Optics: Anomalous physics pulled apart
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NEWS & VIEWS nature physics | VOL 2 | APRIL 2006 | www.nature.com/naturephysics 225 OPTICS Anomalous physics pulled apart For more than a hundred years, optical physicists have been fascinated by the effects that can arise when light interacts anomalously with diffraction gratings. A new experimental study shows how nanofabrication and diagnostic techniques can pull apart the physics behind the so-called anomalies. ROSS C. McPHEDRAN is at the CUDOS ARC Centre of Excellence, School of Physics, University of Sydney, 2006, Australia. e-mail: [email protected] I n 1902 the renowned American optical physicist R. W. Wood discovered an extraordinary aspect of the physics of diffraction gratings 1 : he created a series of bright and dark bands in the spectrum of a white light source. Wood reported with amazement that one of his gratings would display one of the sodium D- lines and not the other — a change in intensity of a factor of 40 or more for a change in wavelength of only 1%. Wood dubbed these bands ‘anomalies’, and experimental and theoretical investigations of their properties for the next 70 years tended more to heighten than to remove their mystery. Two things quickly became clear: the anomalies were much more evident for light polarized with its magnetic vector along the grating grooves than for the orthogonal polarization, and they occurred just after the diffraction angle of one of the diffracted orders of the grating reached 90°. As reported on page 262 of this issue 2 , Gay and colleagues describe powerful fabrication and optical probing techniques that enable the physics behind Wood’s ‘anomalies’ to be investigated in depth, in a way that could have implications for the development of surface wave optics on nanostructured metals. The advent of powerful computers and computational techniques has made it possible to solve Maxwell’s equations to high precision and thereby reproduce these anomalies. But this did not remove the need for a deeper understanding of the underlying physical mechanisms responsible for Wood anomalies, or the need to construct simple models to describe them. Indeed, computational studies have made some anomalous behaviour still more inexplicable and striking. For example, it was shown 3 theoretically and verified experimentally that putting a grating with depth only 7% of its period on a gold surface could make the grating absorb 100% of the light falling on it. This strong absorption shows that the grating converts the incident light into surface plasmons — electromagnetic surface waves that run across the metallic surface of the grating and dissipate energy in the form of heat as they go. The effect of these plasmons can be seen with the naked eye: shine a laser on the surface of a grating placed at the centre of an opalescent sphere, and the scattered light shows organized arcs caused by surface plasmon scattering 4 (see Fig. 1). Plasmons on gratings are just one of the areas in which these electromagnetic surface waves are important. They also crop up in the phenomenon of enhanced transmission of light through subwavelength holes in thin films 5 and in the flat perfect lens 6 . They are being investigated as a means of guiding and manipulating light, and in this quest a simple physical model would be valuable. One contender for this is the coupled diffracted evanescent wave (CDEW) model investigated by Gay et al. 2 in this issue. The model views surface waves as being composed of wave packets, decaying normal to a metal surface, that have a well-defined wavelength along the surface and an amplitude decaying with propagation distance. The characteristic elements of the CDEW model are tested by careful experiments on a range of microstructured geometries, consisting of either a slit and a groove, or a hole and a groove. In either case the groove launches a surface wave on one side of a silver film when it is illuminated. The hole or the slit is on the m = –3(+) m = 3(–) m = –2(+) m = 2(–) m = 1(–) m = –1(+) Figure 1 Light scattered by a diffraction grating. The dark regions show the angles of strong surface plasmon scattering. The dots indicate the directions predicted by the dispersion equation for surface plasmons on a grating, with m numbering the grating orders. (Reproduced with permission from ref. 7.) Nature Publishing Group ©2006
Transcript of Optics: Anomalous physics pulled apart
NEWS & VIEWS
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OPTICS
Anomalous physics pulled apartFor more than a hundred years, optical physicists have been fascinated by the effects that can arise when light interacts anomalously with diffraction gratings. A new experimental study shows how nanofabrication and diagnostic techniques can pull apart the physics behind the so-called anomalies.
ROSS C. McPHEDRANis at the CUDOS ARC Centre of Excellence, School of Physics, University of Sydney, 2006, Australia.
e-mail: [email protected]
I n 1902 the renowned American optical physicist R. W. Wood discovered an extraordinary aspect of the physics of
diffraction gratings1: he created a series of bright and dark bands in the spectrum of a white light source. Wood reported with amazement that one of his gratings would display one of the sodium D-lines and not the other — a change in intensity of a factor of 40 or more for a change in wavelength of only 1%. Wood dubbed these bands ‘anomalies’, and experimental and theoretical investigations of their properties for the next 70 years tended more to heighten than to remove their mystery. Two things quickly became clear: the anomalies were much more evident for light polarized with its magnetic vector along the grating grooves than for the orthogonal polarization, and they occurred just after the diffraction angle of one of the diffracted orders of the grating reached 90°. As reported on page 262 of this issue2, Gay and colleagues describe powerful fabrication and optical probing techniques that enable the physics behind Wood’s ‘anomalies’ to be investigated in depth, in a way that could have implications for the development of surface wave optics on nanostructured metals.
The advent of powerful computers and computational techniques has made it possible to solve Maxwell’s equations to high precision and thereby reproduce these anomalies. But this did not remove the need for a deeper understanding of the underlying physical mechanisms responsible for Wood anomalies, or the need to construct simple models to describe them. Indeed, computational studies have made some anomalous behaviour still more inexplicable and striking. For example, it was shown3 theoretically and verified experimentally that putting a grating with depth only 7% of its period on a gold surface could make the grating absorb 100% of the light falling on it. This strong absorption shows that the grating converts the incident light into surface
plasmons — electromagnetic surface waves that run across the metallic surface of the grating and dissipate energy in the form of heat as they go. The effect of these plasmons can be seen with the naked eye: shine a laser on the surface of a grating placed at the centre of an opalescent sphere, and the scattered light shows organized arcs caused by surface plasmon scattering4 (see Fig. 1).
Plasmons on gratings are just one of the areas in which these electromagnetic surface waves are important. They also crop up in the phenomenon of enhanced transmission of light through subwavelength holes in thin films5 and in the flat perfect lens6. They are being investigated as a means of guiding and manipulating light, and in this quest a simple physical model would be valuable. One contender for this is the coupled diffracted evanescent wave (CDEW) model investigated by Gay et al.2 in this issue. The model views surface waves as being composed of wave packets, decaying normal to a metal surface, that have a well-defined wavelength along the surface and an amplitude decaying with propagation distance. The characteristic elements of the CDEW model are tested by careful experiments on a range of microstructured geometries, consisting of either a slit and a groove, or a hole and a groove. In either case the groove launches a surface wave on one side of a silver film when it is illuminated. The hole or the slit is on the
m = –3(+) m = 3(–)
m = –2(+) m = 2(–)m = 1(–) m = –1(+)
Figure 1 Light scattered by a diffraction grating. The dark regions show the angles of strong surface plasmon scattering. The dots indicate the directions predicted by the dispersion equation for surface plasmons on a grating, with m numbering the grating orders. (Reproduced with permission from ref. 7.)
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other side of the film, and scatters the surface wave, giving light that is detected in the far field. The measurements on groove–hole structures give results that support the CDEW model in quantitative detail, whereas the slit–groove structures are less easily interpreted in terms of it.
What is exciting about these results is that they show the potential of pulling apart the surface waves with careful measurements of purposely designed structures, and testing out ideas for modelling them. It seems that the CDEW model may have to be elaborated to cope with fi ne details of the geometry of the structure that launches these waves, and the
structure that detects them. But it is clear that we have reached the stage of rapid progress, where theory and experiment can be carefully compared. It won’t be long before what was anomalous will become well understood and useful in practical applications.
REFERENCES1. Wood, R. W. Proc. Phil. Mag. 4, 396–408 (1902).2. Gay, G. et al. Nature Phys. 2, 262–267 (2006).3. Maystre, D. & Neviere, M. J. Opt. (Paris) 8, 165–174 (1977).4. Hutley, M. C. Diff raction Gratings (Academic, New York, 1982).5. Ebbesen, T. W. et al. Nature 391, 667–669 (1998).6. Pendry, J. B. Phys. Rev. Lett. 85, 3966–3969 (2000). 7. Loewen, E. G., McKinney, W. R. & McPhedran, R. C. Proc. SPIE 503,
186–197 (1984).
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