Large-scale integration of nanoelectromechanical systems for gas sensing applications

We have developed arrays of nanomechanical systems (NEMS) by large-scale integration, comprising thousands of individual nanoresonators with densities of up to 6 million NEMS per square centimeter. The individual NEMS devices are electrically coupled using a combined series-parallel configuration that is extremely robust with respect to lithographical defects and mechanical or electrostatic-discharge damage. Given the large number of connected nanoresonators, the arrays are able to handle extremely high input powers (>1 W per array, corresponding to <1 mW per nanoresonator) without excessive heating or deterioration of resonance response. We demonstrate the utility of integrated NEMS arrays as high-performance chemical vapor sensors, detecting a part-per-billion concentration of a chemical warfare simulant within only a 2 s exposure period.     

In-line optical fiber sensors based on cladded multimode tapered fibers

The use of uniform-waist cladded multimode tapered optical fibers is demonstrated for evanescent wave spectroscopy and sensors. The tapering is a simple, low-loss process and consists of stretching the fiber while it is being heated with an oscillating flame torch. As examples, a refractive-index sensor and a hydrogen sensor are demonstrated by use of a conventional graded-index multimode optical fiber. Also, absorbance spectra are measured while the tapers are immersed in an absorbing liquid. It is found experimentally that the uniform waist is the part of the taper that contributes most to the sensor sensitivity. The taper waist diameter may also be used to adjust the sensor dynamic range.     

High-sensitivity sensor of low relative humidity based on overlay on side-polished fibers

A low relative humidity (RH) sensor based on overlay on side-polished fiber is presented. The evanescent field from a single-mode optical fiber is coupled to a TiO/sub 2/ waveguide overlay. The transmission response exhibits sharp resonances whose central wavelengths are linearly shifted with RH. This behavior is due to the porous columnar nanostructure of the TiO/sub 2/ film. The water is adsorbed in the pores of the nanostructure changing the refractive index of the layer and causing a shift of the wavelength resonances. The response of the sensor is determined by the shape and size of the pores. The optical fiber evanescent field sensor developed has a linear response and high sensitivity (0.5 nm/% RH) for low RH (RH/spl sim/0%-15%) at 26.1/spl deg/C/spl plusmn/0.6/spl deg/C. The lack of hysteresis in the adsorption-desorption cycle has been checked. The development of a sensor with tailored response is envisaged using properly techniques to control the porosity of the material.     

Imaging single fluorescent molecules at the interface of an optical fiber probe by evanescent wave excitation

We have developed a new fluorescent method for single-molecule detection (SMD) and imaging using an optical fiber probe. The fluorophores were excited by the evanescent wave field produced on the core surface of the optical fiber. This was achieved by exposing a section of the core of the optical fiber probe to the fluorophore solution. Both cylindrical and square optical fiber probes were used for SMD. The fluorescent signals were detected by an intensified charge-coupled device. Single rhodamine 6G molecules have been detected. The number of rhodamine 6G molecules imaged by the optical fiber probe showed an excellent linear relationship with the concentrations of the fluorophores. The SMD scheme was also applied to the imaging of biomolecules, such as molecular beacon DNA molecules, labeled with tetramethylrhodamine. Our results have shown that using an optical fiber is an easy yet effective approach to SMD. It represents a simpler fluorescent method for the detection of single-molecules in solution and at an interface.     

Optical-fiber sensor using tailored porous sol-gel fiber core

A new concept in optical-fiber chemical sensors, the active fiber core optical sensor (AFCOS), is presented. In this sensor, the fiber core acts as a transducer. The sensitivity of an AFCOS sensor is compared with that of an active coating [evanescent wave (EW)] based optical-fiber sensor. Requirements for a fiber core to act as a chemical sensor are discussed. Novel techniques for making a porous sol-gel silica fiber, doping chemical reagents into the fiber, and constructing a chemical sensor using the porous fiber as a transducer have been developed. The microstructure of the fabricated sol-gel silica fiber and the effect of the fiber's microstructure on the capability of the porous sol-gel silica fiber for guiding light are discussed. A humidity sensor employing a CoCl/sub 2/-doped porous sol-gel fiber as a transducer has been constructed as an example. The test results for the humidity sensor verified a theoretical analysis indicating that an optical-fiber chemical sensor using an active fiber core as a transducer has a much higher sensitivity than that of an EW-based sensor.     

Eigenvalue equation and core-mode cutoff of weakly guiding tapered fiber as three layer optical waveguide and used as biochemical sensor

The core-mode cutoff plays a major role in evanescent field absorption based sensors. A method has been proposed to calculate the core-mode cutoff by solving the eigenvalue equations of a weakly guiding three layer optical waveguide graphically. The variation of normalized waveguide parameter (V) is also calculated with different wavelengths at core-mode cutoff. At the first step, theoretical analysis of tapered fiber parameters has been performed for core-mode cutoff. The taper angle of an adiabatic tapered fiber is also analyzed using the length-scale criterion. Secondly, single-mode tapered fiber has been developed to make a precision sensor element suitable for chemical detection. Finally, the sensor element has been used to detect absorption peak of ethylenediamine. Results are presented in which an absorption peak at 1540 nm is observed.     

Transfer function and near-field detection of evanescent waves

We consider characterization of a near-field optical probe in terms of detection efficiency of different spatial frequencies associated with propagating and evanescent field components. The former are both detected with and radiated from an etched single-mode fiber tip, showing reciprocity of collection and illumination modes. Making use of a collection near-field microscope with a similar fiber tip illuminated by an evanescent field, we measure the collected power as a function of the field spatial frequency in different polarization configurations. Considering a two-dimensional probe configuration, numerical simulations of detection efficiency based on the eigenmode expansion technique are carried out for different tip apex angles. The detection roll-off for high spatial frequencies observed in the experiment and obtained during the simulations is fitted using a simple expression for the transfer function, which is derived by introducing an effective point of (dipolelike) detection inside the probe tip. It is found to be possible to fit reasonably well both the experimental and the simulation data for evanescent field components, implying that the developed approximation of the near-field transfer function can serve as a simple, rational, and sufficiently reliable means of fiber probe characterization.     

Planar fiber-optic chips for broadband spectroscopic interrogation of thin films

A planar fiber-optic chip (FOC) has been developed using side-polished optical fibers and characterized for broadband absorbance and fluorescence detection of molecular films. FOC technology combines the sensitivity of an attenuated total reflection (ATR) element with the ease of use of fiber-optic-based spectrometers and light sources to create an improved platform for spectroscopic analysis of molecular adsorbates. A multi-mode optical fiber (core diameter = 50 mum, numerical aperture = 0.22, stepped refractive index profile) mounted in a glass V-groove block was side-polished to create a planar platform that allows access to the evanescent field escaping from the fiber core. For this generation of FOC technology, the exposed evanescent field has an interaction length of approximately 17.2 mm. The FOC platform was independently characterized through measurements of thin-film and bulk absorbing samples. The device performance was compared to the existing ATR technology and methods for increasing sensitivity of the FOC were investigated and validated. Additionally, we have demonstrated the ability of the FOC to both evanescently excite and collect fluorescence through guided modes of the optical fiber for a surface-confined luminescent semiconductor nanoparticle film (4 nm diameter, ligand capped, CdSe core). The FOC described here with a supported planar interface can facilitate the use of conventional planar deposition technologies and provide a robust planar platform that is amenable for incorporation into various sensor technologies.     

Magneto-Nanomechanical Array Biosensor for Ultrasensitive Detection of Oncogenic Exosomes for Early Diagnosis of Cancers

Exosomes are a class of nanoscale vesicles secreted by cells, which contain abundant information closely related to parental cells. The ultrasensitive detection of cancer-derived exosomes is highly significant for early non-invasive diagnosis of cancer. Here, an ultrasensitive nanomechanical sensor is reported, which uses a magnetic-driven microcantilever array to selectively detect oncogenic exosomes. A magnetic force, which can produce a far greater deflection of microcantilever than that produced by the intermolecular interaction force even with very low concentrations of target substances, is introduced. This method reduced the detection limit to less than 10 exosomes mL⁻¹. Direct detection of exosomes in the serum of patients with breast cancer and in healthy people showed a significant difference. This work improved the sensitivity by five orders of magnitude as compared to that of traditional nanomechanical sensing based on surface stress mode. This method can be applied parallelly for highly sensitive detection of other microorganisms (such as bacteria and viruses) by using different probe molecules, which can provide a supersensitive detection approach for cancer diagnosis, food safety, and SARS-CoV-2 infection.     

Multiplexed optical operation of distributed nanoelectromechanical systems arrays

We report a versatile all optical technique to excite and read-out a distributed nanoelectromechanical systems (NEMS) array. The NEMS array is driven by a distributed, intensity modulated optical pump through the photothermal effect. The ensuing vibrational response of the array is multiplexed onto a single probe beam in the form of a high frequency phase modulation. The phase modulation is optically down converted to a low frequency intensity modulation using an adaptive full-field interferometer, and subsequently detected using a CCD array. Rapid and single step mechanical characterization of ～44 nominally identical high-frequency resonators is demonstrated. The technique may enable sensitivity improvements over single NEMS resonators by averaging signals coming from a multitude of devices in the array. In addition, the diffraction limited spatial resolution may allow for position-dependent read-out of NEMS sensor chips for sensing multiple analytes or spatially inhomogeneous forces.     

A monolithic silicon optoelectronic transducer as a real-time affinity biosensor

An optical real-time affinity biosensor, which is based on a monolithic silicon optoelectronic transducer and a microfluidic module, is described. The transducer monolithically integrates silicon avalanche diodes as light sources, silicon nitride optical fibers, and p/n junction detectors and efficiently intercouples these elements through a self-alignment technique. The transducer surface is hydrophilized by oxygen plasma treatment, silanized with (3-aminopropyl)triethoxysilane and bioactivated through adsorption of the biomolecular probes. The use of a microfluidic module allows real-time monitoring of the binding reaction of the gold nanoparticle-labeled analytes with the immobilized probes. Their binding within the evanescent field at the surface of the optical fiber causes attenuated total reflection of the waveguided modes and reduction of the detector photocurrent. The biotin-streptavidin model assay was used for the evaluation of the analytical potentials of the device developed. Detection limits of 3.8 and 13 pM in terms of gold nanoparticle-labeled streptavidin were achieved for continuous- and stopped-flow assay modes, respectively. The detection sensitivity was improved by silver plating of the immobilized gold nanoparticles, and a detection limit of 20 fM was obtained after 20-min of silver plating. In addition, two different analytes, streptavidin and anti-mouse IgG, were simultaneously assayed on the same chip demonstrating the multianalyte potential of the sensor developed.     

Fabrication of Submicron-Diameter and Taper Fibers Using Chemical Etching

The thin, long length and high smoothness silica photonic nanowires and taper optical fiber were fabricated using a simple and low cost chemical etching method. A two-steps wet etch process were used consisting of etching with 30% HF acid to remove cladding and 24% HF acid to decrease fiber core diameter. An approach for on-line monitoring of etching using 1300 nm light power transmitted in the optical fiber was used to determine the diameter of the remaining core and showed a transition between two different operation regimes of nanofiber from the embedded regime, where the mode was isolated from the environment, to the evanescent regime. The data indicated that the diameter of the silica fiber decreased linearly for both 30% and 24% HF acid with 1.2 and 0.1 μm/min grad diameter, respectively at room temperature, and more than 70% of the mode intensity could propagate outside fiber when the core diameter was less than 1 μm. The results of fiber taper showed that the fiber was tapered by a factor of 20 while retaining a thin core structure and leaving about more than 85% of core structure.     

Enumeration of DNA molecules bound to a nanomechanical oscillator

Resonant nanoelectromechanical systems (NEMS) are being actively investigated as sensitive mass detectors for applications such as chemical and biological sensing. We demonstrate that highly uniform arrays of nanomechanical resonators can be used to detect the binding of individual DNA molecules through resonant frequency shifts resulting from the added mass of bound analyte. Localized binding sites created with gold nanodots create a calibrated response with sufficient sensitivity and accuracy to count small numbers of bound molecules. The amount of nonspecifically bound material from solution, a fundamental issue in any ultra-sensitive assay, was measured to be less than the mass of one DNA molecule, allowing us to detect a single 1587 bp DNA molecule.     

Control of reflection at an optical interface in the absence of total internal reflection for a retroreflective display application

Reflection at an interface between two materials can be modulated by means of varying the optical properties at the interface. We have studied this modulation of the reflected light with an aim to develop a flashing retroreflector for roadside conspicuity applications. Reflectance modulation has previously been studied under the conditions of total internal reflection (TIR), where a light-absorbing material placed in the associated evanescent wave region can be used to attenuate the intensity of the reflected light. If instead the light rays strike the interface at an angle that is slightly smaller than the critical angle required for TIR, they instead undergo a substantial, but partial, reflection. We have demonstrated that an analogous attenuation effect to the TIR situation is observed, even though there is no evanescent wave present under these circumstances. We have studied this behavior and have developed a model to describe the motion of the absorbing material and the related interference effects that occur.     

High frequency ultrasound imaging with optical arrays

Dynamically focused and steered high frequency ultrasound imaging systems require arrays with fine element spacing, wide bandwidths, and large apertures. However, these characteristics are difficult to achieve at frequencies greater than 30 MHz using conventional array construction methods. Optical schemes offer a solution. Focused laser beams incident on a suitable surface can generate and detect acoustic radiation. Precisely controlling the position and size of the beams defines points of transmission and detection, making it possible for pulse-echo image formation by synthetic aperture methods. An optical detection array was built, relying on a conventional piezoelectric transducer as an ultrasound source. The detection system, with near optimal resolution over a wide depth of field, demonstrates the potential for high frequency array implementation using optical techniques. A possible application is in pathology, where 2-D or 3-D fine resolution pulse-echo imaging can be performed in situ without the need for biopsies.     

Experimental implementation of fiber optic bundle array wide FOV free space optical communications receiver

A gimbal-free wide field-of-regard (FOR) optical receiver has been built in a laboratory setting for proof-of-concept testing. Multiple datasets are presented that examine the overall FOR of the system and the receiver's ability to track and collect a signal from a moving source. The design is not intended to compete with traditional free space optical communication systems, but rather offer an alternative design that minimizes the number and complexity of mechanical components required at the surface of a small mobile platform. The receiver is composed of a micro-lens array and hexagonal bundles of large core optical fibers that route the optical signal to remote detectors and electronics. Each fiber in the bundle collects power from a distinct solid angle of space and a piezo-electric transducer is used to translate the micro-lens array and optimize coupling into a given fiber core in the bundle. The micro-lens to fiber bundle design is scalable, modular, and can be replicated in an array to increase aperture size.     

不等厚复合构件相控阵超声脱粘检测技术仿真研究

鉴于不等厚复合构件结构的特殊性,采用传统超声检测技术检测此类构件的粘接质量是非常困难的。采用超声相控阵技术实现界面粘接质量的检测。分析研究了不等厚复合构件脱粘检测的特殊性。建立了相控阵超声换能器的辐射声场模型,基于仿真得到的声场,研究了采用超声相控阵技术检测不等厚复合构件界面脱粘的可行性。仿真结果表明,通过控制相控延时,可使检测声束在检测区域内聚焦到任意位置处,保证在厚度变化的界面处回波信号幅度的一致性,可用于此类构件的界面脱粘检测。     

Large defect-induced sub-bandgap photoresponse in semiconductor nanowires via waveguiding excitation

A large defect-induced sub-bandgap photoresponse over a broad spectral range is observed in semiconductor single nanowires via optical waveguiding excitation. Using an evanescent coupling technique, the excitation sub-bandgap light is efficiently transferred from a silica fiber taper into a CdS single nanowire (bandgap ～ 2.46 eV), and is tightly confined and guided through the whole length of the nanowire, which significantly enhances the light-defect interaction compared with the conventional irradiation excitation scheme. Under 593 nm wavelength (～2.09 eV) waveguiding excitation with an input power of 10 pW level at room temperature, a 350 nm diameter 150 μm-length CdS nanowire shows a responsivity of 250 A W( - 1), offering a sub-bandgap photosensitivity five orders of magnitude larger than by irradiation excitation. These results may open opportunities for noninvasive characterization of defect states in semiconductor nanowires, as well as for enabling novel sub-bandgap nanowire devices.     

Resonant-enhanced evanescent-wave fluorescence biosensing with cylindrical optical cavities

We show that the artificial resonances of dielectric optical cavities can be used to enhance the detection sensitivity of evanescent-wave optical fluorescence biosensors to the binding of a labeled analyte with a biospecific monolayer. Resonant coupling of power into the optical cavity allows for efficient use of the long photon lifetimes (or equivalently, the high internal power) of the high-Q whispering gallery modes to increase the probability of photon absorption into the fluorophore, thereby enhancing fluorescence emission. A method to compare the intrinsic sensitivity between resonant cavity and waveguide formats is also developed. Using realistic estimates for dielectric cylindrical cavities in both bulk and integrated configurations, we can expect sensitivity enhancement by at least an order of magnitude over standard waveguide evanescent sensors of equivalent sensing geometries. In addition, the required sample volume can be reduced significantly. The cylindrical cavity format is compatible with a large variety of sensing modalities such as immunoassay and molecular diagnostic assay.     

Sagnac-type fiber-optic array sensor for detection of bulk ultrasonic waves

In this paper, we describe a fiber optic array sensor suitable for detection of bulk ultrasonic waves. This sensor is based on an intrinsic fiber optic Sagnac interferometer. The fiber array is formed by multiple folding of a continuous length of an optical fiber into flat coils. Depending on the orientation of the fiber array with respect to the ultrasonic wave, the proposed sensor can act as a conventional in-phase detector or as a narrowband detector. In the narrowband mode, the center frequency of detection can be tuned by adjusting the spacing of the fiber array elements to be equal to the ultrasonic wavelength of interest. This feature distinguishes this array sensor from conventional hydrophones in which a receiver is typically much smaller than the acoustical wavelength. It is shown that the array sensor provides an enhanced signal-to-noise ratio (SNR) compared with a single element detection scheme. Results are presented for detection of ultrasonic waves in water arising from both piezoelectric and laser ultrasonic sources. Potential areas of application of this sensor include process monitoring, smart structures, bio-medical ultrasound, and chemical sensing.