The objective of this research is to demonstrate a new coupling technique to excite propagating surface plasmons on an unstructured metallic surface by illuminating it with structured light beams. The experimental approach is to focus light from a Titanium Sapphire laser with a cylindrical lens into a narrow diffraction limited stripe on a gold film and to image the surface plasmons by advanced near field scanning optical microscopy techniques. The theoretical approach is to use techniques such as Finite Difference Time Domain simulations for visualizing the propagating surface plasmons.
The intellectual merit of this proposal is that if the proposed effect is demonstrated it will have a major impact on plasmonics. The imaging of two plane-like plasmon waves at an angle would confirm the striking resemblance between this phenomenon and Cherenkov radiation. It will lead to fundamental new understanding on the coupling of light to metallic surfaces and would introduce a completely new approach to excite them, without needing couplers such as gratings or prisms.
The broader impacts of this research stem from its technological, societal and educational importance. It is important because observation of this effect could lead to new optical interconnects on a chip by introducing a simpler and easier/cheaper method. This discovery will thus positively influence on-chip optical communications, leading to significant societal impacts. The findings coming out of this project, by contributing to new basic understanding of surface plasmons and their applications, will definitely also influence the teaching of nanophotonics, including textbooks.
As an accomplished sailor, Lord Kelvin, one of most famous and accomplished physicist of the 19th century, was fascinated by the elegant wave patterns generated behind his boat on the water’s surface. He realized that his boat was always ahead of the waves that it creates, generating a wave pattern of two lines that form the arms of a V, with the boat giving rise to the wake at the apex. In shallow water, the angle between the boat trajectory and the wake varies as the speed of the boat, following a simple mathematical relation: in fact wakes are formed as soon as the speed of the boat exceeds that of the waves. Similar patterns are widely observed in Nature. When a jet exceeds the speed of sound a sonic boom is created; this is the result of shockwaves created by the jet and their angle with respect to the jet’s path is directly related to the ratio of the speed of sound to that of the jet by exactly the same simple mathematical relation describing the wake of a boat or a duck!. Likewise when a charged particle e.g. an electron, traverses a medium at a speed exceeding that of light in that medium the particle emits a cone of light, known as Cherenkov radiation from the Nobel Prize winning physicist who discovered it, where the angle at the cone’s apex again follows the mathematical relation described above. Our project was motivated by the quest to create artificial wakes using nanotechnology. For that we focused on peculiar surface waves that can be created at the interface between a metal and a non-electrically conducting medium like air and known as surface plasmons; they propagate as light attached to the surface with an intensity rapidly dying out away from the surface. They can be created by opening up a long and narrow slit in the metal surface and by illuminating it with light oscillating perpendicularly to it length: the slit will then generate two surface plasmon waves propagating out at right angles to its length. We set out asking the question: Can we create the same type of wake pattern observed in some many different types of waves in the natural world (water, sound and light) but this time artificially, by designing and sculpting in the metal a pattern such that when illuminated by light it will launch surface plasmon wakes at an angle that can be controlled by design? The answer is positive. We found that a repeated linear pattern of nanoscale size rectangular apertures milled in the metal using a beam of ions can generate wakes of surface polaritons emanating at an angle from the row of slits. Remarkably the wakes can be launched in different directions by varying the angle at which the light hits the metal or by changing its polarization. Polarization is a property of light that describes the direction along which it vibrates or more generally its vibration pattern. Equally important is the fact that by changing the design of our nanostructure, for example by varying the repeat period or the shape of the apertures in the metal, the wake pattern can be changed. To image the wakes we used a particular type of optical microscope known as Near Field Scanning Optical Microscope, which is able to resolve features much smaller than the wavelength by scanning a tiny nanoscale size aperture in the immediate vicinity of the surface. The images confirmed very well theoretical predictions such as the wake angles and their dependence on the properties of the incident light. Intellectually the results are exciting because they beautifully illustrate the unity of physics, whereby phenomena occurring in very different systems or situations can be described by the same underlying physical picture and equation. They could also have a future practical impact. Surface plasmon polaritons have been proposed as a tool to fast transmit information on chips (optical interconnects); the ability to tailor and steer the propagation of the wakes could find future applications in photonics.
