Subwavelength Spot Research Articles

Ultraviolet-visible spectroscopy with all-dielectric multi-foci dispersive metalens

Metasurfaces are a cutting-edge development in optical technology miniaturization, enabling the manipulation of light at a microscopic scale while providing unprecedented control over its intrinsic characteristics, such as amplitude, polarization, and phase. Researchers have been making significant progress in advancing and developing high-efficiency and functionally versatile metasurfaces, aiming to achieve miniaturization and overcoming the technological limitations of conventional components. This article is intended to develop a single-layered, highly transmissive all-dielectric multi-foci metalens that serves as a compact spectrometer for portable investigative measurements within the ultraviolet to the visible spectrum, especially for 290–460nm band. The proposed metalens can achieve top-notch performance by precisely mapping the wavelength information into subwavelength diffraction-limited focusing spots. This approach helps in accomplishing simultaneous spectral splitting and focusing on the same plane. The proposed metalens consists of an array of intelligently optimized nanoantennas made of a CMOS-compatible silicon nitride material. It exploits a unique phase and wavelength multiplexing strategy to create highly dispersive multi-foci metalens, making it an ideal choice for an ultra-compact, high-resolution, and dual-band meta-spectrometer. We expect our proposed framework to have numerous applications in the industrial and medical fields, enabling diagnosis, detection, and assessment within a compact platform.

Optically Readable, Physically Unclonable Subwavelength Pixel via Multicolor Quantum Dot Printing for Anticounterfeiting.

We present a secure and user-friendly ultraminiaturized anticounterfeiting labeling technique─the color-encoded physical unclonable nanotag. These nanotags consist of subwavelength spots formed by random combinations of multicolor quantum dots, which are fabricated using a cost-efficient printing method developed in this study. The nanotags support over 170,000 different colors and are inherently resistant to cloning. Moreover, their high brightness and color purity, owing to the quantum dots, ensure an ease of readability. Additionally, these nanotags can function as color-encrypted pixels, enabling the incorporation of labels (such as QR codes) into ultrasmall physically unclonable hidden tags with a resolution exceeding 100,000 DPI. The unique blend of compactness, flexibility, and security positions the color-encoded nanotag as a potent and versatile solution for next-generation anticounterfeiting applications.

Generation of a Focused THz Vortex Beam from a Spintronic THz Emitter with a Helical Fresnel Zone Plate.

Similar to optical vortex beams, terahertz (THz) vortex beams (TVBs) also carry orbital angular momentum (OAM). However, little research has been reported on the generation of TVBs. In this paper, based on the detour phase technique, we design a series of spintronic terahertz emitters with a helical Fresnel zone plate (STE-HFZP) to directly generate focused TVBs with topological charges (TCs) of l = ±1, ±2 and ±3, respectively. The STE-HFZP is a hybrid THz device composed of a terahertz emitter and a THz lens, and it has a high numerical aperture (NA), achieving subwavelength focal spots. Its focus properties are surveyed systemically through accurate simulations. This STE-HFZP can also generate focused TVBs with higher order TCs. More importantly, the components of the focused electric field with OAM make up the majority of the intensity and have potential applications in the field of THz communications, THz imaging and atom trapping.

All-Dielectric Tunable Terahertz Metagrating for Diffraction Control.

Recently, tunable metagratings have attracted substantial attention in manipulating the diffraction of electromagnetic waves with considerable flexibility, but they are usually limited to inherent ohmic loss due to the metal layers. The all-dielectric schemes can address this issue, but its design and optimization remain challenging in the terahertz regime, especially in the 6G communication window. In this work, an all-dielectric tunable terahertz metagrating is demonstrated in theoretical and experimental investigations. The metagrating operating in the 6G communication window bends the electromagnetic waves beam into the T-1 diffraction order by optimizing the unit cell. In the experiments, more than 72.46% of the transmitted energy is concentrated in the desired diffraction order for p-polarized light and more than 66.60% for s-polarized light, which agrees well with the theoretical design. The tunability by angular deflection is reported in this all-dielectric metagrating. Then, based on the all-dielectric metagrating arrays, a metalens with numerical aperture of NA = 0.39 at 0.14 THz is demonstrated. The subwavelength scale focal spot is obtained as 2.0 mm × 2.0 mm with the focusing distance of 117.8 mm. Imaging capability of the metalens is performed utilizing the transmission imaging manner. The measured and anticipated results are satisfactorily congruous with one another, which could validate our design. This work paves the way toward designing highly efficient and tunable devices with potential applications in terahertz communications, sensors, and super-resolution imaging.

Creating and moving nanoantenna cold spots anywhere

Cold spots are sub-wavelength regions which might emerge near a nanoantenna, should one or more components of some far-field illumination cancel out with scattered light. We show that by changing only the polarisation, amplitude, and phase of two plane waves, a unique, zero-magnitude and highly sub-wavelength cold spot can be created and moved anywhere in the space around a nanoantenna of any arbitrary shape. This can be achieved using ultra-fast modulated pulses, or a time-harmonic approximation. Easily disturbed by a change in the nanoantenna’s material or position, a manufactured cold spot is fragile and could be used in nanoscale sensing. Our technique exploits the linearity of Maxwell’s equations and could be adapted to manipulate any phenomena governed by the linear wave equation, including acoustic scattering. This is a means for potentially ultra-fast sub-wavelength electric field manipulation.

Far-field position-tunable trapping of dielectric particles using a graphene-based plasmonic lens.

In this report, a graphene-based plasmonic lens is designed for far-field position-tunable trapping of dielectric particles at a wavelength of 1550 nm, in which target particles can be floated at a variable z-position, using a variable gate voltage applied to the graphene ribbons. Preventing proximity of the trapped particle and the metallic lens structure, we can diminish general thermal issues in plasmonic tweezers, while realizing higher degrees of freedom in studying target characteristics of the particles by achieving position-tunable 3D trapping. These advantageous aspects are impossible in conventional plasmonic tweezers, because of the highly evanescent nature of the plasmonic field at the metal interface. The proposed structure is comprised of two concentric circular slit-sets (S1, S2), each capable of sending a directive beam, which can lead to a constructive interference, and forming a subwavelength focal spot in the far-field. Taking advantage of the epsilon-near-zero (ENZ) behavior of graphene, each of the radiating slit-sets can be switched ON/OFF, with a radiation switching ratio of about 49, by applying a small electric pulse of 80 meV to change the Fermi energy of the corresponding graphene ribbon from 0.535 eV to 0.615 eV. Hence, inverting the radiation state of the designed lens, from (S1:ON, S2:OFF) to (S1:OFF, S2:ON), we can change the z-position of the focal trapping site from 5000 nm to 9800 nm. This configuration can be proposed as a new generation of long-range, electrostatically tunable 3D plasmonic tweezing, without the need for any external bulky optomechanical equipment.

Sub-micrometer direct laser writing using an optimized binary-amplitude zone plate lens.

Direct laser writing (DLW) is a versatile and reliable lithography method widely used in many micro and nano fabrication areas. However, the resolution of DLW is limited by the optical diffraction limit. Many methods have been proposed to improve the lithography resolution, but with either high cost or increasing the complexity of the system. Here, we propose a high numerical aperture binary-amplitude-type zone plate lens that can achieve a sub-wavelength focal spot with a large depth of focus and long working distance. The critical dimension of such a lens is set at micrometer scale for ease of fabrication. By integrating the as-designed planar lens into a DLW system, we experimentally demonstrate less than 300 nm lithography resolution with exposure depth larger than 500 nm. Our results show the possibility of writing sub-micrometer scale structures with the integration of a planar lens into the DLW system, which enables miniaturization and compactness of lithography instruments for many applications.

Focusing of in-plane hyperbolic polaritons in van der Waals crystals with tailored infrared nanoantennas.

Phonon polaritons (PhPs)—light coupled to lattice vibrations—with in-plane hyperbolic dispersion exhibit ray-like propagation with large wave vectors and enhanced density of optical states along certain directions on a surface. As such, they have raised a surge of interest, promising unprecedented manipulation of infrared light at the nanoscale in a planar circuitry. Here, we demonstrate focusing of in-plane hyperbolic PhPs propagating along thin slabs of α-MoO3. To that end, we developed metallic nanoantennas of convex geometries for both efficient launching and focusing of the polaritons. The foci obtained exhibit enhanced near-field confinement and absorption compared to foci produced by in-plane isotropic PhPs. Foci sizes as small as λp/4.5 = λ0/50 were achieved (λp is the polariton wavelength and λ0 is the photon wavelength). Focusing of in-plane hyperbolic polaritons introduces a first and most basic building block developing planar polariton optics using in-plane anisotropic van der Waals materials.

Negative Index Metamaterial Lens for Subwavelength Microwave Detection

Metamaterials are engineered periodic structures designed to have unique properties not encountered in naturally occurring materials. One such unusual property of metamaterials is the ability to exhibit negative refractive index over a prescribed range of frequencies. A lens made of negative refractive index metamaterials can achieve resolution beyond the diffraction limit. This paper presents the design of a metamaterial lens and its use in far-field microwave imaging for subwavelength defect detection in nondestructive evaluation (NDE). Theoretical formulation and numerical studies of the metamaterial lens design are presented followed by experimental demonstration and characterization of metamaterial behavior. Finally, a microwave homodyne receiver-based system is used in conjunction with the metamaterial lens to develop a far-field microwave NDE sensor system. A subwavelength focal spot of size 0.82λ was obtained. The system is shown to be sensitive to a defect of size 0.17λ × 0.06λ in a Teflon sample. Consecutive positions of the defect with a separation of 0.23λ was resolvable using the proposed system.

Cooperative optical wavefront engineering with atomic arrays

AbstractNatural materials typically interact weakly with the magnetic component of light which greatly limits their applications. This has led to the development of artificial metamaterials and metasurfaces. However, natural atoms, where only electric dipole transitions are relevant at optical frequencies, can cooperatively respond to light to form collective excitations with strong magnetic, as well as electric, interactions together with corresponding electric and magnetic mirror reflection properties. By combining the electric and magnetic collective degrees of freedom, we show that ultrathin planar arrays of atoms can be utilized as atomic lenses to focus light to subwavelength spots at the diffraction limit, to steer light at different angles allowing for optical sorting, and as converters between different angular momentum states. The method is based on coherently superposing induced electric and magnetic dipoles to engineer a quantum nanophotonic Huygens’ surface of atoms, giving full 2π phase control over the transmission, with close to zero reflection.

Remote Individual Addressing of Quantum Emitters with Chirped Pulses.

We propose to use chirped pulses propagating near a band gap to remotely address quantum emitters. We introduce a particular family of chirped pulses that dynamically self-compress to subwavelength spot sizes during their evolution in a medium with a quadratic dispersion relation. We analytically describe how the compression distance and width of the pulse can be tuned through its initial parameters. We show that the interaction of such pulses with a quantum emitter is highly sensitive to its position due to effective Landau-Zener processes induced by the pulse chirping. Our results propose pulse engineering as a powerful control and probing tool in the field of quantum emitters coupled to structured reservoirs.

Tensorial phase control in nonlinear meta-optics

Nonlinear metasurfaces have recently brought harmonic generation to subwavelength level, with spectral and polarization control unachievable in bulk crystals. Not only does nonlinear meta-optics enable the investigation of nonlinear physics at the nanoscale, but it also opens promising technological perspectives. To date, however, no full-phase control has been demonstrated on a harmonic field generated with a sufficient efficiency for most practical purposes. In this work, based on the χ ( 2 ) fully tensorial features of AlGaAs metasurfaces, we propose a robust method to generate 0 − 2 π phase-encoded second harmonic light for nonlinear wavefront shaping, experimentally demonstrating nonlinear beam steering and all-dielectric metalenses that generate and focus second harmonic beams to sub-wavelength spot sizes. Nonlinear generation with record efficiency and phase control are achieved in nanostructured arrays that are fully compatible with mature III-V semiconductor technology. This breakthrough paves the way for the development of ultrathin, free-space photonic devices for nonlinear imaging, including night vision.

Active Huygens’ Box: Arbitrary Electromagnetic Wave Generation With an Electronically Controlled Metasurface

This work investigates the generation of arbitrary electromagnetic waveforms inside a geometrical area enclosed by an active metasurface. We introduce the Huygens' box, where a region of space is enclosed by an active Huygens' metasurface. We show that, upon generating the necessary electric and magnetic currents, we can create any desired electromagnetic field inside Huygens' box. Using this method, we demonstrate, through simulation and experiment, the generation of traveling plane waves, a standing plane wave, and a Bessel wave inside a metallic cavity. These waves are generated using the same (reconfigurable) metasurface by aptly controlling the electronic excitations. By linear superposition of these unconventional traveling-wave “modes,” we experimentally demonstrate, for the first time, a subwavelength superoscillation focal spot formed without involving evanescent EM waves and without an accompanying region of exorbitantly high waveform energy. The Huygens' box brings controlled waveform generation to an unprecedented level, with far-reaching implications to imaging, wireless communication, and medical therapy.

Dynamical generation of multiple focal spot pairs with controllable position and polarization

We report a flexible method to dynamically generate multiple sub-wavelength focal spot pairs with adjustable polarization, position and number at the focal plane of a high numerical aperture (NA) objective lens. The desired incident field on the pupil plane can be analytically derived by employing the time-reversed method combined with the dipole antenna radiation theory. The numerical simulations of the corresponding tightly focused field are conducted using the Richards-Wolf vectorial diffraction theory. The validity of the presented method is demonstrated through experimental generation of several designed pupil fields with a versatile vectorial optical field generator and characterization of the produced focused fields with Stokes parameter measurements.

Objective-free excitation of quantum emitters with a laser-written micro parabolic mirror

The efficient excitation of quantum sources such as quantum dots or single molecules requires high NA optics which is often a challenge in cryogenics, or in ultrafast optics. Here we propose a 3.2 um wide parabolic mirror, with a 0.8 um focal length, fabricated by direct laser writing on CdSe/CdS colloidal quantum dots, capable of focusing the excitation light to a sub-wavelength spot and to extract the generated emission by collimating it into a narrow beam. This mirror is fabricated via in-situ volumetric optical lithography, which can be aligned to individual emitters, and it can be easily adapted to other geometries beyond the paraboloid. This compact solid-state transducer from far-field to the emitter has important applications in objective-free quantum technologies.

Three-dimensional-subwavelength field localization, time reversal of sources, and infinitely asymptotic degeneracy in spherical structures

High-resolution field localization in three dimensions is one of the main challenges in optics and has immense importance in fields such as chemistry, biology, and medicine. Time-reversal symmetry of waves has been a fertile ground for applications such as generating a subwavelength focal spot and coherent-perfect absorption. However, in order to generate the time-reversed signal of a monochromatic source, discrete sources that are modulated according to the wave amplitude on a spherical envelope are required, rendering it applicable only in acoustics. Here we approach these challenges by introducing a spherical layer with a resonant permittivity, which naturally generates the spatially continuous time-reversed signal of an atomic and molecular multipole transition at the origin. We start by utilizing a spherical layer with a resonant TM $l=1$ permittivity situated in a uniform medium to generate a free-space-subwavelength focal spot at the origin. We remove the degeneracy of the eigenfunctions of the composite medium by situating a point current source (or polarization) directed parallel to the spherical layer, which generates a focal spot at the origin independently of its location. The free-space focal spot has a FWHM of $0.4\ensuremath{\lambda}$ in the lateral axes and $0.58\ensuremath{\lambda}$ in the axial axis, which is tighter by a factor of $\sqrt{2}$ in each dimension in excitation-collection mode, overcoming the $\ensuremath{\lambda}/2$ far-field resolution limit in three dimensions. We then explore two directions to localize electric field with deep-subwavelength resolution in three dimensions using this setup. Since the imaginary part of the eigenvalue is also realized in the physical parameter and the setup can be in an exact resonance, it can also open avenues in fields such as cavity QED, entanglement, and quantum information. In addition, we show that spherical structures exhibit a unique type of degeneracy in which an infinite number of eigenvalues asymptotically coalesce. This high degeneracy results in a variety of optical phenomena, such as strong scattering and enhancement of absorption and emission from an atom or molecule by orders of magnitude compared with a standard resonance.

Segmented Bayesian optimization of meta-gratings for sub-wavelength light focusing

Using inverse design tools to engineer functional photonic nanostructures has been a subject of great interest over the past several years. We report combining a segmented Bayesian optimization algorithm with the rigorous coupled wave analysis to design meta-gratings for sub-wavelength light focusing. Specifically, the meta-gratings comprise one-dimensional periodic arrays of supercells, each of which consists of dozens of dielectric bars. By optimizing geometry of the structure, we demonstrate two kinds of meta-gratings operating at single and double wavelengths, respectively. Both of them can focus the incoming light into periodic sub-wavelength spots with high energy density. The full width at half-maximum (FWHM) of the focusing spots for the single wavelength ( λ = 633 n m ) case can be as small as 0.36 λ , while FWHMs of the focusing spots at double wavelengths ( λ = 533 n m and 633 nm) are about 0.4 λ . This proposed approach provides an affordable method to tackle the problem of complex photonic structure design.

Non-diffracting light in nature: Anomalously reflected self-healing Bessel beams from jewel scarabs

We report a study of spatial light modulation in the photonic structure of jewel scarabs, revealing the interplay of the polarization and phase control of light, which is not possible with the current optical technology. Phase measurements performed on jewel scarabs demonstrate that the polarization anomalous (helicity-preserving) reflection of light occurs together with alteration of the dynamic phase associated with the optical path length. This control of light differs from the operation of artificially prepared polarization-sensitive structures, shaping light through the geometric phase altered by the polarization transformation. Challenging three-dimensional imaging of the cuticle, requiring high-resolution quantitative mapping of steep phase changes, has been achieved owing to the optical performance of recently developed geometric-phase microscopy. We find that the cuticle of jewel scarabs is formed of micrometer-sized axicon cells, generating thousands of Bessel beams with subwavelength spot size. The nondiffracting features and the self-healing ability of the Bessel beams originating from the beetle Chrysina gloriosa are demonstrated experimentally. Considering Bragg reflection and shaping of RGB components of white light Bessel beams, we explain the spatial structuring of colors in microscopic images of jewel scarabs and reveal the conversion of colors when changing the distance from the cuticle. The functionality and performance of the cuticle axicon cells are discussed in comparison with high-aperture dielectric meta-axicons, and potential applications in colorimetric refractive index sensing are outlined.

Exploiting the circular polarization of light to obtain a spiral energy flow at the subwavelength focus

Using Richards–Wolf formulas, we show analytically that when strongly focusing a circularly polarized axisymmetric non-vortex field (e.g., a Gaussian beam or zero-order Bessel–Gaussian beam), a spiral energy flow is produced around the sub-wavelength focal spot. This phenomenon can be explained by the conversion of spin angular momentum associated with the circular polarization state into the near-focus orbital angular momentum, with the on-axis angular momentum being strictly zero. It is also shown that a linearly polarized optical vortex with topological charge 2 produces an on-axis near-focus energy backflow whose amplitude (defined by the negative longitudinal component of the Poynting vector) is comparable with that of the incident beam.

Tight focusing induced non-uniform polarization change in reflection for arbitrarily polarized incident light

Tight focusing of light through a high numerical aperture (NA) objective can produce a subwavelength focal spot and this technique has thus been widely applied in many microscopic systems. Here we show that the polarization of the light reflected from a high-NA objective shows a significant change when compared with that of the incident light, which would be crucial in polarization-dependent imaging and detection applications. Some typical polarized incident light beams, including linearly, circularly, and elliptically polarized light beams and cylindrical vector beams, are studied both theoretically and experimentally. We find that with the exception of pure radial and azimuthal polarizations, all other polarizations of the incident light show nonuniform changes in the reflected light, particularly in the total internal reflection region. We build a theoretical model of the observed polarization change effect and experimentally verify that this model is valid for use with arbitrarily polarized light. This work could contribute to the design of various high-NA microscopic systems, including those intended for polarization-dependent imaging, and polarization differential sensing applications.