Graphene surface modes enabling terahertz pulling force.

Plasmonic substrates are widely reported for their use in the manipulation of sub-wavelength particles. Here we analyze the optical force in the terahertz (THz) spectrum acting on a dielectric nanoparticle when located close to a graphene monolayer. When lying on a dielectric planar substrate, the graphene sheet enables the nano-sized scatterer to excite a surface plasmon (SP) well confined on the dielectric surface. Under quite general conditions, large pulling forces can be exerted on the particle as a consequence of conservation of linear momentum and a self-action effect. Our results show that the pulling force intensity critically depends on the particle shape and orientation. The low heat dissipation of graphene SPs paves the way for the development of a novel plasmonic tweezer for applications involving biospecimen manipulation in the THz region.

Multi-octave metasurface-based refractory superabsorber enhanced by a tapered unit-cell structure.

An ultra-broadband metasurface-based perfect absorber is proposed based on a periodic array of truncated cone-shaped [Formula: see text] surrounded by TiN/[Formula: see text] conical rings. Due to the refractory materials involved in the metasurface, the given structure can keep its structural stability at high temperatures. The proposed structure can achieve a broadband spectrum of 4.3 µm at normal incidence spanning in the range of 0.2-4.5 µm with the absorption higher than [Formula: see text] and the average absorption around [Formula: see text]. The absorption can be tuned through the angle of the cone. By optimizing geometrical parameters, a super absorption is triggered in the range of 0.2-3.25 µm with the absorption higher than 97.40[Formula: see text] and substantially average absorption over 99[Formula: see text]. In this regard, the proposed structure can gather more than [Formula: see text] of the full spectrum of solar radiation. Furthermore, the absorption of the designed structure is almost insensitive to the launching angle up to [Formula: see text] for TE polarization, while it has a weak dependence on the incident angle for TM polarization. The proposed structure can be a promising candidate for thermal energy harvesting and solar absorption applications.

Giant terahertz pulling force within an evanescent field induced by asymmetric wave coupling into radiative and bound modes.

Manipulation of nano-scale objects by engineering the electromagnetic waves in the environment medium is pivotal for several particle handling techniques using optical resonators, waveguiding, and plasmonic devices. In this Letter, we theoretically demonstrate the possibility of engineering a compact and tunable plasmon-based terahertz (THz) tweezer using a graphene monolayer that is deposited on a high-index dielectric substrate. When a nanoparticle located in a vacuum in the vicinity of the graphene monolayer is illuminated under total internal reflection, as light is launched from the substrate, such a device is shown to be capable of inducing an enhanced rotating dipole in the nanoparticle thus enabling asymmetric, directional near-field coupling into the graphene plasmon mode and the radiative modes in the substrate. As a result of the total momentum conservation, the net force exerted on the particle points in a direction opposite to the pushing scattering force of the exciting evanescent field. Our results can contribute to novel realizations of photonic devices based on polarization-dependent interactions between nanoparticles and electromagnetic mode fields.

Multipolar-sensitive engineering of magnetic dipole spontaneous emission with a dielectric nanoresonator antenna.

We propose an axisymmetric silicon nanoresonator with designed tapered angle well for the extraordinary enhancement of the decay rate of magnetic dipole (MD) emitters. Due to the resonant coupling of a MD emitter and the MD mode of the subwavelength resonator, the Purcell factor (PF) can easily reach 500, which is significantly higher than the PF when using a silicon nanosphere of the same size. The PF and the resonance frequency are conveniently tuned through the resonator diameter and the taper angle of the blind hole. When supported by a metallic substrate, further enhancement ([Formula: see text]) of the MD spontaneous emission is triggered by an image-induced quadrupolar high-Q mode of the nanoantenna. For the sake of comparison we include a critical analysis of the canonical problem that considers a Si spherical shell. Our results might facilitate a novel strategy for promising realizations of chip-scale nanophotonic applications.

Anapole-assisted giant electric field enhancement for surface-enhanced coherent anti-Stokes Raman spectroscopy.

The coherent anti-Stokes Raman spectroscopy (CARS) techniques are recognized for their ability to detect and identify vibrational coherent processes down to the single-molecular levels. Plasmonic oligomers supporting full-range Fano-like line profiles in their scattering spectrum are one of the most promising class of substrates in the context of surface-enhanced (SE) CARS application. In this work, an engineered assembly of metallic disk-shaped nanoparticles providing two Fano-like resonance modes is presented as a highly-efficient design of SECARS substrate. We show that the scattering dips corresponding to the double-Fano spectral line shapes are originated from the mutual interaction of electric and toroidal dipole moments, leading to the so-called non-trivial first- and second-order anapole states. The anapole modes, especially the higher-order ones, can result in huge near-field enhancement due to their light-trapping capability into the so-called "hot spots". In addition, independent spectral tunability of the second Fano line shape is exhibited by modulating the gap distance of the corner particles. This feature is closely related to the electric current loop associated with the corner particles in the second-order anapole state and provides a simple design procedure of an optimum SECARS substrate, where the electric field hot spots corresponding to three involved wavelengths, i.e., anti-Stokes, pump, and Stokes, are localized at the same spatial position. These findings yield valuable insight into the plasmonic substrate design for SECARS applications as well as for other nonlinear optical processes, such as four-wave mixing and multi-photon surface spectroscopy.

Lasing condition for trapped modes in subwavelength-wired PT-symmetric resonators.

The ability to control the laser modes within a subwavelength resonator is of key relevance in modern optoelectronics. This work deals with the theoretical research on optical properties of a PT-symmetric nano-scaled dimer formed by two dielectric wires, one is with loss and the other with gain, wrapped with graphene sheets. We show the existence of two non-radiating trapped modes which transform into radiating modes by increasing the gain-loss parameter. Moreover, these modes reach the lasing condition for suitable values of this parameter, a fact that makes these modes achieve an ultra high quality factor that is manifested on the response of the structure when it is excited by a plane wave. Unlike other mechanisms that transform trapped modes into radiating modes, we show that the variation of gain-loss parameter in the balanced loss-gain structure here studied leads to a variation in the phase difference between induced dipole moments on each wires, without appreciable variation in the modulus of these dipole moments. We provide an approximated method that reproduces the main results provided by the rigorous calculation. Our theoretical findings reveal the possibility to develop unconventional optical devices and structures with enhanced functionality.

Purcell Enhancement and Wavelength Shift of Emitted Light by CsPbI3 Perovskite Nanocrystals Coupled to Hyperbolic Metamaterials.

Manipulation of the exciton emission rate in nanocrystals of lead halide perovskites (LHPs) was demonstrated by means of coupling of excitons with a hyperbolic metamaterial (HMM) consisting of alternating thin metal (Ag) and dielectric (LiF) layers. Such a coupling is found to induce an increase of the exciton radiative recombination rate by more than a factor of three due to the Purcell effect when the distance between the quantum emitter and HMM is nominally as small as 10 nm, which coincides well with the results of our theoretical analysis. Besides, an effect of the coupling-induced long wavelength shift of the exciton emission spectrum is detected and modeled. These results can be of interest for quantum information applications of single emitters on the basis of perovskite nanocrystals with high photon emission rates.

Broadband absorption using all-graphene grating-coupled nanoparticles on a reflector.

In this paper, the hybridized localized surface plasmon resonances (LSPRs) of a periodic assembly of graphene-wrapped nanoparticles are used to design a nanoparticle assisted optical absorber. Bandwidth enhancement of this structure via providing multiple types of plasmonic resonances in the associated unit cell using two densely packed crossly stacked graphene strips is proposed. The designed graphene strips support fundamental propagating surface plasmons on the ribbons, and gap plasmons in the cavity constructed by the adjacent sections. Graphene strips exhibit a hyperbolic dispersion region in the operating spectrum and assist in the bandwidth enhancement. Moreover, since the nanoparticles are deposited on the top strips, real-time biasing of them can be easily conducted by exciting the surface plasmons of the strip without the necessity to electrically connect the adjacent nanoparticles. The overall dynamic bandwidth of the structure, using a two-state biasing scheme, covers the frequencies of 18.16-40.47 THz with 90% efficiency. Due to the symmetry of the structure, the device performs similarly for both transverse electric (TE) and transverse magnetic (TM) waves and it has a high broadband absorption rate regarding different incident angles up to 40°. Due to the presence of 2D graphene material and also using hollow spherical particles, our proposed absorber is also lightweight and it is suitable for novel compact optoelectronic devices due to its sub-wavelength dimensions.

Multi-frequency near-field enhancement with graphene-coated nano-disk homo-dimers.

In this paper, a 3D sub-wavelength graphene-coated nano-disk dimer (GDD) is proposed for multi-frequency giant near-field enhancement. We observed that the dual-band operation originates from the excitation of hybridized localized surface plasmons on top and bottom faces of the disks along with the mutual coupling from the adjacent particle. Due to the sub-wavelength nature of the disks, the excited localized surface plasmons on the sidewalls are weak but they still can affect the dual operating bands. On the other hand, the strength and resonance frequency of the enhanced fields can be simply modulated by tuning the relative distances of 2D graphene disks on top and bottom faces. Adjustable dual-band performance is hardly attainable using simplified 2D graphene disks, however, it naturally comes out through modal hybridization in the subwavelength 3D structure containing multiple resonant units. Our suggested configuration has better optical properties than its noble metal counterparts because of its higher field enhancement and lower ohmic losses. Moreover, the electromagnetic response is reconfigurable by varying the bias voltage. The influence of graphene quality, chemical potential, and dimer gap size on the electric field enhancement and the resonance frequency of the surface plasmons are investigated, as well. To further improve its performance, a double negative metamaterial core is considered. This mechanism of the performance improvement by the core material is feasible thanks to the 3D nature of the structure. Two possible applications of the presented design are in Surface-Enhanced Raman Spectroscopy (SERS) and optical absorbers.

Dielectric metalenses with engineered point spread function.

High-index silicon nanoblocks support excitation of both electric and magnetic resonance modes at telecommunication wavelengths. At frequencies where both electric and magnetic resonance modes are excited simultaneously, changing the geometrical dimensions of the silicon cubes creates a 2π full span over the phase of the transmitted light in different amplitude ranges. We take advantage of the additional power-flux modulation of the scattered signal to focus the incident light with desired full width at half maximum (FWHM) and side lobe levels (SLLs) in both the lateral and axial directions. By implementing proper amplitude filters within the telecommunication working wavelength (1.55 µm), a FWHM less than half of the wavelength (0.42λeff) and apodization with nearly faded SLLs are achievable. Our approach introduced in this paper provides a new way to design high efficiency metalenses with desired FWHM and SLL of focal spot.

Tunable invisibility cloaking by using isolated graphene-coated nanowires and dimers.

We investigate, both theoretically and numerically, a graphene-coated nano-cylinder illuminated by a plane electromagnetic wave in the far-infrared range of frequencies. We have derived an analytical formula that enables fast evaluation of the spectral window with a substantial reduction in scattering efficiency for a sufficiently thin cylinder. This polarization-dependent effect leads to tunable resonant invisibility that can be achieved via modification of graphene chemical potential monitored by the gate voltage. A multi-frequency cloaking mechanism based on dimer coated nanowires is also discussed in detail.

Optimization of multilayered nanotubes for maximal scattering cancellation.

An optimization for multilayered nanotubes that minimizes the scattering efficiency for a given polarization is derived. The cylindrical nanocavities have a radially periodic distribution, and the marginal layers that play a crucial role particularly in the presence of nonlocalities are disposed to reduce the scattering efficiency up to two orders of magnitude in comparison with previous proposals. The predominant causes leading to such invisibility effect are critically discussed.

Ultrathin high-index metasurfaces for shaping focused beams.

The volume size of a converging wave, which plays a relevant role in image resolution, is governed by the wavelength of the radiation and the numerical aperture (NA) of the wavefront. We designed an ultrathin (λ/8 width) curved metasurface that is able to transform a focused field into a high-NA optical architecture, thus boosting the transverse and (mainly) on-axis resolution. The elements of the metasurface are metal-insulator subwavelength gratings exhibiting extreme anisotropy with ultrahigh index of refraction for TM polarization. Our results can be applied to nanolithography and optical microscopy.

Nonparaxial shape-preserving Airy beams with Bessel signature.

Spatially accelerating beams that are solutions to Maxwell equations may propagate along incomplete circular trajectories. Taking these truncated Bessel fields to the paraxial limit, some authors have sustained that it has recovered the known Airy beams (AiBs). Based on the angular spectrum representation of optical fields, we demonstrated that the paraxial approximation rigorously leads to off-axis focused beams instead of finite-energy AiBs. The latter will arise under the umbrella of a nonparaxial approach following elliptical trajectories in place of parabolas. The analytical expression of such a shape-preserving wave field under Gaussian apodization is disclosed by using third-order nonparaxial coefficients. Deviations from full-wave simulations appear more severely in beam positioning rather than its local profile.

Engineered surface waves in hyperbolic metamaterials.

We analyzed surface-wave propagation that takes place at the boundary between a semi-infinite dielectric and a multilayered metamaterial, the latter with indefinite permittivity and cut normally to the layers. Known hyperbolization of the dispersion curve is discussed within distinct spectral regimes, including the role of the surrounding material. Hybridization of surface waves enable tighter confinement near the interface in comparison with pure-TM surface-plasmon polaritons. We demonstrate that the effective-medium approach deviates severely in practical implementations. By using the finite-element method, we predict the existence of long-range oblique surface waves.

Asymmetric transmission of terahertz radiation through a double grating.

We report on experimental evidence of unidirectional transmission of terahertz waves through a pair of metallic gratings with different periods. The gratings are optimized for a broadband transmission in one direction, accompanied with a high extinction rate in the opposite direction. In contrast to previous studies, we show that the zero-order nonreciprocity cannot be achieved. Nonetheless, we confirm that the structure can be used successfully as an asymmetric filter.

Left-handed metamaterial coatings for subwavelength-resolution imaging.

We report on a procedure to improve the resolution of far-field imaging by using a neighboring high-index medium that is coated with a left-handed metamaterial. The resulting plot can also exhibit an enhanced transmission by considering proper conditions to retract backscattering. Based on negative refraction, geometrical aberrations are considered in detail since they may cause a great impact in this sort of diffraction-unlimited imaging by reducing its resolution power. We employ a standard aberration analysis to refine the asymmetric configuration of metamaterial superlenses. We demonstrate that low-order centrosymmetric aberrations can be fully corrected for a given object plane. For subwavelength-resolution imaging, however, high-order aberrations become of relevance, which may be balanced with defocus. Not only the point spread function but also numerical simulations based on the finite-element method support our theoretical analysis, and subwavelength resolution is verified in the image plane.

Considerations on the electromagnetic flow in Airy beams based on the Gouy phase.

We reexamine the Gouy phase in ballistic Airy beams (AiBs). A physical interpretation of our analysis is derived in terms of the local phase velocity and the Poynting vector streamlines. Recent experiments employing AiBs are consistent with our results. We provide an approach which potentially applies to any finite-energy paraxial wave field that lacks a beam axis.

Nondiffracting Bessel plasmons.

We report on the existence of nondiffracting Bessel surface plasmon polaritons (SPPs), advancing at either superluminal or subluminal phase velocities. These wave fields feature deep subwavelength FWHM, but are supported by high-order homogeneous SPPs of a metal/dielectric (MD) superlattice. The beam axis can be relocated to any MD interface, by interfering multiple converging SPPs with controlled phase matching. Dissipative effects in metals lead to a diffraction-free regime that is limited by the energy attenuation length. However, the ultra-localization of the diffracted wave field might still be maintained by more than one order of magnitude.

Three-dimensional point spread function and generalized amplitude transfer function of near-field flat lenses.

We derive a nonsingular, polarization-dependent, 3D impulse response that provides unambiguously the wave field scattered by a negative-refractive-index layered lens and distributed in its image volume. By means of a 3D Fourier transform, we introduce the generalized amplitude transfer function in order to gain a deep insight into the resolution power of the optical element. In the near-field regime, fine details containing some depth information may be transmitted through the lens. We show that metamaterials with moderate absorption are appropriate for subwavelength resolution keeping a limited degree of depth discrimination.