relative intensity noise and harmonic distortion are measured to characterize the dynamic behavior of coherently coupled laser arrays in the locking region. Two-dimensional maps of total relative intensity noise and total harmonic distortion are created to visualize laser array dynamics in the locking region. The data show that the laser array must be carefully designed, biased, and modulated within the locking region to achieve an output signal with low noise, low distortion, and improved modulation performance.

Topologically enabled ultrahigh-Q guided resonances robust to out-of-plane scattering.

Because of their ability to confine light, optical resonators1-3 are of great importance to science and technology, but their performance is often limited by out-of-plane-scattering losses caused by inevitable fabrication imperfections4,5. Here we theoretically propose and experimentally demonstrate a class of guided resonances in photonic crystal slabs, in which out-of-plane-scattering losses are strongly suppressed by their topological nature. These resonances arise when multiple bound states in the continuum-each carrying a topological charge6-merge in momentum space and enhance the quality factors Q of all nearby resonances in the same band. Using such resonances in the telecommunication regime,we experimentally achieve quality factors as high as 4.9×105-12 times higher than those obtained with standard designs-and this enhancement remains robust for all of our samples. Our work paves the way for future explorations of topological photonics in systems with open boundary conditions and for their application to the improvement of optoelectronic devices in photonic integrated circuits.

Disorder-Immune Photonics Based on Mie-Resonant Dielectric Metamaterials.

When the feature size of photonic structures becomes comparable or even smaller than the wavelength of light, the fabrication imperfections inevitably introduce disorder that may eliminate many functionalities of subwavelength photonic devices. Here we suggest a novel concept to achieve a robust band gap which can endure disorder beyond 30% as a result of the transition from photonic crystals to Mie-resonant metamaterials. By utilizing Mie-resonant metamaterials with high refractive index, we demonstrate photonic waveguides and cavities with strong robustness to position disorder, thus providing a novel approach to the band-gap-based nanophotonic devices with new properties and functionalities.

On-chip Fourier-transform spectrometer based on spatial heterodyning tuned by thermo-optic effect

Miniaturized optical spectrometers providing broadband operation and fine resolution have an immense potential for applications in remote sensing, non-invasive medical diagnostics and astronomy. Indeed, optical spectrometers working in the mid-infrared spectral range have garnered a great interest for their singular capability to monitor the main absorption fingerprints of a wide range of chemical and biological substances. Fourier-transform spectrometers (FTS) are a particularly interesting solution for the on-chip integration due to their superior robustness against fabrication imperfections. However, the performance of current on-chip FTS implementations is limited by tradeoffs in bandwidth and resolution. Here, we propose a new FTS approach that gathers the advantages of spatial heterodyning and optical path tuning by thermo-optic effect to overcome this tradeoff. The high resolution is provided by spatial multiplexing among different interferometers with increasing imbalance length, while the broadband operation is enabled by fine tuning of the optical path delay in each interferometer harnessing the thermo-optic effect. Capitalizing on this concept, we experimentally demonstrate a mid-infrared SiGe FTS, with a resolution better than 15 cm−1 and a bandwidth of 603 cm−1 near 7.7 μm wavelength with a 10 MZI array. This is a resolution comparable to state-of-the-art on-chip mid-infrared spectrometers with a 4-fold bandwidth increase with a footprint divided by a factor two.

Topological Nanophotonics: Toward Robust Quantum Circuits

In the last five years, the combination of advances in nanofabrication and a series of ingenious design ideas has enabled the implementation of topological concepts into nanophotonic platforms. In parallel, an unprecedented amount of research effort is being devoted to quantum technologies, with quantum information processing promising exponentially better performance than its classic counterpart for certain tasks. Nanophotonics is bound to play a key role in quantum information technologies given its compactness, low-power requirements, room-temperature operation, and the inherent high speed and low noise of photons. However, the scalability of quantum information systems in integrated photonic platforms is limited by scattering loss and other errors from random fabrication imperfections. The remarkable robustness to disorder and imperfections manifested by photonic states in topological systems emerges as a fascinating research avenue to tackle this issue. Furthermore, the ease of engineering and fabricating nanophotonic platforms with distinctive geometries and symmetries that can effectively generate and transport complex quantum states makes nanophotonics an ideal platform in which to realize topological protection of quantum photonic states. This article highlights the pioneering experimental efforts to understand the potential of topological protection on quantum photonic states and to outline a path toward robust quantum circuits.

Motion Planning for Point-to-Point Navigation of Spherical Robot Using Position Feedback

Nonlinear dynamics, slip, and irregularities in the surface or the spherical hull are the major factors for errors in the spherical robot system model. We attempt a controller design based on the experimental observations on spherical robot motion and identify the primitive motions that are reliable. The objective of the controller is to use these primitive motions and position feedback to guarantee convergence to the destination position. It is shown that the robot converges to a bounded region at the destination, when the modeling and measurement errors are bounded. The control law is applied to the navigation of a pendulum type spherical robot. The proposed navigation technique is not specific to the nonlinear spherical robot model and gives advantage by using limited control commands. Experiments in indoor and outdoor settings support the theory presented in this work. Experiments also show that the proposed controller rejects disturbances due to imperfect fabrication and contact surfaces with changing frictional properties. In particular, the robot navigates to the vicinity of the destination within a known range.

3-D Printed Monolithic GRIN Dielectric-Loaded Double-Ridged Horn Antennas

Monolithic integration of 3-D printed gradient index (GRIN) dielectric loading with a double-ridged rectangular horn standard gain horn (SGH) and its impact on antenna performance are demonstrated. Results show that the GRIN loaded horns improve sidelobe levels (SLLs) and beam symmetry, while maintaining good impedance match and relatively high radiation efficiency over more than an octave bandwidth from 7.5 to 18 GHz. Moreover, they have low weight and show good robustness to fabrication imperfections. This work paves the way for seamless integration of complex wideband antennas in 3-D printed systems such as unmanned aerial vehicles (UAVs).

Sub-decibel silicon grating couplers based on L-shaped waveguides and engineered subwavelength metamaterials

The availability of low-loss optical interfaces to couple light between standard optical fibers and high-index-contrast silicon waveguides is essential for the development of chip-integrated nanophotonics. Input and output couplers based on diffraction gratings are attractive coupling solutions. Advanced grating coupler designs, with Bragg or metal mirror underneath, low- and high-index overlays, and multi-level or multi-layer layouts, have proven less useful due to customized or complex fabrication, however. In this work, we propose a rather simpler in design of efficient off-chip fiber couplers that provide a simulated efficiency up to 95% (-0.25 dB) at a wavelength of 1.55 µm. These grating couplers are formed with an L-shaped waveguide profile and synthesized subwavelength grating metamaterials. This concept jointly provides sufficient degrees of freedom to simultaneously control the grating directionality and out-radiated field profile of the grating mode. The proposed chip-to-fiber couplers promote robust sub-decibel coupling of light, yet contain device dimensions (> 120 nm) compatible with standard lithographic technologies presently available in silicon nanophotonic foundries. Fabrication imperfections are also investigated. Dimensional offsets of ± 15 nm in shallow-etch depth and ± 10 nm in linewidth's and mask misalignments are tolerated for a 1-dB loss penalty. The proposed concept is meant to be universal, which is an essential prerequisite for developing reliable and low-cost optical couplers. We foresee that the work on L-shaped grating couplers with sub-decibel coupling efficiencies could also be a valuable direction for silicon chip interfacing in integrated nanophotonics.

Monolithic Integration of CMOS Temperature Control Circuit and Si3N4 Microring Filters for Wavelength Stabilization Within Ultra Wide Operating Temperature Range

Microring resonators are widely used for both active and passive devices for their small footprint and low- power consumption. However, it is very challenging to achieve precise control of stability of the resonant wavelength, as the microring is very sensitive to fabrication imperfections and temperature variations. In this paper, we propose and demonstrate a new method of monolithic integration of complementary metal-oxide-semiconductor (CMOS) temperature control circuit and Si3N4 microring filters for resonant wavelength stabilization. Through CMOS back-end process, Si3N4 photonic layer is formed vertically above the CMOS circuit chip. Due to the automatic temperature control, the integrated Si3N4 microring filters are almost immune to environment temperature change. The temperature-dependent wavelength shift (TDWS) is decreased from 21.43 pm/°C to 1.43 pm/°C with the environment temperature tuning range from 30 °C to 65 °C.

A 20–26.5-GHz PCB Bandpass Filter Tuned With Contactless Tuners

This letter presents the first substrate integrated waveguide (SIW) ${k}$ -band filters tuned with contactless printed circuit board (PCB) tuners. Unlike previous efforts, the presented filters exhibit two key advantages that make them particularly attractive with respect to manufacturing, performance, and reliability: 1) they are based on low-cost PCB technology and 2) they employ contactless tuners that exhibit no hard contacts with the evanescent-mode (EVA) cavities they control. A proof-of-concept two-pole prototype with 3.2–3.4 dB loss is experimentally demonstrated in the 20–26.5 GHz band. The fabricated resonators employ a unique contactless tuner geometry that minimizes insertion loss (IL) by utilizing extended cavity sidewalls. This design improves IL by an average of 10 dB in the demonstrated band. This letter also discusses fabrication imperfections and their impact on filter performance.

Effects of various loading on the performance of MEMS cantilever beam for in-field tuning of sensors and actuators for high temperature and harsh environment applications

MEMS devices require active mechanism for tuning in field operation because post fabrication, design parameters may change due to residual stress, fabrication imperfections, temperature etc. Application of axial compressive (C) or tensile forces (T) allows one to implement this tuning. Effects of C and T forces, stress gradient (SG) and transverse loading is analyzed for futuristic materials like Gallium Nitride (GaN) and Silicon Carbide (SiC) for use in high temperatures and harsh environments. The effects of above forces on pull in voltage (VPI), bandwidth (BW) and resonance frequency (RF) are analyzed. Results for Aluminum cantilever beam show, that VPI decreases by ~ 1.2 times at low beam lengths of 200 µm and about 5 times at higher length of 800 µm when T force is changed to C under loading. Similar trends are holding for GaN and SiC except that VPI scales up in proportion to material’s Young’s modulus E. An analytical relations of VPI versus E and Poisson’s ratio‘ν’ are predicted. Effect of SG is also studied and it is found that although SG affects VPI within 10% range, application of axial C or T forces further change it within 20% range. Comparison of analytical results for VPI with Coventorware software shows a better agreement for low loading of 10% compared to full loading of 100%. Also, Log–Log plot of VPI versus L can be used to estimate the contribution of charge re-distribution and fringing field. Furthermore, BW decreases by 16 Hz when C is applied and increases by 66 Hz when T is applied for Aluminum cantilever with 400 µm length and 50 µm width. Similarly, RF decreases by 165 Hz in C and increases by 623 Hz for T loading. The predictions of our model agree well with experimental and FEM results (within 4.54% and 6.46% respectively).

Enhancing beam quality and optical intensity of partially coherent VCSEL array by on-chip integrating micro optical phase modulators

Micro electronically controlled liquid crystal optical phase modulators (LCOPMs) are on-chip integrated with partially coherent vertical cavity surface emitting laser (VCSEL) array to improve the array’s beam quality and optical intensity. The initial phase shift among array elements introduced by imperfect fabrication is compensated by adjusting the voltages loaded on the LCOPMs. Experimental results show that the beamwidth of the VCSEL array is reduced by 28% and the intensity ratio of the central lobe to the secondary lobe in the far-field is increased by 50% after phase compensation by the LCOPMs.

Coherency‐Broken Bragg Filters: Overcoming On‐Chip Rejection Limitations

AbstractSelective optical filters with high rejection levels are of fundamental importance for a wide range of advanced photonic circuits. However, the implementation of high‐rejection on‐chip optical filters is seriously hampered by phase errors arising from fabrication imperfections. Due to coherent interactions, unwanted phase‐shifts result in detrimental destructive interferences that distort the filter response, whatever the chosen strategy (resonators, interferometers, Bragg filters, etc.). State‐of‐the‐art high‐rejection filters partially circumvent the sensitivity to phase errors by means of active tuning, complicating device fabrication and operation. Here, a new approach based on coherency‐broken Bragg filters is proposed to overcome this fundamental limitation. Non‐coherent interaction among modal‐engineered waveguide Bragg gratings separated by single‐mode waveguides is exploited to yield effective cascading, even in the presence of phase errors. This technologically independent approach allows seamless combination of filter stages with moderate performance free of active control, providing a dramatic increase of on‐chip rejection. Based on this concept, on‐chip non‐coherent cascading of Si Bragg filters is experimentally demonstrated, achieving a light rejection exceeding 80 dB, the largest value reported for an all‐passive silicon filter.

Statistical properties of defect-dependent detachment strength in bioinspired dry adhesives.

Dry adhesives using surface microstructures inspired by climbing animals have been recognized for their potentially novel capabilities, with relevance to a range of applications including pick-and-place handling. Past work has suggested that performance may be strongly dependent on variability in the critical defect size among fibrillar sub-contacts. However, it has not been directly verified that the resulting adhesive strength distribution is well described by the statistical theory of fracture used. Using in situ contact visualization, we characterize adhesive strength on a fibril-by-fibril basis for a synthetic fibrillar adhesive. Two distinct detachment mechanisms are observed. The fundamental, design-dependent mechanism involves defect propagation from within the contact. The secondary mechanism involves defect propagation from fabrication imperfections at the perimeter. The existence of two defect populations complicates characterization of the statistical properties. This is addressed by using the mean order ranking method to isolate the fundamental mechanism. The statistical properties obtained are subsequently used within a bimodal framework, allowing description of the secondary mechanism. Implications for performance are discussed, including the improvement of strength associated with elimination of fabrication imperfections. This statistical analysis of defect-dependent detachment represents a more complete approach to the characterization of fibrillar adhesives, offering new insight for design and fabrication.

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Initial frequency split reduction of MEMS ring gyroscope based on cascaded springs geometrical compensation

Small initial frequency split is preferred to reduce the complexity of control circuits and enhance the device sensitivity for mode-matching MEMS gyroscopes. A microring gyroscope with cascaded rectangular beams as supporting springs fabricated by (100) single crystal silicon (SCS) is presented. Frequency split due to the anisotropic of Young's modulus of (100) SCS is geometrically compensated by adjusting the width of cascaded springs and adding an extra mass. The rectangular beams are designed to be wide enough to improve the immunity to fabrication imperfections, obtaining good manufacturing repeatability. The proposed method can largely attenuate the frequency difference of the gyroscope by using (100) SCS, which can reduce the processing cost and be easily integrated with CMOS circuit compared with (111) SCS. The test results show that the average initial frequency split of 15 different devices is 5.1 Hz for the ~10,430 Hz resonant frequency. The minimum and maximum splits are 2.3 Hz (0.022%) and 7.3 Hz (0.069%), respectively, with the standard deviation of 1.3 Hz.

Selectively toluene-filled photonic crystal fiber Sagnac interferometer with high sensitivity for temperature sensing applications

Abstract In this paper, we theoretically and numerically investigate toluene-filled polarization maintaining photonic crystal fibers (PMPCFs) for temperature sensing application via Sagnac interferometer. We designed a PMPCF with a birefringence of 8.68 × 10−5, a relative birefringence sensitivity of 26% for 5 °C variation and sensitivity of −11 nm/°C around 25 °C. We study the effects of hole dimensions and fabrication imperfections on the performance of the proposed PMPCF and demonstrate that the PMPCF has a good toleration to any geometrical induced imperfections in the fabrication process of the fiber. Furthermore, we show that the spectral range of the sensor can be easily improved (to values larger than 137 nm) by decreasing the infiltration length of the PMPCF.