Research Article www.acsami.org
Tunable Diffractive Optical Elements Based on Shape-Memory Polymers Fabricated via Hot Embossing Senta Schauer,† Tobias Meier,†,‡ Maximilian Reinhard,† Michael Röhrig,† Marc Schneider,† Markus Heilig,† Alexander Kolew,† Matthias Worgull,† and Hendrik Hölscher*,† †
Institute of Microstructure Technology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany S Supporting Information *
ABSTRACT: We introduce actively tunable diffractive optical elements fabricated from shape-memory polymers (SMPs). By utilizing the shape-memory effect of the polymer, at least one crucial attribute of the diffractive optical element (DOE) is tunable and adjustable subsequent to the completed fabrication process. A thermoplastic, transparent, thermoresponsive polyurethane SMP was structured with diverse diffractive microstructures via hot embossing. The tunability was enabled by programming a second, temporary shape into the diffractive optical element by mechanical deformation, either by stretching or a second embossing cycle at low temperatures. Upon exposure to the stimulus heat, the structures change continuously and controllable in a predefined way. We establish the novel concept of shape-memory diffractive optical elements by illustrating their capabilities, with regard to tunability, by displaying the morphing diffractive pattern of a height tunable and a period tunable structure, respectively. A sample where an arbitrary structure is transformed to a second, disparate one is illustrated as well. To prove the applicability of our tunable shape-memory diffractive optical elements, we verified their long-term stability and demonstrated the precise adjustability with a detailed analysis of the recovery dynamics, in terms of temperature dependence and spatially resolved, time-dependent recovery. KEYWORDS: shape-memory polymer, diffractive optical elements, tunable microoptics, spatially resolved recovery, recovery dynamics, hot embossing DOE at a fixed length.9−13 In contrast, DOEs made from shapememory polymers (SMPs) can resolve this issue without additional components, because defined shape changes can be programmed and recalled to actively control the diffractive structure of the devices.14−17 SMPs are a class of polymers that can change their shape in a predefined way when they are triggered by an external stimulus such as heat or light.18−23 In this work, we used a medical-grade cycloaliphatic thermoplastic polyether urethane (Tecoflex EG72D, Lubrizol, USA), which exhibits a thermally activated oneway shape-memory effect. It is completely amorphous and therefore clear, and, because of the thermoplastic properties, it is processable with common polymer processing techniques. The special molecular structure enables this polymer to remember its original, permanent shape while it is stable at a second, temporarily defined shape until it is exposed to a proper stimulus, whereupon it morphs back into its original shape. Compared with shape-memory alloys, which allow a displacement in the range of, at maximum, 8% of the material dimensions,24 SMPs have a significantly higher shape-changing capability. Acrylate-based SMPs with chemical crosslinks that can show shape-changing capabilities of >800%25 (1000% with
1. INTRODUCTION Since their invention in the 18th century, diffraction gratings have gained growing importance for various applications. With the development of lithographic fabrication techniques and their adaption to optics, the field of diffractive micro-optics evolved.1 Nowadays, diffractive optical elements (DOEs) are versatile components with a huge variety of optical applications, ranging from holograms2 to diffractive mirrors for lasers3 and optical tweezers.4 DOEs are typically fabricated using subsequent lithographic steps, because they exhibit periodic or quantized phase profiles in contrast to the continuous phase profiles of refractive elements.1 Replication of larger amounts of samples is possible with several techniques,5−7 including hot embossing.8 The spatial period and structure height are the significant characteristics of diffractive micro-optical elements and typically in the range of a few micrometers. DOEs can be manufactured by several different techniques, from a variety of different materials with nanometer precision.5−8 In practically all cases, however, their geometrical shape is fixed at the end of the fabrication process. Every required structure change demands the time- and cost-consuming fabrication of new structures or molds. Thus, tuning is only possible with successive steps. In terms of tuning grating periods, this restriction can be overcome by elastic, rubberlike materials but a constant external force is needed to keep the stretched (or contracted) © 2016 American Chemical Society
Received: January 18, 2016 Accepted: March 21, 2016 Published: March 21, 2016 9423
DOI: 10.1021/acsami.6b00679 ACS Appl. Mater. Interfaces 2016, 8, 9423−9430
Research Article
ACS Applied Materials & Interfaces
Figure 1. Demonstration of the shape-memory effect by tuning the focal point of a convex lens. A flat transparent semicircle of a shape-memory polymer (SMP) foil is stretched until its shape is almost rectangular. Consequently, two laser beams coming from the left pass through the polymer foil almost straight, without refraction. Heating the sample recovers the convex, semicircle shape of the lens. Therefore, the two laser beams are refracted at the left air/polymer interface and cross in the focal point. The paths of the laser beams are emphasized by white dashed lines.
shape-memory natural rubbers) were demonstrated26 and polyethylene blend materials even feature shape-changing capabilities of >1500%.27 In contrast to the previously mentioned SMPs, thermoplastic polyurethane-based SMPs were reported with comparable low shape-changing capabilities of ∼400%;24,28 however, they exhibit the exceptional optical property of high transparency in the visible regime and, therefore, are perfectly suited for optical applications. We demonstrate diffractive optical elements with at least one tunable crucial attribute such as structure period or height and show the unique possibilities given by SMPs to morph between two arbitrary, determinable diffractive structures. Furthermore, we present a detailed analysis of the applicability of a SMP as material for tunable DOEs. Therefore, we characterize the material’s optical properties and its shape-memory ability. To be applicable for practical use, it is important that the optical structures are stable over long time periods. Hence, we analyze the long-term stability of a stretched grating in its permanent, temporary, and an intermediate shape. In addition, we measure the recovery velocity of a grating for different temperatures on the nanometer scale by atomic force microscopy (AFM) and compare the data to a Maxwell−Wiechert model. Moreover, we examine the spatial recovery of a grating using local Joule heating. This allows us to fabricate phase gratings with highly controllable and adjustable diffractive characteristics by tuning their periodicity or structure height on demand.
linked together by completely relaxed switching segments. During the deformation of a SMP at T > Thard, even the bonding forces between hard segments release and one can define a new permanent shape. The programming of the temporary shape must occur at temperatures below Thard, so that the hard segments are unaffected by strain and only the softened switching segments deform. For example, an elongation of a piece of SMP thereby leads to extended, tensioned switching segments. The chains uncoil and rise into a higher energy conformation state to fulfill the forced shape change. Cooling the polymer in this shape increases the intermolecular forces, which then stabilize the momentary shape. By subsequent heating of the polymer to the switching temperature (Tswitch), the mobility of the polymer chains increases until the urge to move back to their lowest energy state exceeds the restrictive interaction forces. The stored mechanical energy is released and all chain segments aspire to their relaxed conformation equivalent to the initial conformation until all segments feature the same length as in the beginning. This results in the same molecular arrangement as in the original SMP sample and, thus, the same macroscopic shape. An illustrative example of a SMP employed for tunable optics is demonstrated in Figure 1. For this example, we produced a convex lens from a SMP and utilized the shape-memory effect of the material to tune its focal length. The lens is a half circle with a diameter of ∼5 cm and a height of 1 mm. The convex curvature diminished, because of stretching the sample in a tensile testing machine at room temperature, yielding in this lengthened, almost-rectangular shape (Figure 1, left). Thus, two parallel mounted laser pointer beams passed the sample without observable refraction. To trigger the recovery of the original convex lens shape, we heated it on a hot plate to Tswitch = 70 °C. When the recovery of the lens’ curvature advances, its refractive properties intensify. Therefore, the two laser pointer beams refract increasingly until finally they intersect at the focal point ∼4 cm to the right of the lens (Figure 1, right). The concept presented in Figure 1 can also be transferred to the nanometer and micrometer scale, which recommends transparent SMPs as a convenient material for tunable micro-optical elements. To fabricate micro-optical elements from SMPs, we used the hot embossing technique 30,31 also known as thermal nanoimprint32,33which is suitable for the high-quality fabrication of nanostructures and microstructures on large areas. With hot embossing, the structure of a metallic master mold can be transferred in high quantities into the surface of a large variety of materials.31,34−36 For the fabrication of the microstructured devices, a SMP foil was placed between a metallic mold possessing the negative of the desired structure and a flat substrate plate. The embossing chamber was evacuated and, subsequently, the temperatures of the mold and the substrate plate were elevated above the polymer’s melting temperature, whereupon a defined molding force was applied and the polymer filled the cavities of the mold. To stabilize the structure, the sample, substrate, and mold were cooled to room temperature under constant force before the sample was removed from the mold. To define the permanent shape of a thermoplastic SMP, the embossing temperature must be T > Thard; to program the temporary shape, embossing at T ≪ Thard is recommended. To emboss Tecoflex EG-72D, the temperature to redefine the permanent shape was chosen as T = 155 °C and a low
2. MATERIALS AND METHODS The SMP that we used for our devices is the completely amorphous thermoplastic cycloaliphatic polyether urethane block copolymer Tecoflex EG-72D. Its shape-memory effect is thermally triggered and caused by the formation of two types of segments that are generated by the separately arranging blocks of the polymer chains.21 Because of the different strength of intermolecular interactions within those segments, each one exhibits different phase-transition temperatures, such as glass-transition and melting temperatures. The segments with the higher transition temperature Thard are termed hard segments, whereas the segments with the lower transition temperature Tswitch are termed switching segments. To activate the shape-memory ability, the polymer must be treated with a certain programming cycle.21 For Tecoflex EG-72D, the hard segments consist of blocks of 1-isocyanato-4-[(4-isocyanatocyclohexyl)methyl]cyclohexane (H12MDI) together with 1,4-butanediol (1,4-BD). H12MDI plus poly(tetramethylene ether) glycol (PTMEG) blocks build the switching segments. The upper and lower transition temperatures were experimentally identified by differential scanning calorimetry (DSC) and dynamic-mechanical analysis up to Thard ≈ 150 °C29 and Tswitch ≈ 40−70 °C (see Figure S1 in the Supporting Information). The molecular mechanism of the shape-memory effect of this polymer relies on the differentiate response to temperature of the two segments. In the original, permanent shape, the hard segments are 9424
DOI: 10.1021/acsami.6b00679 ACS Appl. Mater. Interfaces 2016, 8, 9423−9430
Research Article
ACS Applied Materials & Interfaces
Figure 2. Schematic of the two treatment cycles that tune height and periodicity of micro-optical structures by flattening and stretching, respectively. In both cases, a thermoplastic SMP sheet is placed between a smooth substrate plate and a structured mold insert (middle). At a temperature above Thard and under high pressure, the melted polymer is pressed into the cavities of the mold insert to define the permanent shape. After cooling to room temperature, the embossed structure can be released. Flattening the structured surface by hot embossing is shown on the left. In this case, the permanent shape is flattened with a smooth stamp at Tswitch < T < Thard. This temporary shape is stable as long as the sample’s temperature is kept well below the switching temperature Tswitch. However, subsequent heating of the sample to Tswitch recovers the permanent shape. Another option is the stretching of the foil at Tswitch < T < Thard, as shown on the right. This procedure also deforms the sample from its permanent shape to a temporary shape. Subsequent heating of the temporarily shaped polymer sheet to Tswitch releases the forces that fixed this shape and the sheet recovers to the original, permanent shape. In both cases, the samples can now be temporarily deformed again or permanently restructured. embossing pressure was applied; to program the temporary shape, the temperature was set to T = 60 °C at comparatively high embossing pressure, because of the higher viscosity of the polymer at low temperatures. Our partly self-made hot embossing machines feature large temperature ranges and precisely controllable positioning of tool and substrate plate, as well as accurate adjustability of the embossing pressure and velocity during the embossing process. These characteristics allow one to emboss the permanent and temporary shapes of the SMP with the very same machine. A schematic of two fabrication cycles to produce DOEs with tunable structural features is given in Figure 2. The first step of both cycles involves the hot embossing of a diffractive microstructure as permanent shape. To modify its properties for the temporary shape, the SMP sample can either be flattened to obtain a smooth, unstructured surface, or it can be stretched to increase its period. By increasing the sample’s temperature above Tswitch later, the recovery of the original, permanent shape is initialized. The temporarily flattened structure recovers its former height and thus regains its diffractive properties and the stretched grating’s increased period reduces continuously to the initial value, respectively.
3. RESULTS AND DISCUSSION The shape change of DOEs can be nicely illustrated by the observation of their far-field diffraction. To demonstrate the tunable optical properties of our DOEs, we aligned the beam of a laser pointer with a DOE and observed the resulting far-field diffraction pattern on a screen. To trigger the transformation from the temporary shape to the permanent shape, the structured SMP sheet is heated with a hot air gun to T > Tswitch. A schematic is shown in Figure 3A. Figure 3B exemplifies the recovery of the full structure height of a temporarily flattened SMP DOE. The sample was produced as described by the left fabrication cycle in Figure 2. The upper set of pictures shows the SMP device structured with various fields of different diffractive structures.8 The pictures were taken at four different states of recovery from flat (left) to fully recovered (right). The second row shows the corresponding far-field diffraction patterns gained by pointing a
Figure 3. (A) Schematic of the setup used to visualize the recovery of micro-optical diffractive elements made from a SMP. A laser beam is aligned with the DOE and the resulting far-field diffraction pattern is observed on a screen while the DOE is heated with a hot air gun. (B) Recovery of the full structure height of a temporarily flattened diffractive microstructure. By heating the sample to Tswitch, the diffraction pattern on the screen changes from a blurry spot to the clearly visible European Union flag. (C) Morphing between two different micro-optical structures. The temporary 2D phase grating transforms back to the permanent complex microstructure upon triggering the recovery of the shape-memory DOE. On the screen, the diffraction pattern morphs continuously from a grid of dots into a grid of lines. (D) A mechanically stretched sample with a therefore increased period recovers the original, smaller grating period due to the shape-memory ability of the SMP DOE. Illustrated by the increasing distance between the maxima of the diffraction pattern on the screen, it was proven that by using a SMP-based DOE the period of a linear phase grating can be continuously tuned over three times its initial value. 9425
DOI: 10.1021/acsami.6b00679 ACS Appl. Mater. Interfaces 2016, 8, 9423−9430
Research Article
ACS Applied Materials & Interfaces green laser pointer (532 nm wavelength) at the appendant spot on the sample. With increasing structure height, the diffractive pattern on the screen becomes more and more defined until the European Union flag pattern is clearly visible. In the bottom row, the process is depicted in schematics to visualize the actual structure change during the recovery. In addition, Figure 3C displays the morphing between two different diffractive patterns. Instead of using a smooth tool plate to flatten the permanent structure (Figure 2, left cycle), the temporary shape was programmed by imprinting a second, diffractive structure into the sample’s surface. The left picture shows the pattern of a green laser pointer on the screen generated by the temporarily imprinted chessboard-like two-dimensional (2D) diffractive grating. With advancing recovery induced by heating the sample above Tswitch, one can witness the change of the diffraction pattern from a grid of dots to a complex pattern of lines. The period of a diffraction grating is its main characteristic attribute. To obtain a tunable period, we followed the fabrication cycle depicted in Figure 2 on the right side. We embossed a linear grating with an initially small period of Λ = 3 μm as permanent shape into the surface of a SMP foil and stretched it afterward in a tensile testing machine perpendicularly to the grating lines by a factor of 3. Consequently, the period tripled. To ensure a uniform deformation of the SMP gratings, the samples exhibit a homogeneous substrate of 1 mm thickness and constant width, and after the stretching procedure, they were granted a relaxation time of 30 min under constant load. For continuative experiments, only the middle parts with constant transverse contraction were utilized. A schematic of the process is given in the bottom row of Figure 3D. The diffraction pattern of the grating generated by a red laser pointer beam (wavelength of 650 nm) is a pattern of five maxima, as shown in Figure 3D on the left. The recovery of the grating to its permanent, shorter period was again initialized by increasing the temperature of the SMP to Tswitch. With decreasing grating period, the distance between the zeroth and first-order maxima increased. This change of diffraction patterns reveals that the grating period changed continuously from Λ* ≈ 9 μm to its initial value of Λ ≈ 3 μm within several minutes. To verify the eligibility of Tecoflex EG-72D as material for optical applications, we measured the transmission, absorption, and reflection of a 60-μm-thick foil with a spectrophotometer (PerkinElmer) at wavelengths between 200 and 800 nm (Figure 4). In contrast to reports on other types of SMPs, we did not observe a reasonable change of the optical properties with wavelength14 or temperature16 in the visible range of 380− 780 nm. The almost-constant transmission of 90% for this regime (see the thin solid line in Figure 4) qualifies this SMP for the fabrication of optical devices applicably in a wide spectral range. Equally important as the optical properties for DOEs fabricated from SMPs are the long-term stability and the number of possible programming cycles. We previously demonstrated that the same SMP that we have used here can withstand several treatment cycles where each cycle includes a mechanical deformation step for gaining the temporary shape and the recovery of the previously defined permanent structure.36 In that case, we investigated the recovered height of a repeatedly flattened grating. Even after the 10th test cycle, the recovered structure still exhibits the same height as the initial structure. Extensive cyclic thermomechanical analyses of
Figure 4. Optical characterization of the SMP Tecoflex EG-72D in the visible regime. The transmission, absorption, and reflection spectra of a flat, 60-μm-thick SMP foil were measured with a spectrophotometer. In the visible regime (380−780 nm), the transmission is almost constant, with a value of ∼90%, as indicated by the thin solid line.
Tecoflex EG-72D, in terms of shape recovery and shape fixity ratio, were performed by Schmidt and co-workers29,37 and had proven the durability and applicability of the herein-described SMP. For the long-term stability, we monitored the geometrical shape of four SMP DOE samples each from a different stage of the deformation/recovery cycle (Figure 5) by AFM. Instead of
Figure 5. Long-term stability of four gratings made from a SMP. The original grating had a nominal height of 600 nm and a period of 16 μm. The periods of the four samples, each from a different stage of the deformation/recovery cycle, were determined by atomic force microscopy (AFM) over 12 weeks. There was no significant change of all periods within experimental scatter over the complete observation time. The AFM images of the original, stretched, recovered and partly recovered gratings after the fourth week are shown on the right.
flattening the structures for the temporary shape, the samples were now stretched perpendicularly to the grating lines. To fabricate the test samples, the same mold as that used for the repetition cycle test was used.36 The resulting permanent period and height of the gratings was determined as 16 μm and 600 nm, respectively. The first sample represents the original (i.e., permanent) structure directly after hot embossing (Figure 5, right, bottom profile). The second sample was stretched by a factor of 2 and exhibited a period of 32 μm (Figure 5, right, top profile). The third sample was partly recovered after the stretching by abruptly reducing the temperature from Tswitch to room temperature during the recovery process (Figure 5, right, upper middle profile). In this way, the recovery process can be stopped at any intermediate shape. In the case of the third 9426
DOI: 10.1021/acsami.6b00679 ACS Appl. Mater. Interfaces 2016, 8, 9423−9430
Research Article
ACS Applied Materials & Interfaces
Figure 6. Recovery of a flattened SMP microstructure over time recorded by AFM: (A) AFM image of the flattened grating at the beginning of the heating, (B) analyzing the height of the grating lines for all AFM images results in recovery curves for different temperatures (these datasets can be fitted with eq 1; the fit parameters can be found in Table 1), and (C) after heating the sample for 60 min at a temperature of 60 °C, the height of the grating lines increased to 570 nm.
⎛ t ⎞ ⎛ t⎞ h(t ) = 1.0 − h1 exp⎜ − ⎟ − (1.0 − h1) exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠
sample, we used this to adjust the grating period to 19 μm. The fourth sample was allowed to recover completely (Figure 5, right, lower middle profile). The geometries of all four samples were analyzed by AFM weekly over a time period of three months to trace possible changes in the grating’s period. All samples were stored in a refrigerator at a constant temperature of 4 °C between the measurements to protect them from changing ambient conditions. The results are summarized in Figure 5, left. Within experimental scatter, the periodicity remained constant. Most importantly, the stretched and intermediate shape samples exhibit no change in their periodicity. Therefore, we conclude that temporarily deformed optical devices feature a long-term stability sufficient for optical applications, as long as the working temperature is well below Tswitch. As shown in Figure 5, not only two discrete shapes can be programmed into a SMP device, also the switching between those two shapes can be stopped at arbitrary states, which yields stable intermediate shapes. This can be utilized to fabricate not only switchable, but also adjustable DOEs. In order to quantify the dynamics of the shape change from a temporary shape to the permanent shape, we used the same microstructured phase grating as in Figure 5 and flattened the lines of the grating as shown in Figure 2, left cycle. The restoring process from this temporary shape to the permanent shape was then recorded at different temperatures by AFM to explore the temperaturedependent recovery dynamics of our devices. While the sample is kept constant at an elevated temperature, the AFM measured the topography (i.e., the height of the grating), which changed over time. This procedure was conducted for three temperatures at the upper end of the switching temperature range at 55, 60, and 65 °C. The structure height was determined by a histogram analysis of the captured AFM images and normalized to the height of the permanent shape of 570 nm. Figure 6 displays two AFM images recorded during the recovery process and summarizes the obtained datasets of height versus time. Heuchel et al.28 already discussed that the stress relaxation of a polyurethane-based SMP can be described by a slow recovery process and a fast recovery process. Following their arguments, we describe the height recovery h(t) using a Wiechert model with two Maxwell units.28,38 Furthermore, we must consider that the normalized recovery function must be zero (0) at t = 0 and 1 for t → ∞. Therefore, we fitted the experimental data with the following equation:
(1)
Here, τ1 and τ2 are the characteristic relaxation times and h1 is an arbitrarily defined height parameter describing the relationship between the fast recovery process and the slow recovery process. As shown in Figure 6 with the dashed lines, the recovery process fits this equation well. Table 1 summarizes the Table 1. Parameters Obtained by Fitting the Recovery Data Shown in Figure 6 to eq 1 Tsample (°C)
h1
τ1 [h]
τ2 [h]
55 60 65
0.86639 0.74374 0.58524
0.054487 0.075955 0.10635
1.784 1.8469 2.4966
fitting parameters. The continuous recovery and the moderate time constants show that recovery dynamics of optical devices made from Tecoflex are at moderate temperatures and are slow enough to allow a controlled tuning of the parameters such as grating height and period. As seen previously, the recovery of the permanent shape is both a function of temperature and heating time. This dependency can be used to gain strong control of the recovery process, in terms of recovery velocity and partial recovery, but also spatially resolved recovery. A locally adjustable DOE opens a wide field of applications, such as, e.g., the alignment of multiple beams toward each other using a single device. To prove the feasibility of this concept, we evolved a method to heat the samples that are spatially resolved. We embedded 25-μm-thick gold wires into permanently structured SMP foils to use them as Joule heaters. A schematic showing the setup for the fabrication of the samples is given in Figure 7A. For the permanent shape, we chose a linear grating with a period of 16 μm and placed the gold wire beneath the SMP sheet before the hot embossing process started. To keep the paths of thermal conduction as short as possible, a foil that was only ∼50 μm thick was employed. For the temporary shape, the structure was flattened with a smooth tool plate. A small amperage of 0.4 A at 4 V increased the wire’s temperature enough to trigger the recovery in the adjacent polymer. The longer the heating time lasted, the broader the recovered area became. 9427
DOI: 10.1021/acsami.6b00679 ACS Appl. Mater. Interfaces 2016, 8, 9423−9430
Research Article
ACS Applied Materials & Interfaces
compressed. Thus, when the switching segments loosen, this part gets recovered first and fastest. Figure 7C gives the normalized height h(x)/h, with respect to the original height h of the structure, as a function of the distance from the wire in multiples of the grating’s period of 16 μm. For every heating step, the height of the ridges of the structure was analyzed from scan cross sections in consideration of the distance to the heat source. Position of the heat source is located at 0; hence, the diagram shows the recovery of the structure to the right of the wire. The fits were obtained by feeding the data to an exponential decay equation.
4. CONCLUSION With this work, we demonstrated the unique ability of shapememory polymers (SMPs) to enable the fabrication of tunable micro-optical structures. The combination of hot embossing and SMPs facilitates the time and cost-effective production of diffractive optical elements with tunable characteristics, in terms of structure period, height, or even morphing between two completely disparate microstructures. In a macroscopic example, the shape-memory effect of the used polymer was demonstrated by tuning the refractive properties of a lens. Two parallel laser beams converge proportionately to the recovering curvature of the lens until they intersect at the focal point of the lens. The same principle is transferable to diffractive structures with dimensions in the micrometer or submicrometer regime. Hot embossing with its precisely controllable process parameters such as temperature and pressure enables the highly accurate transfer of microstructured masters to the surface of SMP foils. Therefore, we not only defined the permanent structure with hot embossing but also used the technique to allow a height tunability by programming a temporary structure into the SMP surface., whereas the structure’s period tunability, in terms of linear gratings, is provided by controlled stretching of the samples to their temporary, elongated shapes in a tensile testing machine. By subsequently initializing the recovery process, the shape memory effect of the polymer facilitates the tuning from an increased period back to its original size, from a flattened shape back to its initial structure height and morphing between two arbitrary predefined diffractive structures, respectively. To maximize the applicability of the designed diffractive optical elements (DOEs), in the visible regime, a transparent, biocompatible, thermoplastic polyether urethane was employed, which features a low switching temperature of Tswitch = 40−70 °C to trigger the recovery of its original shape but for all that is stable in its shape at room temperature. The SMP Tecoflex EG-72D is not optimized for a narrow switching range, as a relatively wide range proves to be very forgiving, in terms of precise temperature control in various applications. This allows one to trigger the switching by simple means, such as the heating of a resistor. With a precise temperature control, the switching speed and recovery rate can be controlled equally precisely. With the small dimensions and, therefore, small heat capacity of diverse DOEs and micro-opto-electro-mechanical systems (MOEMS), fast actuation of such a device is possible as the material can be quickly heated to the switching temperature. We confirmed the long-term stability of our fabricated SMP DOEs by tracking the period changes of samples from every step of the fabrication/programming/ recovery cycle over a time span of three months. However, we could not detect any variances. Furthermore, we illustrated the temperature dependency of the recovery dynamics by analyzing
Figure 7. Local heating of the SMP sample allows spatial recovery: (A) a gold wire was embossed into a structured SMP foil and used as a Joule heater to locally heat the foil and therefore enable spatially resolved recovery; (B) the combined sections of AFM scans after various heating steps visualize the continuing and broadening recovery of a linear phase grating, which was temporarily flattened (pictured are the recovered surface topography before and after an additional 5, 45, 90, 300, and 1500 s of heating time; and (C) the change in structure height is illustrated by analyzing ridge height from AFM cross sections subject to the distance to the heat source.
A continuous measurement of the structure while heating was not possible, because the sample height change exceeds the AFM’s vertical range. Nonetheless, to be able to observe the progress of the recovery, the heating was carried out in distinct steps. The drawback here is the loss of potential of this method to use the gathered data to gain some growth function in dependency of the distance of the heat source. However, the subsequent heating steps and measurements impressively demonstrate that the recovery process can be stopped and restarted multiple times without forfeiting the shape-memory ability of the polymer. The structure height is measured with a self-made AFM after every individual heating step and subsequently analyzed as a function of distance from the wire. The time-dependent width of the recovered structured area is shown in Figure 7. A section of the surface scanned after every heating step was combined with a three-dimensional (3D) representation (Figure 7B). First, the surface topography of the temporarily flattened SMP foil at 0 s heating time is pictured. The position of the gold wire can be observed through the tiny bulge. Also, a slight residual structure, with an average height of ∼24 nm, remains. After only 5 s of heating, a distinct change of the surface emerges. The height of bulge and structure above the wire increases further after an additional 45, 90, 300, and 1500 s of heating. In the end, after a total sum of 1950 s, heating the structure above the wire accomplishes ∼87% of its original height, whereas the width of the structured area is still
