A. Maker was supported by an Annenberg Foundation Graduate Research Fellowship, and this work was supported by the National Science Foundation [085281 and 1028440].

1. Microsphere Fabrication
<ol>
 <li>Select a small amount (approximately 5 inches) of optical fiber, strip ~1.5" cladding from one end and clean with either methanol or ethanol (Figure 1a, b).</li>
 <li>If available, cleave the end with an optical fiber cleaver. If not available, cut with wire cutters or scissors such that ~0.5" is left. The advantage of using an optical fiber cleaver is that it produces a very smooth, uniform cut as in Figure 1b. Excessive roughness or defects from a cut may cause uneven reflow, lowering the quality factor of the resulting spheres.</li>
 <li>Expose the cleaned fiber end to 3W of CO2 laser power focused to a ~500&mu;m diameter spot size for ~1 second (Figure 1c, d, e). This produces spheres ~200&mu;m in diameter; however, the size can be tuned by increasing or decreasing the diameter of the optical fiber. Slightly adjusting the laser intensity may also be necessary to reflow larger or smaller spheres.</li>
</ol>
2. Microtoroid Fabrication
<ol>
 <li>Design and make a photomask with dark, solid circles, in the spacing and diameter of your choice. It is important to note that the toroids produced will be 25-30% smaller than the circles on the mask. For example, a solid circle with a diameter of 100 microns will produce a toroid with a diameter of approximately 75 microns. Also, it is recommended to leave at least 1-2mm of space between each circle and at least 5mm of space between arrays of circles and around the edges of the mask. Since the sample wafers must be carefully handled with tweezers, it is important to leave space for the tweezers to grip without damaging the toroids. The extra space also provides room for a tapered optical fiber to couple light into the finished devices, and allows samples to be cut into smaller arrays more easily. For this procedure, we used a mask with rows of 160 &mu;m diameter circles ~1mm apart, with ~5mm of space between each row of circles. The finished toroids are approximately 110 &mu;m in diameter.</li>
 <li>Begin with silicon wafers with a 2 &mu;m thick layer of thermally grown silica. Cleave the wafers to fit the desired microdisk pattern on the photolithography mask, leaving room for photoresist edge bead. Note that at the beginning of fabrication, it is usually most convenient to etch several arrays of circles on larger pieces of silicon wafers (~several cm x several cm). Larger wafers allow photolithography and BOE etching of more samples at a time, and are more easily handled with tweezers. Later, before the XeF2 etching step, it is recommended to cleave the larger wafers into smaller arrays to allow faster, more uniform XeF2 etching.</li>
 <li>In a fumehood, thoroughly clean the wafers by rinsing with acetone, methanol, isopropanol, and deionized water. Blow the samples dry using a nitrogen or filtered compressed air gun, and place them on a hot plate set to 120&deg;C for at least 2 minutes to dry.</li>
 <li>After letting the wafers cool, place them in a flammable/solvent fumehood and expose to HMDS for 2 minutes using the vapor deposition method. A simple vapor deposition method: put a few drops of HMDS in a small 10ml beaker, and then cover the wafers and small beaker with a larger glass container to hold the vapor.</li>
 <li>Place a sample on a spinner with an appropriately sized mount. Using a dropper bottle or syringe and filter, apply photoresist to the sample. Spin coat S1813 photoresist onto each sample for 5 seconds at 500rpm, followed by 45 seconds at 3000rpm. Edge bead removal is not needed if the wafer is sufficiently large so that the edge bead does not interfere with the patterning.</li>
 <li>Soft bake the photoresist on a hot plate at 95&deg;C for 2 minutes.</li>
 <li>Using a UV mask aligner and the desired photomask, expose the photoresist-covered samples to a total of 80mJ/cm2 of UV radiation. </li>
 <li>Immerse the samples in MF-321 developer to remove the photoresist which was exposed to UV light. While developing, closely watch as the photoresist is removed from the wafer and dissolved. It is important to stir/swish the container constantly during this process to ensure the photoresist is removed uniformly. For the given parameters, the photoresist takes approximately 30 seconds to develop.</li>
 <li>When most of the unwanted photoresist has dissolved in the developer, rinse the samples thoroughly under running water, gently blow dry the samples using a nitrogen or air gun, and inspect the samples with a microscope to ensure all undesired photoresist has been removed. If needed, the samples can be immersed again in developer; however, one should be careful not to overdevelop the samples as the desired photoresist patterns could also be damaged. (If the desired patterns are damaged or defective, the photoresist can be removed with acetone and steps 2.1-2.9 can be repeated again).</li>
 <li>After developing, thoroughly rinse the samples in running water, gently blow dry the samples, and hard bake them on a hot plate at 110&deg;C for 2 minutes. This step heats the photoresist above its glass transition temperature, reflowing the photoresist and partially repairing roughness which occurred during the developing process.</li>
 <li>Using Teflon containers and the necessary protective equipment, immerse the samples in improved buffered oxide etchant (BOE). BOE contains HF, which etches the silica not covered by photoresist to form circular silica pads on the silicon wafer (Figure 2a-c). Improved buffered HF produces a smoother etch, minimizing roughness in the resulting silica circles. While it is possible to mix buffered HF starting with 49% HF, this can lead to highly variable results as typically only small quantities are made. </li>
 <li>After approximately 15-20 minutes (depending on the patterns, sample sizes and number of samples), remove the samples from the BOE using Teflon tweezers. Carefully rinse the samples in running water. The silica has been removed when the samples become hydrophobic. </li>
 <li>After etching, rinsing, and drying the samples, inspect them using an optical microscope. Check to make sure the desired patterns have been etched completely and all the unwanted silica has been removed. If needed, return the samples to the BOE for further etching. One should be careful not to overetch the samples, or the circular patterns underneath the photoresist may be damaged.</li>
 <li>Once BOE etching is complete, thoroughly rinse the samples in deionized water and blow dry. If the samples are on large pieces of silicon wafer, it is also recommended to cut them (using a dicing saw or diamond scribe) into smaller pieces with individual rows of silica circles. Individual rows of circles are etched more quickly and uniformly in the XeF2 etching step (2.16). Silicon dust produced by the cutting is removed during cleaning in the next step.</li>
 <li>Remove the photoresist by rinsing with acetone, methanol, isopropanol, and deionized water, and dry the samples using a nitrogen gun and heating on a 120&deg;C hot plate for at least 2 minutes.</li>
 <li>Using a XeF2 etcher, undercut the silicon beneath the circular silica pads to form silica microdisks (Figure 2d-f). The amount etched should be approximately 1/3 of the silica circle&#x2019;s size, so that the resulting microdisk&#x2019;s pillar is approximately 1/3-1/2 of the total disk diameter, as determined by inspection with an optical microscope. The number of XeF2 pulses and the duration of each pulse depends on the amount of silicon in the chamber and the type of XeF2 etcher used.</li>
 <li>After XeF2 etching, expose the samples to a focused CO2 laser beam at approximately 12W intensity for ~3 seconds or until a smooth toroid is formed (Figure 2g-i). Depending on the exact size of the disk and the amount of XeF2 undercut, a slightly higher or lower intensity and exposure time may be needed to form a microtoroid. It is important that the center of the laser beam and the center of the microdisk are aligned, so that the silica microdisk will form a smooth, circular microtoroid.</li>
</ol>
3. Representative Results
The microsphere and microtoroid devices can be imaged using both optical microscopy and scanning electron microscopy (Figure 1d, e and Figure 2h, i). In all images, the uniformity of the device surface is clearly evident.
To verify that the detailed approach creates ultra-high-Q devices, we also characterized the Q factor of several devices by performing a linewidth (&Delta;&lambda;) measurement and calculating the loaded Q from the simple expression: Q=&lambda;/&Delta;&lambda;=&omega;&tau;, where &lambda;=resonant wavelength, &omega;=frequency, and &tau;=photon lifetime. Representative spectra of each device fabricated using the previously detailed procedures1,9 and a comparison graph of several devices is shown in Figure 3. The quality factors of all devices are above 10 million, with the majority being above 100 million.
The spectrum of the microsphere was a single resonance, indicating that the light coupled into either the clockwise or counter-clockwise propagating optical mode. However, the spectrum of the toroid exhibited a split resonance, indicating that light coupled into both the clockwise and counter-clockwise modes simultaneously. This phenomenon occurs when there is a slight imperfection at the coupling site. By fitting the spectrum to a dual-Lorentzian, the Q factor of both modes can be determined. The split resonance phenomena can occur in both sphere and toroid resonators, but is more frequently observed in toroids as they are more susceptible to imperfections and have fewer optical modes compared to spheres. 
<img src="/files/ftp_upload/4164/4164fig1.jpg" alt="Figure 1"/> 
Figure 1. Flow chart of the microsphere cavity fabrication process. a) Rendering and b) optical micrograph of a cleaned and cleaved optical fiber. c) Rendering, d) optical micrograph and e) scanning electron micrograph of a microspere resonator.
<img src="/files/ftp_upload/4164/4164fig2.jpg" alt="Figure 2"/> 
Figure 2. Flow chart of the microtoroid cavity fabrication process. a) Rendering, b) top-view optical micrograph and c) side-view scanning electron micrograph of the circular oxide pad, as defined by photolithography and BOE etching. Note the slight wedge-shape of the oxide that is formed by the BOE. d) Rendering, e) top-view optical micrograph and f) side-view scanning electron micrograph of the oxide pad after the XeF2 etching step. Note that the oxide disk maintains the wedge-shaped periphery. g) Rendering, h) top-view optical micrograph and i) side-view scanning electron micrograph of the microtoroid cavity.
<img src="/files/ftp_upload/4164/4164fig3.jpg" alt="Figure 3"/> 
Figure 3. Representative quality factor spectra of the a) microsphere and b) microtoroid resonant cavities as determined using the linewidth measurement method. In very high Q devices, one may observe mode-splitting or a double peak, in which light reflects off a small defect and circulates in both clockwise and counterclockwise directions. c) Comparison graph showing the Q factors of several microsphere and microtoroid resonant cavities. <a href="https://www.jove.com/files/ftp_upload/4164/4164fig3large.jpg" target="_blank"> Click here for larger figure</a>.
<img src="/files/ftp_upload/4164/4164fig4.jpg" alt="Figure 4"/> 
Figure 4. Schematic of the CO2 laser reflow set-up. The CO2 laser beam (solid blue line) is reflected and then focused on the sample. It passes through the 10.6 &mu;m / 633 nm beam combiner, which transmits 10.6 &mu;m and reflects 633 nm. The optical column images the reflection of the sample off of the beam combiner; therefore, the image is somewhat red. A list of the necessary parts for this setup is in Table 4.
<img src="/files/ftp_upload/4164/4164fig5.jpg" alt="Figure 5"/> 
Figure 5. Incorrectly reflowed a) microsphere and b) microtoroid resonant cavities. Due to incorrect placement within the beam, the device is mal-formed. c) As a result of a poor photomask or poor lithography, the toroid is moon-shaped.

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No conflicts of interest declared.

As with any optical structure, maintaining cleanliness at every step of the fabrication process is of critical importance. As there are numerous textbooks written on the topic of lithography and fabrication, the suggestions below are not intended to be comprehensive, but highlight a few of the more common issues researchers have faced.19-20
Because the uniformity of the microtoroid's periphery is determined by the uniformity of the initial disk, it is very important to pattern very ...

<table border="1">
 <tbody>
 <tr>
 <td>Name of the part</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>Fiber scribe</td>
 <td>Newport</td>
 <td>F-RFS</td>
 <td>Optional</td>
 </tr>
 <tr>
 <td>Optical fiber</td>
 <td>Newport</td>
 <td>F-SMF-28</td>
 <td>Any type of optical fiber can be used.</td>
 </tr>
 <tr>
 <td>Fiber coating stripper</td>
 <td>Newport</td>
 <td>F-STR-175</td>
 <td>Wire strippers can also be used</td>
 </tr>
 <tr>
 <td>Ethanol</td>
 <td>Any vendor</td>
 <td>Solvent-level purity</td>
 <td>Methanol or Isopropanol are substitutes</td>
 </tr>
 </tbody>
</table>
Table 1. Microsphere Fabrication Materials.
<table border="1">
 <tbody>
 <tr>
 <td>Name of the reagent</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>Silicon wafers with 2&mu;m thermally grown silica</td>
 <td>WRS Materials</td>
 <td>n/a</td>
 <td>We use intrinsic8, &lt;100&gt;, 4" diameter</td>
 </tr>
 <tr>
 <td>HMDS (Hexamethyldisilazane)</td>
 <td>Aldrich</td>
 <td>440191</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Photoresist</td>
 <td>Shipley</td>
 <td>S1813</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Developer</td>
 <td>Shipley</td>
 <td>MF-321</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Buffered HF - Improved</td>
 <td>Transene</td>
 <td>n/a</td>
 <td>The improved buffered HF gives a smoother, better quality etch than plain BOE or HF</td>
 </tr>
 <tr>
 <td>Acetone, Methanol, Isopropanol</td>
 <td>Any vendor</td>
 <td>99.8% purity</td>
 <td>&nbsp;</td>
 </tr>
 </tbody>
</table>
Table 2. Microtoroid Fabrication Materials.
<table border="1">
 <tbody>
 <tr>
 <td>Equipment Name</td>
 <td>Manufacturer</td>
 <td>Catalogue number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>Spinner</td>
 <td>Solitec</td>
 <td>5110-ND</td>
 <td>Any spinner can be used.</td>
 </tr>
 <tr>
 <td>Aligner</td>
 <td>Suss Microtec</td>
 <td>MJB 3</td>
 <td>Any aligner can be used.</td>
 </tr>
 <tr>
 <td>XeF2 etcher</td>
 <td>Advanced Communication Devices, Inc.</td>
 <td>#ADCETCH2007</td>
 <td>&nbsp;</td>
 </tr>
 </tbody>
</table>
Table 3. Microtoroid Fabrication Equipment.
<table border="1">
 <tbody>
 <tr>
 <td>Name of the part</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>CO2 Laser</td>
 <td>Synrad</td>
 <td>Series 48</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>3-Axis stage</td>
 <td>OptoSigma</td>
 <td>120-0770</td>
 <td>Available from other vendors as well.</td>
 </tr>
 <tr>
 <td>Si Reflector 1" diameter)</td>
 <td>II-VI</td>
 <td>308325</td>
 <td>Available from other vendors as well.</td>
 </tr>
 <tr>
 <td>Kinematic gimbal mount (for Si reflector)</td>
 <td>Thor Labs</td>
 <td>KX1G</td>
 <td>Available from other vendors as well.</td>
 </tr>
 <tr>
 <td>Beam combiner (1" diameter)</td>
 <td>Meller Optics</td>
 <td>L19100008-B0 </td>
 <td>Available from other vendors as well.</td>
 </tr>
 <tr>
 <td>4" Focal length Lens (1" diameter)</td>
 <td>Meller Optics or II-VI</td>
 <td>&nbsp;</td>
 <td>Available from other vendors as well</td>
 </tr>
 <tr>
 <td>Assorted posts, lens mounts</td>
 <td>Thor Labs, Newport, Edmund Optics or Optosigma</td>
 <td>&nbsp;</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Zoom 6000 machine vision system</td>
 <td>Navitar</td>
 <td>n/a</td>
 <td>Requires generic USB camera and computer for real-time imaging. This is purchased as a kit.</td>
 </tr>
 <tr>
 <td>Focuser for Zoom 6000 system</td>
 <td>Edmund Optics</td>
 <td>54-792</td>
 <td>Available from other vendors as well.</td>
 </tr>
 <tr>
 <td>X-Z Axis Positioners for Zoom 6000</td>
 <td>Parker Daedal</td>
 <td>CR4457, CR4452, 4499</td>
 <td>CR4457 is X-axis, CR4452 is Z-axis, 4499 is mounting bracket.</td>
 </tr>
 </tbody>
</table>
Table 4. CO2 Laser Reflow Set-up.

fabrication of silica ultra high quality factor microresonators

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the quality factor (Q) of the cavity, can be in excess of 500ns for cavities with Q factors above 100 million. As a result of their low material losses, silica microcavities have demonstrated some of the longest photon lifetimes to date1-2. Since a portion of the circulating light extends outside the resonator, these devices can also be used to probe the surroundings. This interaction has enabled numerous experiments in biology, such as single molecule biodetection and antibody-antigen kinetics, as well as discoveries in other fields, such as development of ultra-low-threshold microlasers, characterization of thin films, and cavity quantum electrodynamics studies.3-7
The two primary silica resonant cavity geometries are the microsphere and the microtoroid. Both devices rely on a carbon dioxide laser reflow step to achieve their ultra-high-Q factors (Q&gt;100 million).1-2,8-9 However, there are several notable differences between the two structures. Silica microspheres are free-standing, supported by a single optical fiber, whereas silica microtoroids can be fabricated on a silicon wafer in large arrays using a combination of lithography and etching steps. These differences influence which device is optimal for a given experiment.
Here, we present detailed fabrication protocols for both types of resonant cavities. While the fabrication of microsphere resonant cavities is fairly straightforward, the fabrication of microtoroid resonant cavities requires additional specialized equipment and facilities (cleanroom). Therefore, this additional requirement may also influence which device is selected for a given experiment.
Introduction
An optical resonator efficiently confines light at specific wavelengths, known as the resonant wavelengths of the device. 1-2 The common figure of merit for these optical resonators is the quality factor or Q. This term describes the photon lifetime (&tau;o) within the resonator, which is directly related to the resonator's optical losses. Therefore, an optical resonator with a high Q factor has low optical losses, long photon lifetimes, and very low photon decay rates (1/&tau;o). As a result of the long photon lifetimes, it is possible to build-up extremely large circulating optical field intensities in these devices. This very unique property has allowed these devices to be used as laser sources and integrated biosensors.10
A unique sub-class of resonators is the whispering gallery mode optical microcavity. In these devices, the light is confined in circular orbits at the periphery. Therefore, the field is not completely confined within the device, but evanesces into the environment. Whispering gallery mode optical cavities have demonstrated some of the highest quality factors of any optical resonant cavity to date.9,11 Therefore, these devices are used throughout science and engineering, including in fundamental physics studies and in telecommunications as well as in biodetection experiments. 3-7,12
Optical microcavities can be fabricated from a wide range of materials and in a wide variety of geometries. A few examples include silica and silicon microtoroids, silicon, silicon nitride, and silica microdisks, micropillars, and silica and polymer microrings.13-17 The range in quality factor (Q) varies as dramatically as the geometry. Although both geometry and high Q are important considerations in any field, in many applications, there is far greater leverage in boosting device performance through Q enhancement. Among the numerous options detailed previously, the silica microsphere and the silica microtoroid resonator have achieved some of the highest Q factors to date.1,9 Additionally, as a result of the extremely low optical loss of silica from the visible through the near-IR, both microspheres and microtoroids are able to maintain their Q factors over a wide range of testing wavelengths.18 Finally, because silica is inherently biocompatible, it is routinely used in biodetection experiments.
In addition to high material absorption, there are several other potential loss mechanisms, including surface roughness, radiation loss, and contamination loss.2 Through an optimization of the device size, it is possible to eliminate radiation losses, which arise from poor optical field confinement within the device. Similarly, by storing a device in an appropriately clean environment, contamination of the surface can be minimized. Therefore, in addition to material loss, surface scattering is the primary loss mechanism of concern.2,8
In silica devices, surface scattering is minimized by using a laser reflow technique, which melts the silica through surface tension induced reflow. While spherical optical resonators have been studied for many years, it is only with recent advances in fabrication technologies that researchers been able to fabricate high quality silica optical toroidal microresonators (Q&gt;100 million) on a silicon substrate, thus paving the way for integration with microfluidics.1
The present series of protocols details how to fabricate both silica microsphere and microtoroid resonant cavities. While silica microsphere resonant cavities are well-established, microtoroid resonant cavities were only recently invented.1 As many of the fundamental methods used to fabricate the microsphere are also used in the more complex microtoroid fabrication procedure, by including both in a single protocol it will enable researchers to more easily trouble-shoot their experiments.

Watch this Scientific Journal Video about Fabrication of Silica Ultra High Quality Factor Microresonators at JoVE.com

Fabrication of Silica Ultra High Quality Factor Microresonators

We describe the use of a carbon dioxide laser reflow technique to fabricate silica resonant cavities, including free-standing microspheres and on-chip microtoroids. The reflow method removes surface imperfections, allowing long photon lifetimes within both devices. The resulting devices have ultra high quality factors, enabling applications ranging from telecommunications to biodetection.

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the ...

fabrication-of-silica-ultra-high-quality-factor-microresonators

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16.0K Views.  University of Southern California. We describe the use of a carbon dioxide laser reflow technique to fabricate silica resonant cavities, including free-standing microspheres and on-chip microtoroids. The reflow method removes surface imperfections, allowing long photon lifetimes within both devices. The resulting devices have ultra high quality factors, enabling applications ranging from telecommunications to biodetection.

Video: Fabrication of Silica Ultra High Quality Factor Microresonators

Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures

The physical properties of a material are defined by its electronic structure. Electrons in solids are characterized by energy (&omega;) and momentum (k) and the probability to find them in a particular state with given &omega; and k is described by the spectral function A(k, &omega;). This function can be directly measured in an experiment based on the well-known photoelectric effect, for the explanation of which Albert Einstein received the Nobel Prize back in 1921. In the photoelectric effect the light shone on a surface ejects electrons from the material. According to Einstein, energy conservation allows one to determine the energy of an electron inside the sample, provided the energy of the light photon and kinetic energy of the outgoing photoelectron are known. Momentum conservation makes it also possible to estimate k relating it to the momentum of the photoelectron by measuring the angle at which the photoelectron left the surface. The modern version of this technique is called Angle-Resolved Photoemission Spectroscopy (ARPES) and exploits both conservation laws in order to determine the electronic structure, i.e. energy and momentum of electrons inside the solid. In order to resolve the details crucial for understanding the topical problems of condensed matter physics, three quantities need to be minimized: uncertainty* in photon energy, uncertainty in kinetic energy of photoelectrons and temperature of the sample.
In our approach we combine three recent achievements in the field of synchrotron radiation, surface science and cryogenics. We use synchrotron radiation with tunable photon energy contributing an uncertainty of the order of 1 meV, an electron energy analyzer which detects the kinetic energies with a precision of the order of 1 meV and a He3 cryostat which allows us to keep the temperature of the sample below 1 K. We discuss the exemplary results obtained on single crystals of Sr2RuO4 and some other materials. The electronic structure of this material can be determined with an unprecedented clarity.

The overall goal of this method is to determine the low-energy electronic structure of solids at ultra-low temperatures using Angle-Resolved Photoemission Spectroscopy with synchrotron radiation.

The physical properties of a material are defined by its  electronic structure. Electrons in solids are characterized by energy (&omega;)  and momentum ...

Synthesis and Microdiffraction at Extreme Pressures and Temperatures

High pressure compounds and polymorphs are investigated for a broad range of purposes such as determine structures and processes of deep planetary interiors, design materials with novel properties, understand the mechanical behavior of materials exposed to very high stresses as in explosions or impacts. Synthesis and structural analysis of materials at extreme conditions of pressure and temperature entails remarkable technical challenges. In the laser heated diamond anvil cell (LH-DAC), very high pressure is generated between the tips of two opposing diamond anvils forced against each other; focused infrared laser beams, shined through the diamonds, allow to reach very high temperatures on samples absorbing the laser radiation. When the LH-DAC is installed in a synchrotron beamline that provides extremely brilliant x-ray radiation, the structure of materials under extreme conditions can be probed in situ. LH-DAC samples, although very small, can show highly variable grain size, phase and chemical composition. In order to obtain the high resolution structural analysis and the most comprehensive characterization of a sample, we collect diffraction data in 2D grids and combine powder, single crystal and multigrain diffraction techniques. Representative results obtained in the synthesis of a new iron oxide, Fe4O5 1 will be shown.

The laser heated diamond anvil cell combined with synchrotron micro-diffraction techniques allows researchers to explore the nature and properties of new phases of matter at extreme pressure and temperature (PT) conditions. Heterogeneous samples can be characterized in situ under high pressure by 2D mapping and combined powder, single-crystal and multigrain diffraction approaches.

High pressure compounds and polymorphs are investigated for a broad range of purposes such as determine structures and processes of deep planetary ...

Convergent Polishing: A Simple, Rapid, Full Aperture Polishing Process of High Quality Optical Flats &amp; Spheres

Convergent Polishing is a novel polishing system and method for finishing flat and spherical glass optics in which a workpiece, independent of its initial shape (i.e., surface figure), will converge to final surface figure with excellent surface quality under a fixed, unchanging set of polishing parameters in a single polishing iteration. In contrast, conventional full aperture polishing methods require multiple, often long, iterative cycles involving polishing, metrology and process changes to achieve the desired surface figure. The Convergent Polishing process is based on the concept of workpiece-lap height mismatch resulting in pressure differential that decreases with removal and results in the workpiece converging to the shape of the lap. The successful implementation of the Convergent Polishing process is a result of the combination of a number of technologies to remove all sources of non-uniform spatial material removal (except for workpiece-lap mismatch) for surface figure convergence and to reduce the number of rogue particles in the system for low scratch densities and low roughness. The Convergent Polishing process has been demonstrated for the fabrication of both flats and spheres of various shapes, sizes, and aspect ratios on various glass materials. The practical impact is that high quality optical components can be fabricated more rapidly, more repeatedly, with less metrology, and with less labor, resulting in lower unit costs. In this study, the Convergent Polishing protocol is specifically described for fabricating 26.5 cm square fused silica flats from a fine ground surface to a polished ~&#955;/2 surface figure after polishing 4 hr per surface on a 81 cm diameter polisher.

A novel optical polishing process, called &#8220;Convergent Polishing&#8221;, which enables faster, lower cost polishing, is described. Unlike conventional polishing processes, Convergent Polishing allows a glass workpiece to be polished in a single iteration and with high surface quality to its final surface figure without requiring changes to polishing parameters.

Convergent Polishing is a novel polishing system and method for finishing flat and spherical glass optics in which a workpiece, independent of its initial ...

Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve the solar conversion efficiency of a concentrated photovoltaic system. The proposed system will also lower the solar cell cost in the concentrated photovoltaic system by replacing the expensive tandem solar cells with the cost-effective single junction solar cells. The structures and the parameters of high contrast gratings for the dispersive elements were numerically optimized. The large-area fabrication of high contrast gratings was experimentally demonstrated using nanoimprint lithography and dry etching. The quality of grating material and the performance of the fabricated device were both experimentally characterized. By analyzing the measurement results, the possible side effects from the fabrication processes are discussed and several methods that have the potential to improve the fabrication processes are proposed, which can help to increase the optical efficiency of the fabricated devices.

The fabrication of high contrast gratings as the parallel spectrum splitting dispersive element in a concentrated photovoltaic system is demonstrated. Fabrication processes including nanoimprint lithography, TiO2 sputtering and reactive ion etching are described. Reflectance measurement results are used to characterize the optical performance.

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve ...

The Evolution of Silica Nanoparticle-polyester Coatings on Surfaces Exposed to Sunlight

Corrosion of metallic surfaces is prevalent in the environment and is of great concern in many areas, including the military, transport, aviation, building and food industries, amongst others. Polyester and coatings containing both polyester and silica nanoparticles (SiO2NPs) have been widely used to protect steel substrata from corrosion. In this study, we utilized X-ray photoelectron spectroscopy, attenuated total reflection infrared micro-spectroscopy, water contact angle measurements, optical profiling and atomic force microscopy to provide an insight into how exposure to sunlight can cause changes in the micro- and nanoscale integrity of the coatings. No significant change in surface micro-topography was detected using optical profilometry, however, statistically significant nanoscale changes to the surface were detected using atomic force microscopy. Analysis of the X-ray photoelectron spectroscopy and attenuated total reflection infrared micro-spectroscopy data revealed that degradation of the ester groups had occurred through exposure to ultraviolet light to form COO&#903;, -H2C&#903;, -O&#903;, -CO&#903; radicals. During the degradation process, CO and CO2 were also produced.

Two types of surfaces, polyester-coated steel and polyester coated with a layer of silica nanoparticles, were studied. Both surfaces were exposed to sunlight, which was found to cause substantial changes in the chemistry and nanoscale topography of the surface.

Corrosion of metallic surfaces is prevalent in the environment and is of great concern in many areas, including the military, transport, aviation, ...

High Temperature Fabrication of Nanostructured Yttria-Stabilized-Zirconia (YSZ) Scaffolds by In Situ Carbon Templating Xerogels

We demonstrate a method for the high temperature fabrication of porous, nanostructured yttria-stabilized-zirconia (YSZ, 8 mol% yttria - 92 mol% zirconia) scaffolds with tunable specific surface areas up to 80 m2&#183;g-1. An aqueous solution of a zirconium salt, yttrium salt, and glucose is mixed with propylene oxide (PO) to form a gel. The gel is dried under ambient conditions to form a xerogel. The xerogel is pressed into pellets and then sintered in an argon atmosphere. During sintering, a YSZ ceramic phase forms and the organic components decompose, leaving behind amorphous carbon. The carbon formed in situ serves as a hard template, preserving a high surface area YSZ nanomorphology at sintering temperature. The carbon is subsequently removed by oxidation in air at low temperature, resulting in a porous, nanostructured YSZ scaffold. The concentration of the carbon template and the final scaffold surface area can be systematically tuned by varying the glucose concentration in the gel synthesis. The carbon template concentration was quantified using thermogravimetric analysis (TGA), the surface area and pore size distribution was determined by physical adsorption measurements, and the morphology was characterized using scanning electron microscopy (SEM). Phase purity and crystallite size was determined using X-ray diffraction (XRD). This fabrication approach provides a novel, flexible platform for realizing unprecedented scaffold surface areas and nanomorphologies for ceramic-based electrochemical energy conversion applications, e.g. solid oxide fuel cell (SOFC) electrodes.

High Temperature Fabrication of Nanostructured Yttria-Stabilized-Zirconia YSZ Scaffolds by In Situ Carbon Templating Xerogels

A protocol for fabricating porous, nanostructured yttria-stabilized-zirconia (YSZ) scaffolds at temperatures between 1,000 &#176;C and 1,400 &#176;C is presented.

We demonstrate a method for the high temperature fabrication of porous, nanostructured yttria-stabilized-zirconia (YSZ, 8 mol% yttria - 92 mol% zirconia) ...

Scalable Solution-processed Fabrication Strategy for High-performance, Flexible, Transparent Electrodes with Embedded Metal Mesh

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a polymer film. This paper also presents a low-cost, vacuum-free fabrication method for this novel TE; the approach combines lithography, electroplating, and imprint transfer (LEIT) processing. The embedded nature of the EMTEs offers many advantages, such as high surface smoothness, which is essential for organic electronic device production; superior mechanical stability during bending; favorable resistance to chemicals and moisture; and strong adhesion with plastic film. LEIT fabrication features an electroplating process for vacuum-free metal deposition and is favorable for industrial mass production. Furthermore, LEIT allows for the fabrication of metal mesh with a high aspect ratio (i.e., thickness to linewidth), significantly enhancing its electrical conductance without adversely losing optical transmittance. We demonstrate several prototypes of flexible EMTEs, with sheet resistances lower than 1 &#937;/sq and transmittances greater than 90%, resulting in very high figures of merit (FoM) &#8211; up to 1.5 x 104 &#8211; which are amongst the best values in the published literature.

This protocol describes a solution-based fabrication strategy for high-performance, flexible, transparent electrodes with fully-embedded, thick metal mesh. Flexible transparent electrodes fabricated by this process demonstrate among the highest reported performances, including ultra-low sheet resistance, high optical transmittance, mechanical stability under bending, strong substrate adhesion, surface smoothness, and environmental stability.

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a ...

Method for Recording Broadband High Resolution Emission Spectra of Laboratory Lightning Arcs

Lightning is one of the most common and destructive forces in nature and has long been studied using spectroscopic techniques, first with traditional camera film methods and then digital camera technology, from which several important characteristics have been derived. However, such work has always been limited due to the inherently random and non-repeatable nature of natural lightning events in the field. Recent developments in lightning test facilities now allow the reproducible generation of lightning arcs within controlled laboratory environments, providing a test bed for the development of new sensors and diagnostic techniques to understand lightning mechanisms better. One such technique is a spectroscopic system using digital camera technology capable of identifying the chemical elements with which the lightning arc interacts, with these data then being used to derive further characteristics. In this paper, the spectroscopic system is used to obtain the emission spectrum from a 100 kA peak, 100 &#181;s duration lightning arc generated across a pair of hemispherical tungsten electrodes separated by a small air gap. To maintain a spectral resolution of less than 1 nm, several individual spectra were recorded across discrete wavelength ranges, averaged, stitched, and corrected to produce a final composite spectrum in the 450 nm (blue light) to 890 nm (near infrared light) range. Characteristic peaks within the data were then compared to an established publicly available database to identify the chemical element interactions. This method is readily applicable to a variety of other light emitting events, such as fast electrical discharges, partial discharges, and sparking in electrical equipment, apparatus, and systems.

Emission spectroscopy techniques have traditionally been used to analyze inherently random lightning arcs occurring in nature. In this paper, a method developed to obtain the emission spectroscopy from reproducible lightning arcs generated within a laboratory environment is described.

Lightning is one of the most common and destructive forces in nature and has long been studied using spectroscopic techniques, first with traditional ...

Fabrication of Ultra-thin Color Films with Highly Absorbing Media Using Oblique Angle Deposition

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.&#160; We present an advanced method for fabricating ultra-thin color films with improved characteristics. The proposed process addresses several fabrication issues, including large area processing. Specifically, the protocol describes a process for fabricating ultra-thin color films using an electron beam evaporator for oblique angle deposition of germanium (Ge) and gold (Au) on silicon (Si) substrates.&#160; Film porosity produced by the oblique angle deposition induces color changes in the ultra-thin film. The degree of color change depends on factors such as deposition angle and film thickness. Fabricated samples of the ultra-thin color films showed improved color tunability and color purity. In addition, the measured reflectance of the fabricated samples was converted into chromatic values and analyzed in terms of color. Our ultra-thin film fabricating method is expected to be used for various ultra-thin film applications such as flexible color electrodes, thin film solar cells, and optical filters. Also, the process developed here for analyzing the color of the fabricated samples is broadly useful for studying various color structures.

We present a detailed method for fabricating ultra-thin color films with improved characteristics for optical coatings. The oblique angle deposition technique using an electron beam evaporator allows improved color tunability and purity. Fabricated films of Ge and Au on Si substrates were analyzed by reflectance measurements and color information conversion.

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.&#160; We present ...

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 ...