User:Steve Quinn/Superlens work page

'''This Is A SUB PAGE. This is being used to DEVELOP AN ARTICLE. This article is not developed yet.'''

Optical subwavelength imaging
Optical subwavelength imaging, which occurs at nanoscales, means spatial resolution beyond the diffraction limit (about 500 nm). Subwavelength imaging is based on the excitation of surface plasmon polaritons, which are related to the previously discussed evanescent waves.

According to Abbe's diffraction theory the optical resolution of the microscope, d is defined as the minimum distance between two elements. For example, one element is the object is to be imaged, the other the physical surface of the lens of a microscope. Then view this computation as two objects instead of one. Abbe found that


 * $$d = \frac { \lambda } { NA }$$,

where $$\lambda$$ (lambda) is the wavelength of light and NA is the numerical aperture of the objective, defined as the sine of the half aperture angle multiplied by the refractive index of the medium filling the space between the cover glass and the front lens.

The only variable physical quantity is the wavelength of light because the lens's angular aperture and the refractive index have intrinsic limitations. It is the wavelength that must be controlled and altered to achieve subwavelength focusing. To achieve the desired spatial resolution, not constrained by the diffraction limit, the wavelength needs to be shortened. At the same time, it is necessary to stay in the visible or infrared region of the light spectrum in order to image the detailed features of the object. This means that the frequency or the energy of the light must not be changed.

Evanescent wave and imaging


The space "d" (Abbe's diffraction theory)  acts like a low pass filter for highly evanescent components. This means that evanescent waves very short-lived, because these waves usually decay exponentially with the distance from surface interface, or when considering the radiator - the light source. An evanescent wave is essentially a near field wave component which means it is not propagating away from the metal surface but remains standing in its local area. Evanescent fields are responsible for the capacitive or inductive energy at the surface interface, and generally decay quickly with distance. Just like radiating waves, evanescent waves are a general property of wave equations. As these form the near field of the suface interface, they are needed for the proper reconstruction of the image. As stated earlier, in conventional optical microscopy  the evanescent wave components are not utilized for image formation. If evanescent waves are not used in the reconstruction, such as done by a typical lens, which operate in the far field, then there is a diffraction limited image where the minimum resolution size is commensurate with the wavelength. Hence, the evanescent field spectrum forms the very near field. For any given nanometer sized point on an object that is reflecting light, this means substantially enahnced resolution, with a minimum required size that is, distinctly, a smaller fraction of the wavelength than the conventional 1/2. For a nano-sized reflection this means, in principle, highly enhanced resolution.

Metamaterial lens
A metamaterial lens uses metamaterials to go beyond the diffraction limit, which is inherent in conventional optical devices or lenses.

Early subwavelength imaging attempts
Metamaterial lenses are able to compensate for the exponential Evanescent wave decay via negative refractive index, and in essence reconstruct the image. Prior to metamaterials, proposals were advanced in the 1970s to avoid this evanescent decay. For example, in 1974 proposals for two-dimensional, fabrication techniques were presented. These proposals included contact imaging to create a pattern in relief, photolithography, electron lithography, X-ray lithography, or ion bombardment, on an appropriate planar substrate.

In 1981 two different techniques of contact imaging of planar (flat) submicroscopic metal patterns with visible blue light (400 nm) were demonstrated. One demonstration resulted in an image resolution of 100 nm and the other a resolution of 50 to 70 nm.

Contact images (contact lithography) can be corrupted if a speck of dust, dirt or other debris, gets mixed in during the process. The resultant resolution will be compromised to a greater or lesser degree.

After other interim steps John Pendry proposed using a metamaterial lens to, in essence, ameliorate wave decay.

Perfect lens
When the world is observed through conventional lenses, the sharpness of the image is determined by and limited to the wavelength of light. Around the year 2000, a slab of negative index metamterial is theorized to focus all Fourier components of 2D image to create a lens with capabilities beyond conventional (positive index) lenses. "The function of the lens is to apply a phase correction to each of the Fourier components so that at some distance beyond the lens the fields reassemble to a focus, and an image of the dipole source appears." A slab of silver is to be used. Theoretically, this is a breakthrough in that the optical version resolves objects as minuscule as nanometers across. Pendry predicted that NIMs with a refractive index of n < 0, can act, at least in principle, as a "perfect lens" allowing imaging resolution which is limited not by the wavelength but rather by material quality. This type of lens was proposed compensate wave decay and reconstruct images in the near-field.

Optical super-imaging with silver
In 2005 researchers discovered that a thinner slab of silver was best for sub–diffraction-limited imaging, which results in one-sixth of the illumination wavelength. This type of lens was to compensate for wave decay and reconstruct images in the near-field.

50-nm planar silver layer
Also in 2005 another group achieved super-resolution imaging, by refining techniques with a 50-nm thick planar silver layer as a near-field lens at wavelengths around 365 nanometers (nm).

Transmission properties of an optical far-field superlens
Also in 2005 a group proposed a theoretical way to overcome the near-field limitation using a new device termed a far-field superlens (FSL), which is a properly designed periodically corrugated corrugated metallic slab-based superlens.

Metamaterial crystal lens
Theoretically explore an idea for a far-field scanless optical microscopy with a subdiffraction resolution by exploiting the special dispersion characteristics of an anisotropic metamaterial crystal.

Metamaterial lens goes from near-field to far-field
Imaging is experimentally demonstrated in the far-field, taking the next step after near-field experiments. The key element is termed as a Far-field SuperLens (FSL) which consists of a conventional superlens and a nanoscale coupler.

Plasmon-assisted microscopy
Plasmon assisted microscopy.

Focusing beyond the diffraction limit with far-field time reversal
An approach is presented for subwavelength focusing of microwaves using both a time-reversal mirror placed in the far field and a random distribution of scatterers placed in the near field of the focusing point.

Sub-diffraction imaging in the far field
In 2007 researchers used an anisotropic medium experimentally demonstrate far field imaging. Magnification of the subwavelength object is achieved by transforming the scattered evanescent waves into waves which propagate through the medium and projects the image (high-resolution) at the far field. Possibilities in applications such as real-time biomolecular imaging and nanolithography.

Super-imaging in the Visible Frequency Range
Also in 2007 researchers demonstrated super imaging using materials, which create negative refractive index and lensing is achieved in the visible range.

Continual improvements in optical microscopy are needed to keep up with the progress in nanotechnology and microbiology. Advancement in spatial resolution is key. Conventional optical microscopy is limited by a diffraction barrier which is on the order of 200 nanometer (wavelength). This means that viruses, proteins, DNA molecules and many other samples are not visibly accessible., with a regular (optical) microscope. The lens previously demonstrated with negative refractive index material, a thin planar superlens, does not provide magnification beyond the diffraction limit of conventional microscopes. Therefore, images smaller than the conventional diffraction limit will still be unavailable.

However, a new lens capable of magnification beyond the diffraction limit of conventional (optical) microscopes is fabricated and its integration into a regular far-field optical microscope was demonstrated.

Super resolution far-field microscopy techniques
By 2008 the diffraction limit has been surpassed and lateral imaging resolutions of 20 to 50 nm have been achieved by several "super-resolution" far-field microscopy techniques, including stimulated emission depletion (STED) and its related RESOLFT (reversible saturable optically linear fluorescent transitions) microscopy; saturated structured illumination microscopy (SSIM) ; stochastic optical reconstruction microscopy (STORM); photoactivated localization microscopy (PALM); and other methods using similar principles.