Microsphere-assisted microscopy is a new two-dimensional super- resolution imaging technique, which allows the diffraction limit to be overcome by introducing a transparent microsphere in a classical optical microscope. This super-resolution technique makes it possible to reach a lateral resolution of up to one hundred nanometres. Furthermore, microsphere-assisted microscopy distinguishes itself from others by being able to perform label-free and full-field acquisitions and requires only slight modifications of a classical white light microscope. Extended to three-dimensional surface measurement through interference microscopy which has the advantage of providing a high-axial sensitivity, super-resolution topography or the volume distribution of objects can thus be reconstructed depending on the interference method employed. This chapter first presents a brief history of optical microscopy and recent advances in optical nanoscopy. Then, the super-resolution phenomenon through microspheres is introduced and its performance is described. Finally, the combination of optical interferometry with nanoscopy based on microspheres, giving microsphere-assisted interference microscopy, is exposed.

Microsphere-Assisted Interference Microscopy

The optical diffraction limit, also known as Abbe?s limit, has long been a barrier for the development of advanced optical microscopy, thus hampering attempts to explore subdiffraction-scale entities with light. Dielectric microobjects, such as microfibers and microspheres, display unique optical properties including photonic nanojet (PNJ) effects, optical whispering-gallery resonances, and optical directional antenna effects, which are benefits for nanoscale optical engineering applications, such as nanoimaging, nanopatterning, and nanodetection. Dielectric microspherical lens nanoscopy (DMN) has been widely studied for optical superresolution imaging because of its ability of label-free noninvasive nanoscale investigation. In this review article, we present the principles of DMN and recent advances in studies of imaging mechanism and imaging modes. An overview of DMN imaging applications in label-free superresolution imaging is summarized as well. Furthermore, other DMN applications, including microsphere-assisted laser nanopatterning and nonlinear optical effects enhancements, are also discussed.

Advances in Dielectric Microspherical Lens Nanoscopy: Label-Free Superresolution Imaging

An apparatus (100) for carrying out microsphere-enhanced spectroscopy on a sample (15) is shown. Illumination of the sample (15) is provided by an illumination laser (11). A microsphere (1) is attached to the front lens of the objective lens assembly (2) by a column (13) of optically clear material. This allows the illumination radiation to be focused on the sample (15). Scattered radiation from the illuminated area is collected via the microsphere (1) and the objective lens assembly (2). The scattered radiation passes through the objective lens assembly (2), an optional adjustable pinhole (3) and dichroic mirror (4) to a notch filter (8). The filtered scattered radiation is incident upon an optical grating (6), and subsequently a detector. The provision of the microsphere (1) on a column (13) of optically clear material ensures that it can be retained in a desired alignment and at a desired separation from the front lens of the objective lens assembly (2).

Improvements in or Relating to Microsphere-Enhanced Spectroscopy

As water is normally used as the immersion medium in optically trapped microsphere microscopy, the high-refractive-index barium titanate glass (BTG) microsphere shows a better imaging performance than the low-index polystyrene (PS) or melamine formaldehyde (MF) microsphere, but it is difficult to be trapped by single-beam optical trapping due to its overly high refractive index. In this study, coated BTG microspheres with a PS coating have been computationally explored for the combination of optical trapping with microsphere-assisted microscopy. The PS coating thickness affects both the optical trapping efficiency and photonic nanojet (PNJ) property of the coated BTG sphere. Compared to the uncoated BTG sphere, the coated BTG sphere with a proper PS coating thickness has a highly improved trapping efficiency which enables single-beam optical trapping, and a better PNJ with a higher optical intensity Imax and a narrower full width at half maximum (FWHM) corresponding to better imaging performance. These coated BTG spheres also have an advantage in trapping efficiency and imaging performance over conventional PS and MF spheres. The coated BTG microsphere is highly desirable for optically trapped microsphere super-resolution microscopy and potentially beneficial to other research areas, such as nanoparticle detection.

Coated High-Refractive-Index Barium Titanate Glass Microspheres for Optically Trapped Microsphere Super-Resolution Microscopy: A Simulation Study

A microsphere lens assembly (10) comprises a microsphere lens (1) and a base lens (3) connected together by a column of optically clear material (2) which holds the microsphere lens (1) in a fixed position relative to the base lens (3). If the microsphere lens (1) is fixed in the correct position relative to the base lens (3), the assembly (10) can be used, in combination with a suitable microscope, for carrying out super resolution microscopy and/or machining.

Microsphere lens assembly

The basic principle of the SMAL (Super-resolution Microsphere Amplifying Lens) imaging technology that we invented has its roots in the SMON (Submerged Microsphere Optical Nanoscope) pioneering work. SMAL is a novel objective lens whose front lens assembly contains a microsphere and is replaceable. We built a nanoscope prototype with nano XYZ scanning capability, which integrates a SMAL objective lens, allowing us to achieve super resolution imaging (70 nm – 90 nm). We have resolved large area scans (200 µm x 200 µm) by contactless sample scanning.

https://iopscience.iop.org/article/10.1088/1742-6596/902/1/012014/pdf

Imaging with the Super-resolution Microsphere Amplifying Lens (SMAL) Nanoscope

We have demonstrated a novel, non-contacting super resolution imaging setup using optical tweezers designed in-house. We can resolve large areas of samples using laser trapped polystyrene microsphere scanning with resolutions below the diffraction limit.

Optical Tweezers Microsphere-lens Super Resolution Imaging System

We have demonstrated an objective lens attachment comprising a single microsphere lens for achieving super resolution virtual imaging. We have resolved large areas with resolutions below the diffraction limit through contactless sample scanning.

Valerio Galieni

Papers

Imaging with the Super-resolution Microsphere Amplifying Lens (SMAL) Nanoscope

Optical Tweezers Microsphere-lens Super Resolution Imaging System

Optical Scanning Nanoscope with Microsphere Attached Objective Lens for Super Resolution 3D Virtual Imaging