Optical angular momentum conversion in a nanoslit

Abstract

We demonstrate partial conversion of circularly polarized light into orbital angular momentum-carrying vortex light with opposite-handed circular polarization. This conversion is accomplished in a novel manner using the birefringent properties of a circular subwavelength slit in a thin metal film. Our technique can be applied over a very wide range of frequencies and even allows the creation of anisotropic vortices when using a slit without circular symmetry. © 2012 Optical Society of America OCIS codes: 310.6628, 230.7370, 260.1440, 050.1930 The curious phenomenon of optical vortices arising from axial symmetry in birefringent materials has been studied in uniaxial crystals of variable length [1,2] and birefringent plates with a spatially varying optical axis and half-wave retardation ("q-plates") [3][4][5]. This interaction between spin and orbital angular momentum of light by way of a Berry-Pancharatnam phase has also been studied in space-variant gratings [6] and plasmonic nanostructures in the context of selection rules [7], and also outside the domain of optics, in electron beams [8].We present here a novel method for this conversion using a subwavelength slit in a metal film that acts like a quarter-wave plate [9]. We show how this method relaxes the requirement of circular symmetry, allowing greater versatility in the form of the vortex created. In optics, a spin angular momentum of ℏ is associated with a circularly polarized photon. Orbital angular momentum is often associated with an optical vortex beam, where the phase increases azimuthally around the optical axis. These beams have a topological charge Q, equal to the number of full cycles the phase makes in one trip around the optical axis. The expectation value of the orbital angular momentum per photon is Qℏ [10]. The difference between the two forms of angular momentum is beautifully apparent in the interaction of a beam with small particles: interaction with the spin angular momentum in the absence of absorption requires particles that are birefringent; they will start to rotate about their own axis, whereas interaction with a beam carrying orbital angular momentum causes particles, whether birefringent or not, to rotate about the beam's optical axis [11].Recently, we reported on how a subwavelength slit in a metal film can act as an optical retarder [9]. A slit that is subwavelength in one direction and extended in the other has two eigenpolarizations: parallel and perpendicular to the slit. By careful design of the slit's width and depth, it is possible to construct a slit that behaves like a quarter-wave retarder for incident light of a certain wavelength, with its fast axis (i.e., with the lowest index of refraction) parallel to the orientation of the slit. One can achieve similar results using subwavelength structures with different resonances for orthogonal polarization components [12,13].Illuminating the straight slit with circular polarization results in linear polarization being transmitted. The associated change in angular momentum mea...

Experimental control of optical helicity in nanophotonics

“…If we decompose such beam with the collimated modes of Equations (2) and (3), the D mp z components will dominate because of the polarization selectivity of the linear polarizer followed by the quarter waveplate. The RC/LC polarization intensity ratio of a Gaussian beam with well defined L511 will be of the order (wk) 24 , 16 which in our case would be of the order of 10 219 . A comparison of this figure with the typical extinction ratios of commercial polarizers, 10 25 , indicates that the polarizer is the limiting factor when preparing a collimated beam with well defined helicity.…”

“…While similar experimental set-ups and measurements have been previously reported, [22][23][24] in all these cases, the results have been analyzed by means of the spin (S) to orbital (L) angular momentum conversion mechanism. Yet, considering helicity (L) and total angular momentum (J) instead, bears significant advantages.…”

Optical vortices from closed-loop subwavelength slits

“…In practice, this corresponds to the case of a circular subwavelength slit, which locally behaves as a birefringent retarder whose main axes are directed parallel and perpendicular to it [2], as recently reported experimentally [3]. In contrast, when the axial symmetry of the circuit C is broken, the generation of a singular optical field carrying a charge-two vortex with a broad orbital angular momentum spectrum has been predicted [3].…”

Optical angular momentum conversion in a nanoslit: reply

“…We acknowledge this and reiterate our original intention that as long as the dichroism is not too large, then it has little effect on the conversion efficiency in our system. We reply to Dr. Brasselet's Comment on our Letter, "Optical angular momentum conversion in a nanoslit" [1]. We regretfully acknowledge upon rereading the sentence in question on p. 3 of our manuscript that states "We find that the polarization conversion efficiency η is independent of the slit's dichroism," that the text gives the wrong impression.…”