Optical Nanoresonators Research Articles

Efficient method for modeling large-scale arrays of optical nanoresonators based on the coupling theory of quasinormal mode.

We propose an efficient method for calculating the electromagnetic field of a large-scale array of optical nanoresonators based on the coupling theory of quasinormal mode (QNM). In this method, two approaches of the scattered-field reconstruction and stationary-phase-principle calculated plane-wave expansion are developed to obtain the regularized QNM (RQNM) in different regions. This accurate and efficient calculation of RQNM resolves the far-field divergence issue of QNMs in the QNM-coupling theory, thus enabling a rapid computation of the electromagnetic field of a large-scale array of optical nanoresonators, which is a challenging task for full-wave numerical methods. Using this method, we consider the numerical example of the radiation problem of a single point source in a large-scale periodic array of optical nanoantennas. In comparison to full-wave numerical methods, this method significantly reduces the computation time by 1∼2 orders of magnitude while maintaining accuracy. The high computational efficiency and physical intuitiveness of the method enables to clarify the impact of array size (exceeding 50 × 50 wavelengths), period and field-coupling range (far beyond the tight-binding approximation) on the optical response. The proposed method and results can provide an efficient tool and guidance for the design of large-scale arrays of optical nanoresonators.

Twist-tunable polaritonic nanoresonators in a van der Waals crystal

Optical nanoresonators are key building blocks in various nanotechnological applications (e.g., spectroscopy) due to their ability to effectively confine light at the nanoscale. Recently, nanoresonators based on phonon polaritons (PhPs)—light coupled to lattice vibrations—in polar crystals (e.g., SiC, or h-BN) have attracted much attention due to their strong field confinement, high quality factors, and their potential to enhance the photonic density of states at mid-infrared (mid-IR) frequencies, where numerous molecular vibrations reside. Here, we introduce a new class of mid-IR nanoresonators that not only exhibit the extraordinary properties previously reported, but also incorporate a new degree of freedom: twist tuning, i.e., the possibility of controlling their spectral response by simply rotating the constituent material. To achieve this result, we place a pristine slab of the van der Waals (vdW) α-MoO3 crystal, which supports in-plane hyperbolic PhPs, on an array of metallic ribbons. This sample design based on electromagnetic engineering, not only allows the definition of α-MoO3 nanoresonators with low losses (quality factors, Q, up to 200), but also enables a broad spectral tuning of the polaritonic resonances (up to 32 cm−1, i.e., up to ~6 times their full width at half maximum, FWHM ~5 cm−1) by a simple in-plane rotation of the same slab (from 0 to 45°). These results open the door to the development of tunable and low-loss IR nanotechnologies, fundamental requirements for their implementation in molecular sensing, emission or photodetection applications.

Optical Processes behind Plasmonic Applications.

Plasmonics is a revolutionary concept in nanophotonics that combines the properties of both photonics and electronics by confining light energy to a nanometer-scale oscillating field of free electrons, known as a surface plasmon. Generation, processing, routing, and amplification of optical signals at the nanoscale hold promise for optical communications, biophotonics, sensing, chemistry, and medical applications. Surface plasmons manifest themselves as confined oscillations, allowing for optical nanoantennas, ultra-compact optical detectors, state-of-the-art sensors, data storage, and energy harvesting designs. Surface plasmons facilitate both resonant characteristics of nanostructures and guiding and controlling light at the nanoscale. Plasmonics and metamaterials enable the advancement of many photonic designs with unparalleled capabilities, including subwavelength waveguides, optical nanoresonators, super- and hyper-lenses, and light concentrators. Alternative plasmonic materials have been developed to be incorporated in the nanostructures for low losses and controlled optical characteristics along with semiconductor-process compatibility. This review describes optical processes behind a range of plasmonic applications. It pays special attention to the topics of field enhancement and collective effects in nanostructures. The advances in these research topics are expected to transform the domain of nanoscale photonics, optical metamaterials, and their various applications.

Оптические нанорезонаторы

The review presents the analysis and generalization of classical and most modern approaches to the description and development of the principles of operation of open optical nanoresonators, that is, resonators, all sizes of which are smaller than the resonant wavelength of radiation in vacuum. Particular attention is paid to the physics of such phenomena as bound states in a continuum, anapole states, supercavity modes, and perfect nonradiating modes with extremely high-quality factors and localizations of electromagnetic fields. The analysis of the optical properties of natural oscillations in nanoresonators made of metamaterials is also presented in the review. The effects considered in this review are not purely fundamental, but can also find applications in the development of optical nanoantennas, nanolasers, biosensors, photovoltaic devices, and nonlinear nanophotonics.

Robust and Scalable Flat‐Optics on Flexible Substrates via Evolutionary Neural Networks

Flexible Flat-Optics This work, presented in article 2100105 by Andrea Fratalocchi and co-workers, implements an artificial intelligent, end-to-end pipeline for the experimental realization of flexible, broadband ultra-flat optics for vectorial light control. The system takes advantage of “physical” neural networks, engineered into suitably designed optical nanoresonators. The design framework also considers fabrication robustness, furnishing devices that maintain their optical response even under strong stress perturbations and deformations.

Efficient hybrid method for the modal analysis of optical microcavities and nanoresonators.

We propose a novel hybrid method for accurately and efficiently analyzing microcavities and nanoresonators. The method combines the marked spirit of quasinormal mode expansion approaches, e.g., analyticity and physical insight, with the renowned strengths of real-frequency simulations, e.g., accuracy and flexibility. Real- and complex-frequency simulations offer a complementarity between accuracy and computation speed, opening new perspectives for challenging inverse design of nanoresonators.

Preface to the special issue dedicated to Professor Richard P. Van Duyne (1945–2019)

Preface to the special issue dedicated to Professor Richard P. Van Duyne (1945–2019)

Optimized high-speed all-optical 2-bit ADCbased on two-dimensional photonic crystal nanoresonators.

In the present paper, a new 2-bit analog-to-digital converter (ADC) was designed and simulated by using 2D photonic crystal (PC) structures to create a relatively faster and smaller structure. For this purpose, a PC structure with a square lattice and silicon rods in the air bed was used. In the proposed structure, a combination of an optical filter with a linear waveguide, optical nanoresonators, and interference effects was used to create a 2-bit ADC. To create a structure in optimal conditions with maximum output optical power, the size of nanoresonators was scanned to reach the best size. The proposed structure operated at the operating wavelength of 1550 nm with a response time of about 1.63 ps, a sampling rate of about 613 GS/s, and a resolution sampling rate product (RSRP) value of about 2453 ks. Additionally, the size of the structure was about 194µm2, which is small compared with other structures proposed in this field; it also enjoys high simplicity and flexibility like structures with other functions, including 4-bit converters. The amount of power used to create different logic states was at the rate of mW/µm2, which is much lower than the amount used in similar structures and is achieved using nonlinear effects and materials. Therefore, due to the excellent results obtained, this structure is recommended to be used in optical integrated circuits. The plane wave expansion method was used to extract the photonic bandgap, and the finite-difference time-domain method was used to obtain the results related to the output spectrum of the designed structures.

Theoretical treatment of single‐molecule scanning Raman picoscopy in strongly inhomogeneous near fields

AbstractTip‐enhanced Raman spectroscopy (TERS) of a single molecule is commonly described by considering the change in the polarizability of the molecule with respect to a normal coordinate induced by homogeneous illumination. However, the local fields induced by nanoscale and atomic‐scale features at the surface of metallic clusters and nanogaps show strong inhomogeneities in their spatial distribution, which induces breaking of Raman selection rules. In this context, the spatial extension of the molecular electronic states subjected to strongly varying local fields challenges the validity of the point–dipole approximation as an adequate description of TERS in such configurations. Here, we introduce a general treatment to simulate single‐molecule TERS spectra and their energy‐filtered vibrational fingerprints maps, in which the polarization properties of the single molecule and that of the optical enhancing nanoresonator can be calculated separately and then conveniently combined to obtain the total Raman cross section of the molecule under the strongly inhomogeneous field. We apply the general method to study tip‐enhanced scanning Raman picoscopy of a 4,4′‐bipyridine and biphenyl molecules in the proximity of a silver icosahedral cluster with a few atoms at the tip apex mimicking an enhancing picocavity. The polarization of the molecules is calculated within density functional theory (DFT), and the optical response of the tip is calculated within a classical atomistic discrete–dipole approximation. The Raman spectra are found to be extremely sensitive to the spatial distribution of the local fields and to the orientation of the molecule. Our calculations show that the spatial mapping of molecular vibrational fingerprints, as probed by a tip with atomic protrusions, is capable to reveal intramolecular features of a single molecule in real space and thus establish a robust basis for scanning Raman picoscopy.

Light interaction with nanoresonators: mode volume and quasinormal mode expansion

The most general motion of a system is a superposition of its normal modes, or eigenstates. For Hermitian system, classical normal mode theory applies. For non-Hermitian systems, presently a lot of progress is done to describe the response of optical micro and nanoresonators in their quasinormal mode basis. We have developed a rigorous modal analysis of nanoresonators with unprecedented generality and report numerical results for the general case of 3D resonators, made of dispersive materials on substrate with guiding layers.

Gate-Tunable Emission of Exciton–Plasmon Polaritons in Hybrid MoS2-Gap-Mode Metasurfaces

The advance in designing arrays of ultrathin two-dimensional optical nano-resonators, known as metasurfaces, is currently enabling a large variety of novel flat optical components. The remarkable control over the electromagnetic fields offered by this technology can be further extended to the active regime in order to manipulate the light characteristics in real-time. In this contribution, we couple the excitonic resonance of atomic thin MoS2 monolayers with gap-surface-plasmon (GSP) metasurfaces, and demonstrate selective enhancement of the exciton-plasmon polariton emissions. We further demonstrate tunable emissions by controlling the charge density at interface through electrically gating in MOS structure. Straddling two very active fields of research, this demonstration of electrically tunable light-emitting metasurfaces enables real-time manipulation of light-matter interactions at the extreme subwavelength dimensions.

Impedance-matched dielectric metasurfaces for non-discrete wavefront engineering

Metasurfaces can manipulate optical wavefronts by locally shifting the phase of incident light with metallic or dielectric optical nanoresonators that are generally arranged on a lattice with subwavelength spacing. However, such conventional metasurfaces inevitably generate a spatially discrete multi-level phase profile due to the spacing of their building blocks. This directly leads to an efficiency reduction and thus limits their capability. Here, we propose and demonstrate highly efficient transmissive metasurfaces with the ability to form a continuous phase profile. The proposed strategy relies on the fact that high-index dielectric nanobeams with gradually modulated widths can be interpreted to be a virtually impedance-matched material with spatial variations of its refractive index. By highly utilizing such features, one-dimensionally continuous, arbitrary phase profiles can be created in a simple manner with the width profile design. Since spatial transmittance variations can be minimized due to the impedance matching feature, this approach provides a nearly ideal phase profile for spatial light modulation with phase-only filtering operations. We demonstrate that this approach has the capability to improve the performance in various metasurface-based optical components, including polarization-dependent, large-angle beam deflectors and versatile multi-beam splitters. Considering that designing optical phases even in deep-subwavelength regimes is critical for free-space optics, the proposed approach will enable new classes of optical components with complex wavefront engineering.

Magnetic Plasmon-Enhanced Second-Harmonic Generation on Colloidal Gold Nanocups

The magnetic plasmons of three-dimensional nanostructures have unique optical responses and special significance for optical nanoresonators and nanoantennas. In this study, we have successfully synthesized colloidal Au and AuAg nanocups with a well-controlled asymmetric geometry, tunable opening sizes, and normalized depths ( h/ b, where h is depth and b is the height of the templating PbS nanooctahedrons), variable magnetic plasmon resonance, and largely enhanced second-harmonic generation (SHG). The most-efficient SHG of the bare Au nanocups is experimentally observed when the normalized depth h/ b is adjusted to ∼0.78-0.79. We find that the average magnetic field enhancement is maximized at h/ b = ∼0.65 and reveal that the maximal SHG can be attributed to the joint action of the optimized magnetic plasmon resonance and the "lightning-rod effect" of the Au nanocups. Furthermore, we demonstrate for the first time that the AuAg heteronanocups prepared by overgrowth of Ag on the Au nanocups can synergize the magnetic and electric plasmon resonances for nonlinear enhancement. By the tailoring of the dual resonances at the fundamental excitation and second-harmonic wavelengths, the far-field SHG intensity of the AuAg nanocups is enhanced 21.8-fold compared to that of the bare Au nanocups. These findings provide a strategy for the design of nonlinear optical nanoantennas based on magnetic plasmon resonances and can lead to diverse applications ranging from nanophotonics to biological spectroscopy.

Optical Absorption of Dye Molecules in a Spherical Shell Geometry

Dipole–dipole interactions between neighboring dye molecules can cause substantial spectral changes in optical absorption, with a strong dependence on near-neighbors’ relative distances and orientations. Such effects have been previously investigated in dimers, as well as planar arrangements of dipoles, but not to our knowledge in a three-dimensional spherical configuration. This work provides a comprehensive exploration of the effect of dipolar interactions in such a geometry, varying the dye concentration, orientations, and uniformity in coverage. We also contrast this coupled-dipole model to a simpler but often-used homogeneous effective-medium approximation, which ignores effects of orientation and nonuniformity. The results provide a first step toward the full description of light scattering in a complex anisotropic core–shell geometry, which is of strong relevance in surface-enhanced spectroscopy applications, as well as in the strong coupling between molecular emitters and optical nanoresonators.

Measure of general quantum correlations in optomechanics

In this paper, we analyze nonclassical correlations between bipartite states in two optomechanical systems. The first system (Sec. 2) consists of two nanoresonators spatially separated by broadband squeezed light, where each cavity has a fixed mirror and a movable one. The second system (Sec. 3) is an atom-optomechanical system consisting of an atomic ensemble placed inside an optical nanoresonator with a vibrating mirror. For both optomechanical systems, we give the Hamiltonian and the explicit expression of covariance matrix leading to the quantum equations describing the dynamic evolution of the system. Then, the nonclassical correlations are quantified using the logarithmic negativity and Gaussian quantum discord. We propose also a scheme for examining the evolution of Gaussian quantum steering and its asymmetry in each system. We show that the entanglement of the two mechanical modes is very strongly related to the parameters characterizing the environment where the movable mirrors evolve, in particular the squeeze parameter, the optomechanical cooperativity and thermal bath temperature.

Directed self-assembly of a high-chi block copolymer for the fabrication of optical nanoresonators

In this paper, we report on the fabrication of optical nanoresonators using block copolymer lithography. The nanostructured gratings or nanofins were fabricated using a silicon-containing block copolymer on a chromium coated silicon-on-insulator substrate. The etch resistance of the block copolymer template enables a unique patterning technique for high-aspect-ratio silicon nanofins. Integration of the directed self-assembly with nanoimprint lithography provides a well-aligned array of nanofins with a depth of ∼125 nm on a wafer scale. The developed nanopatterning method is an alternative to the previously reported nanopatterning techniques utilizing block copolymers. A dense array of sub-10 nm nanofins is used to realize a photonic guided-mode resonance filter. The nanostructured grating provides high sensitivity in refractive index sensing, as demonstrated by simulations and experiments in measuring varying contents of the tetrahydrofuran solvent.

Optical nanoresonator fabrication by FIB/EB dual-beam lithography

Optical nanoresonator fabrication by FIB/EB dual-beam lithography

Generation of laser radiation by nanostructured solid active elements with selective optical nanoresonators formed in nanoporous aluminum oxide films

Generation of laser radiation by nanostructured solid active elements with selective optical nanoresonators formed in nanoporous aluminum oxide films

Mode Matching for Optical Antennas.

The emission rate of a point dipole can be strongly increased in the presence of a well-designed optical antenna. Yet, optical antenna design is largely based on radio-frequency rules, ignoring, e.g., Ohmic losses and non-negligible field penetration in metals at optical frequencies. Here, we combine reciprocity and Poynting's theorem to derive a set of optical-frequency antenna design rules for benchmarking and optimizing the performance of optical antennas driven by single quantum emitters. Based on these findings a novel plasmonic cavity antenna design is presented exhibiting a considerably improved performance compared to a reference two-wire antenna. Our work will be useful for the design of high-performance optical antennas and nanoresonators for diverse applications ranging from quantum optics to antenna-enhanced single-emitter spectroscopy and sensing.

Using active gain to maximize light absorption

Using an optical nanoresonator, such as a nanoparticle, to concentrate and absorb light at deep subwavelength scale is of both fundamental and technological significance. To quantify absorption, one typically introduces the absorption cross section and the absorption efficiency, i.e., the ratio of the absorption cross section of a local resonator to its geometric cross section. Exploring a general way to enlarge the absorption cross section of a deep subwavelength nanoparticle and increase the absorption efficiency certainly is important and highly desired. In this work, we show that by introducing a gain medium in the structure, the absorption cross section of any deep subwavelength particle could be boosted and even approach a universal maximum value, of the order ${\ensuremath{\lambda}}^{2}$. Our approach for controlling the absorption using gain is general and may be useful both for the design of light harvesting materials and high-speed active nano-optics devices.