We report the color-tuning and switching optical transport characters of CdS hybrid plasmonic waveguide by using near-field optical microscopy. The guided photoluminescence spectra under various waveguide lengths demonstrate a spectroscopic red-shift for the part of CdS nanoribbon placed on the sapphire substrate and an energy compensation at Ag film. Surface plasmon polariton leakage and radiation are explored by near-field characterizations. Finite difference time domain simulations have good agreement with the experimental observations of subwavelength confinement and propagation. With a strong end facet emission, the suggested hybrid plasmonic waveguide can serve as a color-changeable optical nanosource in integrated photonic devices.
© 2009 OSA
Surface plasmon polaritons (SPPs) are electromagnetic surface modes confined at a metal-dielectric interface due to the resonant interaction of the electromagnetic wave with the surface charges of the metal . SPPs have proved to be efficient for confinement and control of optical signals in subwavelength scale [2–6]. SPP waveguide, served as a prospective type of optical information carrier in integrated photonic devices, has become an important topic in nanophotonics and photoelectronics. Metal stripe has been investigated for the plasmonic coupling and propagation , but due to the Ohmic losses, the SPP propagation length is limited to the order of 10 μm in the case of noble metals at visible wavelengths.
The SPP propagation constant β is determined by the corresponding metal and dielectric permittivities ε m and ε d, and the light wavelength λ: (1). The real part of β is related to the effective refractive index, , implying the presence of a dielectric layer on the top of the metal surface can result in a higher SPP index as compared to a metal-air interface . Using SiO2 stripe and Au surface to generate dielectric-loaded SPP (DLSPP) waveguide has been proposed [8,9], and analyzed both theoretically and experimentally [10,11]. Low edge-scattering losses have been achieved in comparison with the bare metallic stripes (diameter < 2 μm), and DLSPP waveguide is expected to be used as the basic building blocks of integrated circuits and plasmonic devices. However, this kind of DLSPP mode can only be excited at near-infrared range, and its architecture is highly depended on the nanofabrication technique. CdS nanoribbons, for their function as device elements and interconnects, are attractive for the assembly of integrated nanosystems. They are found to have photoluminescence and optical transport properties at visible range , can serve as a dielectric layer on the top of metal surface to generate a DLSPP waveguide. Zhang Xiang’s group (UC Berkeley) proposed the idea of hybrid plasmon mode by depositing the semiconductor nanowire on the top surface of metal film . At almost the same time our research group experimentally confirmed the excitation of this kind hybrid plasmon mode by using CdS nanoribbon deposited at the Ag film . Most recently, Zhang’s group reported the plasmon laser based on CdS-Ag structure (MgF2 for the gain layer) realized at deep subwavelength scale in Nature, which open new avenues in the fields of active photonic circuits .
Here, we propose another kind hybrid plasmonic structure by placing CdS nanoribbon partly on the Ag film and sapphire surface. For this architecture, we realize the color-tuning and switching for this hybrid plasmonic waveguide, which are investigated by using scanning near-field optical microscopy (SNOM) at the visible range.
A 100 μm × 100 μm Ag film (50 nm thickness) with the 2 nm average roughness is fabricated on a smooth sapphire substrate by electron beam lithography (EBL) and electron-beam evaporation techniques. The CdS nanoribbon with a cross section about 1 μm × 300 nm, is synthesized based on the physical evaporation in the presence of the Au film catalyst and can be moved to any places on the substrate surface by using poly (methyl methacrylate)-mediated (PMMA) nanotransfer printing technique . This transfer approach is based on the manipulation of individual CdS nanoribbon by handing the PMMA mediator at nano-scale. First, CdS nanoribbon with desired properties on a source substrate are loaded onto PMMA mediator, then the mediator is driven to contact with target substrate at the desired location with an aligning system, finally the PMMA film is removed to release the CdS nanoribbon on to the target surface.
3. Results and discussions
The experimental schematic is shown in Fig. 1 . The laser beam from a laser diode is used to illuminate the cantilevered fiber probe. The reflected beam is detected by the four quadrant position sensitive detector (PSD), which is used to control the probe-sample distance. The fiber probe collects the nonradiative signal from the near-field. With the advantage of the collection mode of SNOM, the waveguide excitation and detection locations can be separated. Due to the subwavelength resolution of SNOM, the SPP wave propagation and emission can be imaged in the near-field. The SNOM spectroscope consists of a scanner NSOM-100 (Nanonics Co.), an electronic controller SPM-1000 (RHK Co.), spectrometer iHR550 (Jobin Yvon Co.) and an avalanche photo diode (APD).
Figure 2(a) is the far-field photoluminescence (PL) image captured by a color CCD for this hybrid plasmonic waveguide. The inset is the bright field optical transmission of the same area, which features a 250 μm long CdS nanoribbon placed on a 100 μm × 100 μm Ag film. The SNOM spectroscopy is used to investigate this CdS-Ag structure under a He-Cd laser (442 nm) in p-polarization.
For the initial measurements, the in situ near-field PL spectra of points A, B and C are recorded with the SNOM detection probe paused at the same excitation laser spot, respectively. Points A, B and C are left end, middle part and right end of the nanoribbon. Point A is not shown in Fig. 2(a) because of the vision limitation of the microscope. The spectroscopic results exhibit similar optical excitation and transport properties for the different nanoribbon locations, as shown in Fig. 2(b).
On the other hand, the guided PL spectra are detected under various waveguide lengths with the SNOM probe always paused at emission end facet (position C). The waveguide length can be controlled by moving the excitation laser spot along the nanoribbon as sketched in Fig. 2(c). With the increment of the propagation length, a 51 meV spectroscopic red-shift from 2.354 eV (526 nm) to 2.303 eV (538 nm) is obtained from the spectra of locations C, D and E, as shown in Fig. 2(d). The points D and E are located on the sapphire and far from point C 30 μm and 50 μm, respectively.
For polar semiconductors, the structural disorder can induce an extension of the density of state into the band gap near the main edges of the bands, and optical transitions assisted by this extension of state give rise to an exponential tail near the fundamental absorption edge at the long wavelength direction, i.e. Urbach tail, which can induce self-absorption of band-edge emission and then leads to a red-shift of the PL spectrum [17–19]. The optical transport in CdS nanoribbon can take place preferentially along the long axis direction, and the positions for excitation and emission can be separated effectively, making the self-absorption phenomena easier to be observed and recorded by SNOM .
On the other hand, the trade-off between waveguide lengths and plasmonic spectroscopic shifting is also investigated by the excitation laser illuminating the CdS nanoribbon on Ag surface (5 points, 20 μm intervals). The guided plasmonic spectra (points 1-5) are recorded with the SNOM paused at emission end (point C). When the laser moves to the Ag film, a spectroscopic energy compensation occurs, all the detected energy peaks are kept around at ~2.34 eV (529 nm) [see Fig. 2(d, e)]. The intensity decay is induced by the dissipation of SPP energy during the wave propagation. However, at the area of Ag/sapphire joint, the emission intensity is enhanced, which we suppose is caused by the SPP radiation at the boundary of Ag and sapphire substrate. It is confirmed by the radiation spot captured by the color CCD at the crossing of the nanoribbon and Ag edge, as indicated in Fig. 2(a).
For this hybrid plasmonic waveguide, the excited SPP modes mixed with CdS PL waves propagate along the nanoribbon at the incident direction, but due to the quenching effect of the Ag film , the CdS PL modes will be gradually absorbed during the wave propagation and only monotone energy emission (~2.34 eV) is recorded by SNOM. By using the dispersion Eq. (1), the SPP energy for CdS-Ag structure is calculated around 2.35 eV (527 nm), which is almost the same as the detected emission result. On the other hand, most of SPP modes are confined at the interfaces of CdS and Ag film, and cannot be involved in the self-absorption of band-edge emission (Urbach tail), which explains the reason why there is no obvious spectroscopic red-shift for the near-field spectra recorded in the Ag region. Thus, by placing a CdS nanoribbon on the Ag surface with the distal end at sapphire substrate, the emission color-tuning and switching property can be realized in a short optical transport region.
The SPP subwavelength confinement and propagation for this hybrid CdS-Ag plasmonic structure are also investigated by the waveguide leakage radiation and end emission in the near-field, as shown in Figs. 3(a) , 3(b). Hybrid plasmonic energy is confined at the interface between the CdS and Ag film and finally emits from the nanoribbon ends. The wave propagation and emission are 3-D reconstructed as inset of Fig. 3(a); it shows a low energy radiation from both edges of the ribbon and a strong light emission at the distal end. The scanning topography of the ribbon end is shown in the inset of Fig. 3(b) at left up corner.
For comparison, the near-field optical image of the CdS nanoribbon entirely placed on the Ag film is also shown as the inset in the middle of Fig. 3(b). The emission intensities at normal and lateral directions are plotted in Figs. 3(c) and 3(d), respectively. The intensity data at lateral direction (line A and line a) can be fitted by the Gauss curve with a FWHM of 250 nm. By the criterion, e −1 of initial optical intensity (e: mathematical constant, 2.72), the emission length for our hybrid plasmonic waveguide can be estimated around 4 μm according to the intensity curve of line B.
In Comparison with the intensity of line a and b, the results of line A and B represent a higher emission intensity and longer emission length, but both curves A and a keep the FWHM always in the subwavelength scale. That means our hybrid plasmonic structure can gather both merits of dielectric and plasmonic waveguide (confinement in the subwavelength). The lower intensity for line a and b is caused by the absorption effect of the Ag film. When increase the incident laser power, the emission length for both waveguides is expended and recorded emission intensity will be increased but still keeps the FWHM in the subwavelength scale. Because the individual nanoribbons can act as laser cavities [15,18], this hybrid plasmonic CdS waveguide under high optical pumping could have a potential application for the plasmonic wavelength-tunable laser in the future.
In order to gain insight of this hybrid plasmonic Ag-CdS structure, finite difference time domain (FDTD) simulations are performed to demonstrate the SPP subwavelength confinement and propagation. The Grid of 2 × 2 nm is utilized to mesh the simulation space. The model geometry is schematically shown in Fig. 4(a) . A CdS nanoribbon (refractive index n = 2.64) is modeled on a Ag layer (50 nm thickness, permittivity ε = −7.6321 + 0.7306i). The simulation is carried out by commercial software XFDTD (Remcom, Inc.).
The subwavelength confinement and propagation of excited SPP modes at the interfaces of nanoribbon and Ag film are illustrated in Fig. 4(a). Two SPP waves are initially launched at the incident spot, and the evolution of the mode is observed during the propagation [See Fig. 4(b)]. It is found that the wave energy can couple into the other wave due to the overlap of the SPP modes. The coupling distance d c (about 2 μm), at which all the SPP modes are coupled to the neighboring wave, is related to the distance between the wave cores (L, 500 nm). The increase of the coupling length with the distance between the wave cores is proved to be exponential, which means the SPP coupling efficiency will become extremely weak when the CdS nanoribbon is infinitely extended into a 2-D film. (This property can be used in waveguide couplers and splitters.) The simulation results correspond with the near-field optical observations of the SPP subwavelength confinement and propagation as shown in Fig. 3(a).
Figure 4(c) is the steady-state image of the electric field for the CdS nanoribbon at the boundary of Ag layer and sapphire surface. Some part of the propagating modes are coupled to the radiation light from the top surface and both sides of the nanoribbon, which corresponds to the light spot observed at the crossing area between the Ag layer and sapphire, as shown in Fig. 2(a). Figure 4(d) is the electric field distribution for the propagation modes beyond the Ag layer and on the sapphire surface. From the image, we can find that most of the energy modes are still confined at the interface between the nanoribbon and sapphire when they propagate away from the Ag layer, only a very small part of energy modes are involved in the Urbach tail, i.e. self-absorption process which should take place in the body of nanoribbon. That is the reason why there is no obvious spectroscopic red-shift for the curves of points 1-5, as shown in Fig. 2(d).
In conclusion, the color-tuning and switching optical transport are realized by a CdS nanoribbon partly deposited on Ag surface with a distal end placed on the sapphire substrate. The guided PL spectra under various waveguide lengths demonstrate a spectroscopic red-shift caused by the semiconductor self-absorption effect (Urbach tail), and an energy compensation at the Ag film induced by the SPP resonance. The SPP sub-wavelength confinement and propagation characters are also observed in the near-field and the results correspond with the FDTD simulations. With a strong distal end emission, the suggested hybrid plasmonic waveguide can serve as an optical nanosource in the photoelectric circuits, and has potential applications for the wavelength-tunable plasmonic laser in the future.
We want to thank Dr. Renmin Ma, Peking University, for the CdS nanoribbon preparation. The work is supported by the National Science Foundation of China (Grant No. 10574002) and National Basic Research Program of China (973 Program) Grant No. 2007CB936800.
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