Letter pubs.acs.org/NanoLett
Gold Nanorod Plasmonic Upconversion Microlaser Ce Shi,† Soheil Soltani,‡ and Andrea M. Armani*,†,‡ †
Mork Family Department of Chemical Engineering and Materials Science and ‡Ming Hsieh Department of Electrical Engineering-Electrophysics, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *
ABSTRACT: Plasmonic−photonic interactions have stimulated significant interdisciplinary interest, leading to rapid innovations in solar design and biosensors. However, the development of an optically pumped plasmonic laser has failed to keep pace due to the difficulty of integrating a plasmonic gain material with a suitable pump source. In the present work, we develop a method for coating high quality factor toroidal optical cavities with gold nanorods, forming a photonic−plasmonic laser. By leveraging the two-photon upconversion capability of the nanorods, lasing at 581 nm with a 20 μW threshold is demonstrated. KEYWORDS: Resonator, plasmonics, laser, nanorod
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industry, in the area of laser development several key technical hurdles remain. The motivation for using gold nanoparticles as a laser gain medium is the large and easily tunable absorption cross section.6,24 This parameter governs the threshold of an optically pumped laser and therefore is a critical consideration in the design of a laser. The absorption spectrum is determined by the geometry of the nanorod and contains two peak wavelengths corresponding to the transverse and longitudinal modes of the nanoparticle.5 By changing the growth conditions, the specific wavelengths of these peaks can be easily tuned. Additionally, previous work has shown that gold nanorods undergo two photon emission.9,10 In this nonlinear process, the nanorod absorbs two photons and emits one photon of higher energy or lower wavelength. As such, by optimizing the nanorod geometry such that the absorption peaks match those of the pump laser, it is possible to develop an optically pumped visible laser that does not rely on a rare earth element. Previous research has demonstrated that nanorod-based twophoton emission is possible by illuminating a coverslip coated with nanoparticles with a high power laser.10 While this approach demonstrates the fundamental principle, the emission
are-earth metal doped glasses have enabled a wide range of optical technologies, including low-threshold integrated lasers and high-transmission optical fiber.1−3 However, unlike many semiconductors or even noble metals these materials are not widely available and, as such, alternatives to rare earth materials are being actively sought. The challenge in developing an alternative is finding a material that has the same optical stability, environmental robustness, and optical properties of the rare earth metal. For example, while many inorganic dyes emit at similar wavelengths, their emission properties slowly degrade over time.4 One emerging alternative is based on gold nanoparticles. Gold nanoparticles can be synthesized in many geometries and sizes, including spheres, rods, prisms, and stars.5−8 As a result of this variety, it is possible to tune the optical properties and absorption of these particles from the visible through the near-IR. Additionally, they have also exhibited many interesting nonlinear phenomena, such as two-photon-based emission.9−13 One reason for the increased interest in gold nanoparticles is their inherent stability against environmental changes due to their minimal reactivity (e.g., low oxidation). Because of this, their optical properties do not change over time. Therefore, they are being studied for a wide range of emerging applications, including plasmonic solar cells, gene delivery, lasers, and sensors.8,14−23 However, while advances in many of these fields have progressed to the point of transitioning to © XXXX American Chemical Society
Received: July 7, 2013 Revised: November 9, 2013
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demonstrated. The nanorods are embedded in a polymer film coating on the surface of the cavity, creating a stable lasing platform. Silica toroidal cavities are chosen for the present work because they are integrated on a silicon wafer and have previously demonstrated Q factors above 100 million.32,33 As mentioned, one of the challenges of using a whispering gallery mode cavity as the basis for creating a plasmonic laser is achieving efficient energy transfer from the evanescent field of the cavity to the plasmonic nanoparticle. To aid in the optimization process, finite element method simulations using COMSOL Multiphysics can be performed. However, because of the asymmetry of the nanorods and the toroidal cavity and the wide range of length-scales, this specific structure is particularly complex to model. Recently, this challenge was solved. Building on the approaches detailed in Kaplan et al,34 we have developed a 3D COMSOL multiphysics finite element method model and simulated both the transverse electric and transverse magnetic modes of our system, both on and away from the plasmonic resonance of the nanoparticle (765 and 1550 nm, respectively). To get acceptable accuracy, the sizes of the mesh elements are chosen to be 0.5, 50, and 3 nm in the gold nanoparticle, near the optical mode and in the thin layer. Additionally, two different orientations are modeled (nanoparticle oriented perpendicular and parallel to the device surface). The nanoparticle and toroid cavity size matched those used in the subsequent experimental work. Additional details on the specific parameters used in the simulations and further analysis are included in the Supporting Information. The results for the transverse magnetic mode are summarized in Figure 2. When the optical cavity is on resonance at 765 nm, the
is very broad, covering hundreds of nanometers. Therefore, this approach is not feasible for a true on-chip microlaser design. One approach for improving the excitation efficiency is to directly intercalate the nanoparticles with the beam path or to create a fiber laser. However, because the melting temperature of gold is significantly below the melting temperature of silica, this combination is not possible. Additionally, simply dip coating a fiber in a nanoparticle solution will not give a sufficiently dense coverage of the nanoparticles to allow lasing at a reasonable threshold. An alternative is to create an optical microcavity-based laser (Figure 1a,b). Whispering gallery mode microresonators
Figure 1. Toroidal optical resonant cavity. (a) Rendering of a toroidal resonant cavity coated with gold nanorods. (b−d) Scanning electron microscope images of a gold nanorod coated cavity at different magnifications.
confine light at the device/air boundary, resulting in large circulating intensities that can strongly interact with and transfer energy to a nanoparticle.25 The circulating intensity is directly proportional to the photon lifetime within the cavity or the quality factor (Q) of the device. As such, high Q devices are ideally suited for this application. Previous research has successfully leveraged these large buildup powers in the design of optically pumped lasers. For example, rare-earth lasers and quantum-dot lasers have been demonstrated using both photonic crystal cavities and whispering gallery mode cavities.26−30 Therefore, they are a promising platform for a metal nanoparticle laser. However, this combination has yet to be shown. One fundamental challenge is designing a system in which the nanorods decorate the surface of the cavity without decreasing the Q of the device. Additionally, it is critical to align the resonant wavelength of the cavity with the plasmonic resonance of the nanoparticle. If these two values are not matched, the energy from the optical cavity will not be efficiently transferred to the nanoparticle. Recent work has demonstrated a method to stably attach spherical metal nanoparticles to the surface of high-quality factor devices without degrading the device performance.31 However, commercially available tunable laser sources are not available at the appropriate pump wavelength for nanospheres. Therefore, in order to achieve a strong overlap between the excitation source and the nanoparticle absorption wavelength, nanorods must be used. In the present work, a plasmonic laser based on the combination of gold nanorods with a high Q silica toroidal microcavity is theoretically modeled and experimentally
Figure 2. COMSOL multiphysics finite element method simulations of the transverse magnetic mode of the microtoroid interacting with a 3.5 AR gold nanorod. The gold nanorod (outlined in white) is (a,c) parallel or (b,d) perpendicular with the boundary of the microtoroid. The surface plasmon resonance of the nanoparticle is only excited when the optical resonance of the cavity overlaps with the optical absorption of the nanoparticle. B
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might distort the shape of the resonance. The Q is calculated by fitting the spectra to a Lorentzian and using the expression Q = λ/δλ, where δλ is the full width at half-maximum (fwhm) determined from the fit. As can be seen in Figure 3b, the Q of the nanorod-decorated cavity is above 10 million. The splitting of the resonance most likely arises from the presence of the nanorods.36 Therefore, while the nanorods do decrease the Q from the nonfunctionalized silica toroid microcavity, it does not substantially degrade the device performance. To characterize the lasing behavior, the side view camera is replaced with a fiber-coupled spectrograph (Andor), and the fiber is aligned with the toroid (Figure 3a).37 The testing setup is enclosed in a black-out curtain, and a background measurement is performed before beginning the experiment and is used to normalize the threshold measurements. The gap between the tapered fiber waveguide and the cavity is kept near critical coupling to get the maximum build-up power, and the lasing threshold is determined by changing the input power using an optical attenuator. The maximum detectable signal on the spectrograph is 65 000 counts. As a control measurement, a nonfunctionalized silica toroid is also characterized. Additional details on the test and measurement system are included in the Supporting Information. The UV−vis and fluorometry spectra are shown in Figure 4. On the basis of the pair of absorption wavelengths, nanorods
longitudinal plasmonic modes are excited in the particle. In contrast, when 1550 nm light is coupled in the optical cavity, the plasmonic modes are not excited, as expected. To experimentally study this system, it is necessary to synthesize and attach gold nanorods to the surface of the optical cavity. First, gold nanorods are synthesized using the seed-mediated, surfactant-directed method.35 The aspect ratio that determines the longitudinal plasmonic resonances of the gold nanorods can be easily tuned from 600 to 850 nm by changing the relative concentration of the silver nitrate to gold seed solution in the synthesis. In the present work, the aspect ratio is optimized for excitation at 765 nm to match with the pump laser used in the device measurements. To transfer the nanorods into the poly(methyl methacrylate) (35k molecular weight, PMMA, Sigma-Aldrich) solution, the surface is functionalized with thiol groups by mixing with mPEG-thiol (5k molecular weight, Laysan Bio) in degassed water for 2 h. Afterward, the particles are transferred to a toluene solution. Finally, the PMMA is added to the nanoparticle solution 0.5% by weight.24 Additional details on the synthesis of the nanorods and results from different nanorod solution concentrations are in the Supporting Information. The absorption/emission characteristics of the nanorods are characterized in water, methanol, toluene, and the PMMA solution using both UV−vis spectrophotometry and spectrofluorometry. The silica toroidal cavities are fabricated using the standard method which combines photolithography with two etching steps (buffered oxide etching and xenon difluoride) to create silica microdisks. This structure is reflowed using a carbon dioxide laser to form the microtoroid cavities with major (minor) diameters of 35 (9) μm.32,33 Finally, the nanorodPMMA film is deposited on the surface of the device by spin coating at 4000 rpm for 1 min (Figure 1b−d). The nanorodPMMA-coated toroid is annealed in a gravity oven at 150 °C for 2 h to remove any residual solvent and to thermally reflow the polymer film. From the SEM images, it appears that all of the nanorods are parallel to the surface, which is the ideal orientation for exciting the longitudinal mode. The quality factor of the device is measured by coupling light from a tunable narrow line width laser centered at 765 nm (Velocity series, Newport) into the cavity using a tapered optical fiber waveguide (Figure 3a).32,33 To measure the Q, the transmission spectrum is recorded using a high speed digitizer/ oscilloscope in the under-coupled regime, and the scan rate and range are optimized to minimize any nonlinear effects that
Figure 4. Nanoparticle analysis. (a) UV−vis spectra of the gold nanorods in toluene and the nanorods in toluene with PMMA. On the basis of the available tunable laser, the synthesis is optimized such that the longitudinal plasmonic resonance is located at approximately 780 nm. The slight red shift after the addition of PMMA is caused by the change in refractive index. (b) Fluorometer spectra of the nanorod in the PMMA solution. The nanorods can be easily excited at both 760 and 780 nm, emitting at 550 nm.
with an aspect ratio of 3.5 are synthesized. The slight shift in the location of the longitudinal mode upon addition of PMMA is due to the increase in the refractive index. It is important to note that the peak remains narrow, indicating that the rods are not agglomerating. Additionally, the absorption and fluorescence measurements are in agreement with the FEM simulations, which show that particles with an AR of 3.5 should interact strongly with optical fields near 780 nm, and have minimal interactions away from this wavelength range. Therefore, based on these measurements and simulations the synthesized particles are ideally suited to be pumped with a 780 nm source. The lasing and threshold results from a nanorod-coated microcavity are in Figure 5. The split emission peak centered 581.5 nm is slightly redshifted from the fluorometry measurement, most likely due to the small change in effective refractive index. Fitting the lasing line to a dual-peak Gaussian, the fwhm values of the laser lines are 0.83 and 1.00 nm. These linewidths
Figure 3. Resonator characterization. (a) Rendering of the device testing setup. A tapered fiber is used to couple a single mode tunable laser into the hybrid microtoroid. A fiber coupled spectrograph detects the lasing signal emitted from the hybrid microtoroid. (b) A typical transmission spectra. The Q is calculated to be 1.5 × 107 and 4 × 107 based on a Lorentzian dual-peak fit in Origin. The resonance is split into clockwise and counter-clockwise modes due to the presence of the nanoparticles. C
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limited to toroidal cavities and can be translated to other optical devices; for example, the deposition of metal nanoparticles on photonic crystals. It will impact researchers over a wide range of disciplines beyond laser development, such as improved signalto-noise for label-free protein detection and the study of photonic-plasmonic interactions.
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ASSOCIATED CONTENT
S Supporting Information *
Materials synthesis and properties, finite element method details, experimental measurement details, quality factor measurement, dependence on threshold on nanorod density, Figures S1−S9. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 5. Lasing spectra and (inset) threshold data of the gold nanorod-coated microtoroid cavity. The gold nanorods are excited by the evanescent field of the microtoroid and lase at 581.5 nm. The wavelength difference of the hybrid device from solution is caused by the slight refractive index change between the solution and the solid thin film. There are 5 points below threshold, and the quadratic relationship between the lasing intensity and the input power is due to the two-photon lasing mechanism. A threshold power as low as 20 μW is achieved.
AUTHOR INFORMATION
Corresponding Author
*E-amil: [email protected]. Phone: (213) 740-4428. Author Contributions
The manuscript was written through contributions of all authors. C.S. performed experiments and developed simulations of nanorod−microcavity interaction. S.S. developed simulations studying the effect of the polymer layer on the microlaser performance and performed analysis. A.A. aided in experimental design and data analysis. All authors have given approval to the final version of the manuscript and Supporting Information.
are significantly narrower than the previous result produced by coating a coverslip by several orders of magnitude.10 The lasing threshold measurement is presented in the inset of Figure 5. The coupled power is defined as output power from fiber times the coupling efficiency. The hybrid device achieves a threshold as low as 20 μW, which is an order of magnitude improvement over the previous approach. While this improvement is not as significant as the change in fwhm, the decrease in threshold is related to both the concentration of the nanorods as well as the circulating intensity. At 20 μW, with the previously defined Q and diameter the circulating optical power in the cavity is approximately 1.3 W, which corresponds to a circulating optical intensity of approximately 60 MW/cm2. While the circulating intensity has significantly increased over the previous approach, the concentration of the gain medium has decreased; as such, these changes nearly balance. In the process of achieving plasmonic lasing, a series of optimization experiments were performed during which the concentration of the nanorods in the initial solution was varied. The results and additional analysis are contained in the Supporting Information. It should be noted that additional peaks generated from Raman lasing from the silica cavity are also present in both the control and nanorod coated cavity emission spectra.38,39 However, the peaks centered at 581.5 nm are not present in the control cavities and are reproducible. Therefore, the lasing results presented in Figure 5 are from the nanorod. In summary, we have theoretically and experimentally demonstrated a visible laser based on a nanorod-coated toroidal microcavity. By embedding the gold nanorods within a conformal polymer coating on the surface of the toroid, we have fabricated hybrid devices with Q factors above 10 million. Through judicious selection of the pump wavelength, the high intensity circulating field within the optical cavity efficiently excites the plasmonic modes within the nanoparticle, enabling two photon emission from the nanorods. This combination results in a 20 μW-threshold laser with an approximate 1 nm line width. This simple approach for device fabrication is not
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Ms. Liubing Huang for her help with the SEM images. This work was supported by the Office of Naval Research [N00014-11-1-0910]. Additional information is available at http://armani.usc.edu.
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ABBREVIATIONS Q, quality factor; SEM, scanning electron micrograph; PMMA, poly(methyl methacrylate); mPEG thiol, O-(2-mercaptoethyl)O′-methyl-hexa(ethylene glycol); fwhm, full width at halfmaximum; δλ, resonance line width
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REFERENCES
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(11) Shalaev, V. M.; Cai, W.; Chettiar, U. K.; Yuan, H.-K.; Sarychev, A. K.; Drachev, V. P.; Kildishev, A. V. Opt. Lett. 2005, 30 (24), 3356− 3358. (12) Wurtz, G. A.; Pollard, R.; Hendren, W.; Wiederrecht, G. P.; Gosztola, D. J.; Podolskiy, V. A.; Zayats, A. V. Nat. Nanotechnol. 2011, 6 (2), 107−111. (13) Yorulmaz, M.; Khatua, S.; Zijlstra, P.; Gaiduk, A.; Orrit, M. Nano Lett. 2012, 12 (8), 4385−4391. (14) Atwater, H. A.; Polman, A. Nat. Mater. 2010, 9 (3), 205−213. (15) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Duyne, R. P. V. Nat. Mater. 2008, 7 (6), 442−453. (16) Hill, M. T.; Oei, Y.-S.; Smalbrugge, B.; Zhu, Y.; Vries, T. D.; Veldhoven, P. J. V.; Otten, F. W. M. V.; Eijemans, T. J.; Turkiewicz, J. P.; Waardt, H. D.; Geluk, E. J.; Kwon, S.-H.; Lee, Y.-H.; Notzel, R.; Smit, M. K. Nat. Photonics 2007, 1 (10), 589−594. (17) Noginov, M. A.; Zhu, G.; Belgrace, A. M.; Bakker, R.; Shalaev, V. M.; Narimanov, E. E.; Stout, S.; Herz, E.; Suteewong, T.; Wiesner, U. Nature 2009, 460 (7259), 1110−1112. (18) Salem, A. K.; Searson, P. C.; Leong, K. W. Nat. Mater. 2003, 2 (10), 668−671. (19) Santiago-Cordoba, M. A.; Boriskina, S. V.; Vollmer, F.; Demirel, M. C. Appl. Phys. Lett. 2011, 99 (7), 073701. (20) Suh, J. Y.; Kim, C. H.; Zhou, W.; Huntington, M. D.; Co, D. T.; Wasielewski, M. R.; Odom, T. W. Nano Lett. 2012, 12 (11), 5769− 5774. (21) Swaim, J. D.; Knittel, J.; Bowen, W. P. Appl. Phys. Lett. 2011, 99 (24), 243109. (22) Alivisatos, P. Nat. Biotechnol. 2004, 22 (1), 47−52. (23) Haddadpour, A.; Yi, Y. Biomed. Opt. Express 2010, 1 (2), 378− 384. (24) Liu, Y.; Mills, E. N.; Composto, R. J. J. Mater. Chem. 2009, 19 (18), 2704−2709. (25) Matsko, A. B.; Ilchenko, V. S. IEEE J. Sel. Top. Quantum Electron. 2006, 12 (1), 3−14. (26) Srinivasan, K.; Painter, O. Phys. Rev. A 2007, 75 (2), 023814. (27) Hsu, H.-S.; Cai, C.; Armani, A. M. Opt. Express 2009, 17 (25), 23265−23271. (28) Min, B.; Kim, S.; Okamoto, K.; Yang, L.; Scherer, A.; Atwater, H.; Vahala, K. Appl. Phys. Lett. 2006, 89 (19), 191124. (29) Reithmaier, J. P.; Sek, G.; Loffler, A.; Hofmann, C.; Kuhn, S.; Reitzenstein, S.; Keldysh, L. V.; Kulakovskii, V. D.; Reinecke, T. L.; Forchel, A. Nature 2004, 432 (7014), 197−200. (30) Wu, Z.; Mi, Z.; Bhattacharya, P.; Zhu, T.; Xu, J. Appl. Phys. Lett. 2007, 90 (17), 171105. (31) Shi, C.; Choi, H. S.; Armani, A. M. Appl. Phys. Lett. 2012, 100 (1), 013305. (32) Armani, D. K.; Kippenberg, T. J.; Spillane, S. M.; Vahala, K. J. Nature 2003, 421 (6926), 925−928. (33) Zhang, X.; Choi, H.-S.; Armani, A. M. Appl. Phys. Lett. 2010, 96 (15), 153304. (34) Kaplan, A.; Tomes, M.; Carmon, T.; Kozlov, M.; Cohen, O.; Bartal, G.; Schwefel, H. G. L. Opt. Express 2013, 21 (12), 14169− 14180. (35) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15 (10), 1957−1962. (36) Knittel, J.; McRae, T. G.; Lee, K. H.; Bowen, W. P. Appl. Phys. Lett. 2010, 97 (12), 123704. (37) Freeman, L. M.; Armani, A. M. IEEE J. Sel. Top. Quantum Electron. 2012, 18 (3), 1160−1165. (38) Chistiakova, M.; Armani, A. M. Opt. Lett. 2012, 37 (19), 4068− 4070. (39) Spillane, S. M.; Kippenberg, T. J.; Vahala, K. J. Nature 2002, 415 (6872), 621−623.
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Gold Nanorod Plasmonic Upconversion Microlaser Ce Shi,† Soheil Soltani,‡ and Andrea M. Armani*,†,‡ †
Mork Family Department of Chemical Engineering and Materials Science and ‡Ming Hsieh Department of Electrical Engineering-Electrophysics, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *
ABSTRACT: Plasmonic−photonic interactions have stimulated significant interdisciplinary interest, leading to rapid innovations in solar design and biosensors. However, the development of an optically pumped plasmonic laser has failed to keep pace due to the difficulty of integrating a plasmonic gain material with a suitable pump source. In the present work, we develop a method for coating high quality factor toroidal optical cavities with gold nanorods, forming a photonic−plasmonic laser. By leveraging the two-photon upconversion capability of the nanorods, lasing at 581 nm with a 20 μW threshold is demonstrated. KEYWORDS: Resonator, plasmonics, laser, nanorod
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industry, in the area of laser development several key technical hurdles remain. The motivation for using gold nanoparticles as a laser gain medium is the large and easily tunable absorption cross section.6,24 This parameter governs the threshold of an optically pumped laser and therefore is a critical consideration in the design of a laser. The absorption spectrum is determined by the geometry of the nanorod and contains two peak wavelengths corresponding to the transverse and longitudinal modes of the nanoparticle.5 By changing the growth conditions, the specific wavelengths of these peaks can be easily tuned. Additionally, previous work has shown that gold nanorods undergo two photon emission.9,10 In this nonlinear process, the nanorod absorbs two photons and emits one photon of higher energy or lower wavelength. As such, by optimizing the nanorod geometry such that the absorption peaks match those of the pump laser, it is possible to develop an optically pumped visible laser that does not rely on a rare earth element. Previous research has demonstrated that nanorod-based twophoton emission is possible by illuminating a coverslip coated with nanoparticles with a high power laser.10 While this approach demonstrates the fundamental principle, the emission
are-earth metal doped glasses have enabled a wide range of optical technologies, including low-threshold integrated lasers and high-transmission optical fiber.1−3 However, unlike many semiconductors or even noble metals these materials are not widely available and, as such, alternatives to rare earth materials are being actively sought. The challenge in developing an alternative is finding a material that has the same optical stability, environmental robustness, and optical properties of the rare earth metal. For example, while many inorganic dyes emit at similar wavelengths, their emission properties slowly degrade over time.4 One emerging alternative is based on gold nanoparticles. Gold nanoparticles can be synthesized in many geometries and sizes, including spheres, rods, prisms, and stars.5−8 As a result of this variety, it is possible to tune the optical properties and absorption of these particles from the visible through the near-IR. Additionally, they have also exhibited many interesting nonlinear phenomena, such as two-photon-based emission.9−13 One reason for the increased interest in gold nanoparticles is their inherent stability against environmental changes due to their minimal reactivity (e.g., low oxidation). Because of this, their optical properties do not change over time. Therefore, they are being studied for a wide range of emerging applications, including plasmonic solar cells, gene delivery, lasers, and sensors.8,14−23 However, while advances in many of these fields have progressed to the point of transitioning to © XXXX American Chemical Society
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demonstrated. The nanorods are embedded in a polymer film coating on the surface of the cavity, creating a stable lasing platform. Silica toroidal cavities are chosen for the present work because they are integrated on a silicon wafer and have previously demonstrated Q factors above 100 million.32,33 As mentioned, one of the challenges of using a whispering gallery mode cavity as the basis for creating a plasmonic laser is achieving efficient energy transfer from the evanescent field of the cavity to the plasmonic nanoparticle. To aid in the optimization process, finite element method simulations using COMSOL Multiphysics can be performed. However, because of the asymmetry of the nanorods and the toroidal cavity and the wide range of length-scales, this specific structure is particularly complex to model. Recently, this challenge was solved. Building on the approaches detailed in Kaplan et al,34 we have developed a 3D COMSOL multiphysics finite element method model and simulated both the transverse electric and transverse magnetic modes of our system, both on and away from the plasmonic resonance of the nanoparticle (765 and 1550 nm, respectively). To get acceptable accuracy, the sizes of the mesh elements are chosen to be 0.5, 50, and 3 nm in the gold nanoparticle, near the optical mode and in the thin layer. Additionally, two different orientations are modeled (nanoparticle oriented perpendicular and parallel to the device surface). The nanoparticle and toroid cavity size matched those used in the subsequent experimental work. Additional details on the specific parameters used in the simulations and further analysis are included in the Supporting Information. The results for the transverse magnetic mode are summarized in Figure 2. When the optical cavity is on resonance at 765 nm, the
is very broad, covering hundreds of nanometers. Therefore, this approach is not feasible for a true on-chip microlaser design. One approach for improving the excitation efficiency is to directly intercalate the nanoparticles with the beam path or to create a fiber laser. However, because the melting temperature of gold is significantly below the melting temperature of silica, this combination is not possible. Additionally, simply dip coating a fiber in a nanoparticle solution will not give a sufficiently dense coverage of the nanoparticles to allow lasing at a reasonable threshold. An alternative is to create an optical microcavity-based laser (Figure 1a,b). Whispering gallery mode microresonators
Figure 1. Toroidal optical resonant cavity. (a) Rendering of a toroidal resonant cavity coated with gold nanorods. (b−d) Scanning electron microscope images of a gold nanorod coated cavity at different magnifications.
confine light at the device/air boundary, resulting in large circulating intensities that can strongly interact with and transfer energy to a nanoparticle.25 The circulating intensity is directly proportional to the photon lifetime within the cavity or the quality factor (Q) of the device. As such, high Q devices are ideally suited for this application. Previous research has successfully leveraged these large buildup powers in the design of optically pumped lasers. For example, rare-earth lasers and quantum-dot lasers have been demonstrated using both photonic crystal cavities and whispering gallery mode cavities.26−30 Therefore, they are a promising platform for a metal nanoparticle laser. However, this combination has yet to be shown. One fundamental challenge is designing a system in which the nanorods decorate the surface of the cavity without decreasing the Q of the device. Additionally, it is critical to align the resonant wavelength of the cavity with the plasmonic resonance of the nanoparticle. If these two values are not matched, the energy from the optical cavity will not be efficiently transferred to the nanoparticle. Recent work has demonstrated a method to stably attach spherical metal nanoparticles to the surface of high-quality factor devices without degrading the device performance.31 However, commercially available tunable laser sources are not available at the appropriate pump wavelength for nanospheres. Therefore, in order to achieve a strong overlap between the excitation source and the nanoparticle absorption wavelength, nanorods must be used. In the present work, a plasmonic laser based on the combination of gold nanorods with a high Q silica toroidal microcavity is theoretically modeled and experimentally
Figure 2. COMSOL multiphysics finite element method simulations of the transverse magnetic mode of the microtoroid interacting with a 3.5 AR gold nanorod. The gold nanorod (outlined in white) is (a,c) parallel or (b,d) perpendicular with the boundary of the microtoroid. The surface plasmon resonance of the nanoparticle is only excited when the optical resonance of the cavity overlaps with the optical absorption of the nanoparticle. B
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might distort the shape of the resonance. The Q is calculated by fitting the spectra to a Lorentzian and using the expression Q = λ/δλ, where δλ is the full width at half-maximum (fwhm) determined from the fit. As can be seen in Figure 3b, the Q of the nanorod-decorated cavity is above 10 million. The splitting of the resonance most likely arises from the presence of the nanorods.36 Therefore, while the nanorods do decrease the Q from the nonfunctionalized silica toroid microcavity, it does not substantially degrade the device performance. To characterize the lasing behavior, the side view camera is replaced with a fiber-coupled spectrograph (Andor), and the fiber is aligned with the toroid (Figure 3a).37 The testing setup is enclosed in a black-out curtain, and a background measurement is performed before beginning the experiment and is used to normalize the threshold measurements. The gap between the tapered fiber waveguide and the cavity is kept near critical coupling to get the maximum build-up power, and the lasing threshold is determined by changing the input power using an optical attenuator. The maximum detectable signal on the spectrograph is 65 000 counts. As a control measurement, a nonfunctionalized silica toroid is also characterized. Additional details on the test and measurement system are included in the Supporting Information. The UV−vis and fluorometry spectra are shown in Figure 4. On the basis of the pair of absorption wavelengths, nanorods
longitudinal plasmonic modes are excited in the particle. In contrast, when 1550 nm light is coupled in the optical cavity, the plasmonic modes are not excited, as expected. To experimentally study this system, it is necessary to synthesize and attach gold nanorods to the surface of the optical cavity. First, gold nanorods are synthesized using the seed-mediated, surfactant-directed method.35 The aspect ratio that determines the longitudinal plasmonic resonances of the gold nanorods can be easily tuned from 600 to 850 nm by changing the relative concentration of the silver nitrate to gold seed solution in the synthesis. In the present work, the aspect ratio is optimized for excitation at 765 nm to match with the pump laser used in the device measurements. To transfer the nanorods into the poly(methyl methacrylate) (35k molecular weight, PMMA, Sigma-Aldrich) solution, the surface is functionalized with thiol groups by mixing with mPEG-thiol (5k molecular weight, Laysan Bio) in degassed water for 2 h. Afterward, the particles are transferred to a toluene solution. Finally, the PMMA is added to the nanoparticle solution 0.5% by weight.24 Additional details on the synthesis of the nanorods and results from different nanorod solution concentrations are in the Supporting Information. The absorption/emission characteristics of the nanorods are characterized in water, methanol, toluene, and the PMMA solution using both UV−vis spectrophotometry and spectrofluorometry. The silica toroidal cavities are fabricated using the standard method which combines photolithography with two etching steps (buffered oxide etching and xenon difluoride) to create silica microdisks. This structure is reflowed using a carbon dioxide laser to form the microtoroid cavities with major (minor) diameters of 35 (9) μm.32,33 Finally, the nanorodPMMA film is deposited on the surface of the device by spin coating at 4000 rpm for 1 min (Figure 1b−d). The nanorodPMMA-coated toroid is annealed in a gravity oven at 150 °C for 2 h to remove any residual solvent and to thermally reflow the polymer film. From the SEM images, it appears that all of the nanorods are parallel to the surface, which is the ideal orientation for exciting the longitudinal mode. The quality factor of the device is measured by coupling light from a tunable narrow line width laser centered at 765 nm (Velocity series, Newport) into the cavity using a tapered optical fiber waveguide (Figure 3a).32,33 To measure the Q, the transmission spectrum is recorded using a high speed digitizer/ oscilloscope in the under-coupled regime, and the scan rate and range are optimized to minimize any nonlinear effects that
Figure 4. Nanoparticle analysis. (a) UV−vis spectra of the gold nanorods in toluene and the nanorods in toluene with PMMA. On the basis of the available tunable laser, the synthesis is optimized such that the longitudinal plasmonic resonance is located at approximately 780 nm. The slight red shift after the addition of PMMA is caused by the change in refractive index. (b) Fluorometer spectra of the nanorod in the PMMA solution. The nanorods can be easily excited at both 760 and 780 nm, emitting at 550 nm.
with an aspect ratio of 3.5 are synthesized. The slight shift in the location of the longitudinal mode upon addition of PMMA is due to the increase in the refractive index. It is important to note that the peak remains narrow, indicating that the rods are not agglomerating. Additionally, the absorption and fluorescence measurements are in agreement with the FEM simulations, which show that particles with an AR of 3.5 should interact strongly with optical fields near 780 nm, and have minimal interactions away from this wavelength range. Therefore, based on these measurements and simulations the synthesized particles are ideally suited to be pumped with a 780 nm source. The lasing and threshold results from a nanorod-coated microcavity are in Figure 5. The split emission peak centered 581.5 nm is slightly redshifted from the fluorometry measurement, most likely due to the small change in effective refractive index. Fitting the lasing line to a dual-peak Gaussian, the fwhm values of the laser lines are 0.83 and 1.00 nm. These linewidths
Figure 3. Resonator characterization. (a) Rendering of the device testing setup. A tapered fiber is used to couple a single mode tunable laser into the hybrid microtoroid. A fiber coupled spectrograph detects the lasing signal emitted from the hybrid microtoroid. (b) A typical transmission spectra. The Q is calculated to be 1.5 × 107 and 4 × 107 based on a Lorentzian dual-peak fit in Origin. The resonance is split into clockwise and counter-clockwise modes due to the presence of the nanoparticles. C
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limited to toroidal cavities and can be translated to other optical devices; for example, the deposition of metal nanoparticles on photonic crystals. It will impact researchers over a wide range of disciplines beyond laser development, such as improved signalto-noise for label-free protein detection and the study of photonic-plasmonic interactions.
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ASSOCIATED CONTENT
S Supporting Information *
Materials synthesis and properties, finite element method details, experimental measurement details, quality factor measurement, dependence on threshold on nanorod density, Figures S1−S9. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 5. Lasing spectra and (inset) threshold data of the gold nanorod-coated microtoroid cavity. The gold nanorods are excited by the evanescent field of the microtoroid and lase at 581.5 nm. The wavelength difference of the hybrid device from solution is caused by the slight refractive index change between the solution and the solid thin film. There are 5 points below threshold, and the quadratic relationship between the lasing intensity and the input power is due to the two-photon lasing mechanism. A threshold power as low as 20 μW is achieved.
AUTHOR INFORMATION
Corresponding Author
*E-amil: [email protected]. Phone: (213) 740-4428. Author Contributions
The manuscript was written through contributions of all authors. C.S. performed experiments and developed simulations of nanorod−microcavity interaction. S.S. developed simulations studying the effect of the polymer layer on the microlaser performance and performed analysis. A.A. aided in experimental design and data analysis. All authors have given approval to the final version of the manuscript and Supporting Information.
are significantly narrower than the previous result produced by coating a coverslip by several orders of magnitude.10 The lasing threshold measurement is presented in the inset of Figure 5. The coupled power is defined as output power from fiber times the coupling efficiency. The hybrid device achieves a threshold as low as 20 μW, which is an order of magnitude improvement over the previous approach. While this improvement is not as significant as the change in fwhm, the decrease in threshold is related to both the concentration of the nanorods as well as the circulating intensity. At 20 μW, with the previously defined Q and diameter the circulating optical power in the cavity is approximately 1.3 W, which corresponds to a circulating optical intensity of approximately 60 MW/cm2. While the circulating intensity has significantly increased over the previous approach, the concentration of the gain medium has decreased; as such, these changes nearly balance. In the process of achieving plasmonic lasing, a series of optimization experiments were performed during which the concentration of the nanorods in the initial solution was varied. The results and additional analysis are contained in the Supporting Information. It should be noted that additional peaks generated from Raman lasing from the silica cavity are also present in both the control and nanorod coated cavity emission spectra.38,39 However, the peaks centered at 581.5 nm are not present in the control cavities and are reproducible. Therefore, the lasing results presented in Figure 5 are from the nanorod. In summary, we have theoretically and experimentally demonstrated a visible laser based on a nanorod-coated toroidal microcavity. By embedding the gold nanorods within a conformal polymer coating on the surface of the toroid, we have fabricated hybrid devices with Q factors above 10 million. Through judicious selection of the pump wavelength, the high intensity circulating field within the optical cavity efficiently excites the plasmonic modes within the nanoparticle, enabling two photon emission from the nanorods. This combination results in a 20 μW-threshold laser with an approximate 1 nm line width. This simple approach for device fabrication is not
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Ms. Liubing Huang for her help with the SEM images. This work was supported by the Office of Naval Research [N00014-11-1-0910]. Additional information is available at http://armani.usc.edu.
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ABBREVIATIONS Q, quality factor; SEM, scanning electron micrograph; PMMA, poly(methyl methacrylate); mPEG thiol, O-(2-mercaptoethyl)O′-methyl-hexa(ethylene glycol); fwhm, full width at halfmaximum; δλ, resonance line width
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