Wideband omnidirectional infrared absorber with a patchwork of plasmonic nanoantennas
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Wideband omnidirectional infrared absorber with a patchwork of plasmonic nanoantennas Patrick Bouchon, 1 Charlie Koechlin, 1,2, * Fabrice Pardo, 2 Riad Haïdar, 1,3 and Jean-Luc Pelouard 2 1 ONERA—The French Aerospace Lab, F-91761 Palaiseau, France 2 Laboratoire de Photonique et de Nanostructures (LPN-CNRS), 91460 Marcoussis, France 3 École Polytechnique, Département de Physique, 91128 Palaiseau, France *Corresponding author: [email protected] Received January 6, 2012; revised January 31, 2012; accepted February 1, 2012; posted February 2, 2012 (Doc. ID 160966); published March 8, 2012 In this Letter, we demonstrate experimentally that a patchwork of four metal–insulator–metal patches leads to an unpolarized wideband omnidirectional infrared absorption. Our structure absorbs 70% of the incident light on a 2.5 μm bandwidth at 8.5 μm. It paves the way to the design of wideband efficient plasmonic absorbers in the infrared spectrum. © 2012 Optical Society of America OCIS codes: 050.6624, 230.1950, 130.3060. For more than three decades, propagative surface plas- mon resonance has been known to induce a total absorp- tion on a nanostructured surface metal [ 1]. However, these structures are sensitive to the angle of incidence. In contrast, localized plasmonic resonances, e.g., in metal- lic nanoslits or nanoparticles, exhibit an absorption independent of the angle of incidence. In particular, sub- wavelength structures based on metal–insulator–metal (MIM) involving Fabry–Perot-like resonances are often used to get a total absorption [ 2– 8]. In the visible range, horizontal MIM can even sustain both horizontal and vertical resonances [ 9]. Such plasmonic antenna are very promising for various applications, such as infrared mi- crobolometers [ 10, 11], infrared photodetectors [ 12], or photovoltaic cells [ 13]. Their absorption spectra can be widened by combining two MIM ribbons with different widths in the same subwavelength period [ 14]. Neverthe- less, the absorption is polarization sensitive and depends on the conical angle of incidence, which are major draw- backs for the mentioned applications. Various works propose the use of a patch antenna whose absorption is unpolarized [ 4, 15, 16]. However, the issue of a tailored absorption spectrum by a square MIM combination has never been addressed: the previously demonstrated devices display a high fill ratio (>25%) incompatible with their juxtaposition inside a subwavelength pat- tern. Besides, the electromagnetic response of their com- bination in a two-dimensional (2D) patchwork remains challenging. In this Letter, we experimentally demonstrate the broadband absorption of light between 7 and 9.5 μm, with an average measured absorption of 70%, by combining four square patch MIM resonators within a subwave- length period. Their individual fill ratios are between 10% and 15%, and the final combination has a fill ratio of 50%. This patchwork exhibits an omnidirectional ab- sorbance diagram of unpolarized light. Moreover, both the metal and dielectric thicknesses are very thin (<λ∕20) and are the same for each resonator of the patch- work, which is a major asset for fabrication since the structure can be produced in a one-step process. To decrease the fill factor of a square MIM grating [de- picted in Fig. 1(a)], we take into account two phenom- ena. First, the period is chosen so as to avoid diffracted orders, even for relatively large angles of inci- dence. If we consider the 8–12 μm band, and a maximum incidence of 30°, the period has a maximum value d 5.3 μm. Second, the resonance wavelength is roughly given by λ r ≃ 2 nw: one can thus reduce the resonator size w by simply choosing an insulator of high- er optical index n. Here we show that, even with a mod- erately high index material, such as zinc sulfide (ZnS, n 2.2 [ 14]), several resonators can be combined into a wideband absorber. The gold permittivity is computed from a Drude model ϵλ 1 − λ p ∕λ iγ λ p ∕λ −1 with λ p 159 nm and γ 0.0048 [ 14]. Thanks to the Fourier Fig. 1. (Color online) (a) Geometry of a square MIM patch. It is made of a stack of ZnS (h ZnS ) and gold (h Au ) deposited on a thick gold layer. The MIM patch has a width w and is periodi- cally arranged in both the x and y directions with a period d. (b) Computed absorption spectrum of a MIM structure of width w 1.62 μm. The other parameters are taken as h ZnS 290 nm, h Au 50 nm, and d 5.3 μm. (c) Top view of the four-MIM structure. The four resonators of different widths are placed in the corner of a rectangle of size c x × c y 4.41 μm× 4.47 μm. (d) Tilted SEM image of the fabri- cated patchwork of square MIM structure. 1038 OPTICS LETTERS / Vol. 37, No. 6 / March 15, 2012 0146-9592/12/061038-03$15.00/0 © 2012 Optical Society of America
Transcript of Wideband omnidirectional infrared absorber with a patchwork of plasmonic nanoantennas
Wideband omnidirectional infrared absorber with apatchwork of plasmonic nanoantennas
Patrick Bouchon,1 Charlie Koechlin,1,2,* Fabrice Pardo,2 Riad Haïdar,1,3 and Jean-Luc Pelouard2
1ONERA—The French Aerospace Lab, F-91761 Palaiseau, France2Laboratoire de Photonique et de Nanostructures (LPN-CNRS), 91460 Marcoussis, France
3École Polytechnique, Département de Physique, 91128 Palaiseau, France*Corresponding author: [email protected]
Received January 6, 2012; revised January 31, 2012; accepted February 1, 2012;posted February 2, 2012 (Doc. ID 160966); published March 8, 2012
In this Letter, we demonstrate experimentally that a patchwork of four metal–insulator–metal patches leads to anunpolarized wideband omnidirectional infrared absorption. Our structure absorbs 70% of the incident light on a2.5 μmbandwidth at 8.5 μm. It paves the way to the design of wideband efficient plasmonic absorbers in the infraredspectrum. © 2012 Optical Society of AmericaOCIS codes: 050.6624, 230.1950, 130.3060.
For more than three decades, propagative surface plas-mon resonance has been known to induce a total absorp-tion on a nanostructured surface metal [1]. However,these structures are sensitive to the angle of incidence.In contrast, localized plasmonic resonances, e.g., inmetal-lic nanoslits or nanoparticles, exhibit an absorptionindependent of the angle of incidence. In particular, sub-wavelength structures based on metal–insulator–metal(MIM) involving Fabry–Perot-like resonances are oftenused to get a total absorption [2–8]. In the visible range,horizontal MIM can even sustain both horizontal andvertical resonances [9]. Such plasmonic antenna are verypromising for various applications, such as infrared mi-crobolometers [10,11], infrared photodetectors [12], orphotovoltaic cells [13]. Their absorption spectra can bewidened by combining two MIM ribbons with differentwidths in the same subwavelength period [14]. Neverthe-less, the absorption is polarization sensitive and dependson the conical angle of incidence, which are major draw-backs for the mentioned applications. Various workspropose the use of a patch antenna whose absorption isunpolarized [4,15,16]. However, the issue of a tailoredabsorption spectrum by a square MIM combination hasnever been addressed: the previously demonstrateddevices display a high fill ratio (>25%) incompatiblewith their juxtaposition inside a subwavelength pat-tern. Besides, the electromagnetic response of their com-bination in a two-dimensional (2D) patchwork remainschallenging.In this Letter, we experimentally demonstrate the
broadband absorption of light between 7 and 9.5 μm, withan average measured absorption of 70%, by combiningfour square patch MIM resonators within a subwave-length period. Their individual fill ratios are between10% and 15%, and the final combination has a fill ratioof 50%. This patchwork exhibits an omnidirectional ab-sorbance diagram of unpolarized light. Moreover, boththe metal and dielectric thicknesses are very thin(<λ∕20) and are the same for each resonator of the patch-work, which is a major asset for fabrication since thestructure can be produced in a one-step process.To decrease the fill factor of a square MIM grating [de-
picted in Fig. 1(a)], we take into account two phenom-ena. First, the period is chosen so as to avoid
diffracted orders, even for relatively large angles of inci-dence. If we consider the 8–12 μm band, and a maximumincidence of 30°, the period has a maximum valued � 5.3 μm. Second, the resonance wavelength isroughly given by λr ≃ 2 nw: one can thus reduce theresonator size w by simply choosing an insulator of high-er optical index n. Here we show that, even with a mod-erately high index material, such as zinc sulfide (ZnS,n � 2.2 [14]), several resonators can be combined intoa wideband absorber. The gold permittivity is computedfrom a Drude model ϵ�λ� � 1 − ��λp∕λ� iγ�λp∕λ�−1 withλp � 159 nm and γ � 0.0048 [14]. Thanks to the Fourier
Fig. 1. (Color online) (a) Geometry of a square MIM patch. It ismade of a stack of ZnS (hZnS) and gold (hAu) deposited on athick gold layer. The MIM patch has a width w and is periodi-cally arranged in both the x and y directions with a period d.(b) Computed absorption spectrum of a MIM structure of widthw � 1.62 μm. The other parameters are taken as hZnS �290 nm, hAu � 50 nm, and d � 5.3 μm. (c) Top view of thefour-MIM structure. The four resonators of different widthsare placed in the corner of a rectangle of sizecx × cy � 4.41 μm × 4.47 μm. (d) Tilted SEM image of the fabri-cated patchwork of square MIM structure.
1038 OPTICS LETTERS / Vol. 37, No. 6 / March 15, 2012
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modal method [17,18], we estimate that the typical size ofa resonator tuned at λr � 7.4 μm is nearly w � 1.62 μm[see the absorption spectrum plotted in Fig. 1(b)]. We arethus able to combine four resonators in each 5.3 μmsquare period. Then, the thickness of the insulator layercan be tuned to match the impedance of the free spaceand thus to set the level of absorption of the resonator[5]. We optimize the thicknesses hZnS � 290 nm andhAu � 50 nm in order to have a nearly total absorptionand find that the resonance wavelength can be tuned onthe 8–12 μm band only by varying the square MIM width.We choose to realize a structure made of four MIM re-
sonators, all having the same thicknesses, and four differ-ent widths. The position of the patches in the periodd � 5.3 μm has been chosen arbitrarily and is repre-sented in Fig. 1(c). The bottom continuous gold layer(200 nm thick) was deposited by electron-beam evapora-tion on a silicon wafer. Square patch patterns were rea-lized by electron-beam lithography on a poly(methylmethacrylate) resist. Then, 290 nm of ZnS was depositedby evaporation, followed by 50 nm of gold. Finally a liftoffis made in dimethyl sulfoxide at 170 °C. Figure 1(d)displays a tilted scanning electron microscope image ofthe structure; the measured widths of the MIM resonatorsare w1 � 1.64 μm, w2 � 1.78 μm, w3 � 1.91 μm, andw4 � 2.07 μm.The far-field reflectivity of the sample was measured
between 6 and 12 μm by a Fourier transform infraredspectrometer (Brucker Vertex 70 v). Figure 2(b) repre-sents the sample absorption spectra at an incidence of13°, and at an azimuthal angle hereafter considered tobe φ � 0°. [The plane of incidence intersects the sample
plane along the x axis as seen in the inset of Fig. 2(b).]Four resonances whose absorption exceeds 80% areclearly observed at λ1r � 7.58 μm, λ2r � 8.17 μm, λ3r �8.9 μm, and λ4r � 9.47 μm. Each resonance is localizedin a specific patch, as confirmed by the computationof absorption maps [see Fig. 2(a)]. Thus each resonatoracts independently and collects more than 80% of the in-coming photons at its resonance wavelength. The absorp-tion volume is very small, less than λ3∕500. Besides, thestructure absorbs more than 70% of the incoming light ona 2.5 μm bandwidth at 8.5 μm.
In the case of one MIM per period, the absorption ofMIM patches is known to be insensitive to the incidenceangle because of the localized nature of the resonance.Furthermore, the patches’ symmetry ensures an indepen-dence of the polarization state of light. Thus, indivi-dual MIMs exhibit an omnidirectional absorption. To as-certain that it is still the case for our multiresonatorstructure, we measured its absorption over the 6–12 μmspectral range, for an angle θ ranging from 13° to 50°, andfor various values of the azimuthal angle φ. Figure 3(a)presents the reflection map as a function of the wave-length and of the angle of incidence θ with φ � 0°.The four resonance wavelengths appear independent ofthe incidence angle. This evidences that each resonanceremains localized in only one patch at a time when chan-ging the incidence angle. Otherwise, the appearance of astrong coupling regime had previously been reportedwhen the wavelength of excitation of the surface plas-mons λSPP ≃ d�1� sin θ� is near to the wavelength of alocalized resonance [19,20]. In our structure, the surfaceplasmons should disturb the Fabry–Perot resonance forincidence θ > 20°, but we have not observed such effect.A detailed study of the coupling between surface plas-mons and the Fabry–Perot-like resonance will be thesubject of a future report.
Figures 3(b)–(d) display measured reflection spectraof the structures for incidence angles of θ � 13°, θ �20°, and θ � 30° and for azimuthal angles of φ � 0°
(red), 45° (green), and 90° (blue). The resonance wave-lengths appear to be as insensitive to the azimuthal angle
Fig. 2. (Color online) (a) Computed mean value of the mag-netic field intensity in the insulator layer of the four-MIM struc-ture at the resonance wavelengths λir for an incident angleθ � 13° and an azimuthal angle φ � 0° in TM polarization. It il-lustrates the localization of the resonances. (b) Measured andcomputed absorption spectra for both polarizations of the mul-tiresonator exhibiting four peaks at λ1r � 7.58 μm, λ2r � 8.17 μm,λ3r � 8.9 μm, and λ4r � 9.47 μm for an incident angle θ � 13° andan azimuthal angle φ � 0°.
Fig. 3. (Color online) (a) Measured reflection as a function ofthe wavelength, and the angle of incidence for an azimuthal an-gle φ � 0°. (b)–(d) Measured reflection spectra for incidenceangles of (b) θ � 13°, (c) θ � 20°, (d) θ � 30° and azimuthalangles φ � 0° (red), φ � 45° (green), and φ � 90° (blue). Theresonance wavelength λir as well as the level of absorptionare quasi-constant.
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as they are to the incidence angle, which confirms theomnidirectional behavior of a patchwork of square MIM.Besides, the level of absorption remains quasi-constantup to relatively large angles (θ � 30°), whatever the azi-muthal angle. The small influence of the azimuthal angleon the level of absorption of the first resonance is theonly sign of the asymmetry of the structure.To conclude, we have shown that a periodic patch-
work of four different square MIM patches leads to anunpolarized, omnidirectional wideband absorption. Eachresonator harvests the photon at its own resonancewavelength and remains almost independent of the threeothers. In the same way, it should be possible to combinenanoantennas with other shapes (e.g., circle or cross) orwith higher index insulators, permitting us to reduceeven further the MIM patch fill factor and making it pos-sible to integrate a higher number of resonators in thesame period. In addition, our structure could be usedas a wideband thermal emitter [21,22]. Moreover, the2D MIM patches can all have the same layer thicknessesand the patchwork can thus be fabricated in a one-stepprocess. We expect that devices integrating several reso-nators in a subwavelength space will have applicationsfor bolometer detectors, and could also be used for otherapplications, such as biochemical sensing [23].
The authors thank Nathalie Bardou and Xavier Lafossefor their help in the clean room and Julien Jaeck for helpin measurements. This work has been supported by theANTARES and INTREPID projects funded by the FrenchNational Research Agency.
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