We acknowledge Paul J. L. Webster for the 3D rendering of optical data with Amira. Phil Mu&ntilde;oz and Benjamin Franta provided feedback on the manuscript throughout its development. The research described in this paper was supported by the Air Force Office of Scientific Research under grants FA9550-09-1-0546 and FA9550-10-1-0402.

1. Preparing Metal-ion Doped Polymer Film
<ol>
 <li>Measure 8 ml of water in a beaker.</li>
 <li>Add 206 mg of PVP to water. Mix using magnetic stirrer or vortex mixer until the solution is clear.</li>
 <li>Add 210 mg of AgNO3 to solution. Mix using magnetic stirrer or vortex mixer until solution is clear.</li>
 <li>Coat glass slide with solution through drop casting.</li>
 <li>Place glass slide in an oven set at 100 &deg;C. Bake sample for 30 min.</li>
 <li>Remove sample from oven and let cool for 30 min.</li>
</ol>
2. Fabrication of Disconnected Silver Structures
<ol>
 <li>Align setup depicted in Figure 1 on optical table with vibration isolators.</li>
 <li>Adjust compressor to obtain 50-fsec pulses after microscope objective.</li>
 <li>Adjust neutral density filters to obtain 3-nJ pulses after the objective.</li>
 <li>Ensure laser spot size is larger than back aperture of microscope objective.</li>
 <li>Set acousto-optic modulator to produce 10-&mu;sec exposure windows during which the sample is irradiated.</li>
 <li>Block laser beam before it reaches the microscope and place sample onto 3-axis translation stage. The beam path of the femtosecond laser pulses should pass through the imaging microscope objective and into the sample.</li>
 <li>Turn on microscope illumination source to observe the sample in-situ using CCD camera.</li>
 <li>Translate z-axis of stage to find interface between glass substrate and polymer film. Then, refocus microscope to the desired depth inside polymer for patterning bottom-most layer. Z-translation during patterning must be in the direction away from the glass-polymer interface to avoid scattering with fabricated structures.</li>
 <li>Unblock laser-beam and set motion-controller software to translate sample in x-, y- and z- directions with speed of 100 &mu;m/sec. Irradiate single voxels for 10 &mu;sec and separate neighboring voxels by at least several micrometers for clear in-situ imaging. Setting acousto-optic modulator repetition rate to 25 Hz will produce 4-&mu;m spacing. Laser exposed areas will contain silver structures.</li>
</ol>

<ol>
	<li>von Freymann, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-Dimensional+Nanostructures+for+Photonics.">Three-Dimensional Nanostructures for Photonics.</a> Advanced Functional Materials. 20, 1038-1052 (2010).</li><li>LaFratta, C. N., Fourkas, J. T., Baldacchini, T., Farrer, R. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multiphoton+Fabrication.">Multiphoton Fabrication.</a> Angewandte Chemie International Edition. 46, 6238-6258 (2007).</li><li>Gattass, R. R., Mazur, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Femtosecond+laser+micromachining+in+transparent+materials.">Femtosecond laser micromachining in transparent materials.</a> Nat. Photon. 2, 219-225 (2008).</li><li>Li, L., Gattass, R. R., Gershgoren, E., Hwang, H., Fourkas, J. T. <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, 910-913 (2009).</li><li>Haske, W., 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=65+nm+feature+sizes+using+visible+wavelength+3-D+multiphoton+lithography.">65 nm feature sizes using visible wavelength 3-D multiphoton lithography.</a> Opt. Express. 15, 3426-3436 (2007).</li><li>Xing, J. F., 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=Improving+spatial+resolution+of+two-photon+microfabrication+by+using+photoinitiator+with+high+initiating+efficiency.">Improving spatial resolution of two-photon microfabrication by using photoinitiator with high initiating efficiency.</a> Appl. Phys. Lett. 90, 131106 (2007).</li><li>Tan, 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=Reduction+in+feature+size+of+two-photon+polymerization+using+SCR500.">Reduction in feature size of two-photon polymerization using SCR500.</a> Appl. Phys. Lett. 90, 071106 (2007).</li><li>Vora, K., Kang, S., Shukla, S., Mazur, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+of+disconnected+three-dimensional+silver+nanostructures+in+a+polymer+matrix.">Fabrication of disconnected three-dimensional silver nanostructures in a polymer matrix.</a> Appl. Phys. Lett. 100, 063120 (2012).</li><li>G&#252;ney, D. &. #. 2. 1. 4. ;., Koschny, T., Soukoulis, C. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intra-connected+three-dimensionally+isotropic+bulk+negative+index+photonic+metamaterial.">Intra-connected three-dimensionally isotropic bulk negative index photonic metamaterial.</a> Opt. Express. 18, 12348-12353 (2010).</li><li>Grigorenko, A. N., 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=Nanofabricated+media+with+negative+permeability+at+visible+frequencies.">Nanofabricated media with negative permeability at visible frequencies.</a> Nat. Photon. 438, 335-338 (2005).</li><li>Grigorenko, A. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Negative+refractive+index+in+artificial+metamaterials.">Negative refractive index in artificial metamaterials.</a> Opt. Lett. 31, 2483-2485 (2006).</li><li>Shalaev, V. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Negative+index+of+refraction+in+optical+metamaterials.">Negative index of refraction in optical metamaterials.</a> Opt. Lett. 30, 3356-3358 (2005).</li><li>Ishikawa, A., Tanaka, T., Kawata, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Magnetic+excitation+of+magnetic+resonance+in+metamaterials+at+far-infrared+frequencies.">Magnetic excitation of magnetic resonance in metamaterials at far-infrared frequencies.</a> Appl. Phys. Lett. 91, 113118 (2007).</li><li>Tanaka, T., Ishikawa, A., Kawata, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two-photon-induced+reduction+of+metal+ions+for+fabricating+three-dimensional+electrically+conductive+metallic+microstructure.">Two-photon-induced reduction of metal ions for fabricating three-dimensional electrically conductive metallic microstructure.</a> Appl. Phys. Lett. 88, 081107 (2006).</li><li>Ishikawa, A., Tanaka, T., Kawata, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improvement+in+the+reduction+of+silver+ions+in+aqueous+solution+using+two-photon+sensitive+dye.">Improvement in the reduction of silver ions in aqueous solution using two-photon sensitive dye.</a> Appl. Phys. Lett. 89, 113102 (2006).</li><li>Cao, Y. -. Y., Takeyasu, N., Tanaka, T., Duan, X. -. M., Kawata, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=3D+Metallic+Nanostructure+Fabrication+by+Surfactant-Assisted+Multiphoton-Induced+Reduction.">3D Metallic Nanostructure Fabrication by Surfactant-Assisted Multiphoton-Induced Reduction.</a> Small. 5, 1144-1148 (2009).</li></ol>

No conflicts of interest declared.

The key to the process is obtaining a doped dielectric matrix that allows high resolution fabrication, but does not degrade soon after preparation. A simple mixture of PVP, AgNO3 and H2O allows the creation of high-resolution silver nanostructures that are embedded inside a support matrix. Varying the PVP to AgNO3 ratio will change the laser energy needed for fabrication, and potentially other properties such as feature resolution. A low ratio leads to faster degradation of the die...

a method to fabricate disconnected silver nanostructures in 3d

The standard nanofabrication toolkit includes techniques primarily aimed at creating 2D patterns in dielectric media. Creating metal patterns on a submicron scale requires a combination of nanofabrication tools and several material processing steps. For example, steps to create planar metal structures using ultraviolet photolithography and electron-beam lithography can include sample exposure, sample development, metal deposition, and metal liftoff. To create 3D metal structures, the sequence is repeated multiple times. The complexity and difficulty of stacking and aligning multiple layers limits practical implementations of 3D metal structuring using standard nanofabrication tools. Femtosecond-laser direct-writing has emerged as a pre-eminent technique for 3D nanofabrication.1,2 Femtosecond lasers are frequently used to create 3D patterns in polymers and glasses.3-7 However, 3D metal direct-writing remains a challenge. Here, we describe a method to fabricate silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm. The method enables the fabrication of patterns not feasible using other techniques, such as 3D arrays of disconnected silver voxels.8 Disconnected 3D metal patterns are useful for metamaterials where unit cells are not in contact with each other,9 such as coupled metal dot10,11or coupled metal rod12,13 resonators. Potential applications include negative index metamaterials, invisibility cloaks, and perfect lenses.
In femtosecond-laser direct-writing, the laser wavelength is chosen such that photons are not linearly absorbed in the target medium. When the laser pulse duration is compressed to the femtosecond time scale and the radiation is tightly focused inside the target, the extremely high intensity induces nonlinear absorption. Multiple photons are absorbed simultaneously to cause electronic transitions that lead to material modification within the focused region. Using this approach, one can form structures in the bulk of a material rather than on its surface.
Most work on 3D direct metal writing has focused on creating self-supported metal structures.14-16 The method described here yields sub-micrometer silver structures that do not need to be self-supported because they are embedded inside a matrix. A doped polymer matrix is prepared using a mixture of silver nitrate (AgNO3), polyvinylpyrrolidone (PVP) and water (H2O). Samples are then patterned by irradiation with an 11-MHz femtosecond laser producing 50-fs pulses. During irradiation, photoreduction of silver ions is induced through nonlinear absorption, creating an aggregate of silver nanoparticles in the focal region. Using this approach we create silver patterns embedded in a doped PVP matrix. Adding 3D translation of the sample extends the patterning to three dimensions.

The acousto-optic modulator and neutral density filters (Figure 1) allow one to control the amount of energy deposited into the sample. Using an exposure of 110 pulses per voxel and 3 nJ per pulse, with the stage translating at 100 &mu;m/sec, the resulting silver structures are readily visible through the in-situ optical microscope. Lower laser exposure levels (by reducing pulse energy and/or pulse number) lead to smaller silver features; we have observed features as small as 300 nm.8...

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A Method to Fabricate Disconnected Silver Nanostructures in 3D

Femtosecond-laser direct-writing is frequently used to create three-dimensional (3D) patterns in polymers and glasses. However, patterning metals in 3D remains a challenge. We describe a method for fabricating silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm.

The standard nanofabrication toolkit includes  techniques primarily aimed at creating 2D patterns in dielectric media. Creating  metal patterns on a ...

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13.7K Views.  Harvard University. Femtosecond-laser direct-writing is frequently used to create three-dimensional (3D) patterns in polymers and glasses. However, patterning metals in 3D remains a challenge. We describe a method for fabricating silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm.