insight review articles by the array, producing evanescent waves that tunnel through the holes, resulting in a small but finite amplitude on the far side of the array. Here the evanescent waves are again diffracted/scattered; the interference of the resulting waves produces the light that propagates away from the structure. Equation(4)acts as a starting point in ately determined by taking into account these diffraction/interfer- ence effects5-55 SPs act to enhance the fields associated with the evanescent waves, thus producing a way to increase the transmit tance. When the metal film is thin enough, this tunneling may become resonant because the sp modes on the two surfaces can over lap and interact via the holes. Interestingly, photon entang ment can be preserved or lost upon transmission through a hole, depending on the experimental conditions Individual subwavelength apertures can alsoexhibit enhanced trans- mission when surrounded by a periodic structure that harvests the incident light. If such nanostructure is added to the output side of the film then, remarkably, the emerging light can be beamed rather than diffracted. In addition, the light is mostly re-emitted from a very small area surrounding the aperture This suggests that a well directed source of light can be generated using a subwavelengthaper ure, an exciting development that is being pursued as a source for a variety of optical technologies, including high-density magneto- 770 optic datastorage. A theoretical treatment shows that just a few well Wavelength (um) designed features around the exit of a slit aperture can give rise to beaming, as confirmed recently in the microwave region"and shown vividly in the numerical simulation presented in Fig. 5 transmission images(top)and spectra(bottom) for three square arrays of subwavelength In the single hole and hole array structures described above, the holes. For the blue, green and red arrays, the periods were 300, 450 and 550 m oles donot supportany propagating modes, that is, the diameter of the respectlvely. the hole diameters were 155. 180 and 225 m and the peak transmission holeissmaller than N/2(halfthe wavelength of thelight thatistransmit wavelengths,538 and 627 nm. The arrays were made ina free standing 300 nm ted), and tunnelling is necessarily part of the transmissionmechanism. thick silver film(courtesy of A Degiron, Universite Louis Pasteur, France). Only the lowest However, when the aperture is larger than N/2inat least one dimension, order peak(ij=0. 1 in equation (4)of the spectrum of each array is shown as it suchas asit, then the aperture can support propagating modes the dominates the colour seen. The figure shows that nanostructures can control the resonant slit can even be wrapped up in the form of an annulus". For instance, wavelength of SP phenomena. investigations of the transmissionspectrum of periodicslit arrays reveal that both the slit modes and the modes caused by the periodic modula tion of the surface(standing wave surface modes) playarole In thecase metalsurface with protrusions, slits orholes, lenses and mirrors for SPs of asingle slit surrounded by linear grooves, the transmissionspectrum ay be integrated into circuits not only on an extended thin film"but is dominated by the modes caused by the periodic groove structure, also directly on a finite-width stripe. An example of such a structure is althoughother modes playan important role nownin Fig 3, where a Bragg reflector for SP modes is demonstrated. Althoughnotyet fully optimized, these early demonstrationsstrength- Future directions en the prospect for SP-based photonicelements In addition to their use in photonic circuits, SPs are being explored for other photonic technologies, notably the generation of light. A partic of subwavelength holes in optically thick metallic films has sparked layer that istypically only 100 nmthick. The close proximity ofthee. . a Hole arrays ular example is that of the organic light-emitting diode. Here excitor The demonstration ofenhanced transmission through periodicarrays are produced by injection of charge into a semi-conducting orga newinterestin SPs-3. Not only is the transmission much higher than tons to the metallic cathode used toinjectelectronsmeans thatmuch pected from classic diffraction theory, it can be greater than the per- the power that would otherwise have been radiated is in fact lost to SP centage area occupied by the holes, implying that even the light modesonthe cathodesurface-, thusdetracting from device efficien- pinging on the metal between the holes can be transmitted. In other cy Managing and, if necessary, recovering this power through the use words, the whole periodic structure acts like an antenna in the optical of a periodic nanostructure is likely to be important for many future regime, nicely demonstrating the benefits that SP modes can provide. device designs, especially for high-efficiency, long-life and full-colour The transmissionspectra of hole arrays display peaks that can be tuned devices. The ocess, that of using SPs to enhance the absorp- by adjusting the period and the symmetry as shown in Fig 4. For a tion of light, for example, in solar cells., is also of interest SP modes array of period ao. the peaks \ma in the normal incidence trans- have even been employed as the lasing mode in quantum cascade relation(equation(1)), and they are given by: To develop SP-based photonics, non-linear components such as switches will also be required. The unique properties ofSPs give them some advantages in thisrespect. We mentioned that they are character Ama vi+f (4) d by an evanescent, near field that is enhanced close to the surface. The use of SPs to enhance non-linear processes such as harmonic gen- eration is well known", but it is only recently that experiments have where indices iand jare the scattering orders from the array been designed tolookat how non-linear effects may be used to provide e2003NaturePublishingGroupNatUrevOl424114AuGusT2003www.nature.com/naturemetal surface with protrusions, slits or holes, lenses and mirrors for SPs may be integrated into circuits not only on an extended thin film18 but also directly on a finite-width stripe. An example of such a structure is shown in Fig. 3, where a Bragg reflector for SP modes is demonstrated. Although not yet fully optimized, these early demonstrations strength￾en the prospect for SP-based photonic elements. Hole arrays The demonstration of enhanced transmission through periodic arrays of subwavelength holes in optically thick metallic films has sparked new interest in SPs47–50. Not only is the transmission much higher than expected from classic diffraction theory, it can be greater than the per￾centage area occupied by the holes, implying that even the light impinging on the metal between the holes can be transmitted. In other words, the whole periodic structure acts like an antenna in the optical regime, nicely demonstrating the benefits that SP modes can provide. The transmission spectra of hole arrays display peaks that can be tuned by adjusting the period and the symmetry as shown in Fig. 4. For a square array of period a0, the peaks lmax in the normal incidence trans￾mittance spectra can be identified approximately from the dispersion relation (equation (1)), and they are given by: lmaxÏi 2 &j w2 .a0 !};m ; & m;d ;d §} (4) where indices i and j are the scattering orders from the array48. For a free-standing hole array, incident light is diffracted/scattered by the array, producing evanescent waves that tunnel through the holes, resulting in a small but finite amplitude on the far side of the array. Here the evanescent waves are again diffracted/scattered; the interference of the resulting waves produces the light that propagates away from the structure. Equation (4) acts as a starting point in analysing the transmission spectrum, a spectrum that is more accu￾rately determined by taking into account these diffraction/interfer￾ence effects51–55. SPs act to enhance the fields associated with the evanescent waves, thus producing a way to increase the transmit￾tance. When the metal film is thin enough, this tunneling may become resonant because the SP modes on the two surfaces can over￾lap and interact via the holes49–51. Interestingly, photon entangle￾ment can be preserved or lost upon transmission through a hole, depending on the experimental conditions56. Single apertures Individual subwavelength apertures can also exhibit enhanced trans￾mission when surrounded by a periodic structure that harvests the incident light. If such nanostructure is added to the output side of the film then, remarkably, the emerging light can be beamed rather than diffracted. In addition, the light is mostly re-emitted from a very small area surrounding the aperture57. This suggests that a well directed source of light can be generated using a subwavelength aper￾ture, an exciting development that is being pursued as a source for a variety of optical technologies, including high-density magneto￾optic data storage. A theoretical treatment58 shows that just a few well designed features around the exit of a slit aperture can give rise to beaming, as confirmed recently in the microwave region59 and shown vividly in the numerical simulation presented in Fig. 5. In the single hole and hole array structures described above, the holes do not support any propagating modes, that is, the diameter of the hole is smaller than l/2 (half the wavelength of the light that is transmit￾ted), and tunnelling is necessarily part of the transmission mechanism. However, when the aperture is larger than l/2 in at least one dimension, such as a slit, then the aperture can support propagating modes60–62; the slit can even be wrapped up in the form of an annulus63. For instance, investigations of the transmission spectrum of periodic slit arrays reveal that both the slit modes and the modes caused by the periodic modula￾tion of the surface (standing wave surface modes) play a role. In the case of a single slit surrounded by linear grooves, the transmission spectrum is dominated by the modes caused by the periodic groove structure, although other modes play an important role64. Future directions In addition to their use in photonic circuits, SPs are being explored for other photonic technologies, notably the generation of light. A partic￾ular example is that of the organic light-emitting diode. Here excitons are produced by injection of charge into a semi-conducting organic layer that is typically only 100 nm thick. The close proximity of the exci￾tons to the metallic cathode used to inject electrons means that much of the power that would otherwise have been radiated is in fact lost to SP modes on the cathode surface65–68, thus detracting from device efficien￾cy. Managing and, if necessary, recovering this power through the use of a periodic nanostructure is likely to be important for many future device designs, especially for high-efficiency, long-life and full-colour devices. The inverse process, that of using SPs to enhance the absorp￾tion of light, for example, in solar cells69,70, is also of interest. SP modes have even been employed as the lasing mode in quantum cascade lasers71. To develop SP-based photonics, non-linear components such as switches will also be required. The unique properties of SPs give them some advantages in this respect. We mentioned that they are character￾ized by an evanescent, near field that is enhanced close to the surface. The use of SPs to enhance non-linear processes such as harmonic gen￾eration is well known72–74, but it is only recently that experiments have been designed to look at how non-linear effects may be used to provide insight review articles 828 NATURE | VOL 424 | 14 AUGUST 2003 | www.nature.com/nature 1 0.5 0 370 470 570 670 770 Wavelength (µm) I/Io (Arbitrary units) Figure 4 Normal incidence transmission for subwavelength holes. Normal incidence transmission images (top) and spectra (bottom) for three square arrays of subwavelength holes. For the blue, green and red arrays, the periods were 300, 450 and 550 nm, respectively, the hole diameters were 155, 180 and 225 nm and the peak transmission wavelengths 436, 538 and 627 nm. The arrays were made in a free standing 300 nm thick silver film (courtesy of A. Degiron, Université Louis Pasteur, France). Only the lowest order peak (i,j40.1 in equation (4)) of the spectrum of each array is shown as it dominates the colour seen.The figure shows that nanostructures can control the resonant wavelength of SP phenomena. © 2003 Nature PublishingGroup