Tuning spin wave modes in yttrium iron garnet films with stray fields.

The nanopatterning of Yttrium Iron Garnets (YIGs) has proven to be a non-trivial problem even with advances in modern lithography techniques due to non-compatibility with a conventional complementary metal oxide semiconductor platform. In an attempt to circumvent this problem, we demonstrate a simple and reliable method to indirectly pattern YIG films on a Gadolinium Gallium Garnet (GGG) substrate. We fabricated exchange-coupled arrays of Py dots onto the underlying YIG films using nanostencil lithography. The stray fields generated from the Py dots were used to transfer patterned magnetic information to the underlying YIG films. The static and dynamic properties of the fabricated hybrid YIG/Py dot structure and reference YIG film were characterized using the focused magneto-optic Kerr effect and by broadband ferromagnetic resonance spectroscopy. For the reference YIG film, as expected, a single field-dependent resonance mode with a narrow linewidth was observed in contrast to the splitting into three distinct resonance modes for the YIG/Py dot structure as predicted by micromagnetic simulations. We have thus shown that it is possible to utilize stray field effects from easily patternable magnetic materials for the development of future YIG-based magnonic devices.

Tailorable infrared emission of microelectromechanical system-based thermal emitters with NiO films for gas sensing.

Infrared gas sensors hold great promise in the internet of things and artificial intelligence. Making infrared light sources with miniaturized size, reliable and tunable emission is essential but remains challenging. Herein, we present the tailorability of radiant power and the emergence of new emission wavelength of microelectromechanical system (MEMS)-based thermal emitters with nickel oxide (NiO) films. The coating of NiO on emitters increases top surface emissivity and induces the appearance of new wavelengths between 15 and 19 µm, all of which have been justified by spectroscopic methods. Furthermore, a sensor array is assembled for simultaneous monitoring of concentrations of carbon dioxide (CO2), methane (CH4), humidity, and temperature. The platform shows selective and sensitive detection at room temperature toward CO2 and CH4 with detection limits of around 50 and 1750 ppm, respectively, and also shows fast response/recovery and good recyclability. The demonstrated emission tailorability of MEMS emitters and their usage in sensor array provide novel insights for designing and fabricating optical sensors with good performance, which is promising for mass production and commercialization.

Chip-Level Integration of Covalent Organic Frameworks for Trace Benzene Sensing.

State-of-the-art chemical sensors based on covalent organic frameworks (COFs) are restricted to the transduction mechanism relying on luminescence quenching and/or enhancement. Herein, we present an alternative methodology via a combination of in situ-grown COF films with interdigitated electrodes utilized for capacitive benzene detection. The resultant COF-based sensors exhibit highly sensitive and selective detection at room temperature toward benzene vapor over carbon dioxide, methane, and propane. Their benzene detection limit can reach 340 ppb, slightly inferior to those of the metal oxide semiconductor-based sensors, but with reduced power consumption and increased selectivity. Such a sensing behavior can be attributed to the large dielectric constant of the benzene molecule, distinctive adsorptivity of the chosen COF toward benzene, and structural distortion induced by the custom-made interaction pair, which is corroborated by sorption measurements and density functional theory (DFT) calculations. This study provides new perspectives for fabricating COF-based sensors with specific functionality targeted for selective gas detection.

Direct mapping of spin wave modes of individual Ni80Fe20 nanorings.

Ferromagnetic nanorings exhibit tunable magnetic states with unique magnetization reversal processes and dynamic behavior that can be exploited in data storage and magnonic devices. Traditionally, probing the magnetization dynamics of individual ferromagnetic nanorings and mapping the resonance modes has proved challenging. In this study, micro-focused Brillouin light scattering spectroscopy is used to directly map the spin wave modes and their intensities in nanorings as a function of ring width and applied magnetic field. Micromagnetic simulations provide further insights into the experimental observations and are in good agreement with the experimental results. These results can help in improving the understanding of spin wave confinement in single elements for magnonic devices and waveguides.

CMOS-compatible all-Si metasurface polarizing bandpass filters on 12-inch wafers.

The implementation of polarization controlling components enables additional functionalities of short-wave infrared (SWIR) imagers. The high-performance and mass-producible polarization controller based on Si metasurface is in high demand for the next-generation SWIR imaging system. In this work, we report the first demonstration of all-Si metasurface based polarizing bandpass filters (PBFs) on 12-inch wafers. The PBF achieves a polarization extinction ratio of above 10 dB in power within the passbands. Using the complementary metal-oxide-semiconductor (CMOS) compatible 193nm ArF deep ultra-violet (DUV) immersion lithography and inductively coupled plasma (ICP) etch processing line, a device yield of 82% is achieved.

Radiation Enhancement by Graphene Oxide on Microelectromechanical System Emitters for Highly Selective Gas Sensing.

Infrared gas sensors have been proven promising for broad applications in Internet of Things and Industrial Internet of Things. However, the lack of miniaturized light sources with good compatibility and tunable spectral features hinders their widespread utilization. Herein, a strategy is proposed to increase the radiated power from microelectromechanical-based thermal emitters by coating with graphene oxide (GO). The radiation can be substantially enhanced, which partially stems from the high emissivity of GO coating demonstrated by spectroscopic methods. Moreover, the sp2 structure within GO may induce plasmons and thus couple with photons to produce blackbody radiation and/or new thermal emission sources. As a proof-of-concept demonstration, the GO-coated emitter is integrated into a multifunctional monitoring platform and evaluated for gas detection. The platform exhibits sensitive and highly selective detection toward CO2 at room temperature with a detection limit of 50 ppm and short response/recovery time, outperforming the state-of-the-art gas sensors. This study demonstrates the emission tailorability of thermal emitters and the feasibility of improving the associated gas sensing property, offering perspectives for designing and fabricating high-end optical sensors with cost-effectiveness and superior performance.

On-Chip Tailorability of Capacitive Gas Sensors Integrated with Metal-Organic Framework Films.

Gas sensing technologies for smart cities require miniaturization, cost-effectiveness, low power consumption, and outstanding sensitivity and selectivity. On-chip, tailorable capacitive sensors integrated with metal-organic framework (MOF) films are presented, in which abundant coordinatively unsaturated metal sites are available for gas detection. The in situ growth of homogeneous Mg-MOF-74 films is realized with an appropriate metal-to-ligand ratio. The resultant sensors exhibit selective detection for benzene vapor and carbon dioxide (CO2 ) at room temperature. Postsynthetic modification of Mg-MOF-74 films with ethylenediamine decreases sensitivity toward benzene but increases selectivity to CO2 . The reduced porosity and blocked open metal sites caused by amine coordination account for a deterioration in the sensing performance for benzene (by ca. 60 %). The enhanced sensitivity for CO2 (by ca. 25 %) stems from a tailored amine-CO2 interaction. This study demonstrates the feasibility of tuning gas sensing properties by adjusting MOF-analyte interactions, thereby offering new perspectives for the development of MOF-based sensors.

Efficient and broadband subwavelength grating coupler for 3.7 µm mid-infrared silicon photonics integration.

A grating coupler is an essential building block for compact and flexible photonics integration. In order to meet the increasing demand of mid-infrared (MIR) integrated photonics for sensitive chemical/gas sensing, we report a silicon-on-insulator (SOI) based MIR subwavelength grating coupler (SWGC) operating in the 3.7 µm wavelength range. We provide the design guidelines of a uniform and apodized SWGC, followed by numerical simulations for design verification. We experimentally demonstrate both types of SWGC. The apodized SWGC enables high coupling efficiency of -6.477 dB/facet with 3 dB bandwidth of 199 nm, whereas the uniform SWGC shows larger 3dB bandwidth of 263.5 nm but slightly lower coupling efficiency of -7.371 dB/facet.

Reconfigurable MEMS Fano metasurfaces with multiple-input-output states for logic operations at terahertz frequencies.

A broad range of dynamic metasurfaces has been developed for manipulating the intensity, phase and wavefront of electromagnetic radiation from microwaves to optical frequencies. However, most of these metasurfaces operate in single-input-output state. Here, we experimentally demonstrate a reconfigurable MEMS Fano resonant metasurface possessing multiple-input-output (MIO) states that performs logic operations with two independently controlled electrical inputs and an optical readout at terahertz frequencies. The far-field behaviour of Fano resonance exhibits XOR and XNOR operations, while the near-field resonant confinement enables the NAND operation. The MIO configuration resembling hysteresis-type closed-loop behaviour is realized through inducing electromechanically tuneable out-of-plane anisotropy in the near-field coupling of constituent resonator structures. The XOR metamaterial gate possesses potential applications in cryptographically secured terahertz wireless communication networks. Furthermore, the MIO features could lay the foundation for the realization of programmable and randomly accessible metamaterials with enhanced electro-optical performance across terahertz, infrared and optical frequencies.

Microelectromechanically tunable multiband metamaterial with preserved isotropy.

We experimentally demonstrate a micromachined reconfigurable metamaterial with polarization independent characteristics for multiple resonances in terahertz spectral region. The metamaterial unit cell consists of eight out-of-plane deformable microcantilevers placed at each corner of an octagon ring. The octagon shaped unit cell geometry provides the desired rotational symmetry, while the out-of-plane movable cantilevers preserves the symmetry at different configurations of the metamaterial. The metamaterial is shown to provide polarization independent response for both electrical inductive-capacitive (eLC) resonance and dipolar resonance at all states of actuation. The proposed metamaterial has a switching range of 0.16 THz and 0.37 THz and a transmission intensity change of more than 0.2 and 0.7 for the eLC and dipolar resonances, respectively for both TE and TM modes. Further optimization of the metal layer thickness, provides an improvement of up to 80% modulation at 0.57 THz. The simultaneously tunable dual band isotropic metamaterial will enable the realization of high performance electro-optic devices that would facilitate numerous terahertz applications such as compressive terahertz imaging, miniaturized terahertz spectroscopy and next generation high speed wireless communication possible in the near future.

Phononic and magnonic dispersions of surface waves on a permalloy/BARC nanostructured array.

Phononic and magnonic dispersions of a linear array of periodic alternating Ni80Fe20 and bottom anti-reflective coating nanostripes on a Si substrate have been measured using Brillouin light scattering. The observed phononic gaps are considerably larger than those of laterally patterned multi-component crystals previously reported, mainly a consequence of the high elastic and density contrasts between the stripe materials. Additionally, the phonon hybridization bandgap has an unusual origin in the hybridization and avoided crossing of the zone-folded Rayleigh and pseudo-Sezawa waves. The magnonic band structure features near-dispersionless branches, with unusual vortex-like dynamic magnetization profiles, some of which lie below the highly-dispersive fundamental mode branch. Finite element calculations of the phononic and magnonic dispersions of the magphonic crystal accord well with experimental data.

Utilizing the unique properties of nanowire MOSFETs for RF applications.

Nanostructures have attracted a great deal of attention because of their potential usefulness for high density applications. More importantly, they offer excellent avenues for improved scaling beyond conventional approaches. Less attention has been paid to their intrinsic potential for distinct circuit applications. Here we discuss how a combination of 1-D transport, operation in the quantum capacitance limit, and ballistic transport can be utilized for certain RF applications. In particular this work explores how the above transport properties can provide a high degree of transconductance linearity at the device level. The article also discusses how device characteristics can be interpreted and analyzed in terms of device linearity if the above conditions are not ideally fulfilled. Using aggressively scaled silicon nanowire field-effect transistors as an example device in this work provides new insights toward the proper choice of channel material to improve linearity through the above-mentioned transport conditions. According to this study, a high degree of linearity occurs feasible while operating at low supply voltages making low-dimensional systems, and here in particular nanowires, an interesting candidate for portable RF applications.

Observation of 1D behavior in Si nanowires: toward high-performance TFETs.

This article provides experimental evidence of one-dimensional behavior of silicon (Si) nanowires (NWs) at low-temperature through both transfer (I(d)-V(G)) and capacitance-voltage characteristics. For the first time, operation of Si NWs in the quantum capacitance limit (QCL) is experimentally demonstrated and quantitatively analyzed. This is of relevance since working in the QCL may allow, e.g., tunneling field-effect transistors (TFETs) to achieve higher on-state currents (I(on)) and larger on-/off-state current ratios (I(on)/I(off)), thus addressing one of the most severe limitations of TFETs. Comparison of the experimental data with simulations finds excellent agreement using a simple capacitor model.

Highly-ordered vertical Si nanowire/nanowall decorated solar cells.

Highly-ordered vertical nanowire and nanowall arrays are studied on Si solar cell surface. The nanowall textured solar cell is found to be more effective in reducing the overall optical reflectance, resulting in higher short circuit current (Jsc = 24.9 mA/cm²) over nanowire structured (Jsc = 23.3 mA/cm²) and planar (Jsc = 17.5 mA/cm²) solar cells. The extracted energy conversion efficiency (η) from planar solar cell is 7.1%, while nanowire/nanowall cells show efficiency of 8.2% and 6.3%, respectively. If corrected with series resistance (Rs), nanowall solar cell exhibits the highest η of 9.8% in this experiment. A careful study of the series resistance from different types of the nanostructures is also presented.

Optical and electrical study of core-shell silicon nanowires for solar applications.

In this work, we report a CMOS comparable fabrication process of core-shell SiNW solar cell from single-crystalline p-type Si(100) test wafers. Optical lithography defined plasma etching was used to form highly ordered vertical SiNW arrays, which display a drastic reduction in optical reflectance over a wide range of wavelengths. BF(2) and P ion implantations were employed for producing a sharp and shallow radial p-n junction. Under AM 1.5G illumination, the device demonstrates a short circuit current density (Jsc) of 14.2 mA/cm(2), an open circuit voltage (Voc) of 0.485 V and a fill factor (FF) of 42.9%, giving a power conversion efficiency (PCE) of 2.95%. The Jsc observed is 52% higher than that in the control device with planar Si p-n junction, indicating significant enhancement in carrier generation and collection efficiency from the core-shell structure. Impact of series resistance (Rs) is also studied, highlighting potential improvement of PCE to 4.40% in the absence of Rs. With top contact optimized, PCE could further increase to 6.29%.

Electrical characteristics and photocurrent spectral response of Si nanowires p-i-n junctions.

P-i-n junctions were fabricated along Si nanowires (SiNWs) via the conventional top-down approach using optical lithography. Each device comprises 500 identical SiNWs connected in parallel, and each SiNW has triangular cross-section with dimensions of ~6 nm (base) by ~8 nm (height). The photodiodes exhibit very good rectifying electrical characteristics with a low reverse bias current of ~0.2 fA per SiNW. The photocurrent spectral response exhibits three peaks between 400 nm to 700 nm, which arise due to local optical field enhancement associated with diffraction by the periodic SiNW array and interference in an air/SiO2/Si cavity.

DNA sensing by silicon nanowire: charge layer distance dependence.

To provide a comprehensive understanding of the field effect in silicon nanowire (SiNW) sensors, we take a systematic approach to fine tune the distance of a charge layer by controlling the hybridization sites of DNA to the SiNW preimmobilized with peptide nucleic acid (PNA) capture probes. Six target DNAs of the same length, but differentiated successively by three bases in the complementary segment, are hybridized to the PNA. Fluorescent images show that the hybridization occurs exclusively on the SiNW surface between the target DNAs and the PNA. However, the field-effect response of the SiNW sensor decreases as the DNA (charge layer) moves away from the SiNW surface. Theoretical analysis shows that the field effect of the SiNW sensor relies primarily on the location of the charge layer. A maximum of 102% change in resistance is estimated based on the shortest distance of the DNA charge layer (4.7 A) to the SiNW surface.

Silicon nanowire arrays for label-free detection of DNA.

Arrays of highly ordered n-type silicon nanowires (SiNW) are fabricated using complementary metal-oxide semiconductor (CMOS) compatible technology, and their applications in biosensors are investigated. Peptide nucleic acid (PNA) capture probe-functionalized SiNW arrays show a concentration-dependent resistance change upon hybridization to complementary target DNA that is linear over a large dynamic range with a detection limit of 10 fM. As with other SiNW biosensing devices, the sensing mechanism can be understood in terms of the change in charge density at the SiNW surface after hybridization, the so-called "field effect". The SiNW array biosensor discriminates satisfactorily against mismatched target DNA. It is also able to monitor directly the DNA hybridization event in situ and in real time. The SiNW array biosensor described here is ultrasensitive, non-radioactive, and more importantly, label-free, and is of particular importance to the development of gene expression profiling tools and point-of-care applications.