This work is financially supported by National Research Foundation grants (NRF-2019R1A2C3003129, CAMM-2019M3A6B3030637, NRF-2018M3D1A1058998, NRF-2015R1A5A1037668) funded by the Ministry of Science and ICT (MSIT), Republic of Korea.

1. Fabrication of the dielectric metasurface
<ol>
	<li>Precleaning of a fused silica substrate
	<ol>
		<li>Prepare a double-side polished, fused silica substrate (length: 2 cm; width: 2 cm; thickness: 500 &#956;m).</li>
		<li>Immerse the fused silica substrate in 50 mL of acetone and conduct the sonication process for 5 min at 40 kHz.</li>
		<li>Immerse the substrate in 50 mL of 2-propanol (IPA) and conduct the sonication process for 5 min at 40 kHz.</li>
		<li>Rinse the substrate with the IPA and blow nitrogen (N2) gas to dry the substrate before the evaporation of the IPA.</li>
	</ol>
	</li>
	<li>Deposition of a-Si:H by PECVD
	<ol>
		<li>Locate the prepared substrates on a zig inside the load lock chamber of the PECVD system.</li>
		<li>On PECVD software, set the chamber temperature to 300 &#176;C and set the radio frequency power to 800 W.</li>
		<li>Set the SiH4 gas flow rate to 10 sccm and the H2 gas flow rate to 75 sccm.</li>
		<li>Set the process pressure to 25 mTorr. Click the Start button to start the deposition process, which takes ~300 s.</li>
	</ol>
	</li>
	<li>Formation of the Cr etching mask
	<ol>
		<li>Load the sample obtained from step 1.2.4 on the sample holder of the spin coater. Release poly(methyl methacrylate) (PMMA) A2 on the sample using a filter-mounted 5 mL syringe and start the coating process with a rotation speed of 2,000 rpm for 1 min. 
		NOTE: Released PMMA should cover the whole substrate; otherwise, the spin-coated film will not be uniform.</li>
		<li>Transfer the sample from the sample holder to a hot plate, and bake the sample with a hot plate at 180 &#176;C for 5 min. Then, cool the sample at room temperature for 1 min.</li>
		<li>Load the sample on the sample holder of the spin coater. Release E-spacer on the sample, using a 1 mL pipette, and start the coating process with a rotation speed of 2,000 rpm for 1 min. 
		NOTE: Released E-spacer should cover the whole substrate; otherwise, the spin-coated film will not be uniform.</li>
		<li>Load and fix the sample on the zig for EBL. Put the zig into the EBL chamber and, then, load it into the main chamber.</li>
		<li>On the EBL console, push the Isolation button and, then, the FC button. Set the magnification to its maximum value using the magnification knob.</li>
		<li>Turn the Zero check button on. Turn the Beam current knob to set the beam current value to 50 pA. Turn the Zero check button off.</li>
		<li>Push the Reference button to move the stage to the reference position. Turn the Blank button off.</li>
		<li>Set the magnification value to 100,000 using the magnification knob. Adjust the focus and stigmatism knobs to obtain the clearest image in the EBL display. Turn the Blank button on.</li>
		<li>On the computer connected to the EBL console, run the Linux terminal. Move the current location to the folder that has the .gds file, using the cd command.</li>
		<li>Enter gds2cel to convert the .gds file to a .cel file and wait until it finishes. Enter job to run the main software.</li>
		<li>Click the Chip size modification menu. Select 600 &#956;m x 600 &#956;m and 240,000 dots. Click Save and then Exit.</li>
		<li>Click the Pattern data creation menu. Enter ps in the command window to load the pattern .cel file generated from step 1.3.10. Enter i in the command window and click the pattern to magnify the pattern image.</li>
		<li>Enter sd in the command window and 3 to set the dose time to 3 &#956;s. Enter sp in the command window and 1,1 to set the exposing pitch to a normal condition. Enter pc in the command window and a filename to create a .ccc file. Click the center of the pattern.</li>
		<li>Enter cp in the command window and click the pattern to apply the exposing conditions from step 1.3.13. Enter sv in the command window and a filename to create a .con file. Enter q in the command window to exit the Pattern data creation menu.</li>
		<li>Click the Exposure menu. Enter i and the .con filename from step 1.3.14. Enter e and click the Exposure button to start the exposing process. 
		NOTE: The exposing time depends on the pattern area and density. General metasurface patterns of a 300 &#956;m x 300 &#956;m area take ~3 h.</li>
		<li>When the exposing process finishes, turn the Isolation button off. Push the EX button to move the stage.</li>
		<li>Unload the sample from the chamber after finishing the exposure. Immerse the sample in 50 mL of deionized (DI) water for 1 min to remove the E-spacer.</li>
		<li>Prepare 10 mL of methyl isobutyl ketone (MIBK):IPA = 1:3 solution in a beaker surrounded by ice. Immerse the sample into the MIBK:IPA = 1:3 solution for 12 min. Then, rinse the sample with the IPA and blow N2 gas to dry the sample.</li>
		<li>Load and fix the sample on the holder of the electron beam evaporator. Mount the holder inside the chamber of the evaporator.</li>
		<li>Load a graphite-crucible-containing piece-type Cr inside the evaporation chamber.</li>
		<li>On the software of the electron beam evaporator, click the Chamber pumping button to create a vacuum on the inside of the chamber, and lower the pressure to 3 x 10-6 mTorr.</li>
		<li>Select Chromium in the material section and click the Material button to apply it. Click the E-beam shutter button to open the source shutter. Click the High voltage and the Source button, in that order.</li>
		<li>Click the upward arrow button to increase the electron beam power slowly, and repeat this until the deposition rate reaches 0.15 nm/s. 
		NOTE: One click per 5 s is slow enough.</li>
		<li>Click the Zero button to reset the thickness gauge. Click the Main shutter button to open the main shutter. When the thickness gauge reaches 30 nm, click the Main shutter button to close the main shutter. 
		NOTE: Deposition time can be easily calculated from the deposition rate. A 30 nm-thick deposition takes ~200 s, in the condition used here.</li>
		<li>Click the E-beam shutter button to close the source shutter. Click the downward arrow button to decrease the electron beam power slowly, and repeat this until the power reaches 0. 
		NOTE: One click per 5 s is slow enough.</li>
		<li>Click the Source and, then, the High voltage button. Wait for 15 min to cool the chamber. Click the Vent button to vent the chamber and unload the sample from the holder.</li>
		<li>Immerse the sample in 50 mL of acetone for 3 min. Conduct the sonication process for 1 min at 40 kHz. Rinse the sample with IPA and blow N2 gas to dry the sample.</li>
	</ol>
	</li>
	<li>Etching process of a-Si:H
	<ol>
		<li>Spread thermal glue to the back of the sample. Attach the sample on the zig and load the zig on the etching system.</li>
		<li>On the software, set the chlorine (Cl2) gas flow rate to 80 sccm and the hydrogen bromide (HBr) gas flow rate to 120 sccm. Set the source power to 500 W and the bias to 100 V. Click the Start button to start the etching process for 100 s.</li>
		<li>Unload the sample and remove the thermal glue with a dustproof wiper.</li>
		<li>Immerse the sample in 20 mL of Cr etchant for 2 min and in 50 mL of DI water for 1 min. Rinse the sample with DI water and blow N2 gas to dry the sample.</li>
	</ol>
	</li>
	<li>Obtaining the scanning electron microscope image of the fabricated metasurface
	<ol>
		<li>Load the sample on the sample holder of the spin coater, release the E-spacer on the sample using a 1 mL pipette, and start the coating process with a rotation speed of 2,000 rpm for 1 min.</li>
		<li>Fix the sample on the sample holder of the scanning electron microscope (SEM), using carbon tape. Put the holder in the load lock chamber of the SEM and create a vacuum in the load lock chamber.</li>
		<li>Transfer the holder from the load lock chamber to the main chamber. Turn the electron beam on with a 15 kV acceleration voltage.</li>
		<li>Move the stage to a 1 cm working distance. Find the metasurface by moving the stage horizontally. Adjust the stigmatism and focal length until the image becomes clear.</li>
		<li>Capture the images.</li>
		<li>Turn off the electron beam. Move the stage to the extraction position. Transfer the holder from the main chamber to the load lock chamber.</li>
		<li>Vent the load lock chamber and unload the sample.</li>
		<li>Immerse the sample into 50 mL of DI water for 1 min to remove the E-spacer. Blow N2 gas to dry the sample.</li>
	</ol>
	</li>
</ol>
2. Optical characterization of the dielectric metasurface
NOTE: Direct radiation of a laser can damage eyes. Avoid direct eye exposure and wear the appropriate laser safety glasses.
<ol>
	<li>Mount a 635 nm-wavelength laser on the optical table (Figure 1a). Turn the laser on and wait for 10 min to stabilize the beam power.</li>
	<li>Adjust the horizontal and vertical alignment of the laser using an alignment screen both near and far from the laser.</li>
	<li>Place a neutral density filter in front of the laser. Mount the first convex lens behind the neutral density filter. Place an iris at the back focal plane of the convex lens to remove noise.</li>
	<li>Mount the second convex lens with twice the focal length from the first convex lens. Place a linear polarizer behind the second convex lens. Place a right-handed circular polarizer behind the linear polarizer.</li>
	<li>Mount the third convex lens behind the circular polarizer. Mount the fabricated metasurface on the holder. Locate the metasurface at the back focal plane of the convex lens. 
	NOTE: The laser beam should be incident from the substrate to the patterned area.</li>
	<li>Place a thick white paper screen, which has a 1 cm-diameter hole in the center, behind the metasurface. Mount a protractor on the optical table aligning the origin with the metasurface.</li>
	<li>Measure the power of the three diffracted beams, which are three bright spots on the screen, using a power meter. 
	NOTE: If the laser beam power is not maintained at a constant, calculate the average beam power over a period of time.</li>
	<li>Replace the right-handed circular polarizer with a left-handed circular polarizer. Measure the three diffracted beam powers, using the power meter.</li>
	<li>Remove the left-handed circular polarizer. Measure three diffracted beam powers, using the power meter.</li>
	<li>Decrease the laser beam power, using the neutral density filter to allow a CCD measurement. Place the right-handed circular polarizer. Capture the three diffracted beam profiles using the CCD. 
	NOTE: Weaker laser beam power is preferred, to prevent CCD damage. A beam power of 300 &#956;W has been used in this work.</li>
	<li>Replace the right-handed circular polarizer with the left-handed circular polarizer. Capture the three diffracted beam profiles, using the CCD.</li>
	<li>Remove the left-handed circular polarizer. Capture the three diffracted beam profiles, using the CCD.</li>
	<li>Replace the 635 nm-wavelength laser with a 532 nm-wavelength laser.</li>
	<li>Repeat steps 2.2 to 2.12.</li>
</ol>

<ol>
	<li>Khorasaninejad, 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=Metalenses+at+visible+wavelengths:+Diffraction-limited+focusing+and+subwavelength+resolution+imaging.">Metalenses at visible wavelengths: Diffraction-limited focusing and subwavelength resolution imaging.</a> Science. 352 (6290), 1190-1194 (2016).</li><li>Chen, W. T., 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=A+broadband+achromatic+metalens+for+focusing+and+imaging+in+the+visible.">A broadband achromatic metalens for focusing and imaging in the visible.</a> Nature Nanotechnology. 13 (3), 220-226 (2018).</li><li>Zheng, G., 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=Metasurface+holograms+reaching+80%+efficiency.">Metasurface holograms reaching 80% efficiency.</a> Nature Nanotechnology. 10 (4), 308-312 (2015).</li><li>Devlin, R. C., Khorasaninejad, M., Chen, W. 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The authors have nothing to disclose.

Some fabrication steps should be conducted carefully, to generate a metasurface that is the same as the original design. In the resist development process, a low-temperature solution is usually preferred. The standard condition is room temperature, but the reaction speed can be slowed down by decreasing the solution temperature to 0 &#176;C. Although the corresponding reaction time becomes longer, a finer pattern can be obtained than with standard conditions. The reaction time control is also easy owing to the low reaction speed. Another critical step for a fine pattern is drying IPA after the resist development. The N2 gas moves and evaporates the rest of the IPA on the sample. Some amount of IPA does not move, creating randomly distributed islands. If the IPA islands are formed and then evaporated, the sample will be damaged. Therefore, to minimize IPA island formation, strong blowing is better than weak blowing, unless the substrate is broken by the strong air flow. An appropriate power of the sonication is helpful to clearly peel off the thin film. After the lift-off step, it is possible to check whether the thin film clearly peels off or not by using a conventional optical microscope. Fortunately, if any Cr thin film remains on the patterned area, it is possible to remove the residue by an additional sonication process. This is a considerable advantage of the Cr mask, because masks made of other materials, such as gold, are extremely difficult to remove once the residue dries.
EBL is an effective method to fabricate nanoscale structures, but this method suffers from a low throughput, impinging large-scale manufacturing. One way to improve productivity is by making master molds, using EBL, and printing the pattern using the master mold. This method is called nanoimprint lithography. Although the fabrication of the mold using EBL takes a long time, the result is that patterns can be transferred in a short time, using the mold that can be used repeatedly. Moreover, it is also possible to transfer the pattern onto a flexible substrate by modifying the printing processes.
In this work, we present a detailed process for the fabrication of dielectric metasurfaces. The method is not limited to the application of beam splitters; other metasurface applications, such as lenses, holograms, and optical cloaks, can be realized via this method. Compared to plasmonic metasurfaces, dielectric metasurfaces provide a much higher efficiency at visible wavelengths due to low optical losses of dielectrics. Hence, this protocol can pave the way to study and develop practical metasurfaces.

Metasurfaces consisting of two-dimensional subwavelength antenna arrays have demonstrated many promising optical functionalities, such as achromatic lenses1,2, holograms3,4,5,6, and optical cloaks7. Conventional bulky optical components can be replaced with ultrathin metasurfaces while maintaining the original functionalities. For example, a beam splitter is an optical device used to separate an incident beam into two beams. Typical beam splitters are made by combining two triangular prisms. Since their interface characteristics determine the beam splitting properties, it is hard to reduce the physical size without functional degradation. On the other hand, ultrathin beam splitters can be realized with metasurfaces encoded with a one-dimensional linear phase gradient8,9. The thickness of metasurfaces is less than their working wavelengths, and separation properties can be controlled by the phase distribution.
We designed a metasurface beam splitter which can generate equal-intensity beams regardless of the incident polarization states10. This characteristic comes from a Fourier hologram. Due to the image of two white spots on a black background, generated hologram from the metasurface is the same as the encoded image. The Fourier hologram does not have a specific focal length, so the encoded image can be observed in the whole space behind the metasurface11. If the same two-spot image is generated behind the metasurface, it also works as a beam splitter. The Fourier hologram by the metasurface creates an inverted image, which is called a twin image, with respect to the orthogonal polarization states. The twin image is typically regarded as noise. However, the two-spot image encoded in this metasurface is origin-symmetric, resulting in a perfect overlap of the original and twin images. Since any polarization states can be represented by a linear combination of right-handed (RCP) and left-handed (LCP) circular polarizations, the device described here shows the polarization-independent functionality.
Here, we present a protocol for the fabrication and optical characterization of dielectric metasurfaces enabling equal-intensity beam generation. The phase distribution of this device is retrieved from the Gerchberg&#8211;Saxton (GS) algorithm, which is generally used for phase-only holograms12. a-Si:H of 300 nm thick is deposited on the fused silica substrate, using PECVD. A Cr mask is defined on the a-Si:H film, using EBL. The mask pattern corresponds to the phase distribution derived from the GS algorithm. ICP-RIE is exploited to etch the a-Si:H film along the Cr mask. The rest of the Cr mask is removed by Cr etchant finalizing the sample fabrication. The optical functionality of the fabricated metasurface is characterized using a customized optical setup. When a laser beam is incident to the metasurface, the transmitted beam is separated into three parts, namely two diffracted beams and one zeroth-order beam. The diffracted beams deviate from an extension of the incident beam path while the zeroth-order beam follows it. To verify the functionality of this device, we measured the beam power, beam profile, and diffracted angle using a power meter, CCD, and protractor, respectively.
All the fabrication processes and materials used are optimized for the target functionality. For visible working frequencies, the individual antenna sizes should be a few hundreds of nanometers, and the material itself should have a low optical loss at visible wavelengths. Only a few kinds of fabrication methods are applicable when defining such small structures. Typical photolithography, as well as direct laser writing, are incapable of the fabrication due to the diffraction limit. Focused ion beam milling can be used, but there are critical issues of gallium contamination, pattern design dependence, and the slow process speed. Practically, EBL is the only way to facilitate the fabrication of metasurfaces working at visible frequencies13.
Dielectrics are usually preferred due to the unavoidable ohmic loss of metals. The optical loss of a-Si:H is low enough for our purpose. Although the optical loss of a-Si:H is not as low as low-loss dielectrics such as titanium dioxide1,4 and crystalline silicon14, the fabrication of a-Si:H is much simpler. Typical evaporation and sputtering processes are not capable of the deposition of an a-Si:H film. PECVD is usually required. During the PECVD process, some hydrogen atoms from SiH4 and H2 gases are trapped among the silicon atoms, resulting in an a-Si:H film. There are two ways to define a-Si:H patterns. One is the deposition of a-Si:H on a patterned photoresist, followed by the lift-off process, and the other is by defining an etching mask on the a-Si:H film, followed by the etching process. The former is well-suited to evaporation processes, but it is not easy to deposit a-Si:H film using evaporation. Hence, the latter is the optimal way to make a-Si:H patterns. Cr is used as the etching mask material because of its high etching selectivity with silicon.

<table>
	<tbody>
		<tr>
			<td>Plasma enhanced chemical vapor deposition</td>
			<td>BMR Technology</td>
			<td>HiDep-SC</td>
			<td></td>
		</tr>
		<tr>
			<td>Electron beam lithography</td>
			<td>Elionix</td>
			<td>ELS-7800</td>
			<td></td>
		</tr>
		<tr>
			<td>E-beam evaporation system</td>
			<td>Korea Vacuum Tech</td>
			<td>KVE-E4000</td>
			<td></td>
		</tr>
		<tr>
			<td>Inductively-coupled plasma reactive ion etching</td>
			<td>DMS</td>
			<td>-</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultrasonic cleaner</td>
			<td>Honda</td>
			<td>W-113</td>
			<td></td>
		</tr>
		<tr>
			<td>E-beam resist</td>
			<td>MICROCHEM</td>
			<td>495 PMMA A2</td>
			<td></td>
		</tr>
		<tr>
			<td>Resist developer</td>
			<td>MICROCHEM</td>
			<td>MIBK:IPA=1:3</td>
			<td></td>
		</tr>
		<tr>
			<td>Conducting polymer</td>
			<td>Showa denko</td>
			<td>E-spacer</td>
			<td></td>
		</tr>
		<tr>
			<td>Chromium etchant</td>
			<td>KMG</td>
			<td>CR-7</td>
			<td></td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>J.T. Baker</td>
			<td>925402</td>
			<td></td>
		</tr>
		<tr>
			<td>2-propanol</td>
			<td>J.T. Baker</td>
			<td>909502</td>
			<td></td>
		</tr>
		<tr>
			<td>Chromium evaporation source</td>
			<td>Kurt J. Lesker</td>
			<td>EVMCR35D</td>
			<td></td>
		</tr>
		<tr>
			<td>Collimated laser diode module</td>
			<td>Thorlabs</td>
			<td>CPS-635</td>
			<td>wavelength: 635 nm</td>
		</tr>
		<tr>
			<td>ND:YAG laser</td>
			<td>GAM laser</td>
			<td>GAM-2000</td>
			<td>wavelength: 532 nm</td>
		</tr>
		<tr>
			<td>power meter</td>
			<td>Thorlabs</td>
			<td>S120VC</td>
			<td></td>
		</tr>
		<tr>
			<td>CCD Camera</td>
			<td>INFINITY</td>
			<td>infinity2-2M</td>
			<td></td>
		</tr>
		<tr>
			<td>ND filter</td>
			<td>Thorlabs</td>
			<td>NCD-50C-4-A</td>
			<td></td>
		</tr>
		<tr>
			<td>Linear polarizer</td>
			<td>Thorlabs</td>
			<td>LPVISA100-MP2</td>
			<td></td>
		</tr>
		<tr>
			<td>Lens</td>
			<td>Thorlabs</td>
			<td>LB1676</td>
			<td></td>
		</tr>
		<tr>
			<td>Iris</td>
			<td>Thorlabs</td>
			<td>ID25</td>
			<td></td>
		</tr>
		<tr>
			<td>Circular polarizer</td>
			<td>Edmund optics</td>
			<td>88-096</td>
			<td></td>
		</tr>
		<tr>
			<td>sample holder</td>
			<td>Thorlabs</td>
			<td>XYFM1</td>
			<td></td>
		</tr>
		<tr>
			<td>PECVD software</td>
			<td>BMR Technology</td>
			<td>HIDEP</td>
			<td></td>
		</tr>
	</tbody>
</table>

demonstration of equal-intensity beam generation by dielectric metasurfaces

The fabrication and characterization protocol for a metasurface beam splitter, enabling equal-intensity beam generation, is demonstrated. Hydrogenated amorphous silicon (a-Si:H) is deposited on the fused silica substrate, using plasma-enhanced chemical vapor deposition (PECVD). Typical amorphous silicon deposited by evaporation causes severe optical loss, impinging the operation at visible frequencies. Hydrogen atoms inside the amorphous silicon thin film can reduce the structural defects, improving optical loss. Nanostructures of a few hundreds of nanometers are required for the operation of metasurfaces in the visible frequencies. Conventional photolithography or direct laser writing is not feasible when fabricating such small structures, due to the diffraction limit. Hence, electron beam lithography (EBL) is utilized to define a chromium (Cr) mask on the thin film. During this process, the exposed resist is developed at a cold temperature to slow down the chemical reaction and make the pattern edges sharper. Finally, a-Si:H is etched along the mask, using inductively coupled plasma&#8211;reactive ion etching (ICP-RIE). The demonstrated method is not feasible for large-scale fabrication due to the low throughput of EBL, but it can be improved upon by combining it with nanoimprint lithography. The fabricated device is characterized by a customized optical setup consisting of a laser, polarizer, lens, power meter, and charge-coupled device (CCD). By changing the laser wavelength and polarization, the diffraction properties are measured. The measured diffracted beam powers are always equal, regardless of the incident polarization, as well as wavelength.

The measurement results show the polarization-independent functionality of the device presented here (Figure 1). Measured beam powers of diffraction orders of m = &#177; 1 are equal regardless of the incident polarization state (i.e., RCP, LCP, and linear polarization). Since any arbitrary polarization states can be decomposed by the linear combination of RCP and LCP, the device&#8217;s functionality can be maintained, regardless of polarization states. The diffraction angles are 24&#176; and 28.5&#176; for the wavelengths of 532 nm and 635 nm, respectively, and the angles can be controlled by changing the encoded hologram.
Diffraction efficiency is defined by the ratio of the diffracted beam powers (m = &#177; 1) to the incident beam power. The device presented here consists of same-sized nanorods with different orientations based on the geometric-phase-resulting broadband operation (Figure 2). Theoretically, the efficiency should be higher than 20% for both wavelengths. However, the measured diffraction efficiencies are 18.3% at &#955; = 532 nm and 9.1% at &#955; = 635 nm. The discrepancy mainly comes from the beam size being larger than the metasurface itself. The measured profiles of the zeroth-order beam clearly show that the incident beam size is larger than the metasurface (i.e., the excess portion of the incident beam directly goes to the power meter without interacting with the metasurface, reducing the diffraction efficiency) (Figure 3).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59066/59066fig1.jpg" /> 
Figure 1: Diffracted beam power measurement. (a) The optical setup for the laser illumination. The next two panels show the measured diffracted beam power (b) at &#955; = 532 nm and (c) at 635 nm. Since the laser beam power was not consistent, the measured beam powers are calculated by time-averaging recorded values. The error bars in the figure represent the maximum and minimum values during the recording time. This figure has been modified from Yoon et al.10. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59066/59066fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59066/59066fig2.jpg" /> 
Figure 2: SEM images of the fabricated metasurface. (a) Top view of the metasurface. (inset) Geometry of the unit cell: length (L) = 150 nm, width (W) = 80 nm, height (H) = 300 nm, and pitch (P) = 240 nm. (b) Perspective view of the metasurface. This figure has been modified from Yoon et al.10. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59066/59066fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59066/59066fig3.jpg" /> 
Figure 3: Captured beam profiles by CCD taken 14 cm behind the metasurface. The beam diameters can be estimated at ~3 mm at &#955; = 532 nm and ~5 mm at &#955; = 635 nm. The corresponding beam diverging angles are approximately 2.5&#176; and 4.1&#176;, respectively. The captured beam profiles have laser speckles, but they can be removed by diffusers1 or Dammann grating3,15. This figure has been modified from Yoon et al.10. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59066/59066fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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Demonstration of Equal-Intensity Beam Generation by Dielectric Metasurfaces

A protocol for the fabrication and optical characterization of dielectric metasurfaces is presented. This method can be applied to the fabrication of not only beam splitters, but also of general dielectric metasurfaces, such as lenses, holograms, and optical cloaks.

The fabrication and characterization protocol for a metasurface beam splitter, enabling equal-intensity beam generation, is demonstrated. Hydrogenated ...

demonstration-equal-intensity-beam-generation-dielectric

Nanyang Technological University

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Engineering

6.2K Views.  Pohang University of Science and Technology (POSTECH). A protocol for the fabrication and optical characterization of dielectric metasurfaces is presented. This method can be applied to the fabrication of not only beam splitters, but also of general dielectric metasurfaces, such as lenses, holograms, and optical cloaks.

Video: Demonstration of Equal-Intensity Beam Generation by Dielectric Metasurfaces

Molecular Beam Mass Spectrometry With Tunable Vacuum Ultraviolet (VUV) Synchrotron Radiation

Tunable soft ionization coupled to mass spectroscopy is a powerful method to investigate isolated molecules, complexes and clusters and their spectroscopy and dynamics1-4. Fundamental studies of photoionization processes of biomolecules provide information about the electronic structure of these systems. Furthermore determinations of ionization energies and other properties of biomolecules in the gas phase are not trivial, and these experiments provide a platform to generate these data. We have developed a thermal vaporization technique coupled with supersonic molecular beams that provides a gentle way to transport these species into the gas phase. Judicious combination of source gas and temperature allows for formation of dimers and higher clusters of the DNA bases. The focus of this particular work is on the effects of non-covalent interactions, i.e., hydrogen bonding, stacking, and electrostatic interactions, on the ionization energies and proton transfer of individual biomolecules, their complexes and upon micro-hydration by water1, 5-9.
We have performed experimental and theoretical characterization of the photoionization dynamics of gas-phase uracil and 1,3-dimethyluracil dimers using molecular beams coupled with synchrotron radiation at the Chemical Dynamics Beamline10 located at the Advanced Light Source and the experimental details are visualized here. This allowed us to observe the proton transfer in 1,3-dimethyluracil dimers, a system with pi stacking geometry and with no hydrogen bonds1. Molecular beams provide a very convenient and efficient way to isolate the sample of interest from environmental perturbations which in return allows accurate comparison with electronic structure calculations11, 12. By tuning the photon energy from the synchrotron, a photoionization efficiency (PIE) curve can be plotted which informs us about the cationic electronic states. These values can then be compared to theoretical models and calculations and in turn, explain in detail the electronic structure and dynamics of the investigated species 1, 3.

Molecular Beam Mass Spectrometry With Tunable Vacuum Ultraviolet VUV Synchrotron Radiation

A molecular beam coupled to tunable vacuum ultraviolet photoionization mass spectrometer at a synchrotron provides a convenient tool to explore the electronic structure of isolated gas phase molecules and clusters. Proton transfer mechanisms in DNA base dimers were elucidated with this technique.

Tunable soft ionization coupled to mass spectroscopy is a powerful method to investigate isolated molecules, complexes and clusters and their spectroscopy ...

The Preparation of Electrohydrodynamic Bridges from Polar Dielectric Liquids

Horizontal and vertical liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields (8-20 kV/cm) and polar dielectric liquids. These bridges are unique from capillary bridges in that they exhibit extensibility beyond a few millimeters, have complex bi-directional mass transfer patterns, and emit non-Planck infrared radiation. A number of common solvents can form such bridges as well as low conductivity solutions and colloidal suspensions. The macroscopic behavior is governed by electrohydrodynamics and provides a means of studying fluid flow phenomena without the presence of rigid walls. Prior to the onset of a liquid bridge several important phenomena can be observed including advancing meniscus height (electrowetting), bulk fluid circulation (the Sumoto effect), and the ejection of charged droplets (electrospray). The interaction between surface, polarization, and displacement forces can be directly examined by varying applied voltage and bridge length. The electric field, assisted by gravity, stabilizes the liquid bridge against Rayleigh-Plateau instabilities. Construction of basic apparatus for both vertical and horizontal orientation along with operational examples, including thermographic images, for three liquids (e.g., water, DMSO, and glycerol) is presented.

Horizontal and vertical electrohydrodynamic liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields and polar dielectric liquids. The construction of basic apparatus and operational examples, including thermographic images, for three liquids (e.g., water, DMSO, and glycerol) is presented.

Horizontal and vertical liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields (8-20 kV/cm) and ...

Speciation and Bioavailability Measurements of Environmental Plutonium Using Diffusion in Thin Films

The biological uptake of plutonium (Pu) in aquatic ecosystems is of particular concern since it is an alpha-particle emitter with long half-life which can potentially contribute to the exposure of biota and humans. The diffusive gradients in thin films technique is introduced here for in-situ measurements of Pu bioavailability and speciation. A diffusion cell constructed for laboratory experiments with Pu and the newly developed protocol make it possible to simulate the environmental behavior of Pu in model solutions of various chemical compositions. Adjustment of the oxidation states to Pu(IV) and Pu(V) described in this protocol is essential in order to investigate the complex redox chemistry of plutonium in the environment. The calibration of this technique and the results obtained in the laboratory experiments enable to develop a specific DGT device for in-situ Pu measurements in freshwaters. Accelerator-based mass-spectrometry measurements of Pu accumulated by DGTs in a karst spring allowed determining the bioavailability of Pu in a mineral freshwater environment. Application of this protocol for Pu measurements using DGT devices has a large potential to improve our understanding of the speciation and the biological transfer of Pu in aquatic ecosystems.

The technique of diffusive gradients in thin films (DGT) is proposed for speciation studies of plutonium. This protocol describes diffusion experiments probing the behavior of Pu(IV) and Pu(V) in presence of organic matter. DGTs deployed in a karstic spring allow assessment of the bioavailability of Pu.

The biological uptake of plutonium (Pu) in aquatic ecosystems is of particular concern since it is an alpha-particle emitter with long half-life which can ...

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 ...

Use of Sacrificial Nanoparticles to Remove the Effects of Shot-noise in Contact Holes Fabricated by E-beam Lithography

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has been attributed to statistical fluctuations in the number of photons/electrons arriving at a given nano-region arising from shot-noise (SN). The SN varies inversely to the square root of a number of photons/electrons. For a fixed dosage, the SN is larger in EUV and E-beam lithographies than for traditional (193 nm) optical lithography. Bottom-up and top-down patterning approaches are combined to minimize the effects of shot noise in nano-hole patterning. Specifically, an amino-silane surfactant self-assembles on a silicon wafer that is subsequently spin-coated with a 100 nm film of a PMMA-based E-beam photoresist. Exposure to the E-beam and the subsequent development uncover the underlying surfactant film at the bottoms of the holes. Dipping the wafer in a suspension of negatively charged, citrate-capped, 20 nm gold nanoparticles (GNP) deposits one particle per hole. The exposed positively charged surfactant film in the hole electrostatically funnels the negatively charged nanoparticle to the center of an exposed hole, which permanently fixes the positional registry. Next, by heating near the glass transition temperature of the photoresist polymer, the photoresist film reflows and engulfs the nanoparticles. This process erases the holes affected by SN but leaves the deposited GNPs locked in place by strong electrostatic binding. Treatment with oxygen plasma exposes the GNPs by etching a thin layer of the photoresist. Wet-etching the exposed GNPs with a solution of I2/KI yields uniform holes located at the center of indentations patterned by E-beam lithography. The experiments presented show that the approach reduces the variation in the size of the holes caused by SN from 35% to below 10%. The method extends the patterning limits of transistor contact holes to below 20 nm.

Uniformly sized nanoparticles can remove fluctuations in contact hole dimensions patterned in poly(methyl methacrylate) (PMMA) photoresist films by electron beam (E-beam) lithography. The process involves electrostatic funneling to center and deposit nanoparticles in contact holes, followed by photoresist reflow and plasma- and wet-etching steps.

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has ...

Optical Trap Loading of Dielectric Microparticles In Air

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly scattered dielectric microparticles adhered to a glass substrate are momentarily detached using ultrasonic vibrations generated by a piezoelectric transducer (PZT). Then, the optical beam focused on a selected particle lifts it up to the optical trap while the vibrationally excited microparticles fall back to the substrate. A particle may be trapped at the nominal focus of the trapping beam or at a position above the focus (referred to here as the levitation position) where gravity provides the restoring force. After the measurement, the trapped particle can be placed at a desired position on the substrate in a controlled manner. 
In this protocol, an experimental procedure for selective optical trap loading in air is outlined. First, the experimental setup is briefly introduced. Second, the design and fabrication of a PZT holder and a sample enclosure are illustrated in detail. The optical trap loading of a selected microparticle is then demonstrated with step-by-step instructions including sample preparation, launching into the trap, and use of electrostatic force to excite particle motion in the trap and measure charge. Finally, we present recorded particle trajectories of Brownian and ballistic motions of a trapped microparticle in air. These trajectories can be used to measure stiffness or to verify optical alignment through time domain and frequency domain analysis. Selective trap loading enables optical tweezers to track a particle and its changes over repeated trap loadings in a reversible manner, thereby enabling studies of particle-surface interaction.

A protocol for launching and stably trapping selected dielectric microparticles in air is presented.

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly ...

Dielectric RheoSANS &#8212; Simultaneous Interrogation of Impedance, Rheology and Small Angle Neutron Scattering of Complex Fluids

A procedure for the operation of a new dielectric RheoSANS instrument capable of simultaneous interrogation of the electrical, mechanical and microstructural properties of complex fluids is presented. The instrument consists of a Couette geometry contained within a modified forced convection oven mounted on a commercial rheometer. This instrument is available for use on the small angle neutron scattering (SANS) beamlines at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). The Couette geometry is machined to be transparent to neutrons and provides for measurement of the electrical properties and microstructural properties of a sample confined between titanium cylinders while the sample undergoes arbitrary deformation. Synchronization of these measurements is enabled through the use of a customizable program that monitors and controls the execution of predetermined experimental protocols. Described here is a protocol to perform a flow sweep experiment where the shear rate is logarithmically stepped from a maximum value to a minimum value holding at each step for a specified period of time while frequency dependent dielectric measurements are made. Representative results are shown from a sample consisting of a gel composed of carbon black aggregates dispersed in propylene carbonate. As the gel undergoes steady shear, the carbon black network is mechanically deformed, which causes an initial decrease in conductivity associated with the breaking of bonds comprising the carbon black network. However, at higher shear rates, the conductivity recovers associated with the onset of shear thickening. Overall, these results demonstrate the utility of the simultaneous measurement of the rheo-electro-microstructural properties of these suspensions using the dielectric RheoSANS geometry.

Here, we present a procedure for the measurement of simultaneous impedance, rheology and neutron scattering from soft matter materials under shear flow.

A procedure for the operation of a new dielectric RheoSANS instrument capable of simultaneous interrogation of the electrical, mechanical and ...

Demonstration of a Hyperlens-integrated Microscope and Super-resolution Imaging

The use of super-resolution imaging to overcome the diffraction limit of conventional microscopy has attracted the interest of researchers in biology and nanotechnology. Although near-field scanning microscopy and superlenses have improved the resolution in the near-field region, far-field imaging in real-time remains a significant challenge. Recently, the hyperlens, which magnifies and converts evanescent waves into propagating waves, has emerged as a novel approach to far-field imaging. Here, we report the fabrication of a spherical hyperlens composed of alternating silver (Ag) and titanium oxide (TiO2) thin layers. Unlike a conventional cylindrical hyperlens, the spherical hyperlens allows for two-dimensional magnification. Thus, incorporation into conventional microscopy is straightforward. A new optical system integrated with the hyperlens is proposed, allowing for a sub-wavelength image to be obtained in the far-field region in real time. In this study, the fabrication and imaging setup methods are explained in detail. This work also describes the accessibility and possibility of the hyperlens, as well as practical applications of real-time imaging in living cells, which can lead to a revolution in biology and nanotechnology.

The use of a hyperlens has been regarded as a novel super-resolution imaging technique due to its advantages in real-time imaging and its simple implementation with conventional optics. Here, we present a protocol describing the fabrication and imaging applications of a spherical hyperlens.

The use of super-resolution imaging to overcome the diffraction limit of conventional microscopy has attracted the interest of researchers in biology and ...

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.

Attainment of high-quality Schottky contacts is imperative for achieving efficient gate modulation in heterostructure field effect transistors (HFETs). We present the fabrication methodology and characteristics of Schottky diodes on Zn-polar BeMgZnO/ZnO heterostructures with high-density two dimensional electron gas (2DEG), grown by plasma-assisted molecular beam epitaxy on GaN templates.

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device ...

Preparing an Isotopically Pure 229Th Ion Beam for Studies of 229mTh

A methodology is described to generate an isotopically pure 229Th ion beam in the 2+ and 3+ charge states. This ion beam enables one to investigate the low-lying isomeric first excited state of 229Th at an excitation energy of about 7.8(5) eV and a radiative lifetime of up to 104 seconds. The presented method allowed for a first direct identification of the decay of the thorium isomer, laying the foundations to study its decay properties as prerequisite for an optical control of this nuclear transition. High energy 229Th ions are produced in the &#945;&#160;decay of a radioactive 233U source. The ions are thermalized in a buffer-gas stopping cell, extracted and subsequently an ion beam is formed. This ion beam is mass purified by a quadrupole-mass separator to generate a pure ion beam. In order to detect the isomeric decay, the ions are collected on the surface of a micro-channel plate detector, where electrons, as emitted in the internal conversion decay of the isomeric state, are observed.

Preparing an Isotopically Pure 229Th Ion Beam for Studies of 229mTh

We present a protocol for the generation of an isotopically purified low-energy 229Th ion beam from a 233U source. This ion beam is used for the direct detection of the 229mTh ground-state decay via the internal-conversion decay channel. We also measure the internal conversion lifetime of 229mTh as well.

A methodology is described to generate an isotopically pure 229Th ion beam in the 2+ and 3+ charge states. This ion beam enables one to investigate the ...

Demonstration of Equal-Intensity Beam Generation by Dielectric Metasurfaces

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