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Abstract
Metal nanomaterials have been widely used to generate photoacoustic (PA) signals because of their high optical absorption characteristics. However, the PA conversion efficiency of metal nanomaterials is limited by the single-wavelength absorption at the resonant peak. To mitigate this issue, a three-layer ultrathin film containing a thin PDMS layer sandwiched between two ultrathin chromium films is proposed. This kind of film structure can attain high optical absorbance (>80%) through the visible light range (450–850 nm). The optical absorption characteristics can be easily modulated by varying the thickness of the PDMS layer. Under the same excitation condition, the PA signal generated by this film structure is twice that of an only Cr film and three times that of an only Au film. This film structure is easily fabricated and can operate with lasers having different central wavelengths or even white light sources, leading to its applications in many fields, including photoacoustic communications and audio transducers.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photoacoustic (PA) conversion has been applied in many fields, such as biological functional imaging [1–5], ultrasound transducers [6], PA driving [7], and PA spectroscopy [8]. Moreover, PA generation can also be used in PA communications and audio transducers. While ultrasound signals produced by traditional sound generation methods based on the piezoelectric principle are limited by electronic components, PA conversion can generate broadband ultrasonic signals with a high frequency of several hundred megahertz when PA materials are excited by optical pulses. In the aforementioned applications, ultrasonic signals with high intensity are desired. Currently, PA conversion is achieved through the thermoelastic scheme, in which the PA signal amplitude is proportional to the volumetric thermal-expansion coefficient (β), light absorption (A), and incident laser fluence (F) [9]. Nowadays, researchers combine lossy materials with high optical absorption and elastic materials with high thermal expansion to improve the PA signal amplitude. Polydimethylsiloxane (PDMS) is usually used as a thermal expansion material for enhancing PA signal due to its high optical transmittance and high thermal expansion coefficient. Embedding light-absorbing materials in PDMS can significantly increase the thermal expansion coefficient of PA composites without influencing absorption. Common light absorbing materials include carbon black [10], reduced graphene oxide (rGO) [11], carbon nanotubes (CNT) [12], and candle soot nanoparticles [13]. Although carbon-based materials can achieve broadband absorption, their applications are limited due to the complex fabrication processes [14], easy aggregation when mixed with polymers [12], and difficulty of thickness control [15].
Another kind of PA composites is with metal nanomaterials based on localized surface plasmon resonance (LSPR) [16–18]. Owing to this characteristic, metal nanomaterials provide distinguished advantages, such as high optical absorption in the resonance band, tunability on LSPR by controlling structural parameters [19], and high thermal conductivity for improving the efficiency of photothermal conversion [20]. Recently, researches have demonstrated that metal nanomaterials can be used in the enhancement of PA signal, such as gold nanoparticles in different shapes [1,4,5,21–23], gold-PDMS composite [15], and silver nanostructures [24]. However, metal nanostructure-based PA composites can usually only achieve single-wavelength optical absorption and only can be excited by a laser source of a particular wavelength. In PA communications for civil applications with illuminating light sources, broadband optical absorption with efficient PA generation is required. Consequently, a broadband absorptive, stable, and polymer-compatible PA material with low-cost and easy fabrications is desired to facilitate the applications of a PA conversion system.
In this work, we present a kind of lithography-free fabrication of an efficient PA conversion material based on a three-layer ultrathin film containing a thin PDMS layer sandwiched between two ultrathin chromium films. This three-layer structure can achieve high broadband absorption through the visible light range, and it can be excited by both laser with different central wavelengths and broadband white light source. The fabrication process is simple, and the structural parameters can be controlled easily to achieve large-scale uniform production.
2. Theoretical analysis of the Cr-PDMS-Cr structure
The generation of PA signal is shown in the schematic diagram in the Fig. 1. The PA conversion structure is illuminated by the light modulated at a frequency. With the high absorbance in the visible band, the structure is heated and naturally cooled down according to the modulation frequency of the illumination. The temperature of the structure is therefore periodically fluctuated at the modulation frequency. The corresponding thermal expansion for the temperature fluctuation leads to periodic displacements, and acoustic signal can be generated at the solid-fluid boundary.
The schematic diagram of the proposed PA conversion structure is shown in Fig. 2(a), which is comprised of a thin PDMS layer sandwiched between two chromium films. This simple three-layer coating works like an asymmetric Fabry-Perot (FP) cavity [25,26]. The optical reflection and transmission spectra can be analyzed through Maxwell equations and boundary conditions at each interface [27]. Assume that the light is normally incident from the silica substrate side and the thicknesses of the bottom metallic layer, dielectric layer and the top metallic layer are d1, d2 and d3, respectively. The refractive index of the silica, chromium and PDMS are denoted as nSilica, nCr and nPDMS, respectively. For TM polarization wave, the electric field component and magnetic field component can be expressed as
Fig. 2. Schematic configuration and optical characteristics of the Cr-PDMS-Cr three-layer thin film structure. (a) Schematic diagram of the Cr-PDMS-Cr three-layer thin film structure. (b) Simulated electric field amplitude and absorbed power along the light propagation direction as a function of wavelength. (c) Simulated absorptivity with the inset for magnified figure. (d) PA signals as a function of the incident wavelength for the Cr-PDMS-Cr three-layer thin-film structures with different PDMS thicknesses, a 100 nm Cr film, and a 100 nm Au film
According to boundary conditions, the tangential components of the electric field and magnetic field are both continuous at interfaces. Assuming A0 = R, B0 = 1 in silica and A3 = 0 in top Cr layer, the reflection coefficients can be calculated as
The Cr layer with a thickness of 100 nm is used as the reflection layer, which is optically opaque for visible light. On the other hand, the Cr layer with a thickness of 3 nm functions as another reflection surface and simultaneous absorption layer due to Cr′s lossy characteristics. The PDMS layer with variable thickness d (70 nm - 100 nm) is selected for its high transmittance in the visible range and high thermal expansion coefficient. The theoretical model and finite element method (FEM) are both used to analyze the optical characteristic of the Cr-PDMS-Cr structure. The permittivity of Cr is fitted by the Drude model [28], and the refractive index of PDMS is set as 1.4. Simulated electric field amplitude and absorbed power along the light propagation direction as a function of wavelength are shown in Fig. 2(b). The heat generated due to the absorption is mainly confined in the bottom thin Cr film. For comparison, optical absorption spectra corresponding to a 100 nm Cr film and a 100 nm Au film are calculated. The absorption spectra of Cr-PDMS-Cr structures with different PDMS thicknesses, a 100 nm Cr film, and a 100 nm Au film are shown in Fig. 2(c). Cr film has a relatively flat absorption from 56% to 60% within the visible light range. According to Fig. 2(c), the Cr-PDMS-Cr structure can achieve broadband strong optical absorption between 450 nm - 850 nm. With the increase of thickness of the PDMS layer, the absorption peak redshifts.
The PA properties of the Cr-PDMS-Cr structure, a Cr film as well as an Au film are also simulated via the FEM method. The excitation light is set as Gaussian-shaped with a full width at half maxima (FWHM) 2.8 ns and the power density 1000 W/m3. The PA signals corresponding to a 100 nm Cr film, a 100 nm Au film, and the Cr-PDMS-Cr structure under different wavelength excitations are shown in Fig. 2(d). The PA signal amplitude of Cr film is proportional to the optical absorption. In a homogeneous medium, higher optical absorption can lead to higher temperature increases, therefore results in larger thermal expansion and stronger PA signal. However, for the Cr-PDMS-Cr structure, PA signal amplitude is not directly proportional to the optical absorption. At 550 nm, the optical absorption of Cr-PDMS-Cr structure with PDMS thickness of 70 nm, 80 nm, 90 nm, and 100 nm is similar, but the structure with PDMS thickness of 70 nm has the strongest PA signal amplitude followed by the structure with PDMS thickness of 80 nm, 90 nm, and 100 nm. This phenomenon can be explained by the fact that the heat is mainly generated on the bottom Cr layer. For the thinner PDMS layer, heat can be transferred quickly into PDMS and the temperature distribution is more uniform, and therefore larger acceleration and PA signal are generated. Consequently, the PA signal amplitude of the Cr-PDMS-Cr structure can be optimized by manipulating both light absorbance and the thickness of the PDMS layer.
3. Experimental result
The Cr-PDMS-Cr structure includes the 3-nm bottom Cr layer, the middle PDMS layer, and the top 100-nm Cr layer. Due to the need for optical and PA characterization, silica quartz glass with high transmittance was selected as the substrate. The first Cr layer is deposited by the magnetron sputtering. Thereafter, the PDMS is mixed with a curing agent (PDMS: curing agent = 10:1) and then diluted by n-hexane. PDMS solution is spin-coated for ultrathin PDMS layers, and the thickness varies with the dilution ratio of PDMS and n-hexane. When the ratio of n-hexane in PDMS is higher, the PDMS layer after spin coating becomes thinner. Samples with different PDMS thicknesses are shown in Fig. 3(a). The marker on the upper left corner indicates the thicknesses of the PDMS layers with the dilution ratio of 1:90,1:120,1:160, and 1:200, respectively [29]. Thereafter, PDMS solution is spin-coated with the rotation rate of 6000 r/min for 120 s on the first Cr layer. After spin coating, PDMS films are solidified under 60°C for 2 hours. In the next step, the ultrathin PDMS layer is treated by oxygen plasma to avoid demoulding problem between the PDMS layer and the 100 nm top Cr layer. Finally, the top Cr layer is deposited by the magnetron sputtering.
Fig. 3. (a) The optical images of the Cr-PDMS-Cr structures corresponding to different PDMS dilution ratios. The marker on the upper left corner indicates the dilution ratio of PDMS and n-hexane. (b) Experimental setup for the PA signal detection experiment. (c) Absorptivity and (d) PA signals as a function of the incident wavelength for the Cr-PDMS-Cr three-layer thin-film structures with different PDMS thicknesses, a 100 nm Cr film, and a 100 nm Au film.
For the optical characterization, an optical microscope (Nikon ECLIPSE 80i) and a spectrometer (Ocean Optics QE65 Pro) are utilized to measure the reflection and transmission spectra. According to the law of energy conservation, $\textrm{A}\; = \; 1 - \textrm{R} - \textrm{T}$, where R represents the reflectivity, T represents the transmittance, and A represents the absorption. Therefore, the absorption of the sample can be obtained by measuring the reflectivity and transmittance of the sample. When measuring the reflectivity, the broad spectrum light is converged on to the sample surface through an objective lens (Nikon CFILU Plan Epi ELWD, 20X). The reflected light from the sample is then collected by the same objective lens and transmitted to the spectrometer and CCD Camera (Nikon DIGITAL CAMERA HEAD DS-Fi1). While measuring the transmittance, the transmitted light from the broadband light source below the sample is collected through another objective lens and guided to the spectrometer and CCD Camera. The absorption spectra of different samples can be calculated by $\textrm{A}\; = \; 1 - \textrm{R} - \textrm{T}$. Optical absorption spectra for the Cr-PDMS-Cr structures with different PDMS film thicknesses, a 100 nm Cr film, and a 100 nm Au film are recorded by a microscope fitted with a spectrometer and are illustrated in Fig. 3(c). The Cr-PDMS(73 nm)-Cr structure shows the highest absorption. Cr film has a relatively flat absorption within the visible light range. On the other hand, the absorption of Au film decreases significantly above 500 nm.
The experimental setup is shown in Fig. 3(b). The PA signal needs to be excited by pulsed light and detected by a sound detector. In this work, the working current of the LDs is modulated by a pulse function arbitrary noise generator (Agilent Technologies, Pulse Function Arbitrary Generator, 81150A). The modulated current is rectangular with a pulse width of 10 µs and a repetition frequency of 7250 Hz. The used laser diodes are 450 nm (Osram PLT 450B, 80 mW), 505 nm (Sharp GH05035A2G, 35 mW), 660 nm (Mitsubishi ML101U29, 130 mW), 780 nm (Sanyo 7140–211n, 80 mW), and 830 nm (Sharp GH0832BA2A, 210 mW). The laser diode is installed on a temperature-controlled mount (Thorlabs, LDM56) and controlled by a current temperature controller (Thorlabs, ITC100D). The incident light is converged through a converging lens (Daheng Optics, GCL-0101B) and focused on the sample surface. The microphone (Takstar, CM-63, 30Hz-20kHz) is placed behind the sample as near as possible without touching it. The acoustic signal is firstly recorded for 200 s by the microphone and then amplified by a preamplifier (Takstar, MA-1C). MATLAB software is used to analyze the PA signal by Fourier transforming the acoustic signal from the time domain to the frequency domain. The amplitude of PA signal can be obtained by monitoring the signal at the light repetition rate (7250 Hz). To minimize the surrounding noise, PA signal amplitude for each sample is determined by the average value of three measurements at different positions on the sample.
The PA signal amplitude is detected at the laser repetition frequency, and the detected signal amplitudes are compared for different incident light wavelengths. As LDs of different wavelengths have different response characteristics, the output powers of different LDs are different even with the same driving current. According to the above optical measurement results, the Cr film has relatively flat absorption. Therefore, the PA signal of a single-layer Cr is used as a reference to normalize the PA signals of other samples at each wavelength. The PA signals of different samples are shown in Fig. 3(d).
According to Fig. 3(d), the PA signal amplitude corresponding to the Cr-PDMS-Cr structure is higher than that of the Cr film. The PA signal amplitude of the gold film is weaker than that of the Cr film and proportional to its optical absorption. When the optical absorption is similar, the Cr-PDMS-Cr structure with a thinner thickness produces a higher PA signal. For example, at 780 nm and 830 nm, the absorption of Cr-PDMS(104 nm)-Cr and Cr-PDMS(68 nm)-Cr are similar, and absorption of Cr-PDMS(85 nm)-Cr and Cr-PDMS(73 nm)-Cr are similar. The PA signals of Cr-PDMS(68 nm)-Cr are stronger than that of Cr-PDMS(104 nm)-Cr at the corresponding wavelengths. Similarly, the PA signals of Cr-PDMS(85 nm) are stronger than that of Cr-PDMS(73 nm)-Cr at the corresponding wavelengths. This is consistent with the simulation results that the photoacoustic signal generated by the sample with a thinner PDMS layer is stronger under the condition of similar optical absorption.
As the Cr-PDMS-Cr structure can achieve broadband absorption within the visible range, it can absorb more energy when excited by a broadband light source. In order to explore its PA characteristics under broadband light excitation, a white light LED is used to excite the PA signal of different samples. The white light LED (QYSSM, 3W) is powered by a laboratory stabilized power supply unit and modulated by a pulse function arbitrary noise generator (Agilent Technologies, Pulse Function Arbitrary Generator, 81150A). To minimize the noise of electrical components, the power supply unit and the pulse function arbitrary noise generator are placed in a different room.
Figure 4(a) shows the absorption power of different samples, which is obtained by multiplying the absorptivity with the light source spectrum. According to the generated PA signals indicated in Fig. 4(b), it is observed that the Cr-PDMS-Cr structure can improve the PA signal significantly. The PA amplitude can also be controlled by varying the thickness of the PDMS layer. In this work, the sample fabricated by the PDMS layer with a thickness of 73 nm shows the highest absorption power and PA signal. Therefore, it can be concluded that the Cr-PDMS-Cr structure can achieve high absorption within the visible light range and can be used to improve PA signal intensity.
Fig. 4. (a) The absorption power and (b) PA signal of different samples. S1, S2, S3, S4, Cr, and Au represent Cr-PDMS(104 nm)-Cr, Cr-PDMS(85 nm)-Cr, Cr-PDMS(73 nm)-Cr, Cr-PDMS(68 nm)-Cr, 100 nm Cr, and 100 nm Au, respectively. PA signals are determined by the average value of three measurements and the error bar represents the maximum and minimum values.
4. Conclusion and discussion
High-frequency ultrasonic signals generated by PA conversion can be applied in many fields. In this work, we present a kind of broadband PA conversion nanostructure based on a three-layer ultrathin film containing a thin PDMS layer sandwiched between two ultrathin chromium films. The experiments show that this nanostructure can achieve high broadband absorption across the visible light range. Compared with metal films, the three-layer film structure shows a significant enhancement in the PA signal both under the excitation of laser diodes with different operation wavelengths and broadband white light. Thereby, it opens up opportunities for PA communications for civil use with a common illuminating light source at a low cost.
Fabrication of this film structure is lithography-free and realized by the magnetron sputtering and spin-coating. With simple structure and easy fabrication, it can be applied to many fields, including photoacoustic communications and audio transducers. Besides, the broadband optical absorption makes it work with lasers with different central wavelengths. It can also be excited by a modulated broadband white light source and generate a PA signal with a high amplitude.
Funding
National Key Research and Development Program of China (2017YFE0100200); National Natural Science Foundation of China (61950410608, 61775194, 61975181).
Disclosures
The authors declare no conflicts of interest.
Supplemental document
See Supplement 1 for supporting content.
References
1. N. Yan, X. Wang, L. Lin, T. Song, P. Sun, H. Tian, H. Liang, and X. Chen, “Gold Nanorods Electrostatically Binding Nucleic Acid Probe for In Vivo MicroRNA Amplified Detection and Photoacoustic Imaging-Guided Photothermal Therapy,” Adv. Funct. Mater. 28(22), 1800490 (2018). [CrossRef]
2. N. Wu, Y. Tian, X. Zou, V. Silva, A. Chery, and X. Wang, “High-efficiency optical ultrasound generation using one-pot synthesized polydimethylsiloxane-gold nanoparticle nanocomposite,” J. Opt. Soc. Am. B 29(8), 2016–2020 (2012). [CrossRef]
3. J. Zhong, L. Wen, S. Yang, L. Xiang, Q. Chen, and D. Xing, “Imaging-guided high-efficient photoacoustic tumor therapy with targeting gold nanorods,” Nanomedicine 11(6), 1499–1509 (2015). [CrossRef]
4. R. Liang, J. Xie, J. Li, K. Wang, L. Liu, Y. Gao, M. Hussain, G. Shen, J. Zhu, and J. Tao, “Liposomes-coated gold nanocages with antigens and adjuvants targeted delivery to dendritic cells for enhancing antitumor immune response,” Biomaterials 149, 41–50 (2017). [CrossRef]
5. J. S. Wi, J. Park, H. Kang, D. Jung, S. W. Lee, and T. G. Lee, “Stacked Gold Nanodisks for Bimodal Photoacoustic and Optical Coherence Imaging,” ACS Nano 11(6), 6225–6232 (2017). [CrossRef]
6. Y. Hou, J.-S. Kim, S. Ashkenazi, S.-W. Huang, L. J. Guo, and M. O’Donnell, “Broadband all-optical ultrasound transducers,” Appl. Phys. Lett. 91(7), 073507 (2007). [CrossRef]
7. Y. Yuan, C. Gu, S. Huang, L. Song, and F. Fang, “Advances on studying optical forces: optical manipulation, optical cooling and light induced dynamics,” J. Phys. D: Appl. Phys. 53(28), 283001 (2020). [CrossRef]
8. D. V. Bageshwar, A. S. Pawar, V. V. Khanvilkar, and V. J. Kadam, “Photoacoustic spectroscopy and its applications–a tutorial review,” Eurasian Journal of Analytical Chemistry 5, 187–203 (2010).
9. T. Lee, H. W. Baac, Q. Li, and L. J. Guo, “Efficient Photoacoustic Conversion in Optical Nanomaterials and Composites,” Adv. Opt. Mater. 6(24), 1800491 (2018). [CrossRef]
10. T. Buma, M. Spisar, and M. O’Donnell, “High-frequency ultrasound array element using thermoelastic expansion in an elastomeric film,” Appl. Phys. Lett. 79(4), 548–550 (2001). [CrossRef]
11. S. Hwan Lee, M.-A. Park, J. J. Yoh, H. Song, E. Yun Jang, Y. Hyup Kim, S. Kang, and Y. Seop Yoon, “Reduced graphene oxide coated thin aluminum film as an optoacoustic transmitter for high pressure and high frequency ultrasound generation,” Appl. Phys. Lett. 101(24), 241909 (2012). [CrossRef]
12. R. J. Colchester, C. A. Mosse, D. S. Bhachu, J. C. Bear, C. J. Carmalt, I. P. Parkin, B. E. Treeby, I. Papakonstantinou, and A. E. Desjardins, “Laser-generated ultrasound with optical fibres using functionalised carbon nanotube composite coatings,” Appl. Phys. Lett. 104(17), 173502 (2014). [CrossRef]
13. W.-Y. Chang, W. Huang, J. Kim, S. Li, and X. Jiang, “Candle soot nanoparticles-polydimethylsiloxane composites for laser ultrasound transducers,” Appl. Phys. Lett. 107(16), 161903 (2015). [CrossRef]
14. H. Won Baac, J. G. Ok, H. J. Park, T. Ling, S. L. Chen, A. J. Hart, and L. J. Guo, “Carbon nanotube composite optoacoustic transmitters for strong and high frequency ultrasound generation,” Appl. Phys. Lett. 97(23), 234104 (2010). [CrossRef]
15. Y. Hou, J.-S. Kim, S. Ashkenazi, M. O’Donnell, and L. J. Guo, “Optical generation of high frequency ultrasound using two-dimensional gold nanostructure,” Appl. Phys. Lett. 89(9), 093901 (2006). [CrossRef]
16. K. A. Willets and R. P. Van Duyne, “Localized surface plasmon resonance spectroscopy and sensing,” Annu. Rev. Phys. Chem. 58(1), 267–297 (2007). [CrossRef]
17. H. Luo, Q. Li, K. Du, Z. Xu, H. Zhu, D. Liu, L. Cai, P. Ghosh, and M. Qiu, “An ultra-thin colored textile with simultaneous solar and passive heating abilities,” Nano Energy 65, 103998 (2019). [CrossRef]
18. W. Wang, Y. Qu, K. Du, S. Bai, J. Tian, M. Pan, H. Ye, M. Qiu, and Q. Li, “Broadband optical absorption based on single-sized metal-dielectric-metal plasmonic nanostructures with high-ɛ″ metals,” Appl. Phys. Lett. 110(10), 101101 (2017). [CrossRef]
19. K. L. Kelly, E. Coronado, L. L. Zhao, and G. C. Schatz, “The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment,” J. Phys. Chem. B 107(3), 668–677 (2003). [CrossRef]
20. Y. Yan, L. Liu, Z. Cai, J. Xu, Z. Xu, D. Zhang, and X. Hu, “Plasmonic nanoparticles tuned thermal sensitive photonic polymer for biomimetic chameleon,” Sci. Rep. 6(1), 31328 (2016). [CrossRef]
21. C. W. Wei, M. Lombardo, K. Larson-Smith, I. Pelivanov, C. Perez, J. Xia, T. Matula, D. Pozzo, and M. O’Donnell, “Nonlinear contrast enhancement in photoacoustic molecular imaging with gold nanosphere encapsulated nanoemulsions,” Appl. Phys. Lett. 104(3), 033701 (2014). [CrossRef]
22. J. Song, X. Yang, Z. Yang, L. Lin, Y. Liu, Z. Zhou, Z. Shen, G. Yu, Y. Dai, O. Jacobson, J. Munasinghe, B. Yung, G. J. Teng, and X. Chen, “Rational Design of Branched Nanoporous Gold Nanoshells with Enhanced Physico-Optical Properties for Optical Imaging and Cancer Therapy,” ACS Nano 11(6), 6102–6113 (2017). [CrossRef]
23. W. Wang, C. Hao, M. Sun, L. Xu, C. Xu, and H. Kuang, “Spiky Fe3O4@Au Supraparticles for Multimodal In Vivo Imaging,” Adv. Funct. Mater. 28, 1800310 (2018). [CrossRef]
24. S. G. Park, S. B. Yang, M. S. Ahn, Y. J. Oh, Y. T. Kim, and K. H. Jeong, “Plasmon enhanced photoacoustic generation from volumetric electromagnetic hotspots,” Nanoscale 8(2), 757–761 (2016). [CrossRef]
25. Z. Li, E. Palacios, S. Butun, H. Kocer, and K. Aydin, “Omnidirectional, broadband light absorption using large-area, ultrathin lossy metallic film coatings,” Sci. Rep. 5(1), 15137 (2015). [CrossRef]
26. Z. Xu, Q. Li, K. Du, S. Long, Y. Yang, X. Cao, H. Luo, H. Zhu, P. Ghosh, W. Shen, and M. Qiu, “Spatially resolved dynamically reconfigurable multilevel control of thermal emission,” Laser Photonics Rev. 14(1), 1900162 (2020). [CrossRef]
27. S. Shu, Z. Li, and Y. Y. Li, “Triple-layer Fabry-Perot absorber with near-perfect absorption in visible and near-infrared regime,” Opt. Express 21(21), 25307–25315 (2013). [CrossRef]
28. C. Wu, B. Neuner III, G. Shvets, J. John, A. Milder, B. Zollars, and S. Savoy, “Large-area wide-angle spectrally selective plasmonic absorber,” Phys. Rev. B 84(7), 075102 (2011). [CrossRef]
29. A. L. Thangawng, R. S. Ruoff, M. A. Swartz, and M. R. Glucksberg, “An ultra-thin PDMS membrane as a bio/micro-nano interface: fabrication and characterization,” Biomed. Microdevices 9(4), 587–595 (2007). [CrossRef]
