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Time-domain analysis of enhanced transmission through a single subwavelength aperture

Amit Agrawal, Hua Cao, and Ajay Nahata

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Abstract

We have measured the enhanced transmission properties of a single subwavelength aperture surrounded by periodically spaced annular grooves using time-domain techniques. While the present measurements utilize terahertz time-domain approaches, with appropriately scaled device parameters, the general observations should be applicable to other spectral ranges. In contrast to measurements that rely on continuous wave excitation and frequency domain measurements, we are able to determine the contribution of each individual groove to the transmitted terahertz waveform. Using structures containing only a single annular groove surrounding the aperture, we find that each groove can couple a large fraction of the incident terahertz bandwidth in the form of a surface wave pulse. When multiple annular grooves surround the aperture, we observe oscillations in the time-domain waveform that are temporally delayed from the initial bipolar waveform in direct relation to the distance of the groove from the aperture. This is further demonstrated by using structures containing defects (absence of annular grooves).

©2005 Optical Society of America

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Figures (4)
Fig. 1.
Fig. 1. (a) Photographs of typical bullseye structures consisting of only one annular groove (b) Cross-sectional line diagrams of the structures used. The radius of the center of the annular groove for the 4 upper structures shown in Fig. 1B vary at RK=KP, where K=3,4,5,6 and P=0.8 mm. (c) Five experimentally observed time-domain waveforms for the structures shown in part (b). The temporal waveforms have been offset from the origin for clarity. The arrows point to the oscillation that arises from coupling of the incident THz pulse to a surface wave pulse. The temporal change in the location of this oscillation corresponds directly to the change in distance of the annular groove from the aperture. The decrease in the magnitude of the oscillations with increasing K is related to the spatial profile of the incident THz beam.

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Fig. 2.
Fig. 2. (a) Schematic drawing showing the two contributions to the transmitted THz time-domain waveform. The component shown by the red arrow corresponds to the non-resonant transmission of the incident THz pulse through the subwavelength aperture. The component shown by the blue arrow corresponds to the contribution that arises from the interaction of the incident THz pulse with the structured surface. This latter component is smaller than the non-resonant component and temporally delayed. (b) Time-domain waveform for K=4 from Fig. 1A. The blue portion of the temporal waveform corresponds to the contribution that arises from coupling of the incident THz pulse by the annular groove (C) Amplitude spectra of the initial bipolar waveform (red) and the time-delayed oscillation (blue). The blue trace has been multiplied by a factor of 3.

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Fig. 3.
Fig. 3. (a) Photographs of typical bullseye structures consisting of periodic multiple annular grooves (b) Cross-sectional line diagrams of the structures used (c) Five experimentally observed time-domain waveforms for the structures shown in part (b). The temporal waveforms have been offset from the origin for clarity. The number of oscillations, after the initial bipolar waveform, matches the number of annular grooves.

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Fig. 4.
Fig. 4. (a) Photographs of typical bullseye structures containing defects (absence of annular grooves) (b) Cross-sectional line diagrams of the structures used (c) Five experimentally observed time-domain waveforms for the structures shown in part (b). The temporal waveforms have been offset from the origin for clarity. The arrows point to the temporal locations of the defects. The suppressed oscillation at the temporal location of the defect can be attributed primarily to the superposition of the trailing portion of the oscillation from the preceding groove and the leading portion of the oscillation from the following groove.

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