Ultrafast plasmonics using transparent conductive oxide hybrids in the epsilon near-zero regime
Daniel Traviss, Roman Bruck, Ben Mills, Martina Abb, Otto L. Muskens
aa r X i v : . [ c ond - m a t . m e s - h a ll ] F e b Ultrafast plasmonics using transparent conductive oxide hybrids in the epsilonnear-zero regime.
Daniel Traviss, Roman Bruck, Ben Mills, Martina Abb, and Otto L. Muskens Physics and Astronomy, Faculty of Physical and Applied Sciences,University of Southampton, Highfield, Southampton SO17, 1BJ, United Kingdom Optoelectronics Research Centre, Faculty of Physical and Applied Sciences,University of Southampton, Highfield, Southampton SO17, 1BJ, United Kingdom (Dated: October 15, 2018)The dielectric response of transparent conductive oxides near the bulk plasmon frequency ischaracterized by a refractive index less than vacuum. In analogy with x-ray optics, it is shownthat this regime results in total external reflection and air-guiding of light. In addition, the strongreduction of the wavevector in the ITO below that of free space enables a new surface plasmonpolariton mode which can be excited without requiring a prism or grating coupler. Ultrafast controlof the surface plasmon polariton mode is achieved with a modulation amplitude reaching 20%.
Epsilon-Near Zero (ENZ) materials are a class of opti-cal materials characterized by a real part of the dielectricfunction close to zero. ENZ materials are of interest fora range of applications including tailoring of directionalemission and radiation phase patterns,[1–3] air-guiding ofelectromagnetic waves,[4] and electromagnetic tunnellingdevices.[5–7] While a lot of effort is aimed at achievingan ENZ response using artificial metamaterial resonators,some naturally occurring materials also show a strong re-duction of the permittivity below that of vacuum. Anexample of naturally occurring low-index materials arenoble metals where the optical permittivity ǫ is governedby the collective motions of the free electron gas knownas bulk plasmons. According to the Drude model, thepermittivity is given by ǫ ( ω ) = ǫ ∞ − ω ω + iωγ , (1)where γ denotes the damping rate of the free elec-trons and the plasma frequency ω pl is given by ω pl =( N e /ǫ m ) / . Around the (screened) bulk plasmon fre-quency ω bp ≡ ω pl / √ ǫ ∞ , the real part of the permittivityshows a transition from negative to positive values. No-ble metals have carrier densities N exceeding 10 cm − ,therefore their bulk plasmon plasma frequency is locatedin the UV region. In contrast, highly doped semiconduc-tors typically have electron densities below 10 cm − and can be well described by the Drude model with a bulkplasmon plasma frequency in the THz range. Transpar-ent conducting oxides (TCOs), with an electron densityinbetween that of bulk metals and doped semiconductors,show a bulk plasmon frequency in the near-infrared. Theresulting combination of a metal-like response in the in-frared and a dielectric optical response in the visible re-gion has stimulated application of TCOs as transparentelectrical contacts and as heat reflecting windows. Re-cently, metal oxides such as indium-tin oxide (ITO) andaluminium-doped zinc oxide have received interest fortheir plasmonic response in relation to metamaterials andtransformation optics.[8, 9] The plasma frequency can betuned by controlling the electron density using electrical or optical methods, opening up opportunities for near-infrared electro-optic or optical modulators [7, 10, 11]and sensing devices.[12, 13]Pioneering studies by Franzen and co-workers haveinvestigated the plasmonic response of ITO and ITO-gold hybrid structures in the metallic (negative epsilon)regime of ITO.[13] Next to a conventional surface plas-mon polariton mode for thick ITO films, a new polari-ton mode associated with the bulk plasmon resonancewas observed for 30 nm thin films. By using hybridITO-gold layers, the balance between the two types ofplasmonic modes could be controlled. In analogy withmetal nanoantennas, ITO and AZO colloidal nanoparti-cles and nanorods revealed pronounced surface plasmonresonances in the infrared range.[12, 14, 15]Here, we explore the intermediate regime where ITOshows a dielectric response with a refractive index belowthat of vacuum. In this studies, we used commercial ITOslides (Lumtec) of 350-nm thickness with a sheet resis-tance of 5 ± / square. Figure 1(a) shows the calculateddispersion relation for these ITO samples (blue line) us-ing the Drude model with experimentally obtained pa-rameters m = 0 . m e , γ D = 0 . N =1 . × cm − . The thin line (red) indicates the dis-persion relation of light in vacuum. A low-index regime isobserved in the frequency range between 0 . − .
27 PHz,which characterized by a superluminal phase velocity anda correspondingly reduced wavevector. The dielectricENZ regime covers the range of (complex) wavevectorsgiven by the condition Im k ITO < Re k ITO < k , as in-dicated by the shaded area in the dispersion relation.Figures of merit for the ENZ response are the refrac-tive index Re n ITO and the propagation length in unitsof wavelength Re n ITO / n ITO , as shown in Fig. 1(b).While the refractive index increases from 0.48 to 1.0 overthe ENZ window, the propagation length of light in theITO layer increases from 0.5 to 3.2 wavelengths.Light incident on an interface between two media fromthe medium with the higher refractive index undergoestotal internal reflection above a critical angle θ crit . Sim-ilarly, at the interface between air and an ENZ medium, FIG. 1: (Color online) (a) Photon dispersion relations for air (red), ITO (blue) and the ITO-Au SPP (dash, black), withimaginary part for ITO (short dash, green). (b) Same for real refractive index Re n ITO and the propagation length in numberof wavelengths, Re n ITO / n ITO . Reflectivity as a function of angle, with (white line) calculated TERF critical angle. Blueshaded area in (a) and dotted lines in (b) indicate epsilon-near zero (ENZ) regime. a similar effect of total external reflection occurs. To-tal external reflection at grazing angles is a well-knownphenomenon in x-ray optics, where it is commonly usedin reflective focusing optics, waveguides, and surfaceanalysis.[17] Figures 1(c,d) show the reflectivity of theITO-glass slide for various angles of incidence for TMand TE polarizations. The dashed lines indicate the es-timated positions of the critical and Brewster’s angles θ crit and θ B in the absence of absorption, i.e. using θ crit = sin − (Re n ITO ) and θ B = tan − (Re n ITO ). Thecritical angle ranges between 30 ◦ and 90 ◦ . For bothpolarizations, an increase in reflectivity is observed inthe ENZ regime above the critical angle. The exactdefinitions for θ crit and θ B loose validity in the pres-ence of absorption, and for strong absorption they tendto shift toward larger angles.[18] To compare the ex-act behavior we calculated Fresnel’s reflectivities usingthe Drude model with optimized free carrier density of N = 1 . × cm − . The resulting Figs. 1(e,f) showgood agreement with the experimental data and con-firm the presence of total external reflection in the NEZregime of ITO.In order to assess the air-guiding effect associated withtotal external reflection in ITO, we changed the configu-ration to a planar waveguide by aligning two ITO-slideswith a tunable separation of several hundred µ m. Theoptical beam was focused to a spot of 50 µ m in diameterusing a focusing lens with numerical aperture of 0.02. By tuning the space between the slides, we achieved up to 8multiple reflections. The resulting transmission spectrumthrough the waveguide is shown in Fig. 1 for different an-gles of incidence. We identify three regimes, respectivelyat energies below, in, and above the ENZ regime. Belowthe ENZ window, the metallic reflectivity of ITO resultsin a weakly varying transmission of several percent. Thistransmission is low because of insertion losses as well asbecause of the limited single-pass reflectivity of up to75% giving rise to a multi-pass transmission below 10%.Above the ENZ regime, the transmission contains con-tributions from the air-ITO and the ITO-glass interface,which are overall relatively small but increasing towardlarge angles of incidence. In the ENZ regime, large vari-ations of the transmission are found when changing theangle of incidence by only a few degrees. A pronouncedspectral dip is observed for TM polarization at angles be-low 80 ◦ , which is caused by a combination of reduced re-flection of the ITO-air interface and increased absorptionin the ITO layer. Above the critical angle, the waveguidetransmission dramatically increases by more than threeorders of magnitude around 0 .
25 PHz. These results in-dicate a potential application of TCO air-guided wavesusing total external reflection, for example in optical sen-sors or modulators.Next, we investigated electromagnetic surface waves atthe interface between ITO and gold in the ENZ regime.Surface plasmon polaritons (SPPs) on metal films are re-
FIG. 2: (Color online) Transmission of air-guided ITO-cladded waveguide for 8 multiple reflections, for TE (a) andTM (b) polarizations and for angles of incidence between 70 ◦ and 85 ◦ . Blue shaded areas indicate epsilon-near zero (ENZ)regime. ceiving enormous interest for their application in sensingand surface plasmon optics.[19] The SPP wavevector isgiven by k SPP = k (cid:18) ǫ d ǫ m ǫ d + ǫ m (cid:19) / . (2)The permittivities of the metal ǫ m and the dielectric ǫ d both depend on frequency. The dashed black line inFig. 1(a) shows the calculated SPP wavevector for theITO-gold interface. The surface plasmon resonance islocated at 0 .
618 PHz, outside the plotted window. Forlow energies, the SPP has a photon-like nature and itsdispersion follows that of the ITO film.For conventional SPPs occurring at the interface be-tween a dielectric and metallic layer, the wavevector islarger than that of free-space radiation; therefore cou-pling to SPPs requires wavevector matching using a glassprism, grating, or near-field scattering object. The ENZ-regime, however, enables a new type of SPP mode at theinterface of Au and ITO, which lies below the light line.Consequently, the SPP can directly couple to free-spaceradiation. Figure 3(a) shows the measured reflectivity ofthe 350-nm thick ITO layer coated with a 50-nm thick(continuous) Au film, for illumination through the ITO-side. A pronounced dip is observed around 0 .
25 PHz,which agrees well with the coupling angle obtained using θ SPP = sin − ( k SPP /k ), which in this frequency rangeclosely follows θ crit . A cross section of the reflectivityspectrum at 65 ◦ , plotted in Fig 3(b) (solid line), shows awell-defined spectral dip with a quality factor of ∼ . ◦ for which light can be coupled into the SPPmode at a frequency of 0.25 PHz.The effect of an additional SiO spacer layer betweenthe ITO and Au layers is shown by the dashed line inFig. 3(b). The refractive index of the spacer slightly in-creases the mode index of SPP, resulting in a redshift FIG. 3: (Color online) (a) Experimental reflectivity R TM / R TE for the ITO-Au multilayer. (b) Spectra at 65 ◦ angle of incidence for a 350 nm ITO - 50 nm Au (solid) and350nm ITO - 40 nm SiO - 50 nm Au multilayer structures. of the SPP dispersion. In addition a 10% narrowing isobserved which we attribute to reduced losses in the di-electric layer. The possibility to include a spacer layeris of interest as it allows electrically isolating the goldfilm from the ITO and holds promise for incorporatingactive layers, or field-effect designs for electro-plasmonicmodulation.[7, 10] Figure 3 shows that, even for a SiO spacer layer of 40 nm thickness, the effective mode in-dex of the SPP lies below unity and the SPP couples tofree-space radiation.As a demonstration of the functionality of the SPPssupported on the Au-ITO interface, we demonstrate ul-trafast control over the plasmon mode using femtosecondpulsed laser excitation. Ultrafast control of plasmons isof interest for applications in optical switching.[20] Herewe use the ultrafast response of the Au-ITO hybrid toproduce a spectral shift of the plasmon mode. The non-linear response was obtained using a regenerative ampli-fier with OPA (Coherent RegA), producing 220-fs laserpulses with a repetition rate of 250 kHz. Modulation ofthe probe pulses was detected using a lock-in amplifierto collect the modulation integrated over the ∼ FIG. 4: (Color online) (a) Reflectivity R TM with and without pump laser, normalized to intensity of TE without pump R TE , .(b) Differential reflectivity ∆ R/R for both TM (line, black) and TE (dash, red) polarizations and for pump-probe delay timeof 0.1 ps, at pump fluence of 10 mJ/cm2. (c) Comparison of ∆
R/R for ITO-Au (dots, black) and ITO-SiO -Au (diamonds,blue) multilayers at pump fluence of 3 mJ/cm . (d) Spectral map of differential reflection for TM obtained using spectrometer,with (e) spectrally integrated pump-probe signal for TM (line, black) and TE (dash, red). (c) and (e) were measured usinglock-in technique. by ultrafast heating of the Au-ITO hybrid. Further-more, the time-response of the TE-reflectivity indicatesthe presence of an additional small instantaneous con-tribution, possibly related to nondegenerate two-photonabsorption, or to the instantaneous Kerr-nonlinearity ofITO.[22]In hybrid nanostructures, hot-electron transfer mayform a new mechanism for enhancing the nonlinearresponse.[11, 23] To investigate this contribution, wetested the response of a multilayer consisting of 50 nmAu, 20 nm SiO and 350 nm ITO. It was found thatthe damage threshold of the Au layer in this samplewas substantially lower than the Au-ITO sample by afactor of three. The reduced pump power required theuse of more sensitive lock-in detection, resulting in thefrequency dependent response shown in Fig. 4(c). Wefind qualitatively the same response for the two Au-ITO(dots) and Au-SiO -ITO (diamonds, blue) samples, in-dicating that in both cases, the signal is governed by aredshift of the SPR. For the Au-SiO -ITO multilayer,the signal is reduced by a factor two. The combinationof an increased damage threshold and higher nonlinearresponse in the Au-ITO hybrid is consistent with the ul-trafast hot-electron transfer mechanism which lowers thestress of the Au-film and increases the hybrid nonlinear response.In conclusion, we have demonstrated new propertiesof ITO in the regime characterized by a refractive indexbelow unity (Epsilon Near-Zero). The ITO layer showspronounced total external reflection which can be usedto obtain waveguiding of light using air as the high-indexmedium. As the refractive index and thus the exter-nal reflection condition strongly depends on carrier den-sity, this effect may be useful for applications in opticalsensing. In addition, we have identified a new surfaceplasmon polariton mode at the interface between ITOand gold. The plasmon polariton is observed to couplestrongly to free-space radiation without requiring addi-tional phase matching, which is attributed to the stronglyreduced wavevector in the ENZ regime. We have demon-strated that this mode can be controlled on ultrafast timescale, opening up potential application in optical switch-ing. Future work may identify other applications of thisnew plasmon mode may be useful for applications suchas outcoupling of light from emitters, local field enhance-ment, or strong light absorption in energy harvesting ap-plications.This work was supported by the EPSRC throughgrants EP/J011797/1 and EP/J016918/1. [1] S. Enoch, G. Tayeb, P. Sabouroux, N. Gu´erin and P.Vincent, Phys. Rev. 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