This work was financially supported by the National Research Foundation (NRF) grants (NRF-2019R1A2C3003129, CAMM-2019M3A6B3030637, NRF-2019R1A5A8080290) funded by the Ministry of Science and ICT of the Korean government. I.K. acknowledges the NRF Global Ph.D. fellowship (NRF-2016H1A2A1906519) funded by the Ministry of Education of the Korean government.

1. Device fabrication
NOTE: Figure 1 shows the fabrication process of a-Si:H metasurfaces17.
<ol>
	<li>Prepare a fused silica wafer piece (size = 2 cm x 2 cm, thickness = 500 &#181;m) as a substrate. Rinse the substrate with acetone and isopropyl alcohol (IPA) then blow nitrogen gas over the substrate to dry it.</li>
	<li>Deposit a 380 nm thick a-Si:H film on the substrate using plasma-enhanced chemical vapor deposition (PECVD) with the following settings: chamber temperature = 300 &#176;C; radio frequency power = 800 W; gas flow rate = 10&#160;sccm for SiH4 and 75 sccm for H2; process pressure = 25 mTorr; time = 30 s.</li>
	<li>Spin-coat an e-beam lithography photoresist. Drop polymethyl methacrylate (PMMA) A2 onto the substrate and spin-coat with a rotation speed of 2,000 rpm for 1 min.</li>
	<li>Bake the resist-coated substrate on a hotplate at 180 &#176;C for 5 min.</li>
	<li>Spin-coat a conductive polymer layer to prevent charge accumulation during the e-beam writing process. Drop the conductive polymer (e.g., Espacer) onto the substrate and spin-coat with a rotation speed of 2,000&#160;rpm for 1 min.</li>
	<li>Run e-beam lithography with an acceleration voltage of 80 kV and a current of 50 pA.</li>
	<li>Immerse the sample in deionized (DI) water for 2 min to remove the conductive polymer layer. Immerse the sample in 1:3 methyl isobutyl ketone (MIBK):IPA solution surrounded by an iced cup for 12 min to develop the exposed pattern. Then rinse the sample with IPA for 30 s.</li>
	<li>Deposit a 30 nm thick chromium (Cr) film by using an e-beam evaporator.</li>
	<li>Immerse the sample in acetone to remove the unexposed photoresist layer and transfer the Cr pattern onto the substrate. Sonicate for 1 min at 40 kHz, then rinse with IPA for 30 s.</li>
	<li>Etch the uncovered a-Si:H layer to transfer the Cr pattern into the a-Si:H layer using a dry etcher with a source power of 500 W, bias of 100 V, gas flow rates of 80 sccm for Cl2 and 120 sccm for HBr.</li>
	<li>Immerse the sample in a Cr etchant solution to remove the Cr etch mask. Then rinse the sample sequentially with acetone, IPA and DI water for 30 s, respectively.</li>
</ol>
2. Scanning electron microscope characterization
<ol>
	<li>Spin-coat a conductive polymer layer to prevent charge accumulation during the electron beam scanning process. Drop the conductive polymer onto the substrate and spin-coat at a rotation speed of 2,000 rpm for 1 min.</li>
	<li>Fix the substrate onto the sample holder using carbon tape. Vent the load lock chamber by pressing the AIR button.</li>
	<li>Put the holder onto the holding rod of the load-lock chamber. Evacuate the load lock chamber by pressing the EVAC button.</li>
	<li>Set the stage height and tilting angle by setting the Z sensor to 8 mm and the T sensor to 0&#176;.</li>
	<li>Open the load lock chamber door by pressing the OPEN button. Press the holding rod to transfer the holder to the main scanning electron microscope (SEM) chamber. Pull out the rod and press the CLOSE button.</li>
	<li>Check the vacuum state before turning on the electron gun. Execute the flashing function by pressing the FLASHING button to remove carbon or dust in the electron gun with an instant high voltage.</li>
	<li>Turn on the electron gun with an accelerating voltage of 5 kV by clicking the ON button in the SEM software.</li>
	<li>Adjust the beam alignment to precisely locate the electron beam in the center position by clicking the BEAM ALIGNMENT panel in the software. Using a stage controller, locate the beam in the center.</li>
	<li>Adjust the aperture alignment and stigma alignment to make a circular electron beam by clicking the APERTURE ALIGNMENT panel in the software. Using a stigma controller, make a stable beam to scan on the same spot.</li>
	<li>Capture SEM images with an appropriate focus and stigmator adjustment.</li>
	<li>Turn off the electron beam by clicking the OFF button in the software. Click the HOME button to return the stage to its original position.</li>
	<li>Open the door of the main chamber and push the rod to pick up the sample holder. Vent the load lock chamber by pressing the AIR button, then unload the holder.</li>
	<li>Rinse the sample with DI water to remove the conductive polymer layer.</li>
</ol>
3. Optical characterization of the spin-multiplexed metahologram
<ol>
	<li>Prepare optical components listed in Table of Materials.</li>
	<li>Attach the diode laser module to an adapter that can be plugged into a 1 inch optical mount. Adjust the height of the diode laser by using a post and a post holder, and fix the position by using a clamp. 
	NOTE: Every optical component should be mounted using a post and a post holder, then fixed in position by using a clamp.</li>
	<li>Assemble the half-wave plate by using a 1 inch rotational mount, then place the plate in front of the laser module to rotate the linearly-polarized light.</li>
	<li>Prepare two mirrors by mounting them on 1 inch kinematic mounts and one alignment disk to align the direction of initial beam.
	<ol>
		<li>Place the alignment disk in front of the laser and set the height. Place the two mirrors so that the beam bends twice at 90&#176; each to be alternating directions.</li>
		<li>Place the alignment disk near the second mirror and adjust the angle of the first mirror by rotating knobs to align the light in the center.</li>
		<li>Place the alignment disk far from the second mirror and adjust the angle of the second mirror by rotating knobs to align the light in the center.</li>
		<li>Repeat steps 3.4.2 and 3.4.3 until the light passes through the center of an alignment disk in both places.</li>
	</ol>
	</li>
	<li>Place a neutral density filter behind the mirror to control the intensity of light. Place an iris behind the neutral density filter to control the diameter of incident light.</li>
	<li>To make a circularly polarized light, place a linear polarizer and a quarter wave plate in order behind the iris. Mount each component on its own rotational mount.</li>
	<li>Attach the fabricated metasurface to a plate with a hole and mount the plate on the XY translation mount for rectangular optics. Adjust the XY translation mount so that light is directed to the pattern in the sample.</li>
	<li>Place a lens after the metasurface. Adjust the position of lens to be placed at the focal length. Place a CCD after the lens to capture a hologram image.</li>
</ol>
4. Optical characterization of the direction-multiplexed metahologram
<ol>
	<li>Prepare two beam splitters, two mirrors, lens and CCD. 
	NOTE: This setup can be built from the spin-multiplexed metahologram setup by adding additional components.</li>
	<li>Place a beam splitter between the quarter-wave plate and the XY translation mount to split the beam into two directions. Place another beam splitter between the XY translation mount and the lens. 
	NOTE: One beam path is the same as the previous spin-multiplexed metahologram setup. Here, another split beam will be aligned to illuminate a sample in the opposite direction to the previous setup.</li>
	<li>Place two mirrors so that the beam bends twice at 90&#176; each to form alternating directions and adjust the beam to be directed into the second beam splitter. Finely align the light so that the beam irradiates the sample correctly in the opposite direction.</li>
	<li>Place another lens at 90&#176; to the right of the first beam splitter and place a CCD to capture a hologram image from the opposite direction.</li>
</ol>

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The a-Si:H metasurfaces were fabricated in three major steps: a-Si:H thin film deposition using PECVD, precise EBL, and dry etching. Among these steps, the EBL writing process is the most important. First, the pattern density on metasurfaces is quite high, so the process requires precise control over the electron dose (energy) and scanning parameters such as number of dots per unit area. The development condition should also be chosen carefully. The density of the pattern is very high, so when the development process is done instantaneously, the nanorod-shaped patterns are not defined well, but are connected to each other. To prevent this problem and to provide an appropriate negative slop of photoresist, which enables easy lift-off, a cold-development technique was used in which the development process is conducted at 2&#8722;4 &#176;C. Furthermore, a bi-layer resist method can be used for easy lift-off process, where two different kinds of resists having different molecular weights and solubility in a development solution are used. Additionally, the side wall profile during the etching process should be made as close to 90&#176; as possible by adjusting the etching process.
SEM and optical characterization of the fabricated metasurfaces should be rigorously conducted. By observing SEM images of the fabricated structures, exact geometric parameters and side-wall profile should be checked to predict the efficiency of metahologram. For the optical experiment, to produce and obtain high quality holographic images, the incident laser beam shape and focusing should be adjusted accurately. Hence, the optical component should be well aligned with each other and properly positioned according to the component specifications such as focal length of lens and angle of polarizer and waveplate.
In this work, we presented a detailed fabrication and characterization method for spin- and direction-multiplexed metaholograms. Increasing the number of functionality on single-layer metasurface is a useful technique for expanding the applications of metasurface. At the same time, however, active functions that can change diverse functions imposed in real time should also be studied. In this experiment, passive methods, such as changing the polarizer angle or optical components, were used to switch holographic images. However, if active material systems such as phase change materials or liquid crystals are combined with the multifunctional metahologram, the holographic video display and anticounterfeiting display technology with metahologram can be commercialized in the near future18. Furthermore, advanced nanoimprinting method will be of great help for scalable manufacturing of metahologram devices.19 Also, new design methodology, such as wavelength-decoupled metasurface design methodology, will enable full-color hologram devices.20

Optical metasurfaces composed of sub-wavelength nanostructures have enabled many interesting optical phenomena, including optical cloaking1, negative refraction2, perfect light absorption3, color filtering4, holographic image projection5, and beam manipulation6,7,8. Optical metasurfaces that have appropriately-designed scatterers can modulate the spectrum, wavefront and polarization of light. Early optical metasurfaces were mainly fabricated using noble metals (e.g., Au, Ag) due to their high reflectivity and ease of nanofabrication, but they have high Ohmic losses, so the metasurfaces have low efficiency at short visible wavelengths.
Development of nanofabrication techniques for dielectric materials that have low losses in visible light (e.g., TiO29, GaN10, and a-Si:H11) has enabled realization of highly efficient flat optical devices with optical metasurfaces. These devices have applications in optics and engineering. One intriguing application is optical holography for projective volumetric display and information encryption. Compared to conventional holograms that use spatial light modulators, the metahologram has numerous advantages such as miniaturization of optical setup, higher resolution of holographic images and larger field of visibility.
Recently, encoding of multiple holographic information in a single-layered metahologram device has been achieved. Examples include metaholograms that are multiplexed in spin12,13, orbital angular momentum14, incident light angle15, and direction16. These efforts have overcome the critical shortcoming of metaholograms, which is a lack of design freedom in a single device. Most conventional metaholograms could only produce single encoded holographic images, but multiplexed device can encode multiple holographic images in real time. Hence, the multiplexed metahologram is a crucial solution platform towards real holographic video display or multifunctional anticounterfeiting holograms.
Reported here are protocols to fabricate spin- and direction-multiplexed all-dielectric visible metaholograms, then to optically characterize them13,16. To encode multiple visual information in a single metasurface device, metaholograms are designed which show two different holographic images when the spin or direction of incident light are changed. To fabricate highly efficient holographic images in a manner comparable with CMOS technology, a-Si:H is used for the metasurfaces and dual magnetic resonances and antiferromagnetic resonances induced inside them are exploited. The fabrication protocol consists of film deposition, electron beam writing, and etching. The fabricated device is characterized using a customized optical setup composed of a laser, a linear polarizer, a quarter waveplate, a lens and a charge-coupled device (CCD).

<table><tbody><tr><td>Aceton</td><td>J.T. Baker</td><td>925402</td><td></td></tr><tr><td>Beam splitter</td><td>Thorlabs</td><td>CCM1-BS013/M</td><td></td></tr><tr><td>Chromium etchant</td><td>KMG</td><td>Cr-7</td><td></td></tr><tr><td>Chromium evaporation source</td><td>Kurt J. Lesker</td><td>EVMCR35D</td><td></td></tr><tr><td>Clamp</td><td>Thorlabs</td><td>CP175</td><td></td></tr><tr><td>Conducting polymer</td><td>Showa denko</td><td>E-spacer</td><td></td></tr><tr><td>Diode laser</td><td>Thorlabs</td><td>CPS635</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>E-beam resist</td><td>Microchem</td><td>495 PMMA A2</td><td></td></tr><tr><td>Electron beam lithography</td><td>Elionix</td><td>ELS-7800</td><td></td></tr><tr><td>Half-wave plate</td><td>Thorlabs</td><td>AHWP05M-600</td><td></td></tr><tr><td>Inductively-coupled plasma reactive ion etching</td><td>DMS</td><td>-</td><td></td></tr><tr><td>Iris</td><td>Thorlabs</td><td>SM1D12</td><td></td></tr><tr><td>Isopropyl alcohol</td><td>J.T. Baker</td><td>909502</td><td></td></tr><tr><td>Kinematic mirror mount</td><td>Thorlabs</td><td>KM100/M</td><td></td></tr><tr><td>Lens</td><td>Thorlabs</td><td>LB1630</td><td></td></tr><tr><td>Lens Mount</td><td>Thorlabs</td><td>LMR2/M</td><td></td></tr><tr><td>Linear polarizer</td><td>Thorlabs</td><td>GTH5-A</td><td></td></tr><tr><td>Mirror</td><td>Thorlabs</td><td>PF10-03-G01</td><td></td></tr><tr><td>Neutral density filter</td><td>Thorlabs</td><td>NDC-50C-4</td><td></td></tr><tr><td>Plasma enhanced chemical vapor deposition</td><td>BMR Technology</td><td>HiDep-SC</td><td></td></tr><tr><td>Post</td><td>Thorlabs</td><td>TR75/M</td><td></td></tr><tr><td>Post holder</td><td>Thorlabs</td><td>PH75E/M</td><td></td></tr><tr><td>Quarter-wave plate</td><td>Thorlabs</td><td>AQWP10M-580</td><td></td></tr><tr><td>Resist developer</td><td>Microchem</td><td>MIBK:IPA=1:3</td><td></td></tr><tr><td>Rotational mount</td><td>Thorlabs</td><td>RSP1/M</td><td></td></tr><tr><td>Scanning electron microscopy</td><td>Hitachi</td><td>Regulus8100</td><td></td></tr><tr><td>XY translation mount</td><td>Thorlabs</td><td>XYF1/M</td><td></td></tr><tr><td>1-inch adapter</td><td>Thorlabs</td><td>AD11F</td><td></td></tr><tr><td>1-inch lens mount</td><td>Thorlabs</td><td>CP02/M</td><td></td></tr></tbody></table>

demonstration of spin-multiplexed and direction-multiplexed all-dielectric visible metaholograms

The optical holography technique realized by metasurfaces has emerged as a novel approach to projective volumetric display and information encryption display in the form of ultrathin and almost flat optical devices. Compared to the conventional holographic technique with spatial light modulators, the metahologram has numerous advantages such as miniaturization of optical setup, higher image resolution and larger field of visibility for holographic images. Here, a protocol is reported for the fabrication and optical characterization of optical metaholograms that are sensitive to the spin and direction of incident light. The metasurfaces are composed of hydrogenated amorphous silicon (a-Si:H), which has large refractive index and small extinction coefficient in the entire visible range resulting in high transmittance and diffraction efficiency. The device produces different holographic images when the spin or direction of incident light are switched. Therefore, they can encode multiple types of visual information simultaneously. The fabrication protocol consists of film deposition, electron beam writing and subsequent etching. The fabricated device can be characterized using a customized optical setup that consists of a laser, a linear polarizer, a quarter waveplate, a lens and a charge-coupled device (CCD).

The a-Si:H metasurfaces enable high cross-polarization efficiency and can be fabricated using a method (Figure 1) that is compatible with CMOS; this trait may enable scalable fabrication and near-future commercialization. The SEM image shows the fabricated a-Si:H metasurfaces (Figure 2). Furthermore, a-Si:H has a larger refractive index than TiO2 and GaN, so even with low aspect ratio nanostructure of around 4.7, an a-SiH meta-hologram with high diffraction efficiency can be realized. The calculated efficiency at 633 nm wavelength was 74% and the measured efficiency was 61%.
A spin-multiplexed metahologram can switch the projected holographic images by simply flipping the handedness of the incident circularly polarized light (Figure 3a). To design such a spin-multiplexed metahologram, two kinds of metasurfaces were used; they can produce different responses depending on whether the light is circularly polarized to the left or to the right. The Gerchberg-Saxton algorithm was used to calculate a phase map that corresponds to the off-axis holographic images. As a result, depending on the input beam polarization states, &#8216;ITU&#8217; and &#8216;RHO&#8217; holographic images (Figure 3c&#8722;e) can be switched in real-time with high fidelity.
A direction-multiplexed metahologram can switch the projected holographic images by changing the incident light direction (Figure 4a). For instance, if the light comes in from the substrate side (forward direction), the holographic &#8216;RHO&#8217; images can be observed (Figure 4b,d), and if the light comes from the metasurface side (backward direction), the holographic &#8216;ITU&#8217; images can be seen (Figure 4c,e). The hologram device that operates in both directions has the advantages of extending the area in which information can be transmitted, and of transmitting and receiving different visual information according to the position of the observer.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/61334/61334fig1.jpg" /> 
Figure 1: Flow chart of a-Si:H metasurface fabrication. The fabrication starts with a double-side-polished fused silica substrate. Using PECVD, 380 nm thick a-Si:H is deposited and followed by spin-coating of the e-beam resist, PMMA A2. Electron beam lithography (EBL) scanning draws nanorod patterns on the resist, which are transferred onto the a-Si:H layer by the Cr lift-off process. A dry etching process transfers the Cr pattern onto the a-Si:H layer, then the Cr etch mask is removed using a Cr etchant. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61334/61334fig1large.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/61334/61334fig2.jpg" /> 
Figure 2: The SEM image of the fabricated device. A tilted view of the SEM image of 380 nm thick a-Si:H metasurfaces is presented. During the etching process, a slanted side-wall profile occurred. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61334/61334fig2large.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/61334/61334fig3.jpg" /> 
Figure 3: A spin-multiplexed metahologram. (a) Schematic of operation of the proposed spin-multiplexed metahologram. (b) Optical microscope and SEM images. The total size of the fabricated metahologram device is 400 &#181;m x 400 &#181;m. A single nanorod has a length of 200 nm, a width of 80 nm, and a height of 380 nm. (c) Experimentally obtained &#8216;ITU&#8217; holographic images with the left circular polarization working at a wavelength of 633 nm. (d) Experimentally obtained &#8216;RHO&#8217; holographic images with the right circular polarization captured with a CCD camera. (e)&#160;Experimentally obtained both holographic images using the elliptically polarized light. This figure was modified from Ansari et al.13. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61334/61334fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/61334/61334fig4.jpg" /> 
Figure 4: A direction-multiplexed metahologram. (a) Schematic of operation of the proposed direction-multiplexed metahologram. (b,c) Fresnel-type metahologram finite-different time-domain simulation results. A left circular polarized light illuminated in forward and backward directions. (d,e) Experimentally obtained holographic images captured with a CCD camera. This figure was modified from Ansari et al.16. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61334/61334fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Demonstration of Spin-Multiplexed and Direction-Multiplexed All-Dielectric Visible Metaholograms at JoVE.com

Demonstration of Spin-Multiplexed and Direction-Multiplexed All-Dielectric Visible Metaholograms

We present a protocol for fabrication of spin- and direction-multiplexed visible metaholograms, then conduct an optical experiment to verify their function. These metaholograms can easily visualize encoded information, so they can be used for projective volumetric display and information encryption.

The optical holography technique realized by metasurfaces has emerged as a novel approach to projective volumetric display and information encryption ...

demonstration-spin-multiplexed-direction-multiplexed-all-dielectric

Nanyang Technological University

Research

JoVE Journal

Engineering

5.7K Views.  Pohang University of Science and Technology (POSTECH). Multifunctionele meta-hologrammen die meerdere holografische beelden kunnen produceren, zijn belangrijke platte optische apparaten voor ultracompacte holografische weergavetoepassingen in het echte leven. Om een multiplexmetaal hologram te genereren, gebruiken we een eenvoudige benadering waarbij twee verschillende holografische beelden kunnen worden weergegeven, afhankelijk van de snelheid van het licht en de voortplantingstractie. Om hun multiplex metal hologram setup te begrijpen, functie ...

Video: Demonstration of Spin-Multiplexed and Direction-Multiplexed All-Dielectric Visible Metaholograms

Patterned Photostimulation with Digital Micromirror Devices to Investigate Dendritic Integration Across Branch Points

Light is a versatile and precise means to control neuronal excitability. The recent introduction of light sensitive effectors such as channel-rhodopsin and caged neurotransmitters have led to interests in developing better means to control patterns of light in space and time that are useful for experimental neuroscience. One conventional strategy, employed in confocal and 2-photon microscopy, is to focus light to a diffraction limited spot and then scan that single spot sequentially over the region of interest. This approach becomes problematic if large areas have to be stimulated within a brief time window, a problem more applicable to photostimulation than for imaging. An alternate strategy is to project the complete spatial pattern on the target with the aid of a digital micromirror device (DMD). The DMD approach is appealing because the hardware components are relatively inexpensive and is supported by commercial interests. Because such a system is not available for upright microscopes, we will discuss the critical issues in the construction and operations of such a DMD system. Even though we will be primarily describing the construction of the system for UV photolysis, the modifications for building the much simpler visible light system for optogenetic experiments will also be provided. The UV photolysis system was used to carryout experiments to study a fundamental question in neuroscience, how are spatially distributed inputs integrated across distal dendritic branch points. The results suggest that integration can be non-linear across branch points and the supralinearity is largely mediated by NMDA receptors.

Digital micromirror devices (DMD) can generate complex patterns in time and space with which to control neuronal excitability. Issues relevant to the design, construction, and operation of DMD systems are discussed. Such a system enabled the demonstration of non-linear integration across distal dendritic branch points.

Light is a versatile and precise means to control neuronal excitability. The recent introduction of light sensitive effectors such as channel-rhodopsin ...

Bioengineering

Patterning via Optical Saturable Transitions - Fabrication and Characterization

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical Saturable Transitions (POST). In this technique the chemical properties of organic photochromic molecules that undergo single-photon reactions are exploited, enabling rapid top-down nanopatterning over large areas at low light intensities, thereby, allowing for the circumvention of the far-field diffraction barrier.4 Simple, cost-effective, high throughput and resolution alternatives to nanopatterning are being explored, such as, two-photon polymerization5,6, beam pen lithography (BPL)7, scanning electron beam lithography (SEBL), and focused ion beam (FIB) patterning. However, multi-photon approaches require high light intensities, which limit their potential for high throughput and offer low image contrast. Although, electron and ion beam lithographic processes offer increased resolution, the serial nature of the process is limited to slow writing speeds, which also prevents patterning of features over large areas. Beam-pen lithography is an approach towards parallel near-field optical lithography. However, the gap between the source of the beam and the surface of the photoresist needs to be controlled extremely precisely for good pattern uniformity and this is very challenging to accomplish for large arrays of beams. Patterning via Optical Saturable Transitions (POST) is an alternative optical nanopatterning technique for patterning sub-wavelength features1-3. Since this technique uses single photons instead of electrons, it is extremely fast and does not require high light intensities1-3, opening the door to massive parallelization.

We report that the diffraction limit of conventional optical lithography can be overcome by exploiting the transitions of organic photochromic derivatives induced by their photoisomerization at low light intensities.1-3 This paper outlines our fabrication technique and two locking mechanisms, namely: dissolution of one photoisomer and electrochemical oxidation.

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical ...

Fabrication and Operation of a Nano-Optical Conveyor Belt

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

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

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

Compact Lens-less Digital Holographic Microscope for MEMS Inspection and Characterization

A micro-electro-mechanical-system (MEMS) is a widely used component in many industries, including energy, biotechnology, medical, communications, and automotive. However, effective inspection and characterization metrology systems are needed to ensure the functional reliability of MEMS. This study presents a system based on digital holography as a tool for MEMS metrology. Digital holography has gained increasing attention in the past 20 years. With the fast development and decreasing cost of sensor arrays, resolution of such systems has increased broadening potential applications. Thus, it has attracted attention from both research and industry sides as a potential reliable tool for industrial metrology. Indeed, by recording the interference pattern between an object beam (which contains sample height information) and a reference beam on a CCD camera, one can retrieve the quantitative phase information of an object. However, most of digital holographic systems are bulky and thus not easy to implement on industry production lines. The novelty of the system presented is that it is lens-less and thus very compact. In this study, it is shown that the Compact Digital Holographic Microscope (CDHM) can be used to evaluate several characteristics typically consider as criteria in MEMS inspections. The surface profiles of MEMS in both static and dynamic conditions are presented. Comparison with AFM is investigated to validate the accuracy of the CDHM.

We present a compact reflection digital holographic system (CDHM) for inspection and characterization of MEMS devices. A lens-less design using a diverging input wave providing natural geometrical magnification is demonstrated. Both static and dynamic studies are presented.

A micro-electro-mechanical-system (MEMS) is a widely used component in many industries, including energy, biotechnology, medical, communications, and ...

Laser-induced Forward Transfer of Ag Nanopaste

Over the past decade, there has been much development of non-lithographic methods1-3 for printing metallic inks or other functional materials. Many of these processes such as inkjet3 and laser-induced forward transfer (LIFT)4 have become increasingly popular as interest in printable electronics and maskless patterning has grown. These additive manufacturing processes are inexpensive, environmentally friendly, and well suited for rapid prototyping, when compared to more traditional semiconductor processing techniques. While most direct-write processes are confined to two-dimensional structures and cannot handle materials with high viscosity (particularly inkjet), LIFT can transcend both constraints if performed properly. Congruent transfer of three dimensional pixels (called voxels), also referred to as laser decal transfer (LDT)5-9, has recently been demonstrated with the LIFT technique using highly viscous Ag nanopastes to fabricate freestanding interconnects, complex voxel shapes, and high-aspect-ratio structures. In this paper, we demonstrate a simple yet versatile process for fabricating a variety of micro- and macroscale Ag structures. Structures include simple shapes for patterning electrical contacts, bridging and cantilever structures, high-aspect-ratio structures, and single-shot, large area transfers using a commercial digital micromirror device (DMD) chip.

We demonstrate the use of the Laser-induced forward transfer technique (LIFT) for the printing of high-viscosity Ag paste. This technique offers a simple, low temperature, robust process for non-lithographically printing microscale 2D and 3D structures.

Over the past decade, there has been much development of non-lithographic methods1-3 for printing metallic inks or other functional materials. Many of ...

Characterization of Anisotropic Leaky Mode Modulators for Holovideo

Holovideo displays are based on light-bending spatial light modulators. One such spatial light modulator is the anisotropic leaky mode modulator. This modulator is particularly well suited for holographic video experimentation as it is relatively simple and inexpensive to fabricate1-3. Some additional advantages of leaky mode devices include: large aggregate bandwidth, polarization separation of signal light from noise, large angular deflection and frequency control of color1. In order to realize these advantages, it is necessary to be able to adequately characterize these devices as their operation is strongly dependent on waveguide and transducer parameters4. To characterize the modulators, the authors use a commercial prism coupler as well as a custom characterization apparatus to identify guided modes, calculate waveguide thickness and finally to map the device's frequency input and angular output of leaky mode modulators. This work gives a detailed description of the measurement and characterization of leaky mode modulators suitable for full-color holographic video.

This work describes fabrication and characterization of anisotropic leaky mode modulators for holographic video.

Holovideo displays are based on light-bending spatial light modulators.  One such spatial light modulator is the anisotropic leaky mode modulator.  This ...

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.

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 ...

Shaping the Amplitude and Phase of Laser Beams by Using a Phase-only Spatial Light Modulator

The aim of this article is to visually demonstrate the utilization of an interferometric method for encoding complex fields associated with coherent laser radiation. The method is based on the coherent sum of two uniform waves, previously encoded into a phase-only spatial light modulator (SLM) by spatial multiplexing of their phases. Here, the interference process is carried out by spatial filtering of light frequencies at the Fourier plane of certain imaging system. The correct implementation of this method allows arbitrary phase and amplitude information to be retrieved at the output of the optical system.
It is an on-axis, rather than off-axis encoding technique, with a direct processing algorithm (not an iterative loop), and free from coherent noise (speckle). The complex field can be exactly retrieved at the output of the optical system, except for some loss of resolution due to the frequency filtering process. The main limitation of the method might come from the inability to operate at frequency rates higher than the refresh rate of the SLM. Applications include, but are not limited to, linear and non-linear microscopy, beam shaping, or laser micro-processing of material surfaces.

We show how to encode the complex field of laser beams by using a single phase element. A common-path interferometer is employed to mix the phase information displayed into a phase-only spatial light modulator to finally retrieve the desired complex field pattern at the output of an optical imaging system.

The aim of this article is to visually demonstrate the utilization of an interferometric method for encoding complex fields associated with coherent laser ...

Recording Ultra-Realistic Full-Color Analog Holograms for Use in a Moving Hologram Display

This paper demonstrates a method to record a set of twelve ultra-realistic full-color analog holograms presenting the same brightness, transparency and homogeneous colors for the fabrication of a Fantatrope, a dynamic holographic 3D display, without the need for special viewing aids. The method involves the use of 3D printer technology, a single-beam full-color Denisyuk optical setup with three low-power lasers (red, green, and blue) and an iso-panchromatic high-sensitive silver-halide holographic emulsion specially designed for recording analog holograms without any diffusion. A cyclic animation is created with a 3D computer graphics program and different elements are 3D printed to form models for the holograms. Holograms are recorded with a full-color holographic setup and developed using two simple chemical baths. To prevent any emulsion thickness variations, the holograms are sealed with optical glue. Results confirm that all holograms recorded with this protocol present the same characteristics, which allow them to be used in the Fantatrope.

We present a protocol to record a set of ultra-realistic full-color analog holograms, showing the same brightness, transparency, and homogeneous colors, on ultra-fine-grain silver-halide holographic emulsions for the fabrication of a dynamic holographic 3D display.

This paper demonstrates a method to record a set of twelve ultra-realistic full-color analog holograms presenting the same brightness, transparency and ...

Transmission of Multiple Signals through an Optical Fiber Using Wavefront Shaping

The transmission of multiple independent optical signals through a multimode fiber is accomplished using wavefront shaping in order to compensate for the light distortion during the propagation within the fiber. Our methodology is based on digital optical phase conjugation employing only a single spatial light modulator, where the optical wavefront is individually modulated at different regions of the modulator, one region per light signal. Digital optical phase conjugation approaches are considered to be faster than other wavefront shaping approaches, where (for example) a complete determination of the wave propagation behavior of the fiber is performed. In contrast, the presented approach is time-efficient since it only requires one calibration per light signal. The proposed method is potentially appropriate for spatial division multiplexing in communications engineering. Further application fields are endoscopic light delivery in biophotonics, especially in optogenetics, where single cells in biological tissue have to be selectively illuminated with high spatial and temporal resolution.

We demonstrate the transmission of multiple independent signals through a multimode fiber using wavefront shaping employing a single spatial light modulator. By modulating the wavefront for each signal individually, spatially separated foci are transmitted. Potential applications are multiplexed data transfer in communications engineering and endoscopic light delivery in biophotonics.

The transmission of multiple independent optical signals through a multimode fiber is accomplished using wavefront shaping in order to compensate for the ...

Demonstration of Spin-Multiplexed and Direction-Multiplexed All-Dielectric Visible Metaholograms

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