Super-focusing of center-covered engineered microsphere

Abstract

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...

Refractive index less than two: photonic nanojets yesterday, today and tomorrow [Invited]

“…There are a number of modifications of photonic structures which allows the control of the nanojet parameters. For example, to engineer the focusing properties one can use an additional pupil mask or microspheres, decorated with concentric rings [87][88][89]. This permits an additional reduction of the nanojet focusing spot size.…”

A Millimetre-Wave Cuboid Solid Immersion Lens with Intensity-Enhanced Amplitude Mask Apodization

“…A variety of methods, including use of the new high-index materials and the innovative structure arrangements, were attempted to lift resolution of a SIL to a super-resolution level for the important applications in imaging and sensing these years [11][12][13][14], however high material cost and harsh control conditions restrict their operations on a wide range of imaging/sensing targets. By contrast, the amplitude mask apodization accomplished by adding a pupil-mask on a SIL was found to be a relatively easy procedure to optimise the resolution of a near-field optical system through waist narrowing (reduction of the full width at half maximum, FWHM) of a photonic jet [15][16][17]. Nevertheless, intensity-loss is always unavoidable in the system applied by this method, because the centred pupil-mask lowers the amount of the incoming rays to be admitted in the SIL.…”

Self-regulating reversible photocatalytic-driven chromism of a cavity enhanced optical field TiO2/CuO nanocomposite

“…The SiO 2 -based core-shell structure has been demonstrated as a successful candidate to improve various optical properties, such as imaging, focusing, and optical nonlinearity. 38,39 However, for various applications that require high light-matter interactions, it is possible to design novel structures with better performances than that of the SiO 2 -based core-shell structure. The key mechanism for the SiO 2 -based core-shell structure is the focusing effect induced by the refraction of the SiO 2 particle lens.…”