Large dynamic resonance transition between surface plasmon and localized surface plasmon modes

Large dynamic resonance transition between surface plasmon and localized surface plasmon

modes

Zhen Tian,1,2

Abul K. Azad,3 Xinchao Lu,

1 Jianqiang Gu,

1,2 Jiaguang Han,

2,4

Qirong Xing,2 Antoinette J. Taylor,

3 John F. O’Hara,

3 and Weili Zhang

1,*

1School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA 2Center for Terahertz waves and College of Precision Instrument and Optoelectronics Engineering, Tianjin

University, and the Key Laboratory of Optoelectronics Information and Technical Science (Ministry of Education), Tianjin 300072, P. R. China

3MPA-CINT, Los Alamos National Laboratory, Los Alamos, NM-87545, USA 4Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore

*[email protected]

Abstract: We present resonant terahertz transmission in a composite plasmonic film comprised of an array of subwavelength metallic patches and semiconductor holes. A large dynamic transition between a dipolar localized surface plasmon mode and a surface plasmon resonance near 0.8 THz is observed under near infrared optical excitation. The reversal in

transmission amplitude from a stop-band to a pass-band and up to 2π

phase shift achieved in the composite plasmonic film make it promising in large dynamic phase modulation, optical changeover switching, and active terahertz plasmonics.

©2010 Optical Society of America

OCIS codes: (240.6680) Surface plasmons; (260.5740) Resonance

References and links

1. T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arrays,” Nature 391(6668), 667–669 (1998).

2. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003).

3. R. Gordon, A. G. Brolo, A. McKinnon, A. Rajora, B. Leathem, and K. L. Kavanagh, “Strong polarization in the optical transmission through elliptical nanohole arrays,” Phys. Rev. Lett. 92(3), 037401 (2004).

4. K. F. MacDonald, Z. L. Samson, M. I. Stockman, and N. I. Zheludev, “Ultrafast active plasmonics,” Nat. Photonics 3(1), 55–58 (2009).

5. C. Rockstuhl, F. Lederer, T. Zentgraf, and H. Giessen, “Enhanced transmission of periodic, quasiperiodic, and random nanoaperture arrays,” Appl. Phys. Lett. 91(15), 151109 (2007).

6. C. Janke, J. G. Rivas, P. H. Bolivar, and H. Kurz, “All-optical switching of the transmission of electromagnetic radiation through subwavelength apertures,” Opt. Lett. 30(18), 2357–2359 (2005).

7. W. Zhang, A. K. Azad, J. Han, J. Xu, J. Chen, and X.-C. Zhang, “Direct observation of a transition of a surface plasmon resonance from a photonic crystal effect,” Phys. Rev. Lett. 98(18), 183901 (2007).

8. E. Hendry, F. J. Garcia-Vidal, L. Martin-Moreno, J. G. Rivas, M. Bonn, A. P. Hibbins, and M. J. Lockyear, “Optical control over surface-plasmon-polariton-assisted THz transmission through a slit aperture,” Phys. Rev. Lett. 100(12), 123901 (2008).

9. E. J. Smythe, E. Cubukcu, and F. Capasso, “Optical properties of surface plasmon resonances of coupled metallic nanorods,” Opt. Express 15(12), 7439–7447 (2007).

10. C. L. Haynes, A. D. Mcfarland, L. Zhao, R. P. Van Duyne, G. C. Schatz, L. Gunnarsson, J. Prikulis, B. Kasemo, and M. Käll, “Nanoparticle optics: the importance of radiative dipole coupling in two-dimensional nanoparticle arrays,” J. Phys. Chem. B 107(30), 7337–7342 (2003).

11. X. Lu, J. Han, and W. Zhang, “Resonant terahertz reflection of periodic arrays of subwavelength metallic rectangles,” Appl. Phys. Lett. 92(12), 121103 (2008).

12. X. Lu, and W. Zhang, “Terahertz localized plasmonic properties of subwavelength ring and coaxial geometries,” Appl. Phys. Lett. 94(18), 181106 (2009).

13. J. Cesario, R. Quidant, G. Badenes, and S. Enoch, “Electromagnetic coupling between a metal nanoparticle grating and a metallic surface,” Opt. Lett. 30(24), 3404–3406 (2005).

14. X. Lu, J. Han, and W. Zhang, “Transmission field enhancement of terahertz pulses in plasmonic, rectangular coaxial geometries,” Opt. Lett. 35(7), 904–906 (2010).

#127749 - $15.00 USD Received 28 Apr 2010; revised 18 May 2010; accepted 19 May 2010; published 26 May 2010(C) 2010 OSA 7 June 2010 / Vol. 18, No. 12 / OPTICS EXPRESS 12482

15. W. Fan, S. Zhang, B. Minhas, K. J. Malloy, and S. R. J. Brueck, “Enhanced infrared transmission through subwavelength coaxial metallic array,” Phys. Rev. Lett. 94(3), 033902 (2005).

16. A. K. Azad, Y. Zhao, and W. Zhang, “Transmission properties of terahertz pulses through an ultrathin subwavelength silicon hole array,” Appl. Phys. Lett. 86(14), 141102 (2005).

17. B. Ferguson, and X.-C. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1(1), 26–33 (2002). 18. D. Qu, D. Grischkowsky, and W. Zhang, “Terahertz transmission properties of thin, subwavelength metallic hole

arrays,” Opt. Lett. 29(8), 896–898 (2004). 19. X. Shou, A. Agrawal, and A. Nahata, “Role of metal film thickness on the enhanced transmission properties of a

periodic array of subwavelength apertures,” Opt. Express 13(24), 9834–9840 (2005). 20. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer-Verlag, Berlin, 1988). 21. D. Grischkowsky, S. Keiding, M. van Exter, and Ch. Fattinger, “Far-infrared time-domain spectroscopy with

terahertz beams of dielectrics and semiconductors,” J. Opt. Soc. Am. B 7(10), 2006 (1990). 22. H.-T. Chen, J. F. O'Hara, A. K. Azad, A. J. Taylor, R. D. Averitt, D. B. Shrekenhamer, and W. J. Padilla,

“Experimental demonstration of frequency-agile terahertz metamaterials,” Nat. Photonics 2(5), 295–298 (2008). 23. F. Gao, L. Carr, C. D. Porter, D. B. Tanner, G. P. Williams, C. J. Hirschmugl, B. Dutta, X. D. Wu, and S.

Etemad, “Quasiparticle damping and the coherence peak in Yba2Cu3O7-δ,” Phys. Rev. B 54(1), 700–710 (1996). 24. K. P. H. Lui, and F. A. Hegmann, “Ultrafast carrier relaxation in radiation-damaged silicon-on-sapphire studied

by optical-pump-terahertz-probe experiments,” Appl. Phys. Lett. 78(22), 3478 (2001). 25. K. Das, “Comment on published carrier lifetime data on silicon-on-insulator (SOI) materials,” Electron. Lett.

23(11), 579 (1987). 26. A. K. Azad, and W. Zhang, “Resonant terahertz transmission in subwavelength metallic hole arrays of sub-skin-

depth thickness,” Opt. Lett. 30(21), 2945–2947 (2005). 27. A. K. Azad, Y. Zhao, W. Zhang, and M. He, “Effect of dielectric properties of metals on terahertz transmission

subwavelength hole arrays,” Opt. Lett. 31(17), 2637–2639 (2006). 28. H.-T. Chen, W. J. Padilla, M. J. Cich, A. K. Azad, R. D. Averitt, and A. J. Taylor, “A metamaterial solid-state

terahertz phase modulator,” Nat. Photonics 3(3), 148–151 (2009). 29. M. C. Beard, G. M. Turner, and C. A. Schmuttenmaer, “Sub-picosecond carrier dynamics in low-temperature

grown GaAs as measured by time-resolved THz spectroscopy,” J. Appl. Phys. 90(12), 5915 (2001). 30. A. K. Azad, H.-T. Chen, S. R. Kasarla, A. J. Taylor, Z. Tian, X. Lu, W. Zhang, H. Lu, A. C. Gossard, and J. F.

O’Hara, “Ultrafast optical control of terahertz surface plasmons in subwavelength hole arrays at room temperature,” Appl. Phys. Lett. 95(1), 011105 (2009).

1. Introduction

The recent progress in terahertz generation, spectroscopy, and imaging promises potential applications in a wide range of disciplines. It thereby challenges the development of next-generation optical components necessary for the effective manipulation of terahertz radiation. Fortunately, the recent advance in subwavelength plasmonics [1–5] has demonstrated great potential in delivering integrated terahertz components, such as modulators, filters, and switches. In particular, active plasmonic devices, being mostly made from semiconductors, have attracted much attention. Through carrier injection, temperature control, or optical excitation, the density of free carriers in semiconductors can be actively modified. High carrier densities enable semiconductors to exhibit metallic properties and thus facilitate resonant excitation of surface plasmon polaritons (SPPs), leading to anomalous transmission in plasmonic arrays of subwavelength holes [1–3]. Optical switching of terahertz SPPs has been extensively demonstrated in recent years, allowing active control of the SPP resonance by intense light illumination [6–8].

Meanwhile, field enhancement due to resonant excitation of dipolar localized surface plasmons (LSPs) also occurs when electromagnetic waves impinge upon subwavelength metallic patches [9–12]. In contrast to SPPs, the LSP resonance in the subwavelength geometries appears as a well-defined minimum in transmission with a central frequency primarily determined by the dimension of the particle and the dielectric function of boarding media, but not sensitive to periodicity [11,12]. The interesting coupling effect between SPPs and the localized modes have also been well explored recently [13–15]. In this paper, we present an active plasmonic film that allows instantaneous, large, and dynamic transitioning of the transmitted terahertz amplitude and phase by switching between the SPP and LSP resonances on a single integrated chip. Such plasmonic film was lithographically fabricated from silicon-on-sapphire (SOS) and is comprised of periodic arrays of metal patches and silicon holes. An evolution from a LSP to a SPP resonance reveals a large dynamic

changeover effect and a giant phase shift (up to 2π ). The optical excitation-dependent

transition characteristics are well reproduced by numerical simulations.


2. Experimental

The composite plasmonic film was patterned on a 350 µm thick undoped SOS substrate having a 500 nm thick epitaxial silicon layer. The sample is schematically illustrated in Figs. 1(a) and 1(b), where the Al patches and silicon holes are periodically fabricated using two-layer photolithography on a single chip. The 200 nm thick Al patches were first metallized on SOS and then the silicon holes adjacent to the patches were formed by reactive ion etching [16]. The resulting 78 × 25 µm

2 rectangular Al patches and 65 × 50 µm

2 rectangular silicon

holes have the same periodicity of 100 µm. The dimensions of the patches and holes were designed to optimize overlapping between the LSP transmission dip and the SPP band-pass peak.

Fig. 1. Schematic of (a) a single unit cell, and (b) the periodic array comprising the composite

plasmonic crystal. (c) Measured fundamental resonances at . πω 2LSP. (dotted curve) and

πω 2SP (solid curve).

Optical excitation-dependent terahertz transmission and phase change are characterized using electro-optic, time-resolved optical pump-terahertz probe measurements [7,17]. A regenerative Ti:Sapphire amplifier that delivers 3.2 mJ, 50 fs pulses at 800 nm and 1 kHz repetition rate is split into two beams for terahertz wave generation-detection and optical excitation of the sample, respectively. The terahertz beam is collimated onto the array sample with a diameter of 3.5 mm, which covers approximately 1,000 unit cells. The p-polarized terahertz electric field, as shown in Fig. 1(a), is parallel to the long axis of the Al patches. The optical beam, with spot size larger than the terahertz spot, provides uniform excitation over the terahertz measurement area. A non-photoexcited, bare SOS wafer serves as a reference by which terahertz transmission measurements are normalized.


Fig. 2. Frequency-dependent amplitude transmission at various optical excitations: (a) experimental results, and (b) simulation results. The measurements and simulation results for different optical excitations are vertically shifted by 0.5 each for clarity. (c) Measured corresponding phase change. (d) Optical excitation dependent phase change at 0.76 THz. Electric field distribution at resonance frequencies under various optical excitations: (e) 0, (f) 400, and (g) 1000 µJ/cm2.

3. Results and discussions

Figure 1(c) shows the resonant terahertz transmission through the composite structure with and without optical excitation. Without optical excitation, a well-defined transmission minimum occurs at 0.78 THz due to dipolar LSPs of the Al patches. The LSP resonance is

described through the relation ( ) 1 22 2

LSP dc Lω π ε −= , where c is the speed of light in

vacuum, L is the length of the island axis parallel to the terahertz electric field, dε is the

dielectric function of the substrate [11]. However, with an optical excitation of fluence 1.2 mJ/cm

2, the transmission is switched from band-stop to a band-pass with a characteristic

transmission peak at 0.86 THz. Due to photo-doping, the silicon array becomes metallic and favors the resonant excitation of SPPs [7]. Such a SPP resonance is ascribed to the metal-

sapphire [ 1,0]± SPP mode with the relation ( ) ( )1 2

2SP m sphr m sphr

c pω π ε ε ε ε−

= + , where p

denotes the lattice constant of the array, mε and

sphrε and are the dielectric function of the

photoexcited silicon and sapphire [18–20]. The measured characteristic transition between the LSP and SPP modes under various

optical excitations is illustrated in Figs. 2(a) and 2(c), respectively, for the frequency-dependent amplitude transmission and the corresponding phase change. In order to tailor the


offset between the LSP and SPP resonance frequencies, we intentionally rotated the chip about the normal of the array plane by 20° to modify the effective length of the patches and the geometrical parameters of the holes. Since the undoped silicon hole array does not support SPPs without optical excitation, only a dipolar LSP mode at 0.81 THz is observed [11], showing a band-stop resonance. With optical excitation, the dielectric function of silicon is modified due to photo-generated free carriers, and the LSP resonance gradually diminishes with a broader linewidth and a less pronounced transmission minimum. The further increase in optical excitation alters the dielectric function of silicon such that it begins to present metallic properties. Thus, the Al patches can be electrically connected by the metallic silicon film and the charges confined to the Al patches will be redistributed over the whole film surface. As a result, the LSP minimum gradually disappears and the SPP resonance peak appears almost at the same frequency. With strong optical excitation, the silicon film along with the Al patches form a metallic hole array, exhibiting an overall band-pass type SPP response. We also observe that the resonance frequency of the SPP mode slightly red-shifts with a narrower linewidth due to reduced damping rate of SPPs. At 1.0 mJ/cm

2 optical

excitation, the resonance is located around 0.83 THz, originating from the metal-sapphire

[ 1,0]± SPP mode [18].

To gain further insight, two-dimensional finite-element simulations using CST Microwave Studio were carried out to model the transition characteristics between the LSP and SPP resonances. In the simulation, the sapphire substrate is treated as a lossless dielectric with

sphrε = 9.61 [21] and the Al film is modeled as a Drude metal. The silicon layer is modeled

with an optical excitation-dependent dc conductivity retrieved from the measured time-domain transmission signals through a blank SOS [22–25]. The variations in transmission and dc conductivity as functions of optical excitation are shown in Fig. 3(a).

Fig. 3. (a) Terahertz transmission and dc conductivity of the bare SOS as functions of optical excitation. (b) Skin depth of Si at 0.81THz under different optical excitations.

The simulation results shown in Fig. 2(b) reproduce very well the characteristic amplitude transition between the two observed resonance modes. The transmission response of the composite structure undergoes three characteristic states, namely, the LSP, SPP, and intermediate states. The LSP state occurs when optical excitation is not yet applied and the chip behaves as an array of the 78 × 25 µm

2 Al patches. The electric field distribution of the

composite structure is calculated at the resonance frequency and illustrated in Fig. 2(e). It indicates that the electric field is mainly confined around the Al patches due to dipolar LSPs and their coupling between adjacent patches.

The intermediate response state takes place at 0.4 mJ/cm2 optical excitation where a nearly

flat response is observed in the transmission, i.e. the signature of the LSP mode disappears while the SPP resonance peak has not yet been established. As optical excitation approaches

0.4 mJ/cm2, the real part of dielectric function

rmε of silicon is gradually altered from positive

to negative. Thus, the LSP minimum gradually broadens and disappears due to damping, a


result of ohmic losses in the silicon layer. Meanwhile, the resonance shifts to lower frequencies due to modification in the dielectric function of the silicon layer [11]. It is interesting to note, although the silicon layer has acquired metallic properties by having a

negative real part of dielectric function rmε at 0.4 mJ/cm

2 optical excitation, the SPP

resonance is not yet established. This is because the thickness of the silicon layer also needs to be larger than one third of the skin depth [26] to support the SPP resonance. At 0.4 mJ/cm

2

optical excitation, the skin depth of silicon is 3.96 µm at the resonance 0.81 THz, as shown in Fig. 3(b), nearly eight times of the layer thickness. It thus does not support the SPP resonance. Nevertheless, as illustrated in Fig. 2(f), a weak SPP electric field appears confined in the silicon holes, while the LSP signature disappears.

When optical excitation is increased above 0.4 mJ/cm2, the silicon layer exhibits improved

metallic properties that lead to reduced skin depth. The SPP state begins to emerge with a resonance peak, which red-shifts and narrows in linewidth with increasing optical excitation. Although the thinner skin depth in the photo-excited semiconductors was found to reduce the effective size of the holes, the effect on the transmission amplitude is negligible when the silicon layer is sufficiently thin [6,7], as in this case. The high density of photo-generated free carriers in the silicon layer enables a relatively high ratio of the real to imaginary parts of

dielectric function rm imε ε− , which plays a crucial role in the improved resonance

enhancement at the maximum optical excitation 1.0 mJ/cm2 [27]. A strong electric field due to

SPPs is now revealed in the holes, as shown in Fig. 2(g). The observed resonance shift and linewidth narrowing effects mainly originate from the improved metallic properties; this leads to higher magnitudes of the silicon dielectric function and less damping.

Fig. 4. (a) Measured terahertz transmissions at two frequencies: 0.25 and 0.81 THz. (b) Simulated SPP resonance of LT-GaAs and SOS with same carrier density.

Along with the amplitude transition between the dipolar LSP and SPP modes, the transmission phase undergoes a strong modulation. Figure 2(c) depicts the measured characteristic evolution of phase change under various optical excitations at 0.76 THz. Due to the Kramers-Kronig relation, the transmission amplitude and phase change are dependent on each other. At 0.76 THz, the amplitude shows minimum modification but the phase undergoes a maximum change, as shown in Fig. 2(d). At 0.81 THz, however, the phase intersects for various optical excitations, but is accompanied with a maximum amplitude variation. Under

1.0 mJ/cm2 optical excitation, a maximum phase shift of 2π is achieved, thus showing great

potential for large dynamic phase modulation at terahertz frequencies [28]. It is worth noting that the composite plasmonic film also exhibits a changeover type

switching effect. Figure 4(a) shows the measured excitation-dependent transmissions at two frequencies, 0.81 and 0.25 THz, respectively. When optical excitation is switched from 0 to 1.0 mJ/cm

2, the transmission at 0.81 THz is switched on (from 0.10 to 0.53) while it is

switched off (from 0.90 to 0.28) at 0.25 THz. The changeover switching frequencies can be engineered by altering the geometrical dimensions of the patches and holes. For example, the


LSP resonance can be widely tuned by varying the length of the Al patches along the incident electric field.

To further improve switching contrast, sharper SPP resonances can be achieved by use of semiconductors of higher carrier mobility. Figure 4(b) illustrates a simulated comparison of SPP resonances in hole arrays made from low-temperature grown GaAs (LT-GaAs) and SOS thin films. The LT-GaAs array not only enables an enhanced contrast and an enhanced phase

change (from 2π to 1.91) at the SPP resonance, but also extensively reduces the excitation

fluence required to obtain same carrier density of SOS [29]. In addition, although the carrier recombination time of SOS is at nanosecond level [25], ultrafast switching between the LSP and SPP modes is feasible in constituent epitaxial films of ultrashort carrier recombination time [30].

4. Conclusion

We demonstrate a composite terahertz plasmonic crystal that exhibits large a dynamic transition between the dipolar LSP and SPP resonances under optical excitation. The reversal in transmission amplitude and up to 2π phase shift observed in the composite plasmonic

crystal make it promising for optical changeover switching and large dynamic phase modulation, as well as active plasmonic device applications. The normal terahertz incidence configuration can also be expanded to in-plane geometry for a plasmon traveling at the sample surface while the optical excitation remains at normal incidence.

Acknowledgements

This work was partly supported by the U.S. National Science Foundation, the National Key Basic Research Foundation of China (Grant Nos. 2007CB310403 and 2007CB310408), the National Natural Science Foundation of China (Grant No. 60578037), and the Tianjin Sci-Tech Programs (Grant Nos. 08ZCKFZC28000 and 07ZCGHHZ01100). The Los Alamos team gratefully acknowledges the support of the U.S. DOE through the LANL/LDRD Program for this work.


Large dynamic resonance transition between surface plasmon and localized surface plasmon modes

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