Subwavelength Focusing Research Articles

Specular-reflection photonic nanojet: physical basis and optical trapping application

A specular-reflection photonic nanojet (s-PNJ) is a specific type of optical near-field subwavelength spatial localization originated from the constructive interference of direct and backward propagated optical waves focused by a transparent dielectric microparticle located near a flat reflecting mirror. The unique property of s-PNJ is reported for maintaining its spatial localization and high intensity when using microparticles with high refractive index contrast when a regular photonic nanojet is not formed. The physical principles of obtaining subwavelength optical focus in the specular-reflection mode of a PNJ are numerically studied and a comparative analysis of jet parameters obtained by the traditional schemes without and with reflection is carried out. Based on the s-PNJ, the physical concept of an optical tweezer integrated into the microfluidic device is proposed provided by the calculations of optical trapping forces of the trial gold nanosphere. Importantly, such an optical trap shows twice as high stability to Brownian motion of the captured nano-bead as compared to the conventional nanojet-based traps and can be relatively easy implemented.

Free space super focusing using all dielectric hyperbolic metamaterial

Despite that Hyperbolic Metamaterial (HMM) has demonstrated sub-wavelength focusing inside of it, sub-wavelength imaging in free space of HMM is rarely introduced. The decay of hyperbolic momentum space outside the hyperbolic medium has hindered the realization of sub-wavelengh focusing in the near field of HMM. Furthermore, manipulating the negatively refracted waves exiting the HMM have addressed another major obstacle to realize free space sub-wavelength focusing. In this work, we report extended sub-wavelength focusing in free space based on negative refraction of light exiting the HMM. The proposed structure is composed of multilayers of doped InAs/intrinsic InAs integrated with metallic slit. We theoretically simulate the doped InAs/intrinsic InAs HMM and investigate the negative refraction behavior outside the HMM. We optimized the structure for achieving high resolution down to 0.2λ, extended to a distance of 3.2 µm in free space. Also, sub-wavelength focusing in free space has been studied at different doping concentrations showing that the small doping concentrations exhibit enhancement in resolution at short distances up to 600 nm away from the HMM. Extending the focusing distance is achieved up to distance 3.5 µm from the hyperbolic structure by manipulating the doping concentration. This proposed lens configuration is expected to find potential usage in mid IR thermal imaging and photolithography application.

Far-field unlabeled super-resolution imaging with superoscillatory illumination

Unlabeled super-resolution is the next grand challenge in imaging. Stimulated emission depletion and single-molecule microscopies have revolutionized the life sciences but are still limited by the need for reporters (labels) embedded within the sample. While the Veselago–Pendry “super-lens,” using a negative-index metamaterial, is a promising idea for imaging beyond the diffraction limit, there are substantial technological challenges to its realization. Another route to far-field subwavelength focusing is using optical superoscillations: engineered interference of multiple coherent waves creating an, in principle, arbitrarily small hotspot. Here, we demonstrate microscopy with superoscillatory illumination of the object and describe its underlying principles. We show that far-field images taken with superoscillatory illumination are themselves superoscillatory and, hence, can reveal fine structural details of the object that are lost in conventional far-field imaging. We show that the resolution of a superoscillatory microscope is determined by the size of the hotspot, rather than the bandwidth of the optical instrument. We demonstrate high-frame-rate polarization-contrast imaging of unmodified living cells with a resolution significantly exceeding that achievable with conventional instruments. This non-algorithmic, low-phototoxicity imaging technology is a powerful tool both for biological research and for super-resolution imaging of samples that do not allow labeling, such as the interior of silicon chips.

Enhanced Sub-wavelength Focusing by Double-Sided Lens with Phase Correction in THz Range

High capacity radio lines operating in the sub-THz and THz ranges often require very efficient optical elements with a focal length to an aperture diameter ratio—f-number—less than 1. Here, we propose a new type of double-sided sub-THz focusing diffractive optical element with f-number equal to 0.2, designed for quasi-monochromatic illumination with carrier frequency equal to 170 GHz. The element is manufactured by 3D printing technology. Its focal spot diameter defined as the Airy disc size is comparable to the used wavelength. In order to optimize numerically the phase distribution on the anterior side of the structure, we proposed a novel idea based on reversal of phase distribution in outer zones with additional constant phase factor (a method called free form phase distribution, FFPD). Moreover, we applied the modified numerical algorithm to obtain an additional phase correction in a form of a corrective kinoform placed on the posterior side of the diffractive system. The resulted diffractive structure, illuminated by a quasi-plane wave, forms an extremely small focal spot. The paper presents the technical and the theoretical backgrounds, the results of the computer simulations and finally the experimental results.

Impedance-Matching Technique of Metasurfaces Generating Evanescent Fields for Subwavelength Focusing

We propose an impedance-matching technique for metasurfaces generating evanescent fields with complex free-space impedance for subwavelength focusing. The proposed technique is mandatory for the subwavelength focusing far beyond the diffraction limit. First, we theoretically show the fact that the wave impedance becomes a complex number due to the generated evanescent fields and give an explicit form of the wave impedance as a ratio of local electric and magnetic fields. Then, we discuss the impedance-matching technique for the distributed complex wave impedance by a metasurface with a position-dependent impedance matrix. To demonstrate the proposed technique, we design a subwavelength focusing 1-D metasurface with the null-to-null beamwidth of one-twentieth of the free-space wavelength at 2.45 GHz, and its performance is verified by wave propagation simulations based on the equivalent circuit model. In order to further verify the validity of the proposed technique, a metasurface is implemented with the trilayer configuration, and the subwavelength focusing is experimentally demonstrated.

Near-UV luminescence tomography with an aperture-free meta super oscillatory lens for single molecule detection.

Medical physics offers super oscillatory lenses (SOL) to attain subwavelength focusing for efficient image resolution in the detection of a single molecule. In this paper, we propose a negative-indexed permeability-controlled meta SOL, which restores the light concentration via exciting surface plasmon resonance and enhances luminescence at our region of interest, i.e., fluid sample (${S_L}$SL). The meta SOL is aperture-free in comparison with other techniques (zero-mode waveguides and nano antennas) and hence avoids the constraints, which are posed by nano apertures, to insert a small volume of $ S_L $SL into these nano apertures for detection purposes. The meta SOL is a unique combination of gold split rings mounted on silicon dioxide (${{\rm SiO}_2}$SiO2) substrate and operational in the near-ultraviolet (UV) region. We utilized the phenomena of negative index of refraction, and our simulated trails exploit the magnetic (${\mu _r}$μr) response of the meta SOL by analyzing its transmission spectra in the frequency range from 3.53 to 3.57 Peta Hz. We observe the methodical response of the meta SOL with its ample potential to surpass the resolution at a working wavelength of $\lambda ={84.173}\;{\rm nm}$λ=84.173nm, which enhances luminescence by restoring the evanescent UV magnetic field (${B_{\rm UV}}$BUV) at ${S_L}$SL. This technique will offer a new and easy approach to uplift the efficiency of super oscillatory lenses in the near-UV regime to benefit single molecule detection techniques and thus the novelty.

Improvement of a Terahertz Detector Performance Using the Terajet Effect in a Mesoscale Dielectric Cube: Proof of Concept

Herein, a simple terahertz (THz) receiver that uses subwavelength focusing of the THz beam on the detector area is proposed. As a proof of concept, a THz detection system with an original optical coupling scheme is implemented, where the signal to be detected is coupled to a THz detector through a mesoscale dielectric particle lens. Coherent detection is successfully demonstrated with an enhancement of the detector sensitivity of about 4.3 dB, compared with that of a direct detection system with the slight decreasing (≈1.67 times) of noise equivalent power value. The results show that the proposed method can reduce the size and increase the sensitivity of various THz systems, including imaging, sensing, and ranging, which would enable significant progress in different fields such as physics, medicine, biology, astronomy, security, etc.

Infrared subwavelength focusing metasurfaces for harvesting heat from the Earth’s back radiation

In this paper, we introduce two different metasurfaces that can achieve subwavelength focusing of the electromagnetic radiation in the infrared range with transmission efficiencies reaching 46% and 74.33% in silicon and thus they can be integrated in heat harvesting applications. We made our designs at the wavelength of 10 μm which is the peak of the back radiation of the Earth. In designing our metasurfaces in the infrared range, we used the phase modulation techniques that proved to be useful in the visible domain. The best focusing resolution achieved in our metasurfaces is 0.26λ (super-focusing). In addition to heat harvesting, the two proposed metasurfaces can be useful in other compact applications that require manipulating the mid-infrared radiation on a very small scale. We used copper and silicon in our metasurfaces which are specifically suitable for the mid-infrared range besides being CMOS compatible. In this paper, we also explained how heat can be harvested from the introduced metasurfaces using the Seebeck effect.

Subwavelength Focusing Beam and Superresolution Ultrasonic Imaging Using a Core-Shell Lens

The ability to manipulate acoustic fields beyond the diffraction limit offers possibilities for many applications, including ultrasound imaging and nondestructive testing. We numerically and experimentally report a simple method for subwavelength ultrasound focusing and superresolution imaging (i.e., images produced with a subwavelength spatial resolution) using a core-shell shaped lens. By changing the mechanical properties and size conditions of the lenses, we can significantly enhance the subwavelength properties of the focusing beams. These properties have a major effect on the image quality of ultrasonic systems. Using a carbon steel core and a Rexolite shell of total diameter of 12.8 mm positioned at a 2 MHz ($\ensuremath{\lambda}=0.74$ mm) plane wave incident field, we show that a beam with intensity gain of 20 dB, full width at half maximum (FWHM) of 0.44 mm ($0.6\ensuremath{\lambda}$), and full length at half maximum (FLHM) of 2.74 mm ($3.7\ensuremath{\lambda}$) can be achieved. Moreover, we experimentally demonstrate that a superresolution imaging system using the core-shell lens can scan objects with subdiffraction information. The results show that the superresolution method can double the spatial resolution and the focal depth can be increased using an appropriate size of lens. The proposed design system can be easily applied to assisted superresolution acoustic microscopy devices.

Ultra-subwavelength focusing and giant magnetic-field enhancement in a low-loss one-way waveguide based on remanence

The subwavelength focusing based on surface plasmon polaritons has been widely explored in tapered metallic structures. However, the efficiency of energy localization is relatively weak, largely due to high propagation loss and strong back reflection. Here, we propose a three-dimensional (3D) straight-tapered one-way surface magnetoplasmon (SMP) waveguide with the end surface of ∼10−4λ0 × 10−4λ0 to achieve energy focusing in the microwave regime. Due to low propagation loss of SMP, we achieve huge magnetic field enhancement in such an ultra-subwavelength area, by five orders of magnitude. Instead of using an external static magnetic field, our proposed SMP waveguide relies on remanence, which is very convenient for operating practical 3D applications. These results show promising applications in magnetic-field enhancing or quenching fluorescence, luminescence or nonlinearity of two-dimensional materials, novel scanning near-field microwave microscopy and energy storage.

Realizing broadband sub-wavelength focusing and a high intensity enhancement with a space-time synergetic modulated acoustic prison

We experimentally demonstrate broadband sub-wavelength focusing and a high sound intensity enhancement using an acoustic prison.

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.

Enhanced subwavelength photonic nanojet focusing via a graded-index round-head microcylinder

Photonic nanojet is a sub-wavelength focus generated by a dielectric microparticle, like microsphere and microcylinder. The formation of nanojet can be tuned by changing the refractive index distribution and geometrical parameters of the structure. Many applications based on photonic nanojet have been explored based on the special property of dielectric microparticles. In this letter, a graded-index structure that combines hemisphere and microcylinder is proposed, which can generate a sub-wavelength nanojet with a full width at a half maximum of 120.9 nm (0.24 λ with 500 nm illumination wavelength). The influences of refraction index distribution and the length of cylinder on the properties of photonic nanojet are simulated and discussed. Moreover, the sensitivity of fabrication errors is analyzed to verify the feasibility of this structure.

Time-delayed acoustic sink for extreme sub-wavelength focusing

After a sound wave enters a Helmholtz resonator, it will be multiply reflected in the cavity and gradually overflow from the opening, just like water slowly flowing out from a water sink. In particular, if such a resonant cavity was arranged near the focus position in a time reversal focusing experiment, the cavity is equivalent to an acoustic sink, and an extreme sub-wavelength focus with a focal size much smaller than the sample size can be achieved. The physical mechanism of this sub-wavelength focusing is that after the sound wave reaches the focus, due to the fact that the arranged cavity has a perfect sound absorbing ability, a large part of the acoustic energy would enter the cavity and then be lost. However, in the loss stage, the localized high-intensity sound energy in the resonant cavity will continuously overflow to the opening. These sounds belong to evanescent wave, and can only be localized near the orifice, so that a very small focus with the size as aperture will be formed. Theoretically, the realization of this extreme sub-wavelength focus resolution depends on the modification of the evanescent wave field. This time-delayed sub-wavelength focusing effect was visually demonstrated by time reversal measurement results in time domain, and the extreme sub-wavelength feature was intuitively demonstrated by results in frequency domain.

Sub-Wavelength Focusing in Inhomogeneous Media with a Metasurface Near Field Plate.

Overcoming the diffraction limit, which enables focusing much less than the wavelength, requires tailoring the evanescent spectrum of an aperture’s field distribution. We model and simulate a corrugated near field plate, which can generate a sub-wavelength focus in inhomogeneous background media. All reactive coupling, between the metasurface near field plate and the focusing domain and among the corrugations in the metasurface, is taken into consideration with the finite element method, which we solve in combination with a constraint to generate a desired focus. Various geometries for the near field plate are considered and we demonstrate that the proposed method can effectively create a deeply sub-wavelength focus within a layered medium having properties resembling brain tissue. Such a device could find use as a detector of biological signals or for hyperthermic treatment near the skin surface.

Subdiffraction focusing of total electric fields of terahertz wave

Terahertz lens is an essential component in terahertz application. We propose a focusing of total electric fields planar lens based on super-oscillation. This planar lens is designed for a wavelength (λ) of 118.8μm with a radius of 160λ, a focal length of 210 λ and a numerical aperture of 0.606. Our experiment demonstrates a subdiffraction and subwavelength focusing of total electric fields. The full width at half-maximum of the focal spot is 0.67 λ, which is smaller than the diffraction limit of 0.825λ. The results shows that it has powerful application for terahertz imaging, especially in the fields of biomedical science.

Polariton nanophotonics using phase-change materials

Polaritons formed by the coupling of light and material excitations enable light-matter interactions at the nanoscale beyond what is currently possible with conventional optics. However, novel techniques are required to control the propagation of polaritons at the nanoscale and to implement the first practical devices. Here we report the experimental realization of polariton refractive and meta-optics in the mid-infrared by exploiting the properties of low-loss phonon polaritons in isotopically pure hexagonal boron nitride interacting with the surrounding dielectric environment comprising the low-loss phase change material Ge3Sb2Te6. We demonstrate rewritable waveguides, refractive optical elements such as lenses, prisms, and metalenses, which allow for polariton wavefront engineering and sub-wavelength focusing. This method will enable the realization of programmable miniaturized integrated optoelectronic devices and on-demand biosensors based on high quality phonon resonators.

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.

Photonic nanojet beam shaping by illumination polarization engineering

Photonic Nanojets (PNJs) have attracted considerable research attention in the fields of super-resolution optical microscopy, nano-photolithography, and single molecule sensors because of their sub-wavelength near-field focusing properties. In this paper, we thoroughly studied how the polarizations and amplitude profiles of the incident beams affect the shape, size, and location of the PNJs generated from the illuminated microspheres. Numerical results showed that the PNJs generated by microspheres were strongly modulated by the polarizations and amplitude profiles of the illumination beams. Therefore, PNJs can be engineered according to the requirements of a specific application by designing the polarizations and amplitude profiles of the illumination light. Various fascinating properties of the PNJs generated with different illumination schemes were demonstrated and their implications for potential applications were discussed as well.

Subwavelength focusing by combining negative-refractive photonic crystal and silicon lens

We propose a new and dynamically focusable subwavelength focusing system, consisting of a negatively refracting photonic crystal and a single silicon lens. The system can adjust the focal length in the range of 370 nm to 4610 nm. We set directional light with a wavelength of 1612nm to incident on the whole system. Numerical simulation through RSOFT shows the smallest focus spot with the half-width of 0.116 λ at a distance of 0.44 λ and calculates focusing efficiency of 45%. In addition, a good result is found that the half-width of the spot is less than 1/2 wavelength at a distance of 2.85 λ from the surface of the structure.