---
title: "Electrical Excitation of Long-Range Surface Plasmons in OLED/PC Structure with Two Metal Nanolayers"
authors: ["Valery N. Konopsky"]
affiliation: "Valery Konopsky Elena Alieva Institute of Spectroscopy Russian Academy of Sciences, Fizicheskaya, 5, Troitsk, 108840 Moscow, Russia. Valery Prokhorov Dmitry Lypenko Artem Dmitriev Frumkin Institute of Physical Chemistry and Electrochemistry Russian Academy of Sciences, Leninsky pr., 31/4, 119071 Moscow, Russia. Giovanni Dietler Sergey Sekatskii Laboratoire de Physique de la Mati `ere Vivante, IPHYS, Ecole Polytechnique F 'ed 'erale de Lausanne, CH-1015 Lausanne, Switzerland."
journal: "Nano-Micro Letters"
year: 2020
volume: ""
issue: ""
article_number: ""
pages: ""
doi: "10.1007/s40820-020-0369-7"
type: journal-article
site_group: ""
url_abstract: ""
url_pdf: "https://valery.konopsky.com/kvnlocal/Konopsky2020_Article_ElectricalExcitationOfLong-Ran.pdf"
language: en
source_tex: "Z:\\ValeryData\\Valery_New\\my_articles\\NanoMicroLetters2020\\NanoMicroLetters\\manuscript\\Konopsky2NML.tex"
source_pdf: "Z:\\ValeryData\\Valery_New\\my_articles\\NanoMicroLetters2020\\NanoMicroLetters\\published\\Konopsky2020_Article_ElectricalExcitationOfLong-Ran.pdf"
---
## Abstract

A current-driven source of long-range surface plasmons (LRSPs) on a duplex metal nanolayer is reported. We experimentally observe electrical excitation of LRSPs in a planar structure, where an organic light-emitting film is sandwiched between two metal nanolayers, which serve as electrodes. In order to achieve the LRSP propagation in these metal nanolayers at the interface with air, this light-emitting structure is bordered by a one-dimensional photonic crystal (PC) on the other side. The dispersion of the light emitted by such hybrid PC/OLED structure comprising two thin metal electrodes is obtained and the LRSP resonance peak is clearly identified. \keywords{Surface plasmons \and Photonic crystal waveguides \and Light-emitting polymers} % \PACS{PACS code1 \and PACS code2 \and more} % \subclass{MSC code1 \and MSC code2 \and more}

Figure1.eps gsave newpath 20 20 moveto 20 220 lineto 220 220 lineto 220 20 lineto closepath 2 setlinewidth gsave .4 setgray fill grestore stroke grestore

# Introduction

Broad and wide interest to the problem of surface plasmons (SPs) amplification and lasing, witnessed during already at least twenty years, is driven first of all by the practical necessity of the miniaturization of electronic and optical devices, lowering their power consumption, increasing operation frequency range, requirements for larger and larger degree of integration, and so force. Nowadays, there is a general believe that optics-based approaches, due to their inherent advantages over electronic ones, will progressively replace the latter. With some abuse of language, it is sometimes said that the twenty-first century will be the “century of photons” in the same sense as the previous one was that of electrons. Of course, profound scientific studies and technological researches in the field of surface plasmon lasing, which started with the SPASER proposal in 2003 [1] and then lead to the experimental realization of such, or similar, devices (see e.g., [2, 3, 4, 5, 6, 7, 8, 9], and a kind of mini-review [10] to cite only some of the related works) are extremely important steps in this direction.

However, without the denial of huge and real progress, two requirements (which as of today are almost never met) should be underlined. First, any surface source of light, which do will work in the field of the future photonics and/or its interface with the electronics, must not depend on whatsoever “bulk” and external (for corresponding operational purposes) laser providing an optical pumping. Second, the generation of exactly the surface plasmons propagating along the corresponding interface(s) rather than, say, lasing in the direction perpendicular to the surface where nanoplasmonic particles/structures are located, is needed.

In the current paper we report our first results in the direction of fabrication of such devices. In a system under the study, a current injection-driven organic light–emitting diode (OLED), which from the “waveguide” point of view is just a certain multilayer dielectric structure, is sandwiched between two thin metal layers (Au and Al electrodes) and deposited on a 1D photonic crystal (PC). Thus, the complete multilayer structure, shown in detail in Fig. 1, comprises the following layers [PC/M$_1$/OLED/M$_2$/air]. The integral thickness of the OLED is selected in such a way that *both* metal electrodes are able to support the long-range propagation of surface plasmons, if their thicknesses are small enough.

The ability of a *single* metal nanolayer to support LRSP propagation in a symmetrical “dielectric / thin metal layer / dielectric” [D/M/D] structure is well known from the early 1980s [11, 12] and is widely used in plasmonics (see recent editorial [13] as a brief review). To simplify the implementation of LRSP in practical applications, where external dielectric is air, a multilayer structure in the form [PC/M/air] was proposed [14]. Such an approach was tested by some of the authors with quite different systems, which among others include thin palladium layers (for ultrasensitive hydrogen detection [15, 16, 17]), thin gold layers in blue spectral range (for nitrogen dioxide detection [18]), and thin ferromagnetic cobalt layers (for magnetoplasmonics [19]), see also [20, 21, 22, 23] as examples of other applications.

Note, however, that in all these aforementioned works, the structures comprising only one thin metal layer were exploited. Recently, it was recognized that in the case of an appropriate design, *two* thin metal layers, provided an adequate dielectric spacing between them is ensured, also can quite effectively support long-range surface plasmons propagation [24]. This is exactly the situation which occurs here and seems barely can be circumvented in some other way given the task: of course, the current injection in OLED is caused by the (DC) voltage applied between two (possibly thin) metal electrodes.

# Plasmons in duplex metal layer

## Experimental setup

The experimental setup is presented in Fig. 1(a). The structure under the study [PC /Au/OLED/Al/ air] was placed onto the quartz right angle prism’s hypotenuse. A thin layer of immersion oil was used to attain optical continuity and refractive index matching between the substrate of the PC chip and the prism. Light coming out from the multilayer structure under study was collected via a multimode optical fiber with input face rigidly attached to the rotating arm of the home-made screw-gear setup allowing angular scanning with the accuracy of 0.25$^{\circ}$. At each fixed registration angle, the spectrum of the collected light was recorded using the AvaSpec 2048 fiber-optic spectrophotometer (Netherlands), with a spectral resolution 0.04 nm.

![(a) Layout of the experiment and (b) the 1D photonic structure with two metal nanolayers.](media/NanoMicroLetters2020/Figure1.eps)

*(a) Layout of the experiment and (b) the 1D photonic structure with two metal nanolayers.*

## 1D Photonic Crystal

The 1D PC-part of the multilayer structure was deposited by magnetron sputtering and has the form [PC]=[substrate/(H L)$^N$/H’], where L is a SiO$_2$ layer with d$_1$=118.6 nm, H is a Ta$_2$O$_5$ layer with thickness d$_2$=86.8 nm and H’ is a Ta$_2$O$_5$ layer with d$_3$=75.8 nm. The prism and the substrate were made from fused silica. The refractive index (RI) of Ta$_2$O$_5$ layers is n$_2$=n$_3$=2.11, whereas RIs of the substrate and prism and SiO$_2$ layers are n$_0$=n$_1$=1.46 (at wavelength $\lambda$=575 nm). When $N$=13 (in our case), this SiO$_2$/Ta$_2$O$_5$ 27-layers structure (started and finished by Ta$_2$O$_5$ layers) permits guided waves, propagating along the outer layers, to be decoupled through a prism at the resonance angle $\theta$ as shown in Fig. 1(a). So this scheme may be called “inverted Kretschmann geometry”. If the number $N$ increases, this decoupling pathway will vanish, and only emission through edges of this multilayer structure would be possible.

## OLED composition

The OLED-part of the multilayer structure has the form: [OLED]=[MoO$_3$/SY/LiF] with thicknesses of the layers pointed in Fig. 1(b). The RIs of OLED layers, at $\lambda$=575 nm, are: the transport layer (MoO$_3$) =2.0, the light-emitting layer (SY) =1.86 and barrier layer (LiF) =1.39. The RIs of Au and Al at this wavelength are equal to n$_\mathrm{Au}$=0.3+2.8$i$ and n$_\mathrm{Al}$=1.1+6.9$i$, correspondingly [25, 26, 27]. The maximum brightness of a test OLED structure with aforementioned parameters, but without 1D PC-part [glass/Au/OLED/Al/air], measured in the direction perpendicular to the layers was 13 500 cd/m$^2$ at 12 V.

Super Yellow (SY, PDY-132, Merck) is an efficient electroluminescent material having a broad luminescence band centered around 570 nm; its emission spectrum is presented in Fig. 2 as a cyan line. It is widely used as a polymer light-emitting layer for OLED manufacture due to its high stability and excellent brightness characteristics [28, 29]. The technical reason for the choosing of MoO$_3$ (Lumtec, Taiwan) as material for the hole injection-transport layer (HTL) is the fact that it is not dissolved in chlorobenzene used for the structure preparation, namely during the spin-coating deposition of SY layer just above the transport layer. Besides this technical reason, MoO$_3$ has quite suitable alignment of energy levels in the band structure with respect to the active layer and Au anode [30]. As is well known, the position of the HTL energy levels is of uttermost importance for OLED functioning, making it possible to reduce the drive voltage by enhancing the charge injection at the interface, thereby improving the power efficiency of the device.

## OLED fabrication on the 1D PC

The OLED manufacturing process was as follows: First, the surface of the 1D PC structure was irradiated by a UV lamp for a time of 15–20 minutes to clean and activate the external PC surface. At the next stage, the auxiliary 90 nm-thick layer of Al was deposited through the specially designed mask onto a part of the PC surface by thermal evaporation in vacuum at a pressure of $6\cdot 10^{-6}$ Tor and with the rate of 0.02–0.04 nm/s. This thick metal layer surrounding the OLED area was used for the subsequent attachment of external electrical contacts (electrodes). Then, the 18 nm-thick gold layer (OLED anode) was deposited through another mask by thermal evaporation using the same vacuum and rate conditions. This layer, at certain places, predefined by the masks design, physically and electrically contacts with the earlier deposited thick Al-layer, and therefore, via this layer, also with external electrodes. Four independent and different OLED structures with the sizes of ca. 4x4 mm$^2$ were prepared on each 1D PC chip with a diameter of 25.4 mm.

Further, MoO$_3$ transport layer 50 nm in a thick was deposited with a rate of 0.03 nm/s in same vacuum conditions, The next step is the spin-coating deposition of the SY layer over the MoO$_3$ layer. 80–90 ul the well preagitated solution of SY in chlorobenzene, with the concentration of 5 mg/ml, rotation speed of 1000 rpm, and about 1 minute exposition time, were used for the spin coating. Then the sample was rest to dry for a time of 12 hours at room temperature, and 4 h at 800 C to remove residual clorobenzene traces. These steps of the preparation of OLED samples, as well as measurements of their spectral and photoelectric characteristics were performed at room temperature in a glove box MBraun (Germany) under argon atmosphere with a controlled content of oxygen and water (below 1 ppm). Finally, 1 nm-thick LiF and 18 nm-thick Al (cathode) layers were thermally deposited, again in the same vacuum and rate conditions as described above with the use of the third mask. Voltage–current and voltage–brightness characteristics were measured with Keithley 2601 SourceMeter, Keithley (USA) 485 pico-ammeter and TKA-04/3 luxmeter-brightness meter (Russia). The thicknesses of the films were determined using MII-4 interferometer (LOMO, St.-Petersburg, Russia).

![Electroluminescence spectrum from the 1D PC with Super Yellow light-emitting layer, which is sandwiched between two metal nanolayers. The standard Super Yellow emission spectrum is presented for comparison.](media/NanoMicroLetters2020/Figure2.eps)

*Electroluminescence spectrum from the 1D PC with Super Yellow light-emitting layer, which is sandwiched between two metal nanolayers. The standard Super Yellow emission spectrum is presented for comparison.*

#  Results and discussion

## LRSP resonance observation

The red curve in Fig. 2 shows a representative example of the recorded spectra at registration angle $\rho$=n$_0\sin(\theta)$=1.005. The LRSP resonance is seen at $\lambda_\mathrm{LRSP}$=575 nm, while two local maxima at 527 nm and 630 nm correspond to band-gap edges of the structure under study. Such measurements were repeated point by point across the angular parameters $\rho$=$0.89\dots1.12$. Due to the finite sizes of the light source and the optical fiber input aperture, the recorded spectra roughly correspond to a 2$^{\circ}$ convolution around the selected angle.

![Dispersion plots of the light-emitting structure under study. (a) Calculated intensity enhancement at the external surface (in a logarithmic color scale), and the dispersion curves of surface modes for a semi-infinite 1D PC (black lines, an error bar represents the imaginary part of ρ). (b) The stack of 29 experimental spectra of electroluminescence taken at different angular parameters ρ. (c) Calculated integral of the optical electric field in the Super Yellow layer. (d) Superimposed experimental spectra (as smoothed contour lines) and the integral of the optical electric field in the SY layer.](media/NanoMicroLetters2020/Figure3.eps)

*Dispersion plots of the light-emitting structure under study. (a) Calculated intensity enhancement at the external surface (in a logarithmic color scale), and the dispersion curves of surface modes for a semi-infinite 1D PC (black lines, an error bar represents the imaginary part of ρ). (b) The stack of 29 experimental spectra of electroluminescence taken at different angular parameters ρ. (c) Calculated integral of the optical electric field in the Super Yellow layer. (d) Superimposed experimental spectra (as smoothed contour lines) and the integral of the optical electric field in the SY layer.*

The stacked set of such angular measurements taken at different $\rho$ provides a two-dimensional picture $I(\rho,\lambda)$, shown in Fig. 3(b), which reveals local maximum at (1.003, 575 nm), corresponding to the LRSP excitation. It was previously demonstrated [14, 31] that the excitation of optical surface waves with an effective RI near
``` math
\begin{equation}
\rho_{1/2}= 
\mathrm{n}_\mathrm{e}
+
\frac{\mathrm{n}_\mathrm{e}^{3}}{2}\,\left[{\pi}{\frac {{\mathrm{d}_\mathrm{m}}}{{\lambda}}}\right]^{2}

\end{equation}
```
is an essential prerequisite for the LRSP propagation along a thin metal film. For the classical LRSP-structure [D/M/D] this condition is satisfied automatically [31, 32]. In [24] it was brought out that the same prerequisite is held for structures containing two metal nanolayers. In this case, the tangential component of the electric field is zero at the centers of *both* metallic nanofilms, and the damping of optical waves is small. In our experiments, both metal nanolayers have a thickness of d$_\mathrm{m}$=18 nm and from (1) we get (for $\lambda$=575 nm and n$_\mathrm{e}\simeq$<!-- -->1) that $\rho_{1/2}$=1.003. Thus, the experimental value given above agrees well with this value.

## Dispersion plots

Figure 3 demonstrates two-dimensional dispersion plots of the light-emitting structure under study in coordinates ($\rho$, $\lambda$). In Fig. 3(a), the magnitude of the optical field enhancement near the external interface with air is depicted in color tones with the logarithmic color scale (shown to the right). This field enhancement was calculated for our real structure containing the 27-layers of SiO$_2$/Ta$_2$O$_5$ in the 1D PC. Two black curves are the dispersion curves that were figured out for a structure with a semi-infinite 1D PC (see [24] for more details).

It is seen from Fig. 3(a) that two corresponding modes display anti-crossing, and one mode is shifted to the light line (shown as the dashed line of total internal reflection (TIR) $\rho_\mathrm{TIR}$). An error bar represents the imaginary part of the effective RI $\rho$. It can be seen that the imaginary part decreases when LRSP curve approaches the light line at the point (1.003, 575 nm).

To simulate the experimentally recorded dispersion of electroluminescence, which is shown in Fig. 3(b) (and which was certainly recorded in the far-field zone), we used the reciprocity theorem [33, 34]. In our case, it means that the optical electric field, created in the far-field zone by dipoles located in the SY layer (with coordinates from 0 to d — see Fig. 1(b) and Fig. 4 below), is the same as the electric field of dipoles from the far-field zone (i.e. from plane optical waves) created in the layer [0 d]. The integral of the intensity distribution in the SY layer (when the structure is excited by plane waves at different angles of incidence $\rho$ and wavelengths $\lambda$) is shown in Fig. 3(c) in color tones. Fig. 3(d) depicts the superposition of the calculated integral of the optical electric field in the Super Yellow (SY) layer (from (c)) and the experimental spectra (from (b)) depicted as a smoothed contour plot. There is a good agreement between these two figures.

## Field Profiles 

The electric field profiles in the outer layers of the structure under study is shown in Fig. 4. These profiles were calculated for LRSP excitation at $\lambda$=575 nm and $\rho$=1.003. One can see that the tangential component of the optical field is indeed equal to zero at the centers of both metal nanolayers (as it should be at $\rho_{1/2}$), and that the total optical field in the metal reaches a minimum with these parameters. More information on designing LRSP-supporting structures with two metal layers can be found in [24].

![The spatial distribution of the optical field components in the outer layers of the structure, at λ=575 nm and ρ=1.003.](media/NanoMicroLetters2020/Figure4.eps)

*The spatial distribution of the optical field components in the outer layers of the structure, at λ=575 nm and ρ=1.003.*

# Conclusion

We have developed a current-driven source of long-range surface plasmons in the multilayer structure containing OLED, bounded by two metal nanolayers, on top of a one-dimensional photonic crystal. Electroluminescence spectra were recorded at set of different decoupling angles, which provide the dispersion profile of the system under study. The LRSP resonance manifests itself at $\lambda$=575 nm, near the light line, as expected from the structure design.

The obvious next step will be an attempt to achieve LRSP amplification and lasing in the structure with two metal nanolayers. The lasing in the classical symmetric LRSP-supporting structure with one metal nanolayer has been reported for InGaAs quantum-well gain media [35]. But, again, this is a device with the optical pumping, like all SPASERs are today, to the best of our knowledge (for review of amplification and lasing in LRSP-supporting systems see [36]). We hope that the presented approach will lead to electrical pumping of future SPASERs. The lasing in organic media is in itself a challenge, but, nevertheless, there is a recent report on the current-injection lasing from an organic semiconductor [37]. Thus, even an organic current-injection SPASER does not seem impossible.

GD and SK thanks Swiss National Science Foundation for support of this work (grant 200021_162767); VK and EA performed this work as a part of the State assignment for the Institute of Spectroscopy of the Russian Academy of Sciences.

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