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Femtosecond laser polymerization of hybrid/integrated micro-optical elements and their characterization
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2010 J. Opt. 12 124010 (http://iopscience.iop.org/2040-8986/12/12/124010) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
JOURNAL OF OPTICS
J. Opt. 12 (2010) 124010 (8pp)
doi:10.1088/2040-8978/12/12/124010
Femtosecond laser polymerization of hybrid/integrated micro-optical elements and their characterization 1 ˇ Mangirdas Malinauskas1 , Albertas Zukauskas , Vytautas Purlys1 , 1 1 Kastytis Belazaras , Andrej Momot , Domas Paipulas1 , Roaldas Gadonas1 , Algis Piskarskas1 , Holger Gilbergs2 , ¯ e3, Ioanna Sakellari3 , Maria Farsari3 Arun˙e Gaidukeviˇciut˙ and Saulius Juodkazis4 1 Laser Nanophotonics Group, Department of Quantum Electronics, Physics Faculty, Vilnius University, Saul˙etekio 9, LT-10222 Vilnius, Lithuania 2 Institute of Applied Optics, Stuttgart University, Pfaffenwaldring 9, 70569 Stuttgart, Germany 3 Foundation of Research and Technology Hellas, Institute of Electronic Structure and Laser, Iraklion 71110, Greece 4 Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, VIC, 3122, Australia
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Received 11 June 2010, accepted for publication 13 July 2010 Published 11 November 2010 Online at stacks.iop.org/JOpt/12/124010 Abstract The femtosecond laser-induced multi-photon polymerization of a zirconium–silicon based sol–gel photopolymer was employed for the fabrication of a series of micro-optical elements with single and combined optical functions: convex and Fresnel lenses, gratings, solid immersion lenses on a glass slide and on the tip of an optical fiber. The microlenses were produced as polymer caps of varying radii from 10 to 90 μm. The matching of refractive indices between the polymer and substrate was exploited for the creation of composite glass-resist structures which functioned as single lenses. Using this principle, solid immersion lenses were fabricated and their performance demonstrated. The magnification of the composite solid immersion lenses corresponded to the calculated values. The surface roughness of the lenses was below ∼30 nm, acceptable for optical applications in the visible range. In addition, the integration of micro-optical elements onto the tip of an optical fiber was demonstrated. To increase the efficiency of the 3D laser polymerization, the lenses were formed by scanning only the outer shell and polymerizing the interior by exposure to UV light. Keywords: nanofabrication, femtosecond laser polymerization, 3D nano-structuring,
micro-optics, solid immersion lenses, fiber-optics, bi-functional optical components S Online supplementary data available from stacks.iop.org/JOpt/12/124010/mmedia (Some figures in this article are in colour only in the electronic version)
fabrication of photonic [2–8], micro-optical [9–14], and microfluidic [15, 16] micro/nanodevices. As new photopolymers, specifically designed for MPP, emerge, detailed investigations on the damage threshold, resolution improvement, and new methods to increase fabrication throughput drives further efforts in the controllable and reproducible manufacturing of
1. Introduction Microfabrication by multi-photon polymerization (MPP) is a relatively new, direct laser-write technique which allows the flexible 3D structuring of photopolymers at the microand nanoscale [1]. To date, it has been employed in the 2040-8978/10/124010+08$30.00
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© 2010 IOP Publishing Ltd Printed in the UK & the USA
J. Opt. 12 (2010) 124010
M Malinauskas et al
Figure 1. (a) Setup of the direct laser writing; PM—powermeter, S—shutter, P—polarizer, M—mirror, DM—dichroic mirror, an alignment telescope made out of a pair of L1 and L2 lenses, LED—light emitting diode illumination, λ/2—half wave-plate, and CMOS—imaging camera. (b) The used fast polymerization sequence: (1) contour scanning for a shell-surface definition, (2) development, and (3) UV exposure for the final volumetric polymerization. (c) Comparison of the time required to fabricate hemispherical lenses of different radii, r , by the proposed shell exposure versus a full-volume raster scan. The step between exposure spots was varied in the range d = 25–200 nm and scan speed was kept in the range v = 50–200 μm s−1 . The curves are calculated by formulas presented in section 2.2.
direct write fabrication routine was made shorter by a factor of up to 200–400 times, depending on the optical element design, by scanning only the outer shell of the optical element and polymerizing the interior by exposure to UV light.
optical components. New designs of complex miniaturized and integrated optical, opto-fluidic devices and sensors are required. For the fabrication of high quality micro-optical elements (MOE), the photopolymer properties play an important role. Hybrid organic–inorganic materials are attractive candidates in this field since they possess good structuring qualities and small achievable feature resolution, are transparent in the visible and near-infrared range, and have refractive indices that can be tuned to match the required value [13, 17, 18]. Additionally, they can be doped with quantum dots [19], nonlinear chromophores [20] or organic dyes [21], thus further increasing their functionality. A low post-fabrication shrinkage for hybrid resists is desirable for the fabrication of microoptical devices and photonic crystals over larger areas with cross-sections above 100 μm, as shrinkage is a primary cause of deformation and detachment at the periphery of polymerized structures with larger cross-sections. This is because the strain, ε = ll , for a small lateral shrinkage length l over length l would cause a force defined by Hooke’s law F = εY and predicts a strong deformation of a material with a lower Young modulus, Y , on the polymer–substrate boundary. As the material employed here, the silicon–zirconium containing hybrid SZ2080 material, suffers less from shrinkage compared to commonly used acrylate based or other sol–gel organic– inorganic materials [18, 22–24], the microstructures fabricated using it are less prone to such failure. Also, its glassy nature makes it compatible with glass substrates for micro-fluidic applications where hydrophilic surfaces are required. Here, we report on the fabrication of MOEs by direct femtosecond (fs) laser writing. A solid immersion lens (SIL) for resolution improvement and aberration-reduced imaging, and micro-optical elements on the tip of optical fibers, as well as bi-functional optical components were fabricated. The
2. Samples and methods 2.1. Materials The hybrid organic–inorganic photopolymer SZ2080 [25] (FORTH, Greece) was used for the fabrication of the MOEs. The samples were prepared by drop-casting SZ2080 onto glass substrates; the resultant droplets were dried on a hotplate at 100 ◦ C for 1 h before the photopolymerization. 4-methyl-2pentanone (isobutyl methyl ketone) mixed with isopropanol in equal quantities was used as a developer. All chemicals were obtained from Sigma-Aldrich Co. and used directly with no further purification. SZ2080 is known to have low shrinkage and sustain high structural rigidity; in addition, its refractive index almost matches that of glass in the visible spectral range (n SZ2080 = 1.504, n Glass = 1.52) [18], making it suitable for the fabrication of micro-optical and integrated structures. The experimental setup employed is shown in figure 1(a). Three different femtosecond lasers were used as light sources for exposure: (1) a frequency-doubled fs-laser amplified system (Pharos, Light Conversion, Lithuania) operating at 515 nm with pulse length approximately 300 fs and tunable repetition rate set at 200 kHz; (2) a fs-laser oscillator (Tsunami, Spectra Physics, USA) operating at 800 nm and producing 80 fs pulses; (3) a fs-laser oscillator (Fusion, Femtolasers, Austria) operating at 800 nm and generating 20 fs pulses. 100× objectives NA = 1.25 (A-Plan, Zeiss), NA = 1.4 (SplanApo, Olympus) and NA = 1.4 (Plan Apochromat, Zeiss) were used in the three MPP setups, respectively. 2
J. Opt. 12 (2010) 124010
M Malinauskas et al
The positioning systems consisted of linear motion stages ( XY directions ALS130-100 and for Z translation ALS13050, Aerotech); three stacked micrometer step-motor stages (Standa) and a piezo nanopositioning stage (Nanocube, Physik Instrumente) mounted on them; and a x – y galvanometric mirror digital scanner (Scanlabs Hurryscan II), respectively. The laser exposure was controlled using a mechanical shutter and laser power attenuation was realized using a λ/2 half wave-plate combined with a polarizer. Wide-field transmission imaging was used to monitor the processing in real time. This is possible as an optical contrast arises from the different densities and hence indices of refraction upon polymerization. For this purpose, a microscope was built by assembling its main components: a LED red light, a CMOS camera (mvBlueFOX-M102G, Matrix Vision), a video monitor, and beam delivery optics. The ability to image the sample while performing structuring is an important feature for a successful fabrication process.
Figure 2. Optical characterization of micro-optical element. The same setup was used to characterize MOE polymerized on the tip of an optical fiber.
design, the polymerized regions can be re-exposed, since the polymerized material is still highly transparent to IR light. An 100 nm spatial overlap of neighboring pulses corresponds to a scanning speed of 100 μm s−1 . The spherical aberration due to refractive index mismatch between the immersion oil (n = 1.515) and the glass substrate is negligible and spherical aberration-free structuring conditions exist for focusing depths up to ∼30 μm [31].
2.2. Femtosecond formation of 3D micro-optics In order to speed up the fabrication process, only the outer shells of the MOEs were made using MPP, with their interiors being polymerized using a post-fabrication UV treatment. The diameter d and height l of single photopolymerized volume pixels (voxels) was controlled by modifying the laser power, scanning speed, and focusing optics [26, 27]. This allowed the fabrication of enclosure shells of sufficient mechanical strength and the formation of unexposed regions inside the optical elements for further processing steps (figure 1(b)). The procedure of shell fabrication, development, and posttreatment with UV light enabled a reduction in the fabrication time by approximately ∼200–400 times. This approach has been already demonstrated in producing sample 3D microstructures of complex geometry and components for micro-fluidics [28, 29]. The time required to fabricate a compact, hemispherical lens of radius, R , by volumetric raster scanning with a pitch, d , (the same pitch in the lateral and axial directions is considered) at a scan speed, v , is: tv = dV2 v , where the volume of hemisphere V = 12 × 4π R 3 ; the pitch is at least two times smaller than the actual fabrication cross section of the voxel. On the contrary, for shell fabrication only, S with the surface of hemisphere the required time is ts = dv 1 4 2 S = 2 × 3 π R . Hence, the efficiency increase factor is f c ≡ ttvs = 3 Rd . As the radius of a lens increases, or the voxel size decreases (hence, the pitch, d , too), the fabrication time becomes impractical for volumetric raster fabrication. In the case of the MOEs discussed in this study, an improvement of approximately (1–3) × 102 times is achieved using the shellfabrication approach (figure 1(c)). The actual exposure time required for the fabrication of a R = 50 μm radius lens of focal length ∼100 μm at volume filling by a raster scan with 200 nm overlap between voxels is 14.7 h; this reduces to approximately 2.6 min by employing the proposed shell exposure (figure 1(b)). The time requirements are similar if a Fresnel lens design is adopted. It is of utmost importance to anchor the fabricated microstructures onto the substrate to withstand the development of the unsolidified resin [30]. When required by
2.3. Optical and surface inspection The focal lengths of the fabricated lenses were varied from several tens to hundreds of micrometers. For measuring such small focal distances a microscope setup mounted on a micrometer step-motor with step size of 2.5 μm (Standa) was used, as shown in figure 2. As the stage moved, the focal plane of the microscope objective crossed through the focus of the fabricated microlenses, which was illuminated by collimated light from a LED. An image of the light distribution was captured using a CCD camera. The images were analyzed by fitting a Gaussian function to the acquired light distribution and the focal length of the lens was calculated. The same approach was used for the fiber-integrated microlenses (figure 2). Spherical lenses with different radii of curvature and, therefore, different focal lengths were fabricated. The glass surface was considered as the reference for the focal distance measurement using the experiment described above and using the thickness of the lens as an offset (fabrication parameter). For validation, spherical lenses were fabricated and their focal length, which should be twice the radius of curvature, was measured. Optical profilometry (PLμ2300, Sensofar) and atomic force microscopy (AFM Dimension 3100, Digital Instruments) were used to characterize the geometry and roughness of the surface.
3. Results and discussion 3.1. Single optical elements: aspherical, Fresnel, and solid immersion lenses Faster processing makes cost effective the fabrication of multiple optical elements and arrays such as those used in micro-imaging and in light collection for solar cells. As recently demonstrated [32], shorter wavelengths of fs-laser irradiation increase the polymerization rate, hence, a smaller overlap between adjacent pulses and a smaller laser power 3
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Figure 3. Control over surface definition: (a) an optical image of polymerized aspherical lenses; see text for details. (b) Image of the laser ablated logo pattern taken by the lenses shown in (a). The inset shows the ablated logo of the Vilnius University Laser Research Center; scale bar 500 μm. (c) SEM images of test samples: a half-Fresnel lens and structure for a surface roughness determination on a cover glass formed by the fast polymerization sequence (figure 1).
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