Near-field properties of light emanated from a subwavelength double slit of finite length in a thin metal film, which is essential for understanding fundamental physical mechanisms for near-field optical beam manipulations and various potential nanophotonic device applications, is investigated by using a threedimensional finite-difference time-domain method. Near-field intensity distribution along the propagation direction of light after passing through the slit has been obtained from the phase relation of transverse electric and magnetic fields and the wave impedance. It is found that the near field of emerged light from the both slits is evanescent, that is consistent with conventional surface plasmon localization near the metal surface. Due to the finite of the slit, the amplitude of this evanescent field does not monotonically approach to than of the infinite slit as the slit length increases,
The famous experiment on the interference of light diffracted by a double slit was performed by Thomas Young, who found that unique interference patterns were related to light sources at a specific separation distance between two slits [1, 2]. This experiment also inspired the development of the particle-wave duality , which became again a trigger for building up the concepts in quantum mechanics.Although this experiment was initially performed more than two centuries ago, even nowadays, intensive research activity on double slit interference is still in progress to explore some novel physical phenomena, including for instance, the light interference characteristics due to a coherence degree of two light sources [2, 4-8], analyses of geometrical effects leading to enhanced light transmission and propagation[9-11]. Recent significant attention to the surface plasmons  has been also related to the double slit interference. The interference pattern produced by the double slit in a plasmonic metal film is quite different from the traditional ones due to the surface plasmon generation and localization near the metal surface at optical wavelengths[2, 4-7, 13-15]. While the plasmonic double slits have also been utilized for novel subwavelength optics applications such as unidirectional surface plasmon excitation [16, 17], optical beam manipulations [18, 19], and sub-diffraction limited optical spot generation in the intermediate field region [20, 21], the studies on the a subwavelength double slit are still inactive compared with those on the single slit structures. Furthermore, most work done on subwavelength slit structures has been limited to two-dimensional approximations or to very long slits compared with operating wavelengths, though finite slit effect is critical to the light interference in practical structures which is essential in the performance of the subwavelength slit based optical devices.
In this work, fundamental near-field behavior of light diffracted by a subwavelength double slit of finite length in a a thin metal film investigated using an intensive threedimensional finite-difference time-domain (3D FDTD) method; in particular, the near-field intensity distributions are obtained from phase relation and field peak ratio between the the electric and magnetic fields transverse to propagation direction. Analysis of the finite slit length effect on the near-field intensity distributions and the comparison of those with two-dimensional infinite slit length case allow us to gain further insights into the physics of the subwavelength plasmonic double slit, which can be potentially useful for various optical devices dealing with subwavelength optical beam manipulation or light focusing in the near-field region.
Figure 1 shows a schematic of the subwavelength double slit with finite slit length, denoted as
[FIG. 1.] Schematic of the subwavelength double slit of finite length in thin metal film. The observation plane (the yz-plane at x=0) in which electromagnetic field distributions are going to be obtained is shown below the metal film. The propagation distance r originates from the bottom surface of the metal film.
be supported at the interface between the bottom surface of the silver film and air, which is one of the reasons that the near-field characteristics are different from those of conventional double slit in a perfect electric conductor(PEC). Thickness of the metal film is 200nm, which is thick enough to block the light passing directly through the metal film and at the same time it is also less than half-the-wavelength to eliminate the Fabry-Perot resonance in the longitudinal
In order to investigate the near-field characteristics of interfered light after passing through the slits in the metal film, the 3D FDTD simulations were performed using a freely available software package , in which material dispersion for real metal has been implemented. Entire computation domain has 1400×1400×400 cells in
Figure 2 shows snapshots of the x-component of the electric field
[FIG. 2.] Snapshots of the electric and magnetic fields when the magnetic fields at the exit of the metal film are close to zero for different slit lengths of sl=600, 1200, and 1800 nm in the observation plane as shown in Fig. 1, i.e., horizontal and vertical axes are z- and y-positions, respectively. [(a)(b)(c): the z-component of the magnetic field (Hz), (d)(e)(f): the x-component of the electric field (Ex),and (a)(d): sl=600 nm, (b)(e): sl=1200 nm, (c)(f): sl=1800 nm].
However, the focused spot for the
Figure 3 shows the intensities of the
[FIG. 3.] The Ex, Hz, and near-field intensities along the propagation direction at the center position (z=0) in the observation plane (shown in Fig. 1) for (a) sl=600 nm, (b) sl=1200 nm, and (c) sl=1800 nm. The near-field intensity distributions along the propagation direction extracted from the subtraction between the Ex and ZHz fields.
number, at the center position (
For the two-dimensional case,
directions along the z-axis due to an oscillation of the fields along z-direction as shown in Fig. 2 (c) and Fig. 2 (f), resulting in even weaker focused fields near the center position than those in the previous two cases as mentioned.
Figure 4 shows the evanescent field intensities, though each of them is already shown separately in Fig. 3, for a clearer comparison with each case. The evanescent field intensities at the bottom surface of the silver film,
Such a behavior of the evanescent field intensity can be a result of the contribution of higher-order eigenmodes. In fact, as the slit length increases, more and more higher order modes can occur in the cavity. Obviously, the odd modes are favorable for increasing the evanescent field at the center, while the even modes are favorable to decrease it. As the slit length approaches infinity, the evanescent field intensity should naturally approach to that of two-dimensional case. However, this approaching is oscillating rather than monotonic because it involves more and more higher order eigenmodes.
In conclusion, the near-field characteristics of light from a subwavelength double slit of finite length in a thin metal film have been investigated by using 3D FDTD method.Near-field intensity along the symmetry axis at the center position of the slit direction has been obtained with phase relation of the transverse electric and magnetic fields and the ratio between them. It is found that the evanescent field distribution is strongly dependent on the slit length.As is shown, the evanescent field intensity distribution along the propagation direction may not necessarily increase toward that of the ideal two-dimensional infinite slit length case as the slit length increases. That might be significant in terms of designing practical subwavelength metal slit based plasmonic devices and the fundamental physics behind it as well. This effect may be attributed to the contribution of higher-order eigenmodes of the cavity, which are capable of critically changing the field distribution in the near zone.