Nanoscale optical imaging

Researchers used several sophisticated imaging tools, such as confocal microscopy, clinical photography, electron microscopy, thermography and geophysical imaging instruments for scientific investigations. However, for non-invasive & 3D-micro-size imaging, optical microscope is the most popular. This article aimed at the successive progress in optical microscope from micro observation to high-resolution nanoscale imaging. It begins with an overview of the present and past of the optical imaging approaches, followed by analysis & examples. In the next section, It will explore the ways in which the high-resolution optical imaging precisely shaped in the far-field region, subsequently, it elucidates the near-field imaging benefits over far-field for molecular study. Towards the end, it will be about the very recent progress and future in nanoimaging by plasmonic nanostructures and metamaterials.

Conventional Optical Microscope and optical resolution limit

By using optical microscope, researchers can enlarge the appearance of live cells, check the material morphology, and examine the chemical properties with great details. However spatial resolution of conventional microscope is limited by diffraction limit of light, which make it undesirable to resolve the specimens details smaller than about the half of light wavelength, also it can achieve maximum magnification typically ranges from 500x to 1500x, which is useful only for a few applications. Electron microscopy, on the other hand, can achieve better resolution and higher magnification (up to 160,000x), but still not a suitable replacement of optical microscope, especially in considering the live cell investigation, or light-matter interaction where effective, fast, safe, and high-quality imaging with nanoscale precision is necessary when tracking the progress of an ongoing scientific studies.

Progress in Far-field Imaging for super resolution

In the early history of high-resolution optical microscope, researchers used shorter wavelength (ultraviolet light) and high numerical aperture (NA) lenses to improve the resolution. However, with the development of confocal laser scanning fluorescence microscopy’ (CLSM) and reflected confocal microscopy, It was possible to create sharp & superior resolution (around 200 nm), still within the limits of diffraction barrier. In the recent years, the emergence of far-field optical techniques overcome the diffraction limit and vastly improved the resolution by making use of specific photophysical properties of fluorescence probes in conjunction with tailored way of illumination. For example, stimulated emission depletion based on reverse saturable transmission on a fluorescent dye can achieve resolution of 70-90 nm. 3D-imaging techniques accomplished by photoactivated localization microscopy & stochastic optical reconstruction microscope offer widefield & multicolor imaging with a resolution of 10 nm, under optimal conditions. Nevertheless, remarkable progress has been achieved in the recent years in nonlinear imaging method such as saturated excitation microscopy. Here, nonlinear fluorescence excitation offer saturation and signal originate from nonlinear region well below the diffraction limit with spatial resolution of 50 nm, smaller than the focal volume of the laser light.

In principle, single-molecular dynamic event can be studied easily using several optical imaging along with spectroscopic approach. However, far-field optics in combination with techniques such as FCS, unfortunately have larger focal illumination volume (1fL) because of diffraction limit of light, preventing the detection and interaction studies at individual molecular level. Also, photobleaching and photochemical damage are the two major issues in the stimulated emission depletion and other nonlinear techniques that is mainly dealt by reducing the input pump power or by using. Recently, strategies like reduction in the number of visible photochromic molecules by reversible photo-switching method or through molecular confinement were used for single-molecule detection.

 Near-field optical microscope for Nanoscale imaging and molecular studies

The most convincing approach to locate one molecule in the given volume to get wealth of spectroscopic information can be accomplished by reducing the size of illumination light. The idea of using a nanoaperture probe with effective illumination volumes a few tens of order smaller than a confocal light volume. Such nanoaperture are made of optical fiber that can couple near-field light formed at the surface when electromagnetic field interacts with the sample, also famously called near-field optical microscope (NSOM). This fiber nanoaperture are capable to funnel light of different wavelengths, allowing multicolor excitation and imaging in transmission and reflection mode. Sadly, the smaller probe size allows very limited light transmission, so an inevitable challenge remains that greater spatial resolution in nanoaperture is obtained at the expense of prolong acquisition time. Therefore, as a trade-off, nanoaperture of 50 nm or above size are mostly used for practical illumination. Recently, more creative optical nanoaperture probes were designed for nanoscale probing using sharp metallic coated tips, also famously called tip enhanced Raman spectroscopy (TERS). Here, complex nanostructures are designed on the TERS tip that can enhanced light field at the tip through plasmonic enhancement, and further boost spatial resolution up to 20-30 nm. Such microscope is used for biomaterial, self-assemble monolayer, and 2D material analysis. Recently TERS is used to study the local modification of chemical properties by introducing force on chemical by tip-pressurized force introduced in 0.1~1nN order. 

However, one challenge still remains that greater spatial resolution in metal coated nanoaperture is obtained at the expense of longer scanning time for a large area measurement. Therefore, it is important to find possible solution to to accelerate the scanning time.

Ultrafast Nanoscale imaging in the near-field region

Few possible strategies that could be directly utilized to accelerate super resolution imaging process without significantly degrading the spatial resolution: possible ideas for such optical imaging could be with the utilization of multiple plasmonic nanoantenna, where multiple metal probes can be utilized and significantly accelerate scanning time. Other possible method could be a new design of hyperlens that could beat the diffraction limit and capture the image of nanoscale size:- Recently proposed and experimentally demonstrated such hyperlens, consists of multiple layers of silver and aluminum oxide placed along the cavity of half a cylinder carved, where evanescent waves travel through the lens and magnified the image by the time it reaches the outer layers of the hyperlens. With a great promise as a new plasmonics-based nanoscale optical imaging technique, plasmonic lens with 3D-tapered arrangement of metallic nanorod chains can be utilized. Here, multiple nanorod chains can transfer the near-field signal originating from a sample to an image at a distance larger than a micro-meter, and nanorod chains with a certain taper angle allows image magnification.

Nanoscale imaging using neural network

With the recent advancement in computational field such as artificial intelligence, deep learning and neural network, super-resolution imaging has been used using a neural lens by tapping in to the power of light technologies and manipulate light at the nanoscale.

Supporting platform for optical imaging

After all, having the technological advancement in the nanoscale optical imaging, it is equally also important to create an imagining platform supported by modern cameras used specifically for optics; software development tools that have better interface with Windows, Linux, MATLAB and LabVIEW, and the rest is history.