Engineered plasmon focusing onfunctional gratings.

We report ontheengineering ofplasmon propagation andfocusing bydedicated curved gratings andnoncollinear phasematching. Gratings werecreated ongoldbyfocused ion beammilling andplasmons weremeasured using phase sensitive PSTM. C2005 Optical Society ofAmerica OCIScodes: (240.6680) Surface plasmons; (070.2580)Fourier optics; (180.5810)Scanning microscopy Surface plasmons canbeexited resonantly whentheplasmon k-vector matches theprojected light k-vector (1). A grating vector shifts thelatter inanengineered continuous way.Anappropriately shaped grating canensure the creation ofplasmons inapredefned direction, e.g. towards acommoncenter (2). Forperpendicular illumination the required grating profile isabulls-eye pattern, where thegrating periodicity matches theplasmon resonance atthe illumination wavelength. Thisgrating ismatched forconverging plasmons aswellasdiverging plasmons. The degeneracy canbelifted byintroduction ofasasymmetry, suchasanangled illumination. Toachieve focusing of plasmons forangled incident illumination thegrating hastoaddless k-vector before thefocal point andmorebehind thefocal point sothatthegrating pattern resembles cutsthrough angled conesorshifted circles (fig1); corresponding totheFourier transform ofacircle ink-space that hasradius corresponding totheplasmon resonant k-vector andisshifted fromthecenter byanamountcorresponding totheprojected light vector. ~~~~~~~~~~~~P - Figure IGrating profiles forfocusing plasmons under increasingly angled illumination andPSTMsetup. Figure 2Phase sensitive Photon Scanning Tunneling Microcoscope (PSTM) setup. Thegrating profile isonthetopside ofthegold layer. Wepresent local probing oftheplasmon propagation byphase sensitive PSTM(fig 2)ofthefield distribution on circles andstretched circles (fig 3,4), showing thefocusing anddirecting ofbeamsofplasmons. Thegratings are created inathin (SOnm) gold layer byFocused IonBeam(FIB) milling.