Active Plasmonic Nanoantennas for Controlling Fluorescence Beams

Aug 22, 2013 - We further extended the tunable antenna design to a bullseye-shaped nanoantenna to achieve tunable complete-collimated beams. The activ...
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Active Plasmonic Nanoantennas for Controlling Fluorescence Beams Haibo Li,† Shuping Xu,† Yuejiao Gu,† Hailong Wang,† Renping Ma,† John R. Lombardi,‡ and Weiqing Xu*,† †

State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China Department of Chemistry, City College of New York, New York, 10031, United States

‡

S Supporting Information *

ABSTRACT: We propose a tunable plasmonic nanoantenna design that achieves steering fluorescence beams via a voltage signal. The configuration is composed of a nanometallic grating structure coated with a thin luminescent layer and a liquid crystals (LC) cell fixed above as a modulator. The angle-scanned fluorescence spectra show that fluorescence emitted from this metallic grating antenna has a high directivity (divergence angle ≈ 3°) and the beams present a high monochromaticity (full width at half-maximum ≈ 14 nm). More importantly, the fluorescence wavelength can be continuously tuned at a high repetition rate according to the electric signal applied on the LC modulation layer. We further extended the tunable antenna design to a bullseye-shaped nanoantenna to achieve tunable complete-collimated beams. The active control strategy of luminescence based on plasmonic nanoantennas has a great practical significance in developing novel tunable nanoscale light sources.

1. INTRODUCTION Metallic optical nanoantennas based on surface plasmons (SPs) have gained much research interest recently for their predominant light modulation ability in subwavelength scale. 1−4 Optical nanoantennas hold great promise in developing high-performance nanoscale optical devices working for light harvesting (e.g., solar cells and photodetectors),5,6 light enhancement (e.g., surface-enhanced spectroscopy and nonlinear optics),7 as well as light emission.8−11 The goal of research on light emitting devices is to gain high luminous efficiency and desirable beam properties.12−15 Since plasmonic nanoantennas are able to enhance the local electromagnetic (EM) intensity in near field and redirect light flow out of emitters into far field, luminescence enhancement and beam shaping can be simultaneously realized by the utilization of optical nanoantennas for emitters.16,17 For example, via a bullseye-shaped nanoantenna, fluorescence beams with 120fold enhancement can be achieved and a full directional emission control can be realized.16,18 Previous reports have shown that the radiation properties of fluorescence, such as colors and radiation directions, can be modulated by changing the physical structural features of antennas.17,18 These regulation factors are limited in precision and flexibility. Accordingly, new strategies for the active control of emission behavior are still in great demand. The nature of manipulation of optical nanoantennas is to control the SPs.19−21 Besides structural features of nanoantennas being varied, plasmons can also be tuned via a change in the refractive indices of surrounding dielectric mediums. This change of refractive indices can be caused by a concentration a © 2013 American Chemical Society

varied concentration of sucrose solutions, electro- and photochromic molecules, etc.22,23 Liquid crystals (LC) are considered to be an ideal material for SP control because of their large refractive index anisotropy and fast dynamic response to electric signal.24−26 In this work, we propose a kind of dynamically modulated nanoantenna design based on LC, through which active-tuned fluorescence beams are obtained. To achieve this, a metallic nanograting antenna is coated with a thin luminescent layer and a LC cell is added above as a modulator. The fluorescence spectra at different applied voltages were recorded via an angle-resolved spectrometer. The results show that the fluorescence from the grating optical nanoantennas exhibits high directivity and monochromaticity. The wavelength of the fluorescence beams can be continuously tuned via an electric signal at a high repetition rate, which is preponderant over other plasmonic beaming modulation strategies via static structure changes. This LC-tuned optical nanoantenna configuration opens up a new opportunity to develop a novel plasmonic light source with tunable emission properties.

2. EXPERIMENTAL SECTION 2.1. Preparation of LC-Tuned Nanoantenna. (1) The metallic grating was prepared using the template method. A 300 nm Ag film was deposited by vacuum evaporation on the Bluray disc (RiTEK Company) at a speed of 0.2 nm/s. Then, the Received: August 16, 2013 Revised: August 22, 2013 Published: August 22, 2013 19154

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Figure 1. (a) Schematic diagram of the electrically modulated fluorescence beams via LC-tuned plasmonic nanoantennas. The red sine wave stands for the SP wave propagating on a metal grating. (b) The AFM image of a Ag nanograting. The period of the Ag grating is 320 nm and the height is 30 nm.

Figure 2. (a) Fluorescence spectra of R640 dye on the LC-tuned antenna at different detection angles. The dashed line is the bulk fluorescence spectrum of R640 in a spin-coated PI film on a glass substrate and it is normalized according to the maximum fluorescence intensity on the LC-tuned antenna. (b) Fluorescence intensity and the Gaussian fit curve at 610 nm wavelength versus detection angle. The marked divergence angle is the angle width at 1/e of maximum. (c) Fluorescence spectra obtained at the detection angle of 16° with a P or S polarizer. P direction is the vertical direction of the grating grooves, and S direction is parallel to the grooves.

and fluorescence spectra at different angles. The gain factor of the ICCD was set to 150, which enhanced the signal about 40 times in counts. The NA of the detection lens is 0.02 to guarantee the high angle resolution. A long pass filter (cutoff wavelength 535 nm, from Semrock, Inc.) was added behind the detection lens to block the Rayleigh scattering. 2.3. FDTD Simulations. The FDTD simulations were carried out by using FDTD solution software (Lumerical Solutions, Inc.). The model is shown in Figure 5a. The dielectric constant of the Ag was according to Palik. The simulation area was 5 μm × 5 μm × 2 μm with PML boundary conditions. The light source is simplified as a dipole which emitted in the wavelength range of 500−600 nm. The polarization direction of the dipole is parallel to the surface of the Ag film. The dipole was put in a centric hole with a diameter of 60 nm and a depth of 100 nm, and the monitor (frequency-domain field and power monitor) was placed 100 nm above the Ag film.

Ag layer was lifted off using ultraviolet curing adhesive. (2) The luminescent layer was prepared by the spin-coating method. A polyimide (PI) solution (0.5 wt % PI in pyrrolidone, from POME Sci-tech Co., Ltd.) with R640 (1.0 mg/mL) was spincoated on the metallic grating (4000 rad/min). Then the grating with luminescent layer was heated at 80 °C for 30 min for thermosetting coatings. (3) The LC cell composed of PI film spacer (50 μm in thickness) and the ITO glass with PI oriented film was fixed on the grating. The PI oriented film was spin-coated with the PI solution (3 wt % PI in pyrrolidone) at 4000 rad/min and heated at 200 °C for 60 min. Then the PI oriented film was repeatedly rubbed along direction of the grooves of the metal grating with a nonwoven fabric for 20 times. (4) LC molecules (TEB300, from ChengzhiYonghua Display Materials Co., Ltd.) were filled into the LC cell at the temperature of 65 °C by capillary action. 2.2. Experiment Setup. The electric signal to drive the LC cell is a sine wave of 1 kHz from a function waveform generator (RIGOL Technologies, Inc.) with a transformer to improve the voltage. The angle-resolved spectrum detection system has been reported in our previous work.29 Briefly, it includes light sources (a 532 nm laser and a bromine tungsten lamp from Ocean Optics, Inc.), a goniometer, and a spectrometer (a monochromator with an ICCD, action SP2300 and PI-MAX2, from Princeton Instruments). The spiral arms (supporting the module of incident light) and the sample stage of the goniometer can rotate separately to obtain the reflectance

3. RESULTS AND DISCUSSION 3.1. Physical Model and Theory. The design of the active optical nanoantennas for controlling fluorescence beams is shown in Figure 1. It is composed of a nanometallic grating structure coated with a thin luminescent layer and a liquid crystals (LC) cell fixed above as a modulator. The Ag nanograting prepared by the template method was spin-coated by a PI layer (about 40 nm in thickness, it is less than the 19155

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with PI oriented film covered the LC. The ITO glass and the Ag nanograting were both employed as electrodes. As shown in Figure 1a, the fluorescent molecule nearby the Ag surface can efficiently excite the SPs on the metallic grating via dipole−dipole interaction.27 The SP wave propagating on the grating will be scattered by the fluctuation on the Ag surface and be coupled to a plane wave traveling in the far field.28 According to the principle of wave optics, each scattering unit can be regarded as a secondary wave source. Similar to phase array antennas, the control of fluorescent beams is based on the interference effect in this model. When the wavelength of the SPs (λSP) equals the period of grating (Λ), the SP-coupled emission in the normal direction of grating will be reinforced by the interference effect since scattering units are all in the same phase, generating a collimating beam vertical to the substrate. Accordingly, when λSP is unequal to Λ, the beam will emit at a defined angle away from the normal direction due to the phase delay (or advance) of adjacent scattering units. Therefore, the direction of the radiation beam depends on the ratio of λSP and Λ. So, there are two strategies for steering fluorescent beams in this antenna, tuning Λ or λSP. It has been reported that changing the periods of antennas can control fluorescent beams.17,18 Tuning the λSP is also a practical way to access this goal. Since λsp = (c/ν)((εm + εd)/(εmεd))1/2 (here ν is the frequency of SPs. εm and εd are the permittivities of the metal and dielectric material, respectively), the value of λSP varies with ν and εd. The variations of λSP for different ν result in the chromatic dispersion of fluorescence emission.30 εd can be easily tuned via changing the surrounding refractive index, allowing an external control of optical nanoantennas. In this work, the LC material was used to link the microcosmic SPs to the external electric signal. Owing to the high refractive indices of LC (λSP is much shorter in LC comparing in air), we adopted a metallic grating with a subwavelength period to ensure fluorescent beams locating in visible region. Fortunately, we found a commercial, cheap, and workable template, Blu-ray discs, which possess a highly uniform subwavelength-scale pattern over a large area. The metallic nanograting pattern with a period of 320 nm and a height of 30 nm was copied from the Blu-ray disc template. The morphology of the metallic nanograting is shown in Figure 1b. 3.2. Fluorescence Beam Properties. The fluorescence beam properties of dye on the metallic nanograting under the manipulation of electric signal were studied via the angleresolved spectroscopy, as shown in Figure 2. R640 dye was excited via a 532 nm laser (0.5 mW, the incident angle equals 20°) and the fluorescence spectra at different detection angles were recorded using an angle-resolved spectrometer fixed with a small numbered aperture (NA, about 0.02) detection lens.29 Figure 2a shows the fluorescence spectra obtained at different detection angles (12° to 19°). It can be found that optical antenna split the bulk fluorescence of R640 into several beams traveling in different spatial directions; in other words, monochromatic fluorescence beams with different wavelengths were obtained at different angles. It indicates that this antenna has a chromatic dispersion function similar as a bulk monochromator, suggesting it can be used as a nanospectrometer. The chromatic dispersion phenomenon indicates that we can obtain a set of monochromatic light beams from a polychromatic source (the frequency selection) via an optical nanoantenna, without using any other bulky optical dispersion elements. Accordingly, the beam can be collimated at a given wavelength, as shown in Figure 2b which displays the

Figure 3. Fluorescence modulations on the LC-tuned optical antenna at varying applied voltage. (a) Electrically modulated fluorescence spectra obtained at the detection angle of 10°. For comparison, fluorescence spectra of the spin-coated R640-PI film on a flat Ag surface and a glass substrate were presented. (b) The comparison of fluorescence spectra and reflectance spectra (detected at 22°) on the LC-tuned antenna.

Figure 4. The continuous tuning of the fluorescence peak positions through changing the applied voltages at different detection angles.

penetration depth of SPs) in which Rhodamine 640 (R640) dye was mixed. A nematic LC layer (no = 1.55 and ne = 1.71, 50 μm in thickness) as the modulation layer was added above the luminescent layer and an indium tin oxide (ITO) glass slide 19156

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Figure 5. FDTD simulations of the tuning 2D-collimated fluorescence beams from a bullseye optical antenna via changing the surrounding refractive index. (a) The FDTD simulation model. The fluorescence emitter is simplified as a dipole which emits the light in the range of 500−600 nm. The surrounding refractive index (n) is 1.55 or 1.71. (b) The far field radiation pattern of |E| intensity (518 nm) in one meter away from the nanoantenna when n = 1.55 (more far field radiation patterns at different conditions are shown in Supporting Information part 4). Panels c and d are the distributions of the normalized light intensity (proportional to |E|2) at 518 and 590 nm when n = 1.55 and n = 1.71, respectively.

shift. It can also be found that the fluorescence intensity of dye on the metallic grating is much stronger than that without the grating. It (energy density more exactly) has been enhanced about 13 times compared with that of the dye on a flat Ag film and 7 times comparing with that of the dye on a glass substrate. The significant improvement in fluorescence intensity is attributed to the local EM field enhancement and the fluorescence redirection by means of optical antennas. Figure 3b presents the relation between the SP excitation process and the emission properties of dye on the LC-tuned nanoantenna. The results show that the emission wavelengths of fluorescence strictly equal the excitation wavelengths of nanoantenna. Since the coupling process of SPs to a beam can be regarded as a reverse process of the SP excitation via a beam, 30 the light emission properties of plasmonic nanoantennas (frequency, direction, etc) are equal to the excitation properties (that is the reciprocity principle of antennas). This may enlighten us on the design of the frequency-response nanophotodetectors by means of optical nanoantennas. It can be expected that these light detection devices could also be electrically controlled by exploiting LC. Figure 4 shows that the fluorescence wavelength can be steered precisely and continuously using an electric signal on LC. In addition, wavelength modulation can also be achieved in a relatively larger spectral range via a change in the collection angle. It is worth mentioning that the time response of electric modulation is very fast (less than 30 ms) and the modulation is repeatable and resumable (shown in SI part 2). These incomparable advantages over the static changes of physical structure allow the application of optical antennas in the fields requiring high response rate. 3.4. Extend to 2D Structures. The wavelength-tunable fluorescence beam which emitted vertical to the substrate was

divergence angle of a fluorescence beam at 610 nm. The divergence angle is as small as about 3° (width at 1/e of maximum), presenting a very high directivity. The fwhm of the fluorescence band is about 14 nm, and its polarization is almost completely in P direction (vertical to the grooves of the grating), as shown in Figure 2c. The good beam properties may be attributed to the high uniformity of the metallic grating substrate and the suitable structure parameters. Here, the ratio of height and period of the prepared metallic grating was ∼0.1, which is able to support the propagation and scattering of the SPs.31 3.3. Control of the Fluorescence Beams via a Voltage Signal. In addition of the good beam properties of the fluorescence on this antenna, more significant is that fluorescence wavelength can be dynamically steered by using an electric signal. Figure 3 shows the modulation of fluorescence with the use of an applied voltage on the LCtuned optical antenna. After the voltages were applied, the fluorescence peak obviously red-shifted, from 597 to 613 nm. The redshift of fluorescence wavelength may be attributed to the chromatic dispersion phenomenon of the antennas and the decrease of fluorescence emission angles after voltage signals are applied (shown in the inserted schematic diagram in Figure 3a). As is shown in Figure 2a, the emitting angles are larger for longer wavelength fluorescence bands. When a voltage is applied, the orientation of LC molecules changes and the effective refractive index of surrounding increases (from no to ne, in fact the change that the SPs sensed was smaller than this due to the influence of the luminescent layer). Accordingly, λSP becomes shorter, resulting in a decrease of the emission angle of the fluorescence beams (here we refer to a condition of λSP > Λ, shown in Supporting Information, part 1). Therefore, the fluorescence wavelength obtained at a fixed angle would red19157

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The Journal of Physical Chemistry C also obtained on this grating nanoantenna (shown in Supporting Information, part 3). However, this collimated fluorescence beam from a nanograting is only in one dimension (1D). To extend the collimating fluorescence beam to 2D, a bullseye nanoantenna structure is considered.12,17,18 As shown in Figure 5, we simulated the far-field radiation patterns of a dipole emitter on the bullseye nanoantenna with different surrounding refractive indices. The simulation model is a round nanohole (diameter = 100 nm) surrounded with a concentric groove structure with a period of 320 nm and a height of 30 nm (Figure 5a). A dipole which emits 500−600 nm light is laid in the centric hole. The surrounding refractive index is changed from 1.55 to 1.71, which equals the no and ne of the LC. Figure 5b presents the far-field radiation pattern of a dipole on this bullseye nanoantenna. It can be seen that the electric field energy (|E|) is collimated into a 2D beam with a very small divergence angle. When changing the surrounding refractive index, as expected, the wavelengths of the collimated beams are different. As shown in Figure 5c,d, a collimated beam at 518 nm forms in the normal direction when n = 1.55. And when n = 1.71, a collimated beam at 590 nm can be observed (divergence angle ≈ 11°). Thus, a 2D collimated beam with tunable wavelength could be obtained on a LC-modulated bullseye structure.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

(1) SPs dispersion relation curves of the LC-modulated nanoantennas at different applying voltages; (2) time response of the LC-tuned nanoantenna; (3) fluorescence beams vertical to the substrate at different applied electric intensity; (4) FDTD simulations of the far field radiation patterns on bullseye optical antennas at different surrounding refractive indices. This material is available free of charge via the Internet at http:// pubs.acs.org.

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ACKNOWLEDGMENTS

This work was supported by the National Instrumentation Program (NIP) of the Ministry of Science and Technology of China (Grant No. 2011YQ03012408), and National Natural Science Foundation of China (NSFC) (Grant Nos. 21073073 and 91027010).

4. CONCLUSION We studied the active control of light emission on a LCmodulated metallic nanoantenna. 1D collimated fluorescence beams with a high directivity and monochromaticity were obtained from a metallic nanograting antenna. The beam wavelengths can be tuned precisely and continuously at high repetition rate using an electric signal. We also successfully extended the LC-based tunable optical antenna to other models, such as a bullseye-shaped nanoantenna, to attain tunable 2D collimated beams. This nanoantenna design for flexibly control luminescence from nano-emitters has broad applications, such as optical spectroscopy, light emitting devices, display, sensing, and optical communication, etc. According to the reciprocity principle of the antenna, it also enlightens us on the designs of smart nano-photodetectors and switchers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-431-85159383. Fax: 86431-85193421. Address: State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun 130012, P. R. China. Notes

The authors declare no competing financial interest. 19158

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