Changing the adjacent medium When Holes of the MHA Filled with UV-gel

The transmission spectra when we attach translucent Scotch^TM tapes on the incident side of 100µm-thick 2D-MHA filled with UV-gel is studied. Peaks shift to the left and decrease as the SPP resonances approach the cutoff frequencies, as shown in Fig. 4-13 (a). Refractive index of the tape is about 1.7 in THz region. These phenomena are analogous with previous reports elucidated in terms of the SPP. However, upon increasing the number of the tape, a side peak on the high frequency side grew gradually and red-shifted shown in Fig. 4-13 (b). For the number of tapes up to fifteen layers, the trend of shift still persists, as shown in Fig. 4-13 (c).

0.10 0.15 0.20 0.25 0.30 0.35 0.40 holes filled with UV-gel attach Scotch^TM tapes on the incident side

no tape

Fig. 4-13 (a) 100µm-thick MHA which holes filled with UV-gel attach different layers of Scotch^TM tapes on the incident side. The number of tapes is from zero to five.

0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.00 holes filled with UV-gel attach Scotch^TM tapes on the incident side

4 tapes

Fig. 4-13 (b) 100µm-thick MHA which holes filled with UV-gel attach different layers of Scotch^TM tapes on the incident side. The number of

0.15 0.20 0.25 0.30 0.35 0.00

0.05 0.10 0.15

0.20 100µm-thick and holes filled with UV-gel attach Scotch^TM tapes on the incident side

13 tapes 14 tape 15 tapes

frequency (THz)

power transmittance

Fig. 4-13 (b) 100µm-thick MHA which holes filled with UV-gel attach different layers of Scotch^TM tapes on the incident side. The number of tapes is from thirteen to fifteen.

The same trend is also found in the case of 400µm-thick 2D-MHA filled with UV-gel, as shown in Fig. 4-14. Owing to the original double peaks in 400µm-thick sample, the situation of attaching tapes exhibited more complication.

0.10 0.15 0.20 0.25 0.30 0.35 0.40 holes filled with UV-gel attach Scotch^TM tapes on the incident side

no tape

Fig. 4-14 (a) 400µm-thick MHA which holes filled with UV-gel attach different layers of Scotch^TM tapes on the incident side. The number of tapes is from zero to five.

0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.00 holes filled with UV-gel attach Scotch^TM tapes on the incident side

6 tapes

Fig. 4-14 (b) 400µm-thick MHA which holes filled with UV-gel attach different layers of Scotch^TM tapes on the incident side. The number of

4.2.4 Verify the Existence of Surface Plasmons Resonance by Observing the Electric Field in Near-Field Range

It is well-known that an in-plane momentum is needed to modify the incident wave to achieve the enhanced transmission. Either the SPP model, or the model of evanescent wave diffracted by the hole edges [27, 28] can provide such a mechanism. In order to identify these contentions, we have calculated distributions of Ez of the electric field amplitude for 400µm-thick 2D-MHA filled with UV-gel, as shown in Fig.

4-15. The frequencies of the incident THz wave are 0.191THz in Fig.

4-15 (a) and 0.276 THz in Fig. 4-15 (b), respectively. At 0.191 THz, the charge distribution on the side wall of each individual hole shows an anti-symmetric pattern between the top and bottom portion of the hole. At 0.276 THz, the charge distribution is symmetric, and surface waves on the top and bottom are coupled together. We also observe that the CW wave at 0.191THz impinges on the MHA, it will induce the maximum SPPs when a wave crest or trough passes through the medium of the MHA shown in Fig. 4-16 (a). Contrary to the first peak at 0.191THz, the CW wave at 0.276THz impinges on the MHA, it will induce the maximum SPPs when a wave crest arrives at the front surface of the MHA and a wave trough arrives at the back surface shown in Fig. 4-16 (b), and vice versa.

+

- + +

-- +

-Fig. 4-15 (a) Simulated z component Ez of the electric field amplitude for 100µm-thick MHA filling with UV-gel. The incident CW wave at 0.191THz.

- + ₊

-Fig. 4-15 (b) Simulated z component Ez of the electric field amplitude for 100µm-thick MHA filling with UV-gel. The incident CW wave at

(1)

(2)

Fig. 4-16 (a) Simulated Ex and Ez for the 400µm-thick 2D-MHA at 0.191THz. (1)-(6) shows a cycle of propagation.

(3)

(4)

Fig. 4-16 (a) Simulated Ex and Ez for the 400µm-thick 2D-MHA at

(5)

(6)

Fig. 4-16 (a) Simulated Ex and Ez for the 400µm-thick 2D-MHA at 0.191THz. (1)-(6) shows a cycle of propagation.

(1)

(2)

Fig. 4-16 (b) Simulated Ex and Ez for the 400µm-thick 2D-MHA at

(3)

(4)

Fig. 4-16 (b) Simulated Ex and Ez for the 400µm-thick 2D-MHA at 0.276THz. (1)-(6) shows a cycle of propagation.

(5)

(6)

Fig. 4-16 (b) Simulated Ex and Ez for the 400µm-thick 2D-MHA at

The intensity of electric field is normalized to the incident CW wave amplitude. The SPP-like surface wave showing in the simulations of the electric field amplitude agrees with the results studied by Miyamaru et al.

[29]. Different SPP modes were also reported by Ebbesen et al. in visible light [30]. However, in our samples, the effective hole depth become deeper and the band-pass frequency region become broader owing to filling UV-gel into the holes of MHAs. Therefore, we consider this is the reason why we can observe both modes at one sample. These might be the causes of multi-peak appearance.

Details of peak frequencies and reasons of the peak drift are still not clear, but it is clear that SPPs play an important role in enhanced transmission characteristics.

5. Conclusions and Future Works

We experimentally and numerically investigate the role of material in the holes on transmission characteristics of the 2D-MHA. New phenomena appeared when holes of the MHA are filled with dielectric material. The effect of filling dielectric material into the holes cannot simply be explained by increased effective hole diameter of the 2D-MHA and SPP model also cannot predict the peak frequencies. When the holes filled with UV-gel, the transmission peak broadened and multi-peak features are observed. The transmittance of the peaks are reduced, but they are still larger than those according to the porosity. Further, with the thickness becomes thinner, all peaks shift to high frequencies and disappear when they approach the diffraction limit. Finally, multi-peaks return to one when the thickness reaches around 100 mµ . Peaks shift to the left and decrease as the SPP resonances approach the cutoff frequencies when we attach translucent Scotch^TM tapes on the incident side of 100µm-thick 2D-MHA filled with UV-gel. Upon increasing the number of the tape, a side peak on the high frequency side grew gradually and red-shifted and for the number of tapes up to fifteen layers, the trend of shift still persists. For deeper holes, SPP can exhibit two distinct modes, coupled and uncoupled types as confirmed in the simulated results for the electric field.

In the future, we want to design a sensor using 2-D MHA for detecting the absorption line.

References

1. Mourou G, “Picosecond microwave pulse generation,” Appl. Phys.

Lett., vol. 38, pp. 470-472, 1981.

2. Auston D H, “Picosecond photoconducting Hertzian dipoles,” Appl.

Phys. Lett., vol.45, pp. 284-286, 1984.

3. Q. Wu, “Ultrafast electro-optic field sensors”, Appl. Phys. Lett., vol.

68, pp. 1604-1606, 1996.

4. G. Gruner, Millimeter and submillimeter wave spectroscopy of solids.

Berlin, Springer-Verlag, 1998.

5. Carsten Winnewisser, “Characterization and Application of Dichroic Filters in the 0.1–3-THz Region,” IEEE Trans. Microwave Theory Tech., vol. 48, pp. 744–749, 2000.

6. Dongmin Wu, “Terahertz plasmonic high pass filter,” Appl. Phys.

Lett., vol.83, pp. 201-203, 2003.

7. Hideaki Kitahara, “Terahertz wave dispersion in two-dimensional photonic crystals,” Phys. Rev. B, vol.64, pp. 042502, 2001

8. T.W. Ebbesen, “Extraordinary optical transmission through sub-wavelength hole arrays,” Nature., vol. 391, pp. 667–669, 1998.

9. M. M. Sigalas, ”Metallic photonic band-gap materials,” Phys. Rev. B, vol. 52, pp. 11744-11751, 1995.

10. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings. Berlin, Springer-Verlag, 1988.

11. Dongxia Qu, “Terahertz transmission properties of thin, subwavelength metallic hole arrays,” Opt. Lett., vol. 29, pp. 896-898, 2004.

12. Masaki Tanaka, “Effect of thin dielectric layer on terahertz transmission characteristics for metal hole arrays,” Opt Lett., to be published. 2005.

13. P. K. Benicewicz, ”Scaling of terahertz radiation from large-aperture biased photoconductors,” J. Opt. Soc. Am. B, vol. 11, pp. 2533–2546,

1994.

14. Zhisheng Piao, “Carrier Dynamics and Terahertz Radiation in Photoconductive Antennas,” Jpn. J. Appl. Phys., vol. 39, pp. 96–100, 2000.

15. P. Uhd Jepsen, “Generation and detection of terahertz pulses from biased semiconductor antennas,” J. Opt. Soc. Am. B, vol. 13, pp.

2424–2436, 1996.

16. S. Nashima, “Temperature dependence of optical and electronic properties of moderately doped silicon at terahertz frequencies,” J.

Appl. Phys., vol. 90, pp. 837–842, 2001.

17. David K. Cheng., Field and wave electromagnetics, 2nd. New York, Addison-Wesley, 1989.

18. William L. Barnes, “Surface plasmon subwavelength optics,” Nature., vol. 424, pp. 824–830, 2003.

19. Hua Cao, “Resonantly enhanced transmission of terahertz radiation through a periodic array of subwavelength apertures,” Opt. Express., vol. 12, pp. 1004–1010, 2004.

20. Allen Taflove, Computational Electrodynamics: The Finite-Difference Time-Domain Method, 2nd. London, Artech House, 2000.

21. K. S. Yee, “Numerical solution of initial boundary value problems involving Maxwell’s equations in isotropic media,” IEEE Trans.

Antennas Propagat., vol. AP-14, pp. 302, 1966.

22. Dennis M. Sullivan, Electromagnetic Simulation Using the FDTD Method. New York, IEEE Microwave Theory and Techniques Society, Sponsor, 2000.

23. M. A. Ordal, “Optical properties of the metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti, and W in the infrared and far infrared,” Appl. Opt., vol. 22, pp. 1099–1120, 1983.

24. Dongfeng Liu, “Carrier dynamics of terahertz emission from low-temperature-grown GaAs,” Appl. Opt., vol. 42, pp. 3678–3683,

25. Martin van Exter, “Terahertz time-domain spectroscopy of water vapor,” Opt. Lett., vol. 20, pp. 1128-1130, 1989.

26. Eugene Hecht, Optics 3rd. Addison-Wesley, 1998.

27. Marek W. Kowarz, “Homogeneous and evanescent contributions in scalar near-field diffraction,” Appl. Opt., vol. 17, pp. 3055–3063, 1995.

28. Henri J. Lezee, “Diffracted evanescent wave model for enhanced and suppressed optical transmission through subwavelength hole arrays,”

Opt. Express., vol. 12, pp. 3629–3651, 2004.

29. Fumiaki Miyamaru, “Anomalous terahertz transmission through double-layer metal hole arrays by coupling of surface plasmon polaritons,” Phys. Rev. B, vol.71, pp. 165408, 2005.

30. A. Degiron, “Effects of hole depth on enhanced light transmission through subwavelength hole arrays,” Appl. Phys. Lett., vol. 81, pp.

4327-4329, 2002.