Design of anapole mode electromagnetic field enhancement structures for biosensing applications

The design of an all-dielectric nanoantenna based on nonradiating “anapole” modes is studied for biosensing applications in an aqueous environment, using FDTD electromagnetic simulation. The strictly confined electromagnetic field within a circular or rectangular opening at the center of a cylindrical silicon disk produces a single point electromagnetic hotspot with up to 6.5x enhancement of |E|, for the 630-650 nm wavelength range, and we can increase the value up to 25x by coupling additional electromagnetic energy from an underlying PEC-backed substrate. We characterize the effects of the substrate design and slot dimensions on the field enhancement magnitude, for devices operating in a water medium. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Although typical surface-based fluorescence assays for detection of protein or nucleic acid molecules for applications that include disease diagnostics [1], genome sequencing [2,3], and pathogen sensing [4] are performed upon glass or plastic surfaces, a variety of nanostructured optical surfaces have demonstrated the ability to increase detected photon output through the mechanisms of enhanced excitation, directional emission and reduced fluorescence lifetimes [5][6][7][8][9].Such structures include plasmonic gratings, nanoantennas, and photonic crystals (PC) [10][11][12][13], which are each capable of efficiently coupling incident light from a laser into surface-confined resonant electric fields (enhanced excitation).PCs have been shown to be especially advantageous because their periodic dielectric structures are comprised of materials without loss at the critical wavelengths, and thus provide moderately high quality factor (Qfactor) resonances that generate strongly confined electric fields near the PC surface.We have shown that PC-enhanced excitation provides >100x improvements in measured detection limits for fluorescent emitters [14].
The use of optically resonant metallic nanostructures to control and concentrate light at sub-diffraction limit scales is currently a well-established capability [15,16].Surface plasmons, generated by the collective oscillation of conduction electrons near the surface of a metal [17,18], have been the subject of enormous interest for biological sensing applications where a host of nanoparticle shapes, surface structures, and materials [19,20] have been applied for coupling electromagnetic energy into molecules for purposes of label-free biosensing [21], fluorescence enhancement [22][23][24], and surface-enhanced Raman scattering (SERS) [25,26].Plasmonic metal nanoantennas' ability to drastically enhance the interaction between a single quantum emitter and its surrounding photonic environment is not only capable of luminescence enhancement, but also ultrafast emission in the picosecond range [27,28] and directional emission control [29,30] -making them nearly ideally suited for ultrasensitive biodetection of single molecules.However, energy transfer to the free electrons in the metal generates losses, which not only quench fluorescence emission, but also cause substantial Joule heating of the antenna and its environment [31] with sufficient magnitude to melt nanoparticles [32] and to kill cells [33,34].Thus, plasmonic nanoantennas face fundamental limitations for applications that require moderate temperatures [35] (such as biosensing of proteins, nucleic acids, and small molecules) and high excitation powers [36] desired for fluorescence excitation.
Nanoantennas capture illumination from a source in the far field and compress its optical energy into volumes smaller than the diffraction limit [37][38][39].The strong local electromagnetic field enhancements (called "hotspots") enable a wide range of useful applications, such as SERS [40], surface-enhanced infrared absorption (SEIRA) [41][42][43][44][45], spontaneous emission enhancement [46], photohermal biosensing [47], nonlinear nanophotonics [48,49], and nanolasing [50,51].To achieve large enhancement of the hotspot localized power, which scales with the square of the magnitude of the electric field |E| 2 , a wide variety of approaches have been reported, including manipulation of the antenna's physical dimensions and materials to tune the resonant wavelength [44,52], using low-loss materials [53,54], impedance matching the input excitation to the nanoantenna [55], and engineering Fano resonances [43,56,57].To achieve the smallest hotspot volumes and the largest field enhancement factors, plasmonic metal dimer structures have been the most widely studied approach, in which reduced gap size between adjacent metal nanostructures generates the greatest amplification factor [58,59].When entering the sub-nanometer gap regime, however, it has recently been recently shown that quantum mechanical effects such as nonlocality and electron tunneling stop the hotspot intensity from further increasing monotonically [60][61][62], and the regime in which biological molecules such as proteins or nucleic acids, no longer fit into the hotspot volume.Thus, to further boost hotspot intensity within a nanoantenna, using designs that do not induce heating of biomolecules, new strategies are needed.
Recently, dielectric nanoantennas comprised of spheres, cylinders, or nanogap dimers of Si, Ge, TiO2, and GaP have been demonstrated as effective alternatives to plasmonic metal antennas [35,[63][64][65].While the selected materials have little or no loss in the visible or near IR wavelength bands, the dielectric nanoantenna structures support spectral (Mie) resonances that can enhance local near-field electromagnetic intensity [66,67].Importantly, the dielectric nanoantennas operate through a fundamentally different physical phenomenon from dielectric microresonators (such as PCs or whispering gallery mode structures) that use high quality (Q)-factors to generate field enhancement.Dielectric nanoantennas use small modal volumes with low Q-factors, which provide a broad spectral range for coupling [68].Furthermore, while metal nanoparticles feature only electric field resonant modes, dielectric nanoantennas have both electric and magnetic modes with similar magnitudes [69][70][71], and thus offer novel opportunities to engineer the light scattering, radiative decay constants of emitters [35,72], control of directional light emission [73,74], and enhancement of the Raman scattering process.Recent theoretical treatments [75] and experimental demonstrations of Si and GaP dielectric nanoantennas clearly prove their capabilities for low heat conversion [63,64], fluorescence lifetime reduction [35], fluorophore emission enhancement (up to 3600x) [35,65], and SERS enhancement [64].
Our recent work that experimentally demonstrates hybrid coupling between PC moderate Q-factor "micro" cavities and nanoantennas on a PC surface opens opportunities to increase available electromagnetic enhancement factors by additional orders of magnitude, while providing imaging detection instrumentation methods that are capable of measuring the output of many nanoantennas in parallel.Integrating nanoantennas with complementary photonic building blocks, such as evanescent diffraction orders [44,76,77], plasmonic crystals [54], photonic crystals [78] and Fabry−Pérot (FP) cavities [45,79,80] can combine the advantages of both deep-subwavelength field localization and extended storage of electromagnetic energy.The nanoantenna-cavity hybrid approach has been shown [81] to boost hotspot intensity up to one order of magnitude, compared to excitation of a solitary nanoantenna that is simply illuminated with a laser in the far field, and therefore can generate more efficient light-matter interactions.Thus, we consider the integration of anapole mode resonators with an external structure to obtain field enhancements greater than achievable with an anapole nanoantenna alone.
Anapoles represent a class of antenna structures that support electric and toroidal dipole moments that result in destructive interference of radiation fields, which is observable in the far field as a pronounced dip in the scattered spectrum at a specific wavelength [82].While anapoles have been the subject of intense research interest and have been studied using structures that operate in the microwave spectrum [83], recently anapole nanoantennas have been reported at optical frequencies [84] using silicon nanodisks fabricated upon quartz substrates [85].
For a full explanation of anapole nanoantenna physics at optical frequencies, the reader is referred to [85,86].Briefly, an anapole mode occurs when an electric dipole and a toroidal dipole can be accommodated in the same structure, so that the respective radiation patterns of the electric and toroidal dipole modes can destructively interfere, leading to total scattering cancellation in the far field, with non-zero near-field excitation.For biosensing applications, anapole dielectric nanoantennas are interesting for several reasons: 1.The anapole mode generates a highly concentrated electromagnetic near-field in the center of the structure, 2. The electric fields associated with the mode are confined within the nanoantenna itself, without extending strongly into the surrounding media or to neighboring nanoantennas.3. The structure is simple to fabricate using well-characterized materials.4. The anapole wavelength is easily measured by observing the far field scattered spectrum of white light from each individual structure.5.The anapole mode can be easily excited by external plane wave illumination.6.As an all-dielectric structure, anapole nanoantennas will not suffer the effects of optical loss and heating that are common to all metal-based plasmonic structures.
To our knowledge, anapole dielectric nanoantenna structures have not been previously considered for biosensing applications.To enable the structure to serve effectively for biodetection, we propose a modification to the silicon nanodisk described in [85] to incorporate a nanohole or nanoslot opening at the disk center [87,88], to coincide with the location of the electric field node of the anapole mode.The hole is a small perturbation to the overall structure, representing a precisely-defined location where light-biomaterial interaction can occur for exciting fluorescent reporters.Further, by fabricating the anapole mode resonator over a dielectric thin film of defined thickness on top of a metal back-reflector, we may take advantage of the hybrid coupling effect to achieve even greater field enhancements.Here, we consider the design of an anapole mode nanoantenna in the context of enhancing localized electromagnetic field intensity for the purpose of enhanced excitation of photon emitters.Our analysis does not consider independent enhancement mechanisms that may also occur due to the Purcell effect or due to enhanced collection efficiency into light collection optics, whose effects are known to multiply with enhanced excitation [89,90].

Device structure
While anapole mode structures have been described in a variety of configurations [91][92][93][94], here we focus specifically on the design, optimization, and achievable field enhancement performance for biosensing applications, where the device surface would be covered in aqueous media, and the enhanced fields are used to excite fluorescent dye molecules attached to biomolecules (such as nucleic acids or proteins), with excitation wavelengths in the visible spectrum.We also choose materials that are readily available for manufacturable microfabrication and dimensions that are within the capabilities of lithography approaches used in silicon integrated circuit manufacturing.Thus, we anticipate a structure that could be produced uniformly with reproducibility in a conventional integrated circuit foundry.
The devic with a centra (length l, wid cylinder can reflector film     ent substrate de ncrease the en been utilized f a silicon nan the interaction noparticle with ported in [98] e field enhance article and the Periodic array monstrated for f ive substrate d e and employ e (ED) and tor tically altered he scattering le ield at a quarte nterference bet plane of the an ore, it can be a greater Q-fact hancing the ele er the conditio tic field does the higher reso length is also es the greatest While a slotted modeled struc he anapole's in upport and liqu enhancement.ngth and field substituting ai fined inside th -factor and the creases.

Figure 2 sho baseline anap center of the coupling whe
cross section, and substrate and surr esign can signif nergy coupling effectively in noparticle and n of the electr h the metal f has been uti ement of the afo e metal film, ys of dielectric field enhancem design can be m ying a method roidal dipole (T when they get evel and energy er wavelength tween the incid napole mode.shown that by tor can be achi ectromagnetic ons for which not intersect w onance frequen shown at the |E| enhanceme disk structure cture was a disk nteraction with uid media, wh .previous resea metallic film ric and magne film.The hyb ilized effectiv foresaid hybrid which is requ c nanoparticles ment [99,100].made by consi d for affecting TD), which ar t close to the su y coupling to t distance of th dent and reflec y decreasing t ieved for the hy field in the an h scattering is with the metal ncy and the sca surface and cr ent occurs wh e has been inve k suspended in h higher refract hich have the ows the effect nt, where a 1 n top of the de creasing the re becoming sma nhancement facto vironments with d y the field redu ole mode struc arch.For exam [97], the nea etic dipole re brid resonance vely to squeez d structures is a uired for a pre s on dielectric idering the con g these modes re the two com urface of a sim the structure.T he mirror is sim cted waves, wh the layer thick ybrid system, w napole structure minimized an of the underly attering cancel ross section of en the slot fits estigated previ n free space, wh tive index surr effect of redu of the media r  ance waveleng by increasing th layer is ascrib As expected, wavelength du 40nm is due to or keeps the Q e storage ene anapole mode e silicon disk he required th he metal with a ed disk is illust 0 degree phase erency in the s eld enhancemen rfect electric effects using t eld enhanceme .structure is illuminated with a normal-incidence plane wave with a magnitude of 1 V/m, an emitter inside the electromagnetic hotspot will emit photons with a scaling factor that multiplies with the power associated with the electromagnetic field.Thus, fluorophore excitation enhancement will scale with the square of the electric field magnitude enhancements predicted in Fig. 10 generating up to 480 gains.Further gains in hotspotassociated emission intensity may also be achieved through the mechanism of Purcell enhancement, in which the fluorescent lifetime of emitters is reduced when they are located within an optical resonator [88].Through this mechanism, a single fluorescent emitter may be "recycled" more quickly through its transition from the excited state to the ground state, and thus more photons/fluorophore are observed in the far field.Finally, emitted photons originating from the center of the anapole mode will be directed outward by the dispersion of the nanoantenna, providing an opportunity for a further "enhanced extraction" effect.Due to the presence of the underlying mirror, emitted photons in the downward direction will have the opportunity to back-reflect to the optics that gather the fluorescent signal (such as a microscope objective).Enhanced extraction effects are a combined function of the spatial distributions of photons originating in the anapole mode center and the numerical aperture of the light collection optics [102], and thus must be considered separately from enhancement of photon excitation due to hotspot electric field intensity.
Because the effects of these enhancement mechanisms are multiplicative, we believe that the structure presented here is promising for enhanced fluorescence single-molecule biosensing applications with very substantial overall enhancement for emitters located within the well-defined hotspot volume, where the volumes shown here range from 0.001 -0.065 fL.The overall enhancement is calculated using the expression , where p F represents the Purcell factor and Ex G represents the gain from enhanced efficiency of extraction of emitted photons from the hotspot into the numerical aperture of light collection optics positioned above the device.For example [88], reports Purcell enhancements of 550x and 740x at two different wavelengths for a perfect dipole emitter in an anapole structure.For the water-immersed structures reported here, we estimate a potential Purcell factor value of 210 , calculated as the ratio of energy dissipation rates of an electric dipole 0 / P P [103].This value is derived from the FDTD software package that offers a methodology for estimating Purcell enhancements [92] in which the structure is excited with a y-polarized dipole placed inside the hotspot of the disk.As an initial estimate of the Ex G available from enhanced extraction effects [104], reports a 20-30x enhanced collection of emission from a dipole upon a PEC backed substrate with a 0.1-1 numerical aperture objective.Therefore, the prospects for achieving overall gains for fluorescent emitters within the anapole mode nanoantenna of ( ) ( ) ( ) ( ) A representative metal-based structure with similar hotspot volume is the subwavelength aperture zero mode waveguides (ZMWs), comprised of a circular hole in an aluminum thin film to provide observation volumes in the zeptoliter range.The small volume of confinement allows for single molecule characterization and further optical application including real-time imaging of protein-protein interactions, real-time observation of enzymatic activity, and single molecule DNA sequencing [105][106][107][108][109], but does not offer the opportunity for resonant field enhancement, except by some considerations [110,111].
We envision applications for the structure in which an array of anapole mode nanoantennas may be prepared with individual capturing molecules located within each central hole, and illuminated by an external excitation source while biomolecules in the liquid media bind/unbind to the molecules in the hotspots.Due to the enhancements afforded to fluorophores captured within the hotspot, biomolecular events occurring there may be observed with high signal-to-noise, representing the emissions from single fluorophores associated with single molecules.Observation of the rates of biomolecule capture/release and fluorophore emission wavelength will enable measurement of single biomolecule binding dynamics and processes that include conformation change and association/dissociation.It is also possible to envision structures in which a pore is included inside the substrate of the anapole mode nanoantenna, exactly beneath of its embedded hole, so that molecules may be rapidly excited while they flow through the structure.
Fig.1with a occurs slot, a dielec3.SimulatioThe Finite D simulation of refractive ind wavelength-d were utilized software pack z-polarized t wavelength ra (PML) bound Fig. 2 water top, (b (Top v The effec Considering explored in a resonance w enhancement.
Fig. 4 differe and su distrib When fix wavelength a Fig. 5 slotted indice An efficie strategy to in systems have consisting of ascribed to t dielectric nan resonator rep structure.The of the nanop spacer layer.also been dem An effecti anapole mod electric dipole mode, is drast can change th the electric fi constructive i energy at the p Furthermo wavelength, a of further enh obtained und electromagnet wavelengths, Fig. 6 throug and un the el 40nm The reson initial value b of the SiO 2 l thicknesses.A illumination w thickness of 4 cross section, and he slotted disk on a ne wave illuminatio bution inside the s gth of the anap he SiO 2 layer th bed to the sca 2x magnifica ue to the phase o the higher Q--factor at a hig rgy and preve on a mirror-ba is responsible hickness for ob a layer of low trated in Fig. 1 e shift of the e slot position.T nt through the conductor (P the dielectric p ent variation o (b) field distribut a PEC mirror for d on from the top sid slotted disk at the ole mode is sh hickness.Scatt attering cancel ation of |E| oc e compensatio -factor which i gh level, wher ents further fi acked substrat e for the great btaining the h index dielectri 1(c).A layer of electric field up The SiO 2 thickn e Q-factor incr PEC) as the u parameters of obtained by ch tion along the x-d different thickness de, (c) top and cro e optimized SiO 2 hifted by the me tering reductio lation of the a ccurs when th on.The greater is obtained at t eas s of the SiO 2 layer oss section view of layer thickness of etal, and it retu n at the lower anapole mode he SiO 2 is λ/ r enhanced fie this thickness.
Fig. 7 PEC-b The Q-fac Q-factors obta of the single SiO 2 thicknes further reduc distribution o anapole mode reduces the st of the single s electric field d SiO 2 layer.