Dynamically reconfigurable terahertz metamaterial through photo ...

Dynamically reconfigurable terahertz metamaterial through photodoped semiconductor Dibakar Roy Chowdhury, Ranjan Singh, John F. O’Hara, Hou-Tong Chen, Antoinette J. Taylor et al. Citation: Appl. Phys. Lett. 99, 231101 (2011); doi: 10.1063/1.3667197 View online: http://dx.doi.org/10.1063/1.3667197 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v99/i23 Published by the American Institute of Physics.

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APPLIED PHYSICS LETTERS 99, 231101 (2011)

Dynamically reconfigurable terahertz metamaterial through photo-doped semiconductor Dibakar Roy Chowdhury,1,a) Ranjan Singh,1 John F. O’Hara,1,2 Hou-Tong Chen,1 Antoinette J. Taylor,1 and Abul K. Azad1

1 Center for Integrated Nanotechnologies, Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 2 School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA

(Received 26 October 2011; accepted 18 November 2011; published online 5 December 2011) We demonstrate reconfigurable terahertz metamaterial (MM) in which constituent resonators can be switched from split-ring resonators (SRRs) to closed-ring resonators via optical excitation of silicon islands strategically placed in the split gap. Both the fundamental and the third-order resonance modes experience monotonic damping due to increasing conductive losses in the photo-doped silicon region. More importantly, increasing the optical fluence (>200 lJ/cm2) results in the excitation of the second-order resonance mode, which is otherwise forbidden in a split-ring resonator for the incidence polarization in our experiments. Such dynamical control of metamaterial resonances could be implemented in active terahertz devices to achieve additional functionalities. C 2011 American Institute of Physics. [doi:10.1063/1.3667197] V During the last decade, metamaterials (MMs) have received enormous enthusiasm and growth because of many exotic properties not permissible by naturally occurring materials.1–4 The operation of most MM based devices depends on the designs to control the fundamental or higher order mode resonances of split-ring resonators (SRRs),5–8 either passively9,10 or actively.11–16 Although passive control of MM resonances has demonstrated many interesting effects,17,18 once the structures are fabricated the properties of the MM are fixed, whereas for the actively controlled MMs, responses can be tuned or controlled by application of an external stimulus even after fabrication. So far, researchers have demonstrated several ways to accomplish actively controllable MM resonances in terahertz (THz) domain by the application of light,11,12,14 bias voltage,13 temperature,15 and/or magnetic field.16 Most of these demonstrations are mainly designed to dynamically control either amplitude or frequency of the fundamental resonance of metamaterials. Dynamical tuning of metamaterial resonances provides an efficient way to manipulate electromagnetic waves. This is of particular interest in the THz regime which still suffers from the shortage of devices required to fully exploit its potential applications. In this letter, we show dynamical modulation of the fundamental inductive capacitive (LC) resonance as well as the higher order resonances simultaneously by employing nearinfrared photoexcitation. The MM studied in this work consists of single gap SRRs with a silicon island strategically inserted in the split gap. This allows us to selectively photodope and modifies the conductivity only in the gap region of the SRR, in contrast to previous schemes where the entire substrate conductivity was modified.11,13 In Ref. 12, photodoped silicon was employed in the SRR structure in order to tune the LC resonance frequency by modifying the gap capacitance whereas in our current design we modify the cona)

Electronic mail: [email protected].

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ductance of the gap. The selective photo-doping inside the split gap has several advantages. First, the external photoexcitation does not change the conductivity of the entire substrate; therefore, it does not attenuate the THz transmission through the entire metamaterial. Second, even under intense photo excitation of the silicon island metamaterial, the higher order resonance does not switch off completely. Therefore, in the current work, the selective excitations of the silicon micro islands in the split gap enables us to simultaneously switch off certain MM resonances and excite additional resonance mode, rather than just simply switching them off. More precisely, we dynamically switch a split-ring resonator to a closed ring resonator (CRR), as we will show below. The active photodoped MM samples studied in this work were fabricated on a silicon-on-sapphire (SOS) wafer, which consists of a 600-nm-thick epilayer of h100i oriented silicon on a 530 -lm-thick sapphire substrate. Followed by photolithography, a 10-nm-thick titanium adhesion layer and a 200-nm-thick gold layer were deposited by electron beam evaporation, after which a lift-off process enables the formation of the single gap SRR array. In the next step, the silicon layer was removed from most areas except in the gap region of the SRR by another round of photolithography followed by plasma etching, as shown in Fig. 1(a). An optical microscope image of the fabricated samples is shown in Fig. 1 with the detailed geometric dimensions. The fabricated samples were characterized using optical-pump THz-probe (OPTP) technique,19 where the polarization of the incident THz electric field is parallel to the gap bearing SRR arm in order to excite the odd mode resonances in the form of fundamental LC and higher order dipolar resonances. A coherent near-infrared (800 nm) laser beam with 50 fs pulse duration was employed to pump the MM sample just a few picosecond prior to the arrival of the THz pulses. Under various photoexcitation fluences, the THz signal was measured in the time domain after transmitting through the

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metamaterial samples, which were transformed to frequency domain and normalized by using a bare sapphire substrate as the reference. All the measurements were done at room temperature and in a dry atmosphere to mitigate water vapour absorption. The measured transmission spectra of the MM samples are shown in Fig. 2(a), with the pump laser power varying from 0 to 1200 mW corresponding to a fluence on the sample between 0 and 1200 lJ/cm2. Without photo excitation, the transmission spectrum of the metamaterial sample shows strong resonances at 0.6 THz and 1.76 THz due to the excitation of the fundamental LC resonance and the third order dipolar resonance, respectively. When increasing optical pump fluences, we observe a gradual diminishing of the LC resonance, along with the frequency tuning of the third order dipolar resonance from 1.76 THz to 1.65 THz with reduced resonance strength. Both resonances disappear and there is a

formation of a new resonance feature at 1.28 THz after further increasing the optical fluence higher than 200 lJ/cm2. The reduction and then disappearance of the LC resonance is due to the increasing photo-induced conductive losses in the silicon micro islands. When the optical fluence is higher than 200 lJ/cm2, the silicon islands become highly metallic and hence transform the SRR to a CRR, in which the THz transmission spectra reveal a strong resonance with significant line reshaping at 1.28 THz, similar to the fundamental resonance of a CRR. The pump fluence dependent transmission amplitude modulation at 0.6 THz and 1.28 THz are shown in Fig. 3. The amplitude modulation of the LC resonance saturates at high pump fluences as the resonance has been completely switched off. On the other hand, the amplitude modulation at 1.28 THz still shows significant improvement at high fluences and at pump power of 1200 lJ/cm2, it reaches 80% which corresponds to a power modulation of 95%. The bandwidth is significantly broader than in purely switching the resonances of the fundamental and third order modes.11 At this fluence level, the SRR is transformed completely into a CRR via the photoconductive silicon islands. One must note that, the phase and amplitude of THz transmission are not independent.13 The pump induced phase change of THz transmission at 0.7 THz and 1.7 THz is shown in Fig. 3(b). At these frequencies, the pump induced amplitude changes remain minimal but the corresponding phase undergoes tremendous shifting, which can be useful in order to design a THz phase shifter. Due to the increase in optical excitation power from 0 mW to 1200 mW, we have observed almost p/2 phase change at 1.7 THz. Conversely, at 1.28 THz, the

FIG. 2. (Color online) Measurements of the transmitted E-field amplitude (a) and the phase (b) for the metamaterial sample using variable optical pump powers with the E-field of the THz radiation aligned parallel to the gap bearing side.

FIG. 3. (Color online) Percentage modulation of the transmitted E-field amplitudes at 0.6 THz and 1.28 THz (a) and the phase changes at 0.7 THz and 1.7 THz (b).

FIG. 1. (Color online) (a) Optical microscope images of single SRR from the fabricated MM samples. The dimensions of the SRR and their features are mentioned in the figure. The active silicon region inside the split gap is indicated in the figure. The array of the fabricated SRRs along with the unit cell periodicity are shown in (b).


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dynamically which was otherwise forbidden due to the asymmetry involved in a single gap SRR. In summary, we have demonstrated a dynamically reconfigurable planar THz metamaterial where the odd resonance modes can be switched off and the otherwise forbidden even resonance mode can be switched on simultaneously. This is accomplished though near-infrared photo excitation of the silicon island region integrated only inside of the split gap of a single gap SRR array. We have also found that the bandwidth is much broader in the even resonance mode than in the odd resonance modes. Using the scheme depicted in this work to modulate the optical properties of MMs could serve as a platform for the design of active micro-scale photonic devices such as THz switches, filters, modulators, and phase shifters with enhanced efficiency.

FIG. 4. (Color online) Simulated surface current distributions at 0 optical pump power for LC resonance (a) and for third order dipolar resonance (b). Similarly, simulated surface current distributions under the maximum optical pump for the second order resonance (c). The black dashed lines are inserted in the figures to indicate the treand of the surface currents. Numerically, simulated transmissions are shown in (d) for two extreme cases of photo induced conductivity.

phase remains constant, although the amplitude shows 80% modulation. The above modes attributions have been further validated through numerical simulations using commercially available software CST MICROWAVE STUDIO.20 The effect of photo excitation is accounted for by introducing conductivity in the silicon island. We obtain a good agreement in transmission spectra between numerical simulations and experimental measurements by increasing the conductivity of silicon from r ¼ 0 to r ¼ 50 000 S/m which represents to the optical pump power of 0 mW and 1200 mW, respectively.21 The simulated surface current distributions are shown in Fig. 4. In the absence of optical pump, the surface current distribution excited by the incident THz radiation, shown in Fig. 4(a), clearly reveals a circular nature for the low frequency LC resonance near 0.6 THz. Similarly, the surface current distribution near 1.7 THz demonstrates a third-order resonance with three identical currents in the SRR arms, shown in Fig. 4(b). At maximum optical pump power, the currents in Figs. 4(a) and 4(b) vanish completely; instead the new resonance mode near 1.28 THz exhibits a strong even current distribution with two current segments of equal length oscillating in-phase. Note that this mode can only be excited in a CRR rather than in an SRR with the configuration of THz polarization in our experiments and simulations, where the transformation is due to photo excited metallic state of silicon islands at the SRR gap. Therefore, this particular MM based on silicon islands shows a very important dynamic effect which was not demonstrated earlier. With increasing pump power, the LC resonance as well as the third order resonance disappears but the second order resonance sets in

We acknowledge support from the Los Alamos National Laboratory LDRD Program. This work was performed, in part, at the Center for Integrated Nanotechnologies, a US Department of Energy, Office of Basic Energy Sciences Nanoscale Science Research Centre operated jointly by Los Alamos and Sandia National Laboratories. Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the US Department of Energy under Contract No. DE-AC5206NA25396. 1

J. B. Pendry, A. Holden, D. Robbins, and W. Stewart, IEEE Trans. Microwave Theory Tech. 7, 2075 (1999). R. A. Shelby, D. R. Smith, and S. Schultz, Science 292, 5514 (2001). 3 T. J. Yen, W. J. Padilla, N. Fang, D. C. Vier, D. R. Smith, J. B. Pendry, D. N. Basov, and X. Zhang, Science 303, 1494 (2004). 4 C. M. Soukoulis, S. Linden, and M. Wegener, Science 315, 47 (2007). 5 R. Singh, E. Smirnova, A. J. Taylor, J. F. O’Hara, and W. Zhang, Opt. Express 16, 6537 (2008). 6 T. Driscoll, G. O. Andreev, D. N. Basov, S. Palit, S. Y. Cho, N. M. Jokerst, and D. R. Smith, Appl. Phys. Lett. 91, 062511 (2007). 7 A. K. Azad, A. J. Taylor, E. Smirnova, and J. F. O’Hara, Appl. Phys. Lett. 92, 011119 (2008). 8 H.-T. Chen, J. F. O’Hara, A. K. Azad, and A. J. Taylor, Laser Photonics Rev. 5, 513 (2011). 9 D. R. Chowdhury, R. Singh, M. T. Reiten, J. Zhou, A. J. Taylor, and J. F. O’Hara, Opt. Express 19, 10679 (2011). 10 O. Sydoruk, E. Tarartschuk, E. Shamonina, and L. Solymer, J. Appl. Phys. 105, 014903 (2009). 11 W. Padilla, A. J. Taylor, C. Highstrete, M. Lee, and R. D. Averitt, Phys. Rev. Lett. 96, 107401 (2006). 12 H.-T. Chen, J. F. O’Hara, A. K. Azad, A. J. Taylor, R. D. Averitt, D. B. Shrekenhamer, and W. J. Padilla, Nature Photon. 2, 295 (2008). 13 H.-T. Chen, W. J. Padilla, M. J. Clich, A. K. Azad, R. D. Averitt, and A. J. Taylor, Nature Photon. 3, 148 (2009). 14 N. Shen, M. Massaouti, M. Gokkavas, J. Manceau, E. Ozbay, M. Kafesaki, T. Koschny, S. Tzortzakis, and C. M. Soukoulis, Phys. Rev. Lett. 106, 037403 (2011). 15 J. Gu, R. Singh, Z. Tian, W. Cao, Q. Xing, M. He, J. W. Zhang, J. Han, H.-T. Chen, and W. Zhang, Appl. Phys. Lett. 97, 071102 (2010). 16 B. Jin, C. Zhang, S. Engelbrecht, A. Pimenov, J. Wu, Q. Xu, C. Cao, J. Chen, W. Xu, L. Kang et al., Opt. Express 18, 17504 (2010). 17 R. Singh, C. Rockstuhl, F. Lederer, and W. Zhang, Phys. Rev. B 79, 085111 (2009). 18 N. Liu, S. Kaiser, and H. Giessen, Adv. Mater. 20, 4521 (2008). 19 R. D. Averitt and A. J. Taylor, J. Phys.: Condensed Matter. 14, R1357 (2002). 20 Computer Simulation Technology (CST), Darmstadt, Germany. 21 Z. Tian, A. K. Azad, X. Lu, J. Gu, J. Han, Q. Xing, A. J. Taylor, J. F. O’Hara, and W. Zhang, Opt. Express 18, 12482 (2010). 2
