Thanks to Michael Knutson, Paul Hamric, Greg Scott, and Chris Landes for helpful discussions and assistance related to the custom inert atmosphere sample holder and assistance in the University of Utah student machine shop. P.C. acknowledges the NSF GRFP under Grant No. 0750758. P.C. acknowledges the University of Utah Nanotechnology Training Fellowship. R.M. acknowledges a NSF CAREER Award No. 1054899 and funding from the USTAR Initiative.

NOTE: perform all the following steps under cleanroom class 100 conditions or better.
1. Sample Preparation
<ol>
	<li>Clean a 2&#8221; diameter silicon wafer with Buffered Oxide Etch (BOE) solution (6 parts 40% NH4F and 1 part 49% HF) for 2 min (Caution: Hazardous chemicals). Choose this etch time to remove any organics or contaminants on the surface. Rinse with deionized (DI) water for approximately 5 min. Dry wafer with dry N2. 
	NOTE: Never work alone when using HF. Always wears eye protection with face shield and personal protective equipment (PPE) in case of spills. Post guidelines for the use and handling of HF waste in the lab where the etching is performed. 
	NOTE: Steps 1.2 to 1.7 are for electrochemical locking only. If performing locking via dissolution proceed to Step 2.</li>
	<li>To lay down the working electrode, sputter 100 nm of Platinum (Pt) onto the clean 2&#8221; diameter silicon wafer.</li>
	<li>Before etching the platinum thin film, clean the RIE chamber of any impurities or leftover photoresist from previous dry etches.</li>
	<li>Pump down the chamber until a base pressure of 1 &#215; 10-5 Torr is achieved. Make sure that the RF power is set to 200 W and the flow rates for the oxygen and argon are set to 50 sccm and 10 sccm respectively. Ignite the Ar/O2 plasma and run for at least 1 hr.</li>
	<li>Turn off the Ar/O2 plasma and allow the chamber vent for approximately 10 min.</li>
	<li>To etch the platinum thin film surface, load the sample into the RIE chamber and pump the chamber down to a base pressure of 1 &#215; 10-5 Torr. This time set the argon flow rate to 0 sccm. Ignite the O2 plasma and let this process run for 30 min.</li>
	<li>Turn off the O2 plasma and let the chamber vent for 10 min.</li>
</ol>
2. Thermal Evaporation of Photochromic Molecule Using Custom Low Temperature Evaporator (LTE)
<ol>
	<li>Fill AlO2 boat with 30 mg of BTE and load into custom LTE source (Figure 6).</li>
	<li>Load silicon wafer into sample mount.</li>
	<li>Seal chamber ports and pump chamber down to a base pressure of 1 x 10-6 Torr.</li>
	<li>Evaporate the BTE at a setpoint temperature of 100 &#176;C, with a film thickness of 30 nm.</li>
	<li>Immediately after evaporation, flood illuminate the sample to 5 min of UV to transform the BTE material to the closed form, 1c.</li>
	<li>In order to define the sample size, cleave a small piece of the wafer using a diamond scribe to scratch a line from the edge of the silicon surface. Grab the wafer on both sides of the scratch line and bend the wafer downwards until it breaks along the crystal plane.</li>
	<li>Perform profilometer measurements to validate BTE thin film thickness. To do this, scratch the sample using a fine edge tweezers. Measure the step height from this scratch, which is the difference in height between the right and left cursor position. 
	NOTE: Inaccuracies in film thickness will result in discrepancies in exposure dose.</li>
	<li>Store remaining sample in N2 filled glovebox.</li>
</ol>
3. Exposures
NOTE: Perform all exposures under inert atmosphere conditions to prevent degradation of sample.
<ol>
	<li>Cleave the sample by following the same procedure as outlined in step 2.6.</li>
	<li>Load sample in inert atmosphere sample holder.</li>
	<li>Mount inert sample holder on stage. Purge sample with N2.</li>
	<li>Expose the sample to the desired exposure time using an interferometer, such as the one shown in Figure 8.</li>
</ol>
4. Electrochemical Oxidation Using Three Electrode Cell
NOTE: Perform electrochemistry under inert atmosphere conditions to prevent degradation of sample.
<ol>
	<li>Clamp a clean glass vial on top of the hot plate. Place a clean stirring bar in the vial. Turn on the stirrer.</li>
	<li>Clean a new copper clip with methanol. Clean the platinum counter electrode with methanol.</li>
	<li>Using a clean copper clip, clip the sample through one of the holes in the Teflon vial cap. Make sure to clip onto the exposed platinum only.</li>
	<li>Place the Teflon vial cap onto the vial. Clip the red lead onto the platinum counter electrode and the black lead onto the copper clip holding the sample.</li>
	<li>Using a clean syringe, fill the vial with filtered deionized (DI) water through the second hole in the Teflon vial cap. Fill as high without immersing any of the bare platinum on the sample.</li>
	<li>Bubble nitrogen through the water for 3-5 min. Turn off the nitrogen.</li>
	<li>Place the reference electrode in the second hole in the Teflon vial cap. Clip the white lead onto the reference electrode. Check to make sure none of the bare platinum on the sample is immersed.</li>
	<li>Using a voltammograph, set the oxidation voltage to 0.5 V/sec.</li>
	<li>After the desired oxidation time has elapsed, turn the power to the voltammograph off.</li>
	<li>Remove the red, black, and white clips from the platinum counter electrode, copper clip, and reference electrode.</li>
	<li>Expose the sample to UV for 5 min.</li>
</ol>
5. Sample Development &#8212; Electrochemical Locking
NOTE: Perform development under inert atmosphere conditions to prevent degradation of sample.
<ol>
	<li>Develop the sample in filtered 5 (wt%) isopropanol, 95 (wt%) ethylene glycol for the desired amount of time. Note: Typically 50 nm samples are developed for 30-60 sec while 80 nm samples are developed for 60-180 sec.</li>
	<li>Dry sample with dry N2.</li>
	<li>Immediately expose sample to 5 min of UV.</li>
</ol>
6. Sample Development &#8212; Dissolution Locking
NOTE: Perform development under inert atmosphere conditions to prevent degradation of sample.
<ol>
	<li>Using 100 ml of ethylene glycol in a clean glass beaker, develop the exposed sample for the desired development time.</li>
	<li>Dry sample with dry N2. Immediately expose sample to 5 min of UV.</li>
</ol>
7. Multiple Exposures
<ol>
	<li>If performing multiple exposures repeat steps 3-6 with a translation of the sample relative to the optics.</li>
</ol>

<ol>
	<li>Brimhall, N., Andrew, T. L., Manthena, R. V., Menon, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Breaking+the+far-field+diffraction+limit+in+optical+nanopatterning+via+repeated+photochemical+and+electrochemical+transitions+in+photochromic+molecules.">Breaking the far-field diffraction limit in optical nanopatterning via repeated photochemical and electrochemical transitions in photochromic molecules.</a> Physical Review Letters. 107 (20), 205501 (2011).</li><li>Cantu, P., 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=Subwavelength+nanopatterning+of+photochromic+diaryethene+films.">Subwavelength nanopatterning of photochromic diaryethene films.</a> Applied Physics Letters. 100 (18), 183103 (2012).</li><li>Cantu, P., Andrew, T. L., Menon, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanopatterning+of+diarylethene+films+via+selective+dissolution+of+one+photoisomer.">Nanopatterning of diarylethene films via selective dissolution of one photoisomer.</a> Applied Physics Letters. 103 (17), 173112 (2013).</li><li>Abbe, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Beitr&#228;ge+zur+Theorie+des+Mikroskops+und+der+mikroskopischen+Wahrnehmung.">Beitr&#228;ge zur Theorie des Mikroskops und der mikroskopischen Wahrnehmung.</a> Archiv f&#252;r mikroskopische Anatomie. 9 (1), 413-418 (1873).</li><li>Li, L., 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=Achieving+&#955;/20+resolution+by+one-color+initiation+and+deactivation+of+polymerization.">Achieving &#955;/20 resolution by one-color initiation and deactivation of polymerization.</a> Science. 324 (5929), 910-913 (2009).</li><li>Fischer, J., von Freymann, G., Wegener, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+materials+challenge+in+diffraction-unlimited+direct-laser-writing+optical+lithography.">The materials challenge in diffraction-unlimited direct-laser-writing optical lithography.</a> Advanced Materials. 22 (32), 3578-3582 (2010).</li><li>Mirkin, C. A., 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=Beam+pen+lithography.">Beam pen lithography.</a> Nature Nanotechnology. 5, 637-640 (2010).</li><li>Xie, X., 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=Manipulating+spatial+light+fields+for+micro-+and+nano-photonics.">Manipulating spatial light fields for micro- and nano-photonics.</a> Physica E: Low-dimensional Systems and Nanostructures. 44, 1109-1126 (2012).</li><li>Leroy, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-speed+metal-insulator+transition+in+vanadium+dioxide+films+induced+by+an+electrical+pulsed+voltage+over+nano-gap+electrodes.">High-speed metal-insulator transition in vanadium dioxide films induced by an electrical pulsed voltage over nano-gap electrodes.</a> Applied Physics Letters. 100 (21), 213507 (2012).</li><li>Carr, D., Sekaric, L., Craighead, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurement+of+nanomechanical+resonant+structures+in+single-crystal+silicon.">Measurement of nanomechanical resonant structures in single-crystal silicon.</a> Journal of Vacuum Science &amp; Technology B. 16 (6), 3821-3824 (1998).</li><li>Wilhelmi, O., 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=Rapid+prototyping+of+nanostructured+materials+with+a+focused+ion+beam.">Rapid prototyping of nanostructured materials with a focused ion beam.</a> Japanese Journal of Applied Physics. 47 (6), 2010-5014 (2008).</li><li>Hell, S. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Far-field+optical+nanoscopy.">Far-field optical nanoscopy.</a> Science. 316 (5828), 1153-1158 (2007).</li><li>Chou, S. Y., Krauss, P. R., Renstrom, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanoimprint+lithography.">Nanoimprint lithography.</a> Journal of Vacuum Science &amp; Technology B. 14, 4129 (1996).</li><li>Guillemette, M. D., 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=Surface+topography+induces+3D+self-orientation+of+cells+and+extracellular+matrix+resulting+in+improved+tissue+function.">Surface topography induces 3D self-orientation of cells and extracellular matrix resulting in improved tissue function.</a> Integrative Biology. 1 (2), 196-204 (2009).</li></ol>

The authors have nothing to disclose.

The fabrication, experimental setup and related operational procedures of Patterning via Optical Saturable Transitions (POST) have been described. By exploiting the linear switching properties of thermally stable photochromic molecules, POST offers new perspectives on circumventing the far-field diffraction limit.1-2,4
Previously long-term storage requirement of the samples was solved by storing the samples under N2, directly after the initial evaporation.2 However, the significant line-edge roughness evident in the gratings, Figures 3 and 5, is likely due to the formation of oxidized products of 1c in the presence of air.2 Improving the uniformity of the patterns requires that the exposure and development process be conducted under an inert atmosphere. To address the exposures done in ambient conditions, a custom inert atmosphere sample holder was designed and fabricated. The sample holder needed to provide a rigorous inert gas blanketing of the air-sensitive BTE in closed form, 1c and maintain an excellent seal for sufficient time to obtain the appropriate exposure conditions. The holder was configured so that it could be easily modified for various optical configurations.
One way to address the exposure of the sample to atmosphere would be to design and fabricate an in-situ setup. The main goal of in-situ experimentation in both the electrochemical and dissolution method is performing the exposure, electrochemical oxidation, and development in an O2 purged environment. Having the experiments performed in-situ would aid in the yield and quality of the desired nanostructures.
Currently the extension of this super-resolution technique, POST, to 3-dimensions, which would provide a promising approach to nanofabrication of nanoporous scaffolds, which act as supports for the initial cell attachment and subsequent tissue formation in biological tissue engineering (TE) is being investigated. The nanolithographic technique used to fabricate these initial scaffolds must produce a high yield and be at a low cost in order to produce any significant impact.14
There are, however, some limitations to the POST technique. The need for a separate locking mechanism (electrochemical or dissolution) requires an immersion-type setup. This could add associated complexities and cost if performed in-situ. Also, the need for a platinum layer for the electrochemical locking step (see Step 1.2) might preclude some already patterned substrates or devices. The organic compound (BTE), which is used as the recording medium, requires a relatively complex synthesis, which may add to the cost of the technique initially. However, with increased volumes, the cost should decrease.

Optical lithography is of key importance in the fabrication of nanoscale structures and devices. Increased advancements in novel lithography techniques has the ability to enable new generations of novel devices.8-11 In this article, a review is presented of a class of optical lithographic techniques that achieve deep sub-wavelength resolution using novel photoswitchable molecules. This approach is called Patterning via Optical-Saturable Transitions (POST).1-3
POST is a novel nanofabrication technique that uniquely combines the ideas of saturating optical transitions of photochromic molecules, specifically (1,2-bis(5,5&#8217;-dimethyl-2,2&#8217;-bithiophen-yl))perfluorocyclopent-1-ene. Colloquially, this compound is referred to as BTE, Figure 1, such as those used in stimulated emission-depletion (STED) microscopy12, with interference lithography, which makes it a powerful tool for large-area parallel nanopatterning of deep subwavelength features onto a variety of surfaces with potential extension to 2- and 3-dimensions.
The photochromic layer is originally in one homogeneous state. When this layer is exposed to a uniform illumination of &#955;1, it converts into the second isomeric state (1c), Figure 2. Then the sample is exposed to a focused node at &#955;2, which converts the sample into the first isomeric state (1o) everywhere except in the near vicinity of the node. By controlling the exposure dose, the size of the unconverted region may be made arbitrarily small. A subsequent fixing step of one of the isomers may be selectively and irreversibly converted (locked) into a 3rd state (in black) to lock the pattern. Next, the layer is exposed uniformly to &#955;1, which converts everything except the locked region back to the original state. The sequence of steps may be repeated with a displacement of the sample relative to the optics, resulting in two locked regions whose spacing is smaller than the far-field diffraction limit. Therefore, any arbitrary geometry may be patterned in a &#8220;dot-matrix&#8221; fashion.1-3

<table><tbody><tr><td>Isopropanol</td><td>Fisher Scientific</td><td>P/7500/15</td><td>CAUTION: flammable, use good ventilation and avoid all ignition sources.</td></tr><tr><td>Buffered Oxide Etch</td><td></td><td></td><td></td></tr><tr><td>Methanol</td><td>Ricca Chemical</td><td>48-293-2&amp;nbsp;</td><td>CAUTION: flammable, use good ventilation and avoid all ignition sources.</td></tr><tr><td>Ethylene Glycol</td><td>Sigma-Aldrich</td><td>324558</td><td>CAUTION: Harmful if swallowed</td></tr><tr><td>Silicon wafer</td><td></td><td></td><td></td></tr><tr><td>Diamond Scribe</td><td></td><td></td><td></td></tr><tr><td>Glass Beakers</td><td></td><td></td><td></td></tr><tr><td>Tweezers</td><td>Ted Pella</td><td>5226</td><td></td></tr><tr><td>Reactive Ion Etching System</td><td>Oxford</td><td>Plasma Lab 80 Plus</td><td></td></tr><tr><td>Inert Atmosphere Sample Holder</td><td>Proprietary In-house Designed</td><td></td><td></td></tr><tr><td>Polarizing beamsplitter cube</td><td>Thorlabs</td><td>PBS052</td><td></td></tr><tr><td>HeNe Laser</td><td>Melles Griot</td><td>25-LHP-171</td><td>CAUTION: Wear safety glasses</td></tr><tr><td>Half-wave plates</td><td>Thorlabs</td><td>WPH05M-633</td><td></td></tr><tr><td>Thermal Evaporator</td><td>Proprietary In-house Designed</td><td></td><td></td></tr><tr><td>TMV Super</td><td>TM Vacuum Products</td><td>TMV Super</td><td></td></tr><tr><td>Voltammograph</td><td>Bioanalytical Systems</td><td>CV-37</td><td></td></tr><tr><td>Shortwave UV lamp 365&amp;nbsp;nm</td><td>UVP Analytik Jena Company</td><td>UVGL-25</td><td>CAUTION: Wear UV safety glasses</td></tr></tbody></table>

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.

Fabricated samples:
Different oxidation times were characterized as illustrated by the atomic-force micrographs in Figure 3 at an oxidation voltage of 0.85 V determined from cyclic voltammetry. The 50 nm-thick films were exposed to a standing wave at &#955; = 647 nm of period 400 nm for 60 sec at a power density of 0.95 mW/cm2. As the oxidation time is increased from 10 min&#160;to 25 min, one can clearly see a loss of contrast as some of the regions comprised of 1o get oxidized as well. The developer (5 (wt%) isopropanol: 95 (wt%) ethylene glycol) dissolves all oxidized portions. Larger oxidation times result in uneven line and increased surface non-uniformities after development. Therefore, a careful choice of the oxidation conditions is critical to patterning high-quality nanostructures.2
The higher dipole moment of the closed form of the molecule, 1c, as compared to the open form, 1o, allows for the closed form to be more soluble in polar solvents. This is represented in Figure 4, where half of the sample was converted to the closed form, 1c, and the other half was converted to the open form, 1o. The sample was then developed in 100 (wt%) ethylene glycol for several different development times and then the thickness of the film remaining was measured using a profilometer. From this graph the high selectivity of the dissolution locking step is seen. To remove the residual layer of the closed form, 1c, a reactive ion etching (RIE) process as used in nanoimprint lithography could be used.13
Since the photochromic film can readily recover to its original state upon exposure to UV, it is straightforward to extend the idea to multiple exposures. This is, of course, required for creating dense patterns. Here, the feasibility of this approach is shown by performing two exposures of the same standing wave, but with a ~45&#176; rotation in between (Figure 5). Each exposure was conducted on the Lloyd&#8217;s-mirror interferometer, with a standing wave of period, 540 nm at &#955; = 647 nm (incident intensity ~2.1 mW/cm2) for 1 min. After the first exposure, the sample was immersed in 100 (wt%) ethylene glycol for 30 min&#160;and exposed to short-wavelength UV lamp for 5 min&#160;to convert the molecules to the original closed-ring isomer 1c. The sample was then rotated approximately 45&#176; relative to the optics, and a second exposure to the standing wave was performed. Again, the sample was immersed in 100 (wt%) ethylene glycol for 30 min. After each development, the sample was rinsed in deionized water and dried with N2. The corresponding atomic-force micrograph resolves lines with spacing as small as ~260 nm or &#955;/2.5, which is less than half of the period of the standing wave.3
To verify the efficacy of the sample holder, several exposures were performed to see if the line edge roughness had improved. Assuming an incident sinusoidal illumination, the resulting feature size can be readily simulated. In Figure 7, this feature size is plotted as a function of the exposure time using the solid blue line. The experimentally measured values are shown using crosses. Using the exposure threshold as the only fitting parameter, it is shown that this simple model can accurately explain our experimental results. The smallest experimentally obtained feature size was ~85 nm, corresponding to a linewidth of ~&#955;/7.4. More precise control of the exposure time should enable even smaller features. Note that as the exposure time is increased, the simulation indicates that feature size should be reduced significantly below the far-field diffraction limit. From the scanning electron microscope (SEM) images, it is shown that the line-edge roughness has improved with the use of the inert atmosphere sample holder.
<img alt="Figure 1" src="/files/ftp_upload/52449/52449fig1highres.jpg" /> 
Figure 1. Organic photochromic molecule structure. Compound 1 exists in open form, 1o and the closed form, 1c. Electrochemical oxidation selectively converts 1c to 1ox.
<img alt="Figure 2" src="/files/ftp_upload/52449/52449fig2highres.jpg" /> 
Figure 2. POST technique. Exposure and patterning &#8220;locking&#8221; steps required for recording feature. (A) Electrochemical oxidation. (B) Dissolution of one photoisomer.
<img alt="Figure 3" src="/files/ftp_upload/52449/52449fig3highres.jpg" /> 
Figure 3. Isolated features. Atomic force micrographs of lines after development for samples at various oxidation times.2 Thin-film thickness of ~50 nm. Reprinted with permission from [Cantu, P., et al. Subwavelength nanopatterning of photochromic diarylethene films. App. Phys. Lett. 100(18), 183103]. Copyright [2012], AIP Publishing LLC. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52449/52449fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/52449/52449fig4highres.jpg" /> 
Figure 4. Dissolution rate. This figure shows the Macro-scale solubility of 1c and 1o in 100 (wt%) Ethylene glycol.3 Thin-film thickness of ~29 nm. Reprinted with permission from [Cantu, P., et al. Nanopatterning of diarylethene films via selective dissolution of one photoisomer. App. Phys. Lett. 103(17) 173112]. Copyright [2013], AIP Publishing LLC.
<img alt="Figure 5" src="/files/ftp_upload/52449/52449fig5highres.jpg" /> 
Figure 5. Experimental demonstration of a double-exposure. Left: Schematic showing orientation of sample for double-exposure using POST. Right: Atomic force micrograph of the resulting pattern. The atomic force micrograph reveals the smallest spacing between the features as ~260nm, which is approximately half the period of the illuminating standing wave.3 Reprinted with permission from [Cantu, P., et al. Nanopatterning of diarylethene films via selective dissolution of one photoisomer. App. Phys. Lett. 103(17) 173112]. Copyright [2013], AIP Publishing LLC. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52449/52449fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/52449/52449fig6highres.jpg" /> 
Figure 6. Custom evaporator. Image of the low temperature thermal evaporator (LTE) used in the POST technique.2 Reprinted with permission from [Cantu, P., et al. Subwavelength nanopatterning of photochromic diarylethene films. App. Phys. Lett. 100(18), 183103]. Copyright [2012], AIP Publishing LLC.
<img alt="Figure 7" src="/files/ftp_upload/52449/52449fig7highres.jpg" /> 
Figure 7. Linewidth vs exposure time for a single development and exposure. The incident simulated sinusoidal illumination is shown as a solid blue line, while the experimental data is shown using crosses. A sinusoidal illumination with period of 457 nm was assumed. Inset: SEM images. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52449/52449fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" src="/files/ftp_upload/52449/52449fig8highres.jpg" /> 
Figure 8. Schematic of the Mach-Zehnder interferometry setup used for exposures. The first half-wave plate is used to control the power in each arm. The second half-wave plate is used to control the polarization.

Watch this Scientific Journal Video about Patterning via Optical Saturable Transitions - Fabrication and Characterization at JoVE.com

Patterning via Optical Saturable Transitions - Fabrication and Characterization

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 ...

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6.8K Views.  The University of Utah. 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.

Video: Patterning via Optical Saturable Transitions - Fabrication and Characterization

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

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

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

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

Fabrication and Characterization of Disordered Polymer Optical Fibers for Transverse Anderson Localization of Light

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. The developed polymer optical fiber is composed of 80,000 strands of poly (methyl methacrylate) (PMMA) and polystyrene (PS) that are randomly mixed and drawn into a square cross section optical fiber with a side width of 250 &mu;m. Initially, each strand is 200 &mu;m in diameter and 8-inches long. During the mixing process of the original fiber strands, the fibers cross over each other; however, a large draw ratio guarantees that the refractive index profile is invariant along the length of the fiber for several tens of centimeters. The large refractive index difference of 0.1 between the disordered sites results in a small localized beam radius that is comparable to the beam radius of conventional optical fibers. The input light is launched from a standard single mode optical fiber using the butt-coupling method and the near-field output beam from the disordered fiber is imaged using a 40X objective and a CCD camera. The output beam diameter agrees well with the expected results from the numerical simulations. The disordered optical fiber presented in this work is the first device-level implementation of 2D Anderson localization, and can potentially be used for image transport and short-haul optical communication systems.

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. This microstructured fiber can transport a small localized beam with a radius that is comparable to the beam radius of conventional optical fibers.

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. The developed ...

Quantum State Engineering of Light with Continuous-wave Optical Parametric Oscillators

Engineering non-classical states of the electromagnetic field is a central quest for quantum optics1,2. Beyond their fundamental significance, such states are indeed the resources for implementing various protocols, ranging from enhanced metrology to quantum communication and computing. A variety of devices can be used to generate non-classical states, such as single emitters, light-matter interfaces or non-linear systems3. We focus here on the use of a continuous-wave optical parametric oscillator3,4. This system is based on a non-linear &#967;2 crystal inserted inside an optical cavity and it is now well-known as a very efficient source of non-classical light, such as single-mode or two-mode squeezed vacuum depending on the crystal phase matching. 
Squeezed vacuum is a Gaussian state as its quadrature distributions follow a Gaussian statistics. However, it has been shown that number of protocols require non-Gaussian states5. Generating directly such states is a difficult task and would require strong &#967;3 non-linearities. Another procedure, probabilistic but heralded, consists in using a measurement-induced non-linearity via a conditional preparation technique operated on Gaussian states. Here, we detail this generation protocol for two non-Gaussian states, the single-photon state and a superposition of coherent states, using two differently phase-matched parametric oscillators as primary resources. This technique enables achievement of a high fidelity with the targeted state and generation of the state in a well-controlled spatiotemporal mode.

We describe the reliable generation of non-Gaussian states of traveling optical fields, including single-photon states and coherent state superpositions, using a conditional preparation method operated on the non-classical light emitted by optical parametric oscillators. Type-I and type-II phase-matched oscillators are considered and common procedures, such as the required frequency filtering or the high-efficiency quantum state characterization by homodyning, are detailed.

Engineering non-classical states of the electromagnetic field is a central quest for quantum optics1,2. Beyond their fundamental significance, such states ...

Fabrication and Testing of Microfluidic Optomechanical Oscillators

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

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

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

Patterning Cells on Optically Transparent Indium Tin Oxide Electrodes

The ability to exercise precise spatial and temporal control over cell-surface interactions is an important prerequisite to the assembly of multi-cellular constructs serving as in vitro mimics of native tissues.  In this study, photolithography and wet etching techniques were used to fabricate individually addressable indium tin oxide (ITO) electrodes on glass substrates.  The glass substrates containing ITO microelectrodes were modified with poly(ethylene glycol) (PEG) silane to make them protein and cell resistive.  Presence of insulating PEG molecules on the electrode surface was verified by cyclic voltammetry employing potassium ferricyanide as a redox reporter molecule.  Importantly, the application of reductive potential caused desorption of the PEG layer, resulting in regeneration of the conductive electrode surface and appearance of typical ferricyanide redox peaks.    Application of reductive potential also corresponded to switching of ITO electrode properties from cell non-adhesive to cell-adhesive. Electrochemical stripping of PEG-silane layer from ITO microelectrodes allowed for cell adhesion to take place in a spatially defined fashion, with cellular patterns corresponding closely to electrode patterns.  Micropatterning of several cell types was demonstrated on these substrates.   In the future, the control of the biointerfacial properties afforded by this method will allow to engineer cellular microenvironments through the assembly of three or more cell types into a precise geometric configuration on an optically transparent substrate.

Non-fouling PEG silane monolayer was desorbed from individually addressable ITO electrodes on glass by application of a reductive potential. Electrochemical stripping of PEG-silane layer from ITO microelectrodes allowed for cell adhesion to take place in a spatially defined fashion, with cellular patterns corresponding closely to electrode patterns.

The ability to exercise precise spatial and temporal control over cell-surface interactions is an important prerequisite to the assembly of multi-cellular ...

Biology

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 ...

Cell Patterning on Photolithographically Defined 
Parylene-C: SiO2 Substrates

Cell patterning platforms support broad research goals, such as construction of predefined in vitro neuronal networks and the exploration of certain central aspects of cellular physiology. To easily combine cell patterning with Multi-Electrode Arrays (MEAs) and silicon-based &#8216;lab on a chip&#8217; technologies, a microfabrication-compatible protocol is required. We describe a method that utilizes deposition of the polymer parylene-C on SiO2&#160;wafers. Photolithography enables accurate and reliable patterning of parylene-C at micron-level resolution. Subsequent activation by immersion in fetal bovine serum (or another specific activation solution) results in a substrate in which cultured cells adhere to, or are repulsed by, parylene or SiO2 regions respectively. This technique has allowed patterning of a broad range of cell types (including primary murine hippocampal cells, HEK 293 cell line, human neuron-like teratocarcinoma cell line, primary murine cerebellar granule cells, and primary human glioma-derived stem-like cells). Interestingly, however, the platform is not universal; reflecting the importance of cell-specific adhesion molecules. This cell patterning process is cost effective, reliable, and importantly can be incorporated into standard microfabrication (chip manufacturing) protocols, paving the way for integration of microelectronic technology.

Cell Patterning on Photolithographically Defined 
Parylene-C: SiO2 Substrates

This protocol describes a microfabrication-compatible method for cell patterning on SiO2. A predefined parylene-C design is photolithographically printed on SiO2 wafers. Following incubation with serum (or other activation solution) cells adhere specifically to (and grow according to the conformity of) underlying parylene-C, whilst being repulsed by SiO2 regions.

Cell patterning platforms support broad research goals, such as construction of predefined in vitro neuronal networks and the exploration of certain ...

Bioengineering

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

Rapid Repetition Rate Fluctuation Measurement of Soliton Crystals in a Microresonator

Temporal solitons have attracted great interest in the past decades for their behavior in a steady state, where the dispersion is balanced by the nonlinearity in a propagation Kerr medium. The development of dissipative Kerr solitons (DKSs) in high-Q microcavities drives a novel, compact, chip-scale soliton source. When DKSs serve as femtosecond pulses, the repetition rate fluctuation can be applied to ultrahigh precision metrology, high-speed optical sampling, and optical clocks, etc. In this paper, the rapid repetition rate fluctuation of soliton crystals (SCs), a special state of DKSs where particle-like solitons are tightly packed and fully occupy a resonator, is measured based on the well-known delayed self-heterodyne method. The SCs are generated using a thermal-controlled method. The pump is a frequency fixed laser with a linewidth of 100 Hz. The integral time in frequency fluctuation measurements is controlled by the length of the delay fiber. For a SC with a single vacancy, the repetition rate fluctuations are ~53.24 Hz within 10 &#181;s and ~509.32 Hz within 125 &#181;s, respectively.

Here, we present a protocol to generate soliton crystals in a butterfly-packaged micro-ring resonator using a thermal tuned method. Further, the repetition rate fluctuations of a soliton crystal with a single vacancy are measured using a delayed self-heterodyne method.

Temporal solitons have attracted great interest in the past decades for their behavior in a steady state, where the dispersion is balanced by the ...

Multicolor Fluorescence Detection for Droplet Microfluidics Using Optical Fibers

Fluorescence assays are the most common readouts used in droplet microfluidics due to their bright signals and fast time response. Applications such as multiplex assays, enzyme evolution, and molecular biology enhanced cell sorting require the detection of two or more colors of fluorescence. Standard multicolor detection systems that couple free space lasers to epifluorescence microscopes are bulky, expensive, and difficult to maintain. In this paper, we describe a scheme to perform multicolor detection by exciting discrete regions of a microfluidic channel with lasers coupled to optical fibers. Emitted light is collected by an optical fiber coupled to a single photodetector. Because the excitation occurs at different spatial locations, the identity of emitted light can be encoded as a temporal shift, eliminating the need for more complicated light filtering schemes. The system has been used to detect droplet populations containing four unique combinations of dyes and to detect sub-nanomolar concentrations of fluorescein.

Multicolor fluorescence detection in droplet microfluidics typically involves bulky and complex epifluorescence microscope-based detection systems. Here we describe a compact and modular multicolor detection scheme that utilizes an array of optical fibers to temporally encode multicolor data collected by a single photodetector.

Fluorescence assays are the most common readouts used in droplet microfluidics due to their bright signals and fast time response. Applications such as ...

Patterning via Optical Saturable Transitions - Fabrication and Characterization

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