AOS News - The Australian Optical Society - Macquarie University

AOS News Volume 25 Number 1 2011Chiral MetamaterialsUnlocking Nonlinear Optical ActivityWe demonstrate a chiral metamaterial exhibiting nonlinear optical activity –polarisation rotation which depends on the magnitude of the incidentfield. This effect is almost negligible in natural materials, but can bemade very strong using artificially structured metamaterials. We utilise this effectto create an optical diode for circularly polarised waves – a device which allowstransmission only in one direction.Chirality has important implications inmany areas of physics and chemistry,including optics. A chiral structure isone which has distinct left and righthandedforms which are the mirrorimages of each other. It is known thatthe interaction of circularly polarisedlight with chiral molecules depend onthe relative handedness between thetwo. This is the basis of the well-knownoptical activity in sugar solutions, wherebya linearly polarised input wave has itspolarisation rotated at the output. Themicroscopic origin of this effect is theexcitation of a magnetic response by theelectric component of the light and viceversa. Although the effect exists in naturalmedia, it can be orders of magnitudestronger in artificially structured media- metamaterials. In place of molecules,one can engineer tiny, sub-wavelengthelements such as resonant spirals thatact as magneto-electric dipoles. Figure 1shows an example of a chiral metamaterial,showing a linearly polarised incidentplane wave, which undergoes polarisationrotation by angle θ.The polarisation rotation in microwavechiral metamaterials can be nearly amillion times stronger than in naturalquartz for optical frequencies, oncethe sample thickness is normalised tothe wavelength of radiation [1]. Forsuch chiral metamaterials, the effect ofoptical activity can be so strong thatthe refractive index becomes negativefor one circular polarisation. There aremany other examples in the literatureof how metamaterials can be engineeredto achieve linear material parameters farbeyond those of natural media. An evenmore exciting feature of metamaterialsis that they can achieve exotic nonlinearparameters. Previous results have shownthat a nonlinear inclusion in an opticalor microwave metamaterial can resultin a nonlinear response much strongerthan that in the corresponding bulknonlinear media [2]. This occurs due tothe resonant enhancement of fields, andthe local hot spots which develop withinthe structure.by David A. Powell, Ilya V. Shadrivov, Vassili A.Fedotov, Nikolay I. Zheludev and Yuri S. KivsharNonlinear Chiral MetamaterialOur aim is to combine the strongchirality and nonlinearity of metamaterialsto develop a structure with nonlinearoptical activity - polarisation rotationwhich depends on the strength of theincident field. This effect has previouslybeen proposed [3], and subsequentlyobserved in LiIO3 crystals [4], however insuch materials the nonlinear optical activitywas smaller than its linear counterpart bya factor of 10-6. This required samplesseveral centimetres in length and lightintensities of 100 MW/cm2, which isclose to the optical breakdown of thecrystal. Such a small level of nonlinearityis not sufficient for demonstrating anypractically important functionality.Using metamaterials, we can overcomethese difficulties, by engineering thechiral response and carefully placing thenonlinear elements within the structure.The metamaterial is designed to operateat microwave frequencies, and consistsof a pair of metallic wires, twisted so thatthey are no longer parallel, as shown inFigure 2. We can see that the structure ischiral, because if we take its mirror image,we end up with a non-identical object.Nonlinearity is introduced by cuttingeach wire and inserting a varactor diode.The structure is placed inside a circularwaveguide, and excited with a high powermicrowave source in the 5-7GHz range.Figure 1. Example chiral metamaterial showing polarisationrotation.Figure 2. Schematic of the nonlinear chiral metamaterial.28