Absorption of electromagnetic energy by a dissipative material is one of the most fundamental electromagnetic processes that underlies a plethora of applied problems, including sensing and molecular detection, radar detection, wireless power transfer, and photovoltaics. Perfect absorption is a particular regime when all of the incoming electromagnetic energy is absorbed by the system without scattering. Commonly, the incident energy is delivered to the absorbing system by a plane wave, hence perfect absorption of this wave requires an infinitely extended planar structure. Here, we demonstrate theoretically that a confined incident beam carrying a finite amount of electromagnetic energy can be perfectly absorbed by a finite size deep subwavelength scatterer on a substrate. We analytically solve the self-consistent scattering problem in the dipole approximation and find a closed-form expression for the spatial spectrum of the incident field and the required complex polarizability of the particle. All analytical predictions are confirmed with full-wave simulations.
A general concept of perfect nonradiating modes in dielectric nanoparticles of an arbitrary shape is put forward. It is proved that such modes exist in axisymmetric dielectric nanoparticles and have unlimited radiative Q-factors. With smart tuning of the excitation beams, perfect modes appear as deep minima in the scattered radiation spectra (up to complete disappearance), but at the same time, they have a giant amplification of the fields inside the particle. Such modes have no analogs and can be useful for the realization of nanosensors, low threshold nanolasers, and other strong nonlinear e…
We provide a quantitative explanation of the mechanism of the far-field intensity modulation induced by a nanoparticle in a focused Gaussian laser beam, as was demonstrated in several recent direct detection studies. Most approaches take advantage of interference between the incident light and the scattered light from a nanoparticle to facilitate a linear dependence of the signal on the nanoparticle volume. The phase relation between the incoming field and the scattered field by the nanoparticle is elucidated by the concept of Gouy phase. This phase relation is used to analyze the far-field si…
A high index dielectric nano-sphere can be excited and yet remain radiationless. A method to excite the non-radiating anapole mode of a high index isotropic dielectric nanosphere is presented. With tightly focused radially polarized beam illumination, the main-contributing electric dipole mode and magnetic modes can be zero with only a weak electric quadruple contributing to the total scattering. Further, with a standing wave illumination formed by two counter-propagating focused radially polarized beam under $4π$ configuration, the ideal radiationless ananpole can be excited.
Optical tweezers, the three-dimensional confinement of a nanoparticle by a strongly focused beam of light, have been widely employed in investigating biomaterial nanomechanics, nanoscopic fluid properties, and ultrasensitive detections in various environments such as inside living cells, at gigapascal pressure, and under high vacuum. However, the cryogenic operation of solid-state-particle optical tweezers is poorly understood. In this study, we demonstrate the optical trapping of metallic and dielectric nanoparticles in superfluid helium below 2 K, which is two orders of magnitude lower than …
The heat-shielding properties of random metasurface, composed of spherical or spheroidal nanoparticles with random displacements and/or random deformation, were theoretically investigated using the finite difference time domain method. The effective coverage was defined using the total area of nanoparticles in the metasurface, and the robustness of the near-infrared light reflection against randomness was investigated. When the effective coverage was high, the near-infrared light reflection was reduced by at least 20% in both nanoparticle arrangement and shape randomness compared to the hexago…
We investigate the interaction of tightly focused light with the surface-plasmon-polariton resonances of metal nanospheres. In particular, we compute the scattering and absorption ratios as well as transmission and reflection coefficients. Inspired by our previous work in Ref. [1], we discuss how well a metal nanoparticle approximates a point-like dipolar radiator. We find that a 100 nm silver nanosphere is very close to such an ideal oscillator. Our results have immediate implications for single nanoparticle spectroscopy and microscopy as well as plasmonics.
We experimentally observe the dipole scattering of a nanoparticle using a high numerical aperture (NA) imaging system. The optically levitated nanoparticle provides an environment free of particle-substrate interaction. We illuminate the silica nanoparticle in vacuum with a 532 nm laser beam orthogonally to the propagation direction of the 1064 nm trapping laser beam strongly focused by the same high NA objective used to collect the scattering, which results in a dark background and high signal-noise ratio. The dipole orientations of the nanoparticle induced by the linear polarization of the i…
We derive envelope equations which generate the Maxwell-Lorentz model and describe the interaction of optical pulses with plasmonic oscillations in metal nanoparticle composites. A family of solitary wave solutions is found which is analogous to self-induced transparency in Maxwell-Bloch. The evolution of incident optical pulses is studied numerically as are the collision dynamics of the solitary waves. These simulations reveal that the collision dynamics vary from near perfectly elastic to highly radiative depending on the relative phase of the initial pulses.
The principle of optical trapping is conventionally based on the interaction of optical fields with linear induced polarizations. However, the optical force originating from the nonlinear polarization becomes significant when nonlinear optical nanoparticles are trapped by ultrafast laser pulses. Herein we establish the time-averaged optical forces on a nonlinear optical nanoparticle using high-repetition-rate ultrafast laser pulses, based on the linear and nonlinear polarization effects. We investigate the dependence of the optical forces on the magnitudes and signs of the refractive nonlinear…
A theory of light reflection and transmission by an optically thin nanocomposite slab which contains randomly distributed metal nanoparticles (NPs) is developed. The underlying model takes into account the reflection of light scattered by NPs from the slab boundaries, enhanced decay of localized surface plasmons in dense NP arrays and light scattering at the slab surface -- the factors which are beyond the scope of the Maxwell Garnett approximation. It is demonstrated that the first two effects lead to broadband perfect absorption observed in such nanocomposites, whereas the last one is respon…
Light's orbital angular momentum (OAM) with inherent mode orthogonality has been suggested as a new way to the optical encryption. However, the dependence of annular intensity profiles on the topological charge complicates nanoscale light-matter interactions and hampers the ultra-secure encryption application. In this paper, we demonstrate ultra-secure image encryption by tightly focusing perfect optical vortex (POV) beams with controllable annular intensity profiles and OAM states. A simple scheme composed of single spatial light modulator is proposed to generate radius-controllable POV in ti…
There is a study of coherent perfect absorption (CPA) in composite medium (CM) which is made of metal-dielectric composition. The CM, an orthorhombic shaped slab, is illuminated by two identical Gaussian beams (GB) incident at equal angle of incidence and focused on the two interfaces between wall of slab and air from opposite side. It has determine the necessary conditions required for CPA and shows that perfect CPA is possible with GBs. Author also shows broader beams exhibit more absorption than comparatively tightly focused beams.
We study the modification of the far-field cross sections and the near-field enhancement for gold and silver nanospheres illuminated by a tightly focused beam. Using a multipole-expansion approach we obtain an analytical solution to the scattering problem and provide insight on the effects of focusing on the optical response. Large differences with respect to Mie theory are found especially when the nanoparticle supports quadrupole or higher-order resonances.
We demonstrate the measurement of nanoparticle shape by angularly resolved Rayleigh scattering of single optical levitated particles that are oriented in space via the trapping light in vacuum. This technique is applied to a range of particle geometries, from perfect spherical nanodroplets to octahedral nanocrystals. We show that this method can resolve shape differences down to a few nanometers and be applied in both low-damping environments, as demonstrated here, and in traditional overdamped fluids used in optical tweezers.
Long-term observations of photoluminescence at the single-molecule level were until recently very diffcult, due to the photobleaching of organic ?uorophore molecules. Although inorganic semiconductor nanocrystals can overcome this diffculty showing very low photobleaching yield, they suffer from photoblinking. A new marker has been recently introduced, relying on diamond nanoparticles containing photoluminescent color centers. In this work we compare the photoluminescence of single quantum dots (QDs) to the one of nanodiamonds containing a single-color center. Contrary to other markers, photol…
High-Q optical resonators allow label-free detection of individual nanoparticles through perturbation of optical signatures but have practical limitations due to reliance on random diffusion to deliver particles to the sensing region. We have recently developed microfluidic optomechanical resonators that allow detection of free-flowing particles in fluid media with near perfect detection efficiency, without requiring labeling, binding, or direct access to the optical mode. Rapid detection of single particles is achieved through a long-range optomechanical interaction that influences the scatte…
We study the interaction of focused radially-polarized light with metal nanospheres. By expanding the electromagnetic field in terms of multipoles, we gain insight on the excitation of localized surface plasmon-polariton resonances in the nanoparticle. We show that focused radially-polarized beams offer more opportunities than a focused plane wave or a Gaussian beam for tuning the near- and far-field system response. These results find applications in nano-optics, optical tweezers, and optical data storage.
Biosynthesis of four samples of colloidal suspensions of gold nanoparticles is achieved using hydroalcoholic extract and three different separated compounds of the plant Bacopa procumbens. The nonlinear optical properties of each sample are characterized with the Z-scan technique. In all cases, the Z-scan curves indicate a negative or self-defocusing response, which is mainly attributed to thermal effects. Among the four samples, the hydroalcoholic extract was noted to have the highest nonlinear optical response and was selected to demonstrate the formation of self-collimated beams (SCBs). Thi…
Enabling perfect light absorption in ultrathin materials promises the development of exotic photonic devices. Here we demonstrate new strategies that can provide capabilities to rationally design ultrathin (thickness < λ/10~λ/5) semiconductor perfect absorbers for arbitrary wavelengths, including those at which the intrinsic absorption of the semiconductor is weak, e.g. Si for near-IR wavelengths. This is in stark contrast with the existing studies on ultrathin perfect absorbers, which have focused on metallic materials or highly-absorptive semiconductors. Our design strategies are built upon …
A thorough theoretical study of the optical properties of periodic Si nanosphere arrays is undertaken, placing particular emphasis on the synergy between electric and magnetic Mie resonances, which occur in high-refractive-index nanoparticles and can lead to a rich variety of phenomena ranging from perfect reflection to controlled diffraction. By means of systematic calculations using the layer-multiple-scattering method that we properly extended so as to describe periodic arrays with many scatterers per unit cell, in conjunction with finite-element simulations, we optimized surfaces of Si nan…
Feedback cooling of a levitated nanoparticle in a radially polarized vector beam trap
University College London（伦敦大学学院）；David Potter Institute for Quantum Information and Spacetime, University College London（伦敦大学学院戴维·波特量子信息与时空研究所）；Department of Physics and Astronomy, University College London（伦敦大学学院物理与天文学系）
Highly confined vectorial electromagnetic field distributions represent an excellent tool for detailed studies in nano-optics and high resolution microscopy, such as nonlinear microscopy, advanced fluorescence imaging or nanoplasmonics. Such field distributions can be generated, for instance, by tight focussing of polarized light beams. To guarantee high quality and resolution in the investigation of objects with sub-wavelength dimensions, the precise knowledge of the spatial distribution of the exciting vectorial field is of utmost importance. Full-field reconstruction methods presented so fa…
We demonstrate experimentally that disordered scattering can be used to improve, rather than deteriorate, the focusing resolution of a lens. By using wavefront shaping to compensate for scattering, light was focused to a spot as small as one tenth of the diffraction limit of the lens. We show both experimentally and theoretically that it is the scattering medium, rather than the lens, that determines the width of the focus. Despite the disordered propagation of the light, the profile of the focus was always exactly equal to the theoretical best focus that we derived.
This paper quantifies the rate at which laser shot noise energy is delivered to a nanoparticle for the various scenarios commonly encountered in levitated optomechanics. While previous articles have the same form and dependencies, the proportionality constants often differ in the literature. This paper resolves these discrepancies. The rate at which energy is delivered to an optically trapped particle's respective degrees of freedom depends on the radiation pattern of scattered light as well as the direction of laser propagation. For a traveling plane wave with linearly polarized light, in the…
The ability to drive a system with an external input is a fundamental aspect of light-matter interaction. The coherent perfect absorption (CPA) phenomenon extends to the general multibeam interference phenomenology the well known critical coupling concepts. This interferometric control of absorption can be employed to reach full delivery of optical energy to nanoscale systems such as plasmonic nanoparticles, and multi-port interference can be used to enhance the absorption of a nanoscale device when it is embedded in a strongly scattering system, with potential applications to nanoscale sensin…
Spin-orbit interactions in optics traditionally describe an influence of the polarization degree of freedom of light on its spatial properties. The most prominent example is the generation of a spin-dependent optical vortex upon focusing or scattering of a circularly polarized plane-wave by a nanoparticle, converting spin to orbital angular momentum of light. Here, we present a mechanism of conversion of orbital-to-spin angular momentum of light upon scattering of a linearly polarized vortex beam by a spherical silicon nanoparticle. We show that focused linearly polarized Laguerre-Gaussian bea…
Optical absorption is usually considered deleterious, something to avoid if at all possible. We propose a broadband nanoabsorber that completely eliminates the diffracting wave, resulting in a subwavelength enhancement of the field. Broadband operation is made possible by engineering the dispersion of the complex dielectric function. The local enhancement can be significantly improved compared to the standard plane wave illumination of a metallic nanoparticle. Our numerical simulation shows that an optical pulse as short as 6 fs can be focused to a 11 nm region. Not only the local field, but a…
Plasmonic vortices (PV) excited by a highly focused radially polarized optical vortex (RPOV) beam on a metal surface are investigated experimentally and theoretically. The proposed method reveals a direct phase singularity and orbital angular momentum (OAM) transfer from an incident structured beam to its counterpart in surface plasmon with dynamic, reconfigurable and high-efficiency advantages. The plasmonic field pattern, phase distributions, Poynting vector and focusing efficiency of PV are studied in detail. Experimental verification further shows that nanoparticles can be confined and man…
One highly desirable function of a diffraction grating is its ability to deflect incident light into a specific diffraction order with near-perfect efficiency. While such asymmetry can be achieved in a variety of ways, e.g., by using a sawtooth (blazed) geometry, a recently emerged approach is to use a planar metagrating comprised of designer multi-resonant periodic units (metamolecules). Here we demonstrate that a bianisotropic unit cell supporting four resonances interfering in the far field can be used as a building block for achieving the prefect deflection. A coupled mode analysis shows t…
