Observing and controlling a Tamm plasmon at the interface with a metasurface

Abstract We demonstrate experimentally that Tamm plasmons in the near infrared can be supported by a dielectric mirror interfaced with a metasurface, a discontinuous thin metal film periodically patterned on the sub-wavelength scale. More crucially, not only do Tamm plasmons survive the nanopatterning of the metal film but they also become sensitive to external perturbations as a result. In particular, by depositing a nematic liquid crystal on the outer side of the metasurface, we were able to red shift the spectral position of Tamm plasmon by 35 nm, while electrical switching of the liquid crystal enabled us to tune the wavelength of this notoriously inert excitation within a 10-nm range.

A Tamm plasmon (TP) is a localized resonant optical state, a quasi-particle, which exists at the interface between a metal and a dielectric (or semiconductor) Bragg mirror.It was theoretically predicted in [1] and experimentally observed in [2].The TP dispersion lies completely within the light cone and therefore, in contrast to an ordinary surface plasmon polariton, a Tamm plasmon can be excited with both TE and TM polarized light at any angle of incidence [1].Another advantage of a Tamm plasmon over a surface plasmon polariton is that the former appears to be almost insensitive to dissipative losses in the metal film since its electromagnetic fields are localized predominantly in the non-absorbing Bragg mirror [3].Because of its robust nature, a Tamm plasmon has been regarded as a viable alternative to conventional surface plasmons in a wide range of applications, including refractive index sensing, optical switches, semiconductor lasers, and temperature sensors [4,5,6,7,8,9,10].For many practical applications it is important to realize an external dynamic control over the TP wavelength.Such a task, however, presents a formidable challenge given that the fields of a Tamm plasmon reside inside the Bragg mirror and, therefore, are very difficult to access from the outside.Correspondingly, most of the approaches proposed so far involved the integration of a control element into the very structure of the Bragg mirror [4,6,11,12,13,14], which may not always appear feasible.It has also been shown that the wavelength of a Tamm plasmon could change (although irreversibly) as a result of the lateral confinement of its fields achieved by patterning the metal film on the microscale [3,15,16].
In this Letter we report on the first experimental observation of a near-IR Tamm plasmon at the interface between a Bragg mirror and a nano-patterned metal film acting as a nondiffracting optical metasurface.We found that the discrete framework of the metasurface exposed Tamm plasmon to external perturbations, such as changes of the refractive index in an adjacent medium, which enabled us to dynamically control the wavelength of this weakly coupled optical state in a simple yet efficient way.
Figure 1a presents the design of the structure that was used to observe Tamm plasmons in our experiments.The structure was based on a silver-coated dielectric Bragg mirror designed to exhibit a 0.5 μm wide reflection band centred at the wavelength λ = 1.45 μm.It was formed by a stack of alternating 11 layers of Nb 2 O 5 and 10 layers of SiO 2 .The niobium pentoxide and silicone dioxide layers had the thickness of correspondingly 159 ± 2 nm and 246 ± 2 nm, and were deposited onto a double-side polished fused silica substrate using magnetron sputtering (Kurt J. Lesker PVD 225), as detailed in [17].The silver coating had the thickness of 37 ± 2 nm, and was applied to a section of the Bragg mirror by magnetron sputtering at room temperature, working pressure of 2.2 mTorr and deposition rate of about 11 nm / min using an Ag planar sputtering target (99.99%purity).A 30 μm x 30 μm patch of the silver film was turned into a metallic metasurface by nanopatterning the film with a focussed ion beam (Helios Nanolab 600).The pattern of the metasurface featured a square array of 500 nm large disks with the period of 600 nm (see Fig. 1b).Such a pattern, although very simple, was sufficient to ensure that the metasurface would act as a non-diffracting narrow-band mirror above λ = 1.34 μm with its reflection band centred at the wavelength of ~ 1.55 μm, as defined by the plasmon resonance of silver nano-disks.(c) Reflectivity spectra of the Bragg mirror acquired while it was in a pristine state (blue), after it was interfaced with a 37 nm thick continuous silver film (black), and after the silver film was nano-patterned to become a metasurface (red).(d) Wavelengths of Tamm plasmons measured for the silver film and metasurface as functions of applied voltage after the structure was integrated with a liquid-crystal cell.
The spectral response of the fabricated sample was characterized in reflection at normal incidence using a commercial microspectrophotometer developed by CRAIC Technologies on the basis of a ZEISS Axio microscope.It employed a cooled near-IR CCD array with spectral resolution of 0.8 nm and featured a tungsten-halogen light source equipped with a broadband linear polarizer.Light was focused onto the sample as well as collected using a x15 reflective objective with NA 0.28.The reflectivity spectra were acquired through a 22 μm x 22 μm square aperture installed in the image plane of the microscope.
Figure 1c compares the reflectivity spectra taken at three different areas of the sample corresponding to an uncoated (i.e.pristine) Bragg mirror, a Bragg mirror with a continuous silver film, and a Bragg mirror with the metasurface.As per design, the pristine mirror is seen to exhibit a characteristic, spectrally flat reflection band spanning from about 1.22 to 1.66 μm (blue curve in Fig. 1c).The reflectivity spectrum of the silver-covered area of the mirror reveals the appearance of a narrow reflectivity dip located within the band of the pristine mirror and centred near λ = 1.49μm (black curve in Fig. 1c).Such a conspicuous transformation of the Bragg mirror's reflectivity spectrum signifies the excitation of the Tamm plasmon, as has previously been shown in a number of works [2,18,19,20,21].Quite remarkably, the Bragg mirror, when combined with the metasurface, also appeared to support Tamm plasmons, exhibiting a similar reflectivity dip in the same spectral window (red curve in Fig. 1c).Such an outcome can be readily appreciated if one recalls that the metasurface, although structurally discontinuous, was patterned on the sub-wavelength scale and so electromagnetically behaved as a continuous film.Furthermore, within the reflection band of the metasurface (centered at λ ≈ 1.55 μm by design) the polarizability of silver nano-disks was naturally larger than that of the unpatterned film owing to the plasmon resonance [22], which effectively made up for the loss of metal due to nanopatterning, and thus minimized the distinction between the metasurface and silver film in terms of their optical properties around 1.50 μm.
While the nano-pattering of the silver film did not seem to affect the ability of the structure to support a Tamm plasmon, it naturally exposed the surface of the Bragg mirror.Consequently, that should have locally broken the confinement of the Tamm plasmon, allowing a direct access to its fields (which would otherwise remain difficult to couple to residing under the continuous silver film [7,23,24]).To verify that assumption experimentally, we introduced an electrically controlled liquid-crystal (LC) cell into the structure of the sample, as schematically shown in Fig. 1a.The cell was assembled by placing an ITO cover glass approximately 10 µm above the silver-coated surface of the mirror.The cover glass served as the top (transparent) electrode of the cell, while the silver film played the role of its bottom electrode.The cell was vacuum-filled with E7 (Merck), a widely used and commercially available LC mixture with high optical anisotropy (n o = 1.50, n e = 1.70 [25]).The surface of the cover glass facing the mirror had been coated with a thin film of uniformly rubbed polyimide to ensure planar alignment of LC molecules in the cell (i.e., parallel to the mirror and along the direction of rubbing, n).By increasing the voltage across the cell, U, we gradually switched E7 from the planar to homeotropic state in which LC molecules were oriented perpendicular to the mirror.Due to optical anisotropy of LC molecules the switching of the cell was accompanied by the change of the LC refractive index from n e to n o for light polarized parallel to the direction of rubbing (E || n).Correspondingly, for light polarized perpendicular to the direction of rubbing (E ⊥ n) the refractive index remained n o .
Figure 2 presents reflectivity spectra of the sample integrated with the LC cell, which were measured under linearly polarized light, while sweeping the voltage across the cell from 0 to 2 V. Quite evidently, in the case of unstructured silver film the spectral location of the Tamm plasmon was unaffected by filling the cell with E7, as well as by changing the state of the liquid crystal in the cell (see Figs. 2a and 2b).As noted above, such behaviour resulted from strong confinement of the TP fields, which were effectively screened by the unstructured film from the ambient medium and, expectedly, remained inert to external perturbations [7,23,24].By contrast, the Tamm plasmon excited at the interface with the metasurface appeared to be quite sensitive to the presence of the liquid crystal (see Figs. 2c and 2d).In particular, filling the cell with E7 red-shifted the TP reflectivity dip by about 35 nm (E || n) and 25 nm (E ⊥ n), which was consistent with the increase of the refractive index above the metasurface from 1 to n e and n o , respectively.Also, for npolarized illumination the TP reflectivity dip was seen to blue-shift as soon as the applied voltage had exceeded 0.5 V with the extent of the shift reaching 10 nm at U = 2.0 V (see Figs. 2c and 1d).Our observations, thus, appeared to agree with the assumption we made earlier that the nano-patterning of the silver film would allow external coupling to the fields of the Tamm plasmon.Note that for the orthogonal polarization (E ⊥ n) sweeping the voltage did not have any effect on the TP wavelength (see Figs. 2d and 1d).Indeed, in that case the re-orientation of LC molecules occurred in the plane perpendicular to the polarization of light and, therefore, could not affect the relevant component of the refractive index tensor.Apart from changes in the spectral location of the TP reflectivity dip, we observed consistent, voltage-induced changes of its magnitude under n-polarized illumination.
Interestingly enough, such changes occurred also in the case of unstructured film (see Fig. 2c).The exact nature of those changes remains unclear to us.We speculate that the effect might result from mild focusing of light produced by the objective of our microspectrophotometer.Indeed, for light focused on a metal-coated Bragg mirror a change in the refractive index immediately above the structure would move its image from the image plane of the instrument and could, in principle, affect the level of the measured reflectivity.Another possible explanation is that a change in the refractive index of the ambient medium would change the reflectivity of its interface with the metal film and, therefore, could modify the conditions for the excitation of a Tamm plasmon.Such an explanation, if true, suggests a different approach to sensing involving Tamm plasmons.A detailed analysis of the above noted effect is currently underway and will be published elsewhere.
In conclusion, we showed experimentally that a Tamm plasmon could be excited in the near-IR at the interface between a dielectric Bragg mirror and a nano-structured nondiffracting metasurface (which effectively replaced a continuous metal film conventionally used as the second mirror).Our findings also indicate that the metasurface, through its discrete framework, had enabled an external access to, otherwise, weakly coupled fields of the Tamm plasmon.More specifically, we found that placing a dielectric, such as a liquid crystal, in direct contact with the outer side of the metasurface red-shifted the TP wavelength by as much as 35 nm (while no spectral shift could be detected when the liquid crystal was applied to a continuous metal film in the conventional configuration).Furthermore, we managed to tune the TP wavelength within a 10 nm range by changing the LC refractive index above the metasurface with an externally applied electric field.We note in passing that some of the currently available LC materials exhibit optical birefringence as high as 0.8 [26] and, hence, will enable even higher tuneability range.We also argue that the demonstrated ability to control the spectral location of Tamm plasmon opens up a viable route to exploiting this peculiar excitation in many real-life applications, including refractive-index sensing, enhancement of optical nonlinearity, and surfaceenhanced spectroscopy.

Figure 1 .
Figure 1.(a) Schematic of the structure used in our experiments.The white arrow indicates the direction of rubbing, n, which controlled the alignment of the liquid crystal.(b) Scanning electron micrograph of a fragment of the metasurface fabricated on top of a Bragg mirror.(c)Reflectivity spectra of the Bragg mirror acquired while it was in a pristine state (blue), after it was interfaced with a 37 nm thick continuous silver film (black), and after the silver film was nano-patterned to become a metasurface (red).(d) Wavelengths of Tamm plasmons measured for the silver film and metasurface as functions of applied voltage after the structure was integrated with a liquid-crystal cell.

Figure 2 .
Figure 2.Reflectivity spectra of the silver-coated Bragg mirror acquired with linearly polarized light at different voltages once the structure was integrated with a liquid-crystal cell (solid curves).Data in panels (a) and (b) correspond to an area of unpatterned silver film, while in panels (c) and (d) -to the metasurface.Dashed curves show the reflectivity spectra of the sample before the cell was filled with the liquid crystal.