Packaging consideration of two dimensional polymer-based. photonic crystals for laser beam steering

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1 Pckging considertion of two dimensionl polymer-sed photonic crystls for lser em steering Xinyun Dou, Xionn Chen, Mggie Yihong Chen, IEEE Senior Memer, Aln Xiolong Wng, Wei Jing c, Ry T. Chen *, IEEE Fellow Microelectronics Reserch Center, Deprtment of Electricl nd Computer Engineering, University of Texs t Austin, Austin, TX, 78758, USA Omeg Optics, Inc. Austin, TX c Deprtment of Electricl nd Computer Engineering, nd Institute for Advnced Mterils, Devices, nd Nnotechnology, Rutgers University, Pisctwy, NJ *Emil: chen@mil.utexs.edu ABSTRACT In this pper, we report the theoreticl study of polymer-sed photonic crystls for lser em steering which is sed on the superprism effect s well s the experiment friction of the two dimensionl photonic crystls for the lser em steering. Superprism effect, the principle for em steering, ws seprtely studied in detils through EFC (Equifrequency Contour) nlysis. Polymer sed photonic crystls were fricted through doule exposure hologrphic interference method using SU The experiment results were lso reported. Key words: photonic crystls, em steering, superprism effect, hologrphic interference 1. INTRODUCTION Photonic crystls re rtificil periodic dielectric structures to control light propgtion [1]. Recently method hs een demonstrted for controlling the propgtion of light inside photonic crystl, involving the mnipultion of the nisotropy of the nds [2,3]. Due to this nisotropy, the propgtion direction of light inside photonic crystl cn e extremely sensitive to the prmeters such s the wvelength or the incident ngle of the light em. This effect, known s the superprism phenomenon, is oserved, where nisotropy in photonic nd structure is strongest nd it ehves like negtive refrction. Superprism effect is one of the unusul properties of the photonic crystls, which could led to lrge chnge of propgtion direction of the light em within the photonic crystls due to smll chnge of the incident em, such s its direction or the wvelength. A prism mde up of photonic crystl would hve dispersion cpility 500 times stronger thn prism mde of conventionl crystl [2]. Vrious theoreticl predictions nd experimentl studies hve een reported regrding nomlous ngulr devition t high frequencies ner the photonic nd gp [4-6]. In this work, we will consider superprism configurtion where the input nd Photonics Pckging, Integrtion, nd Interconnects IX, edited y Alexei L. Gleov, Ry T. Chen, Proc. of SPIE Vol. 7221, SPIE CCC code: X09$18 doi: Proc. of SPIE Vol

output surfces re perpendiculr to ech other. Here we give detiled theoreticl study of this effect through EFC (Equi-frequency Contour) nlysis for the polymer sed photonic crystls.

MODELING AND SIMULATION The lyout of the polymer sed 2D photonic crystls with n re of 100 100 is shown in Fig. 1, where is the lttice constnt of the PC structures.

6, which corresponds to the index of the SU8 polymer we use t the wvelength of interest. The 50 ngle corresponds to the recording result during the experiment.

2 output surfces re perpendiculr to ech other. Here we give detiled theoreticl study of this effect through EFC (Equi-frequency Contour) nlysis for the polymer sed photonic crystls. The friction procedures of the photonic crystls re illustrted nd the superprism effect-sed em steering is demonstrted. 2. MODELING AND SIMULATION The lyout of the polymer sed 2D photonic crystls with n re of is shown in Fig. 1, where is the lttice constnt of the PC structures. In our simultion, the input em wvelength λ is 1.55μm nd the lttice constnt is 1.0μm (λ=0.6452), the mteril index n is ssumed to e 1.6, which corresponds to the index of the SU8 polymer we use t the wvelength of interest. The 50 ngle corresponds to the recording result during the experiment c d TE pitf ds Y 2.82 Fig.1. () () Lyout of the photonic crystls in xoz plne. (c) 3D nd structure of the polymer sed photonic crystls. The x,y xis represents k x nd k z, respectively. z represents the frequency. (d) The EFC (Equi-frequency Contour) t certin freqeuency(ω2πc=λ=0.6452) for nd 3. Fig.1c shows 3D view of the first four nds of the two dimensionl polymer sed photonic crystls. The lowest nd is nd 0, then nd 1, nd 2 nd so on, indicted y numers in Fig.1c. The EFC (Equi-frequency Contour) cn e chieved y intercepting the 3D nd structure y certin frequency plne which is perpendiculr to the Z-xis. In the EFC nlysis (Fig.1d) we re interested in the nd 3(Fig.1d) ecuse it corresponds the em wvelength t 1.55μm nd it hs the highest possiility of superprism effect. Proc. of SPIE Vol

3 I, IF equfrequen nds 0-6 kout Contours: cof2,toa Bnd c krr I nd interfoes Fig. 2. Anlysis of the refrction process when the input nd output surfces re prllel. In Fig.2, we show the EFC nlysis for the two prllel interfces. k in is the input wve vector, k pc is the wve vector inside the photonic crystls, k out is the output wve vector. The smll nd ig circles re the ir contour nd medium contour, respectively. If the medium is lso ir, then the two circles re the sme. The output wve vector is given y the conservtion lw of the tngentil wvevector component t the interfces [5]. Therefore the finl results of the input nd output wve vectors stisfy the Snell s Lw. In this wy, the photonic crystl superprism effect is cncelled y the two prllel interfces. To void this effect, we hve to use the photonic crystl effect using two nonprllel interfces of the photonic crystls. IF eqifreqoetcy plot t eods 0-6 TE equifreqerc p'ot o erds 0-6 ittetfeoe interfce Fig.3 EFC nlysis for () the input ngle chnge nd () input wvelength chnge In Fig.3, we show the EFC nlysis of the refrction process, or the superprism effect process with two perpendiculr interfces. The lser em first enters the photonic crystls re from the ir or medium. After two refrctions, the lser em enters the ir or medium gin. The input or 1 st nd the output or the Proc. of SPIE Vol

4 2 nd interfces re perpendiculr to ech other. k in is the input wve vector, k pc is the wve vector inside photonic crystls. k out is the output wve vector fter the 2 nd interfce. k in nd k pc hve equl k x component due to the momentum conservtion t the 1 st interfce. k pc nd k out hve equl k z component due to the momentum conservtion t the 2 nd interfce. Momentum conservtion is indicted y the dshed line in the figure. When the input ngle or wvelength is chnged, the corresponding wve vector k in will e chnged, so will k pc e chnged. k out will lso e chnged ccording to the momentum conservtion. In this nlysis, we cn find the exct cross point of EFC contour with the conservtion dshed lines, so we cn find the output ngle chnge. Tht is lso the em steering principle. In Fig.4, we gve the simulted em steering ngle curves with respect to the input ngle nd input wvelength. Both of these two curves re chieved t round 1.55μm wvelength, 10º input ngle. From these two curves, we cn see when the input ngle chnges 2º, the output ngle cn chnge round 2.5º. When the input wvelength chnges round 30nm, the output ngle cn chnge round 4º Input Angle Chnge Input Wvelength Chnge 20.0 Output Angle(deg) Output Angle(deg) Input Angle(deg) Input Wvelength (nm) Fig.4 The output ngle chnge vs () input ngle chnge nd () input wvelength chnge. 3. PHOTONIC CRYSTAL FABRICATION In order to demonstrte the lser em steering, we fricted the two dimensionl polymer sed photonic crystls through doule-exposure hologrphic interference method [7-8]. The photoresist we used in the friction is photoresist SU nd the lser is He-Cd lser t 325nm. A mirror ws used to crete the interference pttern. In order to crry out the em steering test using lrge collimted em(1-2mm dimeter), we mde the photonic crystls prllel stcked with respect to the glss sustrte. The scheme in Fig. 5 shows the complete friction process for the horizontlly stcked photonic crystls. A pttern of Cr ws deposited to lock UV light to open trenches for the developing process. First Cr-ptterned glss sustrte ws prepred using photolithogrphy nd lift-off technique. After coting the dhesion promoter, film of 10μm thick SU ws spin-coted on the glss sustrte. After king the smple y rmping the hotplte from 65ºC to 95ºC, the doule exposure ws crried out t 325nm lser em. Post exposure ke ws t the sme condition s the pre-ke. At lst the smple ws developed t room temperture. Proc. of SPIE Vol

Glss sustrte Cot PR nd do the photolithogrphy

100nm Cr Remove PR Post-ke develop nd Fig.

photonic crystl on the glss sustrte. Fig.

2D photonic crystl structures with fether size round

Fig. 6 () low nd () high mgnifiction view SEM

5 Glss sustrte Cot PR nd do the photolithogrphy Spin-cot SU8 Bckside doule exposure t 325nm Cot 100nm Cr Remove PR Post-ke develop nd Fig.5 The scheme of friction of horizontlly stcked photonic crystl on the glss sustrte. Fig.6 shows the SEM pictures, which show the period of 2D photonic crystl structures with fether size round 1.0μm. E F-Bem Spot Mg Det FWD Tilt Scn 2 pm 10.0 kv kx TLD-S S H c y d (0,1) (0,0) (0,1) x (0,-1) z (0,0) (0,-1) Fig. 6 () low nd () high mgnifiction view SEM pictures of the horizontlly stcked photonic crystl structures fricted using SU8. (c) The schemtic lser diffrction setup. (d) The 442nm lser diffrction pttern of the fricted photonic crystls. Proc. of SPIE Vol

6 Fig.6c shows the schemtic view of the lser diffrction setup. Fig.6d shows the 442nm lser diffrction pttern. From the diffrction pttern, we cn clerly see the center spots, which is corresponding with the zero order (0,0) em diffrction nd the two first orders (0,1) nd (0,-1) order. The first order diffrction ngle t 27.2º indictes the period of the photonic crystls to e 0.97μm, which is consistent with the simultion. The periodicl photo polymer elts with period of 30um lso hve grting effect, which produces the discrete spots horizontlly in z direction (Fig.6c,d). The photonic crystl diffrction effect is in the verticl y direction, which ppers in line of weker spots on the top (0,1) nd ottom(0.-1) of the screen. Bems (0,1) nd (0,-1) re symmetricl round the input em nd hve the sme intensity. In this wy, we hve successfully chieved the horizontlly stcked 2D polymer sed photonic crystls structures through doule-exposure hologrphic interference. Note tht this diffrction experiment with input nd output surfces prllel to ech other ws solely intended for the confirmtion of photonic crystl lttice structure. The superprism effect must e investigted with perpendiculr input nd output surfces nd the detils will e presented in the next section. 4. MEASUREMENT RESULTS The em steering experiment ws crried out t 1.55μm. The setup is shown in Fig.7. Fig.7 is the schemtic view of the opticl em steering setup. First, the input em enters the glss sustrte, nd then the photonic crystl region. Some prt of the em will e reflected t the glssir interfce nd some prt will directly pss through the photonic crystl. The em which hs refrctions t the glsspc interfce nd the PCir interfce t the edge cn e cptured y the IR cmer. Fig.7 is the ctul experimentl setup. We oserved three output ems, the trnsmitted em, reflected em nd the superprism steering em. The steering em ws oserved y IR cmer. PC region Sustrte IR Cmer Input em Fig.7 () Schemtic view nd () experimentl setup of the em steering test Proc. of SPIE Vol

three different input ngles or three different input wvelengths.

spots move simultneously in horizontl direction, which corresponds to

9 shows the mesured opticl em steering experiment results for the input

The steering ngle is round 10º for 30nm input wvelength chnge nd 6º for

Compring with the simultions, the experiment results hve some

4 9.6 9.8 10.0 10.2 10.4 10.6 10.8 11.0 11.

26 25 Input Wvelength Chnge 1530 1535 1540 1545 1550 1555 1560 1565

7 - Fig.8 Typicl IR cmer view of the steering output ems for three different input ngles t 1.55μm wvelength. In Fig.8, we show the typicl output steering em imge cpture y the IR cmer t three different input ngles or three different input wvelengths. The verticl discrete spots re due to the polymer elts or rs grting effect. When the input ngle or input wvelength chnges, the steering discrete em spots move simultneously in horizontl direction, which corresponds to the output ngle chnge. Fig.9 shows the mesured opticl em steering experiment results for the input wvelength chnge nd the input ngle chnge. The steering ngle is round 10º for 30nm input wvelength chnge nd 6º for 1.5º input ngle chnge. Compring with the simultions, the experiment results hve some difference ecuse the photonic crystls in the simultion re for idel configurtions nd shpes. Output Angle (deg) Input Angle Chnge Input Angle (deg) Output Angle(nm) Input Wvelength Chnge Input Wvelength(nm) Fig.9. The em steering experiment result sed on the () input ngle chnge nd () input wvelength chnge. 4. CONCULUSION In conclusion, we reported the polymer-sed photonic crystl em steering simultion nd the friction of horizontlly stcked photonic crystl structures through hologrphic interference method using photo polymer SU8. The em steering ws studied in detil using EFC (Equi-frequency Contour) method. SEM Proc. of SPIE Vol

8 pictures nd lser diffrction pttern of the fricted photonic crystl were lso shown. Opticl em steering t 1.55μm ws crried out for the input ngle chnge nd the input wvelength chnge. The experimentl results show strong superprism effect. This reserch is supported y AFRL. Helpful discussion with Jiqi Chen, Dr. Lnln Gu nd Dr. Roert Nelson is cknowledged. REFERENCES 1. E. Ylonovitch, Phys. Rev. Lett. 58, 2059 (1987) 2. H. Kosk, T. Kwshim, A. Tomit, M. Notomi, T. Tmmur, T. Sto nd S. Kwkmi, Phys. Rev. B 58, R10096 (1998) 3. H. Kosk, T. Kwshim, A. Tomit, M. Notomi, T. Tmmur, T. Sto nd S. Kwkmi, Appl. Phys. Lett. 74, 1370 (1999) 4. S. Lin et l., Opt. Lett. 21, 1771 (1996) 5. W. Jing, R. T. Chen, nd X. Lu, Phys. Rev. B 71, (2005) 6. W. Jing nd R. T. Chen, Phys. Rev. B 77, (2008) 7. JQ Chen, W. Jing, XN Chen, L. Wng, SS Zhng, nd R. T. Chen, Appl. Phys. Lett. 90, (2007) 8. T. Kondo, S. Juodkzis, V. Mizeikis nd H. Misw, Optics Express, 14, 7943 (2006) Proc. of SPIE Vol

Light and Optics Propagation of light Electromagnetic waves (light) in vacuum and matter Reflection and refraction of light Huygens principle

$Light and Optics Propagation of light Electromagnetic waves (light) in vacuum and matter Reflection and refraction of light Huygens principle$ Light nd Optics Propgtion of light Electromgnetic wves (light) in vcuum nd mtter Reflection nd refrction of light Huygens principle Polristion of light Geometric optics Plne nd curved mirrors Thin lenses