Resonance properties of Ag-ZnO nanostructures at terahertz frequencies John E. Sanchez,1 Ramón Díaz de León,2 Fernando Mendoza-Santoyo,1 Gabriel González,3 Miguel José-Yacaman,1 Arturo Ponce,1 and Francisco Javier González3,* 1
Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio 78249, USA 2 Instituto Tecnológico de San Luis Potosí, San Luis Potosi 78437, Mexico 3 Coordinación para la Innovación y la Aplicación de la Ciencia y la Tecnología, Universidad Autónoma de San Luis Potosí, San Luis Potosí, 78210, Mexico * [email protected]
Abstract: Nanoantennas have been fabricated by scaling down traditional antenna designs using nanolithographic techniques and testing them at different optical wavelengths, these particular nanoantennas have shown responses in a broad range of frequencies going from visible wavelengths to the range of the terahertz. Some self-assembled nanostructures exist that exhibit similar shapes and properties to those of traditional antenna structures. In this work the emission and absorption properties of selfassembled nanostructures made of zinc oxide nanorods on silver nanowires, which resemble traditional dipole antennas, were measured and simulated in order to test their antenna performance. These structures show resonant properties in the 10-120 THz range, with the main resonance at 60 THz. The radiation pattern of these nanostructures was also obtained by numerical simulations, and it is shown that it can be tailored to increase or decrease its directivity as a function of the location of the energy source of excitation. Experimental measurements were performed by Raman spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR) in order to show existing vibrational frequencies at the resonant frequencies of the nanostructures, measurements were made from ~9 to 103 THz and the results were in agreement with the simulations. These characteristics make these metal-semiconductor Ag/ZnO nanostructures useful as self-assembled nanoantennas in applications such as terahertz spectroscopy and sensing at terahertz frequencies. ©2015 Optical Society of America OCIS codes: (040.2235) Far infrared or terahertz; (160.4236) Nanomaterials.
References and links 1. 2. 3. 4. 5. 6. 7.
T. H. Taminiau, F. D. Stefani, F. B. Segerink, and N. F. van Hulst, “Optical antennas direct single-molecule emission,” Nat. Photonics 2(4), 234–237 (2008). F. J. González and G. D. Boreman, “Comparison of dipole, bowtie, spiral and log-periodic IR antennas,” Infrared Phys. Technol. 46(5), 418–428 (2005). J. E. Sanchez, F. Mendoza-Santoyo, J. Cantu-Valle, J. Velazquez-Salazar, M. José Yacaman, F. J. González, R. Diaz de Leon, and A. Ponce, “Electric radiation mapping of silver/zinc oxide nanoantennas by using electron holography,” J. Appl. Phys. 117(3), 034306 (2015). S. Kim, J. Jin, Y.-J. Kim, I.-Y. Park, Y. Kim, and S.-W. Kim, “High-harmonic generation by resonant plasmon field enhancement,” Nature 453(7196), 757–760 (2008). R. M. Bakker, V. P. Drachev, Z. Liu, H.-K. Yuan, R. H. Pedersen, A. Boltasseva, J. Chen, J. Irudayaraj, A. V. Kildishev, and V. M. Shalaev, “Nanoantenna array-induced fluorescence enhancement and reduced lifetimes,” New J. Phys. 10(12), 125022 (2008). P. Anger, P. Bharadwaj, and L. Novotny, “Enhancement and quenching of single-molecule fluorescence,” Phys. Rev. Lett. 96(11), 113002 (2006). K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S. Feld, “Single molecule detection using surface-enhanced Raman scattering (SERS),” Phys. Rev. Lett. 78(9), 1667–1670 (1997).
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25111
8. 9. 10. 11. 12. 13. 14. 15.
G. Das, F. Mecarini, F. Gentile, F. De Angelis, H. Mohan Kumar, P. Candeloro, C. Liberale, G. Cuda, and E. Di Fabrizio, “Nano-patterned SERS substrate: application for protein analysis vs. temperature,” Biosens. Bioelectron. 24(6), 1693–1699 (2009). F. Neubrech, A. Pucci, T. W. Cornelius, S. Karim, A. García-Etxarri, and J. Aizpurua, “Resonant plasmonic and vibrational coupling in a tailored nanoantenna for infrared detection,” Phys. Rev. Lett. 101(15), 157403 (2008). R. Adato, A. A. Yanik, J. J. Amsden, D. L. Kaplan, F. G. Omenetto, M. K. Hong, S. Erramilli, and H. Altug, “Ultra-sensitive vibrational spectroscopy of protein monolayers with plasmonic nanoantenna arrays,” Proc. Natl. Acad. Sci. USA 106(46), 19227–19232 (2009). L. Razzari, A. Toma, M. Clerici, M. Shalaby, G. Das, C. Liberale, M. Chirumamilla, R. P. Zaccaria, F. De Angelis, M. Peccianti, R. Morandotti, and E. Di Fabrizio, “Terahertz dipole nanoantenna arrays: Resonance characteristics,” Plasmonics 8(1), 133–138 (2013). G. Pellegrini, P. Mazzoldi, and G. Mattei, “Asymmetric plasmonic nanoshells as subwavelength directional nanoantennas and color nanorouters: A multipole interference approach,” J. Phys. Chem. C 116(40), 21536 (2012). E. R. Encina and E. A. Coronado, “Plasmonic nanoantennas: Angular scattering properties of multipole resonances in noble metal nanorods,” J. Phys. Chem. C 112(26), 9586–9594 (2008). H. Aouani, M. Rahmani, H. Sipova, V. Torres, K. Hegnerova, M. Beruete, J. Homola, M. Hong, M. NavarroCía, and S. A. Maier, “Plasmonic nanoantennas for multispectral surface-enhanced spectroscopies,” J. Phys. Chem. C 117(36), 18620–18626 (2013). C. Fumeaux, M. A. Gritz, I. Codreanu, W. L. Schaich, F. J. González, and G. D. Boreman, “Measurement of the resonant lengths of infrared dipole antennas,” Infrared Phys. Technol. 41(5), 271–281 (2000).
1. Introduction Antennas have been used for more than a century to control the emission and collection of radio and microwave radiation. Traditional antenna designs have been scaled down and tested at terahertz and optical wavelengths, where their dimensions are proportional to the desired resonant wavelength [1] e.g., in the case of optical wavelengths is less than 1 μm. In order to fabricate these structures at the nanoscale level, different methods such as electron-beam lithography and microwave synthesis have been used with a precise control of the assembling between metal and semiconductor materials [2, 3]. Additionally, optical nanoantennas have the ability to focus light beyond the diffraction limit, which can enhance different optical processes such as high-harmonic generation [4], fluorescence [5, 6], Raman scattering [7, 8] and infrared absorption [9, 10]. Nanoantennas at terahertz frequencies are of interest since they can improve ultra-sensitive terahertz spectroscopy and antenna-enhanced terahertz nonlinear experiments by combining both strongly localized field enhancement with a spatially extended interaction of the incoming radiation [11–13]. It is well known that metallic nanoparticles support resonant plasmonic modes at optical wavelengths making them natural optical antennas [1, 14], also some self-assembled nanostructures have similar shapes to traditional antenna structures and could be used as emitting and receiving devices at optical and terahertz wavelengths, avoiding the need of nanolithography to fabricate them. Structures like these can be used to control the absorption and emission at the nanometer scale [2]. In a previous work, experimental evidence of the radiation pattern of self assembled nanoantennas was presented by introducing an external electrical signal at different frequencies in the range of megahertz under an operando mode using off-axis electron holography in real time within a transmission electron microscope [3]. In this work, the radiation properties of Ag/ZnO nanoantennas are determined by computer simulations and compared with Raman and Fourier Transform Infrared (FTIR) measurements in the range of terahertz, in order to demonstrate their potential use in optoelectronic devices. 2. Method The structure shown in Fig. 1 corresponds to a nanoantenna assembled with Zinc Oxide (ZnO) nanorods on silver nanowires (Ag-NWs) synthetized by a microwave method [3]. In order to study the radiation, emission and absorption properties associated with the optical response of these nanostructures at optical wavelength excitation, the geometries of Ag/ZnO nanostructures were simulated using a commercial software based on the finite element method (COMSOL Multiphysics). The Finite Element Method (FEM) is used to solve partial
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25112
differential equations under diverse boundary conditions. Two different scenarios were simulated, the first one consisted in considering the nanostructures as receivers, in this case a linearly polarized plane wave with frequencies going from 10 THz to 300 THz (1 μm a 30 μm in wavelength), and a 1 V/m E-field amplitude and polarization set parallel to one of the ZnO nanorods was simulated, the plane wave was launched perpendicular to one of the structure's star-shaped faces and the E-field pattern near the nanostructure was plotted.
Fig. 1. Nanostructures made of Ag nanowires with ZnO nanorods.
A second set of simulations consisted in considering the nanostructure as an electromagnetic emitter, for this simulation a sinusoidal voltage source was placed at one end of the structure over the tip of the silver nanowire and the electric field emitted due to this source was plotted for different source frequencies ranging from 10 to 300 THz. 3. Results The first set of simulations was performed by calculating the Electric Field generated near one end of the nanostructure due to a plane wave that impinges on that particular end of the Ag/ZnO nanostructure, these simulations were performed at different frequencies. The induced electric field was measured by integrating the electric field over the cross-section of the star-shaped end of the nanostructure. Matched boundary conditions were used in the FEM simulations and tetrahedral elements were used to discretize the computational domain. These simulations give the resonant properties of the star-shaped structure and give insight on how the size of the ZnO nanorods can influence the resonance of the whole structure. Figure 2 shows the results of this first set of simulations where we can see the E-field near the starshaped end of the nanostructure for different incident wave frequencies. The polarization of the incident wave is along the x-axis, each graph at a different incident wave frequency has an arrow that indicates the polarization of the incident wave, as it is evident in the graph the arms parallel to the polarization of the incident wave are the ones that have a higher response at the main resonance.
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25113
Fig. 2. E-field distribution on a star-shaped Ag-ZnO structure due to electromagnetic irradiation of different frequencies, the arrow on each figure illustrates the polarization of the incident Electric Field.
Figure 3 shows the normalized induced electric field over the star-shaped nanostructure as a function of frequency. The induced electric field was calculated by integrating the electric field across the antenna plane. From the figure it can be seen that there is a main resonance at 60 THz (5 μm in wavelength) and a secondary resonance at 200 THz (1.5 μm in wavelength). Since the size of each ZnO rod is around 0.8 μm and the overall height of the structure is around 1.6 μm, classical antenna theory would predict a main resonance at around 3.2 μm. Furthermore, classical antenna theory applies for low frequencies and highly conductive materials, in this case, the rods are made of a semiconductor material, which increases the conduction losses and shifts the resonance to larger wavelengths [15].
Fig. 3. Normalized Electric Field induced by a plane wave as a function of the plane-wave's frequency.
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25114
Figure 4(a)-4(c) shows the radiation pattern of the whole silver-zinc oxide nanostructure when the structure was excited with a current source at the bottom end of the structure. In this emission case since the silver nanowire is larger than the ZnO star-like structure the whole structure first resonates at lower frequencies, from Fig. 4(a) it can be seen how the silver nanowire dominates in the emission of electromagnetic radiation giving a dipole-like radiation pattern at 10 THz, at higher frequencies the zinc oxide nanorods become dominant shifting the directivity of the radiation pattern, as can be seen in Fig. 4(c). Figure 4(d)-4(f) shows the radiation pattern from the tip of the nanostructure for the same emission case, with a current source being located at the bottom of the structure, as in the previous figures the radiation pattern indicates that the nanorod structures resonate at higher frequencies, while at lower frequencies the main resonance is consistent with the one of a single dipole.
Fig. 4. (a)-(c) shows the radiation pattern of the whole Ag-ZnO nanostructure at three different frequencies, (d)-(f) shows the radiation pattern of the tip of the Ag-ZnO nanostructure, showing the dominant effect of the ZnO nanorods at higher frequencies.
In order to better understand the effect of an internal current source in these nanostructures, the radiation characteristics of the Ag/ZnO nanostructure were obtained for the case where the sinusoidal excitation is located inside the Ag nanowire. In this case the radiation pattern shows that the location of the source affects its directivity. For the specific case where the source is inside the nanowire the nanorods located behind the source reflects the electromagnetic energy and increases the directivity of the structure, these radiation characteristics resembles a traditional Yagi-Uda antenna. Experimental measurements in the Ag-NWs/ZnO nanoantennas were obtained by Raman spectroscopy and FTIR. Raman spectroscopy exhibits vibrational frequencies for E2 (High) stretching vibrational mode (434 cm−1) which correspond to a vibrational frequency of 13 THz. The spectrum is shown in Fig. 5(a), which is correlated with the theoretical Raman shift of ZnO (Fig. 5(b)). This frequency, in the infrared regime, is associated with the in-plane Zn – O (as shown the in the inset of the Fig. 5(c) vibrations stimulated with a polarized excitation of λ = 785 nm. This frequency activates the internal vibrational modes on the ZnO molecule.
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25115
During the process, the quantum energy absorbed by the molecule is released and a phonon is emitted at a specific frequency. In this way, the spectrum collected corresponds to the shift of the incident radiation wave number ( v ). The transversal (TO) and longitudinal (LO) optical modes around 325 and 680 cm−1 correspond to the out-of-plane Zn – O vibrations. The transversal and longitudinal vibration modes in the Raman spectrum correspond to 17 and 19 THz, respectively. The experimental spectrum shows two additional vibrational modes at 1050 and 1100 cm−1 (31.7 and 34 THz), which do not appear in the theoretical spectrum of ZnO, however it matches an increase in response at 30THz obtained by numerical simulations (Fig. 3). These two bands represent the scatter enhancement Raman spectrum (SERS), which is generated by the collective resonant vibrations of the whole system (Ag/ZnO). The frequencies above 1000 cm−1 are associated with the highest vibrational frequencies of the nanoantennas according to the numerical simulation described above. The corresponding frequencies of the nanoantennas related to the vibrational energies are included in the Fig. 5.
Fig. 5. Raman spectrum of Ag-ZnO nanostructures. The E2h-I and E2High vibrational modes around 320 and 434 cm-1 correspond to a frequency of 9.7 and 13 THz, respectively. Additionally, the enhance optical overtones modes around 1057 and 1147 cm-1 have frequencies 31.7 and 34 THz. The inset table on the figure shows the corresponding values of the frequencies corresponding to each vibrational mode.
As a complementary technique the samples were measured by FTIR to show the absorption spectrum of the nanoantennas in the infrared region. FTIR provides a higher response of the nanoantennas measured by Raman spectroscopy. Figure 6(a) shows the absorbance spectrum and their corresponding transmittance spectrum (Fig. 6(b)). The frequencies registered in the spectra around 550 cm−1 and 1600 cm−1, correspond to vibrational frequencies of 16.5 and 48 THz, respectively. In fact, the spectrum in Fig. 6(a) shows a characteristic absorption broad bands around 3450 and 1570 cm−1, with frequencies 103.5 and 47.1 THz, respectively. It is worth mentioning that the net effect on the resonance of the whole nanostructure in the infrared region matches closely a characteristic resonant frequency around 50 THz when compared with the calculated frequencies found by using the numerical methods between 10 and 120 THz (Fig. 3). Furthermore, from table inset on Fig.
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25116
6(b) it is identified vibrational frequencies (marked with the symbol *) around 27 THz that also matches closely with the lower region of the Normalized response of the electric in function of the frequency in Fig. 3
Fig. 6. FTIR spectra for Ag-ZnO nanoantennas.
4. Conclusions The resonance at terahertz frequencies of self-assembled nanostructures (Ag/ZnO) working as nanoantennas have been demonstrated by experimental measurements and simulations. Simulations show a main resonance at 60 THz, which has been identified in the ZnO nanorods as well as the effect of a frequency variation due to the influence of the silver nanowires. We found that Ag nanowires dominate the radiation pattern of the whole structure at lower frequencies, at higher frequencies the ZnO nanorods become dominant and modify the radiation pattern of the whole structure. Two experimental techniques, Raman and FTIR, were used to compare the resonances of the nanoantennas, which are in agreement with the prediction of the simulated nanoantennas. In both techniques the frequencies are clearly defined and two different ranges can be detected, Raman from 9 to 34 THz and FTIR from 16 to 103 THz. The nanoantennas measured and simulated are promising candidates for novel application for emitting and receiving nano-systems in the terahertz range. Acknowledgments This work was supported by the program “Cátedras CONACYT”, by project 32 of “Centro Mexicano de Innovación en Energía Solar” from Fondo Sectorial CONACYT-Secretaría de Energía-Sustentabilidad Energética and by the National Laboratory program from CONACYT through the Terahertz Science and Technology National Lab (LANCYTT). The authors would like to acknowledge the NSF PREM # DMR 0934218. J.E. Sanchez thanks to COLCIENCIAS for the Scholarship Francisco Jose de Caldas. The microscopy work was supported by the NIH RCMI Nanotechnology and Human Health Core (G12MD007591), Department of Defense #64756-RT-REP and the Welch Foundation grant award # AX-1615.
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25117
Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio 78249, USA 2 Instituto Tecnológico de San Luis Potosí, San Luis Potosi 78437, Mexico 3 Coordinación para la Innovación y la Aplicación de la Ciencia y la Tecnología, Universidad Autónoma de San Luis Potosí, San Luis Potosí, 78210, Mexico * [email protected]
Abstract: Nanoantennas have been fabricated by scaling down traditional antenna designs using nanolithographic techniques and testing them at different optical wavelengths, these particular nanoantennas have shown responses in a broad range of frequencies going from visible wavelengths to the range of the terahertz. Some self-assembled nanostructures exist that exhibit similar shapes and properties to those of traditional antenna structures. In this work the emission and absorption properties of selfassembled nanostructures made of zinc oxide nanorods on silver nanowires, which resemble traditional dipole antennas, were measured and simulated in order to test their antenna performance. These structures show resonant properties in the 10-120 THz range, with the main resonance at 60 THz. The radiation pattern of these nanostructures was also obtained by numerical simulations, and it is shown that it can be tailored to increase or decrease its directivity as a function of the location of the energy source of excitation. Experimental measurements were performed by Raman spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR) in order to show existing vibrational frequencies at the resonant frequencies of the nanostructures, measurements were made from ~9 to 103 THz and the results were in agreement with the simulations. These characteristics make these metal-semiconductor Ag/ZnO nanostructures useful as self-assembled nanoantennas in applications such as terahertz spectroscopy and sensing at terahertz frequencies. ©2015 Optical Society of America OCIS codes: (040.2235) Far infrared or terahertz; (160.4236) Nanomaterials.
References and links 1. 2. 3. 4. 5. 6. 7.
T. H. Taminiau, F. D. Stefani, F. B. Segerink, and N. F. van Hulst, “Optical antennas direct single-molecule emission,” Nat. Photonics 2(4), 234–237 (2008). F. J. González and G. D. Boreman, “Comparison of dipole, bowtie, spiral and log-periodic IR antennas,” Infrared Phys. Technol. 46(5), 418–428 (2005). J. E. Sanchez, F. Mendoza-Santoyo, J. Cantu-Valle, J. Velazquez-Salazar, M. José Yacaman, F. J. González, R. Diaz de Leon, and A. Ponce, “Electric radiation mapping of silver/zinc oxide nanoantennas by using electron holography,” J. Appl. Phys. 117(3), 034306 (2015). S. Kim, J. Jin, Y.-J. Kim, I.-Y. Park, Y. Kim, and S.-W. Kim, “High-harmonic generation by resonant plasmon field enhancement,” Nature 453(7196), 757–760 (2008). R. M. Bakker, V. P. Drachev, Z. Liu, H.-K. Yuan, R. H. Pedersen, A. Boltasseva, J. Chen, J. Irudayaraj, A. V. Kildishev, and V. M. Shalaev, “Nanoantenna array-induced fluorescence enhancement and reduced lifetimes,” New J. Phys. 10(12), 125022 (2008). P. Anger, P. Bharadwaj, and L. Novotny, “Enhancement and quenching of single-molecule fluorescence,” Phys. Rev. Lett. 96(11), 113002 (2006). K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S. Feld, “Single molecule detection using surface-enhanced Raman scattering (SERS),” Phys. Rev. Lett. 78(9), 1667–1670 (1997).
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25111
8. 9. 10. 11. 12. 13. 14. 15.
G. Das, F. Mecarini, F. Gentile, F. De Angelis, H. Mohan Kumar, P. Candeloro, C. Liberale, G. Cuda, and E. Di Fabrizio, “Nano-patterned SERS substrate: application for protein analysis vs. temperature,” Biosens. Bioelectron. 24(6), 1693–1699 (2009). F. Neubrech, A. Pucci, T. W. Cornelius, S. Karim, A. García-Etxarri, and J. Aizpurua, “Resonant plasmonic and vibrational coupling in a tailored nanoantenna for infrared detection,” Phys. Rev. Lett. 101(15), 157403 (2008). R. Adato, A. A. Yanik, J. J. Amsden, D. L. Kaplan, F. G. Omenetto, M. K. Hong, S. Erramilli, and H. Altug, “Ultra-sensitive vibrational spectroscopy of protein monolayers with plasmonic nanoantenna arrays,” Proc. Natl. Acad. Sci. USA 106(46), 19227–19232 (2009). L. Razzari, A. Toma, M. Clerici, M. Shalaby, G. Das, C. Liberale, M. Chirumamilla, R. P. Zaccaria, F. De Angelis, M. Peccianti, R. Morandotti, and E. Di Fabrizio, “Terahertz dipole nanoantenna arrays: Resonance characteristics,” Plasmonics 8(1), 133–138 (2013). G. Pellegrini, P. Mazzoldi, and G. Mattei, “Asymmetric plasmonic nanoshells as subwavelength directional nanoantennas and color nanorouters: A multipole interference approach,” J. Phys. Chem. C 116(40), 21536 (2012). E. R. Encina and E. A. Coronado, “Plasmonic nanoantennas: Angular scattering properties of multipole resonances in noble metal nanorods,” J. Phys. Chem. C 112(26), 9586–9594 (2008). H. Aouani, M. Rahmani, H. Sipova, V. Torres, K. Hegnerova, M. Beruete, J. Homola, M. Hong, M. NavarroCía, and S. A. Maier, “Plasmonic nanoantennas for multispectral surface-enhanced spectroscopies,” J. Phys. Chem. C 117(36), 18620–18626 (2013). C. Fumeaux, M. A. Gritz, I. Codreanu, W. L. Schaich, F. J. González, and G. D. Boreman, “Measurement of the resonant lengths of infrared dipole antennas,” Infrared Phys. Technol. 41(5), 271–281 (2000).
1. Introduction Antennas have been used for more than a century to control the emission and collection of radio and microwave radiation. Traditional antenna designs have been scaled down and tested at terahertz and optical wavelengths, where their dimensions are proportional to the desired resonant wavelength [1] e.g., in the case of optical wavelengths is less than 1 μm. In order to fabricate these structures at the nanoscale level, different methods such as electron-beam lithography and microwave synthesis have been used with a precise control of the assembling between metal and semiconductor materials [2, 3]. Additionally, optical nanoantennas have the ability to focus light beyond the diffraction limit, which can enhance different optical processes such as high-harmonic generation [4], fluorescence [5, 6], Raman scattering [7, 8] and infrared absorption [9, 10]. Nanoantennas at terahertz frequencies are of interest since they can improve ultra-sensitive terahertz spectroscopy and antenna-enhanced terahertz nonlinear experiments by combining both strongly localized field enhancement with a spatially extended interaction of the incoming radiation [11–13]. It is well known that metallic nanoparticles support resonant plasmonic modes at optical wavelengths making them natural optical antennas [1, 14], also some self-assembled nanostructures have similar shapes to traditional antenna structures and could be used as emitting and receiving devices at optical and terahertz wavelengths, avoiding the need of nanolithography to fabricate them. Structures like these can be used to control the absorption and emission at the nanometer scale [2]. In a previous work, experimental evidence of the radiation pattern of self assembled nanoantennas was presented by introducing an external electrical signal at different frequencies in the range of megahertz under an operando mode using off-axis electron holography in real time within a transmission electron microscope [3]. In this work, the radiation properties of Ag/ZnO nanoantennas are determined by computer simulations and compared with Raman and Fourier Transform Infrared (FTIR) measurements in the range of terahertz, in order to demonstrate their potential use in optoelectronic devices. 2. Method The structure shown in Fig. 1 corresponds to a nanoantenna assembled with Zinc Oxide (ZnO) nanorods on silver nanowires (Ag-NWs) synthetized by a microwave method [3]. In order to study the radiation, emission and absorption properties associated with the optical response of these nanostructures at optical wavelength excitation, the geometries of Ag/ZnO nanostructures were simulated using a commercial software based on the finite element method (COMSOL Multiphysics). The Finite Element Method (FEM) is used to solve partial
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25112
differential equations under diverse boundary conditions. Two different scenarios were simulated, the first one consisted in considering the nanostructures as receivers, in this case a linearly polarized plane wave with frequencies going from 10 THz to 300 THz (1 μm a 30 μm in wavelength), and a 1 V/m E-field amplitude and polarization set parallel to one of the ZnO nanorods was simulated, the plane wave was launched perpendicular to one of the structure's star-shaped faces and the E-field pattern near the nanostructure was plotted.
Fig. 1. Nanostructures made of Ag nanowires with ZnO nanorods.
A second set of simulations consisted in considering the nanostructure as an electromagnetic emitter, for this simulation a sinusoidal voltage source was placed at one end of the structure over the tip of the silver nanowire and the electric field emitted due to this source was plotted for different source frequencies ranging from 10 to 300 THz. 3. Results The first set of simulations was performed by calculating the Electric Field generated near one end of the nanostructure due to a plane wave that impinges on that particular end of the Ag/ZnO nanostructure, these simulations were performed at different frequencies. The induced electric field was measured by integrating the electric field over the cross-section of the star-shaped end of the nanostructure. Matched boundary conditions were used in the FEM simulations and tetrahedral elements were used to discretize the computational domain. These simulations give the resonant properties of the star-shaped structure and give insight on how the size of the ZnO nanorods can influence the resonance of the whole structure. Figure 2 shows the results of this first set of simulations where we can see the E-field near the starshaped end of the nanostructure for different incident wave frequencies. The polarization of the incident wave is along the x-axis, each graph at a different incident wave frequency has an arrow that indicates the polarization of the incident wave, as it is evident in the graph the arms parallel to the polarization of the incident wave are the ones that have a higher response at the main resonance.
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25113
Fig. 2. E-field distribution on a star-shaped Ag-ZnO structure due to electromagnetic irradiation of different frequencies, the arrow on each figure illustrates the polarization of the incident Electric Field.
Figure 3 shows the normalized induced electric field over the star-shaped nanostructure as a function of frequency. The induced electric field was calculated by integrating the electric field across the antenna plane. From the figure it can be seen that there is a main resonance at 60 THz (5 μm in wavelength) and a secondary resonance at 200 THz (1.5 μm in wavelength). Since the size of each ZnO rod is around 0.8 μm and the overall height of the structure is around 1.6 μm, classical antenna theory would predict a main resonance at around 3.2 μm. Furthermore, classical antenna theory applies for low frequencies and highly conductive materials, in this case, the rods are made of a semiconductor material, which increases the conduction losses and shifts the resonance to larger wavelengths [15].
Fig. 3. Normalized Electric Field induced by a plane wave as a function of the plane-wave's frequency.
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25114
Figure 4(a)-4(c) shows the radiation pattern of the whole silver-zinc oxide nanostructure when the structure was excited with a current source at the bottom end of the structure. In this emission case since the silver nanowire is larger than the ZnO star-like structure the whole structure first resonates at lower frequencies, from Fig. 4(a) it can be seen how the silver nanowire dominates in the emission of electromagnetic radiation giving a dipole-like radiation pattern at 10 THz, at higher frequencies the zinc oxide nanorods become dominant shifting the directivity of the radiation pattern, as can be seen in Fig. 4(c). Figure 4(d)-4(f) shows the radiation pattern from the tip of the nanostructure for the same emission case, with a current source being located at the bottom of the structure, as in the previous figures the radiation pattern indicates that the nanorod structures resonate at higher frequencies, while at lower frequencies the main resonance is consistent with the one of a single dipole.
Fig. 4. (a)-(c) shows the radiation pattern of the whole Ag-ZnO nanostructure at three different frequencies, (d)-(f) shows the radiation pattern of the tip of the Ag-ZnO nanostructure, showing the dominant effect of the ZnO nanorods at higher frequencies.
In order to better understand the effect of an internal current source in these nanostructures, the radiation characteristics of the Ag/ZnO nanostructure were obtained for the case where the sinusoidal excitation is located inside the Ag nanowire. In this case the radiation pattern shows that the location of the source affects its directivity. For the specific case where the source is inside the nanowire the nanorods located behind the source reflects the electromagnetic energy and increases the directivity of the structure, these radiation characteristics resembles a traditional Yagi-Uda antenna. Experimental measurements in the Ag-NWs/ZnO nanoantennas were obtained by Raman spectroscopy and FTIR. Raman spectroscopy exhibits vibrational frequencies for E2 (High) stretching vibrational mode (434 cm−1) which correspond to a vibrational frequency of 13 THz. The spectrum is shown in Fig. 5(a), which is correlated with the theoretical Raman shift of ZnO (Fig. 5(b)). This frequency, in the infrared regime, is associated with the in-plane Zn – O (as shown the in the inset of the Fig. 5(c) vibrations stimulated with a polarized excitation of λ = 785 nm. This frequency activates the internal vibrational modes on the ZnO molecule.
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25115
During the process, the quantum energy absorbed by the molecule is released and a phonon is emitted at a specific frequency. In this way, the spectrum collected corresponds to the shift of the incident radiation wave number ( v ). The transversal (TO) and longitudinal (LO) optical modes around 325 and 680 cm−1 correspond to the out-of-plane Zn – O vibrations. The transversal and longitudinal vibration modes in the Raman spectrum correspond to 17 and 19 THz, respectively. The experimental spectrum shows two additional vibrational modes at 1050 and 1100 cm−1 (31.7 and 34 THz), which do not appear in the theoretical spectrum of ZnO, however it matches an increase in response at 30THz obtained by numerical simulations (Fig. 3). These two bands represent the scatter enhancement Raman spectrum (SERS), which is generated by the collective resonant vibrations of the whole system (Ag/ZnO). The frequencies above 1000 cm−1 are associated with the highest vibrational frequencies of the nanoantennas according to the numerical simulation described above. The corresponding frequencies of the nanoantennas related to the vibrational energies are included in the Fig. 5.
Fig. 5. Raman spectrum of Ag-ZnO nanostructures. The E2h-I and E2High vibrational modes around 320 and 434 cm-1 correspond to a frequency of 9.7 and 13 THz, respectively. Additionally, the enhance optical overtones modes around 1057 and 1147 cm-1 have frequencies 31.7 and 34 THz. The inset table on the figure shows the corresponding values of the frequencies corresponding to each vibrational mode.
As a complementary technique the samples were measured by FTIR to show the absorption spectrum of the nanoantennas in the infrared region. FTIR provides a higher response of the nanoantennas measured by Raman spectroscopy. Figure 6(a) shows the absorbance spectrum and their corresponding transmittance spectrum (Fig. 6(b)). The frequencies registered in the spectra around 550 cm−1 and 1600 cm−1, correspond to vibrational frequencies of 16.5 and 48 THz, respectively. In fact, the spectrum in Fig. 6(a) shows a characteristic absorption broad bands around 3450 and 1570 cm−1, with frequencies 103.5 and 47.1 THz, respectively. It is worth mentioning that the net effect on the resonance of the whole nanostructure in the infrared region matches closely a characteristic resonant frequency around 50 THz when compared with the calculated frequencies found by using the numerical methods between 10 and 120 THz (Fig. 3). Furthermore, from table inset on Fig.
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25116
6(b) it is identified vibrational frequencies (marked with the symbol *) around 27 THz that also matches closely with the lower region of the Normalized response of the electric in function of the frequency in Fig. 3
Fig. 6. FTIR spectra for Ag-ZnO nanoantennas.
4. Conclusions The resonance at terahertz frequencies of self-assembled nanostructures (Ag/ZnO) working as nanoantennas have been demonstrated by experimental measurements and simulations. Simulations show a main resonance at 60 THz, which has been identified in the ZnO nanorods as well as the effect of a frequency variation due to the influence of the silver nanowires. We found that Ag nanowires dominate the radiation pattern of the whole structure at lower frequencies, at higher frequencies the ZnO nanorods become dominant and modify the radiation pattern of the whole structure. Two experimental techniques, Raman and FTIR, were used to compare the resonances of the nanoantennas, which are in agreement with the prediction of the simulated nanoantennas. In both techniques the frequencies are clearly defined and two different ranges can be detected, Raman from 9 to 34 THz and FTIR from 16 to 103 THz. The nanoantennas measured and simulated are promising candidates for novel application for emitting and receiving nano-systems in the terahertz range. Acknowledgments This work was supported by the program “Cátedras CONACYT”, by project 32 of “Centro Mexicano de Innovación en Energía Solar” from Fondo Sectorial CONACYT-Secretaría de Energía-Sustentabilidad Energética and by the National Laboratory program from CONACYT through the Terahertz Science and Technology National Lab (LANCYTT). The authors would like to acknowledge the NSF PREM # DMR 0934218. J.E. Sanchez thanks to COLCIENCIAS for the Scholarship Francisco Jose de Caldas. The microscopy work was supported by the NIH RCMI Nanotechnology and Human Health Core (G12MD007591), Department of Defense #64756-RT-REP and the Welch Foundation grant award # AX-1615.
#242746 (C) 2015 OSA
Received 11 Jun 2015; revised 31 Jul 2015; accepted 7 Aug 2015; published 17 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.025111 | OPTICS EXPRESS 25117
