High Fabrication Tolerance Research Articles

Design of freeform geometries in a MEMS accelerometer with a mechanical motion preamplifier based on a genetic algorithm

This paper describes a novel, semiautomated design methodology based on a genetic algorithm (GA) using freeform geometries for microelectromechanical systems (MEMS) devices. The proposed method can design MEMS devices comprising freeform geometries and optimize such MEMS devices to provide high sensitivity, large bandwidth, and large fabrication tolerances. The proposed method does not require much computation time or memory. The use of freeform geometries allows more degrees of freedom in the design process, improving the diversity and performance of MEMS devices. A MEMS accelerometer comprising a mechanical motion amplifier is presented to demonstrate the effectiveness of the design approach. Experimental results show an improvement in the product of sensitivity and bandwidth by 100% and a sensitivity improvement by 141% compared to the case of a device designed with conventional orthogonal shapes. Furthermore, excellent immunities to fabrication tolerance and parameter mismatch are achieved.

Vortex Beam Detection Based on Plasmonic in Plane Zone-Plate

A plasmonic in-plane zone-plate (IPZP) is analytically proposed and numerically analyzed. The proposed scheme consists of several discretely metallic nano-slits, which couple incident vortices into SPP and form focal spots. Wave front information of incident beam is reflected in the field distribution along “focal plane” of IPZP, and therefore, the proposed structure can be used for topological charge detection. Besides, an orthogonal slit array IPZP achieving simultaneous spin and orbital angular momentum detection is elaborated. IPZP structure has relatively high fabrication tolerance and the parameters can be adjusted to fit various operating wavelength. The proposed structure is expected to find potential applications in orbital angular momentum communication and high-dimensional quantum entanglement.

Broadband and high-extinction-ratio polarization beam splitter on tilted subwavelength gratings waveguides.

A compact asymmetrical directional coupler (ADC), which is composed of a strip and a tilted subwavelength gratings (TSWG) waveguide, is proposed to realize a broadband and high-extinction-ratio polarization beam splitter (PBS). In addition to conventional SWG structural parameters, such as its period and duty cycle, a TSWG waveguide provides an extra degree of freedom to solely adjust the TE modal effective index for more accurate phase matching. Hence the TE polarization state is coupled more efficiently in the ADC system, and then the extinction ratio (ER) is consequently improved. The simulation of our proposed device demonstrated a higher coupling efficiency and an ultrahigh ER of 34.2 dB for the TM polarization state at the center wavelength around 1550 nm. The operating bandwidth of the TM polarization state defined as the spectral range, with ER higher than 20 dB, is determined to be as broad as 75 nm. And, with an assisted hybrid plasmonic waveguide at the end of the cross port, the achieved bandwidth of the TE polarization state is as broad as 133 nm. They are much broader than those of other DC/ADC implemented PBS devices. Moreover, further analysis shows that the tilting angle of the TSWG waveguide has much higher fabrication tolerance than changing SWG duty cycle.

A design method for high fabrication tolerance integrated optical mode multiplexer

The tapered asymmetric directional coupler has the potential to realize high fabrication tolerance and high transmission efficiency on-chip mode multiplexer. However, the geometry parameter selection of tapered structure remains empirical. In this paper, we propose a design method for the tapered structure based on genetic algorithm. Combined with the adjusted coupling equations and interpolation method, low-time-cost optimization can be realized. Three mode multiplexers ($\mathrm{TE}_{0}\&~\mathrm{TE}_{1}$, $\mathrm{TE}_{0}\&~\mathrm{TE}_{2}$ and $\mathrm{TE}_{0}\&~\mathrm{TE}_{5}$) are designed by our method. According to simulation results, the insertion loss of the designed devices is lower than 1.8 dB and the crosstalk is lower than $-15$ dB when the fabrication error is within required range ($\pm$10 nm for $\mathrm{TE}_{0}\&~\mathrm{TE}_{2}$ and $\mathrm{TE}_{0}\&~\mathrm{TE}_{5}$, and $\pm$20 nm for $\mathrm{TE}_{0}\&~\mathrm{TE}_{1}$) in the bandwidth of 1.5–1.6 $\mu$m. In addition, the entire optimization process takes only 2 h for each device, which is around the time cost of a single 3D simulation.

Compact Multi-Photons Quantum Interference Component for Integrated Quantum Optic Device

Complex multi-photon quantum interference states are indispensable requisites for integrated quantum optic processing and quantum communication technologies. Here, two mode interference (TMI) coupler has been presented to demonstrate HOM model of multi-photons (N > 2) Quantum interference. The experiment of three and four photons quantum interference have been performed and coincidence rate of three and four photons quantum interference have been measured and quantum visibilities are obtained as ∼ 0.783 ± 0.009 and ∼ 0.85 ± 0.009 respectively at 3 dB TMI coupling length of 11.5 μm. High fabrication tolerance of quantum fidelity of TMI coupler has been achieved due to having less number of design parameters in comparison to directional coupler and multimode interference coupler. Our results pave the way to achieve multi-photons quantum entanglement in compact device.

Silicon-embedded photonic crystal fiber for high birefringence and nonlinearity

In this study, we propose a hexagonal photonic crystal fiber (PCF) including an elliptical Si core for high nonlinearity and birefringence, simultaneously. The propagation properties are rigorously investigated by numerical full-vector analysis. The designed fiber provides high nonlinearity and birefringence for a wide range of wavelengths, from 1.25 to 2 μm, maintaining low propagation loss by virtue of a high nonlinear coefficient and large Si refractive index. The optimal values of nonlinearity and birefringence are 2 × 105 W−1 km−1 and 0.92, respectively. In addition, the proposed hexagonal PCF can be easily fabricated with a conventional draw technique compared with other PCFs composed of complex structures to achieve high nonlinearity. It also has a high fabrication tolerance for the rotational angle of the elliptical Si core. We believe that this type of the fiber could be applied to various efficient nonlinear applications.

Quasi-Phase Matched Second Harmonic Generation in Plasmonic–Organic Hybrid Structures

In this article, we propose the design and analyze a plasmonic-organic-hybrid (POH) waveguide structure for efficient forward and backward second harmonic generation (SHG) to bridge the mid-infrared (IR) to near-IR. The required phase matching is satisfied by quasi-phase matching (QPM) technique through periodically altering the material of the metal-insulator-metal slot region between a nonlinear polymer (with second-order nonlinear coefficient of χ (2) ≠ 0) and Al2O3 (with χ (2) = 0) leading to modulation of χ (2) coefficient along the propagation direction. The strong field confinement offered by surface plasmon polaritons (SPPs) significantly enhances the normalized efficiency up to 1.75 × 106 W−1cm−2 in the case of backward SHG over a short device length of 17 μm with only 20 mW input power. Also, a slightly lower efficiency is achieved for forward configuration with broadband response, high fabrication tolerance, and small temperature-variation sensitivity. Furthermore, our proposed structure is based on standard nanofabrication technologies and can be used for all-optical signal processing, low-noise detection of a mid-IR signal in the near-IR windows, and (mirrorless) optical parametric oscillator.

Silicon-Based Mode Converter and Demultiplexer for Wavelength Division Multiplexing Transmission by Using Multimode Interference Couplers

We present a mode converter and demultiplexer structure for wavelength division multiplexing (WDM) transmission by employing multimode interference (MMI) on Silicon-on-Insulator (SOI) platform. The mode converter and demultiplexer have a compact size of less than 2.7 μm × 43.7 μm. Moreover, the crosstalk between neighboring wavelength channel within C band (1530 nm to 1570 nm) can be reduced by utilizing the tapered phase shifter cascaded with MMI. The simulated results indicate that this structure has a low insertion loss of less than 1 dB, a low crosstalk of better than ?15 dB and a relatively high fabrication tolerance of ~10 nm. Such structure may find many potential applications in silicon photonic integrated devices.

Surface Plasmon Resonance Induced High Sensitivity Temperature and Refractive Index Sensor Based on Evanescent Field Enhanced Photonic Crystal Fiber

An ultra high-sensitivity temperature and refractive index (RI) sensor based on surface plasmon resonance (SPR) effect in D-shape high birefringence photonic crystal fiber (PCF) is presented and discussed by the finite element method (FEM). The designed PCF is polished to enhance the interaction of the evanescent field with the external medium, ultimately improving the sensing performance. The influence of three different plasma materials, gold, silver and aluminum, on the sensing performance was investigated. Finally, the chemically stable plasma material gold is selected to coat on flat surface of D-shape PCF for SPR sensing, realizing the sensitive real-time monitoring of RI and temperature. The sensing performance was greatly improved, with a wide RI (1.43 to 1.50) and temperature (36 $^{\circ }$ C to 86 $^{\circ }$ C) detection range, and the corresponding maximum sensitivity was 44850 nm/RIU and −16.875 nm/ $^{\circ }$ C, respectively. Furthermore, the sensitivity is not particularly sensitive to the sizes of the designed air holes, which leads to a sensor with high structure tolerance. Numerical results show that the coating thickness not only affects the sensitivity but also the detection range of the sensor, so we can tune the RI and temperature detection range by adjusting the gold film thickness to meet specific detection requirements. The sensor with enhanced sensitivity, tunable detection range, and high fabrication tolerance has a wide application prospect in the field of life science research, chemical production, and environmental monitoring.

Switching behavior engineerable, electro-optic directional couplers in aperiodic optical superlattice waveguides

We report on the first study and a proof-of-principle demonstration of aperiodically alternating-Δβcouplers in LiNbO3 waveguides whose switching behavior can be engineered with a high degree of freedom. A high fabrication-tolerance and broad working-bandwidth electro-optic (EO) coupler is developed based on a unique Δβ scheme derived using a simulated annealing algorithm and realized in Ti-diffused aperiodically poled lithium niobate (APPLN) waveguides. The waveguide fabrication tolerance and the working bandwidth (149.2 nm) of the APPLN EO coupler are found to be increased by 2.7 and 2.4 times, respectively, over that of a conventional periodically alternating-Δβ (PPLN) coupler for ≥99% power transfer efficiency under a fixed switching voltage at the 1550 nm band. The results even hold for a domain poling-width error of ≤9%. An interesting APPLN EO coupler capable of working on a certain coupling state over a broad voltage operating range is also studied. The technology developed in this work should enable the production of coupler devices whose switching characteristics can be tailored and realized with much more relaxed fabrication and bandwidth control, which would be beneficial for the application in efficient and compact integrated-optic circuits/systems.

Broadband and compact polarization beam splitter based on an asymmetrical directional coupler with extra optimizing designs.

In this paper, a novel, to the best of our knowledge, polarization beam splitter (PBS) based on an asymmetrical directional coupler (DC) was proposed, which consists of a strip waveguide (WG) and a ${{\rm Si}_3}{{\rm N}_4}$Si3N4 loaded horizontal slot WG. By carefully adjusting the geometric parameters of the DC, the phase match condition between these two WGs can be satisfied for the transverse magnetic (TM) polarization, while the coupling efficiency of the transverse electric (TE) polarization is frustrated due to the large phase mismatch. The extra optimizing designs include adding filters to the output ports as well as introducing the tapered structure into the DC, which is settled by the particle swarm optimizing (PSO) algorithm so that the performance of the proposed PBS is improved over a broadband range. Numerical simulations show that the bandwidths for the extinction ratio (ER) $ \gt {20}\;{\rm dB}$>20dB, 30 dB, and 40 dB are 160 nm, 95 nm, and 50 nm, respectively, with insertion loss (IL) $ \lt {1}\;{\rm dB}$<1dB for the wavelength of 1.49-1.58 µm. The analysis of the deviations demonstrates that the proposed PBS allows high fabrication tolerances.

High fabrication tolerance and broadband silicon polarization beam splitter by point-symmetric cascaded fast quasiadiabatic couplers

We propose a high fabrication tolerance and broadband silicon polarization beam splitter (PBS) by cascading point-symmetric 3-dB couplers designed using fast quasiadiabatic dynamics (FAQUAD). The FAQUAD strategy only requires access to a single control parameter, and it can shorten the conventional adiabatic designs, maintaining the broadband characteristic and robustness to fabrication errors. The 3-dB FAQUAD coupler for the TM0 mode is made invisible to the TE0 mode due to a large difference in their adiabaticity parameters. Moreover, the system of cascaded point-symmetric devices further enhances the bandwidth and fabrication tolerance of the FAQUAD PBS. The total length of the FAQUAD PBS is 89.4 µm. We find that the FAQUAD PBS exhibits > 10 dB extinction ratio (ER) for the input TM0 mode and the input TE0 mode over a bandwidth of 240 nm and 260 nm, with excess losses below 0.4 dB and 0.021 dB, respectively. The PBS also exhibits excellent fabrication tolerance, showing an ER > 24 dB and an excess loss 16.8 dB and an excess loss 15.3 dB and excess loss 15.9 dB and excess loss < 0.015 dB for the TE0 mode are observed.

Broadband absorption enhancement of monolayer graphene by prism coupling in the visible range

We experimentally demonstrated the broadband absorption enhancement of monolayer graphene embedded in the center of a dielectric cavity of the prism coupling configuration in the visible spectral range. The absorption peak is enhanced up to 86.1% at λ = 650 nm with a 3 dB bandwidth of 314 nm (from 542 nm to 856 nm). The absorption performance is the best experimental result for monolayer graphene in the visible range, to the best of knowledge. The improved absorption is attributed to enhanced light-graphene interaction via the cavity mode formed in the dielectric layer with the properly chosen graphene position. The proposed structure is suitable to a high-efficiency broadband photodetector based on monolayer graphene due to the simple fabrication process and the high fabrication tolerance.

Low-loss fiber-to-chip interface for lithium niobate photonic integrated circuits.

Integrated lithium niobate (LN) photonic circuits have recently emerged as a promising candidate for advanced photonic functions such as high-speed modulation, nonlinear frequency conversion, and frequency comb generation. For practical applications, optical interfaces that feature low fiber-to-chip coupling losses are essential. So far, the fiber-to-chip loss (commonly >10 dB/facet) has dominated the total insertion losses of typical LN photonic integrated circuits, where on-chip losses can be as low as 0.03-0.1dB/cm. Here we experimentally demonstrate a low-loss mode size converter for coupling between a standard lensed fiber and sub-micrometer LN rib waveguides. The coupler consists of two inverse tapers that convert the small optical mode of a rib waveguide into a symmetrically guided mode of a LN nanowire, featuring a larger mode area matched to that of a tapered optical fiber. The measured fiber-to-chip coupling loss is lower than 1.7dB/facet with high fabrication tolerance and repeatability. Our results open the door for practical integrated LN photonic circuits efficiently interfaced with optical fibers.

Fabrication Tolerant Integrated Polarization Rotator Design Using the Jones Calculus

A fabrication tolerant polarization rotator (PR) building block is added to our generic photonic integration technology. Using asymmetric waveguides, we propose a PR with guided modes that do not enclose a 45° angle with the substrate. Rather, we find that polarization performance and loss can be improved substantially by allowing the mode angles to vary along the device. We numerically optimize PR designs, targeting at high extinction and high fabrication tolerance. The optimization makes use of the Jones formalism and the fact that Jones calculus is numerically much faster than beam propagation methods. A comparison of our Jones model and commercial simulation software shows excellent agreement. Fabricated devices show a polarization extinction between 10 and 14 dB across different wafers, at an excess loss of 1 dB. Compared to PR designs without tapers, the loss is 3 dB lower and the polarization extinction is now reproducible from wafer to wafer.

High fabrication-tolerant narrowband perfect graphene absorber based on guided-mode resonance in distributed Bragg reflector

We propose the narrowband perfect absorbers with enormously high fabrication tolerance, which consists of a low-contrast grating and a finite distributed Bragg reflector (DBR) layer with an ultrathin absorbing medium (graphene). It is numerically shown that the proposed perfect absorber outperforms the previously proposed schemes in fabrication tolerance. According to the rigorous coupled wave analysis (RCWA) and coupled mode theory (CMT) fitting, over a considerably wide range of grating width and thickness, the proposed absorber provides a proper ratio of leakage rate to loss rate while preserving resonant condition, so that almost perfect absorption (>99.9%) can be obtained. This result is attributed to the strong electric field confinement in the DBR region rather than the grating layer owing to lower index of grating compared to DBR. In addition, without degrading the fabrication tolerance, the bandwidth of the proposed absorber can be controlled by the DBR thickness (the number of pairs) and a narrow absorbing bandwidth of sub-nanometer is achieved with 8.5 Si/SiO2 pair stacked DBR.

Dynamically reconfigurable topological edge state in phase change photonic crystals

The observation of topological edge states (TESs) revolutionized our understanding of scattering and propagation of electromagnetic (EM) waves. Supported by topological robustness, the TES at the interface between trivial and non-trivial insulators was not reflected from the structural disorders and imperfections. Recently topological photonic crystals (PhCs) were demonstrated to obtain remarkable one-way propagation of the TES, having the advantages of lossless propagation, dense integration, and high fabrication tolerance over conventional PhCs. Nevertheless, the lack of reversible switching of TES possesses significant limitations in helicity/spin filtering and tunable photonic devices. We proposed a topological PhC based on a prototypical phase-change material, Ge2Sb2Te5 (GST225) to solve the problem. We find that at a particular frequency, the TES within the structure can be reversibly switched between “on” and “off” by transiting the GST225 structural state between amorphous and crystalline. Moreover, the topology of the PhC was maintained since the tuning of TES was achieved by varying the refractive index of GST225 instead of the structural geometry. This provides a continuous change of the spectral position of the photonic bandgap and TES by gradually crystallising the GST225. We show that the phase change of GST225 from amorphous to crystalline and vice versa can be engineered in nanoseconds. Our proof of concept may offer a platform for dynamically tuning the TESs that might otherwise be challenging to attain in photonic systems. We expect it to have potential applications for photonic devices in topological optical circuits and scatter-free one-way light propagation.

Development of Novel Thin Film Solar Cells: Design and Numerical Optimisation

The development of cost-effective solar cells requires on the one hand to master the elaboration techniques, and on the other hand, an adequate design to optimise the photovoltaic efficiency. These two research topics are closely linked and their association in the research work is the key in the development of novel thin film solar cells. The design associated with numerical optimisation gives the set of optimal physical and geometrical parameters, taking into account the technological feasibility. This will allow elaboration to target the most efficient structures in order to speed up the final device realisation. In this work, we used a new approach, based on rigorous multivariate mathematical global Bayesian algorithm, to optimise a Schottky based solar cell (SBSC) using InGaN as the absorber. The obtained photovoltaic efficiency is close to the conventional structures efficiency while being less complex to elaborate. In addition, the results have shown that the optimised SBSC structure exhibits high fabrication tolerances.

Thermooptic reconfigurable Mach Zehnder quantum interference device

Abstract Integrated Mach-Zehnder (MZ) quantum optic circuit is one of most promising approaches for generation and reconfiguration of complex quantum interference. Here, multi photons quantum entanglement is formulated in terms of thermooptic phase change in MZ interferometer having 3 dB two mode interference couplers (MZ-TMI). The manipulation of quantum entanglement is demonstrated theoretically, opening a way to design silicon based quantum optic technology platform for quantum processing applications. Our results also provide multi-photon quantum interference with high fabrication tolerance and fidelity of 0.98 ± 0.01 within smaller dimension than previous implementation of MZ component.

A Compact Surface Plasmonics Polariton Quantum Entanglement Device

Large-scale quantum integrated circuit requires its basic component at nanoscale beyond the diffraction limit. To obtain nanodimensional quantum operation, we introduce two surface plasmonic polariton (SPP) mode interference couplers as a fundamental component. The Handel-Ou-Mandel (HOM) interference of the coupler of 50:50 splitting ratio provides high-fidelity quantum entanglement. It also enhances the visibility in quantum regime in comparison to that in classical regime. The degradation of fidelity due to small deviation of waveguide width is lower than that of a SPP mode directional coupler. Our results promise to achieve the complexity of quantum plasmonic circuit with high fabrication tolerances.