Left-handed Metamaterials

Imaging by a Flat Lens


Patanjali Parimi, Wen-tao Lu, Plarenta Vodo and Srinivas Sridhar (Northeastern University)
John Derov and Beverly Turchinetz (AFRL Hanscom)
Supported by the Air Force Research Labs and the National Science Foundation

Experimental observation of focussing by a Flat Lens made of Photonic Crystal

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            Our lab is dedicated to fabricating and elucidating the fundamental characteristics of Left-handed Metamaterials (LHM) that possess very interesting electromagnetic properties. Exciting results inlcuding negative refraction by a photonic crystal and imaging by a flat lens have been observed in "our three dimensional approach": experiment, theory and simulations.
Backward wave propagation in a lefthanded material
        Artificial materials with negative refractive index are called left handed materials becaue of the unusual electromagnetic wave propagation in them. In these material the wave travel backwards while the energy propagates along the incident direction, contrary to the naturally available materials.  Photonic crystals and metamaterials are currently being investigated for such left handedness of EM wave, while a few other artificial materials are feasible. Click here for more.



 Experimental observation of Negative Refraction by Metallic Photonic Crystal         An intriguing property of the left-handed material is negative refraction.   The optical properties of materials that are transparent to electromagnetic (EM) waves can be characterized by an index of refraction. Given the direction of the incident beam  at the interface of vacuum and the material, the direction   of the outgoing beam can be determined using Snell’s formula. All naturally available materials possess a positive refractive index. In lefthanded materials EM waves bend negatively  and consequently a negative index of refractive index can be assigned to such materials without violating Maxwell’s equations. This negative bending allows considerable control over light propagation and opens the door for new approaches to a variety of applications from microwave to optical frequencies. We have demonstrated neagative refraction by a metallic photonic crystal. Click here to see some movies of negtive refraction.

        Negative refarcation can be exploted to make novel lenses having  flat surfaces. Smulation: Image formation by a falt lens of a point source All conventional lenses have curved surfaces due to  positive index of refraction.  However,  negative index of refraction allows a flat slab of a material to behave as a lens and focus electromagnetic waves as well as produce a real 3-D image. We have demonstrated this unique feature of  imaging by a flat lens, using the phenomenon of all-angle negative refraction in a photonic crystalline material.
 
Experiment: Imging by a Flat Lens made of Photonic Crystal Conventional optical systems have a single optical axis, limited aperture and cannot focus light onto an area smaller than a square wavelength.  In contrast the present flat lens does not have a unique optical axis and is not restricted by the aperture size. We have demonstrated both these features by moving the object by 4 cm: the image moves by a corresponding amount in the same direction (see Figure on right)  Note that for the sub-wavelength source we have observed an image of similar size. The unique properties of the flat lens  lead to entirely new perspectives on imaging and potentially new applications. A particular advantage of the photonic crystalline material is its scalability to sub-micron dimensions ensuring several possible applications from microwave to optical frequencies.  Click here to see some simulations of Imaging by Flat Lens.
a) Photonic Crystal
Photonic crystal made of a periodic array of Alumina rods
Photonic crystal is an array of either dielectric or metallic rods  or holes in a dielectric medium or any material that has a periodic dielectric or magnetic contrast. The PC can be designed to possess left handed behavior in a chosen  frequency of intrerest, for example optical to microwave frequencies.     



b) Metamaterial

NIM used for measurements The metamerial was fabricated by interleaving  split ring  resonators and metallic wire strips, following the design of David Smith and collaborators at UCSD. Printed circuit board was etched using lithography techniques to produce arrays of split ring resonators (SRR) on  one side and wire strips (WS) on the other side of the board. An array of SRR and WS should result in an effective negative permeability and negative permittivity  material. Therefore a lattice of material was assembled with unit cell consisting of two sets paired at right angles of three SRR and one WS each  to produce a material that notionally has  negaive index.  The thickness of the WS is 30 microns and width is 1mm. Each SRR has two rings of thickness 30 microns with the larger ring having each side 2.6 mm.