Applied Materials Today 23 (2021) 101052 Contents lists available at ScienceDirect Applied Materials Today journal homepage: www.elsevier.com/locate/apmt Ultrabroadband light absorbing Fe/polymer flexible metamaterial for soft opto-mechanical devices Pau Güell-Grau a,b, Francesc Pi c, Rosa Villa a,d, Josep Nogués b,e, Mar Alvarez a,d,∗, Borja Sepúlveda b,∗ a Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC), Campus UAB, Bellaterra 08193, Barcelona, Spain b Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra 08193, Barcelona, Spain c Departament de Física, Facultat de Ciències, Universitat Autònoma de Barcelona, Bellaterra 08193, Barcelona, Spain d Networking Research Centre on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Spain e ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain a r t i c l e i n f o a b s t r a c t Article history: Ultrabroadband light absorbers are attracting increasing interest for applications in energy harvesting, Received 19 March 2021 photodetection, self-regulated devices or soft robotics. However, current absorbers show detrimental in- Revised 16 April 2021 sufficient absorption spectral range, or light angle and polarization dependence. Here we show that the Accepted 22 April 2021 unexplored optical properties of highly-damped plasmonic materials combined with the infrared absorp- tion of thin polymer films enable developing ultrabroadband light-absorbing soft metamaterials. The Keywords: developed metamaterial, composed of a nanostructured Fe layer mechanically coupled to a thin poly- Ultrabroadband absorption dimethylsiloxane (PDMS) film, shows unprecedented ultrabroadband and angle-independent optical ab- Soft metamaterials sorption (averaging 84% within 30 0–180 0 0 nm). The excellent photothermal efficiency and large thermal- Highly-damped plasmonics expansion mismatch of the metamaterial is efficiently transformed into large mechanical deflections, Iron nanostructures which we exploit to show an artificial iris that self-regulates the transmitted light power from the ultra- Optomechanical devices violet to the long-wave infrared, an untethered light-controlled mechanical gripper and a light-triggered electrical switch. © 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) 1. Introduction resonances can be tuned by size increasing, this imposes a large scattering/absorption ratio enhancement in most plasmonic mate- Efficient broadband light absorption is crucial for many rele- rials. This effect is more noticeable for resonances in the infrared vant photonic devices, such as solar cells [1,2], photodetectors [3,4] (IR), where these materials behave as near perfect conductors with or soft-actuators [5]. As their performance depends on the abil- minimal radiation penetration, thereby drastically increasing the ity to collect and convert light into other types of energies, novel reflected and scattered light. To expand the absorption range of concepts are being studied to maximize the absorption spectral plasmonic materials, the use of plasmonic nanoparticles of differ- window [6]. Recent broadband absorbing metamaterials include ent sizes [12–15], their combination with dielectric layers [16,17], microstructures with resonators [7,8], plasmonic nano-composites or the nanometric control of their near-field interaction [18,19] [9,10], or graphene-based metamaterials [11]. Plasmonic nanostruc- have been proposed. Additionally, apart from conventional pho- tures are particularly interesting to achieve strong absorbing meta- tonic/plasmonic materials, there are other materials, like transition materials due to their easily tunable optical properties. However, metals, which have been barely explored for photonic applications highly efficient light absorption is restricted to small particles com- and/or efficient light absorption, as is the case of metallic iron (Fe). pared to the light wavelength and to the excitation of localized Fe shows a negative real part of the dielectric constant from the ul- surface plasmon resonances in a narrow spectral band. Although traviolet (UV) to the far-IR, which is an essential condition for the generation of plasmonic resonances. However, its imaginary part is much larger than in classical plasmonic materials. This results in a ∗ Corresponding authors. large plasmonic damping, which prevents the generation of sharp E-mail addresses: mar.alvarez@csic.es (M. Alvarez), borja.sepulveda@icn2.cat (B. resonances, hampering its application in typical plasmonic applica- Sepúlveda). https://doi.org/10.1016/j.apmt.2021.101052 2352-9407/© 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) P. Güell-Grau, F. Pi, R. Villa et al. Applied Materials Today 23 (2021) 101052 tions such as refractometric sensors or biosensors [20], or the gen- silicon substrate. After drying, the metal film (gold/iron) is de- eration of enhanced electromagnetic fields at the nanostructures posited (40/80 nm) by electron beam physical vapor deposition edges. However, these broad resonances make Fe ideal for broad- (UNIVEX 450, Leybold). The nanostructured metal films were then band absorption applications. Fe enables a deeper electromagnetic transferred to the cured PDMS by contact transfer printing. A laser field penetration inside the nanostructures, thus minimizing light cutter (Epilog Mini 24, Epilog Laser) was used to define and cut the reflectance/scattering. This feature is particularly appealing to com- self-suspended structures into the desired geometry and dimen- bine nanostructured-Fe absorption with highly absorbing materi- sions. The structures were then released from the substrate by us- als in the far-IR, like flexible polymers, whose enhanced absorp- ing a 0.5 mm thick poly(methyl methacrylate) with double-sided tion bands in the midwave-IR–longwave-IR (MWIR-LWIR) range pressure-sensitive adhesive as base support. have been rarely exploited for practical photonic applications. In- corporating ultrabroadband light absorption into polymers is par- 2.3. Morphological characterization ticularly appealing for developing light-controlled soft actuators, as a base for innovative applications in soft robotics [21–25], en- The morphology of the nanostructured metal films on PDMS ergy harvesters [26–28] or self-regulated devices [29–31]. Impor- was examined by Scanning Electron Microscopy (Quanta FEG650, tantly, these applications demand strong ultrabroadband light ab- FEI (ThermoFisher)). sorption, incident light angle and polarization independence (due to the large mechanical deflections), and high compliance between 2.4. Optical characterization the metal and the flexible polymer substrate. However, all the soft actuators reported to date fail in at least one of these require- The UV-Visible-NIR transmission spectrometry was carried out ments. using a fiber-coupled halogen light source (HL20 0 0, Ocean Optics), Here we show that the highly-damped plasmons in nanos- with an optical fiber (Ø = 400 μm, NA = 0.5; M45L01, Thorlabs) tructured Fe films (hexagonal close-packaged Fe semi-shells) ex- and an aspheric lens (Thorlabs) to collimate the light. The opti- hibit extremely efficient broadband absorption in the UV–near- cal spectra were recorded by two spectrometers for the UV-visible infrared (UV-NIR) with minimal reflectance in the MWIR-LWIR. (Flame, Ocean Optics®) and near-infrared (NIRQuest, Ocean Optics) This, combined with flexible polydimethylsiloxane (PDMS) films ranges. The UV-Visible-NIR reflection spectrometry was carried out showing enhanced light absorption in the MWIR-LWIR, enables using a bifurcated optical fiber bundle (M25L01, Thorlabs), a silver developing soft metamaterials with an unprecedented ultrabroad- mirror as a reference (PF10-03-P01, Thorlabs) and the same light band absorption range. This inexpensive novel metamaterial shows source and spectrometers as for transmission. Infrared spectrome- an angle-independent and efficient ultrabroadband optical absorp- try was carried out by a Fourier Transform Infrared Spectrometer tion range (average 84% within 30 0-180 0 0 nm), which results (FTIR) (Bruker Vertex80), using a gold film as a reference for reflec- in an excellent photothermal conversion efficiency. Due to the tion and a KBr (FT-IR grade, Sigma Aldrich) sample substrate as a large thermal-expansion-coefficient mismatch between both ma- reference for transmission. terials, the photoinduced temperature increase is efficiently trans- formed into large mechanical deflections. We exploit these features 2.5. Photothermal characterization to demonstrate three soft photo-thermo-mechanical devices: (i) a power-free, ultrabroadband, self-regulated artificial iris that adjusts Temperature monitoring of the sample was carried out us- its aperture to the light intensity (from UV to far-IR), thus mimick- ing a non-contact infrared thermometer (MLX90614, Melexis) and ing a natural iris or acting as a self-regulated radiation protector; a computer with Labview data acquisition software. The infrared (ii) an optically-controlled mechanical gripper, enabling picking, sensing characterization was carried out using a white light LED transporting and releasing cargos, and (iii) an untethered optically- (MWWHLP1, Thorlabs), a NIR laser diode with emission wave- triggered electrical switch. length at 808 nm (B1-808-1500-15A, Sheaumann) and another laser with emission at 1470 nm (QSM-1470-3, QPhotonics). A ger- 2. Materials and methods manium filter (WG90530-G, Thorlabs) was used to block the in- terferences at the infrared thermometer coming from the infrared 2.1. Simulations light sources. Sunlight photothermal characterization was cali- brated using an optical power meter (S310C, PM100D, Thorlabs) The finite-difference time-domain (FDTD) simulations were car- and measured using an infrared thermal camera (FLIR). ried out by the Lumerical Software, having a mesh of 2 nm in the region containing the metal structures. For the forward/back- 2.6. Optomechanical characterization scattering simulation, a 900 × 900 nm2 detector plane was placed 100 nm before/after the semi-shell structure. The FDTD calculation The angle change of the cantilever and aperture change of the volume is a cube of 1 μm edge. artificial iris was recorded using a conventional USB camera (Di- noLite) and DinoCapture as the image capture software. The light 2.2. Fabrication sources were the same as for the photothermal characterization. The IR thermal emission was produced by heating a PDMS and alu- Firstly, the PDMS thin layer was obtained by spin-coating liq- minum sample using a single stage TEC Element (TECD6, Thorlabs). uid PDMS (Sylgard 184) at 10 0 0 rpm for 90 seconds on a thick (5 The transmitted power was measured using two different optical mm) PDMS substrate, previously coated with (Tridecafluoro-1,1,2,2- power meters for visible-NIR light (S310C, PM100D, Thorlabs) and tetrahydrooctyl)trichlorosilane (97%, ABCR), and curing it at 80°C for for thermal IR (S401C, Thorlabs). 30 minutes. In parallel, to obtain the nanostructured metal layers, a hexagonal close-packed colloidal self-assembly of polystyrene 3. Results and discussion nanospheres (diameter 20 0/30 0/50 0 nm) (Life Technologies) on a silicon substrate is prepared. To do the assembly, the close-packed To highlight the key differences between classical plasmonic monolayer of spheres is first formed at the air/water interface that absorbers and highly-damped plasmonic materials for ultrabroad- is on the immersed silicon wafer. Once the monolayer is fully band absorption applications, we compare the scattering and ab- formed, the water is drained and the layer is deposited on the sorption cross sections of identical gold (Au) and Fe structures 2 P. Güell-Grau, F. Pi, R. Villa et al. Applied Materials Today 23 (2021) 101052 Fig. 1. (a) FDTD simulations comparing the scattering (Cscat ) and absorption (Cabs ) cross-sections for 500 nm diameter Au and Fe semi-shell structures. (b) Electric field distribution in the central plane for Au (left) and Fe (center) semi-shells and the ratio of the electric fields observed in each structure (right), evidencing the much larger light penetration inside the Fe structure. Scale-bar = 100 nm. (c) Comparison of the Cabs for Fe structures of different shapes and constant size (diameter = 500 nm). by finite-difference time-domain (FDTD) simulations. As a rule of sorbed by the Fe structures) to be transmitted and totally absorbed thumb, to achieve strong absorption, the absorption/scattering ra- by another material, such as a thin flexible polymer layer with tio of the structures should be maximized, whereas very broad/flat strong absorption bands in this range. Notably, the structure ge- spectra are required to reach the ultrabroadband absorption. From ometry also plays a major role to maximize the spectral absorption the FDTD calculations, we observe that Fe semi-shell structures of range, where other more common geometries, such as spheres or 500 nm diameter exhibit approximately 7-fold larger absorption discs, show substantially lower absorption bands (Fig. 1c). Finally, cross-section compared to their it is worth highlighting that this type of highly damped plasmonic Au A Au counterparts (Fig. 1a). Moreover, the large scattering Fe semi-shell structures can be easily obtained by colloidal self- observed in the Au structures is dramatically reduced (between 2 assembly and highly directional Fe evaporation. In addition, the in- and 3-fold) in the Fe semi-shell. herent potential to adjust the colloidal size and separation distance Overall, the absorption/scattering ratio of Fe semi-shells can be between self-assembled structures can also be exploited to attain up to 25-fold larger than Au semi-shells in the NIR, thus highlight- even broader absorption bands. The combination of self-assembled ing their potential to achieve intense light absorption efficiency. Fe semi-shells with a polymer film gives rise to an absorbing meta- The reason behind this stark difference is the much higher pen- material enabling the efficient light absorption in the whole UV to etration of the light inside the highly damped plasmonic Fe semi- IR regime, which otherwise would be almost totally reflected by a shell compared to the Au structures, as shown by the electric field continuous iron film. distribution around and inside the metal structures, which is al- Considering the previous theoretical results, the fabrication pro- most 6-fold higher at the central thickest part of the Fe metal cess of the nanostructured-Fe/PDMS metamaterial is based on structure (Fig. 1b). The larger light penetration in Fe is endowed by a scalable combination of colloidal self-assembly, physical vapor both the lower absolute value of the real part of the dielectric con- deposition and polymer casting (Fig. 2a). The nanostructured-Fe stant and the much higher imaginary part (up to 20-fold larger at film is prepared by the hexagonal close-packed self-assembly of 800 nm wavelength compared to Au). This large difference in the a monolayer of polystyrene (PS) nanospheres (500 nm diameter) absorption/scattering ratio is also clearly observed in other com- followed by the electron beam deposition of the Fe layer (80 nm), mon plasmonic materials, like Ag and Al semi-shells (Fig. S1a). leading to a “semi-shell” structure [32,33]. The nanostructured-Fe Interestingly, the Fe semi-shells also enable stronger absorp- film is mechanically coupled to the PDMS layer (50 μm) by trans- tion in a much broader spectral range (from the UV to the NIR), fer printing (Fig. 2b). Once the metamaterial is fabricated, the gen- even covering wavelengths shorter than the size of the nanostruc- eration of the opto-mechanical structures for soft actuation appli- tures. In addition to the scattering reduction compared to Au, the cations is performed by defining the desired geometries by laser FDTD simulations show that the forward scattering is favored with writing and releasing them from the substrate, thus yielding free- respect to the back scattering (Fig. S1b). This is essential to al- standing structures. Note that the released structures have an ini- low the MWIR and LWIR radiation (that is not so efficiently ab- tial curvature due to the generated residual stress during the re- 3 P. Güell-Grau, F. Pi, R. Villa et al. Applied Materials Today 23 (2021) 101052 Fig. 2. (a) Schematic of the fabrication steps of the Fe/PDMS metamaterial. (b) SEM image and schematic representation of the nanostructured-Fe layer in contact to the PDMS layer (scale-bar = 500 nm). leasing step. It is worth emphasizing that this fabrication process The photothermal conversion efficiency, η = dT/dI (where T is is straightforward, fast, inexpensive and easily scalable. temperature and I is the light irradiance), was determined by irra- In agreement with the FDTD simulations, the experimental diating the metamaterials with four different light sources: white- characterization of the Fe/PDMS metamaterial demonstrated a very LED (410–700 nm), 808 nm and 1470 nm lasers, and natural sun- high optical absorption of the nanostructured-Fe layer within the light. The change of temperature was found to be proportional to Ultraviolet–Visible–Near Infrared (UV-vis- NIR) ranges (30 0 – 20 0 0 the light intensity for all the light sources (Fig. 3c). Moreover, the nm) where the PDMS is transparent [34] (Fig. 3a). Moreover, the photothermal conversion efficiency of the Fe/PDMS-metamaterial nanostructured-Fe layer minimizes the reflectance in the MWIR did not significantly differ for any light source, which is in ac- and LWIR infrared regions (20 0 0 - 180 0 0 nm) (Fig. S2), thereby cordance with its nearly-flat broadband absorption. Importantly, enabling the transmission and posterior absorption of these wave- the η of the Fe/PDMS-metamaterial exhibited minimal dependence lengths by the PDMS layer, which is an efficient absorber in this on the light incidence angle, which is critical to provide a linear range (Fig. 3b). The synergistic combination of the optical prop- photo-thermo-mechanical response in structures with high curva- erties of both layers results in an outstanding ultrabroadband ab- tures, thus overcoming the angular limitation inherent to other sorption range from the UV to the LWIR, 300 – 180 0 0 nm, with absorbers [34,44]. As expected from the absorption spectra, the an average 84% absorption (Fig. 3a,b). In comparison with previ- Fe/PDMS-metamaterial showed substantially higher η (ca. 2-fold) ously reported strong (>80%) absorbers, the Fe/PDMS metamate- than the analogous Au/PDMS-metamaterial at normal incidence. rial shows a competitive absorption (from 75% to 94%) within a Moreover, the nanostructured-Au layer presented strong angular much broader wavelength range than any other reported absorber dependence due to the diffraction-coupling to surface-plasmon- within the UV to LWIR range (see Table S1) [7,9–11,18,22,35,36- polaritons. 43]. In contrast, the Au/PDMS metamaterial showed substantially The optomechanical response was assessed in Fe/PDMS meta- weaker absorption (Fig. 3a,b) because of the increased reflection of material cantilevers (Fig. 4a) by measuring the curvature changes the nanostructured-Au (Fig. S2a,b), as suggested by the FDTD sim- as a function of the light intensity (Fig. 4b). In bimorph struc- ulations. The different performance between both metals is due tures the curvature change is produced by the thermal strain ε to the highly damped plasmonic behavior of Fe, which provides generated by the light-induced temperature increase T, ε = deeper electromagnetic field penetration inside the metal to max- (αPDMS − αF e )T , where αPDMS and αF e are the thermal expansion imize the absorption in the UV-vis-NIR (as previously shown in coefficients of PDMS and Fe, respectively. Since the PDMS layer Fig. 1b), and the transmittance in the MWIR and LWIR. Oppositely, thickness is much larger than the Fe layer (tPDMS >> tFe ), the equa- the nanostructured-Au layer behaves as a near-perfect conductor tion of the curvature (1/r) change can be simplified to [22]: in the NIR-LWIR, thereby enhancing the reflectance and decreasing 1 (αPDMS − αFe )T even further the absorption efficiency. The superior absorption ef- = (1) r EPDMS tPDMS 2 ficiency in the Fe/PDMS with respect to the Au/PDMS metamaterial 6EFe tFe + 2tPDMS 3 was also observed for semi-shells of other diameters and different Where r is the curvature radius (Fig. 4b, inset), and EPDMS and EFe metal thicknesses (Fig. S3). Note that by selecting the appropriate are the Young’s-modulus of the PDMS and nanostructured-Fe lay- thickness of the Fe layer or the diameter of the nanostructures in ers, respectively. the nanostructured-Fe layer, the minimum absorption of the meta- The parameter “1/r” was chosen to quantify the curvature vari- material in the UV – NIR could be further enhanced to above 80% ations and to compare with the literature results, since it is inde- (Fig. S3b). pendent of the cantilever length. The cantilever curvature changed 4 P. Güell-Grau, F. Pi, R. Villa et al. Applied Materials Today 23 (2021) 101052 Fig. 3. (a,b) Absorption spectra of the Fe/PDMS- and Au/PDMS- metamaterials (a) in the UV-visible-NIR range, and (b) in the MWIR- LWIR range. (c) Photothermal conversion efficiency of the Fe/PDMS- and Au/PDMS- metamaterials for light sources of different emission wavelengths and incident angles (“S” = natural sunlight). linearly with the light-induced temperature changes, and therefore tions (i.e., less than 15%) from the UV to the LWIR, whereas the with the light intensity, showing a responsivity of 0.012 cm·mW−1 . carbon ink suffers an absorption decrease of 2.5 orders of magni- The excellent photo-thermo-mechanical actuation of the Fe/PDMS- tude from the blue to 2.6 μm wavelength. Such flatter and higher metamaterial is achieved by: i) the highly-efficient light absorption absorption efficiency enables using a 15-fold thinner iron absorp- in an unprecedentedly broad spectral range to maximize the light- tion layer, with respect to the carbon ink layer. The main limitation induced temperature changes, and ii) the large thermal expansion of the Fe/PDMS metamaterial comes from the difficulty to achieve coefficient mismatch between both materials (α Fe = 1.2×10−5 K−1 , mechanically stable PDMS layers with thickness below 20 μm, thus α PDMS = 3×10−4 K−1 ). limiting the achievable curvature changes from the bimetallic ef- By fitting the experimental curvature to Eq. (1) (assuming fect, as predicted by Eq. (1). tPDMS = 50 μm, tFe = 80 nm, EPDMS = 1.4 MPa, α Fe = 1.2×10−5 To exploit the excellent optomechanical responsivity of the K−1 , α PDMS = 3×10−4 K−1 ), the effective-Young’s-modulus of the Fe/PDMS metamaterial, we explored the application of this meta- nanostructured-Fe layer was extracted, yielding 1.2 GPa. This value material in three different light-controlled devices. Specifically, we demonstrates that the nanostructured-Fe layer behaves as a con- designed, (i) a self-regulated artificial iris, (ii) a light-regulated tinuous layer but with a substantially lower Young’s-modulus than soft-robotic gripper and iii) an optically-triggered electrical switch. bulk-Fe (10 0–20 0 GPa), thereby highlighting its behavior as both Firstly, we developed a self-regulated iris mimicking the behav- optical and mechanical metamaterial. ior of the human iris. A power-free, self-regulated iris might be a Compared to other bimorph actuators, the Fe/PDMS metama- solution to ophthalmological problems related to the size and dy- terial presents a better responsivity than other actuators of sim- namics of the pupil/iris, by controlling the amount of light that ilar thickness (Fig. 4c). Moreover, according to Eq. (1), the opto- reaches the retina [56]. In addition, the same design can be used mechanical responsivity can be further enhanced by reducing the for an automatic light transmission control (i.e., an optical attenu- PDMS thickness [45]. We could demonstrate this ability by reduc- ator for light protection) of any source emitting in the 30 0–180 0 0 ing the PDMS thickness to 20 μm, which yielded a responsivity in- nm range. Note that our iris design was geometrically inspired by a crease up to 0.018 cm·mW−1 , thus surpassing again the responsiv- previously reported artificial iris based on liquid crystal elastomers ity reported in the literature in that thickness range (Fig. 4c). Com- (LCE) [30] working in the 40 0–60 0 nm range. Using the standard pared to the carbon-ink based bimorph actuator, which has the dimensions of a natural iris [57] the device consisted of a circular highest reported responsivity, the Fe/PDMS metamaterial exhibits Fe/PDMS membrane fixed at its outer diameter (11 mm) with an a substantially flatter absorption spectra, showing minimal varia- inner circular hole of 3 mm. The circular membrane was divided 5 P. Güell-Grau, F. Pi, R. Villa et al. Applied Materials Today 23 (2021) 101052 Fig. 4. (a) Schematic of the ultrabroadband opto-mechanical structure principle of operation. (b) Curvature variation in response to light induced temperature changes (bot- tom axis) and associated white-LED irradiance (top axis). Inset: actual cantilever cross-view picture indicating the curvature radius/angle. Scale-bar = 2 mm. (c) Comparison of the light-to-mechanical conversion efficiency of different photothermal bimorph actuators reported in the literature [21,23,30,31,46–55] (see Table S2 for the description of the different materials). into 8 segments presenting an initial curvature of 108 ± 6 degrees dence. As observed in Fig. 5c, the iris also responded with similar (Fig. S4), which automatically folded/unfolded depending on the responsivity to the illumination with monochromatic near-infrared incoming light intensity, thereby opening/closing the inner aper- light from the 808 nm laser. ture (Fig. 5a). The biomimetic optomechanical response was char- In addition to the visible and NIR, the efficient absorption in the acterized by measuring the inner circle diameter of the iris (i.e., MWIR-LWIR range can also be applied to develop opto-mechanical the pupil size) as a function of the incoming light irradiance from a devices, such as thermal imaging systems [59–61]. Remarkably, the white-LED (Fig. 5b). The iris responded linearly to the light irradi- iris also exhibited self-regulation at the MWIR-LWIR range, which ance, reaching the nearly closed configuration with 234 mW·cm−2 , was demonstrated by its response to the weak emission from a which corresponds to 1.7-sun intensity [58]. This system was ca- material heated from room temperature to 120°C (Fig. S6a), in pable of self-adjusting the pupil aperture diameter from 6.9 to 3.8 which the blackbody radiation wavelength peaks at 7500 nm. To mm, which is comparable to the standard dynamic range of a nat- rule out any contribution from the temperature increase of the ural pupil. The transmitted power through the iris (Fig. 5b, red air surrounding the iris, two samples of very different emissivity line) showed a linear self-regulation reaching a relative transmit- (ε ), PDMS (ε = 0.86) and aluminum (ε = 0.04), were heated at ted power of 56% in the nearly closed configuration, which approx- the same temperature (Fig. S6b). The iris diameter was reduced imately corresponded to the relative area variation of the pupil. a 9% for the PDMS good-emitter, while it barely changed for the This transmission range can be readily tuned by adjusting the in- Al poor-emitter. Considering the low thermal irradiance at the iris ner hole diameter. To show the ultrabroadband self-regulation of (29.7 mW·cm−2 ), the responsivity to the thermal IR light was even the iris, we analyzed its responsivity, i.e., the variation of the pupil higher than the other light sources, due to the increased absorp- diameter as a function of the incident intensity, for light sources tion efficiency of the Fe/PDMS metamaterial in the MWIR-LWIR with emission from the visible to the LWIR (Fig. 5c), including the range. Additionally, the temperature increase at the iris upon di- white-LED (Fig. 5b), sunlight, an 808 nm laser (Fig. S5a), and an rect sunlight irradiation was 14.3 °C (Fig. S7), which was enough infrared thermal emitter. Although the wavelength distribution of to actuate the optomechanical system, and reasonably low to be the white-LED and the natural sunlight are different, their respon- easily insulated and safe for biomedical applications. sivity was nearly equal due to the flat broadband absorption of Finally, the dynamic response of the biomimetic iris was tested the Fe/PDMS metamaterial that minimized any wavelength depen- during an on-off cycle using the white-LED (Fig. 5d) and the 808 6 P. Güell-Grau, F. Pi, R. Villa et al. Applied Materials Today 23 (2021) 101052 Fig. 5. (a) Schematic of the artificial iris working principle. (b) Diameter of the iris aperture upon white-LED irradiation with increasing irradiance (black squares) and related fraction of the transmitted light power (red circles), together with the actual pictures of the artificial iris with/without white-LED illumination (irradiance 234 mW·cm−2 ). Scale-bar = 2 mm. (c) Ultrabroadband iris responsivity to different light sources with emission from the visible to the LWIR. (d) Dynamic response of the artificial iris actuated by the white-LED (234 mW·cm−2 ). nm laser (Fig. S5b, Video S1). In both cases, the aperture abruptly controlling residual stress in the fabrication process. On the other changed during the initial 4 seconds, followed by a slower ther- hand, the opening strength and velocity can be controlled by the mal stabilization during 10 seconds. Once the light was switched incident light intensity. off, the iris returned to the initial position. The transmitted power Finally, we tested the Fe/PDMS actuator as a remote optically- during the same on-off cycle showed similar dynamic response as triggered electrical switch. In this approach, we exploited the the iris diameter. The reaction time was slightly slower than the optically-controlled bending of the actuator to open/close an elec- human iris [62], but it could be matched by optimization of the trical circuit and turn off/on a LED (Fig. 6b, Video S3). Since metamaterial stiffness or geometry. Notably, compared to the pre- the nanostructured-Fe/PDMS is not conductive, we applied a thin viously reported self-regulating artificial iris based on LCE [30], our layer of conductive silver paste on the edge of the structure. Even device presents several advantages, such as a faster and more ho- though the actuator’s performance was slightly altered due to its mogeneous response to small light intensities and a much broader weight and stiffness increase, the photo-thermo-mechanical forces spectral response range. However, it could be of great interest to were sufficiently strong to bend the free-standing structure and combine the control on the thermo-mechanical response of LCEs close the electrical circuit. However, in future demonstrations, the with the ultrabroadband absorber metamaterial presented in this incorporation of a conductive pad could be optimized using more work. sophisticated micropatterning techniques such as inkjet-printing, To show the versatility of the Fe/PDMS opto-mechanical meta- thus opening the path to light-controlled electrical switches for en- material, we also developed a simple soft mechanic gripper ergy efficiency systems. [21,49], consisting of two cantilevers facing each other (Fig. 6a, Video S2). In its relaxed state, the initial bending due to the resid- 4. Conclusion ual stress configures the gripper in its closed state (step 1). When the gripper is irradiated (step 2), its arms open making it possible In summary, we have presented a novel flexible ultrabroad- to encase the cargo (step 3). As the light irradiation stops, the grip- band absorbing nanostructured-Fe/PDMS metamaterial. The opti- per closes and clutches the cargo, which can be then transported cal synergy between the highly damped plasmonic behavior in to a different position (steps 4-7). To release the cargo, the gripper the nanostructured-Fe layer and the efficient IR absorption in the can be opened again by light irradiation (steps 8-9). It is worth PDMS enables achieving angle-independent and strong (average highlighting the gripper loading capacity, as the cargo weight was 84%) ultrabroadband absorption from the UV to LWIR (300 - 18000 remarkably higher (54 mg) than the actuator itself (1 mg). More- nm), thus overcoming the limitations of typical plasmonic ab- over, the clutching force of the actuator, which depends on the ini- sorbers and multi-layered systems. The angle-independent opti- tial stress, can be adjusted by tuning the actuators’ length and by cal absorption in a large wavelength range translates to an excel- lent photothermal conversion efficiency. This, combined with the 7 P. Güell-Grau, F. Pi, R. Villa et al. Applied Materials Today 23 (2021) 101052 Fig. 6. (a) Pictures of the optically controlled mechanical gripper showing the opening, clutching, transport and release of the cargo. (b) Pictures of the light-triggered electrical switch. The inset pictures show the opto-mechanical actuator that open/closes the electrical circuit and turns on the LED. The incident light (λ = 808 nm) irradiance in (a) and (b) is 242 mW•cm−2 . large thermal expansion mismatch between Fe and PDMS, provides Declaration of Competing Interest an outstanding opto-mechanical response in suspended structures made of this metamaterial. To exploit these unique features, we The authors declare that they have no known competing finan- demonstrate a self-regulated biomimetic iris capable of responding cial interests or personal relationships that could have appeared to to an ultrabroadband wavelength range, including natural sunlight influence the work reported in this paper. and IR thermal radiation. The iris reversibly changes its aperture size following the same dimensions of a natural pupil, and achieves CRediT authorship contribution statement fast, efficient, homogeneous and linear opto-mechanical regulation of the transmitted power. Furthermore, we show the versatile ap- Pau Güell-Grau: Conceptualization, Investigation, Formal analy- plicability of the metamaterial by developing a light-controlled soft sis, Methodology, Writing – review & editing. Francesc Pi: Investi- mechanical gripper and a light-triggered electrical switch. Over- gation. Rosa Villa: Supervision, Funding acquisition. Josep Nogués: all, the exceptional photo-thermo-mechanical properties and the Supervision, Funding acquisition, Writing – review & editing. Mar versatility and cost-effectiveness of this metamaterial bring new Alvarez: Conceptualization, Investigation, Formal analysis, Method- opportunities to develop different types opto-mechanical compo- ology, Supervision, Funding acquisition, Writing – review & editing. nents, such as dynamic prosthetics, self-regulated and power free Borja Sepúlveda: Conceptualization, Investigation, Formal analysis, light protectors (at any wavelength from UV to LWIR) or light- Methodology, Supervision, Funding acquisition, Writing – review & controlled soft-robots. Furthermore, its ultrabroadband absorption editing. could be exploited in other fields where light collection is crucial, as thermophotovoltaics or photodetection, and the ferromagnetic Acknowledgments properties of the iron film could be used to develop multi-stimuli responsive actuators. We acknowledge funding from Generalitat de Catalunya Appendix A. Supplementary data through the 2017-SGR-292 project. The funding from the Span- Supplementary data associated with this article can be found, ish Ministerio de Ciencia, Innovación y Universidades (MICINN) in the online version, at through the MAT2016-77391-R, PID2019-106229RB-I00, PCIN2016- 093 (M-ERA-NET), DPI2015-68197-R and DPI2015-72948-EXP projects and Ramon y Cajal Fellowship (RyC2013-14479) is ac- Data availability knowledged. ICN2 is funded by the CERCA programme/Generalitat de Catalunya. The ICN2 is supported by the Severo Ochoa Centres The raw/processed data required to reproduce these findings of Excellence programme, funded by the Spanish Research Agency cannot be shared at this time due to technical or time limitations. (AEI, grant no. SEV-2017-0706). 8 P. Güell-Grau, F. Pi, R. Villa et al. Applied Materials Today 23 (2021) 101052 Supplementary materials [22] H. Lim, T. Park, J. Na, C. Park, B. Kim, E. 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