Squeezing Millimeter Waves into Microns

Microstructured metallic devices will play a vital role in the continuing search to manipulate the passage of electromagnetic radiation relevant to optical, microwave, and communication technologies. Here, we investigate the electromagnetic response of a completely novel and ultrathin ($\ll$ wavelength) structure within which is buried a metal-clad waveguiding layer (“core”) of subwavelength width. By removing metal from the core cladding to form a periodic array of slits, radiation is coupled into a standing wave within the layer and the structure resonantly absorbs or transmits radiation of wavelength more than 100 times its thickness. Additionally, such structures display the truly remarkable capability of compressing half of the standing-wave wavelength into a fraction of the expected distance.

Figure 1

Experimental reflectivity spectra and the numerically modeled results from the structure illustrated schematically in (a). (b) The $R_{ss}$ response of the structure when orientated with the slit direction parallel to the plane of incidence ($\phi =90°$). (c)–(e) The $R_{pp}$ response with $\phi =0°$ at three different angles of incidence. The change in noise at 10 GHz corresponds to a switch in the set of waveguides and horn antennas used.

Figure 2

(a)–(c) Electric field enhancement throughout the structure on resonance of the $N=1$ mode (7 GHz) visible in Fig. 1e. (a) The time-averaged field magnitude where white regions represent enhancements of at least 40 times. The region of the structure shown corresponds to a unit cell. (b),(c) The instantaneous fields at a phase corresponding to the strongest enhancement. (b) illustrates the $y$ and $z$ components along a line at the midheight of the core, where the grey vertical lines correspond to the edges of the slit. (c) A vector representation of the field distribution in a magnified region around the slit. (d) The time-averaged electric field magnitudes associated with the resonance of the $N=2$ mode at 14.05 GHz with $\theta =50°$ [Fig. 1e]. White regions represent field magnitude enhancements of at least 20 times.

Figure 3

Experimental frequency-dependent transmissivity data from (a) a structure whose slits on either side of the core are directly opposite each other ($d=0\mathrm{mm}$), and (b) a structure with $d={\lambda }_g/2=5\mathrm{mm}$. Also shown are the numerically modeled results. Radiation is normally incident ($\theta =0°$) and detected, and the sample is orientated so that the slit direction is perpendicular to the electric field vector.

Figure 4

Electric field enhancement throughout the two transmitting structures on resonance of each of their lowest frequency modes. (a),(b) $N=1$, $d=0\mathrm{mm}$ $f=7.2\mathrm{GHz}$; (c), (d) $N=2$ $d={\lambda }_g/2=5\mathrm{mm}$, $f=13.3\mathrm{GHz}$. The grey scale plots, (a) and (c), illustrate the time-averaged field magnitudes, plotted over an entire unit cell (pitch) of the sample, where white regions correspond to enhancements of 20 times or more. Parts (b) and (d) show the instantaneous vector field distribution in magnified regions around the slits at a phase corresponding to the strongest enhancement.