Superlens imaging and light concentration in mesoscale photonics: design and implementation

Progress in nanofabrication made possible development of metamaterials and nanoplamonics two decades ago. The area of mesoscale photonics where the characteristic dimensions of spherical, pyramidal or other building blocks are on the order of several wavelengths remained relatively less studied. However, the optical properties of such structures are extremely interesting due to their ability to create tightly focused beams, so-called “photonic nanojets”, and to resonantly trap light inside their building blocks. In this dissertation, we focus on two main applications of such structures. It is proposed to use dielectric microcons for concentrating light on photodetector focal plane arrays (FPAs) and it is proposed to use contact high-index dielectric microspheres (also termed ball lenses) for improving resolution in cellphone-based microscopy.
We proposed and developed three designs of silicon (Si) microconical arrays which can be used as light concentrators for integration with FPAs operating in mid-infrared (MWIR) region. Such structures can be fabricated by anisotropic wet etching of Si. The spectral and angular dependencies of power enhancement factors (PEFs) provided by such high-index (n~3.5) Si microcones are calculated using finite-difference time-domain modeling. In addition, we observed and studied resonant trapping of photons inside such microcones which can lead to their applications in multispectral imaging devices with a large angle-of-view (AoV).
It is shown that similar microconical light concentrators formed by low-index materials (n = 1.6) which can be fabricated by Nanoscirbe or by plastic injection molding. It is demonstrated that PEFs ~100 times can be achieved in such structures with optimized geometry. It was demonstrated a good agreement of our numerical modeling results with the experiments performed previously on structures with a suboptimal geometry.
We proposed a novel label-free cellphone microscopy assisted by high index contact ball lenses. Resolution of the cellphones is limited by the pixilation of the images. Previous microoptics-based imaging solutions provided insufficient magnification and suffered from spherical aberrations and pincushion distortion. In our work, it is shown that use of ball lenses with n~2 specially designed to provide maximal magnification values (up to 50 times) allows to reduce the role of pixilation and reach diffraction limited resolution values of ~600 nm based on rigorous resolution quantification criteria. It is demonstrated that dispersion properties of the ball lens material significantly influence the magnification in such cellphone imaging. It is shown a semi-quantitative agreement of observed magnification with a simplified geometrical optics model. We demonstrated imaging of various biomedical samples by using proposed cellphone microscopy. It is shown that it can be used as a compact and inexpensive replacement of conventional microscopes to diagnose diseases such as melanoma in vivo without invasive biopsy.
To extend FoV, we assembled centimeter-scale arrays of ball lenses using either micromanipulation or air suction through microhole arrays obtained by laser burning or micromachining. It was found this technology allows obtaining ordered arrays for sufficiently large (>300 μm) ball lenses, but assembling smaller microspheres can in principle be also achieved in future work. It was demonstrated that such microspherical arrays can be used as: a) superresolution coverslips with wide FoV (after embedding in plastic), and b) retroreflectors with ultranarrow reflection cone and highly dispersive properties.