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This article aims to study the varying diffraction spectrum phenomenon influenced by surface plasmon of nanomaterials. Experiments used silver nanoparticles to build the echelon-like grating as test samples, where the grating spacing of line pattern is 10 μm and within the width of silver nanoparticles line is 5 μm alternately. In this work, the silver stripes with gradient thickness were first formed line pattern alternately with glancing angle deposition and photolithography, and then annealed at temperature of 250 °C for 3 mins to fabricate the silver nanoparticles as bowl-like forms. Thicknesses of the silver nanoparticles in the echelon-like grating increase from 0 nm to 40 nm and their diameters grow from 0 nm to 35 nm with quasilinear increment simultaneously. Analyzing the varying diffraction spectrum was focused on observing the first order diffraction by changing the beam size and the probe position of the incident light. The significant results show that the bigger the incident light beam size, the larger the peak wavelength difference (Δλ) of the two first order diffractions. Moreover, the peak spectrum is shifted to long wavelength and Δλ is diminished by probing area with big size of nanoparticles.
© 2015 Optical Society of America
1. Introduction
Collective oscillation of electron charge in metallic nanoparticles via light excitation known as localized surface plasmon resonance (SPR) has been studied extensively [1, 2]. In general, the surface plasmon of nanomaterial is seriously related to its element, shape, and environmental dielectric media [1–6]. Due to the particular optical characteristics of SPR, metallic nanoparticles have been widely used as optical filters [7], plasmonic waveguides [8], biosensors [9, 10], and substrates of surface-enhanced spectroscopies [11–14]. To manipulate the special optical property of SPR, many ingenious techniques to establish metallic nanoparticles in 1D and 2D array have been developed successfully [15–20]. The coupling effect of surface plasmon among the metallic nano-dots often causes optical behavior changes in nanoscale systems, which is very important in the nano-optical field and also helps many optical devices such as sub-diffraction limit image [19], optical signal propagation via metal nanoparticles array [20] to work well.
Grating with spatially modulated refractive index or coefficient of absorption can determine the spectral composition of light by recording the diffracted intensity with wavelength [21]. Diffraction spectrum via grating with excellent light dispersion technique is widely used in optical fields like spectrography, holography, and fiber optics. A lot of devices such as blazed grating, echelon grating, and sinusoidal grating [22] have been designed to concentrate the diffraction energy to a single principle maximum by modifying profile of the grating element in some special application fields.
This work has successfully developed a new grating built in metallic nanoparticles for adjusting diffraction spectrum. The developed grating called echelon-like grating used the quasilinear gradient thickness of nanoparticles to yield the asymmetrical intensities and dependent wavelengths of the two first order diffractions. In process, adjusting beam size or probe position of the incident light on the grating can effectively modulate the diffraction spectrum.
2. Experimental details
Figure 1 shows schematics of the developed echelon-like grating in this work, where thicknesses and sizes of the coated nanoparticles are quasilinear distribution. Samples preparation first deposited the photoresist on the glass as line grating pattern using photolithography. The grating spacing is 10 μm, where widths of the opaque and the transparent areas are 5 μm respectively. In process, a L-form glass slab was perpendicularly attached to the glass substrate for Ag deposition, which was functioned as a shielding plate. The commissure of the glass slab and the glass substrate was utilized as a rotation axis to control the gradient thickness of the Ag film with 0.14 o/s rotation speed. The deposition rate is 0.3 nm/s. After that, the photoresist on the glass substrate was peeled off by embedding the sample into acetone and a raw array with gradient thickness of Ag stripes was obtained. The samples were annealed at temperature of 250 °C for 3 mins. Finally, the quasilinear gradient thickness of Ag nanoparticles varied along the grating vector on the line pattern alternately was fabricated. The measured positions of the two first order diffractions were labeled + 1 and −1 as shown in the figure to indicate their locations relative to principle maximum of label 0 at right and left.
Fig. 1 Schematics of the echelon-like grating, where the labels of 0, + 1, and −1 represent the locations of zero and two first order diffractions respectively.
Figure 2 shows schematics of the apparatus for optical measurements. In process, the monochromatic incident light with wavelength from 400 nm to 800 nm via monochromator is individually applied on the samples for observing the first order diffraction spectra. The yielded monochromatic light was collimated by a lens of 50 mm focal length to impinge onto the sample. The intensity of the first-order diffraction light was measured by a photo detector which was set on a rotation stage to collect light in a normal direction. A lock-in amplifier connected with PC was used to record the signal automatically.
3. Results and discussion
Figure 3 shows relationship of thicknesses measured by N & K analyzer for the echelon-like grating with and without annealing relative to distance along grating vector direction called the x-axis. The origin of the coordinate x = 0 is set at the intersected line between the front of the L-form glass slab and the glass substrate. The Ag nanoparticles taken shape by annealing as shown in Fig. 3 with solid line indicates that the gradient thickness of the nanoparticles is quasilinear increased from 0 nm to 40 nm with the measured position from 0 mm to 10 mm along the x-axis. The raw sample of the echelon-like grating without annealing has the same inclination as shown in Fig. 3 with dashed line. In comparison with the raw samples of the echelon-like grating without annealing, thickness of the annealed echelon-like grating averagely increases ca. 2.3 times due to the stripes split into small pieces and aggregate.
Fig. 3 Relationship between thickness and position of the coated Ag film with and without annealing on the echelon-like grating.
Figures 4(a)-4(d) show the SEM images of the echelon-like grating focused on the Ag nanoparticles stripe at positions of x = 3, 5, 7, and 9 mm respectively, which clearly exhibit that the nanoparticles are distributed randomly and well confined in the stripe. The insets at top-right in Fig. 4 show grating and the corresponding positions of the nanoparticles stripe on grating. The average diameters of the Ag nanoparticles via particle analysis are 7.0, 13.69, 21.20, and 27.21 nm respectively. In comparison with thickness measurements in Fig. 3 using N & K analyzer, the measured thicknesses were individually 9.3, 19.7, 26.4, and 35.2 nm which are something bigger than particle analysis in Fig. 4. Both analyses reveal that the sizes of nanoparticles gradually increase in the x-direction. If the average shape of the nanoparticles is considered to be the bowl-like form, the height of the bowl-like nanoparticles is larger than the diameter.
Fig. 4 The SEM images of the echelon-like grating focused on the Ag nanoparticles stripe at positions of x = 3, 5, 7, and 9 mm respectively, where the insets at top-right are grating and the corresponding positions of the nanoparticles stripe on grating.
Figure 5 shows first order diffraction intensities of + 1 and −1 orders via echelon-like grating excited by visible light with wavelength range of 400 ~800 nm respectively. The probe light beam with diameter of 6 mm was centered at position of x = 3 mm on the test sample, which covered ca. 600 Ag nanoparticles stripes where the nanoparticles sizes were gradually increased from 0 to 20 nm. Figure 5 clearly indicates that the exciting peak diffraction intensities of + 1 and −1 diffraction orders appear at wavelengths of 600 nm and 530 nm respectively. The peak wavelength difference (Δλ) is ca. 70 nm. The grating made in this work with diffraction intensity of + 1 order larger than −1 order is similar to the commercial echelon-like grating due to the increasing size of nanoparticles along the + x direction [23].
Fig. 5 The intensities of + 1 and −1 first order diffractions via echelon-like grating excited by visible light with wavelength range of 400 ~800 nm respectively.
Analyzing the experimental results should consider the configuration of the echelon-like grating. The echelon-like grating composed of Ag nanoparticles stripes consists of scattering areas and transparent areas which are distributed alternately. In process, the incident light was scattered by the nanoparticles area and unchanged ran through the transparent area. The detected signal of diffraction spectrum was superposition of all excited light waves via the whole echelon-like grating. That is, the diffraction spectrum shift between + 1 and −1 diffraction orders is mainly caused by the light scattered areas. C. F. Bohren and D. R. Huffman had pointed out that the field scattered isotropic dipole oscillators in a thin slab of matter is proportional to , where α is the polarizability, θ is the phase shift between the displacement of an oscillator and the field that excited it [24]. The optical field excited by one nanoparticle transmitted to the image plane is proportional to , where k is the wave vector and l is the transmitted distance from nanoparticle to image plane. The phase difference between two adjacent scattered stripes denoted by suffix 1 and 2 has the form of for ± 1 diffraction orders. In this work, the diffraction spectra of + 1 and −1 diffraction orders are asymmetric due to the different phase difference of the resultant field from the adjacent light scattered areas. The peak diffraction intensities of + 1 and −1 diffraction orders are individually shifted to the larger and the shorter wavelength values in comparison with 0 diffraction order of principal maximum.
Figure 6 mainly shows curves by changing the size of light beam to measure the peak wavelength of the two diffraction orders. The probe light beam with diameters from 2 mm to 6 mm at an interval of 1 mm was centered at position of x = 3 mm on the test sample, which covered grating stripe numbers of the light scattered areas with gradient increment from ca. 100 to ca. 600. Moreover, the inset illustrates relationship of the peak wavelength difference (Δλ) relative to the light beam size. Figure 6 indicates that the peak wavelength of + 1 diffraction order is shifted to longer wavelength from ca. 550 nm to ca. 600 nm, but that of −1 diffraction order appears blue shift from ca. 550 nm to ca. 530 nm with the increase of light beam size. The peak wavelengths of + 1 and −1 diffraction orders for the light beam with diameter of 2 mm approach to the same value of 550 nm due to the unapparent size gradient of nanoparticles. In addition, the relationship of the peak wavelength difference (Δλ) relative to the light beam size is quasilinear.
Fig. 6 Curves between sizes of the incident light beam and peak wavelength of two first order diffractions, where the inset is relationship of the peak wavelength difference (Δλ) relative to the light beam size.
Figure 7 mainly shows curves between probe position and peak wavelength of the incident light. Moreover, the inset illustrates relationship of the peak wavelength difference (Δλ) relative to the light probe position. The probe light beam with diameter of 3 mm was centered at positions along the x-axis from 1.5 mm to 8.5 mm at an interval of 0.5 mm on the test sample, which covered ca. 300 stripes of the light scattered areas. Figure 7 indicates that the peak spectrum is shifted to long wavelength at the probe positions from 1.5 mm to 5.5 mm and then has no apparent shift. Moreover, the peak wavelength difference (Δλ) increases to a maximum value at the probe position of ~3 mm where the covered nanoparticles sizes are ca. 6.3 ~17.1 nm according to Fig. 2 and then gradually decreases to zero at the probe position range of 5.5 ~8.5 mm. U. Kreibig and M. Vollmer had pointed out that the full width at half maximum (FWHM) of absorption spectrum gradually increases with increment of silver nanoparticles size [1]. In other works, the field excited by SPR with specific wavelength has less sensitivity for big silver nanoparticles which leads to Δλ decreasing along + x direction as shown in the inset of Fig. 7. Moreover, the light probe beam centered at x = 1.5 mm appears the smaller value of Δλ than that at x = 3.0 mm due to the size of nanoparticles is much smaller than the wavelength of the incident light.
Fig. 7 Curves between probe position of the incident light and peak wavelength of two first order diffractions, where the inset is relationship of the peak wavelength difference (Δλ) relative to the light probe position.
In addition, the quasilinear size distribution of nanoparticles formed by wedge shaped silver coating is also analyzed. Figures 8(a)-8(e) show the SEM images of the sample at positions of x = 1, 3, 5, 7, and 9 mm with 15,000 magnification respectively as well as Fig. 8(f) is at position of x = 1 mm with 50,000 magnification. Figures 4 and 8 indicate that a portion of nanoparticles slightly formed elongated ellipsoidal or abnormal shapes with irregular distribution which leads to be difficult to give an actual analysis. In general, the plasmon coupling effect could be analyzed with spherical and isometric nanoparticles. Plasmon hybridization provides an intuitive method to describe the resonances of strong electromagnetic coupling between closely neighbor nanoparticles [25–28]. Analyzing Fig. 4 and Fig. 8 shows that the interparticle distance is diminished as a whole and some portions are smaller than 10 nm at the probe positions along the negative direction of x-axis. For a dimer, the dipolar mode of the individual particle interacted with the other will result in hybridized bonding and anti-bonding plasmon modes which cause near-field coupling to dominate specific spectrum. The influence of dimer plasmon might be functioned at probe positions of x-axis smaller than 3 mm.
Fig. 8 SEM images photographed at positions of (a) x = 1 mm, (b) x = 3 mm, (c) x = 5 mm, (d) x = 7 mm, (e) x = 9 mm, and (f) x = 1 mm, respectively
Figure 9 further shows the relationship between absorbance and wavelength for Fig. 8, which clearly indicates that the peak wavelengths are shifted to long wavelength and the full width at half maximum are increased with the probe position along the positive direction of x-axis. It indicates that the shapes of nanoparticles are more uniform at the probe position near x = 0 mm as shown in Fig. 8. The nanoparticles at positions smaller than x = 3 mm might have a weak near-field coupling to influence the diffraction spectrum, which causes the peak wavelength difference (Δλ) to be apparent at probe positions smaller than 3 mm in Fig. 7.
4. Conclusion
The new echelon-like grating built in silver nanoparticles for adjusting diffraction spectrum and the diffraction spectra analysis via light excitation are successfully carried out. The results show that the developed grating employs the quasilinear gradient thickness of nanoparticles to yield the asymmetrical intensities and the dependent wavelengths of the two first order diffractions. The asymmetric configuration of the developed echelon-like grating yields the different phase shifts for + 1 and −1 diffraction orders and causes the superimposition of diffraction light by the scattered light to result the different spectra of these two diffraction orders. The relative diffraction intensities of + 1 and −1 diffraction orders as well as the spectrum peaks of the two first order diffractions can be modulated by adjusting beam size or probe position of the incident light on the grating. In addition, the peak wavelength difference (Δλ) can be easily adjusted by changing the beam size or probe position of the incident light. In this work, a maximum variation Δλ ~70 nm was received.
Acknowledgments
The authors would like to thank the Ministry of Science and Technology of Taiwan for financially supporting this research under the Contract No. MOST 104-2221-E-027-068.
References and links
1. U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters (Springer, 1995).
2. L. M. Liz-Marzán, “Tailoring surface plasmons through the morphology and assembly of metal nanoparticles,” Langmuir 22(1), 32–41 (2006). [CrossRef] [PubMed]
3. S. A. Maier and H. A. Atwater, “Plasmonics: Localization and guiding of electromagnetic energy in metal/dielectric structures,” J. Appl. Phys. 98(1), 011101 (2005). [CrossRef]
4. M. Pelton, J. Aizpurua, and G. Bryant, “Metal-nanoparticle plasmonics,” Laser Photonics Rev. 2(3), 136–159 (2008). [CrossRef]
5. T. K. Sau, A. L. Rogach, F. Jäckel, T. A. Klar, and J. Feldmann, “Properties and applications of colloidal nonspherical noble metal nanoparticles,” Adv. Mater. 22(16), 1805–1825 (2010). [CrossRef] [PubMed]
6. J. Kottmann, O. Martin, D. Smith, and S. Schultz, “Spectral response of plasmon resonant nanoparticles with a non-regular shape,” Opt. Express 6(11), 213–219 (2000). [CrossRef] [PubMed]
7. S. Daniel, K. Saastamoinen, T. Saastamoinen, J. Rahomäki, A. T. Friberg, and T. D. Visser, “Dynamic control of optical transmission through a nano-slit using surface plasmons,” Opt. Express 23(17), 22512–22519 (2015). [CrossRef] [PubMed]
8. A. Apuzzo, M. Février, R. Salas-Montiel, A. Bruyant, A. Chelnokov, G. Lérondel, B. Dagens, and S. Blaize, “Observation of near-field dipolar interactions involved in a metal nanoparticle chain waveguide,” Nano Lett. 13(3), 1000–1006 (2013). [CrossRef] [PubMed]
9. L. Dykman and N. Khlebtsov, “Gold nanoparticles in biomedical applications: recent advances and perspectives,” Chem. Soc. Rev. 41(6), 2256–2282 (2012). [CrossRef] [PubMed]
10. M. Holzinger, A. Le Goff, and S. Cosnier, “Nanomaterials for biosensing applications: a review,” Front Chem. 2, 63 (2014). [CrossRef] [PubMed]
11. A. Campion and P. Kambhampati, “Surface-enhanced Raman scattering,” Chem. Soc. Rev. 27(4), 241–250 (1998). [CrossRef]
12. 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). [CrossRef]
13. S. Nie and S. R. Emory, “Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering,” Science 275(5303), 1102–1106 (1997). [CrossRef] [PubMed]
14. M. Moskovits, “Surface-enhanced spectroscopy,” Rev. Mod. Phys. 57(3), 783–826 (1985). [CrossRef]
15. W. Nomura, M. Ohtsu, and T. Yatsui, “Nanodot coupler with a surface plasmon polariton condenser for optical far/near-field conversion,” Appl. Phys. Lett. 86(18), 181108 (2005). [CrossRef]
16. S. K. Ghosh and T. Pal, “Interparticle Coupling Effect on the Surface Plasmon Resonance of Gold Nanoparticles: From Theory to Applications,” Chem. Rev. 107(11), 4797–4862 (2007). [CrossRef] [PubMed]
17. S. A. Maier, M. N. Brongersma, P. G. Kik, and H. A. Atwater, “Observation of near-field coupling in metal nanoparticle chains using far-field polarization spectroscopy,” Phys. Rev. B 65(19), 193408 (2002). [CrossRef]
18. H. Ditlbacher, J. R. Krenn, G. Schider, A. Leitner, and F. R. Aussenegg, “Two-dimensional optics with surface plasmon polaritons,” Appl. Phys. Lett. 81(10), 1762–1764 (2002). [CrossRef]
19. N. Fang, H. Lee, C. Sun, and X. Zhang, “Sub-Diffraction-Limited Optical Imaging with a Silver Superlens,” Science 308(5721), 534–537 (2005). [CrossRef] [PubMed]
20. R. M. Dickson and L. A. Lyon, “Unidirectional plasmon propagation in metallic nanowires,” J. Phys. Chem. B 104(26), 6095–6098 (2000). [CrossRef]
21. H. J. Eichler, P. Gunter, and D. W. Pohl, Laser-Induced Dynamic Gratings (Springer, 1986).
22. K. K. Sharma, Optics: Principles and Applications (Elsevier, 2006).
23. Y. Tachikawa, “Spectral analysis of transmission echelon grating filters for photonic networks,” Opt. Rev. 6(2), 131–138 (1999). [CrossRef]
24. C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles, (Wiley-VCH, 2004).
25. E. Prodan, C. Radloff, N. J. Halas, and P. Nordlander, “A hybridization model for the plasmon response of complex nanostructures,” Science 302(5644), 419–422 (2003). [CrossRef] [PubMed]
26. J. A. Fan, C. Wu, K. Bao, J. Bao, R. Bardhan, N. J. Halas, V. N. Manoharan, P. Nordlander, G. Shvets, and F. Capasso, “Self-assembled plasmonic nanoparticle clusters,” Science 328(5982), 1135–1138 (2010). [CrossRef] [PubMed]
27. Y. Wang, Z. Li, K. Zhao, A. Sobhani, X. Zhu, Z. Fang, and N. J. Halas, “Substrate-mediated charge transfer plasmons in simple and complex nanoparticle clusters,” Nanoscale 5(20), 9897–9901 (2013). [CrossRef] [PubMed]
28. A. Ahmadivand, N. Pala, and D. Ö. Güney, “Enhancement of photothermal heat generation by metallodielectric nanoplasmonic clusters,” Opt. Express 23(11), A682–A691 (2015). [CrossRef] [PubMed]
