In order to investigate the effects of plasmonic environments on spontaneous emission of magnetic and electric dipoles, we have studied luminescence of Eu3+ ions in close vicinity to gold nanostrip arrays. Significant changes in the emission kinetics, emission polarization, and radiation patterns have been observed in the wavelength range corresponding to the plasmonic resonance. The effect of the plasmonic resonance on the magnetic dipole transition 5D0→7F1 is found to be very different from its effect on the electric dipole transitions. This makes Eu3+-containing complexes promising for mapping local distributions of magnetic and electric fields in metamaterials and plasmonic systems.
© 2013 Optical Society of America
Unique properties of plasmonic metamaterials and structures are associated with strong modification of the spectrum of electromagnetic modes due to electromagnetic responses of engineered subwavelength inclusions [1–3]. In such media, electric and magnetic components of electromagnetic fields can be altered to a different degree and have different spatial distributions. As it has been discussed in [4–6], rare earth or transition metal ions with both electric dipole and magnetic dipole transitions in the luminescence spectra can be used to experimentally probe and separately map the distributions of optical electric and magnetic fields. Placing such an emitter in a magnetic or electric resonance environment, one can expect modification of spontaneous emission of, correspondingly, magnetic dipole transitions or electric dipole transitions due to the Purcell effect . The goal of this work is to study the effect of plasmonic excitations in nanostructured metallic systems on electric dipole and magnetic dipole emission transitions.
2.1. Eu3+ organic systems and sample preparation
Trivalent Europium ions doped into organic hosts, in particular, europium nitrate Eu(NO3)3·Bpy2, Bpy,2,2’ bipyridine, and an alkylated europium complex, tris(α-thenoyltrifluoroacetonato)(1-octadecyl-2(−2-pyridyl) benzimidazole) europium(III), Eu(TTA)3(L18), are convenient systems to probe the effects of local electromagnetic environments. They can be excited with ultraviolet (UV) light, and have highly efficient luminescence in the visible part of the spectrum with well-distinguishable electric dipole and magnetic dipole transitions [4, 8, 9].
The Eu(NO3)3·Bpy2 and Eu(TTA)3(L18) chromophore materials have been synthesized in house, following the routine described in [4, 9]. We used the Langmuir-Blogdett technique to deposit thin films of amphiphilic Eu(TTA)3(L18) complex. Spin coating technique was used to fabricate thin films of Eu(NO3)3·Bpy2 in polyvinylpyrrolidone (PVP) (1:1 by weight) (see [5,6] for the fabrication details). The thickness of the fabricated films ranged between 20 nm and 70 nm, as confirmed by atomic force microscopy (AFM) measurements.
Gold and silver films used in our experiments were fabricated with thermal deposition technique. Arrays of gold strips on glass substrates have been fabricated using interferometric lithography to form a photoresist pattern in SU-8 on top of a metal film and then transferring the pattern into the metal with a reactive ion etching, a dry etch [10–12]. The energy level diagram of Eu3+-containing complexes and their luminescence spectra are shown in Fig. 1. Several emission lines ranging from 570 nm to 850 nm belong to transitions between the same excited state 5D0 and multiple components of the ground state multifold 7Fn. The strongest line, which accounts for ~80% of the total emission, is observed around 610 nm (611 nm in Eu(TTA)3(L18) complex, and at 614 nm in Eu(NO3)3·Bpy2) and corresponds to the 5D0 – 7F2 transition. The transition 5D0 – 7F1 at λ~590 nm is primarily associated with a magnetic dipole. The other spectral lines are related to the primarily electric dipole transitions . The emission kinetics lifetimes are ~0.8 ms in Eu(TTA)3(L18), and ~0.7 ms in Eu(NO3)3·Bpy2.
2.2. Modification of Eu3+ emission near planar metal-dielectric interface
Modification of magnetic and electric dipole emission in a modified local optical environment has been studied in literature, both theoretically and experimentally [4–6, 12–23]. Commonly, the case of planar geometry has been considered, when emitters were placed at a certain distance from metallic or dielectric surfaces. The theoretical models describing spontaneous emission in inhomogeneous environments can be found in [23, 24]. For the planar geometry, the results have been often discussed in terms of the image dipole approximation [12–14]. In order to describe interplay of spectral line intensities more accurately, a competition between different transitions originating from the same excited state has been taken into account in [18–20].
In the dipole image approximation, in the vicinity of a mirror, the transition probability of an electric dipole oriented parallel to the surface is suppressed, and that of a dipole oriented perpendicular to the interface is enhanced, Fig. 2(a). The situation is opposite for a magnetic dipole, Fig. 2(b). Correspondingly, when the luminescence spectrum is recorded in the direction normal to the surface, one can expect an increase of the relative intensity of a magnetic-dipole transition and reduction in the relative intensity of an electric-dipole transition.
Figures 3(a) and 3(b) show typical experimental results observed in relatively thick (50-70 nm) films of Eu(NO3)3·Bpy2:PVP in vicinity of a flat silver mirror [4, 5]. Lower emission intensity was recorded for these films at the spectral maximum (614 nm) for light polarized with electric field E parallel to the interface (s polarization), as compared with the orthogonal p polarization. This observation was in accord with the predictions of the image dipole approximation model for electric dipoles [12–14]. The ratio of emission intensities between p and s polarizations, Ip /Is, grew with an increase of the observation angle, Fig. 3(c), due to the higher contribution of perpendicularly oriented dipoles to p-polarized emission. The intensity of the magnetic dipole transition, I590, normalized to that of the strongest electric dipole transition, I610, was higher on the metal film than on the top of a glass substrate, Fig. 3(a). This was also predicted by the dipole image approximation.
At the same time, a very different emission behavior has been observed in thin (~20-30 nm) Eu3+ doped organic films deposited on gold or silver surfaces . In particular, (i) the intensity of p-polarized emission was lower than that of s polarized emission, (ii) the ratio, Ip/Is, recorded at the spectral maximum, decreased with increase of the observation angle, Fig. 3(d), and (iii) the magnetic dipole emission line at 590 nm was relatively smaller than that of films of the same thickness on dielectric surfaces, Fig. 3(b). According to our preliminary studies, the difference between the experiment and the predictions of the image model was particularly pronounced when the metallic film was thin or rough, pointing to a possible role of plasmonic excitations. The goal of the current work is to study in more detail the effect of plasmonic environment on the emission of magnetic and electric dipoles. In order to clearly distinguish between the plasmonic effect and other possible contributing factors, here we focus on the system (array of metallic nanostrips), whose plasmonic response has a pronounced spectral signature. Modification of Eu3+ luminescence in close vicinity of thin and rough metallic films will be discussed elsewhere.
2.3. Array of gold nano-strips – anisotropic plasmonic system
A scanning electron microscope (SEM) image of the array of gold strips is shown in Fig. 4(a). The period of the structure was 340 nm, the width of individual strips was 170 nm, and their height was ~100 nm. The size of the patterned structure was of about 8 mm in diameter. Optical properties of similar nanostrip arrays were studied in [26, 27]. It was shown, that the reflection and scattering of light in such systems is different for different polarizations due to localized surface plasmons which can be excited by an optical electric field (E) oriented perpendicular to the strips.
The reflection spectra of our nanostrip array and the gold film (collected at the incidence angle of 8 degrees) are shown in Fig. 4(b). One can see that light polarized with E parallel to the strips, E||, was reflected with almost the same efficiency as the light reflected off a continuous gold film. A considerably weaker reflection was observed at the perpendicular polarization, E⊥, (E perpendicular to strips). This result is similar to that observed in , where it was ascribed to enhanced scattering and absorption of the perpendicular polarization (E⊥) due to excitation of localized plasmons. To confirm this mechanism in our samples, we measured the spectral dependence of scattered light as a function of polarization.
In this particular experiment, the strips, oriented either vertically or horizontally, were illuminated at a close-to-normal angle of incidence. Scattered light with vertical polarization was collected at an angle of ~70 degrees off normal, see Fig. 4(c). The ratio of intensities I⊥/I|| is shown in Fig. 4(d) for the light scattered forward and backward. As one can see, in the forward direction, a maximum in I⊥/I|| is observed at ~720 nm. In line with [27, 28], we explain increased transmission of light polarized perpendicular to the strips (E⊥) in the spectral range of the plasmon resonance in terms of plasmon-mediated extraordinary transmission through subwavelength slits. In the Appendix, using simple equivalent circuit arguments, we show that enhanced transmission and reduced reflection of a generic resonant cavity can qualitatively describe the experimental results of Fig. 4(d).
2.4. Eu3+ spontaneous emission in the nanostrip arrays
We started the series of spontaneous emission experiments with the study of the effect of Au nanostrips on the Eu3+ emission kinetics. Polymeric films doped with Eu3+ ions were deposited on the nanostrip arrays. The samples were excited with the 4th harmonics of the Q-switched Nd:YAG laser (λ = 266 nm, tpulse≈5 ns). The kinetics were recorded at the emission maximum (611 nm or 614 nm, depending on a particular system) using 1 GHz digital oscilloscope.
The emission kinetics are shown in Fig. 5. They are multi-exponential in the beginning and become nearly single exponential at longer times. In the Eu(TTA)3(L18) complex, the longest decay time constants, measured at the “tails” of the emission kinetics, vary from ~0.8 ms on glass, to 0.5 ms on gold film, and to 0.3 ms on gold nanostrips. In Eu(NO3)3·Bpy2, the corresponding decay times are equal to 0.66 ms (glass), 0.5 ms (gold), and 0.4 ms (gold nanostrips).
In the spectroscopic measurements performed in the Fluoromax 3 spectrofluorometer, the sample was mounted on the rotation stage, which allowed one to set strips vertically or horizontally. The emission was excited at λ = 320 nm and collected, in both “backward” and “forward” directions, at vertical polarization and nearly normal detection angle, see schematic in Fig. 6(a). In the latter case the emission was an order of magnitude weaker than the emission collected in the backward direction.
Figures 6(b) and 6(c) show the spectra of the magnetic dipole transition (5D0 –7F1) collected at vertical orientation of the strips (E parallel to strips, E||) and horizontal orientation of the strips (E perpendicular to strips, E⊥) in the backward (B) and forward (F) directions. The spectra are normalized to the spectral maximum of the electric dipole transition (5D0 –7F2) at λ≈0.61 μm.
As one can see, in the backward direction, the relative intensity of the magnetic dipole line is significantly lower for parallel polarization (E||) than for perpendicular polarization (E⊥) or for the Eu3+ doped film deposited onto a glass substrate (compare traces B|| and B⊥ and “glass”). However, light emitted in the forward direction shows a completely different behavior, with the ratio I590/I610 higher for parallel polarization (E||) than for perpendicular polarization (E⊥), (traces F|| and F⊥).
Let us now compare absolute emission intensities at two polarizations. The polarized emission spectra collected in the backward direction are shown in Fig. 7(a).
The intensity was always higher at E parallel to the strips. We estimated the ratio of intensities at perpendicular and parallel polarizations, I⊥/I||, for each spectral line, which was clearly resolved in the luminescence spectrum. As shown in Fig. 7(b), for the emission recorded in the backward direction, I⊥/I|| varied from ~0.75 to 0.27, depending on the wavelength. A broad minimum was observed at ~650 - 700 nm. To the contrary, in the emission collected on the other side of the sample (in the forward direction), the ratio I⊥/I|| was greater than unity for the majority of spectral lines. Another important observation was that the ratio I⊥/I|| measured for the magnetic dipole transition (≈590 nm) was noticeably off the range of the values calculated for the electric dipole transitions.
The same result can alternatively be presented in terms of the ratio of emission intensities in the forward and backward directions, IF/IB, Fig. 7(c). It has a maximum around 650-700 nm at the perpendicular polarization, and a minimum at the parallel polarization. Framed this way, a distinct behavior of the magnetic dipole emission line is seen only for the parallel polarization.
To summarize the experimental results, strong modification of the emission kinetics, as well as polarization and radiation patterns have been observed in spontaneous emission of Eu3+ on top of arrays of gold nanostrips in the spectral range of localized plasmon resonance.
The most important effects include:
- 1) Significant shortening of the emission kinetics of Eu3+ ions.
- 2) Changes in the directionality and polarization of Eu3+ emission. A clear spectral signature is observed in the range of the plasmon resonance (λ≈650-700 nm), with the minimum of I⊥/I|| clearly seeing in the backward direction, and I⊥/I|| >1 in the emission transmitted through the strips in the forward direction, Fig. 7(b).
- 4) Strictly different behavior of the magnetic dipole emission, 5D0→7F1, and the electric dipole emission.
Strong shortening of the emission decay kinetics and sharp decrease of the emission intensity (quenching of radiation) in close vicinity of metal are commonly explained by emission to surface plasmon polariton modes, which have high propagation ohmic loss [29,30], as well as quenching at interband transitions of bound electrons. In our experiments, the kinetics was faster in vicinity of strips than in vicinity of planar gold. This points to the presence of an additional relaxation channel, such as coupling to localized plasmons, which are excited with E perpendicular to strips.
Coupling between surface plasmons and radiative modes can significantly modify the emission properties . The similarity between the far field light scattering and the electric dipole emission in our experiments can be explained assuming that only electric dipoles oriented perpendicular to strips are efficiently coupled with the localized plasmons. As the result, their emission is partially transmitted. This is not the case for the parallel dipoles. Correspondingly, we see in Fig. 7(b) that in the forward scattering direction, I⊥/I|| >1, with the maximum around the plasmon resonance frequency.
The difference between the electric dipole and the magnetic dipole transitions is most pronounced when we analyze the emission spectrum transmitted forward through the strips as the function of polarization, Fig. 7(c). At the parallel polarization, IF/IB is very low for all electrical transitions (see the bottom curve in Fig. 7(c)). This can be expected, assuming that the electric dipoles parallel to strips do not excite the plasmonic mode. The fact that we see much larger ratio IF/IB for the magnetic transition may indicate a possible coupling of the magnetic dipoles to the plasmons.
In conclusion, we have studied modification of magnetic and electric dipole emission of Eu3+ ions placed in close vicinity of plasmonic nanostrips. Significant shortening of the emission kinetics as well as changes in the spectra, polarization and emission patterns have been observed. These effects are attributed to coupling of emitting ions to the plasmonic and radiative (photonic) modes, which is different for electric and magnetic dipoles. These results may be important for applications involving probing and mapping electric and magnetic field distributions in photonic metamaterials and plasmonic structures.
Let us consider a simple equivalent circuit where the plasmonic resonance is presented by a resonant cavity coupled to input (incident light) and output (transmitted light) transmission lines. In our model, P0 is the input power, ω0 is the resonance frequency, β1, β2 are the coupling coefficients, Q0 = ω0/(2δ) is the unloaded quality factor, and δ is the half-width of the resonance curve.
In such a circuit, reflection and transmission coefficients can be expressed in terms of impedances of the transmission lines, Z01, Z02 (for simplicity, Z01 = Z02 = Z0) and the cavity, . Here R is the series cavity resistance and n is the transformer factor (typically, n<<1). Reflected and transmitted powers (Pr and Pt, respectively) can be found as [32, 33]
In our experiments, a role of the resonance cavity is played by gold nanostrips, where localized plasmon resonances are excited only for E perpendicular to the strips. The orthogonal polarization, with E parallel to the strips, is not efficiently coupled with the resonance, and not transmitted forward into the “output line”.
The authors acknowledge the support by the NSF PREM grant # DMR 1205457, NSF IGERT grant #DGE 0966188, and AFOSR grant # FA9550-09-1-0456.
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