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We present an experimental demonstration of a quantum dot (QD)-based plasmon emitter controllably integrated in designed patterns on a thin metal film. The generation of surface plasmons polaritons (SPPs) from optically excited QDs on a thin metal film is experimentally demonstrated. Long-range, low-dispersion, two-dimensional isotropic guiding, as well as efficient coupling of the SPPs are also shown. The realization of planar, low loss and efficient plasmon emitter-waveguide integration will offer further development of plasmon circuits.
© 2013 Optical Society of America
1. Introduction
Surface plasmon polaritons (SPPs) are transverse magnetic (TM)-polarized electromagnetic waves coupled to collective electron oscillations on a metal surface, and can propagate in metal nanostructures beyond the diffraction limit [1, 2]. For guiding SPPs, various types of metal nanostructure have been proposed over the past decade [3]. They are now key elements in nanophotonic components, including waveguides [1, 4–8], resonators [9], logic gates [10], lasers [11, 12], detectors [13], modulators [14, 15] and amplifiers [16], which provides unique prospects for the design of nanophotonic circuits well known as “plasmon circuits” [17, 18].
The current challenge of plasmonics is the realization of a plasmon emitter (an optical source directly coupled to SPPs) [19, 20]; it is critical topic for the further development of SPPs and also the realization of plasmon circuits. Recently, strong interactions between SPPs and quantum dots (QDs) open a topic of great interest for a plasmon emitter. The emission properties of QDs can be significantly modified by the proximity of a metal structure supporting SPPs [21, 22]. In this phenomenon, partial energy of excited QDs can be directly coupled to SPPs, which has been demonstrated as a QD-based plasmon emitter with high efficiency in QD-metal nanowire coupling systems [23, 24]. However, integration of complex plasmonic components and routing of optical signals on a chip require the realization of power-efficient plasmon emitters coupled to planar waveguides with long-range and dispersionless guiding [16, 25, 26]. For the accomplishment of these goals, further advances in a design concept of emitter-waveguide integration and fabrication for integration of emitters with significantly flexible design on one-chip circuits are greatly needed.
In this paper, we experimentally demonstrate QD-based plasmon emitters controllably integrated in a designed pattern on a thin metal film supporting long-range, low-dispersion and two-dimensional isotropic plasmon guiding. We achieve the controlled deposition of colloidal QDs into designed patterns on a thin metal film by using lithography process combined with chemical functionalization. When the QDs are optically excited, emission from the QDs directly couples to long guiding SPPs supported by the thin metal film, without specific polarization and momentum matching; it results in the emission from an output slit (Fig. 1). The excited SPPs propagate along the metal surface in all directions, and its propagation properties can be well controlled by the design of a guiding structure. By the formation of the symmetric distribution of the refractive index in a waveguiding structure, we demonstrate a plasmon emitter with SPPs that show low-dispersion and ∼20-fold longer propagation than that of single-interface SPPs and nanowire SPPs. We also discuss the potential for power-efficient emitters. The QD-based plasmon emitters analysed here will lead to the realization of planar, low loss, controllably integrated and efficient plasmon emitter-waveguide system, and greatly increase the applicability range of plasmon circuits.
Fig. 1 Direct coupling between QDs and SPPs on a thin metal film. (a) Schematic of the QD-based plasmon emitter. SPPs can be excited directly by illuminated QDs, resulting in the emission from an output slit. (b) Energy structure of the QD-SPP coupling system. The arrows show the transitions. Photons are first converted into excitons of QDs, and a part of the excitons can be coupled to SPPs in thin metal films. The excited plasmons then propagate and finally are absorbed in metal layers or coupled to far fields at an output coupler.
2. Fabrication and experimental setup
The QD-based plasmon emitters are fabricated by using electron beam (EB) lithography process combined with chemical functionalization and focused ion beam (FIB) milling (Fig. 2(a)). First, Al2O3-Ag-Al2O3 layer (the Ag thickness has three variation: tAg = 20 nm, 25 nm, 30 nm, and the Al2O3 thickness is 20 nm) is evaporated on SiO2 substrate (Fig. 2(b)). The Al2O3 layers can be used for the bond of QDs, the prevention of quenching of QD emission, and forming the symmetrical index distribution. For the controlled deposition of colloidal QDs, a template polymer (ZEP520A, the thickness is 300 nm) is formed on the three-layer structure with EB lithography, as shown in Fig. 2(c). CdSe/ZnO QDs in water (80 nM, modified with carboxyl groups, emission wavelength: 625 nm ± 15 nm, Invitrogen) are then poured onto the template polymer. During evaporation of the solution, the majority of QDs can be trapped in the template holes due to the hole configuration and the hydrophilic contrast between Al2O3 and polymer (Al2O3:hydrophilic, polymer:hydrophobic) [27]. After the evaporation, the trapped QDs are immobilized on the Al2O3 surface with chemical bonds, and a few QDs remained on the polymer are removed by rinsing with water. We confirmed that the QDs are selectively trapped in fabricated holes, from the fluorescence image as shown in Fig. 2(d). Also, it is expected that the QDs form a multi-layer in a hole, from the nonuniformity of the fluorescence intensity. This QD deposition technique enables us to fabricate integrated QD-based plasmonic components with significantly flexible design on a chip.
Fig. 2 The fabrication of colloidal QD-based plasmon emitters. (a) Schematic drawing of the fabrication procedure, EB lithography, deposition of QDs and FIB milling. (b) Scanning electron microscope (SEM) image of the cross-section of Al2O3-Ag-Al2O3 layer (each thickness is 20 nm, and the image was obtained by SU9000, Hitachi High-Tech). (c) Optical image of the patterned polymer. (d) Fluorescence image of QDs trapped in fabricated holes. The image is observed with wide-field excitation by a 532 nm laser. (e) Scanning ion microscope (SIM) image of the fabricated system. QD area is 5 μm squares and the distance between the edge of the QD area and a slit is 15 μm.
After the controlled deposition of colloidal QDs, an output slit (600 nm width) for SPP observation is finally fabricated by FIB milling (40 keV, 0.74 nA, FB2200, Hitachi High-Tech). In this process, we use a conductive polymer as a cover layer for the prevention of charging and the reduction of the damage of QDs. This FIB process can also be used for formation of various types of waveguide configuration and their routing. Typical fabricated structure is shown (Fig. 2(e)) with tAg = 25 nm, a 5 μm squares of QDs area and 15 μm distance between the edge of QDs area and an output slit.
The SPP excitation from the deposited QDs is characterized with a fluorescence microscope with two dedicated detection branches: fluorescence imaging and spectroscopy. The sample is excited by a linearly polarized laser beam (wavelength: 532 nm), which is focused onto the QDs through an objective (50×, NA = 0.55). The resulting fluorescence is separated from the excitation wavelength with a dichroic mirror and a long-pass filter, and then recorded by a complementary metal oxide semiconductor (CMOS, with a linearity) detector and a spectroscope.
3. Results and discussion
Figure 3 presents an experimental demonstration of the direct plasmon excitation in the QDs-metal film systems. Figure 3(b) shows the fluorescence image of the system (Fig. 3(a)), when the QDs are directly illuminated by the laser. Besides the direct fluorescence of QDs (left side), we observed the emission light from the output slit (right side). We made sure that the emission light from the slit is seen when only the QDs are illuminated by the laser, and there is no emitter in and around the slit. Also, we confirmed that there is no propagation mode except SPP modes in the system, which was analysed by finite element method (FEM) calculation (Fig. 3(c)). Although the transverse electric (TE) modes (Fig. 3(e): dielectric modes) are effectively pushed away from the metal film as the polymer thickness decreases and have a cutoff level at around 450 nm polymer thickness, TM modes (Fig. 3(d): SPP modes) are free to propagate in the plane even in the fabricated system (300 nm polymer thickness). Note that this TM mode corresponds to a ‘long-range mode’ in our system, which shows a relatively low propagation loss. Also, a ‘short-range mode’ is present in the system, but the mode never reach the output slit due to its high propagation loss (the calculated propagation length: 0.49 μm). From the above discussion, we concluded that the emission from the slit is due to the propagation of SPPs (long-range modes) directly excited by the QDs. This paper therefore focuses on the long-range mode excited by QDs.
Fig. 3 Excitation and propagation of SPPs in QDs-metal film systems. (a) SIM image of the structure. The structural parameters are the same as for Fig. 2(e). (b) Fluorescence image with an excitation laser focused onto the QD layer. The arrow shows the polarization of an excitation laser. (c) Effective index of TM (SPP) and TE (dielectric) modes versus polymer thickness. (d,e) Field distribution of TM mode (300 nm polymer thickness) (d) and TE mode (600 nm polymer thickness) (e).
The measurement of the SPP emission direction (Fig. 4(b),(e)) demonstrates the two-dimensional isotropic propagation of excited SPPs. We observed a fluorescence image with a linearly polarized laser, in a QDs-metal film system with concentric output slits (Fig. 4(a)): 2 μm diameter QD area, 600 nm slit width and 10 μm distance between the QDs edge and the slit. We found that the fluorescence intensity from the slits is almost equal in all directions, which means that the excited SPPs show the isotropic propagation. In other words, the results show that there is no dependence on the laser polarization for SPP excitation and its guiding. This is due to the coupling mechanism between photons and surface plasmons. In the presented system, photon-plasmon coupling can be simply explained by the energy transition: photons-excitons-plasmons [28] (Fig. 1(b)), and there is no need for specific polarization and momentum matching, as the optical dipolar near-field contains polarized and momentum components matching those of SPPs. The system does not need prescribed polarization and incident angle for the excitation, showing the flexibility of optical pumping in the emitter.
Fig. 4 Two-dimensional isotropic plasmon guiding. (a) SIM image of the fabricated structure (tAg = 25 nm) with concentric output slits. (b) Fluorescence image in the system with concentric output slits. The emission direction angle (θp) and the polarization of an excitation laser are indicated. (c,d) Distribution of the emission intensity from a concentric slit when only collecting with the electric field component polarized along the transverse (c) and longitudinal (d) axis, respectively. The arrows show the transmission axis of the polarizer. (e,f) Polar plot of the emission intensity (a.u.) from the concentric slit at different points of θp. (e) and (f) present the results from (b) and (c), respectively. θp (degrees) is noted by numbers around the circle. The polarization direction of collection is highlighted by the blue dashed line in (f).
In contrast to the input, the output intensities from the concentric slits vary with the orientation of a linear polarization filter inserted between a detector and a sample. When the transmission axis of the polarizer is aligned along the transverse direction, the emission can be strongly detected from the bilateral slits and the upper and lower slits appear dark (Fig. 4(c)). The reverse situation occurs when the transmission axis is rotated by 90° (Fig. 4(d)). Figure 4(f) presents a polar plot of the emission intensity from the image of Fig. 4(c), which reveals the polarization dependence of the output emission. This result is consistent with the notion of plasmon excitation and its isotropic guiding. SPPs are TM-polarized wave, and have a longitudinal (the propagation direction) electrical field component. When the TM-wave is coupled to far fields at an output coupler, it results in a free space wave polarized along the propagation direction, that is, the vertical direction along the slit in the present case. The obtained result is a consequence of the nature of SPPs, highlighting that we successfully demonstrated SPP guiding from the QD-based plasmon emitter.
The unique characterizations shown above can be used for the quantitative measurement of a propagation property of excited SPPs, presented in Fig. 5. To obtain a propagation length of SPPs, we fabricated the system with output slits placed concentrically at distance Lp =10 ∼ 30 μm that is the length from the emitter (Fig. 5(a)). This structure enables us to measure an output intensity (an integrated intensity along a slit; the damping effect due to the two-dimensional isotropic propagation [29] is canceled) at each distance at the same time, which can evaluate the accurate propagation length. Figure 5(c) presents experimentally observed output intensity (measured from the fluorescence image in Fig. 5(b)) as a function of the distance Lp. From the statistical exponential fit
to the experimental points, the power propagation length LSP = 23.2 ± 1.8 μm is obtained, which is in good agreement with FEM calculation. The obtained value means that the considered system supports the SPPs with greatly longer propagation compared with other SPP modes, including single-interface SPPs, gap SPPs and nanowire SPPs (for example, a 100 nm diameter Ag-nanorod in air presents LSP = 1.84 μm at the wavelength 625 nm, calculated by FEM). This long-range guiding can be understood through the distribution of the electric field in the propagating SPP mode (Fig. 3(d)), showing the almost symmetrical field distribution along y-axis. Such field distribution is well known to present long propagation (in the case of the perfect symmetrical field, a structure supports the so-called long-range SPPs [30, 31]), and it can be significantly controlled by the design of the refractive index distribution. We note that the presented system, consisting of the designed index distribution (SiO2-Al2O3-Ag-Al2O3-polymer-air), led to the direct excitation of SPPs with long-range guiding.Fig. 5 Propagation length of excited SPPs. (a) SIM image of a fabricated structure for the measurement of the propagation length for tAg = 25 nm. Slits are located concentrically in the place of each distance (10 μm, 15 μm, 20 μm, 25 μm and 30 μm). (b) The experimentally observed fluorescence from each slit. (c) The experimental (red dot) and theoretical (dashed curve) dependences of normalized emission from the slits plotted on the distance Lp. The theoretical results are calculated at the vacuum wavelength 625 nm by FEM. Error bars indicate ±1 s.d.
To obtain further insight into the propagation properties of excited SPPs, we observed LSP of three systems with different Ag film thicknesses: 20 nm, 25nm and 30 nm (Fig. 6). This measurement demonstrates that the propagation length rapidly increases as the thickness decreases, and reaches LSP ≈ 35 μm for the 20 nm thickness. This strong dependence on a metal layer shows good agreement with FEM calculation, illustrating once more that we achieved the controlled coupling of QDs into SPPs with long-range guiding in the QDs-metal film systems.
Fig. 6 Propagation length as a function of the Ag film thickness tAg (the thickness of the polymer layer and the Al2O3 layers is fixed). The red dots and black dashed curve represent experiments and FEM calculations, respectively. Error bars indicate ±1 s.d. Inset: Fluorescence images of the system for two different Ag film thicknesses.
Further demonstration of the characterization of the plasmon emitter is presented in Fig. 7. The broadband coupling and guiding are demonstrated by comparing the optical spectra associated with emission from the QDs and from the output slit. We observed identical spectra with ∼30 nm width for both the QDs and output emission (Fig. 7(a)). The calculated dispersion relation around the emission wavelength (580 nm–680 nm) is also presented in Fig. 7(b). It can be seen that the dispersion of both parts of the effective index is low, and there is no resonant effect. These results are a consequence of the fact that the system supports the low-dispersion broadband guiding and the equivalent coupling efficiency in a wide range of optical frequencies. Furthermore, we mention that the characterization of long-range guiding with low-dispersion can be seen even in the near infrared region (∼ 1000 nm). This fact highlights the wide frequency range of the plasmon emitter applicability.
Fig. 7 Broadband coupling and guiding. (a) Spectra from the QDs-metal film system. The black curve shows the fluorescence spectrum of deposited QD layers. The red curve represents the fluorescence spectrum acquired via emission from an output slit upon plasmon excitation by QDs. Inset: Observed fluorescence image. The structural parameters are the same as for Fig. 2(e). (b) Theoretical dependences of the real (black curve) and imaginary (red curve) parts of the effective index for the fundamental SPP mode (tAg = 25 nm) plotted on vacuum wavelength. The dark part indicates the QD spectrum width (625 nm ± 15 nm).
We also evaluated the efficiency of QD emission into SPPs in the plasmon emitter. The efficiency can be estimated by comparing the fluorescence intensity from the QDs and the intensity of whole excited SPPs,
where IQD is the directly observed intensity from the QDs, and ILR and ISR are the intensity of the excited long-range mode and short-range mode, respectively. Ifree and Isub represent the scattered intensity from the concentric slit (see Fig. 4(b)) into free space (substantial emission) and a substrate, respectively. In experiments, the SPP emission via only long-range mode is observed at the output, thus we consider a lower bound on the efficiency [23] (ηl, the right side of Eq. (2)) in this discussion. Here we estimated that Ifree and Isub are nearly equal (ILR = Ifree + Isub ≈ 2 Ifree), because the amount of emission into the upper and lower directions from the slit will be similar due to the symmetry of the field distribution (see Fig. 3(d)) and the coupler configuration. ηl ≈ 15 ± 4 % was then obtained from the fluorescence of the system. This value does not account for the out-coupling efficiency and the propagation loss of SPPs. Considering the calculated coupling efficiency (1 − R − T: the SPP reflectance R = 0.03 and transmittance T = 0.45 at the slit were obtained by finite-difference time-domain (FDTD) analysis) and the experimentally measured loss factor, the correct efficiency reaches ηl ≈ 34±8 %, directly demonstrating efficient coupling to SPPs. We note that this coupling efficiency can be further improved by the modification of the QD layers. In the presented emitter, the efficiency difference between the lower QD layer (in the proximity of a metal film) and the upper QD layer (far from a metal film) is expected. Although only the lower layer will effectively couple to SPPs due to Purcell effect, the upper layer will not mainly contribute to the SPP excitation, which will lead to reduction of the total efficiency. Therefore, the decrease of the number of the QD layer (a single layer will be better) will significantly increase overall system efficiency.4. Conclusion
We have experimentally demonstrated the design of functional and controllably integrated QD-based plasmon emitters coupled to a thin metal film supporting long-range (∼ 35 μm), low-dispersion and two-dimensional isotropic plasmon guiding. The QD-based plasmon emitters analysed here have high potential for power-efficient emitters, and can also be applied to a wide range of frequencies by selection of QD size and composition. Our concept will be implemented with a plasmon emitter coupled to other planar plasmonic waveguides with same configurations such as metal slab plasmonic waveguides and dielectric-loaded plasmonic waveguides; these waveguides are now one of the best options for plasmonic routing and processing. Furthermore, the emitter can be integrated with other QD-based components, such as modulators [32] and amplifiers [33] with similar dimensions, in one-chip circuits by using our fabrication technique. We believe that the present work will be the basis of these applications and offer further development of a wide range of plasmonics.
A part of this work was supported by “Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University)” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan [No. F-12-OS-0003, No. S-12-OS-0012].
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