Engineered microsphere possesses the advantage of strong light manipulation at sub-wavelength scale and emerges as a promising candidate to shrink the focal spot size. Here we demonstrated a centercovered engineered microsphere which can adjust the transverse component of the incident beam and achieve a sharp photonic nanojet. Modification of the beam width and working distance of the photonic nanojet were achieved by tuning the cover ratio of the engineered microsphere, leading to a sharp spot size which exceeded the optical diffraction limit. At a wavelength of 633 nm, a focal spot of 245 nm (0.387 λ) was achieved experimentally under plane wave illumination. Strong localized field with Bessel-like distribution was demonstrated by employing the linearly polarized beam and a centercovered mask being engineered on the microsphere.Micro-lenses, with a size of a few wavelengths, exhibit excellent abilities to confine incident light and generate small focal spot which exceeds the optical diffraction limit at around half of the incident wavelength. Among which, the most investigated micro-lenses are microspheres and microcylinders. It has been demonstrated early in 2000 by Lu and Luk'aynchuk et al. 1 . Later, Chen et al. studied the field enhancement at the shadow side of a microcylinder under plane wave illumination and termed it as "photonic nanojet" 2 . Excellent optical properties of the photonic nanojet, such as non-diffracting, strong localized field intensity and sharp focal spot, have proved to be beneficial for various applications: nanoparticle detection, optical nanolithography, and super-resolution imaging. Among which, a small beam waist of the focal spot is the most desired property as it characterizes the converging ability of the microlens and plays a key role in these applications. It is found that when a nanoparticle is located within the focal region of a microsphere, the back-scattering intensity can be greatly enhanced. This enhancement is applied for detecting nanoparticles in liquid and nanoparticles at a size of 20 nm can be identified 3,4 . Furthermore, it is concluded that the detection sensitivity can be greatly enhanced when the beam width of the photonic nanojet is small. On the other hand, the photonic nanojet generated by the microsphere is applied as the exposure beam in optical lithography [5][6][7][8][9][10] , where the minimum line width of the fabricated patterns is directly dominated by the beam width of the photonic nanojet. Therefore, when a sharp photonic nanojet is achieved, the pattern size can be reduced correspondingly. Also, in optical super-resolution imaging, the sample interacts with the electric field of the photonic nanojet, and generates scattered wave which propagates through the microsphere to form the image. When combined with a confocal microscope, resolution of 25 nm in air can be achieved, pushing the super-resolution ability of microspheres to a new limit 11,12 .Approaches to modify the optical properties of the photonic nanojet, including changing the refrac...

Liquid metal droplets, such as eutectic gallium–indium (EGaIn), are important in many research areas, such as soft electronics, catalysis, and energy storage. Droplet contact on solid surfaces is typically achieved without control over the applied force and without optimizing the wetting properties in different environments (e.g., in air or liquid), resulting in poorly defined contact areas. In this work, we demonstrate the direct manipulation of EGaIn microdroplets using an atomic force microscope (AFM) to generate repeated, on-demand making and breaking of contact on self-assembled monolayers (SAMs) of alkanethiols. The nanoscale positional control and feedback loop in an AFM allow us to control the contact force at the nanonewton level and, consequently, tune the droplet contact areas at the micrometer length scale in both air and ethanol. When submerged in ethanol, the droplets are highly nonwetting, resulting in hysteresis-free contact forces and minimal adhesion; as a result, we are able to create reproducible geometric contact areas of 0.8–4.5 μm2 with the alkanethiolate SAMs in ethanol. In contrast, there is a larger hysteresis in the contact forces and larger adhesion for the same EGaIn droplet in air, which reduced the control over the contact area (4–12 μm2). We demonstrate the usefulness of the technique and of the gained insights in EGaIn contact mechanics by making well-defined molecular tunneling junctions based on alkanethiolate SAMs with small geometric contact areas of between 4 and 12 μm2 in air, 1 to 2 orders of magnitude smaller than previously achieved.

Confining electric fields to a nanoscale region is challenging yet crucial for applications such as high-resolution probing of electrical properties of materials and electric-field manipulation of nanoparticles. State-of-the-art techniques involving atomic force microscopy typically have a lateral resolution limit of tens of nanometers due to limitations in the probe geometry and stray electric fields that extend over space. Engineering the probes is the most direct approach to improving this resolution limit. However, current methods to fabricate high-resolution probes, which can effectively confine the electric fields laterally, involve expensive and sophisticated probe manipulation, which has limited the use of this approach. Here, we demonstrate that nanoscale phase switching of configurable thin films on probes can result in high-resolution electrical probes. These configurable coatings can be both germanium–antimony–tellurium (GST) as well as amorphous-carbon, materials known to undergo electric field-induced nonvolatile, yet reversible switching. By forming a localized conductive filament through phase transition, we demonstrate a spatial resolution of electrical field beyond the geometrical limitations of commercial platinum probes (i.e., an improvement of ∼48%). We then utilize these confined electric fields to manipulate nanoparticles with single nanoparticle precision via dielectrophoresis. Our results advance the field of nanomanufacturing and metrology with direct applications for pick and place assembly at the nanoscale.