Unusual 3D lithography approaches for fabrication of polymeric photonic microstructures

Unusual 3D lithography approaches for fabrication of polymeric photonic microstructures

Sara Coppola, Veronica Vespini, Francesco Merola, Biagio Mandracchia, Simonetta Grilli,

Pietro Ferraro

[email protected]

Istituto Nazionale di Ottica del CNR (CNR-INO), Via Campi Flegrei, 34, 80078 Pozzuoli (Napoli), Italy

Abstract

Novel and intriguing lithographic approaches based on instabilities of liquid polymers and electro-hydro-dynamic at nanoscale have been developed. The unusual fabrication methods were aimed at fabricating 3D polymeric microstructures. A variety of microstructures were fabricated and tested for applications in different fields

1. LIQUID NANO DISPENSER

A new opto-nanofluidic approach named Pyro-EHD is presented for dispensing liquid nano-pico-droplets through pyroelectric effect activated by hot tip source or an IR laser into a dielectric crystal using a non-invasive simple and powerful electrode-less configuration. The manipulation of small amounts of liquids at micro to nanometer scale is of great interest in many fields of technology: biotechnology, patterning by deposition of inorganic, organic and biological inks and photnics [1-4]. We show a new and simple system where the liquid actuation and dispensing has been achieved through electrode-less configurations using polar dielectric crystals such as Lithium Niobate (LN) crystal and by exploiting the pyroelectric effect [5,6]. The technique presented allows one to avoid the use of high-voltage power supplies and electrical circuits, and moreover there is no need to design and fabricate nanocapillary nozzles. The functionalization of the lithium niobate (LN) is obtained by micro-engineering the ferroelectric domains and by inducing the pyroelectric effect through the use of appropriate heat sources such as a IR laser beam [7]. The set-up consists basically of a polished 500-µm thick z-cut LN substrate (from Crystal Technology, Inc.) placed over a microscope glass slide at a specific distance fixed by appropriate spacers (Figure 1). A liquid drop or film is first deposited on the glass slide and successively the upper surface of the LN wafer is placed in contact with an heated-tip that can be scanned in order to induce point-wise thermal stimuli. The heated-tip is in axis with the droplet reservoir on the microscope glass slide. A conventional heated soldering tip was used as heated-tip-source. LN reacts to the thermal-stimuli by building-up an electric potential across the z-cut LN crystal’s surfaces because of the pyroelectric effect, that consists in the spontaneous polarization change ∆Ps following to a temperature gradient ∆T. At equilibrium, the crystal Ps is fully screened by the external screening charge and no electric field exists [8]. When the heating source locally heats the crystal, a sudden surface charge density σ immediately appears given by neglecting losses, where Pc is the material-specific pyroelectric coefficient (Pc= -8.3 x 10 -5C/°C/m2 for LN @ 25°C). The electric field exerts an attractive force on the liquid . When the liquid starts to deform under the action of the electric field, two evolutions are possible. Case (I): if the liquid volume and the separation distance D between the two

Invited Paper

Optical Components and Materials XI, edited by Michel J. F. Digonnet, Shibin Jiang,Proc. of SPIE Vol. 8982, 89820N · © 2014 SPIE · CCC code: 0277-786X/14/$18

doi: 10.1117/12.2036628

Proc. of SPIE Vol. 8982 89820N-1

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 04/05/2014 Terms of Use: http://spiedl.org/terms

Substrate

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plates are appropriate then a stable liquid bridge can be formed. For a given volume, the critical distance below which a bridge can be established is expressed by [9].

3/1)4/1( VDc ϑ+= (1)

where θ is the contact angle and V is the volume. If the separation D is above the critical value, a stable liquid bridge cannot be established between the plates. In the case of a sufficiently strong electric field, thin liquid jets can be released from conical tip structures (case II) (similar to Taylor’s cone usual in electro-spray). We designed our experiments with the aim at using such instability for dosing and dispensing liquid drops. A photoresist coated cover glass was inserted in between and mounted onto a computer controlled x-y translation stage. A hot tip was locally in contact with the LN crystal and induced a point-wise thermal-stimulus to dispense separate droplets or lines in case of shorter distance between the reservoir and the resist coated substrate.

Figure 1: Arrangement for liquid dispensing onto a dielectric substrate. A photoresist spin coated cover glass is inserted in between the LN upper substrate and the glass plate in order to avoid the droplet spreading and to dispense droplets on different substrates.

The control of translation direction and speed of the substrate permits to obtain droplets aligned along straight or curved lines and distributed with different distances and/or periods, respectively. Moreover the volume variation of the drop reservoir allows one to dose the printed droplets with different volumes and sizes (Figure 2). It is important to point out that the technique is able to print droplets with much reduced dimensions by decreasing the volume of the drop reservoir (for example after a certain amount of shots).



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Figure 2: a. Simple liquid patterns consisting of continuous straight line (width around 40 µm), periodic separate droplets with different sizes (diameters around 40 µm and 25 µm) and periods and continuous lines with curved directions. b. Continuous (up) and dotted (bottom) greek fret. c. Continuous and dotted patterned Greek fret.

1.1 Dispenser activation by plasmon resonance of gold nanorods

The results reported open a new way for compact, cost-effective, and integrated systems and can be exploited for photonics applications. We propose an additional laser-assisted EHD (LA-EHD) technique [10] based on the successful combination of a near infrared (NIR) source with the plasmon resonance of gold nanorods (GNRs) patterned onto the surface of a LN crystal. This system exhibits additional advantages compared to the electrode and nozzle-free pyro-EHD configuration [7].The resonance was achieved by NIR irradiation, thus enabling the direct activation of the pyroelectric effect into a LN crystal through a compact, versatile, and light source. In fact, thanks to the availability on one side of a laser exiting an optical fiber and, on the other side, of activation sites of the pyroelectric effect patterned directly onto the driving crystal, the dispensing is highly versatile and resolved. Figure 3 shows the side view of typical multiple dispensing events. Five almond oil sessile droplets (about 1 µl each) are positioned on the lower glass slide in correspondence to the GNR spots on the driving plate. Laser radiation lighted an area of about 1.5 cm diameter with a 5.6W power, so that the spots were illuminated simultaneously from above.



3

Figure 3. Side view of a multi dispenser of five almond oil droplets (about 1 µl each). These drops are positioned on the lower glass slide right below the GNR films placed onto LN wafer.

1.2 Ink-jet printing of active microlens array

Furthermore the different way of applications of this method, based on a pyro-electrohydrodynamic mechanism, open the way to the possibility of dispensing viscous liquids and drawing polymer microlenses directly from the reservoir [11]. The reliability of the microlenses and the tunability of their focal properties were demonstrated through profilometric and inteferometric analyses. In particular interferometric methods allow accurate measurement of refractive index and focal length [12,13]. The main advantage of this approach consists in avoiding the nozzle because the liquid is drawn directly from the liquid reservoir (drops or layers). Being nozzlefree, it can be applied also for high viscosity liquids, greatly extending the fabrication capabilities of the conventional inkjet printing processes. In Figure 4 3D plots of the optical microstructures obtained by microprofilometer analysis (Tencor P10, vertical resolution 10 Å) with PMMA inks at different NMP: TOL mixing ratios (10∶0, 7∶3) are reported.

Figure 4. (Color online) 3D image of the microlens obtained printing PMMA dissolved in (a) pure NMP; (b) NMP:TOL 7∶3.

Experimenting the fabrication of optical microlenses we also introduce new active micro-optical elements made by a mixture of rod-shaped inorganic NanoCrystals (NCs) dispersed into poly-dimethylsiloxane (PDMS). In fact, the experimental perspective to disperse emitting colloidal NCs into polymers has allowed to further engineer hybrid organic-inorganic materials introducing innovative functionalities as for instance photoluminescence conversion capabilities. This has proved of great interest for novel applications such as the fabrication of photonic crystals [14] and, notably, of innovative solar cells showing enhanced efficiency [15-17]. In fact, in the last few years, the interest in polymer-based microlenses and devices has increased specially motivated by the possibility of embedding active media like colloidal inorganic nanocrystals (NCs) into polymers thus transforming originally passive micro-optical elements into active



components. For this reason the pyro nozzle-less approach is applied to the fabrication and optical characterization of nanorod-incorporating and light-converting micro-lens arrays of different shapes and heights as visible in Figure 5.

Figure 5. (a) Fluorescence image of the NCs emission in a microlens array and (b) microlens profile

1.3 Controlling fragmentation and assembling of nematic liquid crystal

Also nematic liquid crystal (LC) droplets over a PPLN crystals covered with PDMS polymer are subject to the pyroelectric field. In fact, the strong electric fields pyroelectrically generated allow to manipulate liquid crystal in 2D on a substrate or even in 3D for dispensing liquids from one liquid reservoir to a dispensable substrate. Various applications exist by taking advantage of this effect as the spatial modulation of the wettability (i.e., wettability patterning) and realization of arrays of tunable liquid lenses. The method is non invasive and because LC used in the experiment is a polar molecule, it undergoes a force due to the electric fields generated by the surface charges that is able to move the LC droplets. As a consequence of the pyro-electric field the drops of LC are first uniformly fragmented in smaller droplets by heating the sample while after a few minutes, fragmented droplets coalesce to form bigger droplets at fixed locations. These drops behave as microlenses with focal lengths of tens or hundreds of microns Figure 6. The whole sample could be viewed as a dynamical optical microelement able to switch from a diffuser state (fragmentation state) to a microlens array (coalescence state) without the need for an external voltage [16].

Figure 6. Several minutes after cooling, the droplets coalesce inside hexagonal domains of sample B and behave as LC microlenses. The inset shows an image taken in the focal plane of the lens.

2. POLYMER PATTERNING

We extend the use of the pyro-dispenser also to high viscous polymer, in particular we defined an innovative electrospinning (ES) approach as a powerful method for realizing well-ordered patterns in processing and engineering functionalized polymers. We simplify the ES systems through a novel concept and approach that stands out from all the existing electrospinning apparatus, because it operates in a nozzle-less modality. Consequently it is much more compact, flexible, simpler and could led to design the architecture of polymer patterns, controlling morphological and topographical characteristics with good resolution. The pyroelectric effect activated onto the LN crystal exerts an attractive force on the polymer drop deforming it into a Taylor’s cone thus generating liquid jet emission. The fibers are directly produced and



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stretched from the elongation of the Taylor cone and deposited onto the target substrate of interest. No syringes are required for the formation of the spinning cone making easier also the multi jetting [19,20] applications, even in case of simultaneous processing of different materials. The position of the receiving substrate used as target could be fine controlled in space with computer-controlled x,y axes translation stage making possible the fabrication of well-ordered patterns Figure 7 and embroidery printing with good resolution, as visible in Figure 8.

Figure 7 : (a ) ordered square grid with period of 55 µm and (b) fluorescence image

The characteristic of this fibers lead us to experiment their possible exploitations in biotechnology and photonics. In particular, we show experiments of direct writing of the poly-co-glycolic acid (PLGA) ink using the pyroelectrodynamic approach. The PLGA is a biopolymer widely used in tissue engineering and regenerative medicine. DMC (Dimethyl Carbonate) is used as solvent and a fluorochrome Nile Red is introduced for modeling active organic materials for photonics waveguides demonstration. With this procedure we also introduce optically active scaffolds where the fibers composing the scaffold, besides controlling the cells growth and fate, could deliver optical stimuli to the cells and could be utilized for monitoring the cells growth in real time. The nanoengineered functional substrates could represent a new tools in nanobiophotonics for the fabrication of active and smart materials opening the way to innovative optogenesys studies.

Figure 8: typical embroidery patterns (a) white lamp and (b) fluorescence image of micro-beads

3. UNUSUAL 3D LITHOGRAPHY

Working on polymeric materials we introduce an innovative approach that exploits the pyro-electrohydrodynamic effect for controlling liquid instabilities and self-assembling of polymeric liquids for fabricating single or arrays of complex 3D microstructures. In fact, the pyro-electric effect activated onto a ferroelectric crystal generates an electric filed so high that could be used not only for the dispensing and ink-jet printing of liquids, as already described, but also for the manipulation

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of high viscous materials. Liquid instabilities in polymers are first driven and then quickly cured to obtain permanent 3D microstructures, by the same thermal treatment, opening the way to a previously underscribed paradigm in fabrication of 3D polymeric microstructures. This new and unique technique is named "pyro-electrohydrodynamic (PEHD) 3D-lithography", meaning the generation of structures by using forces produced by electric fields generated by the pyroelectric effect. The fabrication of polymer wires, needles, pillars, cones, or microspheres is reported, and practical proofs of their use in photonics are presented. In particular, we report a rapid way to freeze nanoliter liquid instabilities producing micro-elements that could find application in different fields of technology from nano to bio photnincs. The set-up used for this experiments is the same described for the dispensing application but the drop reservoir is in this case a high viscous polymer, polydimethylsiloxane (PDMS). The pyro-electric field is able to exert a force on the liquid PDMS creating a bridge across the two substrates, as shown in Figure 9. The fluid dynamics causes liquid depletion from the unstable bridge and thus the formation of various temporary liquid silhouettes. The dynamic evolutions were observed by a high-speed (CMOS) camera imaging system controlling in real time the polymeric dynamic evolution. The final stage of such unstable liquid column is the collapsing point but we are able to freeze the microstructures by a rapid-curing approach in a well defined moment in time, designing geometries and shapes. An external thermal source is used for stimulating the generation of the structures and for their rapid curing [21].

Figure 9: Experimental procedure. (1-6) Temporal evolution of an unstable liquid bridge during the PEHD process, leading to the formation of polymeric pillar (3), cone (4), and microspheres on a wire (5-6).

Typical 3D microstructures we can realize are soft solid-like bridges with different aspect ratios, solid-like wired structures connected by conical terminations, conical structures with and without needle tip and beads-on-a-string (BOAS) structures (Figure 10).



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Figure 10: (a) Beads-on-a-string (BOAS) micro-structures and polymeric microresonator.

3.1 Active micro-resonators

Many of these structures find application in different fields. For example, the wires are potential optical waveguides similar to optical fibers and could be used for collecting or distributing light signals in lab-on-a-chip optofluidic devices. In order to better functionalize this optical micro-components we present also the possibility of fabricating active optics elements embedding active molecule inside the polymer used for the experiments. In Figure 11 is reported an example of active microresonator embedding CdSe QD nanocrystals (Lumidots CdSe 590 nm) in PDMS.

Figure 11: BOAS as active microresonators under UV and white light illumination

Various QD-PDMS microstructures have been fabricated successfully: solid-like bridges with different aspect ratios, solid-like wired structures connected by conical terminations, conical structures with and without needle tip and beads-on-a-string (BOAS) structures. The fabrication process is easier to accomplish compared to other methods where the QDs are embedded into the periphery of polymer microspheres through complicated chemical procedures and enables the inclusion of the QDs into deeper regions of the structures.

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3.2 Stretchable micro-sensors

An additional application regarding micro resonators is that of tuning the optical whispering gallery modes in a PDMS microsphere resonator by more than a THz. The PDMS microsphere realized through 3D lithography is subjected to mechanical stretching resulting in tuning of the whispering gallery modes generated inside the cavity by one free spectral range, Figure 12. The investigations demonstrate that the whispering gallery mode shift has a higher sensitivity (0.13 nm/µN) to an applied force when the resonator is in its maximally stretched state compared to its relaxed state. Moreover potential applications exist in optical and biological sensing if the microspheres are appropriately functionalized [22].

Figure 12: (a) WGM shift for increased stretching and shift versus motor position ( the total shift is ~15 nm). (b) Fine tuning of the WGM shift using the piezo actuator.

3.3 Biodegradable active microaxicon

In terms of micro-engineering the optical properties of this micro structures additional experiments show the possibility of combining the use of a biopolymer and the 3D lithography approach. We define a smart way of fabrication of biodegradable active microaxicon (Figure 13), this optical microelements could be inserted in lab-on-chip devices [23]. In fact, axicon lenses are of great interest because of their particular optical properties: they change an incident Gaussian beam into a Bessel one. It is well known that Bessel beams exhibit little or no diffraction over a limited distance and a transverse ring-like shape intensity pattern that can be exploited in various applications. For example, Bessel beams are extremely useful for optical tweezing of cells or particles, as a narrow Bessel beam will maintain its required property of strong focus over a relatively long distance.



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Figure 13: (a) Fluorescence image of a PLGA microaxicon

An axicon is able to produce Bessel beams having high depth of focus when compared to focused Gaussian beams obtained by high numerical aperture microscope objective. This is a clear advantage for particles trapping by the light forces compared to the use of microscope objectives. In fact, an axicon is very useful as optical tweezers for trapping and sorting microparticles or biological cells, Figure 14.Overall they could be used for trapping light and delivery optical signals from and to cells in biophotonics applications [24].

Figure 14: (a) Drawing of the trapping system, (b) The red circle represents the axicon’s base, and is due to the white bulb illuminating the system. The trapped particle confined in the x–y plane by the axicon, but free to move along the axicon’s line of focus (i.e. for more than 200 μm along the z direction)

4. CONCLUSION AND PERSPECTIVE

In conclusion, we have described an unusual and smart way for the manipulation of liquid and polymeric materials. In 1D,2D and 3D. Different approaches related to the application of the pyro-electric effect activated onto the Lithium Niobate crystal are used for the fabrication of polymeric photonic microstructures. Nanoliquid instabilities could be “fluidynamically” designed a priori with the aim of fabricating even complex shapes exploitable in many fields. In summary, the fabrication of optically active elements, such as nanodroplets, microlenses and microstructures is described with different fields of application in photonics. Overall we think about the possibility of using this structures for biophotonics applications in fact, besides controlling the cells growth and fate, this active microelements could deliver optical stimuli to the cells and could be utilized for monitoring the cells growth in real time. In conclusion the



nanoengineered functional elements could represent a new tools in nanobiophotonics for the fabrication of active and smart materials opening the way to innovative optogenesys studies.

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Unusual 3D lithography approaches for fabrication of polymeric photonic microstructures

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