Optically tunable terahertz metamaterials on highly ...

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Optically tunable terahertz metamaterials on highly flexible substrates Article in IEEE Transactions on Terahertz Science and Technology · November 2013 DOI: 10.1109/TTHZ.2013.2285619

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IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 3, NO. 6, NOVEMBER 2013

Optically Tunable Terahertz Metamaterials on Highly Flexible Substrates Kebin Fan, Xiaoguang Zhao, Jingdi Zhang, Kun Geng, George R. Keiser, Huseyin R. Seren, Student Member, IEEE, Grace D. Metcalfe, Michael Wraback, Xin Zhang, and Richard D. Averitt (Invited Paper)

Abstract—We present optically tunable metamaterials (MMs) on flexible polymer sheets operating at terahertz (THz) frequencies. The flexible MMs, consisting of electric split-ring resonators (eSRRs) on patterned GaAs patches, were fabricated on a thin polyimide layer using a transfer technique. Optical excitation of the GaAs patches modifies the metamaterial response. Our experimental results revealed that, with increasing fluence, a transmission modulation depth of was achieved at the LC resonant frequency of 0.98 THz. In addition, a similar modulation depth was obtained over a broad range from 1.1 to 1.8 THz. Numerical simulations agree with experiment and indicate efficient tuning of the effective permittivity of the MMs. Our flexible tunable device paves the way to create multilayer nonplanar tunable electromagnetic composites for nonlinear and multifunctional applications, including sensing, modulation, and energy harvesting. Index Terms—Metamaterial, optical tuning, spectroscopy.

I. INTRODUCTION

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URING the past several years, terahertz metamaterials (MMs) operating at far-infrared frequencies from 0.1 to 10 THz, have garnered considerable interest, not only due to their exotic engineerable electromagnetic response, but also because of their promise as terahertz devices. Recent progress has led to tremendous results, including invisibility cloaks [1], negative refractive index [2], [3], terahertz detectors [4], imaging devices [5], [6], perfect absorbers [7]–[10], waveplates [11]–[15], and switches and filters [16]–[28]. Quite generally,

Manuscript received July 04, 2013; revised September 28, 2013; accepted October 01, 2013. Date of publication October 30, 2013; date of current version November 22, 2013. This work was supported in part by the Air Force Office of Scientific Research under Contract FA9550–09-1–0708, the National Science Foundation under Contract ECCS 0802036, and DTRA under Contract W911NF-06–2-0040 administered by the Army Research Laboratory. K. Fan, X. Zhao, H. R. Seren, and X. Zhang are with the Department of Mechanical Engineering, Boston University, Boston, MA 02215 USA (e-mail: [email protected]; [email protected]; hseren; [email protected]). J. Zhang, K. Geng, G. Keiser, and R. D. Averitt are with the Department of Physics, Boston University, Boston, MA 02215 USA (e-mail: [email protected]; [email protected]; [email protected]; [email protected]). G. D. Metcalfe and M. Wraback are with the U.S. Army Research Laboratory, Adelphi, MD USA 20783 (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TTHZ.2013.2285619

dynamically tunable MMs offer new opportunities for terahertz applications in sensing, imaging, and telecommunications, amongst others [29]–[31]. Tunable metamaterials are typically created by fabricating MMs on active substrates. Since the first successful demonstration of tunable metamaterials through optical carrier excitation in GaAs [32], numerous strategies to dynamically tune the response of metamaterials have been studied, such as applying bias voltage [5], [16], optical excitation [18], [23], [26]–[28], thermal heating [22], [33], [34], and MEMS actuation [20], [24], [25]. For instance, the metamaterial transmission response (phase and amplitude) can be effectively modulated by applying a voltage to a Schottky diode, which is formed between the gold metamaterial layer and the highly doped semiconductor substrate [16]. This tuning scheme has been successfully implemented for terahertz imaging [5]. Similarly, hybridization of MMs with phase-change materials such as vanadium dioxide VO , MM optical properties can be modified temperature changes [33]. This real-time tuning method shows promise in creating memory devices for future high-performance computing [35]. As a final example, developments in terahertz generation have enabled explorations of nonlinear MMs [36]–[38]. However, the majority of these tunable MMs are patterned on rigid substrates, such as GaAs, silicon, or sapphire. The accurate characterization of these metamaterials generally requires a blank substrate of the same type as a reference, which complicates their characterization and application in THz systems. MMs on rigid substrates are also limited for applications on curved surfaces such as spherical lenses and mirrors. Recent progress on transfer-printing techniques patterning metallic or semiconductor structures onto flexible polymeric substrates shows considerable promise to overcome these limitations [39]–[41]. Flexible and stretchable substrates exhibit advantages that include transparency, lightweight, low cost, conformable adhesion, and biocompatibility, germinating applications such as bio-inspired mimickry [42], [43] and biomedical sensing [44]. Recently, several examples of tunable MMs on flexible substrates have been demonstrated in the mid-infrared and terahertz ranges [45]–[47]. For example, in [45], the response of metamaterials exhibits approximately a 10% LC resonant frequency shift through stretching PDMS substrates to which photonic metamaterials had been transferred. In this paper, we demonstrate flexible optically tunable metamaterials. This is accomplished by transferring an array

2156-342X © 2013 IEEE

FAN et al.: OPTICALLY TUNABLE TERAHERTZ METAMATERIALS ON HIGHLY FLEXIBLE SUBSTRATES

Fig. 1. Illustration of flexible tunable MM. (a) MMs with GaAs thin film on polyimide illuminated by an 800-nm beam (red); the response of MMs is probed by incident terahertz radiations (blue). (b) Structure of eSSR (top view and cross-sectional view).

of split ring resonators (SRRs) with thin semiconductor pads to a flexible polyimide substrate. Through photoexcitation of free carriers in the semiconductor layer, the metamaterial transmission amplitude can be effectively modulated by up to 60% at 0.98 THz. Numerical simulations match experiments and indicate, through parameter extraction, efficient tuning of the effective permittivity. Our results exhibit great potential to create multilayer nonplanar tunable MMs, such as invisible cloaks and conformable perfect absorbers, fitting onto complex surfaces. II. FABRICATION OF ULTRATHIN TUNABLE METAMATERIALS SRRs, the canonical metamaterial atom, are usually described as capacitor–inductor circuits, with a resonant frequency . For optically tunable metamaterials, the incorporation of photoactive materials can be used to either short the capacitive gap or modify the inductance. The tuning mechanism presented in this paper is capacitive-based, similar to those on rigid substrates [18], [28]. Fig. 1(a) schematically shows flexible MM arrays with GaAs patches photoexcited by an incident pump beam. The MM response is interrogated by terahertz waves. The unit cell of the flexible MMs shown in Fig. 1(b) is composed of three stacked layers, consisting of one layer of polyimide, a metallic electric split-ring resonator (eSRR), and a thin layer of GaAs patches. The eSRR layer is sandwiched between the polymer and GaAs layers. To release the GaAs easily from a rigid substrate, two etching windows are intentionally designed

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Fig. 2. Fabrication process flow. (a) Pattern Au eSRR structure and wet etch of GaAs patches. (b) Etch AlGaAs layer. (c) Spin polyimide on ESRR. (d) Etch polyimide with O plasma to form the releasing holes. (e) Etch AlGaAs layer to release the flexible metamaterial structure.

on the GaAs layer. The total thickness of the device is a few micrometers, which is sufficiently thin to conformally wrap curved surfaces while maintaining the integrity of the MMs and semiconductor [48], [49]. With above bandgap excitation of GaAs, electrons in the valence band will be excited to the conduction band, increasing the conductivity of the GaAs. With a sufficient photoinduced increase in the carrier density, the conductivity increase will shunt the ESRR capacitance thereby damping the LC resonance. In turn, the transmission response of the flexible metamaterials will be modulated. Fig. 2 shows the fabrication process. First, a 300-nm sacrificial layer of Al Ga As, 400-nm SI-GaAs, and 150-nm n-type GaAs with carrier density of 1 10 cm were epitaxially grown on a 2-in semi-insulating (SI) GaAs wafer. Then, an 8 8 mm array of ESRRs with 150 nm of gold film was patterned on the epitaxial layer, with an overall sample size of 1 1 cm to facilitate handling during the characterization. Next, wet etching of the epilayer with citric acid: H O (10:1) solution was used to define the GaAs patches in the center of the eSRRs after photoresist S1813 was coated and patterned [Fig. 2(a)]. Because of the highly selective etching solution for GaAs on AlGaAs layer, after the GaAs had been etched away, the AlGaAs sacrificial layer was exposed. The following step is to dip the wafer into a diluted HF solution to remove part of the sacrificial layer, as shown in Fig. 2(b). The isotropic HF etching of AlGaAs yielded an undercut 500 nm beneath the epi-GaAs layer, which facilitates the subsequent transfer process. After removing the photoresist, a 4- m-thick polyimide layer was spun on the wafer followed by a curing process [Fig. 2(c)].

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Fig. 4. Experimental results of transmission response at different pump powers using OPTP spectroscopy. Fig. 3. Microscope images of the eSRRS before releasing from GaAs substrate. 52 m, 56 m, (a) Arrayed SRRs. (b) Close-up view of the SRR 4 m, 2.5 m, 68 m. (c) Flexible MMs with GaAs epilayer wrapped around a vial. The area of metamaterial is 8 8 mm .

During this process, the undercut area was also filled with polyimide. Next, RIE etching of polyimide with oxygen gas was performed to define the etching windows so that the following wetetching process can remove the exposed GaAs layer and form release holes [Fig. 2(d)]. Finally, after sacrificial layer etching in a diluted HF solution, the polyimide was released from the GaAs substrate and the metamaterials with the epilayer was successfully transferred to the polyimide substrate [Fig. 2(e)]. It should be mentioned that our process can also be used to transfer other semiconductors including silicon, InAs, or twodimensional electron gas (2DEG) layers onto a flexible substrate. Fig. 3 shows the microscope images of MM before releasing and a photograph of flexible metamaterials with GaAs epilayers wrapped onto a vial. III. EXPERIMENT AND RESULTS Optical-pump THz-probe (OPTP) spectroscopy was used to characterize the response of the flexible tunable MMs. To characterize the electromagnetic response, sample and reference scans were obtained with air as the reference. The flexible MM was mounted on a holder such that the THz pulses were at normal incidence to the sample with electric field normal to the gaps of the ESRRs. The OPTP spectroscopy system provided amplified, 35 fs, near infrared laser pulses at center wavelength of 800 nm and a repetition rate of 1 kHz to photoexcite the charge carriers in the GaAs patches. The optical beam was focused onto the sample with a diameter of 4 mm. Since the photocarrier lifetime in GaAs is 1 ns, the THz probe beam was set to arrive on the sample 10 ps after the 1.55-eV pump pulse such that a quasi-steady-state accumulation of the carriers is established. The experiment was carried out in a humidity-controlled environment. Fig. 4 shows the experimental terahertz transmission spectra of the flexible metamaterials at various excitation powers. In the absence of 1.55-eV pump excitation, the excited LC resonance is at 0.98 THz with transmission amplitude of 25% as

shown in Fig. 4(a) (black curve). The 4.7- m thickness of the sample is 0.015 of the corresponding free space wavelength at 0.98 THz. As the pump power is increased from 0 to 1 mW (equivalent to a fluence of 8 J cm ), the resonance strength weakens and transmission increases to . In comparison with previously reported optically tunable metamaterials on silicon thin film [18], [21], [26], [28], the pumping power is approximately two orders of magnitude smaller. Further increasing the pump power results in an LC resonance that is significantly damped and finally disappears as the orange line shows in Fig. 4. The transmission amplitude at 0.98 THz does not increase notably, yet the resonant frequency blueshifts. The transmission at 1.25 THz decreases with increasing pumping power. Fig. 5(a) is a plot of the transmission as a function of fluence at 0.98 THz and 1.25 THz. With increasing fluence from 0 to 64 J cm , the transmission at 1.25 THz decreases from approximately 80% to 25%. To explicitly show the metamaterial tunability, we also plot the differential transmission , or modulation depth, in Fig. 5(b). This is defined as , where T(p) is the incident-power dependent transmission amplitude and corresponds to the transmission without pumping power. At the resonant frequency of 0.98 THz, the maximal modulation reaches over 60% as the incident power increases from 0.25 to 8 mW. When the frequency is larger than 1.1 THz, the modulation depth is relatively flat yet with a modulation depth above 60% in a broad range from 1.1 to 1.8 THz. This could be useful for broadband transmission modulation. IV. NUMERICAL SIMULATIONS AND DISCUSSION To explain the nature of the amplitude tuning and blueshift of the resonance, we performed finite-difference time-domain (FDTD) simulations using CST commercial software. In our model, the dimensions of the ESRRs are based on measurements of the fabricated samples. The gold layer was simulated as a lossy metal with conductivity of 4.5 10 cm . The 4- m polyimide was modeled as a lossy dielectric material with a dielectric constant of 2.88 and a loss tangent of 0.03,


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Fig. 6. Simulation results of transmission as a function of carrier density of GaAs thin film.

Fig. 5. (a) Transmission as function of pump fluence at 0.98 and 1.25 THz. (b) Power dependence of the differential transmission.

as determined from polyimide substrates with thicknesses of 5.5 and 11 m [48]. The modeling of the GaAs patch requires more attention since there are two layers of GaAs. When there is no pump beam exciting the carriers, the top 400-nm semi-insulating GaAs layer can be modeled as a simple lossy dielectric material with a dielectric constant of 12.9, while the bottom 150-nm n-type GaAs, under the ESRRs, is modeled as a Drude type material [50], in which the plasma frequency , where n is the carrier density. Initially, for the n-type GaAs thin film, is set to cm . After photoexcitation, since the total thickness of GaAs epilayer is thinner than the penetration depth of 1.55-eV beam in GaAs (around 750 nm) [51], carriers are generated in both of the SI GaAs layer and n-type GaAs layer. In our simulations, we assumed, for simplicity, homogeneous photoexcitation of carriers in both layers. Therefore, the Drude model was applied for both layers but with different carrier densities due to the initial doping in the n-type GaAs layer. Fig. 6 shows the simulated transmission as a function of carrier density in the SI-GaAs layer. As the induced carrier density increases from 10 cm to 1.6 10 cm , the transmission at the resonance increases and the resonant frequency blueshifts due to the decreased real part of permittivity of

Fig. 7. Extracted effective permittivity dependent on photoexcited carrier density in GaAs epilayer. (a) Real part of permittivity. (b) Imaginary part of permittivity.

GaAs. However, with further increasing the carrier density, the transmission around 1.2 THz decreases and the LC resonance

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is strongly damped. With a density of 1.3 10 cm , there is a very broad resonance at 1.6 THz, which is attributed to the dipole-like resonance of the eSRRs with the gap shorted. The corresponding induced carrier density of 1.3 10 cm is consistent with the experimental results in [50] showing the carrier density on the order of cm under 70- J cm photoexcitation on 630 m-thick highly resistive GaAs. In general, our simulation results agree with the measurements. The tuning transmission response of MMs also leads to modulation of the effective permittivity of the MMs. Fig. 7 shows the real and imaginary part of the extracted effective permittivity depending on the carrier density of excited GaAs thin film (assuming a total thickness of m equal to its physical thickness). Without photoexcitation, the permittivity shows a strong Lorentzian response at 0.98 and 2.5 THz (black curve), which corresponds to fundamental LC resonance and high-order dipole resonance, respectively. As the excited carriers increase to about 4 10 cm , the resonant frequency blueshifts and the resonance is significantly damped by the conductive GaAs layer. The GaAs layer has two effects on the response of MMs. According to Drude model, the larger carrier density increases the plasma frequency , and thereby, decreases and becomes negative, whereas (or real part of conductivity ) increases notably. Consequently, the reduced results in the resonant frequency blueshifting, while the increase introduces loss into the resonator, damping the resonance. Further increasing the carrier density, the resonance close to 1.5 THz becomes stronger since the significantly increased conductivity of GaAs shorts the SRR gap, resulting in a modified dipolar resonance around 2 THz. Therefore, the of metamaterials around 1.5 THz shown in Fig. 7(b) increases and broadens due to the enhanced dipole resonance. V. CONCLUSION A novel flexible tunable MM was fabricated employing a semiconductor transfer technique and characterized by optical-pump terahertz-probe spectroscopy. The total thickness of sample is 4.7 m, equivalent to 0.015 of the resonant wavelength at 0.98 THz. Our experimental results show that low-fluence photoexcitation of the GaAs layer yields a transmission modulation depth of 60%. Numerical simulations agree with the experimental results and reveal the details of the modulation of the effective permittivity. Our flexible tunable device enables the creation of tunable multilayered nonplanar electromagnetic composites and potential sensing applications on nonplanar structures. ACKNOWLEDGMENT The authors would like to thank Boston University Photonics Center for technical support. REFERENCES [1] F. Zhou, Y. Bao, W. Cao, C. T. Stuart, J. Gu, W. Zhang, and C. Sun, “Hiding a realistic object using a broadband terahertz invisibility cloak,” Sci. reports, vol. 1, p. 78, 2011.

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Kebin Fan received the Ph.D. degree in mechanical engineering from Boston University, Boston, MA, USA, in 2012. He is currently a Research Associate with Boston University, Boston, MA, USA, and Boston College, Chestnut Hill, MA, USA. His research interests include micro/nanoelectromechanical systems (MEMS/NEMS) design and fabrication, metamaterials, and plasmonics.

Xiaoguang Zhao received the M.S. degree from Tsinghua University, Beijing, China, in 2011. He is currently working toward the Ph.D. degree at the Department of Mechanical Engineering, Boston University, Boston, MA, USA. His research interests include micro-electromechanical Systems (MEMS) design and fabrication and metamaterials.

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Jingdi Zhang received the M.A. degree in Physics from Boston University, Boston, MA, in 2009. He is pursuing the Ph.D. degree at the Department of Physics, Boston University. His research interests include ultrafast spectroscopy of strongly correlated materials, metamaterials and light-control (visible to THz) of transition metal oxides.

Kun Geng received the M.A. degree in physics from Boston University, Boston, MA, USA, in 2012, where he is currently working toward the Ph.D. degree at the Department of Physics. His research interests include terahertz metamaterials and photoinduced phase transitions in complex materials.

George R. Keiser received the B.S. degree in physics from The University of Scranton, Scranton, PA, USA, in 2009, and the M.A. degree in physics from Boston University, Boston, MA, USA, in 2011, where he is currently working toward the Ph.D. degree in the Department of Physics. His research interests focus on near-field interactions within a metamaterial unit cell and includes metamaterial field enhancement, tunable metamaterials, and nonlinear metamaterials at terahertz frequencies.

Huseyin R. Seren (S’10) received the B.S. and M.S. degrees in electrical engineering from Koç University, Koç, Turkey, in 2007 and 2009, respectively. He is currently working toward the Ph.D. degree at the Mechanical Engineering Department, Boston University, Boston, MA, USA. His research interests include THz metamaterials, THz plasmonics, perfect absorbers, optical MEMS, and RF MEMS. Mr. Seren was the recipient of The Scientific and Technological Research Council of Turkey (TUBITAK) Graduate Scholarship in 2007 and the Boston University Dean’s Scholarship in 2009.

Grace D. Metcalfe received the Ph.D. degree in physics from Yale University, New Haven, CT, USA, in 2005. Her dissertation research included design, development, and characterization of unidirectionally emitting ultraviolet microcavity semiconductor lasers and amplifiers. Following a two-year appointment as an Oak Ridge Associated Universities Postdoctoral Fellow, she officially joined the Nitride Semiconductor Optoelectronics team at the Army Research Laboratory (ARL), Adelphi, MD, USA, in 2007. She has authored and presented more than 40 papers on terahertz radiation and nitride materials, including a chapter entitled “Terahertz Radiation from Nitride Semiconductors,” in Advanced Series in Applied Physics Volume 6, and holds one patent, with two pending. Dr. Metcalfe was the recipient of the 2009 Department of the Army Research and Development Achievement Award for her work on terahertz sources with enhanced emission from spontaneously forming nanostructures in nonpolar nitride semiconductors.

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Michael Wraback received the Ph.D. degree in physics from Brown University, Providence, RI, USA. As an NRC Postdoctoral Research Associate and subsequent Member of the Technical Staff at the Army Research Laboratory (ARL), Adelphi, MD, USA, he has used optical techniques to investigate both coherent and incoherent excitonic optical nonlinearities in optically anisotropic semiconductor heterostructures, and nonequilibrium electron dynamics and transport in semiconductor optoelectronic devices. He is currently presiding over research and development of semiconductor ultraviolet and visible light sources and detectors, and optical generation and detection of terahertz radiation. He has authored or coauthored more than 200 papers and presentations addressing the physics of semiconductor materials and devices and holds 12 U.S. patents. Dr. Wraback is a Fellow of the American Physical Society, the Optical Society of America, and the Army Research Laboratory. He was the recipient of the ARL Award for Scientific Achievement in 2005 and the Department of the Army Research and Development Achievement Awards in 1994, 1997, 2002, 2005, and 2009.

Xin Zhang received the Ph.D. degree from Hong Kong University of Science and Technology, Hong Kong, in 1998. From 1998 to 2001, she was a Post-Doctoral Researcher and then a Research Scientist with the Massachusetts Institute of Technology, Cambridge, MA, USA. She then joined Boston University, Boston, MA, USA, where she is now a Professor with the Department of Mechanical Engineering. Her research interests are in the fundamental and applied aspects of microelectromechanical Systems (MEMS) and microfabrication and nanofabrication technologies. Specifically, she seeks to understand and exploit interesting characteristics of micro/nanomaterials, micro/nanomechanics, and micro/nanomanufacturing technologies with forward-looking engineering efforts and practical applications ranging from energy to healthcare to homeland security.

Richard D. Averitt received the Ph.D. degree in applied physics from Rice University, Houston, TX, USA, in 1998. Following this, he was a Los Alamos National Laboratory Directors Post-Doctoral Fellow. His postdoctoral work focused on time-resolved far-infrared spectroscopy of strongly correlated electron materials. In 2001, he became a Member of the Technical Staff with Los Alamos National Laboratory, Los Alamos, NM, USA, and, in 2005, a member of the Center for Integrated Nanotechnologies colocated at Los Alamos and Sandia National Laboratories. In 2007, he joined Boston University, Boston, MA, USA, as a Faculty Member with the Department of Physics and the Boston University Photonics Center. Starting in 2014, he will be a Professor with the Department of Physics, University of California at San Diego, La Jolla, CA, USA. His research interests are primarily directed towards characterizing the optical and electronic properties of materials including metamaterials and plasmonic composites, quantum cheese, transition metal oxides, and other complex materials using experimental techniques which span from the far-infrared through the visible.