Near-field plate overcomes diffraction limits

10 Aug 2007

Modulated plates that focus light beyond its diffraction limit could herald benefits for near-field applications.

Near-field diffraction

A near-field plate is helping researchers in the US to focus light well beyond its diffraction limit. With the ability to focus radiation ranging from microwaves to the UV, the device could have applications in near-field data storage, non-contact sensing, imaging and nanolithography. (Science Express July 12 1143884)

"This device opens up the possibility of using light to image features that are smaller than the wavelength of light," Roberto Merlin, a professor at the University of Michigan, told optics.org. "Current methods using visible light can image features around one micron in size. With our near-field plate, you can go well below that using visible light."

The cost of lithography could benefit from this technology. "Until now, in order to perform lithography on such small scales you had to use electron beams or ion beams which require an ultra-high vacuum," explained Merlin. "Using light instead means an enormous reduction in the cost in the equipment required for lithography."

The key to the achievement is diffracting the evanescent modes of light in the near-field. "The device is conceptionally similar to a Fresnel zone plate except that there are no propagating or radiative modes," explained Merlin. "This plate involves manipulating the evanescent or exponentially decaying modes instead where the mathematics of interference is very different."

Merlin begins the design process by solving the inverse scattering problem to calculate the beam that needs to exit the plate in order to give the desired focal spot. "If you suppose light is traveling from left to right and somewhere down this beam I want to have a spot. I can back-propagate from this spot to determine what the field should be at the exit phase of the plate," explained Merlin. "Then it is a question of designing the plate that will do this job."

The structure of the plate relies on introducing modulation to a standard periodic grating. The modulation depends upon the result required and the wavelength of the radiation being focused. "If you want to produce a line, you start with a non-standard periodic grating where the distance between the grating lines is much smaller than the wavelength," explained Merlin. "The modulation is introduced into the thickness of the grating lines and is varied from the central line to the edges in a certain fashion."

Alternatively, to produce a single point, Merlin starts with concentric rings positioned at particular places and again introduces a modulation to these rings. Particular modulations enable the team to produce a point.

The material used for the near-field plate depends on the frequency range of the radiation. "For microwave applications, the plate consists of capacitive and inductive elements made primarily with metallic wires," explained Merlin. "For the visible range, a combination of materials with negative and positive permittivity such as silver and silicon is used."

The team is now looking to commercialize the device and is in discussion with various companies interested in licensing this technology.