Efiect on Transmissivity of Interaction between Subwavelength Slits

When light goes through a subwavelength multi-slit structure, there is inevitably interaction between slits that greatly in∞uence the transmissivity of the structure. We have studied the efiect of the interaction. Our model structure is, in xz plane, that an ideal metal fllm extends inflnitely along the x direction, with a thickness w in the z direction. There are N identical slits, each with width a, are arranged with equal spaced b along the x direction. The transmissivity with various structural parameters is calculated. When the light wavelength ‚ is flxed while the a varies, the transmissivity shows only one peak. Transmissivity oscillates with the spacing b, and the oscillation period is just ‚. The maximum values of the peaks and minimum values of the valleys of the curve are denoted by Tmax and Tmin, respectively, and they can be fltted by expressions Tmax = Tmax 0e ib=fi1 + c1 and Tmin = Tmin 0e ib=fi2 + c2. The parameters depend on structural parameters and ‚. Especially, Tmax 0 increase and Tmin 0 decreases with N. Tmin 0 can even touch zero. As the slit number N increases, transmissivity varies in three manners depending on b value: oscillatorily increase, attenuate or basically unchanged. The interaction between slits is re∞ected by the distribution of the magnetic flelds of the light, or electric current, on the metal surface between adjacent two slits. The waveform can be well expressed by the formula j = j10e i(x+x 0)=a1 cos(q1(x + x0) + '1) + j20e (x+x 0)=a2 cos(q2(x + x0) i '2); where x0 merely depends on the choice of the origin. The formula shows that the current is the superposition of two components, each starting at one slit corner and oscillatorily decaying toward the center between the slits. In the case that the two slits are arranged symmetrically, all the parameters in the two terms are the same. It is this cosine waveform of the current between slits that causes the transmissivity oscillation with the variation of spacing b. Of cause, when b becomes large, the transmissivity behavior of the multi-slit structure degrades to that of a single structure. It is well-known that a single subwavelength slit is of extraordinary optical transmission. The interaction in a multi-slit structure can cause constructive or destructive interference, which may be of potential application in nano optical devices. ACKNOWLEDGMENT