Anomalous Light Absorption around Subwavelength Apertures in Metal Films

In this Letter, we study the heat dissipated at metal surfaces by the electromagnetic field scattered by isolated subwavelength apertures in metal screens. In contrast to the common belief that the intensity of waves created by local sources should decrease with the distance from the sources, we reveal that the dissipated heat at the surface remains constant over a broad spatial interval. This behavior that occurs for noble metals at near infrared wavelengths is observed with nonintrusive thermoreflectance measurements and is explained with an analytical model, which underlines the intricate role played by quasicylindrical waves in the phenomenon. Additionally, we show that, by monitoring the phase of the quasicylindrical waves, the total heat dissipated at the metal surface can be rendered substantially smaller than the heat dissipated by the launched plasmon. This interesting property offers an alternative to amplification for overcoming the loss issue in miniaturized plasmonic devices.

Figure 1

(a) Sketch of the subpicosecond pump-probe slit experiment. The inset shows the multilayer structure of the sample. (b) Scanning electron microscope image of the slit used in the experiment. (c) Time-resolved response of a 200-nm-thick gold layer as a function of the pump-probe delay.

Figure 2

Thermoreflectance data for a 300-nm-wide slit. (a) $\mathrm{\Delta }R$ map obtained for TM polarization (the inset is obtained for TE). (b) Plot of normalized reflectivity $\mathrm{\Delta }R/R$ (black dots) are obtained by averaging 40 line scans from (a). The solid red curve represents the calculated absorption profile $A(x)$ convoluted by a Gaussian with a FWHM equal to the waist ($2.8\mu \mathrm{m}$) of the probe beam.

Figure 3

Computed results obtained for the absorption at a gold-air interface illuminated by the fundamental ${\mathrm{TM}}_0$ mode of a slit. (a) Absorbed power density $A(x,z)$ in a linear scale at $\lambda =800\mathrm{nm}$ for $w=300\mathrm{nm}$. (b) The solid curves represent the absorption profile $A(x)=\int A(x,z)dz$. The dashed curves represent the absorption profile $A_{\mathrm{SPP}}(x)$, which would be achieved if the metal field was composed of only two SPPs launched in opposite directions. Results are presented for several wavelengths and for the same slit-width-to-wavelength ratios, $w/\lambda =3/8$. The dots are fitted data using Eq. (3)). Note that the plateau length varies from $20\mu \mathrm{m}$ for $\lambda =0.8\mu \mathrm{m}$ to $135\mu m$ for $\lambda =1.5\mu \mathrm{m}$ and that the SPP propagation length $L_{\mathrm{SPP}}$ rapidly increases with the wavelength ($L_{\mathrm{SPP}}=4.7$, 45, 97, and $192\mu \mathrm{m}$ for $\lambda =0.6$, 0.8, 1, and $1.5\mu \mathrm{m}$).

Figure 4

Absorbance at a gold-air interface illuminated by a point dipole source located at $x=y=0$, just above ($z=0^{+}$) the metal plane and emitting at $\lambda =800\mathrm{nm}$. (a) Absorbance $A(\rho ,\theta )$ for a dipole perpendicular (upper panel) or parallel (lower panel) to the surface and oriented along the $x$ axis. (b) Solid curves: $\rho A(\rho ,\theta )$ for several $\theta$. The dashed curves correspond to the associated SPP absorbance $\rho A_{\mathrm{SPP}}(\rho ,\theta )$. The dotted light blue curve represents the azimuthally averaged absorbance $\int \rho A(\rho ,\theta )d\theta /2\pi$. It is virtually superimposed with the circles that represent the absorption profile $A(x)$ of the slit [red curve in Fig. 3] after vertical rescaling.