The work presented here was conducted in the framework of the FFG funded project &#34;Green Sensing&#34; and the NATO SPS program &#34;Photonic Nano Particle Sensors for Detecting CBRN events&#34;. The work was also supported by TU Graz Open Access Publishing Fund.

Fabrication of a Fabry-P&#233;rot Etalon for Application in Trace-Gas Spectroscopy

<ol>
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C., Geskus, D., Pollnau, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabry-P&#233;rot+resonator:+Spectral+line+shapes,+generic+and+related+Airy+distributions,+linewidths,+finesses,+and+performance+at+low+or+frequency-dependent+reflectivity.">Fabry-P&#233;rot resonator: Spectral line shapes, generic and related Airy distributions, linewidths, finesses, and performance at low or frequency-dependent reflectivity.</a> Optics Express. 24 (15), 16366-16389 (2016).</li><li>Eklund, E. J., Shkel, A. M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Factors+affecting+the+performance+of+micromachined+sensors+based+on+Fabry-Perot+interferometry.">Factors affecting the performance of micromachined sensors based on Fabry-Perot interferometry.</a> Journal of Micromechanics and Microengineering. 15 (9), 1770-1776 (2005).</li><li>Rees, D., Fuller-Rowell, T. J., Lyons, A., Killeen, T. L., Hays, P. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stable+and+rugged+etalon+for+the+Dynamics+Explorer+Fabry-P&#233;rot+interferometer.+1:+Design+and+construction.">Stable and rugged etalon for the Dynamics Explorer Fabry-P&#233;rot interferometer. 1: Design and construction.</a> Applied Optics. 21 (21), 3896-3902 (1982).</li><li>Killeen, T. L., Hays, P. B., Kennedy, B. C., Rees, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stable+and+rugged+etalon+for+the+Dynamics+Explorer+Fabry-P&#233;rot+interferometer.+2:+Performance.">Stable and rugged etalon for the Dynamics Explorer Fabry-P&#233;rot interferometer. 2: Performance.</a> Applied Optics. 21 (21), 3903-3912 (1982).</li><li>Marques, D. M., Guggenheim, J. A., Munro, P. R. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analysing+the+impact+of+non-parallelism+in+Fabry-P&#233;rot+etalons+through+optical+modelling.">Analysing the impact of non-parallelism in Fabry-P&#233;rot etalons through optical modelling.</a> Optics Express. 29 (14), 21603-21614 (2021).</li><li>Marques, D. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modelling+Fabry-P&#233;rot+etalons+illuminated+by+focussed+beams.">Modelling Fabry-P&#233;rot etalons illuminated by focussed beams.</a> Optics Express. 28 (5), 7691-7706 (2020).</li><li>Islam, M. R., Ali, M. M., Lai, M. -. H., Lim, K. -. S., Ahmad, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chronology+of+Fabry-Perot+interferometer+fiber-optic+sensors+and+their+applications:+A+review.">Chronology of Fabry-Perot interferometer fiber-optic sensors and their applications: A review.</a> MDPI Sensors. 14 (4), 7451-7488 (2014).</li><li>Preisser, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=All-optical+highly+sensitive+akinetic+sensor+for+ultrasound+detection+and+photoacoustic+imaging.">All-optical highly sensitive akinetic sensor for ultrasound detection and photoacoustic imaging.</a> Biomedical Optics Express. 7 (10), 4171-4186 (2016).</li><li>Chen, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Micro-fiber-optic+acoustic+sensor+based+on+high-Q+resonance+effect+using+Fabry-P&#233;rot+etalon.">Micro-fiber-optic acoustic sensor based on high-Q resonance effect using Fabry-P&#233;rot etalon.</a> Optics Express. 29 (11), 16447-16454 (2021).</li><li>Bialkowski, S. E., Astrath, N. G. C., Proskurnin, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Photothermal Spectroscopy Methods. , (2019).</li><li>Campillo, A. J., Petuchowski, S. J., Davis, C. C., Lin, H. -. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabry-P&#233;rot+photothermal+trace+detection.">Fabry-P&#233;rot photothermal trace detection.</a> Applied Physics Letters. 41 (4), 327-329 (1982).</li><li>Breitegger, P., Lang, B., Bergmann, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intensity+modulated+photothermal+measurements+of+NO2+with+a+compact+fiber-coupled+Fabry-P&#233;rot+interferometer.">Intensity modulated photothermal measurements of NO2 with a compact fiber-coupled Fabry-P&#233;rot interferometer.</a> MDPI.Sensors. 19 (15), 1424 (2019).</li><li>Waclawek, J. P., Kristament, C., Moser, H., Lendl, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Balanced-detection+interferometric+cavity-assisted+photothermal+spectroscopy.">Balanced-detection interferometric cavity-assisted photothermal spectroscopy.</a> Optics Express. 27 (9), 12183-12195 (2019).</li><li>Pevec, S., Donlagic, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Miniature+all-fiber+Fabry-P&#233;rot+sensor+for+simultaneous+measurement+of+pressure+and+temperature.">Miniature all-fiber Fabry-P&#233;rot sensor for simultaneous measurement of pressure and temperature.</a> Applied Optics. 51 (19), 4536-4541 (2012).</li><li>Dudzik, G., Krzempek, K., Abramski, K., Wysocki, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Solid-state+laser+intra-cavity+photothermal+gas+sensor.">Solid-state laser intra-cavity photothermal gas sensor.</a> Sensors and Actuators B: Chemical. 328, 129072 (2021).</li><li>Chen, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Micro-fiber-optic+acoustic+sensor+based+on+high-Q+resonance+effect+using+Fabry-P&#233;rot+etalon.">Micro-fiber-optic acoustic sensor based on high-Q resonance effect using Fabry-P&#233;rot etalon.</a> Optics Express. 29 (11), 16447-16454 (2021).</li><li>H&#228;lg, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+silicon+pressure+sensor+with+a+low-cost+contactless+interferometric+optical+readout.">A silicon pressure sensor with a low-cost contactless interferometric optical readout.</a> Sensors and Actuators A: Physical. 30 (3), 225-230 (1992).</li><li>Fischer, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+microphone+hears+ultrasound.">Optical microphone hears ultrasound.</a> Nature Photonics. 10 (6), 356-358 (2016).</li><li>Waclawek, J. P., Bauer, V. C., Moser, H., Lendl, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=2f-wavelength+modulation+Fabry-P&#233;rot+photothermal+interferometry.">2f-wavelength modulation Fabry-P&#233;rot photothermal interferometry.</a> Optics Express. 24 (25), 28958-28967 (2016).</li><li>Chen, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Acoustic+performance+study+of+fiber-optic+acoustic+sensors+based+on+Fabry-P&#233;rot+etalons+with+different+Q+factors.">Acoustic performance study of fiber-optic acoustic sensors based on Fabry-P&#233;rot etalons with different Q factors.</a> MDPI Micromachines. 13 (1), 118 (2022).</li><li>Meyendorf, N., Ida, N., Singh, R., Vran, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Handbook of Nondestructive Evaluation 4.0. , (2022).</li><li>Kim, Y. J., Celliers, P. M., Eggert, J. H., Lazicki, A., Millot, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interferometric+measurements+of+refractive+index+and+dispersion+at+high+pressure.">Interferometric measurements of refractive index and dispersion at high pressure.</a> Scientific Reports. 11, 5610 (2021).</li><li>Stollberger, W. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single+particle+photothermal+interferometry.">Single particle photothermal interferometry.</a> Technical University Graz. , (2022).</li><li>Radeschnig, U., Bergmann, A., Lang, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flow-enhanced+photothermal+spectroscopy.">Flow-enhanced photothermal spectroscopy.</a> MDPI Sensors. 22 (19), 7148 (2022).</li></ol>

There are no conflicts of interest.

As the FPE fabricated following the protocol given here is optimized for a specific application, possible adaptations and critical steps are explained in this chapter. First of all, the FPE and the measurement cell are designed for PTI measurements. Therefore, a gas inlet and outlet, as well as a channel for the excitation laser, which is perpendicular to the probe laser, are added to the cell. All the openings of the cell are either made air-tight via O-rings and/or covered via UVFS windows to allow laser propagation. If used differently, the cell, as given in Supplementary Coding File 1, can be redesigned and adapted to the specific application. The threading in step 1.4 is done post printing. The threads could also be 3D-printed, but as these tend to wear out fast, only holes with the appropriate core hole diameter are printed, and these are threaded afterward.
The choice of material for the spacers in step 2.1 is crucial. The parallelism of the spacers determines the parallelism of the etalon mirrors and, hence, influences the finesse7. A &#189; inch UVFS precision window, as provided in the Table of Materials, with a parallelism of &#8804;5 arcsecs and a surface flatness of &#955;/10 over the clear aperture was used in this study. The coefficient of thermal expansion of UVFS is 0.55 x 10&#8722;6/&#176;C. The temperature stability can be further increased by using, for example, Zerodur5 spacers, with a coefficient of thermal expansion lower than&#160;0.1 x 10&#8722;6/&#176;C; however, this has the disadvantage of higher costs.
The FPE is formed by one fully reflecting mirror, as well as a beamsplitter. The beamsplitter has one 70% reflecting surface, as well as an anti-reflective-coated back side. This enables the coupling of the light in and out of the etalon. Additionally, the beamsplitter&#39;s substrate features one wedged side to prevent unwanted etalon effects. The back side of the mirror is roughened for the same reasons.
In step 5.1, the optoelectronic setup for tracking the alignment process is described. All the fibers used are standard SMF-28 fibers with FC/APC connectors. Due to the designated application for PTI, a balanced photodetector was readily available in this study, but this is not necessary in general. A conventional photodetector can be used instead; in this case, using a 1 x 2 coupler is obsolete. These changes do not affect the other components of the setup, as presented in Figure 5. The triangular current modulation of the probe laser, as described in step 5.4, corresponds to a wavelength sweep. A current range sufficient to sweep over at least one reflectance peak of the FPE has to be chosen. Therefore, one FSR can serve as a rule of thumb. Calculations for the FSR of an ideal FPE can be found in the introduction section. Together with the current tuning coefficient (nm/mA) of the laser, given in the respective manual, the current range covering one FSR can be calculated. As an example, the laser used in this work had a current tuning coefficient of 0.003 nm/mA and emitted at a wavelength of 1,550 nm. The expected FSR of an ideal FPE with 3 mm mirror spacing, d, is approximately 0.4 nm. This gives a current tuning range of 133 mA.
In this work, the modulation frequency was set to 100 Hz for convenient display at the oscilloscope. As the desired current tuning range is rather large, a fixed-fiber attenuator can be used to remain within the power limits of the used detector. The attenuator can be mounted directly after the isolator.
The UV-curing adhesive used in step 6 and step 7 is transparent to laser light and has a refractive index of 1.56. The alignment process, as described in step 7.1,&#160;is dependent on the available photodetector. The balanced detector used in this setup generates a negative voltage &#34;Signal&#34; output. For reasons of generality, a positive voltage output is assumed for the description of step 7.10 and in Figure 6. For a well-aligned etalon, the reflectance peak will go toward zero, while the triangular function will increase its peak-to-peak ratio.
For the etalon characterization in step 8.1, numerical calculation software is used (see Table of Materials). The measured voltage for each temperature step is averaged and plotted, as shown in Figure 7. To convert the temperature steps into wavelength steps, the temperature tuning coefficient of the probe laser is used. Signal analysis libraries have integrated peak-finding algorithms, which can be used for that purpose. As the data analysis strongly depends on the data format, no code is provided here, but it can be made available by the corresponding author upon request.
A possible limitation of the fabrication technique presented here is the thermal and mechanical stability in changing environments. As the scope of this instructional paper is the low-cost prototyping of FPEs for laboratory applications, no tests concerning mechanical and temperature stability are given here. If the FPE is used for mobile applications or in changing environments, additional measures have to be taken in order to mechanically stabilize the fiber-GRIN lens system relative to the etalon.
A new method to fabricate and characterize an FPE is demonstrated here with standard optical components available in every photonic laboratory. The presented FPE has a finesse of approximately 15 and a sensitivity sufficient for detecting approximately 5 ppmV of water vapor. Besides the presented application for PTI, this FPE could be used in applications such as building optical microphones20, which are commonly applied in the field of non-destructive testing23, refractive index measurements24,25, or hygrometers26, just to name a few.

In its most basic form, an FPE consists of two plane-parallel partially reflecting mirror surfaces1. In the following explanations, when referring to mirrors, the optical substrate and the reflective coating are addressed as one. In most applications, the mirrors used feature one wedged surface2 to prevent unwanted etalon effects. Figure 1 illustrates the formation of the interference pattern of an air-spaced etalon (Figure 1A), as well as the reflectance function for different mirror reflectivities (Figure 1B).
The light enters the cavity through one mirror, undergoes multiple reflections, and leaves the cavity by reflection as well as transmission. As this article focuses on the fabrication of an FPE operated in reflectance, the further explanations refer to reflection specifically. The waves leaving the cavity interfere, depending on the phase difference, q = 4&#960;nd/&#955;. Here, n is the refractive index inside the cavity, d is the mirror spacing, and &#955; is the wavelength of the interferometer&#39;s light source, here called the probe laser. A minimum reflectance occurs when the optical path difference matches the integer multiple of the wavelength, <img alt="Equation 2" src="/files/ftp_upload/65174/65174eq02.jpg" style="margin: auto;" />. The finesse of an ideal plane-parallel etalon is determined by the mirror reflectivities R1 and R2 only3:
<img alt="Equation 3" src="/files/ftp_upload/65174/65174eq03.jpg" style="margin: auto;" />
However, a real etalon is subject to many losses, which degrade the theoretically achievable finesse4,5,6. Deviation of the mirror parallelism7, non-normal incidence of the laser beam, beam shape8, mirror surface impurities, and scattering, among others, lead to a reduction in the finesse. The characteristic interference pattern can be described by the Airy function1:
<img alt="Equation 4" src="/files/ftp_upload/65174/65174eq04.jpg" style="margin: auto;" />
The full width at half maximum (FWHM), as well as the free spectral range (FSR) of the reflectance function, can be calculated as follows:
<img alt="Equation 5" src="/files/ftp_upload/65174/65174eq05.jpg" style="margin: auto;" />
<img alt="Equation 6" src="/files/ftp_upload/65174/65174eq06.jpg" style="margin: auto;" />
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/65174/65174fig01.jpg" /> 
Figure 1: Fabry-P&#233;rot interferometer theory. (A) A schematic depiction of the multi-beam interference for an air-spaced etalon with wedged windows. A plane wave, E0, enters the cavity under a certain angle, &#966;, through an anti-reflection (AR)-coated surface and subsequently undergoes multiple reflections between the highly reflecting (high R) surfaces spaced at a distance, d. With each reflection, part of the light is out-coupled of the etalon either in transmission or reflection, where it interferes with the other waves. (B) The reflectance function of an ideal Fabry-P&#233;rot etalon for different mirror reflectivities (y-axis). <a href="https://www.jove.com/files/ftp_upload/65174/65174fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
FPEs can be found in a wide range of applications9,10,11. In the case presented here, the FPE is used in a photothermal interferometry (PTI) setup. In PTI, small density and, hence, refractive index changes, induced by the periodic excitation followed by the fast thermalization of a target gas via a second laser, are measured interferometrically12. The amount of heat and, thus, the magnitude of the refractive index change are proportional to the gas concentration. When measuring the intensity of the reflectance function of the FPE at its steepest point (operation point), these refractive index changes shift the reflectance function, thereby altering the measured intensity. As the reflectance function can be assumed to be linear in the region around the operation point, the measured signal is then proportional to the gas concentration. The sensor&#39;s sensitivity is determined by the slope of the reflectance function and is, therefore, proportional to the finesse. PTI, in combination with FPEs, has proven to be a sensitive and selective method to detect trace amounts of gases and aerosols13,14,15,16,17,18. In the past, many sensors for pressure and acoustic measurements relied on the use of moveable parts, like membranes, substituting the second mirror of the FPE19. Deflections of the membrane lead to a change in the mirror distance and, thus, the optical path length. These instruments have the disadvantage of being prone to mechanical vibrations. In recent years, the development of optical microphones using solid FPEs has reached a commercial level20. By abstaining from the use of moveable parts, the measurand changed from distance to the refractive index inside the Fabry-P&#233;rot cavity, thus increasing the ruggedness of the sensors significantly.
Commercially available air-spaced FPEs cost beyond what is acceptable for prototyping and testing, as well as high-volume production instrument integration. Most scientific publications constructing and using such FPEs discuss the topic of fabrication only minimally21,22. In most cases, specific equipment and machines (e.g., clean rooms, coating facilities, etc.) are necessary; for example, for fully-fiber-integrated FPEs, special micromachining equipment is necessary. To reduce the manufacturing costs and enable the testing of multiple different FPE configurations to enhance their suitability for PTI setups, a new fabrication method was developed, which is described in detail in the following protocol. By using only commercially available, standard bulk-optic and telecom fiber-optic components, the manufacturing costs could be reduced to less than &#8364;400 euros. Every facility working with standard photonic equipment should be able to reproduce our fabrication scheme and adapt it to their applications.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Auto-Balanced Photoreceiver Nirvana</td>
			<td>New Focus, Inc.</td>
			<td>2017</td>
			<td>Balanced Photodetector</td>
		</tr>
		<tr>
			<td>Benchtop laser diode/TEC controller, 1A/96 W</td>
			<td>Thorlabs</td>
			<td>ITC4001</td>
			<td></td>
		</tr>
		<tr>
			<td>Butterfly laser diode mount</td>
			<td>Thorlabs</td>
			<td>LM14S2</td>
			<td></td>
		</tr>
		<tr>
			<td>Clamping fork</td>
			<td>Thorlabs</td>
			<td>CF175</td>
			<td></td>
		</tr>
		<tr>
			<td>compactRIO</td>
			<td>National Instruments</td>
			<td></td>
			<td>For data aquisition</td>
		</tr>
		<tr>
			<td>Dust remover</td>
			<td>RS Components</td>
			<td>168-1644</td>
			<td></td>
		</tr>
		<tr>
			<td>FC/APC to FC/APC Single L-Bracket Mating Sleeve</td>
			<td>Thorlabs</td>
			<td>ADAFCB3</td>
			<td>Multiple needed</td>
		</tr>
		<tr>
			<td>Fiber cleaning fluid</td>
			<td>Thorlabs</td>
			<td>RCS3</td>
			<td></td>
		</tr>
		<tr>
			<td>Fiber optic SM circulator</td>
			<td>AFW technologies</td>
			<td>CIR-3-15-L-1-2</td>
			<td></td>
		</tr>
		<tr>
			<td>Fiber optic SM coupler 1 x 2, 90/10</td>
			<td>AFW technologies</td>
			<td>FOBC-1-15-10-L-1-S-2</td>
			<td>Only if balanced photodetector is used</td>
		</tr>
		<tr>
			<td>Fiber optic SM isolator</td>
			<td>AFW technologies</td>
			<td>ISOD-15-L-1-2</td>
			<td></td>
		</tr>
		<tr>
			<td>Fiber optic storage reels</td>
			<td>Thorlabs</td>
			<td>FSR1</td>
			<td>Multiple needed</td>
		</tr>
		<tr>
			<td>Fixed fiber optic attenuator</td>
			<td>Thorlabs</td>
			<td>FA15T-APC</td>
			<td>Different attenuation levels used</td>
		</tr>
		<tr>
			<td>GRIN/Ferrule Sleeve, 1.818 mm Internal Diameter, 10 mm Length, Borosilicate Glass</td>
			<td>Thorlabs</td>
			<td>51-2800-1800</td>
			<td>Fiber-GRIN-lens system</td>
		</tr>
		<tr>
			<td>GRIN Lens, &#216;1.8 mm, 0.23 Pitch, 8&#176;, 1560 nm Design Wavelength, AR Coated: 1250 - 1650 nm</td>
			<td>Thorlabs</td>
			<td>GRIN2315A</td>
			<td>Fiber-GRIN-lens system</td>
		</tr>
		<tr>
			<td>Handheld UV-LED lamp</td>
			<td>RS Components</td>
			<td>220-6819</td>
			<td>Lamp for curing the adhesive&#160;</td>
		</tr>
		<tr>
			<td>High precision stage and base</td>
			<td>Newport</td>
			<td>9062-X-M</td>
			<td>Three nedded</td>
		</tr>
		<tr>
			<td>Hose conector</td>
			<td>RS Components</td>
			<td></td>
			<td>M5 threaded</td>
		</tr>
		<tr>
			<td>Large Goniometer, 44.5 mm Distance to Point of Rotation, &#177;5&#176;, Metric</td>
			<td>Thorlabs</td>
			<td>GNL18/M</td>
			<td>Two needed</td>
		</tr>
		<tr>
			<td>L-Bracket Mating Sleeve</td>
			<td>Thorlabs</td>
			<td>ADAFCB3</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnetic button clamps</td>
			<td>Thorlabs</td>
			<td>BM075</td>
			<td>Multiple needed</td>
		</tr>
		<tr>
			<td>Micrometer screw</td>
			<td>Newport</td>
			<td>9355</td>
			<td>Three nedded</td>
		</tr>
		<tr>
			<td>MIL-A-3920 Optical Adhesive with Resiliency, 1 oz.</td>
			<td>Thorlabs</td>
			<td>NOA61</td>
			<td>UV-curing adhesive</td>
		</tr>
		<tr>
			<td>Mounting Base, 50 mm x 75 mm x 10 mm</td>
			<td>Thorlabs</td>
			<td>BA2/M</td>
			<td></td>
		</tr>
		<tr>
			<td>O-Rings</td>
			<td>Haberkorn</td>
			<td></td>
			<td>Sizes given in text</td>
		</tr>
		<tr>
			<td>Passive component fiber tray</td>
			<td>Thorlabs</td>
			<td>BFCT</td>
			<td>Multiple needed</td>
		</tr>
		<tr>
			<td>Pedestal base adapter</td>
			<td>Thorlabs</td>
			<td>BE1</td>
			<td></td>
		</tr>
		<tr>
			<td>Pigtailed Ferrule, &#216;1.8 mm, 8&#176;, FC/APC, AR Coated: 1310/1550 nm</td>
			<td>Thorlabs</td>
			<td>SMPF0115-APC</td>
			<td>Fiber-GRIN-lens system</td>
		</tr>
		<tr>
			<td>Post holder</td>
			<td>Thorlabs</td>
			<td>PH30/M</td>
			<td></td>
		</tr>
		<tr>
			<td>Post-Mountable &#216;2.5 mm Ferrule Clamp, M4 Tap</td>
			<td>Thorlabs</td>
			<td>FCM/M</td>
			<td></td>
		</tr>
		<tr>
			<td>Python</td>
			<td>Python</td>
			<td>3.9</td>
			<td>Numerical data analysis software</td>
		</tr>
		<tr>
			<td>Right-angled-bracket</td>
			<td>Newport</td>
			<td>9062-A-M</td>
			<td></td>
		</tr>
		<tr>
			<td>Self-centering lens mount</td>
			<td>Thorlabs</td>
			<td>SCL03</td>
			<td></td>
		</tr>
		<tr>
			<td>Silberschnitt 3001</td>
			<td>Bohle&#160;</td>
			<td>3001</td>
			<td>Glas cutter set</td>
		</tr>
		<tr>
			<td>SM1-threaded standard cage plate</td>
			<td>Thorlabs</td>
			<td>CP33/M</td>
			<td></td>
		</tr>
		<tr>
			<td>UV-curing device</td>
			<td>Formlabs</td>
			<td>Form Cure</td>
			<td></td>
		</tr>
		<tr>
			<td>1550 nm 20 mW butterfly DFB laser diode</td>
			<td>AeroDiode</td>
			<td>1550LD-5-0-0-2</td>
			<td></td>
		</tr>
		<tr>
			<td>3D-printer</td>
			<td>Formlabs</td>
			<td>3+</td>
			<td></td>
		</tr>
		<tr>
			<td>&#216;1/2&#34; UVFS Broadband Precision Window, Uncoated, t = 3 mm</td>
			<td>Thorlabs</td>
			<td>WG40530</td>
			<td>Spacers</td>
		</tr>
		<tr>
			<td>&#216;1/2&#34; Broadband Dielectric Mirror, 1280 - 1600 nm</td>
			<td>Thorlabs</td>
			<td>BB05-E04</td>
			<td>Mirror</td>
		</tr>
		<tr>
			<td>&#216;1/2&#34; 70:30 (R:T) UVFS Plate Beamsplitter, Coating: 1.2 - 1.6 &#181;m, t = 3 mm</td>
			<td>Thorlabs</td>
			<td>BST06</td>
			<td>Beamsplitter</td>
		</tr>
	</tbody>
</table>

fabrication of a low-cost, fiber-coupled, and air-spaced fabry-p&#233;rot etalon

Fabry-P&#233;rot etalons (FPE) have found their way into many applications. In fields such as spectroscopy, telecommunications, and astronomy, FPEs are used for their high sensitivity as well as their exceptional filtering capability. However, air-spaced etalons with high finesse are usually built by specialized facilities. Their production requires a clean room, special glass handling, and coating machinery, meaning commercially available FPEs are sold for a high price. In this article, a new and cost-effective method to fabricate fiber-coupled FPEs with standard photonic laboratory equipment is presented. The protocol should serve as a step-by-step guide for the construction and characterization of these FPEs. We hope this&#160;will enable researchers to conduct fast and cost-effective prototyping of FPEs for various fields of application. The FPE, as presented here, is used for spectroscopic applications. As shown in the representative results section via proof of principle measurements of water vapor in ambient air, this FPE has a finesse of 15, which is sufficient for the photothermal detection of trace concentrations of gases.

As can be seen in Figure 7, an FPE with a well-defined reflectance function could be fabricated.
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/65174/65174fig07.jpg" /> 
Figure 7: Measured reflectance function of the finished FPE. A temperature sweep, corresponding to a wavelength sweep of the laser, was performed to measure the reflectance function of the FPE. This is used to evaluate metrics like the full width at half maximum (FWHM) and the free spectral range (FSR) of the fabricated device. Relative reflectance refers to the relative proportion of light being back-reflected into the fiber after passing the FPE. <a href="https://www.jove.com/files/ftp_upload/65174/65174fig07large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The measured metrics of the FPE are listed in Table 1 and compared to the calculated values of an ideal etalon with the same specifications. The formulas for an ideal FPE can be found in the introduction section.
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td></td>
			<td>Measured</td>
			<td>Ideal FPE</td>
		</tr>
		<tr>
			<td>Finesse</td>
			<td>12.8</td>
			<td>17.1</td>
		</tr>
		<tr>
			<td>FWHM</td>
			<td>0.0268 nm</td>
			<td>0.0234 nm</td>
		</tr>
		<tr>
			<td>FSR</td>
			<td>0.3441 nm</td>
			<td>0.4004 nm</td>
		</tr>
		<tr>
			<td>Sensitivity</td>
			<td>14 1/nm</td>
			<td>21 1/nm</td>
		</tr>
	</tbody>
</table>
Table 1: Comparison of the measured and calculated metrics of the fabricated FPE etalon.
To validate the aptitude for a designated application, the FPE is used for PTI measurements of water vapor in ambient air. Therefore, an excitation laser with a wavelength of 1,364 nm is guided into the cell perpendicularly to the probe laser. Both lasers intersect inside the FPE. The excitation laser is modulated sinusoidally with a frequency of 125 Hz. By stabilizing the probe laser on the steepest slope of the FPE, via constant current, the highest sensitivity of the sensor is achieved. For water vapor measurements, the cell is operated with open windows and exposed to ambient air with a concentration of 13,762 ppmV, as measured by a reference device (temperature = 21.4 &#176;C, pressure = 979.9 hPa, relative humidity = 52.2%). The signal is extracted by means of a fast Fourier transform (FFT) and compared to the background signal with the excitation laser turned off, as shown in Figure 8. A signal-to-noise ratio of more than 7,000 can be obtained, corresponding to a detection limit of approximately 5 ppmV (3&#963;).
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/65174/65174fig08.jpg" /> 
Figure 8: PTI measurements of water vapor in ambient air. In black, the FFT signal of a measurement with 125 Hz laser excitation is shown. In blue, the background signal without excitation is depicted. The inset shows the measured peak at 125 Hz in more detail. <a href="https://www.jove.com/files/ftp_upload/65174/65174fig08large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Supplementary Coding File 1: Measurement_cell.SLDPRT. CAD file for the measurement cell. The cell can be adapted to the requirements of the specific application and subsequently 3D-printed. <a href="https://www.jove.com/files/ftp_upload/65174/measurement_cell.zip" target="_blank">Please click here to download this File.</a>
Supplementary Coding File 2: cap_etalon.SLDPRT. CAD file for fixing the etalon inside the measurement cell. <a href="https://www.jove.com/files/ftp_upload/65174/cap_etalon.zip" target="_blank">Please click here to download this File.</a>
Supplementary Coding File 3: cap_window.SLDPRT. CAD file for fixing the laser windows onto the measurement cell. <a href="https://www.jove.com/files/ftp_upload/65174/cap_window.zip" target="_blank">Please click here to download this File.</a>

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Author Spotlight: Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-P&#233;rot Etalon  

This protocol describes the construction of a low-cost, discrete, fiber-coupled, and air-spaced Fabry-Perot etalon with various applications, such as in trace-gas spectroscopy. The fabrication is possible in any facility with standard optical laboratory equipment available.

Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-P&#233;rot Etalon

Fabry-P&#233;rot etalons (FPE) have found their way into many applications. In fields such as spectroscopy, telecommunications, and astronomy, FPEs are ...

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5.1K Views.  Technical University of Graz. This protocol describes the construction of a low-cost, discrete, fiber-coupled, and air-spaced Fabry-Perot etalon with various applications, such as in trace-gas spectroscopy. The fabrication is possible in any facility with standard optical laboratory equipment available.

Video: Author Spotlight: Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-Pérot Etalon  

Fabrication, Densification, and Replica Molding of 3D Carbon Nanotube Microstructures

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip devices, and their applications. In particular, capabilities for cost-effective fabrication of polymer microstructures were transformed by the advent of soft lithography and other micromolding techniques 1, 2, and this led a revolution in applications of microfabrication to biomedical engineering and biology. Nevertheless, it remains challenging to fabricate microstructures with well-defined nanoscale surface textures, and to fabricate arbitrary 3D shapes at the micro-scale. Robustness of master molds and maintenance of shape integrity is especially important to achieve high fidelity replication of complex structures and preserving their nanoscale surface texture. The combination of hierarchical textures, and heterogeneous shapes, is a profound challenge to existing microfabrication methods that largely rely upon top-down etching using fixed mask templates. On the other hand, the bottom-up synthesis of nanostructures such as nanotubes and nanowires can offer new capabilities to microfabrication, in particular by taking advantage of the collective self-organization of nanostructures, and local control of their growth behavior with respect to microfabricated patterns. 
Our goal is to introduce vertically aligned carbon nanotubes (CNTs), which we refer to as CNT "forests", as a new microfabrication material. We present details of a suite of related methods recently developed by our group: fabrication of CNT forest microstructures by thermal CVD from lithographically patterned catalyst thin films; self-directed elastocapillary densification of CNT microstructures; and replica molding of polymer microstructures using CNT composite master molds. In particular, our work shows that self-directed capillary densification ("capillary forming"), which is performed by condensation of a solvent onto the substrate with CNT microstructures, significantly increases the packing density of CNTs. This process enables directed transformation of vertical CNT microstructures into straight, inclined, and twisted shapes, which have robust mechanical properties exceeding those of typical microfabrication polymers. This in turn enables formation of nanocomposite CNT master molds by capillary-driven infiltration of polymers. The replica structures exhibit the anisotropic nanoscale texture of the aligned CNTs, and can have walls with sub-micron thickness and aspect ratios exceeding 50:1. Integration of CNT microstructures in fabrication offers further opportunity to exploit the electrical and thermal properties of CNTs, and diverse capabilities for chemical and biochemical functionalization 3.

We present methods for fabrication of patterned microstructures of vertically aligned carbon nanotubes (CNTs), and their use as master molds for production of polymer microstructures with organized nanoscale surface texture. The CNT forests are densified by condensation of solvent onto the substrate, which significantly increases their packing density and enables self-directed formation of 3D shapes.

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip ...

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals1-4 and the enhancement of both linear5-7 and nonlinear devices.8-11
Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically12 and electrically13 pumped ultra-low threshold lasers and the enhancement of nonlinear effects.14-16 Furthermore, passive filters17 and modulators18-19 have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption.
To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues.
The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum20-21 and interferometric techniques.22-25 Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.26 Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans.
When characterising photonic crystal cavities, techniques involving internal sources21 or external waveguides directly coupled to the cavity27 impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.28 and further developed by Galli et al.29

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view ...

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including ...

Fabrication and Operation of a Nano-Optical Conveyor Belt

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological and physical sciences over the past few decades. The progress made in this field invites further study of even smaller systems and at a larger scale, with tools that could be distributed more easily and made more widely available. Unfortunately, the fundamental laws of diffraction limit the minimum size of the focal spot of a laser beam, which makes particles smaller than a half-wavelength in diameter hard to trap and generally prevents an operator from discriminating between particles which are closer together than one half-wavelength. This precludes the optical manipulation of many closely-spaced nanoparticles and&#160;limits the&#160;resolution of optical-mechanical systems. Furthermore, manipulation using focused beams requires beam-forming or steering optics, which can be very bulky and expensive. To address these limitations in the system scalability of conventional optical trapping our lab has devised an alternative technique which utilizes near-field optics to move particles across a chip. Instead of focusing laser beams in the far-field, the optical near field of plasmonic resonators produces the necessary local optical intensity enhancement to overcome the restrictions of diffraction and manipulate particles at higher resolution. Closely-spaced resonators produce strong optical traps which can be addressed to mediate the hand-off of particles from one to the next in a conveyor-belt-like fashion. Here, we describe how to design and produce a conveyor belt using a gold surface patterned with plasmonic C-shaped resonators and how to operate it with polarized laser light to achieve super-resolution nanoparticle manipulation and transport. The nano-optical conveyor belt chip can be produced using lithography techniques and easily packaged and distributed.

The scalability and resolution of conventional optical manipulation techniques are limited by diffraction. We circumvent the diffraction limit and describe a method of optically transporting nanoparticles across a chip using a gold surface patterned with a path of closely spaced C-shaped plasmonic resonators.

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological ...

Fabrication Process of Silicone-based Dielectric Elastomer Actuators

This contribution demonstrates the fabrication process of dielectric elastomer transducers (DETs). DETs are stretchable capacitors consisting of an elastomeric dielectric membrane sandwiched between two compliant electrodes. The large actuation strains of these transducers when used as actuators (over 300% area strain) and their soft and compliant nature has been exploited for a wide range of applications, including electrically tunable optics, haptic feedback devices, wave-energy harvesting, deformable cell-culture devices, compliant grippers, and propulsion of a bio-inspired fish-like airship. In most cases, DETs are made with a commercial proprietary acrylic elastomer and with hand-applied electrodes of carbon powder or carbon grease. This combination leads to non-reproducible and slow actuators exhibiting viscoelastic creep and a short lifetime. We present here a complete process flow for the reproducible fabrication of DETs based on thin elastomeric silicone films, including casting of thin silicone membranes, membrane release and prestretching, patterning of robust compliant electrodes, assembly and testing. The membranes are cast on flexible polyethylene terephthalate (PET) substrates coated with a water-soluble sacrificial layer for ease of release. The electrodes consist of carbon black particles dispersed into a silicone matrix and patterned using a stamping technique, which leads to precisely-defined compliant electrodes that present a high adhesion to the dielectric membrane on which they are applied.

This manuscript shows the fabrication process for the manufacture of dielectric elastomer soft actuators based on silicone membranes. The three key stages of production are presented in detail: blade casting of thin silicone membranes; pad printing of compliant electrodes; and the assembly of all the components.

This contribution demonstrates the fabrication process of dielectric elastomer transducers (DETs). DETs are stretchable capacitors consisting of an ...

Fabrication and Operation of Acoustofluidic Devices Supporting Bulk Acoustic Standing Waves for Sheathless Focusing of Particles

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must be smaller than the incident wavelength of sound and the width of the fluidic channels are typically tens to hundreds of micrometers across, acoustofluidic devices typically use ultrasonic waves generated from a piezoelectric transducer pulsating at high frequencies (in the megahertz range). At characteristic frequencies that depend on the geometry of the device, it is possible to induce the formation of standing waves that can focus particles along desired fluidic streamlines within a bulk flow. Here, we describe a method for the fabrication of acoustophoretic devices from common materials and clean room equipment. We show representative results for the focusing of particles with positive or negative acoustic contrast factors, which move towards the pressure nodes or antinodes of the standing waves, respectively. These devices offer enormous practical utility for precisely positioning large numbers of microscopic entities (e.g., cells) in stationary or flowing fluids for applications ranging from cytometry to assembly.

Acoustofluidic devices use ultrasonic waves within microfluidic channels to manipulate, concentrate and isolate suspended micro and nanoscopic entities. This protocol describes the fabrication and operation of such a device supporting bulk acoustic standing waves to focus particles in a central streamline without the aid of sheath fluids.

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must ...

Fabrication and Characterization of Superconducting Resonators

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.

Superconducting microwave resonators are of interest for detection of light, quantum computing applications and materials characterization. This work presents a detailed procedure for fabrication and characterization of superconducting microwave resonator scattering parameters.

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors ...

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 ...

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. 
In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. 
The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.

In this paper, flow assisted dielectrophoresis is demonstrated for the self-assembly of nanowire devices. The fabrication of a silicon nanowire field effect transistor is shown as an example.

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of ...

Fabrication of Refractive-index-matched Devices for Biomedical Microfluidics

The use of microfluidic devices has emerged as a defining tool for biomedical applications. When combined with modern microscopy techniques, these devices can be implemented as part of a robust platform capable of making simultaneous complementary measurements. The primary challenge created by the combination of these two techniques is the mismatch in refractive index between the materials traditionally used to make microfluidic devices and the aqueous solutions typically used in biomedicine. This mismatch can create optical artifacts near the channel or device edges. One solution is to reduce the refractive index of the material used to fabricate the device by using a fluorinated polymer such as MY133-V2000 whose refractive index is similar to that of water (n = 1.33). Here, the construction of a microfluidic device made out of MY133-V2000 using soft lithography techniques is demonstrated, using O2 plasma in conjunction with an acrylic holder to increase the adhesion between the MY133-V2000 fabricated device and the polydimethylsiloxane (PDMS) substrate. The device is then tested by incubating it filled with cell culture media for 24 h to demonstrate the ability of the device to maintain cell culture conditions during the course of a typical imaging experiment. Finally, quantitative phase microscopy (QPM) is used to measure the distribution of mass within the live adherent cells in the microchannel. This way, the increased precision, enabled by fabricating the device from a low index of refraction polymer such as MY133-V2000 in lieu of traditional soft lithography materials such as PDMS, is demonstrated. Overall, this approach for fabricating microfluidic devices can be readily integrated into existing soft lithography workflows in order to reduce optical artifacts and increase measurement precision.

This protocol describes the fabrication of microfluidic devices from MY133-V2000 to eliminate artifacts that often arise in microchannels due to the mismatching refractive indices between microchannel structures and an aqueous solution. This protocol uses an acrylic holder to compress the encapsulated device, improving adhesion both chemically and mechanically.

The use of microfluidic devices has emerged as a defining tool for biomedical applications. When combined with modern microscopy techniques, these devices ...

Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-Pérot Etalon

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