Active metasurfaces

Active metasurfaces a Ada-Simona Popescu* , Tamelia Alia, Igor Bendoymc, Simeon Bikorimanaa, Roger Dorsinvillea, Linda Marchesed, Alain Bergerond, Marc Terrouxd, Andrii B. Golovina,b, and David T. Crousea,b a Department of Electrical Engineering, The City College of New York, New York, NY, USA 10031 b Center for Metamaterials, 160 Convent Avenue, Steinman Hall, New York, NY, USA 10031 c Phoebus Optoelectronics, New York, NY, USA 10031 d Institut National d'Optique INO, Quebec, Canada, G1P 4S4 ABSTRACT An innovative method of examining properties of metasurfaces is presented. A pump-probe technique is used to create a metasurface composed of conductive shapes on a silicon surface. A wave-front of intense pulse of 82 fs from Ti:Sa laser with wavelength of 800 nm is shaped by a spatial light modulator and then focused into a preprogrammed array of vshaped features on a high purity float zone silicon substrate. The laser pulse generates electron-hole pairs on the silicon substrate, thus a metasurface consisting of an array of metal-like v-shaped antennas is inscribed on the silicon substrate. The lifetime of v-shaped antennas is in millisecond time range. In the meantime, the second, less intense pulse, also of wavelength 800 nm is converted to a pulse of terahertz radiation with a peak-power at wavelength approximately 800 m and used to probe the metasurface inscribed in the silicon. Tracing the position of the refracted terahertz beam is achieved with a specially designed INO video camera for terahertz radiation. Keywords: metasurface, pump-probe technique, spatial light modulator, terahertz radiation, terahertz video camera 1. INTRODUCTION The field of metamaterials has been gathering increasing attention due to its promise of being able to control light in extraordinary ways that are impossible with conventional optical materials. One important subset of metamaterials is metasurfaces. As the name implies, they are composed of 2D arrays of surface structures which work together to control light. Compared to 3D metamaterial structures, metasurfaces have the benefit of being single layer structures that are easier to fabricate and integrate to advanced photonic systems. The objective of this project is to explore techniques of dynamic lithography of metasurfaces inscription and applications for control over the terahertz radiation. For this purpose, one can use the so-called pump-probe technique. The 800 nm laser pulse is split into two beams: “pump” and “probe”. The pump beam has its wavefront shaped by a spatial light modulator (SLM) and then projected onto a high purity float zone (FZ) silicon substrate. The SLM is programmed to modify the wavefront into the desired pattern, a metasurface composed of v-shaped antennas. In the meantime, the probe pulse is converted into a THz pulse, which interacts with the metasurface constituted of the v-shaped antennas. Relevant applications of this project can go into a few separate directions. One engineering subject receiving much attention lately has been the applications in terahertz domain. In this realm, the most important applications are the beam forming and beam steering. Some of the more particular applications starting from the general concept of beam steering are: directed energy for remote sensing, remote charging and imagining radars, which can be very valuable for operations such as guided descent or advancing through visually degraded environments. 2. TERAHERTZ METASURFACE 1 In 2011, Yu et. al. reported a metasurface structure which allowed for a point-by-point control of the phase shifts which an incident beam accumulates upon transmission through a patterned surface. The metasurface was constructed of an array of micro/nano antennas, similar to the one depicted in Fig.1. The antennas located on the surface are shaped and oriented in particular ways to control light by using a coordinate dependent phase profile Φ=Φ(x). The concept of a generalized law of reflection and refraction was introduced and mathematically described by the Eq.(1)1: *[email protected]; phone 1 212 650-7280; fax 1 212 650-7058; http://www.centerformetamaterials.org Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications VII, edited by Laurence P. Sadwick, Créidhe M. O'Sullivan, Proc. of SPIE Vol. 8985, 89850Z 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2040372 Proc. of SPIE Vol. 8985 89850Z-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/06/2014 Terms of Use: http://spiedl.org/terms sin sin 2 Φ , 1 where is the t wavelength h of light, and a are the incident angle and the refracctive index of the superstratee, and and are thhe refraction an ngle and the reffractive index of o the substratee, respectively.. The left handd side of this equation is, as opposed o to thee traditional reppresentation off Snell’s law off refraction, a nonzero, n dΦ position depeendent value. The term m on right handd side represennts the accumuulations of phaase along the interface i dx between the superstrate an nd substrate. The proposed array composed of the v-shaped antennnas of sub-wavvelength dimensions is i capable of producing p the traditional refflection and refraction as weell as steering light in an annomalous direction, as depicted in Fig g.2(a). o sub-wavelenggth v-shaped anttennas, which inntroduces a certaain amount of coordinate depenndent Fig.1. Arrray composed of phase acccumulation Φ across its surfaace1. ( (a) (b) Fiig.2. Optical tracces of normal annd anomalous reffraction (a) and its i 3D perspectivve illustration (bb). 2.1 gn of v-shaped d Antennas Modelling and Desig In order to model m the operration of the v-shaped antennnas, High Freqquency Structuural Simulator (HFSS) from ANSYS was employeed. The antenn nas were charaacterized indivvidually and thhen the 2D struucture of metaasurface was modeled, m which demonnstrated that it was capable to produce a controllable reflection r and refraction, inccluding the annomalous ones. The buildingg block of the metasurface was w a unit celll composed off eight differennt v-shaped anttennas. Each v-shaped v antenna wass a sub-wavelength resonatoor with a cerrtain resonant frequency annd phase shift.. Across the resonant frequency innterval, each v--shaped antennna is capable of o introducing the 180 degreees phase shift, as depicted inn Fig.4a. This phase shift is the phaase difference between b the exxcitation planee wave and thee scattered wavve emanating from f the s must be b designed wiith each antennna producing a phase shift off π/4 radians, reelative to antenna. Thee metasurface structure the neighboriing antennas, as a depicted in Fig.3. F In orderr to achieve thee phase shift of π/4 radians inn between neigghboring antennas, parrametric sweep ps of the arm leength of v-shapped antennas were w performedd. Proc. of SPIE Vol. 8985 89850Z-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/06/2014 Terms of Use: http://spiedl.org/terms Fig.3. Unnit cell of the metasurface m arrayy. Each v-shaped antenna introdduces an additioonal π/4 phase shift s relative to the chevron before b it1. In order to acccurately modeel the metasurffaces, real expeeriment conditiions were takeen in an accounnt. The first steep was to model the anntennas with air a as a surrouunding medium m. For the firsst two antennaas from Fig.3, simulation results are presented in Fig.4. A moree realistic moddel was simulaated by incorpoorating the silicon substrate on o which the antennas a would be insscribed, Fig.5. The antenna magnitudes m andd the phase shiffts were shiftedd after adding the silicon subbstrate to the simulatioons. leo antenna -75.00 400.00425.00 - 150.00- / 1 ¢.n5.001800.00- t825.00-250.00 -275.00 300.00 190.00 230.00 (a) (b) Fig.4. HF FSS snapshots off the v-shaped antennas a resonannces when havinng the antennas surrounded s by vacuum. v Straightt-line antenna (a) ( and v-shapeed antenna withh 60 degrees anngle between the arms (b). Sollid and dashed lines corresponnd to amplitudee and phase shift ft, respectively. As mentioneed previously, the t magnitude and phase shift of the antennnas must be acccurately tailorred in order too provide full control over o the refractted wave. Systematically, eacch v-shaped anntenna was sim mulated by varyying dimensionns, while monitoring thhe magnitude and phase shifft of the resonance. This alloowed for the selection of thee dimensions of o the all eight v-shapeed antennas su uch that they exhibit e almost similar scatteering amplitudees and the π/44 radian phase shift, to achieve contrrollable refracttion on the mettasurface. fiOVaMpalMilano 0.00100 in 10&0300 0m13 m3 10&0300 ABn1 uunnn Antenna .a00090 ÿanoau- s 10.00090 - saooaugauunu- auunuauunu3 00 110m 115.00 NO 166.00 06.00 110A0 WW1 lumi 10Óm 10Ó.m0 110 00 11000 Ann1m001 lumi (a) (b) Fig. 5. Zoomed Z images of the antennaa resonances. Sttraight-line anteenna, named as 180 because teechnically the angle a between v arms is 180 degrees d (a) and v-shaped antennna with 60 deggrees between thhe arms (b). Sollid and dashed lines mplitudes and phhases corresponnd to the amplittude and the phaase shift, respecctively. The marrks m1 and m2 demonstrate am where thhe phase difference between radiation scatteered by the strraight-line and 60 degrees v-shaped antennaas is approxim mately π/4 radian n. Proc. of SPIE Vol. 8985 89850Z-3 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/06/2014 Terms of Use: http://spiedl.org/terms The next steep was to mod del the complette unit cell as an infinite arrray. The metassurfaces are caapable of anom malously refracting and reflecting thee incident lightt. The anomaloous reflection and a refraction, depicted in Figg.6, occurs only for the component off the electric field is depictedd in Fig.6. x componentt of the electricc field1. The Fig.6. Annomalous interacction of light with w v-shaped anttennas metasurfface: the wave (not shown in thhe picture) incom ming from the bottom is split in i two waves whhere one is anom malously refractted into silicon substrate s (above the dotted line)) and other onee is reflected in air-superstrate a (bbelow the dotted line). 3 3. EXPER RIMENTAL L SETUP The basis of our optical settup is a pump-pprobe techniquue. The pump beam b was the stronger s beam,, while the probe beam is the weakerr one. The stronger beam is used u to “write” the pattern off the v-shaped antennas a on thee silicon substrrate. The weaker beam m is used to gen nerate THz radiation in order to probe the innscribed metasurface. E dSSSL ........................ RR Fig. 7. Experimental E setu up: Laser is a Ti:Sa T pulsed laser, M1 - M6 arre silver coated flat mirrors, M77 and M8 are silver coated paarabolic mirrorss, BS is a beam m splitter, L1 and a L2 are lenses, PCA is a photoconductive antenna, RR is a retrorefleector, P1 and P2 are polarizers, Si S is a silicon subbstrate, SLM is a spatial light modulator, m THz camera c is a uncoooled bolometeer based video caamera. Proc. of SPIE Vol. 8985 89850Z-4 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/06/2014 Terms of Use: http://spiedl.org/terms The Ti:Sa laser beam at 80 00 nm is split by b the beam spplitter BS into two beams, puump and probee, as depicted in Fig.7. LM (BNS, Coloorado, USA). The T SLM channges the wave front of The wave froont of pump beam is modulaated by the SL the beam succh that the patttern of intensitty on the silicoon sample has the appearancce shown in Figg.1. The actuaal area of the v-shapedd antennas is illluminated, whhereas the rest of the area iss not. The enerrgy of the phootons forming the t laser beam is highher than the ban nd gap of silicoon. Therefore, electron-hole pairs p are generrated in the regions where raddiation is present. How wever, the featu ures become metal-like m only if i the intensity of the radiatioon is sufficientlly high. The liffetime of these carriers is short, beiing limited by the exposure of the siliconn substrate to the t radiation and a the recom mbination s Duringg the time intterval, when these t carriers are present, the t v-shaped antennas a lifetime charracteristic to silicon. demonstrate metallic-like properties, p beinng able to support a current duue to the free charges. c The probe beeam is less stro ong than the puump beam, carring only 10% of the total poower of the oriiginal laser beaam. This beam is usedd for the generration of the teerahertz pulse, which is achiieved with a phhotoconductive antenna (PC CA) from Batop Optoeelectronics, Gerrmany. The TH Hz radiation iss collimated affter reflection from f the parabbolic mirror M7. M Then, the plane waave of teraherttz radiation intteracts with thhe v-shaped anntenna structurre inscribed onn the silicon suubstrate. After refractiion on the metaasurface, the parabolic mirrorr M8 focuses terahertz beam to the video caamera. A key detaill of this techn nique is to illuuminate the sillicon wafer with the teraherrtz beam exacttly after “writting” the antenna patteern and beforee those carrierss get diffused in the neighbooring areas. In order to fulfilll this requirem ment, the paths of bothh beams should d be approximately equal. Thhus, for the puurpose of balanncing the optical lengths of thhe pump and the probee beams, a delaay line with a retro r reflector (RR) ( is used. 3.1 Dynam mic lithograph hy The above deescribed technique was suggestively calledd “dynamic lithhography”. In essence, e it allow ws for metasurrfaces to be optically created c and ch haracterized in a timely fashion. In what follows, the techhniques, proceddures, and calcculations employed in our experimen nts are describeed. 3.1.1 Saxton3 algoritthm Thee Gerchberg-S The Gerchbeerg-Saxton algo orithm allows one o to calculate the phase maap needed to bee programed innto the SLM inn order to achieve the pattern p of inten nsity corresponnding to 2D strructure of v-shhaped antennass in the plane of o the silicon suubstrate. 3 It is a recursive algorithm, based on Fastt Fourier Transsform (FFT) off Cooley and Tukey T , which is implementeed in our programs. The algorithm m starts by assuming a randoom phase map at the SLM plaane, with givenn pattern of inttensities in the plane of the silicon suubstrate and thee input image, as depicted in Fig.8. After seeveral iterationns, the phase map m at the SLM M plane is reshaped succh that evaluatted intensity map m in the plaane of silicon substrate matcches the desireed pattern of v-shaped v antennas. Fig.8. Diiagram of the optical o setup whhich allows for the projection of the v-shappeed antenna patteern onto the sillicon substrate in the setup dep picted in Fig.7. SLM is the spaatial light modullator, f is the foocal length of leens L2, and Si iss the silicon suubstrate. Proc. of SPIE Vol. 8985 89850Z-5 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/06/2014 Terms of Use: http://spiedl.org/terms 3.1.2 Spaatial light mod dulator The spatial light l modulato or (SLM) from m Boulder Nonnlinear System ms (Lafayette, Colorado) useed in our experiments operates in reeflection modee. Technical paarameters of SLM are as nexxt: the pitch off pixels is 15x115 μm , the nuumber of pixels in the array is 512x512 pixels, the spatial resolutiion is 33l p/mm m and the arrayy size is 7.68x77.68 mm . Thee SLM is =800 nm: the face f surface of the SLM is cooated with antireflection coatting with optimized for operating at wavelength λ= maximum traansmission on 800 nm and the t backplane of the SLM iss coated with a dielectric mirrror with maxiimum of reflection alsso on 800 nm. The most im mportant limitattion introducedd by usage of the SLM is thhe appearance of the zeroth order o of diffraction. In order to redduce the intensity of the zeeroth order, a series of calibration proceedures were conducted prioor to the experiments with the opticcal setup depiccted in Fig.7. Our O calibrationn procedures included i measurements of thhe phase retardation measured m as fu unction of graay level for tw wo angles of incident i beam m, namely 0 annd 9.5 degreess. These calibration prrocedures weree performed inn two optical seetups that incluuded Michelsonn and Mach-Zeehnder interferoometers. 3.1.2.1 Macch-Zehnder In nterferometerr Our first caliibration proced dure with the Michelson M inteerferometer repproduced the sttandard calibraation setup useed by the manufacturerr company (B BNS), which allows only measurementss with incideent angle 0 degree. d Howevver, our experimentall setup in Fig.7 7 requires a sm mall incident anngle (approxim mately 9.5 degrees). Thereforre, the Look-U Up-Table (LUT)5 generated by using measurementss from Michelsson interferometer is not accuurate for our exxperimental seetup. The high contrast of v-shaped antennas patttern on siliconn can be only achieved by using the LU UT generated by b using measurementts from the in nterferometer setup s with sam me incident anngle. In order to generate thhe new LUT, a MachZehnder interrferometer setu up with the SL LM was assembbled, Fig.9. Fig.9. Caalibration setup with w Mach-Zehnnder interferometter: laser is a minni cw laser diodee operating at wavelegnth 7 785 nm, M1, M2, M3 are flatt mirrors, L1 andd L2 are lenses of o spetial filter, Ph is a pinhole, I is an aperture iris, P1 and P2 are two polarrizers used to atttenuate intensity of light, BS1 annd BS2 are beam m splitters, SLM is a spatial lightt modulator and BP is a beam m profiler. In Fig.9, thee SLM introdu uces a variablee phase shift too the linearly polarized inciddent wave by changing an effective e refractive inddex of the liqu uid crystal4. Thhe phase shift of o the SLM waas calibrated as a a function off the gray leveel, which represents thhe amplitude of the applied voltage v in the SLM’s softwaare. For these calibration c meaasurements, grray level patterns weree generated, where w the right hand side of the image wass kept constantt at a gray level 0, while forr the left hand side thee gray level waas varied, from 0 to 255, as deemonstrated inn Fig.10. Proc. of SPIE Vol. 8985 89850Z-6 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/06/2014 Terms of Use: http://spiedl.org/terms Fig.10. Examples E of threee patterns, whichh demonstrate thhe fixed gray levvel 0 on the rightt hand side and have h gray levels of 0 (left), 1222 (center), and 255 (right) all on the left hand sidde. The interfereence pattern after a Mach-Zehnder interferometer was reecorded with a beam profiller BC 106-V VIS from Thorlabs. Figg.11 demonstraates three expeerimental pictuures of interferrence patterns captured whenn the SLM waas loaded with three im mages depicted in Fig.10. For each interferennce pattern thee phase shift beetween the fringes was measuured. (a) (b) (c) Fig.11. Innterference patteerns captured witth Mach-Zehndeer interferometerr when SLM wass loaded with paatterns from Fig.10. The phase shhifts between the two sides of the interferrence patterns were measureed and recordeed for all the 255 2 gray levels and then converted to o the phase shiift plot depictedd in Fig.12. The phase shhifts introduced d with normal and a tilted inciddent beams are different, as illlustrated in Figg.12(a). The maximum m phase shift was w of 4.9π rad dians in the caase of the Michhelson interferoometer and 4.771π radians in the case of thhe MachZehnder interrferometer. (a) (b) Fig.12. Phhase shift versus applied gray levvel based on thee data collected with the beam profiler p for the Michelson M and MachM Zehnder innterferometer settups with waveleengths 785 nm cw c laser (a) and 800 8 nm pulse lasser (b). From these phase p measureements we werre able to creaate a 16-bit LU UT5, used to control c the phaase shift from 0 to 2π. Similar calibbration procedu ure was perform med for Mach-Zehnder interfferometer equippped with Ti:S Sa pulse laser operating o at the waveleength of 800 nm m, Fig.12(b). Proc. of SPIE Vol. 8985 89850Z-7 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/06/2014 Terms of Use: http://spiedl.org/terms 3.1.2.2 Corrrection coeffiicients In order to fuurther reduce th he intensity off zeroth diffracction order, we calculated a set of correctionn coefficients by b using the procedure described by y Ronzitti, et. al a 6. Once generrated, these corrrection coefficcients were applied to the phhase map generated wiith the Gerchbeerg-Saxton algoorithm, which was to be loadded in the SLM M, as discussed in section 3.1.1. Vertical, horrizontal and ch heckerboard gratings g with high h spatial freequencies suchh as 2, 4 and 6 pixels period were generated and loaded into the t SLM. For these t gratings, one half of thee period was keept with constaant phase retarddation Φ and the phasee of other half of the period was w varied from m Φ to Φ+2π. The intensitiess of the zeroth order and first order of diffraction foor all these graatings were theen measured, which w allowed for the assessm ment of the difffraction efficieency and cross talk between the pixeels. These valuues were used to t calculate thee correction cooefficients thatt allow for maxximizing diffraction effficiencies. By applying this correction to thhe phase profilles of the gratinngs, the intensity of zeroth orrder was minimized doown to 4.75%, as depicted inn Fig.14. A comparisoon of the v-shap ped antennas pattern p recordedd with the beam m profiler is prresented for thrree different sittuations: using the BN NS’s LUT, LU UT created withh Mach-Zehndder interferomeeter, and lastly,, applying the correction coeefficients and LUT created with Mach-Zehnder inteerferometer. III u (a) (b) (c) Fig.13. Im mages of the v-sshaped antenna pattern p recorded with the beam profiler p after appplying: BNS’s LUT(a), L LUT creeated with Macch-Zehnder interrferometer(b), LU UT created with Mach-Zehnder interferometer and a correction cooefficients (c). A comparisoon of the imprrovements intrroduced to thee zeroth order of diffractionn order in the form of a diaagram is illustrated in Fig.14, wheree the ordinate axis a representss the intensity of o the zeroth order o of diffracction normalizeed to the total intensityy of the laser beam. b Verticall bars in Fig.144 correspond to t the intensities of zeroth orrders of the difffraction patterns depiicted in Fig.13.. Fig.14. Comparison between the inteensity of zeroth order diffractionn in cases illustraated above situaations. Proc. of SPIE Vol. 8985 89850Z-8 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/06/2014 Terms of Use: http://spiedl.org/terms hertz radiation n Terah 3.2 3.2.1 Gen neration of terrahertz radiattion The photocoonductive anteenna PCA-44--06-10-800-h used u for terahhertz generation in our exxperimental setup was purchased froom BATOP Op ptoelectronics,, Germany. The wavelength of o laser excitation for this PC CA model is λ≈ ≈800 nm. The arm lenngth of the PCA P is 44 μm m and the gaap in betweenn arms is 6 μm. μ The PCA A is equippedd with a hyperhemisppherical silicon lens to initially collimate terrahertz radiatioon. Fig. 15. Schem matic of the PC CA antennas useed to generate terahertz radiaation. LT-GaAs is i low temperatuure GaAs. Fig.16. Spectrum off terahertz signnal emitted by the PCA antennna. This specttrum was meaasured with a detection schem me, which includded a second PC CA antenna as deetector. The short puulse from Ti:S Sa laser was focused f into thhe gap of the antenna by leens L1 (Fig.7). The pulse radiation r generates carrriers in the area a between thhe two contaccts of the antennna, therefore closing the circuit. Concurrrently, a voltage is applied to two co ontacts of the antenna a allowinng for the carriers to be accelerated. The cuurrent generateed in this way radiates a terahertz waave. This lens allows for alm most all the teraahertz radiationn emitted by thhe PCA to be collected c and used. Thhe parabolic miirror M7 was inntroduced in thhe optical setupp to collimate the terahertz raadiation. 3.2.2 hertz radiation Detection of terah Detection off the terahertz radiation r is perrformed with a terahertz cam mera IRXCAM M-INO384 desiggned and fabricated by Institut Natioonal d'Optiquee (INO), Canadda7. The mainn component of o the camera is i the uncooleed bolometric terahertz t detector that is used with th he resolution off 640x480 pixeels. (b) (a) Fig.17. Phhoto of the terah hertz video cameera7 (courtesy of o INO, Canada)) (a) and snapshhot of terahertz beam generatedd by PCA antenna (bb) used in the exp perimental setupp. Proc. of SPIE Vol. 8985 89850Z-9 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/06/2014 Terms of Use: http://spiedl.org/terms 4. DISCUSSION In this stage of our experiments, the beam steering produced by the v-shaped antenna pattern excited with the terahertz radiation is not demonstrated yet. However, the limitation introduced by the SLM, namely the zeroth order of diffraction, is minimized. Currently, our efforts are concentrated on the generation of the v-shaped antennas on surface of silicon. The most promising feature of this method is dynamic lithography of a metasurface on a semiconductor substrate which will allow for the non-mechanical beam steering of terahertz radiation. 5. CONCLUSION Current stage of experiments with the dynamic lithography of metasurfaces for beam steering applications in the terahertz range of radiation is presented. ACKNOWLEDGEMENTS This work was supported by the NSF Industry/University Cooperative Research Center for Metamaterials (IIP1068028). The authors thank Isroel Mandel for his help with HFSS simulations. 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[7] Chavalier, C., Mercier, L., Duchesne, F., Gagnon, L., Tremblay, B, Terroux, M., Ge ń e ŕ eux, F., Paultre J.-E, Provencal, Desroches, Marchese,L., Jerominek, H., Bergeron, A., ” Introducing a 384×288 Pixel Terahertz Camera Core”, Proc. SPIE 8624, Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications VI, 86240F (2013). Proc. of SPIE Vol. 8985 89850Z-10 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/06/2014 Terms of Use: http://spiedl.org/terms