This project was supported in part by the National Science Foundation and by the Air Force Research Laboratory through the CONTACT program.

1. Sensor Design and Fabrication
<ol>
 <li>Design a parallel-plate waveguide with one or more integrated cavities (or "grooves"). See Figures 1 and 2. Geometry may be based on that given in our previous publications 8,9 or specially designed for the particular application. The following general guiding principles are suggested:
 <ol>
 <li>Plate Spacing: In this experiment a plate spacing of 1mm is used for effective coupling to the TE1 mode without the need for special optics. It also ensures single-mode propagation at the frequencies of interest. When using other plate spacings, multimode propagation, dispersion and coupling efficiency should be considered.</li>
 <li>Spacers: This plate spacing is maintained using dielectric spacers. Small pieces of glass with very uniform thickness make excellent spacers - in our case, we use shards from a shattered microscope slide, with have a thickness of 1 mm +/- 3 &mu;m.</li>
 <li>Plate size: The plates themselves should be wide enough that they can be considered infinite compared to the input beam. (In our case, 4.75 cm for a 1.2 cm beam.) The thickness of each plate must be much thicker than the skin depth, and thicker plates (&gt; 1 cm) are recommended to reduce the possibility of energy passing above or below the waveguide and reaching the detector. Propagation length should be enough that the groove is at least twice its own width away from the input and output faces, but minimized to reduce dispersion.</li>
 <li>Bottom Plate Geometry: To allow for easy access to the groove, the bottom waveguide plate should be significantly wider than the top plate, while the groove extends almost (but not quite) the entire width of the plate. (See Figure 1) This makes it much easier to access the groove and monitor the level of filling.</li>
 <li>Screws: Both top and bottom plate have an extension so that screws may be inserted to hold the waveguide together without obstructing either the grooves or the propagation path. (See Figure 1) The holes in the bottom plate are threaded while top are not.</li>
 <li>Cavity Geometry: Design for the groove will depend on the desired resonant frequency, the desired linewidth, and the chosen plate spacing, among other factors. It is important to consider the limitations of your fabrication techniques for very narrow or very shallow grooves. Multiple grooves for multichannel sensing have additional requirements 10.</li>
 <li>Ungrooved Version: a design identical in every aspect WITHOUT a groove should also be fabricated, to be used as a reference.</li>
 </ol>
 </li>
 <li>Fabrication of the waveguide can be done by machining. IMPORTANT: do not blunt the edges of the plates, particularly on the input face. Rounded edges are standard practice in many machine shops for safety reasons but a rounded edge on the input face will distort the signal.</li>
 <li>Assembly Procedure. After the two plates have been fabricated, they should be assembled into the waveguide.
 <ol>
 <li>Use an L-bracket or other flat objects to create a structure with two flat surfaces perpendicular to one another. Place the bottom plate on the horizontal surface and press it flush against the vertical surface. Place the dielectric spacers as close to the screw holes as possible (two per screw, one on each side), being careful not to obstruct the groove or to extend beyond the input face.</li>
 <li>Carefully place the top plate flush against the vertical surface and slide it down to sit on the bottom plate and spacers. Holding both plates flush against the vertical surface, insert the screws. Screw them down incrementally in an alternating pattern. This procedure leads to a waveguide with a perfectly flat input face and uniform plate spacing.</li>
 </ol>
 </li>
</ol>
2. Experimental Apparatus
This protocol assumes the user has access to a transmission-geometry terahertz time-domain spectrometer (in our case, the Picometrix T-Ray 4,000) and is familiar with obtaining time-domain waveforms and Fourier transforming to the frequency-domain.
<ol>
 <li>Confocal Configuration. If not already present, four lenses should be introduced into the beam path in a confocal orientation in order to provide a tight focus at the midpoint of the path.</li>
 <li>Place an aperture at the focal point. The aperture should be large enough to block all radiation from propagating except through the waveguide. The size of the aperture will determine the beam size propagating in the waveguide (in our case, 12 mm).</li>
 <li>Place waveguide immediately behind the aperture, with the input face in contact with the aperture and with the waveguide propagation axis aligned as closely as possible with the optical axis. The alignment here is critical - reflections, dispersion, variation in the cut-off and resonant frequencies, and other issues may arise due to improper alignment of the waveguide. Use a secure holder to ensure REPEATABLE PLACEMENT.</li>
 <li>Syringe holder: it is useful to have a structure that holds the syringe in place so that the tip is aligned with the groove. By doing this you can reduce the possibility of mistakes in the filling due to the motion of the syringe in your hands.</li>
</ol>
3. Sample Preparation
<ol>
 <li>Cleaning Procedure: Disassemble the waveguide. Wash both plates of the waveguide thoroughly in an appropriate solvent to remove any residue from the experiment. Blow dry with compressed air. Reassemble as in 1.3.</li>
 <li>Syringe Preparation. For best results, we recommend using a different syringe for each material to prevent cross-contamination. If this is not possible, the syringe should also be cleaned with the same solvent.</li>
 <li>Fill syringe to appropriate fill volume with the liquid to be tested. Try to eliminate any bubbles.</li>
</ol>
4. Experimental Procedure
<ol>
 <li>Place the ungrooved reference waveguide in the apparatus as described in (2.3). Take a reference waveform of the ungrooved waveguide, then remove. This is only necessary once every few hr during each experimental session, depending on the long-term stability of the time-domain spectrometer signal.</li>
 <li>Place clean grooved waveguide in apparatus, as described in (2.3)</li>
 <li>Take a waveform for the empty grooved waveguide. NOTE: This must be done every time the waveguide is removed and cleaned. The process of removal and disassembly can lead to very small variations in the geometry of the waveguide. These variations will affect the absolute resonant frequency of the empty and filled grooves but not the observed shift; therefore each "full" measurement requires its own "empty" reference to calculate the shift.</li>
 <li>WITHOUT MOVING THE WAVEGUIDE, put the filled syringe in place in the holder. Slowly fill the groove, keeping watch that the fill is good, with no bubbles or overflow. (How to determine the correct fill amount is described in the Discussion section.) Take another waveform.</li>
 <li>If the system has more than one groove, continue filling grooves and taking waveforms as desired.</li>
 <li>Remove waveguide and clean (as in Step 3).</li>
 <li>Repeat as many times as necessary. For best results, several data sets for each sample are recommended to reduce the error.</li>
</ol>
5. Representative Results
Data analysis of these waveforms is straightforward and can follow the experimenter's usual techniques for transforming to the frequency domain. Frequency spectra such as those given in Figure 3 should result. These can be squared and divided by the reference waveform to obtain power transmission spectra such as Figure 4. The linewidth and central frequency of the resonances for the empty and full waveguides can be measured from these spectra, or Lorentzian fits can be performed to increase the accuracy.
The resonant shift caused by the liquid is merely the difference between the observed central frequencies of the resonances for the empty and full waveguides. To convert this to a refractive index measurement, the relationship between the shift and the RI must be established. This can done experimentally by following this procedure with samples of known index, or computationally by conducting simulations of the groove filled with samples of known index 9, or analytically using mode-matching techniques 8. Once a shift vs. RI curve is established, RI measurements of unknown samples can be accurately performed.
There are a few particular errors that may occur during this procedure. Bubbles or mistakes in the filling of the groove can result in noisy or incorrect data, which is why we recommend multiple data sets for each sample material. Another frequent source of error is in the placement of the waveguides. If the reference and sensor waveguides are placed in exactly the same alignment, any reflections or other artifacts will be the same for both and will divide out of the transmission spectrum. If the alignment is slightly off, the reflections will not divide out and ringing will be observed in the transmission spectra (some minor ringing can be seen in Figure 4). If it is not desirable to retake the data, it is possible to eliminate this ringing by trimming the time-domain waveform before the reflection appears, but this greatly reduces the spectral resolution and therefore the refractive index resolution is limited as well.
<img alt="Figure 1" src="/files/ftp_upload/4304/4304fig1.jpg" /> 
 Figure 1. Photograph of the waveguide with relevant parts marked. Note that the groove does not extend the entire length or width of the waveguide and the structure is designed so that the mounting hardware will not obstruct the groove or the path of radiation propagation.
<img alt="Figure 2" src="/files/ftp_upload/4304/4304fig2.jpg" /> 
 Figure 2. Schematic of the grooved waveguide.
<img alt="Figure 3" src="/files/ftp_upload/4304/4304fig3.jpg" /> 
 Figure 3. (a) Sample frequency spectra for the reference waveguide (black), the grooved waveguide with no liquid fill (blue), and the grooved waveguide with liquid, in this case tetradecane (red). The cutoff frequencies for the TE1 and TE<samp>3</samp> propagation modes are shown, as are the water vapor absorption lines. (b) Closeup of the resonances for the empty and full grooved waveguides.
<img alt="Figure 4" src="/files/ftp_upload/4304/4304fig4.jpg" /> 
 Figure 4. Power transmission spectra for the empty and full grooved waveguides. The difference in frequency between the two resonant features is the resonant shift (&Delta;f), which relates to the refractive index.

<ol>
	<li>Kuswandi, B., Nuriman, ., Huskens, J., Verboom, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+sensing+systems+for+microfluidic+devices:+A+review.">Optical sensing systems for microfluidic devices: A review.</a> Ana. Chim. Acta. 601, 141-155 (2007).</li><li>Zhu, H., White, I. M., Suter, J. D., Zourob, M., Fan, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Integrated+refractive+index+optical+ring+resonator+detector+for+capillary+electrophoresis.">Integrated refractive index optical ring resonator detector for capillary electrophoresis.</a> Anal. Chem. 79, 930-937 (2007).</li><li>Kurt, H., Citrin, D. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coupled-resonator+optical+waveguides+for+biochemical+sensing+of+nanoliter+volumes+of+analyte+in+the+terahertz+region.">Coupled-resonator optical waveguides for biochemical sensing of nanoliter volumes of analyte in the terahertz region.</a> Appl. Phys. Lett. 87, 241119 (2005).</li><li>Debus, C., Bolivar, P. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Frequency+selective+surfaces+for+high+sensitivity+terahertz+sensing.">Frequency selective surfaces for high sensitivity terahertz sensing.</a> Appl. Phys. Lett. 91, 184102 (2007).</li><li>Harsha, S. S., Laman, N., Grischkowsky, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-Q+terahertz+Bragg+rsonances+within+a+metal+parallel+plate+waveguide.">High-Q terahertz Bragg rsonances within a metal parallel plate waveguide.</a> Appl. Phys. Lett. 94, 091118 (2009).</li><li>Mendis, R., Mittleman, D. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparison+of+the+lowest-order+transverse-electric+(TE1)+and+transverse-magnetic+(TEM)+modes+of+the+parallel-plate+waveguide+for+terahertz+pulse+applications.">Comparison of the lowest-order transverse-electric (TE1) and transverse-magnetic (TEM) modes of the parallel-plate waveguide for terahertz pulse applications.</a> Optics Express. 17, 14839-14850 (2009).</li><li>Mendis, R., Mittleman, D. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+investigation+of+the+lowest-order+transverse-electric+(TE1)+mode+of+the+parallel-plate+waveguide+for+THz+pulse+propagation.">An investigation of the lowest-order transverse-electric (TE1) mode of the parallel-plate waveguide for THz pulse propagation.</a> JOSA B. 26, A6-A13 (2009).</li><li>Astley, V., McCracken, B., Mendis, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analysis+of+rectangular+resonant+cavities+in+terahertz+parallel-plate+waveguides.">Analysis of rectangular resonant cavities in terahertz parallel-plate waveguides.</a> Opt. Lett. 36, 1452 (2011).</li><li>Mendis, R., Astley, V., Liu, J., Mittleman, D. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Terahertz+microfluidic+sensor+based+on+a+parallel-plate-waveguide+resonant+cavity.">Terahertz microfluidic sensor based on a parallel-plate-waveguide resonant cavity.</a> Appl. Phys. Lett. 95, 171113 (2009).</li><li>Astley, V., Reichel, K., Jones, J., Mendis, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Terahertz+multichannel+microfluidic+sensor+based+on+parallel-plate+waveguide+resonant+cavities.">Terahertz multichannel microfluidic sensor based on parallel-plate waveguide resonant cavities.</a> Appl. Phys. Lett. , (2012).</li></ol>

No conflicts of interest declared.

It should be noted that the refractive index of the liquid under test is determined only at the frequency of the cavity resonance, not over a broad bandwidth. This has a few distinct advantages. First, although our measurements have made use of a broadband terahertz source for characterization purposes, one could also build an equivalent sensing system with a single-frequency THz source with only a limited degree of frequency tunability, an approach that could be much less expensive and more compact. Second, the ...

<table border="1">
 <tbody>
 <tr>
 <td>Name of the reagent</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments (optional)</td>
 </tr>
 <tr>
 <td>10 &mu;l syringe</td>
 <td>Hamilton</td>
 <td>80314</td>
 <td>High precision syringe</td>
 </tr>
 <tr>
 <td>Liquid alkanes </td>
 <td>Acros Organics</td>
 <td></td>
 <td>Samples for calibration and testing</td>
 </tr>
<tr>
 <td></td>
<td></td>
<td></td>
<td>No specific equipment is required. Suitable test materials and solvents are left to the experimenter's discretion. The high-precision syringes used in this procedure are listed in the table below, but the experimenter may wish to use syringes of a different volume or design, including digital syringes for improved accuracy. The test alkanes used in this experiment are also listed.
</td>
</tr>
 </tbody>
</table>

terahertz microfluidic sensing using a parallel-plate waveguide sensor

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic samples with a wide range of possible sensor designs such as interferometers and resonators 1,2. Most of the existing RI sensing applications focus on biological materials in aqueous solutions in visible and IR frequencies, such as DNA hybridization and genome sequencing. At terahertz frequencies, applications include quality control, monitoring of industrial processes and sensing and detection applications involving nonpolar materials.
Several potential designs for refractive index sensors in the terahertz regime exist, including photonic crystal waveguides 3, asymmetric split-ring resonators 4, and photonic band gap structures integrated into parallel-plate waveguides 5. Many of these designs are based on optical resonators such as rings or cavities. The resonant frequencies of these structures are dependent on the refractive index of the material in or around the resonator. By monitoring the shifts in resonant frequency the refractive index of a sample can be accurately measured and this in turn can be used to identify a material, monitor contamination or dilution, etc.
The sensor design we use here is based on a simple parallel-plate waveguide 6,7. A rectangular groove machined into one face acts as a resonant cavity (Figures 1 and 2). When terahertz radiation is coupled into the waveguide and propagates in the lowest-order transverse-electric (TE1) mode, the result is a single strong resonant feature with a tunable resonant frequency that is dependent on the geometry of the groove 6,8. This groove can be filled with nonpolar liquid microfluidic samples which cause a shift in the observed resonant frequency that depends on the amount of liquid in the groove and its refractive index 9.
Our technique has an advantage over other terahertz techniques in its simplicity, both in fabrication and implementation, since the procedure can be accomplished with standard laboratory equipment without the need for a clean room or any special fabrication or experimental techniques. It can also be easily expanded to multichannel operation by the incorporation of multiple grooves 10. In this video we will describe our complete experimental procedure, from the design of the sensor to the data analysis and determination of the sample refractive index.

Watch this Scientific Journal Video about Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor at JoVE.com

Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor

The procedure for implementing a refractive index sensor for terahertz frequencies based on a grooved parallel-plate waveguide geometry is described here. The method yields a measurement of the refractive index of a small volume of liquid through monitoring of the shift in the resonant frequency of the waveguide structure

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic ...

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10.7K Views.  Rice University. The procedure for implementing a refractive index sensor for terahertz frequencies based on a grooved parallel-plate waveguide geometry is described here. The method yields a measurement of the refractive index of a small volume of liquid through monitoring of the shift in the resonant frequency of the waveguide structure

Video: Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor

Design, Fabrication, and Experimental Characterization of Plasmonic Photoconductive Terahertz Emitters

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on photoconduction, which has been one of the most commonly used techniques for terahertz generation 1-8. Terahertz generation in a photoconductive emitter is achieved by pumping an ultrafast photoconductor with a pulsed or heterodyned laser illumination. The induced photocurrent, which follows the envelope of the pump laser, is routed to a terahertz radiating antenna connected to the photoconductor contact electrodes to generate terahertz radiation. Although the quantum efficiency of a photoconductive emitter can theoretically reach 100%, the relatively long transport path lengths of photo-generated carriers to the contact electrodes of conventional photoconductors have severely limited their quantum efficiency. Additionally, the carrier screening effect and thermal breakdown strictly limit the maximum output power of conventional photoconductive terahertz sources. To address the quantum efficiency limitations of conventional photoconductive terahertz emitters, we have developed a new photoconductive emitter concept which incorporates a plasmonic contact electrode configuration to offer high quantum-efficiency and ultrafast operation simultaneously. By using nano-scale plasmonic contact electrodes, we significantly reduce the average photo-generated carrier transport path to photoconductor contact electrodes compared to conventional photoconductors 9. Our method also allows increasing photoconductor active area without a considerable increase in the capacitive loading to the antenna, boosting the maximum terahertz radiation power by preventing the carrier screening effect and thermal breakdown at high optical pump powers. By incorporating plasmonic contact electrodes, we demonstrate enhancing the optical-to-terahertz power conversion efficiency of a conventional photoconductive terahertz emitter by a factor of 50 10.

We describe methods for the design, fabrication, and experimental characterization of plasmonic photoconductive emitters, which offer two orders of magnitude higher terahertz power levels compared to conventional photoconductive emitters.

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on ...

How to Build a Vacuum Spring-transport Package for Spinning Rotor Gauges

The spinning rotor gauge (SRG) is a high-vacuum gauge often used as a secondary or transfer standard for vacuum pressures in the range of 1.0 x 10-4 Pa to 1.0 Pa. In this application, the SRGs are frequently transported to laboratories for calibration. Events can occur during transportation that change the rotor surface conditions, thus changing the calibration factor. To assure calibration stability, a spring-transport mechanism is often used to immobilize the rotor and keep it under vacuum during transport. It is also important to transport the spring-transport mechanism using packaging designed to minimize the risk of damage during shipping. In this manuscript, a detailed description is given on how to build a robust spring-transport mechanism and shipping container. Together these form a spring-transport package. The spring-transport package design was tested using drop-tests and the performance was found to be excellent. The present spring-transport mechanism design keeps the rotor immobilized when experiencing shocks of several hundred g (g = 9.8 m/sec2 and is the acceleration due to gravity), while the shipping container assures that the mechanism will not experience shocks greater than about 100 g during common shipping mishaps (as defined by industry standards).

Here we describe how to build a robust spring-transport mechanism for a spinning rotor gauge. This device securely immobilizes the rotor and keeps it under vacuum during transportation. We also describe packaging that minimizes the risk of damage during transport. Tests show our design works for typical shocks during transport.

The spinning rotor gauge (SRG) is a high-vacuum gauge often used as a secondary or transfer standard for vacuum pressures in the range of 1.0 x 10-4 Pa to ...

Solvent Bonding for Fabrication of PMMA and COP Microfluidic Devices

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each thermoplastic of interest. Solvent bonding is a simple and versatile method that can be used to fabricate devices from a variety of plastics. An appropriate solvent is added between two device layers to be bonded, and heat and pressure are applied to the device to facilitate the bonding. By using an appropriate combination of solvent, plastic, heat, and pressure, the device can be sealed with a high quality bond, characterized as having high bond coverage, bond strength, optical clarity, durability over time, and low deformation or damage to microfeature geometry. We describe the procedure for bonding devices made from two popular thermoplastics, poly(methyl-methacrylate) (PMMA), and cyclo-olefin polymer (COP), as well as a variety of methods to characterize the quality of the resulting bonds, and strategies to troubleshoot low quality bonds. These methods can be used to develop new solvent bonding protocols for other plastic-solvent systems.

Solvent bonding is a simple and versatile method for fabricating thermoplastic microfluidic devices with high quality bonds. We describe a protocol to achieve strong, optically clear bonds in PMMA and COP microfluidic devices that preserve microfeature details, by a judicious combination of pressure, temperature, an appropriate solvent, and device geometry.

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each ...

Fabrication of Nanopillar-Based Split Ring Resonators for Displacement Current Mediated Resonances in Terahertz Metamaterials

Terahertz (THz) split ring resonator (SRR) metamaterials (MMs) has been studied for gas, chemical, and biomolecular sensing applications because the SRR is not affected by environmental characteristics such as the temperature and pressure surrounding the resonator. Electromagnetic radiation in THz frequencies is biocompatible, which is a critical condition especially for the application of the biomolecular sensing. However, the quality factor (Q-factor) and frequency responses of traditional thin-film based split ring resonator (SRR) MMs are very low, which limits their sensitivities and selectivity as sensors. In this work, novel nanopillar-based SRR MMs, utilizing displacement current, are designed to enhance the Q-factor up to 450, which is around 45 times higher than that of traditional thin-film-based MMs. In addition to the enhanced Q-factor, the nanopillar-based MMs induce a larger frequency shifts (17 times compared to the shift obtained by the traditional thin-film based MMs). Because of the significantly enhanced Q-factors and frequency shifts as well as the property of biocompatible radiation, the THz nanopillar-based SRR are ideal MMs for the development of biomolecular sensors with high sensitivity and selectivity without inducing damage or distortion to biomaterials. A novel fabrication process has been demonstrated to build the nanopillar-based SRRs for displacement current mediated THz MMs. A two-step gold (Au) electroplating process and an atomic layer deposition (ALD) process are used to create sub-10 nm scale gaps between Au nanopillars. Since the ALD process is a conformal coating process, a uniform aluminum oxide (Al2O3) layer with nanometer-scale thickness can be achieved. By sequentially electroplating another Au thin film to fill the spaces between Al2O3 and Au, a close-packed Au-Al2O3-Au structure with nano-scale Al2O3 gaps can be fabricated. The size of the nano-gaps can be well defined by precisely controlling the deposition cycles of the ALD process, which has an accuracy of 0.1 nm.

A protocol for the design and fabrication of a novel nanopillar-based split ring resonator (SRR) is presented.

Terahertz (THz) split ring resonator (SRR) metamaterials (MMs) has been studied for gas, chemical, and biomolecular sensing applications because the SRR ...

Fabrication of Flexible Image Sensor Based on Lateral NIPIN Phototransistors

Flexible photodetectors have been intensely studied for the use of curved image sensors, which are a crucial component in bio-inspired imaging systems, but several challenging points remain, such as a low absorption efficiency due to a thin active layer and low flexibility. We present an advanced method to fabricate a flexible phototransistor array with an improved electrical performance. The outstanding electrical performance is driven by a low dark current owing to deep impurity doping. Stretchable and flexible metal interconnectors simultaneously offer electrical and mechanical stabilities in a highly deformed state. The protocol explicitly describes the fabrication process of the phototransistor using a thin silicon membrane. By measuring I-V characteristics of the completed device in deformed states, we demonstrate that this approach improves the mechanical and electrical stabilities of the phototransistor array. We expect that this approach to a flexible phototransistor can be widely used for the applications of not only next-generation imaging systems/optoelectronics but also wearable devices such as tactile/pressure/temperature sensors and health monitors.

We present a detailed method to fabricate a deformable lateral NIPIN phototransistor array for curved image sensors. The phototransistor array with an open mesh form, which is composed of thin silicon islands and stretchable metal interconnectors, provides flexibility and stretchability. The parameter analyzer characterizes the electrical property of the fabricated phototransistor.

Flexible photodetectors have been intensely studied for the use of curved image sensors, which are a crucial component in bio-inspired imaging systems, ...

Femtosecond Laser Filaments for Use in Sub-Diffraction-Limited Imaging and Remote Sensing

Probing remote matter with laser light is a ubiquitous technique used in circumstances as diverse as laser-induced breakdown spectroscopy and barcode scanners. In classical optics, the intensity that can be brought to bear on a remote target is limited by the spot size of the laser at the distance of the target. This spot size has a lower bound determined by the diffraction limit of classical optics. However, amplified femtosecond laser pulses generate intensity sufficient to modify the refractive index of the ambient air and undergo self-focusing. This self-focusing effect leads to the generation of highly intense laser filaments which maintain their intensity and small sub-millimeter diameter size at distances well beyond the classical Rayleigh length. Such intensity provides the capability of remote scanning, imaging, sensing, and spectroscopy with enhanced spatial resolution. We describe a technique for generating filaments with a femtosecond regenerative chirped-pulse amplifier, and for using the resulting filament to conduct imaging and spectroscopic measurements at remote distances of at least several meters.

High-intensity femtosecond pulses of laser light can undergo cycles of Kerr self-focusing and plasma defocusing, propagating an intense sub-millimeter-diameter beam over long distances. We describe a technique for generating and using these filaments to perform remote imaging and sensing beyond the classical diffraction limits of linear optics.

Probing remote matter with laser light is a ubiquitous technique used in circumstances as diverse as laser-induced breakdown spectroscopy and barcode ...

A Silicon-tipped Fiber-optic Sensing Platform with High Resolution and Fast Response

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This FOSP relies on a silicon Fabry-Perot interferometer (FPI) attached to the fiber end, referred to as Si-FOSP in this work. The Si-FOSP generates an interferogram determined by the optical path length (OPL) of the silicon cavity. Measurand alters the OPL and thus shifts the interferogram. Due to the unique optical and thermal properties of the silicon material, this Si-FOSP exhibits an advantageous performance in terms of sensitivity and speed. Furthermore, the mature silicon fabrication industry endows the Si-FOSP with excellent reproducibility and low cost toward practical applications. Depending on the specific applications, either a low-finesse or high-finesse version will be utilized, and two different data demodulation methods will be adopted accordingly. Detailed protocols for fabricating both versions of the Si-FOSP will be provided. Three representative applications and their according results will be shown. The first one is a prototype underwater thermometer for profiling the ocean thermoclines, the second one is a flow meter to measure flow speed in the ocean, and the last one is a bolometer used for monitoring exhaust radiation from magnetically confined high-temperature plasma.

This work reports an innovative silicon-tipped fiber-optic sensing platform (Si-FOSP) for high-resolution and fast-response measurement of a variety of physical parameters, such as temperature, flow, and radiation. Applications of this Si-FOSP span from oceanographic research, mechanical industry, to fusion energy research.

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This ...

Terahertz Imaging and Characterization Protocol for Freshly Excised Breast Cancer Tumors

This manuscript presents a protocol to handle, characterize, and image freshly excised human breast tumors using pulsed terahertz imaging and spectroscopy techniques. The protocol involves terahertz transmission mode at normal incidence and terahertz reflection mode at an oblique angle of 30&#176;. The collected experimental data represent time domain pulses of the electric field. The terahertz electric field signal transmitted through a fixed point on the excised tissue is processed, through an analytical model, to extract the refractive index and absorption coefficient of the tissue. Utilizing a stepper motor scanner, the terahertz emitted pulse is reflected from each pixel on the tumor providing a planar image of different tissue regions. The image can be presented in time or frequency domain. Furthermore, the extracted data of the refractive index and absorption coefficient at each pixel are utilized to provide a tomographic terahertz image of the tumor. The protocol demonstrates clear differentiation between cancerous and healthy tissues. On the other hand, not adhering to the protocol can result in noisy or inaccurate images due to the presence of air bubbles and fluid remains on the tumor surface. The protocol provides a method for surgical margins assessment of breast tumors.

Freshly excised human breast cancer tumors are characterized with terahertz spectroscopy and imaging following fresh tissue handling protocols. Tissue positioning is taken into consideration to enable effective characterization while providing analysis in a timely manner for future intraoperative applications.

This manuscript presents a protocol to handle, characterize, and image freshly excised human breast tumors using pulsed terahertz imaging and spectroscopy ...

Measuring Sub-23 Nanometer Real Driving Particle Number Emissions Using the Portable DownToTen Sampling System

The current particle size threshold of the European Particle Number (PN) emission standards is 23 nm. This threshold could change because future combustion engine vehicle technology may emit large amounts of sub-23 nm particles. The Horizon 2020 funded project DownToTen (DTT) developed a sampling and measurement method to characterize particle emissions in this currently unregulated size range. A PN measurement system was developed based on an extensive review of the literature and laboratory experiments testing a variety of PN measurement and sampling approaches. The measurement system developed is characterized by high particle penetration and versatility, which enables the assessment of primary particles, delayed primary particles, and secondary aerosols, starting from a few nanometers in diameter. This paper provides instruction on how to install and operate this Portable Emission Measurement System (PEMS) for Real Drive Emissions (RDE) measurements and assess particle number emissions below the current legislative limit of 23 nm.

Presented here is the DownToTen (DTT) portable emission measurement system to assess real driving automotive emissions of sub-23 nm particles.

The current particle size threshold of the European Particle Number (PN) emission standards is 23 nm. This threshold could change because future ...

Electroantennography-based Bio-hybrid Odor-detecting Drone using Silkmoth Antennae for Odor Source Localization

Small drones with chemical or biosensor devices that can detect airborne odorant molecules have attracted considerable attention owing to their applicability in environmental and security monitoring and search-and-rescue operations. Small drones with commercial metal-oxide-semiconductor (MOX) gas sensors have been developed for odor source localization; however, their real-time-odor-detection performance has proven inadequate. However, biosensing technologies based on insect olfactory systems exhibit relatively high sensitivity, selectivity, and real-time response with respect to odorant molecules compared to commercial MOX gas sensors. In such devices, excised insect antennae function as portable odorant biosensor elements and have been found to deliver excellent sensing performance. This study presents experimental protocols for odorant-molecule detection in the air using a small autonomous bio-hybrid drone based on a mountable electroantennography (EAG) device incorporating silkmoth antennae.
We developed a mountable EAG device including sensing/processing parts with a Wi-Fi module. The device was equipped with a simple sensor enclosure to enhance the sensor directivity. Thus, odor source localization was conducted using the spiral-surge algorithm, which does not assume an upwind direction. The experimental bio-hybrid odor-detecting drone identified real-time odorant-concentration differences in a pseudo-open environment (outside a wind tunnel) and localized the source. The developed drone and associated system can serve as an efficient odorant molecule-detection tool and a suitable flight platform for developing odor source localization algorithms owing to its high programmability.

This study introduces experimental protocols for a bio-hybrid odor-detecting drone based on silkmoth antennae. The operation of an experimental electroantennogram device with silkmoth antennae is presented, in addition to the structure of a bio-hybrid drone designed for odor source localization using the spiral-surge algorithm.