Tunable photonic nanojet formed by generalized Luneburg lens

Broadband achromatic aberration general conformal Luneburg lens with quasi-far-field highly efficient super-focusing

Super-focusing light using metamaterials and metasurfaces is of paramount importance in several applications, from integrated optics to microwave engineering and sensing. However, there are still some difficulties to realize broadband achromatic aberration highly efficient super-focusing from the far field to far field or quasi far field. In this Letter, based on conformal transformation optics, we propose a generalized conformal Luneburg lens (GCLL), which provides a new, to the best of our knowledge, strategy for quasi-far-field super-focusing with broadband (0.9-1.3 THz) achromatic aberration and high efficiency (above 60%). A relatively high numerical aperture (NA of 0.63) and sub-diffraction-limited resolution (FWHM of 0.45λ) are also obtained. The sample of the GCLL was designed using gradient metamaterials. The numerical simulation results verify that the focusing effects of the designed samples are consistent with the performance of the ideal GCLL.

Optical Force on a Metal Nanorod Exerted by a Photonic Jet

In this article, we study the optical force exerted on nanorods. In recent years, the capture of micro-nanoparticles has been a frontier topic in optics. A Photonic Jet (PJ) is an emerging subwavelength beam with excellent application prospects. This paper studies the optical force exerted by photonic jets generated by a plane wave illuminating a Generalized Luneburg Lens (GLLs) on nanorods. In the framework of the dipole approximation, the optical force on the nanorods is studied. The electric field of the photonic jet is calculated by the open-source software package DDSCAT developed based on the Discrete Dipole Approximation (DDA). In this paper, the effects of the nanorods' orientation and dielectric constant on the transverse force Fx and longitudinal force Fy are analyzed. Numerical results show that the maximum value of the positive force and the negative force are equal and appear alternately at the position of the photonic jet. Therefore, to capture anisotropic nanoscale-geometries (nanorods), it is necessary to adjust the position of GLLs continuously. It is worth emphasizing that manipulations with nanorods will make it possible to create new materials at the nanoscale.

Broadband ultra-long acoustic jet based on double-foci Luneburg lens

In this paper, a gradient index acoustic metamaterial is proposed based on the concept of the optical modified generalized Luneburg lens (MGLL). With the MGLL, double-foci and high energy density between the two foci can be achieved, which enables the realization of an ultra-long acoustic jet between the two foci. This capability of the MGLL is theoretically and numerically demonstrated with an acoustic metamaterial lens. Numerical simulation results show that based on this design, ultra-long acoustic jets with a jet length of up to 30 λ can be achieved, covering both the near field and far field.

Structured light engineering using a photonic nanojet

In this Letter, we present the photonic nanojet as a phenomenon in a structured light generator system that is implemented to modify the source focal spot size and emission angle. The optical system comprises a microlens array that is illuminated by a focused Gaussian beam to generate a structured pattern in the far field. By introducing a spheroid with different aspect ratios in the focus of the Gaussian beam, the source optical characteristics change, and a photonic nanojet is generated, which will engineer the far-field distribution. To probe the light fields, we implement a high-resolution interferometry setup to extract both the phase and intensity at different planes. We both numerically and experimentally demonstrate that the pattern distribution in the far field can be engineered by a photonic nanojet. As an example, we examine prolate, sphere, and oblate geometries. An interesting finding is that depending on the spheroid geometry, a smaller transverse FWHM of a photonic nanojet with a higher divergence angle produces an increased pattern field of view at the same physical size of the optical system.

All-dielectric concentration of electromagnetic fields at the nanoscale: the role of photonic nanojets

The photonic nanojet (PNJ) is a narrow high-energy beam that was originally found on the back side of all-dielectric spherical structures. It is a unique type of energy concentration mode. The field of PNJs has experienced rapid growth in the past decade: nonspherical and even pixelized PNJ generators based on new physics and principles along with extended photonic applications from linear optics to nonlinear optics have driven the re-evaluation of the role of PNJs in optics and photonics. In this article, we give a comprehensive review for the emerging sub-topics in the past decade with a focus on two specific areas: (1) PNJ generators based on natural materials, artificial materials and nanostructures, and even programmable systems instead of conventional dielectric geometries such as microspheres, cubes, and trihedral prisms, and (2) the emerging novel applications in both linear and nonlinear optics that are built upon the specific features of PNJs. The extraordinary features of PNJs including subwavelength concentration of electromagnetic energy, high intensity focusing spot, and lower Joule heating as compared to plasmonic resonance systems, have made PNJs attractive to diverse fields spanning from optical imaging, nanofabrication, and integrated photonics to biosensing, optical tweezers, and disease diagnosis.

Stepwise Luneburg Lens for Bloch Surface Waves

In order to enlarge the capability for in-plane manipulation of the Bloch surface wave (BSW), we investigate 2D gradient index (GRIN) optical components using a finite-difference time-domain (FDTD) numerical method. To ease difficulties in fabrication to acquire a continuous index profile of GRIN optical components, we propose a stepwise index profile. For 2D surface wave devices, such discrete index steps can be achieved by stepwise structuring of the top layer, also called the device layer. For the demonstration of the stepwise GRIN optics concept, we consider a Luneburg lens, which is a good example of the GRIN optical component that produces a strong focal spot on the shadow-side curvature of the lens. The limited index contrast of the BSW systems loosens the confinement of the focal spot. A mitigation plan is to elongate the circular geometry to the prolate ellipse. BSW-based Luneburg lenses with a relatively small number of steps and an elliptical geometry are demonstrated with comparable performances to a standard Luneburg lens.

Temperature-controlled photonic nanojet via VO2 coating

In this work, a numerical investigation of how temperature can tune the FWHM and working distance (WD) of a photonic nanojet (PNJ) is conducted. Vanadium oxide (VO₂), a phase change material, is coated onto the top half-surface of a glass microsphere and illuminated with incident light at a wavelength of 800 nm. As VO₂ changes from semiconducting to metallic phase, the refractive index of the VO₂ layer changes at its transition temperature of 68°C. It is found that a coating of 75 nm on a 5.0 μm diameter microsphere with a refractive index of 1.50 is the most optimal, as it tunes the FWHM the greatest while remaining thin enough to have a high transmission. When temperature is raised from 20°C to 90°C, the FWHM varies from 0.43 to 0.37 μm, corresponding to a 14.0% change. The WD varies from 0.29 to 0.20 μm, corresponding to a 31.0% change. Tunable PNJs have potential applications in tunable nanolithography and imaging.