Active and regular monitoring of structural components is a critical safety precaution, particularly as it concerns infrastructure, aerospace, industry, and many other applications. Solutions for effective structural health monitoring need to be able to probe surface and bulk properties using durable and low-power engineering. Towards these ends it is possible to build smarter, highly networked, and secure materials in order to enable remote real-time material sensing with a lesser-known form of 3D printing, ultrasonic consolidation (UC). This approach harnesses sound waves to weld metal layers in a low temperature process that does not damage sensors and electronics as they are embedded into a solid metal structure. This makes UC perfectly suited for designing solid metal parts with active embedded sensing components. However, the process parameters, material influences and mechanical factors that result in high-quality UC metal components are difficult to control or are loosely understood. The use of trial-and-error optimization during fabrication represents the chief hurdle between its current state of use and its potential to transform rapid prototyping and manufacturing of high-impact technologies (e.g., metal smart structures, wearable sensors, lab-on-a-chip, etc.). In this paper we will describe how embedded sensors may be used for in situ process monitoring and optimization. We will also discuss efforts towards standardizing UC welding for similar and dissimilar metal bonding and for embedding active sensors that can be used to create smart structures that will enable long-term structural health monitoring and other high impact applications.
In microbiology research, there is a strong need for next-generation imaging and sensing instrumentation that will enable minimally invasive and label-free investigation of soft, hydrated structures, such as in bacterial biofilms. White-light interferometry (WLI) can provide high-resolution images of surface topology without the use of fluorescent labels but is not typically used to image biofilms because there is insufficient refractive index contrast to induce reflection from the biofilm’s interface. The soft structure and water-like bulk properties of hydrated biofilms make them difficult to characterize in situ, especially in a nondestructive manner. We build on our prior description of static biofilm imaging and describe the design of a dynamic growth flow cell that enables monitoring of the thickness and topology of live biofilms over time using a WLI microscope. The microfluidic system is designed to grow biofilms in dynamic conditions and to create a reflective interface on the surface while minimizing disruption of fragile structures. The imaging cell was also designed to accommodate limitations imposed by the depth of focus of the microscope’s objective lens. Example images of live biofilm samples are shown to illustrate the ability of the flow cell and WLI instrument to (1) support bacterial growth and biofilm development, (2) image biofilm structure that reflects growth in flow conditions, and (3) monitor biofilm development over time nondestructively. In future work, the apparatus described here will enable surface metrology measurements (roughness, surface area, etc.) of biofilms and may be used to observe changes in biofilm structure in response to changes in environmental conditions (e.g., flow velocity, availability of nutrients, and presence of biocides). This development will open opportunities for the use of WLI in bioimaging.
White light interferometry (WLI) is not typically used to image bacterial biofilms that are immersed in water because
there is insufficient refractive index contrast to induce reflection from the biofilm’s interface. The soft structure and
water-like bulk properties of hydrated biofilms make them difficult to characterize in situ by any means, especially in a
non-destructive manner. Here we describe a new method for measuring and monitoring the thickness and topology of
live biofilms using a WLI microscope. A microfluidic system was used to create a reflective interface on the surface of
biofilms. Live biofilm samples were monitored non-destructively over time. The method enables surface metrology
measurements (roughness, surface area) and a novel approach to measuring thickness of the thin hydrated biofilms.
Increase in surface roughness preceded observable increase in biofilm thickness, indicating that this measure may be
used to predict future development of biofilms. We have also developed a flow cell that enables WLI biofilm imaging in
a dynamic environment. We have used this flow cell to observe changes in biofilm structure in response to changes in
environmental conditions - flow velocity, availability of nutrients, and presence of biocides.
We present the design, modeling and performance of a proof-of-concept tamper indicating approach that exploits newlydeveloped subwavelength-patterned films. These films have a nanostructure-dependent resonant optical reflection that is wavelength, angle, and polarization dependent. As such, they can be tailored to fabricate overlay transparent films for tamper indication and authentication of sensitive or controlled materials not possible with currently-known technologies. An additional advantage is that the unique optical signature is dictated by the geometry and fabrication process of the nanostructures in the film, rather than on the material used. The essential structure unit in the subwavelength resonant coating is a nanoscale Open-Ring Resonator (ORR). This building block is fabricated by coating a dielectric nanoscale template with metal to form a hemispherical shell-like structure. This curved metallic shell structure has a cross-section with an intrinsic capacitance and inductance and is thus the optical equivalent to the well-known “LC” circuit where the capacitance and inductance are determined by the nanoshell dimensions. For structures with sub 100 nm scale, this resonance occurs in the visible electromagnetic spectrum, and in the IR for larger shells. Tampering of the film would be visible though misalignment of the angle-sensitive features in the film. It is additionally possible to add in intrinsic oxidation and strain sensitive matrix materials to further complicate tamper repair and counterfeiting. Cursory standoff readout would be relatively simple using a combination of a near-infrared (or visible) LED flashlight and polarizer or passively using room lighting illumination and a dispersive detector.
We present optical subwavelength nanostructure architecture suitable for standoff radiation dosimetry with remote optical readout in the visible or infrared spectral regions. To achieve this, films of subwavelength structures are fabricated over several square inches via the creation of a 2D non-close packed (NCP) array template of radiationsensitive polymeric nanoparticles, followed by magnetron sputtering of a metallic coating to form a 2D array of separated hemispherical nanoscale metallic shells. The nanoshells are highly reflective at resonance in the visible or infrared depending on design. These structures and their behavior are based on the open ring resonator (ORR) architecture and have their analog in resonant inductive-capacitive (LC) circuits, which display a resonance wavelength that is inversely proportional to the square root of the product of the inductance and capacitance. Therefore, any modification of the nanostructure material properties due to radiation alters the inductive or capacitive behavior of the subwavelength features, which in turn changes their optical properties resulting in a shift in the optical resonance. This shift in resonance may be remotely interrogated actively using either laser illumination or passively by hyperspectral or multispectral sensing with broadband illumination. These structures may be designed to be either anisotropic or isotropic, which can also offer polarization-sensitive interrogation. We present experimental measurements of a radiation induced shift in the optical resonance of a subwavelength film after exposure to an absorbed dose of gamma radiation from 2 Mrad up to 62 Mrad demonstrating the effect. Interestingly the resonance shift is non-monotonic for this material system and possible radiation damage mechanisms to the nanoparticles are discussed.
Energy and climate change represent significant factors in global security. Atmospheric carbon dioxide levels, while global in scope, are influenced by pore-scale phenomena in the subsurface. We are developing tools to visualize and investigate processes in pore network microfluidic structures that serve as representations of normally-opaque porous media. These structures enable, for example, visualization of water displacement from pore spaces by hydrophobic fluids, including carbon dioxide, in studies related to carbon sequestration. In situ fluorescent oxygen sensing methods and fluorescent cellulosic materials are being used to investigate processes related to terrestrial carbon cycling involving cellulolytic respiring microorganisms.
We present a novel subwavelength nanostructure architecture that may be utilized for optical standoff sensing applications. The subwavelength structures are fabricated via a combination of nanoimprint lithography and metal sputtering to create metallic nanostructured films encased within a transparent media. The structures are based on the open ring resonator (ORR) architecture which has a characteristic resonance frequency. Any perturbation of the nanostructured films due to chemical or environmental effects can shift the resonant frequency and provide an indication of the external stimulus. This shift in resonance can be interrogated remotely either actively using either laser illumination or passively using hyperspectral or multispectral sensing. These structures may be designed to be either anisotropic or isotropic, which can also provide polarization-sensitive interrogation. Due to the nanometer scale of the structures, they can be tailored to be optically responsive in the visible or near infrared spectrum with a highly reflective resonant peak that is dependent solely on structural dimensions and material characteristics. We present experimental measurements of the optical response of these structures as a function of wavelength, polarization, and incident angle demonstrating the resonant effect in the near infrared region. Numerical modeling data showing the effect of different fabrication parameters such as structure parameters are also discussed.
Reliable active and passive hyperspectral imaging and detection of explosives and solid-phase chemical residue on
surfaces remains a challenge and an active area of research. Both methods rely on reference libraries for material
identification, but in many cases the reference spectra are either not available or do not sufficiently resemble the
instrumental signals of light reflected, scattered, or emitted from real-world objects. We describe a physics-based model
using the complex dielectric constant to explain what is often thought of as anomalous behavior of scattered or nonspecular
signatures encountered in active and passive sensing of explosives or chemicals on surfaces and show modeling
and experimental results for RDX.
We present a study of the spectral and angular dependence of the diffuse scatter of mid-infrared (MIR) laser light from
explosives residues on surfaces. Experiments were performed using an external cavity quantum cascade laser (ECQCL)
tunable between 7 and 8 μm (1270 to 1400 cm-1) for surface illumination. A mercury cadmium telluride (MCT) detector
was used to detect backscattered spectra as a function of surface angle at a 2 meter standoff. A ferroelectric focal plane
array was used to build hyperspectral images at a 0.5 meter standoff. Residues of RDX, tetryl, and TNT were
investigated on surfaces including a painted car door for angles between zero (specular) and 50 degrees. We observe
spectral signatures of the explosives in the diffuse scattering geometry which differ significantly from those observed in
transmission geometries. Characterization of the scattered light spectra of explosives on surfaces will be essential for
understanding the performance of standoff explosives detection instruments and developing robust spectral analysis
techniques.
Using infrared hyperspectral imaging, we demonstrate microscopy of small particles of the explosives compounds RDX,
tetryl, and PETN with near diffraction-limited performance. The custom microscope apparatus includes an external
cavity quantum cascade laser illuminator scanned over its tuning range of 9.13-10.53 μm in four seconds, coupled with a
microbolometer focal plane array to record infrared transmission images. We use the hyperspectral microscopy
technique to study the infrared absorption spectra of individual explosives particles, and demonstrate sub-nanogram
detection limits.
Microfluidic lasers, which utilize liquid as a gain medium, are of great interest for lab-on-a-chip devices due to their
small size, tunability, and cost-effectiveness. We demonstrate a soft-lithography-based opto-fluidic ring resonator
(OFRR) laser which can be produced in arrays of identical rings in polydimethyl siloxane (PDMS). The PDMS
structures are produced from a silicon mold fabricated using reactive ion etching (RIE) and are both robust and reusable.
Using rhodamine 6G in a tetraethylene glycol (TEG) dye solvent provides enough refractive index contrast with PDMS
to generate a multimode lasing signal from rings 200 to 400 microns in diameter and lasing thresholds of 2.7 μJ/mm2
centered around 580 nm. These rings are coupled to liquid waveguides which conveniently direct the lasing emission to
other on-chip devices. Since the rings and waveguides are not in fluidic contact, many rings may potentially be coupled
into a single waveguide for multi-color emission. Separating the ring and waveguide fluidics also prevents unwanted
absorption of the lasing signal by extra dye molecules.
We overview the recent progress on optofluidic ring resonator (OFRR) dye lasers developed in our research group. The
fluidics and laser cavity design can be divided into three categories: capillary optofluidic ring resonator (COFRR),
integrated cylindrical optofluidic ring resonator (ICOFRR), and coupled optofluidic ring resonator (CpOFRR). The
COFRR dye laser is based on a micro-sized glass capillary with a wall thickness of a few micrometers. The capillary
circular cross-section forms the ring resonator and supports the whispering gallery modes (WGMs) that interact
evanescently with the gain medium in the core. The laser cavity structure is versatile to adapt to the gain medium of any
refractive index. Owing to the high Q-factor (>109), the lasing threshold of 25 nJ/mm2 is achieved. Besides directly
pump the dye molecules, lasing through fluorescence resonance energy transfer (FRET) between the donor and acceptor
dye molecules is also studied in COFRR laser. The energy transfer process can be further controlled by designed DNA
scaffold labeled with donor/acceptor molecules. The ICOFRR dye laser is based on a cylindrical ring resonator fused
onto the inner surface of a thick walled glass capillary. The structure has robust mechanical strength to sustain rapid gain
medium circulation. The CpOFRR utilizes a cylindrical ring resonator fused on the inner surface of the COFRR capillary.
Since the capillary wall is thin, the individual WGMs of the cylindrical ring resonator and the COFRR couples strongly
and forms Vernier effect, which provides a way to generate a single mode dye laser.
KEYWORDS: Capillaries, Resonators, Sensors, Curium, Biological research, Statistical analysis, Cancer, Proteins, Molecules, Simulation of CCA and DLA aggregates
We demonstrate the utility of the opto-fluidic ring resonator (OFRR) sensor for the purpose of analyzing the degree of
methylation in sample oligonucleotides. Cytosine methylation, a regular epigenetic function in cellular growth and
metabolism, is prone to abnormal behavior that may lead to uncontrolled suppression of key genes involved with cellular
proliferation. Such behavior is suspected to be strongly related to the occurrence of several types of cancers. The OFRR
is demonstrated as a tool to explore and monitor the degree of methylation in DNA. Two different approaches are
explored, using either bisulfite modification or immunoprecipitation. The methods are compared and the signal response
for both methods is characterized.
Rapid and accurate detection of biomolecules is important for medical diagnosis, pharmaceuticals,
homeland security, food quality control, and environmental protection. A simple, low cost and highly
sensitive label-free optical biosensor based on opto-fluidic ring resonator (OFRR) has been developed that
naturally integrates microfluidics with ring resonators. The OFRR employs a piece of fused silica capillary
with a diameter around 100 micrometers. The circular cross section of the capillary forms the ring resonator
and light repeatedly travels along the resonator circumference in the form of whispering gallery modes
(WGMs) through total internal reflection. When the capillary wall is as thin as a couple of micrometers (< 4 μm), an evanescent field of the WGMs exists at the OFRR inner surface and interacts with the sample when
it flows through the OFRR. In order to detect the target molecules with high specificity, the OFRR inner
surface is functionalized with receptors, such as antibodies, peptide-displayed bacteriophage or
oligonucleotide DNA probes. The WGM spectral position shifts when biomolecules bind to the OFRR
inner surface and change the local refractive index, which provides quantitative and kinetic information
about the biomolecule interaction near the OFRR inner surface. The OFRR has been successfully
demonstrated for detection of various types of biomoelcuels. Here, we will first introduce the basic
operation principle of the OFRR as a sensor and then application examples of the OFRR in the detection of
proteins, disease biomarkers, virus, DNA molecules, and cells with high sensitivities will be presented.
In this work we present a method for creating an integrated optofluidic ring resonator (OFRR) laser system by
embedding it in a low index polymer, polydimethylsiloxane (PDMS). Packaging the OFRR inside PDMS enhances
portability, mechanical stability, and the ability to connect it to chip-based microfluidics. The OFRR retains high Q-factors
even in the polymer (> 106) and exhibits a low lasing threshold (<1 μJ/mm2). Additionally, the laser emission can
be efficiently and directionally coupled out through an optical fiber or fiber prism in touch with the ring resonator. At 2.2
μJ/mm2 pump intensity, the laser output from the fiber is 80 nW, corresponding to 50% power extraction efficiency. Our
work will lead to novel design in lab-on-a-chip devices and micro total analysis systems for biological and chemical
detection.
A rapid, label-free on-line virus detection method has been developed based on opto-fluidic ring resonator (OFRR). The OFRR employs a fused silica capillary with a diameter around 100 μm. The circular cross section of the capillary forms the ring resonator that supports the whispering gallery modes (WGMs). The OFRR wall is only a few micrometers. Thus, the evanescent field of the WGMs extends into the core and interacts with the sample flowing in the core. The WGM spectral position shifts in response to the binding of biomolecules to the OFRR inner surface, providing quantitative and kinetic information about the biomolecule interaction. In this work, M13 filamentous phage and anti-M13 antibody are chosen as a model system to demonstrate the detection and quantification of virus in liquid samples. Anti-M13 antibodies are first covalently attached on the aminosilane coated OFRR surface to provide a bioselective layer. The detection is then performed when the virus concentration varies from 1011 pfu/mL down to 103 pfu/mL. Our experimental results show that the OFRR is capable of detecting M13 at a concentration as low as 1000 pfu/mL. Control experiments are carried out to show the specificity of the detection. A theoretical model is developed to analyze the experimental results. The OFRR are advantageous in virus detection, as it integrates the ring resonator with fluidic channels and provides continuous on-line monitoring capability. It also has great potential for sensitive, rapid, and low-cost micro total analysis devices for biomolecule detection.
The liquid core optical ring resonator (LCORR) integrates an array of optical ring resonators into a microfluidics
channel. The LCORR is made of a micro-sized glass capillary; the circular cross-section of the capillary acts as an
optical ring resonator while the resonating light interacts with the fluid sample passing through the core. Q-factors
larger than 107 have been achieved in LCORRs on the order of 100 micrometers in diameter. This implies an effective
interaction length between the evanescent field of the resonator and the fluidic core of over 10 cm.
The novel integrated architecture and excellent photonic performance lead to a number of applications in sensing,
analytical chemistry, and photonics. For the last decade, optical ring resonators have been explored for label-free
bio/chemical detection. The LCORR architecture possesses the same capabilities as other optical ring resonator
bio/chemical sensors while also integrating micro-capillary-based fluidics with the sensor head. The integrated fluidics
design in combination with the micro-sized sensor head and pico-liter sample volume lead to a lab-on-a-chip sensor for
biomolecules, such as biomarkers and specific DNA sequences. Also, because the ring resonator creates a high-intensity
field inside the microfluidic channel, the LCORR is an excellent microfluidic platform for surface-enhanced
Raman scattering (SERS) detection in silver colloids. Finally, the high quality factor of the capillary-based resonator
enables novel opto-fluidic devices, such as dye lasers. We will discuss the details of these concepts and present our
research results in each of these applications.
We present a novel label-free method of quantifying single stranded DNA concentrations in solution using the Liquid
Core Optical Ring Resonator (LCORR). The LCORR is a glass capillary that is capable of evanescent sensing of
analytes while providing fluidic delivery through its hollow core. An evanescent field is excited in the ring-shaped
circumference of the LCORR by externally coupled photons, which circulate via total internal reflection in the form of
Whispering Gallery Modes (WGM's). When the wall of the capillary is etched to under 5 μm, the evanescent field from
the WGM's is exposed in both the internal and external media.
Chemical modification of the interior of the LCORR enables specific capture of target oligonucleotides by hybridization
with a covalently bound probe. Refractive index changes at this interface are shown to produce a measurable change in
the optical signal by shifting the resonance condition of the cavity.
Real time kinetic analysis of the hybridization between the two complimentary strands is demonstrated as well as a
thorough calibration of the sensor response to strand lengths between 25 and 100 bases and bulk concentration from 0.5
nM to 10 μM. Non-specific binding of completely mismatched oligonucleotides is shown to be minimal and single base
mismatch detection is also demonstrated definitively.
The liquid core optical ring resonator (LCORR) sensor is a newly developed capillary-based ring resonator that
integrates microfluidics with photonic sensing technology. The circular cross-section of the capillary forms a ring
resonator that supports whispering gallery modes (WGM), which interact with the sample as it passes through the
capillary. As in previous ring resonator sensor implementations, the interaction between the WGM evanescent field and
the sample enables label-free detection.
With a prototype of an LCORR sensor, we have achieved a refractive index detection limit of 10-6 RIU and a detection
limit for protein of 2 pg/mm2. Several engineering developments have been accomplished as well, including a thermal
noise characterization, a thermal stabilization implementation, integration of the LCORR with a planar waveguide array,
and electro-kinetic sample delivery. In the near future, the LCORR will be integrated into a dense 2-dimensional
sensing array by integrating multiple capillaries with a chip-based waveguide array. This lab-on-a-chip sensing system
will have a number of applications, including environmental sensing for defense purposes, disease diagnostics for
medical purposes, and as a lab tool for analytical chemistry and molecular analysis.
In parallel to a stand-alone microsphere resonator and a planar ring resonator on a wafer, the liquid core optical ring
resonator (LCORR) is regarded as the third type of ring resonator that integrates microfluidics with state-of-the-art
photonics. The LCORR employs a micro-sized glass capillary with a wall thickness of a few microns. The circular cross
section of the capillary forms a ring resonator that supports the whispering gallery modes (WGMs), which has the
evanescent field in the core, allowing for repetitive interaction with the analytes carried inside the capillary. Despite the
small physical size of the LCORR and sub-nanoliter sensing volume, the effective interaction length can exceed 10 cm
due to high Q-factor (106), significantly improving the LCORR detection limit. The LCORR is a versatile system that
exhibits excellent fluid handling capability inherent to capillaries and permits non-invasive and quantitative
measurement at any location along the capillary. Furthermore, the LCORR uses the refractive index change as a
transduction signal, which enables label-free detection. Therefore, the LCORR is a promising technology platform for
future sensitive, miniaturized, lab-on-a-chip type sensors. In this paper, we will introduce the concept of the LCORR and
present the theoretical analysis and the experimental results related to the LCORR sensor development.
Label-free optical biosensors offer advantages for many applications due to their simplicity and low cost compared to
fluorescence detection. Thus, it is desirable to develop label-free sensors that can be integrated with advanced
microfluidic systems into dense, multi-purpose biosensor arrays. One candidate technology is ring resonators, which
utilize the resonating whispering gallery modes to create a strongly enhanced optical field in the sensing volume.
Because of the high Q-factor of ring resonators, the optical field can be enhanced by 2-3 orders of magnitude, which
leads to much smaller required light-matter interaction length and sensing volume. These are critical characteristics for
dense integration into lab-on-a-chip systems.
We have developed a novel label-free ring resonator sensor based on a liquid core optical ring resonator (LCORR).
This system uses a glass capillary as both the fluidics and the ring resonator. With the LCORR, we have demonstrated a
measurable whispering gallery mode spectral shift of 30 pm/refractive-index-unit (RIU), which leads to a detection limit
of approximately 10-6 RIU. Additionally, we have achieved an estimated detection limit for protein molecules of 10
pg/mm2. These experimental demonstrations of this novel sensing system will lead to the development of highly
sensitive label-free sensors that are well-suited for dense integration with advanced microfluidics for lab-on-a-chip
systems.
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