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Copyright (c) 2011 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing [email protected]. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. XX, NO. XX, APRIL 2012 1 Metamaterial-based photonic devices for terahertz technology Benjamin Reinhard, Oliver Paul, and Marco Rahm Abstract—We review recent research on metamaterial-based devices for the terahertz (THz) frequency range. Possible appli- cations for THz metamaterials include high-speed modulators, highly sensitive devices for refractometry or thin film sensing, efficient polarization optics, THz-absorbing materials and high- performance gradient index optics. Due to the high flexibility in metamaterial design, these devices have the potential to outperform existing devices based on conventional materials. Index Terms—Terahertz, metamaterials. I. I NTRODUCTION M ETAMATERIALS are a class of artificial materials which derive their response to electromagnetic waves from their structure rather than from their composition. Usually, metamaterials consist of small metallic inclusions in a dielectric matrix. In contrast to photonic crystals, which possess a periodicity in the order of the wavelength, the size and the distance of the individual inclusions in metamaterials are subwavelength. This allows one to describe the optical response of metamaterials by effective material parameters. This is an analogy to wave propagation in conventional materials, where the interaction of an electromagnetic wave with an ensemble of atoms or molecules is described by spatially averaged, macroscopic quantities such as the permittivity and the permeability μ. Metamaterials take this concept one step further: In a second spatial averaging, the electromagnetic response of the metamaterial is described by an effective permittivity eand an effective permeability μ e. From eand μ e, the effective refractive index n eand the effective wave impedance z ecan be calculated and used to describe the propagation of a wave through the metamaterial. In the late 1990’s, two fundamental metamaterial designs were described: an array of thin conducting wires, showing an effective plasmonic response at gigahertz frequencies [1], and a material composed of split- ring resonators (SRRs) which exhibits a magnetic resonance [2]. Later, the electric LC-resonator (ELC) was introduced as a third basic metamaterial element [3] with an electric resonance. Most of today’s metamaterials are based on variations of these three designs. Since then, metamaterials have emerged as versatile tools for the implementation of optical components. The advantage of metamaterials as compared with conventional materials B. Reinhard and O. Paul are with the Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, 67663 Kaiserslautern, Germany. M. Rahm is with the Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, 67663 Kaiserslautern, Germany and the Fraun- hofer Institute for Physical Measurement Techniques IPM, 79110 Freiburg, Germany. is that by changing the size and shape of the subwavelength constituents, the optical response of a metamaterial can be easily designed to whichever values are desired for an application. Even optical properties that are not readily available in conventional materials, such as a high-frequency magnetic response can be obtained [2], [4]. Of a rather academic interest are materials with a negative index of refraction, which occurs if the permittivity and the permeability are negative at the same time. This behavior had been predicted already in the 1960’s [5], but only with the advent of metamaterials could be experimentally demonstrated in 2000 [6], [7]. With the rapidly developing computational methods of the last decade, metamaterial research has received an additional boost in recent years. Numerical techniques offer a fast, reliable, and cost- efficient way of designing and optimizing metamaterial devices [8]. While much pioneering work in metamaterial research was done for microwave frequencies, the terahertz frequency range (frequencies between approx. 0.1 THz and 10 THz, also termed submillimeter wave range or far infrared range), located between the microwave and infrared spectral region, is a particularly interesting playground for the development of metamaterials [9]–[11], because terahertz radiation offers many applications in security, quality inspection, chemical sensing, imaging, astronomy, and fundamental research [12], [13]. Due to the difficulty of efficient generation and detection of terahertz waves, the terahertz frequency range was also called the “terahertz gap” in the electromagnetic spectrum. This gap was closed in the 1980’s with the arrival of the photoconductive switch [14], [15] and the development of phase-sensitive terahertz time-domain spectroscopy (THz-TDS) [16]. Because of the scale invariance of Maxwell’s equations, metamaterial designs for the microwave range can be easily adapted for operation at terahertz frequencies by simple geometric scaling. Owing to the relatively large wavelengths of terahertz radiation (1 THz corresponds to a vacuum wavelength of 300 μm), terahertz metamaterials can be easily fabricated with standard microfabrication techniques such as UV lithography, because the smallest structure dimensions are typically in the order of few micrometers. Furthermore, in the terahertz range, the losses in materials are relatively low compared to higher frequencies like the infrared or visible region. Most metals behave almost like perfect conductors at terahertz frequencies, and many low-loss dielectric materials exist that can be used as background materials for the fabrication of metamaterials. Because of the low material losses at terahertz frequencies, the attenuation of the resonances of the metamaterial is relatively weak. This extends the range of accessible values of the

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Copyright (c) 2011 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing [email protected].

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication.

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. XX, NO. XX, APRIL 2012 1

Metamaterial-based photonic devices for terahertztechnology

Benjamin Reinhard, Oliver Paul, and Marco Rahm

Abstract—We review recent research on metamaterial-baseddevices for the terahertz (THz) frequency range. Possible appli-cations for THz metamaterials include high-speed modulators,highly sensitive devices for refractometry or thin film sensing,efficient polarization optics, THz-absorbing materials and high-performance gradient index optics. Due to the high flexibilityin metamaterial design, these devices have the potential tooutperform existing devices based on conventional materials.

Index Terms—Terahertz, metamaterials.

I. INTRODUCTION

METAMATERIALS are a class of artificial materialswhich derive their response to electromagnetic waves

from their structure rather than from their composition. Usually,metamaterials consist of small metallic inclusions in a dielectricmatrix. In contrast to photonic crystals, which possess aperiodicity in the order of the wavelength, the size and thedistance of the individual inclusions in metamaterials aresubwavelength. This allows one to describe the optical responseof metamaterials by effective material parameters. This is ananalogy to wave propagation in conventional materials, wherethe interaction of an electromagnetic wave with an ensembleof atoms or molecules is described by spatially averaged,macroscopic quantities such as the permittivity ε and thepermeability µ. Metamaterials take this concept one step further:In a second spatial averaging, the electromagnetic responseof the metamaterial is described by an effective permittivityεeff and an effective permeability µeff . From εeff and µeff , theeffective refractive index neff and the effective wave impedancezeff can be calculated and used to describe the propagationof a wave through the metamaterial. In the late 1990’s, twofundamental metamaterial designs were described: an array ofthin conducting wires, showing an effective plasmonic responseat gigahertz frequencies [1], and a material composed of split-ring resonators (SRRs) which exhibits a magnetic resonance[2]. Later, the electric LC-resonator (ELC) was introduced as athird basic metamaterial element [3] with an electric resonance.Most of today’s metamaterials are based on variations of thesethree designs.

Since then, metamaterials have emerged as versatile toolsfor the implementation of optical components. The advantageof metamaterials as compared with conventional materials

B. Reinhard and O. Paul are with the Department of Physics andResearch Center OPTIMAS, University of Kaiserslautern, 67663 Kaiserslautern,Germany.

M. Rahm is with the Department of Physics and Research Center OPTIMAS,University of Kaiserslautern, 67663 Kaiserslautern, Germany and the Fraun-hofer Institute for Physical Measurement Techniques IPM, 79110 Freiburg,Germany.

is that by changing the size and shape of the subwavelengthconstituents, the optical response of a metamaterial can be easilydesigned to whichever values are desired for an application.Even optical properties that are not readily available inconventional materials, such as a high-frequency magneticresponse can be obtained [2], [4]. Of a rather academic interestare materials with a negative index of refraction, which occursif the permittivity and the permeability are negative at thesame time. This behavior had been predicted already in the1960’s [5], but only with the advent of metamaterials couldbe experimentally demonstrated in 2000 [6], [7]. With therapidly developing computational methods of the last decade,metamaterial research has received an additional boost in recentyears. Numerical techniques offer a fast, reliable, and cost-efficient way of designing and optimizing metamaterial devices[8].

While much pioneering work in metamaterial researchwas done for microwave frequencies, the terahertz frequencyrange (frequencies between approx. 0.1 THz and 10 THz, alsotermed submillimeter wave range or far infrared range), locatedbetween the microwave and infrared spectral region, is aparticularly interesting playground for the development ofmetamaterials [9]–[11], because terahertz radiation offers manyapplications in security, quality inspection, chemical sensing,imaging, astronomy, and fundamental research [12], [13]. Dueto the difficulty of efficient generation and detection of terahertzwaves, the terahertz frequency range was also called the“terahertz gap” in the electromagnetic spectrum. This gap wasclosed in the 1980’s with the arrival of the photoconductiveswitch [14], [15] and the development of phase-sensitiveterahertz time-domain spectroscopy (THz-TDS) [16]. Becauseof the scale invariance of Maxwell’s equations, metamaterialdesigns for the microwave range can be easily adapted foroperation at terahertz frequencies by simple geometric scaling.

Owing to the relatively large wavelengths of terahertzradiation (1 THz corresponds to a vacuum wavelength of300 µm), terahertz metamaterials can be easily fabricated withstandard microfabrication techniques such as UV lithography,because the smallest structure dimensions are typically in theorder of few micrometers. Furthermore, in the terahertz range,the losses in materials are relatively low compared to higherfrequencies like the infrared or visible region. Most metalsbehave almost like perfect conductors at terahertz frequencies,and many low-loss dielectric materials exist that can be usedas background materials for the fabrication of metamaterials.Because of the low material losses at terahertz frequencies, theattenuation of the resonances of the metamaterial is relativelyweak. This extends the range of accessible values of the

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2 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. XX, NO. XX, APRIL 2012

optical constants which can be reached with metamaterialsand increases the possibilities for the design of optical devices.

Here, we review current trends towards applications ofmetamaterials for terahertz waves. We especially focus onterahertz optical components based on metamaterials.

II. MODULATORS FOR TERAHERTZ WAVES

Many applications in THz technology require optical ele-ments for the modulation of the amplitude and/or phase of aTHz beam. In principle, there are two classes of modulatorsthat can be differentiated into electrically driven and opticallydriven modulators. In electronically driven modulators, thetransmitted amplitude and/or phase of the THz wave dependson an electric current or voltage that is applied to the modulatorwhile in optically driven modulators the electromagneticresponse is controlled by the power or intensity of an incidentoptical modulation beam. Further methods rely on thermalmodulation [17]–[19], modulation by external magnetic fields[20] or mechanically reconfigurable metamaterials [21]. Mostof the earlier developed optically and electronically drivenmodulators that were based on quantum well structures had thedisadvantage to operate at cryogenic temperatures only [22],[23]. In 2004, an electrically tunable THz wave modulator wasproposed which operates at room temperature, however onlyachieved modulation depths of the order of a few percent of thesignal amplitude [24]. Other theoretical approaches pursue thedesign of THz wave modulators by means of a photonic crystalthe bandgap of which is modulated by an external electric field[25]. As an experimental demonstration, optically driven ultra-fast photonic-crystal-based modulators have been reported byFekete et al. [26]. Most recent developments focus on theconception of THz wave modulators by electronically drivengraphene structures [27], [28]. In this light, metamaterials haveshown the potential to be promising competitors for the imple-mentation of both electrically and optically driven THz wavemodulators that might even outperform the aforementionedmethods. The most notable designs refer to electrically drivenmodulators that potentially serve as spatial light modulatorsfor compressed sensing and imaging in the THz frequencyrange. In the following, we explain the basic principle behindelectrically driven metamaterial modulators for THz waves andreview their application to compressed sensing and fast imagedata acquisition in THz technology.

A. Electrically driven terahertz wave modulators and com-pressed sensing

Compressed sensing is an intriguing methodology to speedup the data acquisition time of THz imaging systems whichusually strongly lag behind the performance of imagingsystems in other frequency ranges [29], [30]. This is especiallysignificant before the background that the low data acquisitionrate currently hinders THz imaging systems from beingimplemented in real-world industrial applications. In a basicand simple approach of compressed sensing (among many moresophisticated derivatives of this technique), an object underinvestigation is basically illuminated by a light source andthe scattered fields are imaged on a spatial mask. In the most

(a)

(b)

(c)

Fig. 1. (a) Scheme of a metamaterial-based THz wave modulator. Themetamaterial unit cell consists of electric LC-resonators. The resonators forma Schottky contact with an n-doped GaAs layer grown on a semi-insulatingGaAs substrate. The THz transmission through the modulator can be controlledby applying a bias voltage between the Schottky and an Ohmic contact [35].(b) The application of a reverse bias voltage between the Schottky and theOhmic contact induces a depeletion zone in the capacitance of the electricLC-circuit, thus changing the transmission characteristics of the unit cell [35].(c). The area of a single pixel of the spatial light modulator is 4 × 4 mm2.Each pixel consists of 2500 unit cells. The electric LC-resonators of a pixel areelectrically interconnected and constitute a Schottky contact with the substrate.By applying a bias voltage between an individual pad (V1–V16) and the ohmiccontact the transmittance of each single pixel can be independently controlled[34]. (Reprinted with permission from Ref. 34. Copyright 2009, AmericanInstitute of Physics.)

extreme case, the mask consists of an array of pixels amongwhich a certain number is transparent and the remaining pixelsare opaque. Hereby, the spatial distribution of transparent andopaque pixels is random. The radiation penetrating through thetransparent pixels of the mask is collected by a lens and focusedonto a single-pixel detector. By taking a series of measurementswhere the spatial pattern of transparent and opaque pixels isvaried randomly from measurement to measurement, the imageof the sample can be reconstructed in amplitude and phase by anumeric algorithm. The compressed sensing technique providestwo essential advantages in comparison with conventional THzimaging techniques. First of all, a significantly lower numberof measurements is required to record an image that carries allthe necessary spatial and spectral information about the objectthan by any conventional raster scanning method. Since thenumeric retrieval algorithms are very fast and time-efficient thisleads to a tremendous decrease of image data acquisition timein comparison to conventional imaging techniques. Second,the image can be recorded by a single-pixel THz detectorwhich is much easier to implement than a THz detectorarray. So far, most experimental demonstrations of compressedsensing in the terahertz frequency range were proof-of-principleexperiments where the spatial light modulator was mimicked byphysically interchanging masks with different patterns betweenthe measurements [31]–[33]. In this respect, the measurementsdid not result in lower data acquisition times and the approachwas not suitable for any practical applications. It is obviousthat the method of compressed THz sensing thrives and fallswith the ability to design and implement a spatial modulator forterahertz waves. As a crucial step towards this goal, recently ametamaterial-based 4-by-4 array of independently switchablepixels has been demonstrated [34].

Fig. 1(c) shows a metamaterial-based spatial light modulator(SLM) with 4×4 pixels. Each individual pixel of the SLM

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REINHARD et al.: METAMATERIAL-BASED PHOTONIC DEVICES FOR TERAHERTZ TECHNOLOGY 3

(a) (b)

Fig. 2. (a) Transmission pattern of a 4×4 spatial light modulator array with2 non-transparent pixels (biased) and 14 transparent pixels (unbiased) [34]. (b)Difference signal of the transmission through a spatial light modulator with thetwo border columns switched from transparent (unbiased) to opaque (biased),while the two middle columns remain transparent. The spatial distribution ofthe signal at 0.36 THz shows the typical diffraction pattern of a double slit[34]. (Reprinted with permission from Ref. 34. Copyright 2009, AmericanInstitute of Physics.)

comprises 2500 electric LC-resonators (ELCs) that form theunit cells of the metamaterial. The ELCs are fabricated on ann-doped gallium arsenide (GaAs) layer grown on top of a semi-insulating GaAs substrate. This is schematically shown in Fig.1(a) for a reduced number of unit cells. For each pixel, the 2500ELCs are electrically interconnected to set them to an identicalelectric potential and attached to an electric pad. That way theELCs and the electric pad form a Schottky contact with then-doped GaAs. By applying an external bias voltage betweenthe electric pad and an Ohmic contact, the strength of theresonance of the ELCs in a pixel and thus its transmittance canbe controlled by an external bias voltage. The tunability of theresonance strength relies on the formation of carrier depletionzones within the capacitance gap of the ELCs upon applicationof an external bias field (Fig. 1(b)). In consequence, the volumeof the depletion zone determines the capacitive loss of the ELCsand thus changes the resonance strength. The basic conceptbehind electrically driven metamaterial modulators has beenexplained in many publications. A review article summarizingthe physical aspects behind metamaterial-based modulators canbe found in Ref. 10. For more details, the reader is referred toRef. 35.

Because each pixel is connected to an individual electricpad (V1–V16 in Fig. 1(c)) and since the different pixels areelectrically insulated from each other to avoid cross talking, thetransmittance of each pixel can be independently controlled. Anexample of an arbitrary transmittance pattern with two opaquepixels and 14 transparent pixels at a frequency of 0.36 THzis shown in Fig. 2(a). In this case, the dark pixels are biasedwhile the bright pixels are unbiased. As a further example, Fig.2(b) depicts the transmitted power through a double slit at 0.36THz. For this purpose the two vertical middle lines of the 4×4SLM array were kept unbiased, while the two border verticallines were switched from unbiased (bright) to biased (dark).The difference signal shows the typical diffraction pattern of adouble slit. Analytical calculations confirmed the observations.From this point of view, a metamaterial-based SLM seems to

(a)

(b) (c) (d)

Fig. 3. (a) Terahertz single pixel camera based on compressed sensing. TheTHz wave is transmitted through an object mask and imaged through a randompattern array on a single pixel THz receiver. (b) White light image of the objectmask. (c) Compressed image based on 300 magnitude measurements (30 % oftotal number of image pixels). (d) Compressed image based on 600 magnitudemeasurements (60 % of total number of image pixels) [31]. (Reprinted withpermission from Ref. 31. Copyright 2008, American Institute of Physics.)

be perfectly suitable as a modulation mask for compressedsensing. Although such a modulator has not been successfullyapplied in compressed sensing so far, first proof-of-principleexperiments demonstrated that compressed sensing enables theretrieval of the amplitude and phase of the THz wave from acompressed image [31], [32].

Fig. 3(a) illustrates the typical setup of a THz compressedsensing scenario. An object is illuminated by a THz waveand imaged through a random pattern mask on a single pixelTHz detector. In this case the random 32×32 square pixelpatterns were printed on a standard printed-circuit board (PCB)resulting in a total number of 32 × 32 = 1024 image pixels.Instead of raster scanning the image to obtain amplitudeand phase information about all N = 1024 pixels in Nmeasurements, compressed sensing allows to take a muchsmaller number M < N of individual measurements whilestill being able to reconstruct the image. For each singlemeasurement, however, the random pattern of the spatiallight modulator must be changed. In this proof-of-principleexperiment this was accomplished by physically interchangingthe PCB between the individual measurements. It is obvious thatcompressed sensing can lead to significant time savings as soonas an electrically driven spatial light modulator is applied. Basedon a time-efficient optimization and reconstruction algorithmthe image can be retrieved from M < N measurements. Itshould be noted at this point that a detailed description ofcompressed sensing is out of the scope of this paper. To get anoverview about the basic principles behind compressed sensingthe reader is referred to Refs. 29 and 30. Fig. 3(b) showsthe white-light image of a Chinese character that was laterimaged by the compressed sensing setup in Fig. 3(a). Figs.3(c) and 3(d) depict the reconstructed compressed images that

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were obtained from 300 and 600 magnitude measurements,respectively, which corresponds to 30 % and 60 % of thetotal number of image pixels. A comparison of the retrievedimages in Figs. 3(c) and 3(d) reveals that, although the contrastincreases with the number of measurements, 300 measurementsare already sufficient to resolve the details of the Chinesecharacter. Further investigations evidenced that, in additionto the image amplitude, also the phase of the image canbe reconstructed from the compressed image [31]. All theseinvestigations clearly demonstrate that compressed sensingin combination with metamaterial-based electronically drivenspatial light modulators promises a considerable increase ofdata acquisition rate and thus can break new ground for THzimaging applications in an industrial environment.

B. Optically driven and mechanically reconfigurable terahertzwave modulators

The second class of metamaterial-based modulators areoptically driven modulators. The advantage of optical modu-lation lies in the high modulation speed that can be achievedby all-optical switching of the electromagnetic properties ofa metamaterial. For this purpose, light-sensitive media areusually embedded into the metamaterial structures that changetheir optical properties and, by interaction, the electromagneticproperties of the metamaterial upon illumination by an externallight source. Typically, the wavelength of the external sourceis different from the wavelength of the THz radiation andis rather located in the infrared or visible frequency range.Most commonly, semiconductors are engineered into themetamaterial structure and the electromagnetic properties arevaried by changing the power of an incident laser beam thatexcites free carriers in the semiconductor. This idea dates backto at least 1996, when theoretical investigations on the opticalmodulation of the response of a frequency-selective surface inthe millimeter-wave range were reported [36]. Probably the firstexperimental demonstration of all-optical modulation of theelectromagnetic response of a metamaterial was accomplishedby Padilla et al. who dynamically controlled the electricalresponse of SRRs via photoexcitation of free carriers in ahigh-resistivity GaAs substrate [37], succeeded by furtherinvestigations on optically modulated metamaterials [38]–[42].A thorough study of resonance tuning of a single split ringresonator in the microwave regime can be found in Ref. 43. Fig.4(a) shows the spectral transmission of a THz wave throughan ELC metamaterial on a high-resistivity GaAs substratefor different powers of the photo-doping pulsed laser. Thetransmission minimum at lower frequencies corresponds to theexcitation of an asymmetric magnetic mode with oscillating ringcurrents in the ELCs while the transmission minimum at higherfrequencies relates to a symmetric electric resonance with in-phase current oscillations in the opposing strip lines with andwithout gap. An increase of the photo-doping power leads toa decrease of the overall transmission and visibly bleachesthe minimum at lower frequencies. It is obvious that higherphoto-doping power is required to influence the magnitudeof the second transmission minimum at higher frequencieswhich can be explained by the different nature of the electric

Fig. 4. (a) Spectral transmission through a metamaterial THz wave modulatorfor different photo-doping powers of the pulsed laser. (b) Real part of thepermittivity in dependence on the photo-doping power of the pulsed laser [37].(Reprinted with permission from Ref. 37. Copyright 2006 by the AmericanPhysical Society.)

resonance. The change of the transmittance is attended by avariation of the real part of the dielectric permittivity as can beseen in Fig. 4(b) which induces a phase modulation of the THzwave. To even further increase the phase modulation depth, atechnologically more advanced concept is to incorporate smallphotoconducting strip lines into metamaterials. This changesthe effective shape of the metamaterial constituents when thesample is illuminated, and can therefore influence the resonancefrequency of the metamaterial [44], [45].

Another smart methodology to modulate THz waves is theuse of mechanically reconfigurable metamaterials. In suchdevices, the effective geometric parameters of the metamaterialstructure, e.g. the angle of incidence, is changed by use ofbimaterial cantilevers. It was shown that the electric andmagnetic resonance in the transmission spectrum at a frequencyof 0.5 THz could be tuned within a range of 30 % and 50 %respectively by reorientation of the unit cells of a split ringresonator metamaterial. The tunability of the electromagnetic

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REINHARD et al.: METAMATERIAL-BASED PHOTONIC DEVICES FOR TERAHERTZ TECHNOLOGY 5

response of such media renders them suitable for use as angle-dependent filters and absorbers [21].

III. METAMATERIALS FOR TERAHERTZ OPTICALCOMPONENTS

A. Polarization optics

Efficient manipulation of the polarization state of terahertzradiation has long been a difficult task, because conventionalmaterials usually show only a weak birefringence in theterahertz frequency range. Polarization optics constructed fromconventional birefringent materials therefore have to be severalwavelengths thick at least, which limits their applicability [46]–[48]. More strongly birefringent optical components can bedevised by structuring a slab of material with straight grooves,which causes an artificial polarization sensitivity of the material.This has been demonstrated with polyethylene [49] and silicon[50] as a basic material. This concept provides a certain amountof design freedom by changing the width and depth of thegrooves, but the refractive index of the bulk material remainsa limiting factor for the refractive index contrast experiencedby opposite polarizations. A similar effect has been achievedby stacking layers of paper [51].

Many metamaterials have an inherently polarization-sensitiveresponse because of the symmetry properties of their constituentelements. This renders them predestined to construct efficientterahertz polarization optics such as polarizers and waveplates. The probably best-known metamaterial componentfor polarization control of terahertz waves is the wire-gridpolarizer, which provides broadband behavior, low loss anda high extinction ratio [52]–[54]. Furthermore, birefringentmetamaterial structures have been demonstrated which actas quarter- or half-wave plates for terahertz radiation [55]–[57]. Several designs for THz wave plates are pictured inFigs. 5(a)–5(d). Because the birefringence of metamaterialsis designable, the refractive index difference for oppositelypolarized waves can reach very high values. A refractive indexcontrast as high as ∆n ≈ 3.1 has been reported in a wire-pair metamaterial [57]. Fig. 5(d) shows the unit cell of theemployed structure. Incident waves that are polarized parallelto the wires experience a negative index of refraction, while therefractive index is positive for perpendicularly polarized waves.By this means, quarter- and half-wave plates with thicknesseswell below the operating wavelength were fabricated. Furthernotable polarization-sensitive metamaterial designs are chiralstructures which show differences in their optical responsefor left and right circularly polarized incident terahertz waves,respectively [58], [59].

For many applications, however, a polarization-sensitiveresponse of metamaterials is not desired. In order to providea polarization-independent response, a metamaterial structuremust possess a three- or morefold rotational symmetry [60].Because the simplest resonant elements, like the split-ringresonator and the electric LC-resonator, are not rotationallysymmetric, efforts have been made to design resonant metama-terial structures that are based on a symmetric unit cell [55],[61].

(a) (b)

(c) (d)

Fig. 5. Various designs for terahertz wave plates. (a) Electric LC resonator[56], (b) meanderline [56], (c) elliptical split ring resonator [55], (d) wirepairs [57]. One unit cell of each design is shown. (Reprinted with permissionfrom Refs. 55–57. (a)–(c) Copyright 2009, Optical Society of America. (d)Copyright 2009, American Institute of Physics.)

B. Gradient index optics

The possibility of changing the effective optical constantsof metamaterials at the unit cell level provides a powerfultool for the conception of gradient index (GRIN) optics. Bydesigning the refractive index distribution of a metamaterial,optical components such as lenses, beam steerers and beambends can be constructed. GRIN lenses, for example, whencompared to conventional, “constant index” lenses, have theadvantage that they do not require curved surfaces and thereforeare free of spherical aberration. Because metamaterials allowone to obtain high values of the refractive index near resonance[62], it is possible to create large refractive index gradients. Bythis means, efficient GRIN optics of few wavelengths thicknesscan be devised by judicious design. A different design approachfor the implementation of novel optical components, such aselectromagnetic cloaking devices [63], [64], is transformationoptics [65]–[67]. Since transformation-optical devices areusually inhomogeneous and demand a spatial variation of theoptical constants of the implementing material, metamaterialsare the tools of choice for building such devices.

There exist numerous publications on GRIN materialsoperating in the microwave regime [68]–[74]. In contrast, arelatively small amount of examples for GRIN optics has beendemonstrated in the terahertz frequency region so far [75], [76].A reason for this is that the experimental realization becomesmore challenging when moving to higher frequencies, since thestandard photo- or electron beam lithography techniques restrictthe designs to planar structures with a limited number of layers.In order to compensate the layer restriction, it is essential to usemetamaterial designs that allow tuning of the refractive index in

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(a) (b)

(c) (d)

Fig. 6. (a) Unit cell of a gradient index metamaterial based on an arrayof annular slots in a metal sheet. (b) The effective refractive index of themetamaterial design strongly depends on the radius of the slots. (c) Microscopeimage of a gradient index lens comprised of three layers of annular slots. (d)Numerical calculation of the focusing effect of the gradient index lens [75],[76]. (Reprinted with permission from Refs. 75, 76. (a), (c) Copyright 2010,Optical Society of America. (b), (d) Copyright 2010, American Institute ofPhysics.)

a wide range. Many resonant metamaterial structures, however,are very lossy and provide only a limited index contrast withinan acceptable transmission window. For the construction ofGRIN optics, it is therefore advantageous to use metamaterialelements with a resonance frequency well above the operatingfrequency, where the loss of the metamaterial is low [71], [74],[75].

As a promising design for gradient index metamaterials in theterahertz regime, an array of annular slots in a metal sheet hasbeen proposed [75]. Its unit cell is illustrated in Fig. 6(a). Theeffective refractive index of the metamaterial strongly dependson the radius of the slots, as shown in Fig. 6(b). By variationof the inner radius of the slots, a refractive index contrastof ∆n = 1.5 was obtained. Based on this design, a gradientindex lens operating in a frequency range between 1.2 THz and1.5 THz has been realized and experimentally characterized[76]. The lens consisted of three functional layers and had adiameter of 1.5 mm. Figure 6(c) shows a microscope image ofthe fabricated lens. Incident terahertz waves were focused to aspot size of 250 µm, which corresponds to approximately onewavelength at the operating frequency. In Fig. 6(d), a numericalcalculation of the focusing effect is shown for a frequency of1.2 THz.

C. Terahertz absorbers

Based on metamaterials, absorbing materials can be con-ceived which exhibit a terahertz reflectance of almost zero ina certain frequency range. This may be useful for terahertzstealthing [77] or applications where shielding against terahertzwaves is required. Another possible field of application is theimplementation of room-temperature thermal terahertz sensors.

(a) (b)

Fig. 7. (a) Microscope image of a metamaterial absorber. (b) Spectralabsorptivity of the metamaterial absorber [78]. (Reprinted with permissionfrom Ref. 78. Copyright 2008, Optical Society of America.)

Because the relative permittivity εeff and permeability µeff

of a metamaterial can be tuned independently, it is possibleto design metamaterial structures with εeff = µeff , which leadsto a relative wave impedance of zeff = 1. By this means, themetamaterial is impedance matched to the surrounding air,and reflections are minimized. If the metamaterial itself oran embedded medium in the metamaterial exhibits high loss,the transmission also tends to be near zero and almost all theenergy of the incident wave is absorbed in the material. Inother words, the metamaterial acts as a perfect absorber. Thatway, very compact metamaterial absorbers can be constructedwith a thickness of only a fraction of the terahertz wavelength(< λ/10).

After a first theoretical investigation by Landy et al. in2008 [79], the first metamaterial-based terahertz absorber wasfabricated and experimentally characterized in 2008 by Taoet al. [78]. The metamaterial consisted of two metallic layersseparated by a polyimide spacer layer. The top layer comprisedan array of electric resonators, while the bottom layer impliedcut wires (Fig. 7(a)). With this design, a maximum absorptionof 70 % was measured at a frequency of 1.3 THz (Fig. 7(b)).In the same year, this design was significantly improved byexchanging the layer of cut wires by a continuous metal sheet[80]. With the advanced design, a maximum absorptivity of97 % could be demonstrated. In both cases, the frequencyrange of operation was relatively small. Another improvementwas that this design worked without a thick semiconductorwafer and even for non-normal incidence. The early designshad the disadvantage that they only operated as absorbers forone polarization of the incident wave due to the polarizationdependence of the constituent resonant elements. To overcomethis drawback, several designs have been proposed that operateindependently of the polarization of the incident beam due totheir structural symmetry [81]–[85]. Another path of effortsconcentrated on the design and implementation of metamaterialabsorbers that offer a wider bandwidth of absorption [85]–[88].Furthermore, a design for an electrically tunable metamaterial-based THz absorber has been recently suggested and discussedon the basis of numerical calculations [89]. For more detailson metamaterial absorbers the reader is referred to Ref. 10 andthe references therein.

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REINHARD et al.: METAMATERIAL-BASED PHOTONIC DEVICES FOR TERAHERTZ TECHNOLOGY 7

D. Terahertz filtersBecause the functionality of most metamaterials roots in

the resonances of their constituent elements, metamaterialsgenerally exhibit a strong frequency dependence of theirtransmission and reflection characteristics. Hence, it is notsurprising that wavelength-filtering devices were among thefirst metamaterial optical components for terahertz radiation.THz filters can be used for spectrally resolved detection ofTHz radiation from incoherent sources, e. g. as input filtersfor bolometers, in order to select a specific frequency bandor to suppress noise. Furthermore, they are an importantprerequisite for the development of duplexers, multiplexers,and demultiplexers for multi-channel wireless communicationsystems with THz carrier frequencies. Metamaterial-basedfilters are very versatile because it is easy to adapt the designsto a desired operating frequency range by simple scaling ofthe geometric structure.

For several decades, terahertz wave filters consisting of thinstructured metal sheets have been well-known. Among theearliest reported designs are single- or multi-layer wire meshesand their complements [90]–[92] and arrays of metallic crossesor cross-shaped holes [91]. Until today, numerous terahertz filterdesigns have been proposed that are variations of these earlyconcepts. These include photonic band gap filters constructedfrom metallic meshes [93], arrays of holes of different shapesin metallic sheets [94], and a closely-spaced composite of ametallic mesh and a complementary mesh [95]. As a different,technologically more sophisticated approach for terahertz filters,microfabricated pillar arrays have been demonstrated [96], [97]which show a low-pass behavior in reflection and a high-pass characteristic in their transmission spectrum. Recently,several designs of filters with two or more stop bands havebeen suggested [98]–[100] as well as bandpass filters witha spectrally broad transmission [101]–[104]. In contrast tothis, THz filters based on resonances with high quality factorshave been presented, which can be used in applications wherenarrow-band filtering is required [105], [106].

A notable advanced design for efficient terahertz bandpassfilters is a metal sheet perforated with thin circular, squareor cross-shaped slots [75], [95], [107]. The metal area fillingfactor of these structures is between 85 % and 96 %. Despite thesmall free aperture, the transmission at the design frequency ofthese filters is extraordinarily high (over 80 % of the incidentamplitude). This can be explained by the excitation of so-called trapped modes [108] which couple only very weakly toincident radiation. Such trapped modes are known to also playa prominent role in a classical analogue of electromagneticallyinduced transparency [109], [110]. Figures 8(a)–8(e) show thedesign as well as the transmission and reflection characteristicsof a bandpass filter comprised of cross-shaped slots. The passband of the filter is centered at 1.3 THz. By stacking twoidentical filters, the frequency selectivity is further increasedwithout a significant decrease in the transmittance.

IV. METAMATERIAL-BASED INTEGRATED TERAHERTZPLASMONICS

It has been shown that interfaces between dielectric (ε > 0)and conducting materials (ε < 0) support confined electro-

(a) (b)

(c)

(d) (e)

Fig. 8. Metamaterial-based terahertz bandpass filter consisting of an array ofcross-shaped slots in a thin metal sheet [95]. (a) Unit cell of the metamaterial,(b) microscope image of a fabricated bandpass filter, (c) measurement geometry,(d) measured and calculated transmission spectra, (e) calculated reflectionspectra.

magnetic waves that are called surface plasmon polaritons(SPPs). These are of great interest in research, because theirpropagation properties depend on the nature of the boundarythat they are confined to. By means of metamaterials, theelectromagnetic properties of the boundaries can be designed atpurpose and thus allows unprecedented control over the spatialand spectral properties of SPPs with potential applications inthe field of subwavelength integrated optics [111]. Anotherapplication of SPPs is the development of sensors which utilizethe confinement of the electromagnetic energy and the extendedinteraction length to increase the sensitivity. Terahertz SPPshave been investigated experimentally for some decades [112]–[115].

Tight confinement of SPPs, which is essential for mostapplications, can only be obtained at frequencies not too faraway from the plasma frequency of the metal surface. Formost metals, the plasma frequency lies in the UV frequencyrange, which restricts the existence of laterally localizedSPPs to the visible and near infrared frequency ranges. Atlower frequencies like the terahertz range, the surface wavesare only very weakly confined to the interface and do notsignificantly differ from grazing plane waves, which limits theirusefulness considerably. The most obvious way to overcomethis limitation is to use surfaces with lower conductivities,such as doped semiconductors [115], [116]. A much more

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8 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. XX, NO. XX, APRIL 2012

(a) (b)

(c)

Fig. 9. Terahertz wave propagation on a structured metal sheet (THz spoofplasmon), as demonstrated by Williams et al. [120]. (a) Schematic of thestructured surface, (b) measurement setup, (c) numerical calculation of theelectric field distribution in the vicinity of the surface at a frequency of 1.3 THz.(Reprinted by permission from Macmillan Publishers Ltd: Nature Photonics[120], copyright 2008.)

flexible method to obtain strong confinement to a surfaceat low frequencies, however, is the structuring of a metalsurface with subwavelength holes or grooves, a method whichis even applicable for perfectly conducting metal surfaces[117], [118]. The holes lower the effective conductivity of thesurface and enhance the confinement of the electromagneticfields. The propagating waves on such structures have beentermed “spoof plasmons” in literature because of their similarityto SPPs. Using this approach, localized terahertz waves onstructured surfaces have been experimentally demonstratedrecently [119]–[121], as shown in Figs. 9(a)–9(c). In similarworks, terahertz waves were confined to structured metal sheets[122] or arrays of metallic cylinders [123]. Terahertz waveswhich are confined in both the in-plane and out-of planedirection were demonstrated as well [124]. Spatial variationof the size of the holes can be used to further design thepropagation properties of the surface waves [125], [126].

Thin metamaterial sheets, or metafilms, have been shown tosupport localized guided waves that are very similar to SPPs onmetal surfaces. Metamaterials offer more possibilities to designthe confinement, polarization, and dispersion of the surfacewaves. For example, in contrast to SPPs, which can only existwith an electric field vector normal to the surface, surface waveson metamaterials are not restricted in this way. Utilizing themagnetic resonance behavior of split-ring resonators, surfacewaves with the magnetic field perpendicular to the surfacehave been experimentally demonstrated in the microwaverange [127] and, more recently, at terahertz frequencies [128],[129] (Fig. 10). Furthermore, surface waves on metamaterialstructures composed of complemetary split-ring resonators havebeen reported [130], [131]. The high tunability of the effectiveparameters of meta-surfaces may pave the way to metamaterial-based optics for terahertz surface waves, which are an importantprerequisite for integrated terahertz wave circuits.

V. TERAHERTZ METAMATERIAL SENSING DEVICES

Because many materials have a specific optical response atterahertz frequencies, terahertz radiation is particularly suitablefor sensing applications [132], [133]. However, when it comes

zx

yx

Hz

+

0

(a) mode 1 mode 2

(b) mode 1 mode 2

Fig. 10. Magnetic terahertz surface waves on a metafilm composed ofsplit-ring resonators [128], (a) side view, (b) top view. The magnetic fieldis polarized perpendicular to the metafilm. The metamaterial supports twosurface modes which differ in the degree of confinement.

to the detection of very small amounts of a sample material,the signal change caused by the sample may be very small anddifficult to detect. In this respect, metamaterial-based sensingis a promising approach to increase the interaction betweenthe terahertz wave and a sample material and thus to increasethe change of the measurement signal. Due to the resonantnature of the constituting elements, metamaterials exhibit astrongly enhanced electromagnetic near field. In consequence,the resonance is strongly influenced by a change in the dielectricenvironment. This property can be exploited to devise sensorswith superior performance.

The functionality of most metamaterial-based terahertzsensors originates from the shift of a resonance feature inthe presence of a sample material. In capacitive resonators,this shift can be understood as an increase of the effectivecapacitance of the resonant elements of the metamaterial,which lowers the resonance frequency of the sensor. Thefrequency shift can be exploited as a measure that containsinformation about the sample material, such as the thicknessof a dielectric layer or the refractive index of a material.Because a frequency measurement is less prone to noisethan an amplitude measurement, metamaterial sensors mayexhibit superior performance regarding measurements in anoisy environment. Several metamaterial designs have beenproposed for the sensing of thin dielectric films or minuteamounts of sample material [134]–[143]. In Figs. 11(a)–11(g),a sensor consisting of split-ring resonators is depicted whoseresonance frequency shifts with the gradual thickness increaseof a silicon layer.

It has been shown that metamaterial designs on thinsubstrates and/or substrates with a relatively low refractiveindex display a superior performance when compared tometamaterials on conventional wafers (e. g. silicon) [138],[141]. A recent experimental demonstration revealed that deep-subwavelength thin layers of a sample material can be detectedwith metamaterial sensing devices even for sample materialsthat would not cause a significant signal change in a simpletransmission or reflection measurement. Layer thicknessesdown to λ/16000 could be resolved with metamaterial-basedsensors [143].

Asymmetric split-ring resonators show particularly narrowresonances in their transmission and reflection spectra becauseof the excitation of trapped modes in the resonator [108].A narrow resonance is advantageous for metamaterial-basedsensing because a small frequency shift can be more easily

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REINHARD et al.: METAMATERIAL-BASED PHOTONIC DEVICES FOR TERAHERTZ TECHNOLOGY 9

(a) (b)

(c) (d)

(e) (f)

(g)

Fig. 11. Metamaterial-based sensor for the detection of thin layers of silicon[136]. (a)–(f) Silicon is gradually added to the sensor, then removed. (g) Theresonance frequency of the sensor shifts in dependence of the silicon thickness.(Reprinted with permission from Ref. 136. Copyright 2007, American Instituteof Physics.)

detected if the width of the resonance feature is small, thus in-creasing the resolution of the metamaterial sensor. In a terahertzmetamaterial consisting of asymmetric SRRs, resonances withquality factors of up to 50 have been demonstrated [106]. Withan optimized design, the performance of metamaterial-basedsensors may be further improved.

VI. SUMMARY

In summary, we reviewed various aspects of the latestresearch developments in the field of terahertz metamaterialsand set them into context with respect to possible applicationsin terahertz technology. We don’t claim completeness of ourreview at any point since we are convinced that no concisereview article can satisfy this goal. We rather focused on a moredetailed description of a number of different advances which,in our opinion, open promising routes to extend the applicationfields of terahertz technology in conjunction with metamaterial-based devices. Facing the current limitations in the dataacquisition rate of state-of-the-art terahertz imaging systems, wehighlighted recent advances in the field of metamaterial-basedspatial light modulators and their application to compressedsensing which potentially promises a strong decrease of dataacquistion time and therefore paves the way to industrialapplications for terahertz imaging. Furthermore, we reviewedlatest progress in metamaterial-based terahertz optical com-ponents including polarization optics, gradient index optics,perfect absorbers, and optical filters. As a great advantage,all these components are extremely compact and are suitedfor the implementation in budget-priced, compact terahertzmeasurement systems. Furthermore, we discussed the role ofmetamaterial-based integrated terahertz plasmonics that enablesthe conception and implementation of integrated plasmoniccircuits on a chip. Due to a high spatial concentration of theterahertz fields near the surface of such chips, this technologypromises the realization of highly sensitive sensor circuits. Asa last example, we discussed the performance of terahertzmetamaterial sensing devices that allow the measurementand sensing of substances in an extremely thin layer whosethickness can be 1/16000 of the wavelength of the terahertzradiation.

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12 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. XX, NO. XX, APRIL 2012

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Benjamin Reinhard received the Diploma degreein physics from the University of Kaiserslautern,Kaiserslautern, Germany, in 2007.

He is currently involved in the investigation ofplasmonic sensing devices and terahertz surfacewaves on meta-surfaces.

Oliver Paul received the Diploma degree in physicsand the Doctorate in natural sciences from the Uni-versity of Kaiserslautern, Kaiserslautern, Germany,in 2006 and 2010, respectively.

The central topic of his PhD research was theinvestigation of plasmonic metamaterials in the THzfrequency range. In his studies, he focused on therealization of negative refractive media, tunable meta-materials, gradient index optics and metamaterial-based photonic devices for the THz technology. From2010 to 2012 he was a postdoctoral fellow in the

junior research group “Metamaterials and Transformation Optics” of Jun.-Prof. Dr. Marco Rahm at the university of Kaiserslautern, Germany, wherehe worked on cloaking devices and the transformation optics of nonlinearmedia. In 2012 he joined the research department of the Carl Zeiss AG inOberkochen, Germany.

Marco Rahm received the Diploma degree inphysics and the Doctorate in natural sciences fromthe University of Kaiserslautern, Kaiserslautern, Ger-many, in 2001 and 2006, respectively. From 2006 to2008, he was a post-doctoral fellow in the researchgroup of Prof. David R. Smith in the Departmentof Electrical and Computer Engineering at DukeUniversity, Durham, North Carolina (USA). In 2008,he was appointed a junior professor (equivalent toan assistant professor) in the Physics Department ofthe University of Kaiserslautern. At the same time,

he was granted a Fraunhofer Attract Award of the Fraunhofer Associationto work with the Department of Terahertz Systems and Measurement inKaiserslautern, Germany, a branch of the Fraunhofer Institute for PhysicalMeasurement Systems IPM, Freiburg, Germany. In 2012, he received a call fora full professorship in the Department of Electrical and Information Technologyat the University of Kaiserslautern, Kaiserslautern, Germany. His researchinterests include artificial electromagnetic materials, transformation optics andthe application of metamaterials to terahertz technology.