Collective optical effect in complement left-handed material

Transcription

1 Collective optical effect in complement left-hane material S.-C. Wu, C.-F. Chen, W. C. Chao, W.-J. Huang an H. L. Chen, National Nano Device Laboratories, Ta-Hsueh Roa, Hsinchu, Taiwan R.O.C A.-C. Hsu an J.-L. Chern Institute of Electro-Optical Engineering, National Chiao Tung University, 1001, Ta-Hsueh Roa, Hsinchu, Taiwan Abstract: The Babinet s effect occurs at a long wavelength span of incient light uring FTIR (Fourier Transform Infrare) spectroscopy scanning in complementary LHM (Left-Hane Material) samples. The occurring wavelength ranges from 4 to 20 µ m uner a scanne light source in 2.5~25 µ m regime. The LHM sample consists of an SRR (Split Ring Resonator) aacent to wire array in perioic orer. Complement samples were prepare using stanar integrate-circuit processes on a Si wafer. Gol nano particles were eposite on the conucting parts of patterns. The positive pattern is a sample that patche with perioic Au wire on the Si wafer. An the negative one is aopte with a reverse way, the wire part is empty of Au an the surrouning space is fille with Au particles. Transmittance-enhance behavior was ientifie from those consequent ata. The surface plasmon resonate effect on Au nano particle on top of surface seems to contribute the enhancement. Key-wors: Babinet s effect, Left-Hane Material, Gol nano particle. 1. Introuction In the avent of nano era, all technologies step into mini-size worl to get the biggest benefits by scale reuction. Especially, in IC inustry, the bigger wafer an the smaller size are aopte, the larger competing an income can be mae. Meanwhile, there are some of istinct effects will emerge simultaneously uner imension of evice through a big step shrunk. In IC evices, they exist the parasitic effects an even quantum phenomena, which more or less influence on the evices performance. Therefore it is urgent neee to evaluate the circuit behaviors using simulation methos uring esign processes. On the other han, in the fiel of optics, the evice ensity is also increasing with the rate of communication. Thus the multifunction of optical evice attracts the interest of wireless esigners,. Designers hope to inclue laser, grating, wave-guie, coupler, sensor an other functions into a monolith an emonstrate a complete performance. The probable effects of scaling own on optics of a evice are

2 worthy of stuying. For instance, the specula phenomena involve sub-wavelength transmitting behavior [1, 2], oubly negative permeability an permittivity effects [3-5] Woo s anomaly [6], an even the Babinet s effect [7]. Babinet s effect is that the iffraction pattern on an aperture is the same as the pattern for opaque obect of the same shape illuminate in the same manner. In aition, the two spectra of complementary pattern must be out of phase an have the same amplitue at each point on a etector. The transmittance enhance is ue to resonance of the surface plasmon in the metal film on top of sub-wavelength grating [8, 9]. The egree of enhancement is epening on incient wavelength [10]. Aitionally, the relation between the wave vector of illuminating light an the reciprocal lattice of grating array nees to obey the momentum conservation [11]. Accoringly, the scale of wire grating is very important when the wavelength of light is reuce to IR regime, or even into visible range. It acts as an optical filter but intensity of passing light is enhance [12]. Therefore the authors anticipate that IC technology will be further scale own until a nano evice can have a completely optical application. The LHM is a specific pattern, which inicates the unique characteristics of a negative refraction inex [13]. The pattern is a combining of the SRR pattern at µ < 0 an the wire grating with < 0 [14]. Therefore, FTIR is use herein to evaluate the effect of a negative inex at the esigne frequency. Interestingly, a comparison of FTIR spectra emonstrates the Babinet s behavior between the complementary patterns. Meanwhile, a large range of Babinet s behavior effects are observe among a long regime of incient wavelengths. This phenomenon motivates this work. The canceling behaviors of opposite responses in spectra will be investigate by overlapping the both complement patterns together. Thus, the istribution of electromagnetic fiel can be observe using near fiel probing metho at polarize light. In aition, the Woo s anomaly an the transmittance enhancement ue to surface plasmon resonance are also observe. This stuy also involves Au nano particle with specific behaviors an unique properties [15], which are very useful in bios, optics an nano-electronics, for instance. These particles were incorporate to a fabricate LHM to eluciate new effects. Therefore this work inclue this technique into this LHM array fabrication an expects to fin something new at this combination. A series of meta-material samples were fabricate using the mature IC technology an Au nano particle treatment. One set of results investigate by CO 2 laser has been accepte at OSA Optical Annual Meeting in 2003 [16]. The SRR patterns measure using FTIR were also presente at Aphys 2003 Conference [17]. This work aresse only the optical properties on LHM array obtaine using FTIR. The escription in this phenomenon will be presente following up the experiment Section 2 states the experiment an characterization etails. In Section 3, we report the main results an iscuss the experimental ata. Section 4 raws the conclusions. 2. Experiment The patterne samples were fabricate using stanar IC (integrate-circuit) processes. The substrate was a 6 ouble polishe n-type silicon wafer. The wafer was cleane by RCA cleaning. Reference [17] escribes negative an positive patterning of samples [16]. For positive one, the

3 conucting parts in the pattern are fille with Au metal. Therefore, in the negative samples, the space along the conucting parts is fille with the gol particles. The conucting parts are thus empty an not covere by Au film; only SiO 2 remains on the Si substrate. Photographs were taken using SEM (Scanning Electronic Microscopy) instruments iniviually. The optical microscopy tool is very har to view the negative patterns owing to scattering effect from the gol particles. c g 1.84 w Fig.1 LHM pattern an Y 6.8 Dimensions. (unit=µm) X1 3.2 Figure 1shows the size LHM array herein. The thickness of Au was ~ 20nm while the thickness of the Si substrate was ~0.7mm. The horizontal istance between the two LHM units was 3.2 µm an the vertical istance was 6.8 µm. Typically, the area of an entire LHM array sample is about 4 mm X 4 mm. An entire sample has = 30,000 LHM units. The FTIR equipment use herein was QS 300 prouce by Bio-Ra Laboratories, Inc. The range of wavenumbers of the light source was 4000~400 cm -1 (The wavelengths range from 2.5 to 25 µm). Each sample was obtaine by carefully cutting with iamon pen. In this experiment the incient light was perpenicular to the pattern surface. Each of spectra was obtaine using 16scans.The time resolution was 2 sec an the scanning length was 30 sec. The purpose of FTIR is to fin out the existing components or constituents of the film eposite on Si wafer. Therefore the response of signal uner such as measuring is usually aopte by intensity ratio, which is ivie the response intensity of testing sample with that of pure Si wafer. It is so-calle as absorption ratio uring the testing sample inserte. On the other han, it is ust the absorption signal resulte from the film component top of Si wafer. But in this stuy, for clearly unerstaning, we aopte the transmittance instea of absorption to evaluate the SRR behavior. This paper consiers only the results that correspon to LHM pattern of the minimum size. 3. Results an Discussion Figure 2 shows the SEM photographs of positive LHM array an negative one respectively. The magnifications are 1000x an 3500x in positive an negative array, respectively. (a) (b) Fig. 2. SEM photographs of LHM array pattern 1000x in positive pattern (a), 3500x in negative pattern (b). Figure 3 isplays the FTIR transmittance spectra of both LHM patterns. Babinet s behaviors are obvious in many regions. The range of wave numbers is from 696~710 cm -1 (14.1~14.4 µm), to 2926 cm -1 (3.4 µm). Babinet s behavior is clearly exhibite in over 25 positions. In orer to further showing the relationship of the complementary behavior, the FTIR spectra were zoome in at two regions. One region is at 600~3000 cm -1 shown as Fig. 4, an another at 1200~2800 cm -1.exhibite as Fig. 5, respectively. At those regions, the transmittance (T) values are taken to be T p (λ) = T 0 ( ) ± T (λ) an T n (λ)= T 0 (λ) ± T (λ) for that of

4 positive an negative LHM patterns, respectively. T p (λ), an T n (λ) are the peak or valley transmittance values of the positive an negative patterns in the regions of Babinet s effects, an T 0 is the approximate transmission of the uniffracte light, which is consiere to be the average value of T p (λ)+ T n (λ). Some T 0 values excee 100%, probably because of the effect of transmission enhance on both types of patterns. Where T (λ), i.e., the ifference between T n an T 0, or T p an T 0, is erive from the ifference in iffraction from opaque obstacles in both SRR patterns. Transmittance wave number Negative LHM Positive LHM Fig. 3. (Color) FTIR transmittance spectra of both types of LHM patterns. Wavelength span at incient light source is 400~4000 cm -1. Notably, the complementary behaviors of the spectra are very clear at some wavelengths. For instance, at aroun 700, 1220, 1393, 1444, 1530, an 1800 cm -1, the phase ifferences between waveforms are almost equal to π. On the other han, it is selom foun that Babinet s behavior is still exhibite uner a sophisticate an perioic patterne array. Especially the incient wavelength is also compatible with the size of array. The annihilation effect of precise overlapping of two reciprocal patterns is thus observe. This fining is being investigate an will be iscusse in a later stuy. Transmittance wave number Negative LHM Positive LHM Fig. 4. (Color) Magnifie spectrum of FTIR ata that clearly shows the Basinet s effect in the 600~3000 cm -1 regime. Woo [18] an Fano [19] escribe maximum an minimum of the iffracte light at metal grating experiments. Therefore the x an y imension of the LHM array are examine to eluciate the relationship of anomaly with the array imension. The contribution of the surface plasmon (SP) to the enhancement of transmittance at this peculiar wavelength ue to resonance with the incient light is also consiere. Transmittance wave num ber Negative LHM Positive LHM Fig. 5. (Color) Magnifie spectrum of FTIR ata that clearly shows the Basinet s effect in the 1200~2800 cm -1 regime. In the mi-ir region, the ielectric parts can be recognize as holes, as in other work [20] in the visible regime. Therefore, the moel of the coupling of light with an SP on a perioically

5 corrugate metal surface is consistent with the conservation of momentum, as escribe in [21]. That is, K sp = K x + ig x + G y, where the K sp is the SP wave vector, K x = x ) (2π / λ) sinθ is the x ) component of the incient light s wave vector, an G x, G y are the reciprocal lattices for a rectangular-lattice vectors with G x = 2π/a x, G y = 2π/a y ; i, are integers. Where a x an a y are the lattice constants of the unit cell of array in x-axis an y-axis, respectively. The θ value is 0 uner normal incient. The SP ispersion [9] is given by ω 1 Κ = [ + ] 2 SP m /( m ), c where ω is the angular frequency of the incient light, m is the real part of the ielectric constant of the metal, an is that of ielectric material. In our case, the incient light is in mi-inferre regime, whereas the value of m in Au metal [22] is negative an very larger than. That is, the value of Κ SP can be simplifie to a formula of an the reciprocal vectors. The refraction inex n is not a fixe value uner the range of mi-ir [23]. Especially at the region of 6~10µm, the refraction inex n is ecreasing with wavelength increasing, herein the n value can own to Therefore, in this work, we use the value 1.9 as refraction inex value of SiO 2 to preict the SP resonate wavelength. An the values of a x an a y are µm an µm, respectively, in the sample. Accoringly, there can exhibit many of the resonate behaviors of SP uner wavelength of 25µm. Uner the conition i + = 1, the resonance at the Au/SiO 2 interface is at a wavelength greater than 25 µm an is therefore invisible. Where i an are the Miller inices associate with the x an y-axes, respectively. The most istinct peak at µm in the positive an negative patterns correspons to the resonance of the surface plasmon in the interface of the air an the Au. The value of this peak is consistent with the calculation mae using the SP formula. The other peaks at 10.9 µm in the positive pattern an 10.4 an, 11.2 µm in the negative pattern follow from the i + = 2 conition at the Au/SiO 2 interface. When the sum i + excees 3, the corresponing wavelength will be smaller than the lattice imensions of the wire array. Thus the iffraction behavior is likely to be exhibite in this region. The resonance is also too weak to be manifest uner this higher orer conition. The ispersion of Au nano particles on the conucting surface in array pattern was observe using SEM. The istribution of Au particles is like that of cluster in a semicontinuous metal shown as Fig. 6. Accoringly, the Au nano particles contribute to the enhancement of transmittance ue to percolation-enhance nonlinear scattering (PENS) effect [24]. Because the metal line in positive LHM array or surrouning parts in negative one are of same effect as semicontinuous metal film. Thus the local fiel E coul be enhance with the collective plasmon resonance uner ensembles of metal particles [25]. Fig. 6 SEM photograph of ispersion of Au nano particle on conucting surface in LHM pattern. Hereon, we can also fin out the phenomena of maxima an minima in spectrum that calle as Woo s anomaly [18] as Fig.3. These anomalies now are well known since the work of Fano [19], that are ue to the excitation of surface plasmon propagating along the metallic-ielectric interface. In fact the conitions for the occurring of Woo s

6 anomaly are similar with SP resonance, except that the wave vector of the grazing light replaces Ksp. The magnitue of this wave vector is k iff = 2πn /λ, where n = 1/2. In normal incient these wavelength are given by λ λ i = 0 m 1/ 2 max (, ) ( ) 2 2 m + ( i, min ) = i i 2 a a The loci of Woo s anomaly is almost following with the SP ispersion branches in the (E, k x ) iagram, an the minima occurring at slightly shorter wavelength than the maxima. Two ip valley positions aroun 21.8 µm in spectra of positive an negative patterns are ue to the Si-O-Si bon bening motion an can fin from the FTIR ata sheet. Transmittance enhance factor can be evaluate base on the true area ratio of conucting part in LHM to the complete area in our sample. The area ratio of the positive pattern is 0.22, explaining what the enhancement factor for the highest peak is about 2. However, it is aroun 6.6 for the negative pattern. The non-coherent irraiating light thus has a marke effect, which may be more obvious when polarize coherent light source is use. This claim is being investigate further. arise in the same positions. This result has selom been reporte an shoul be stuie further. Whether the complementary spectra coul be ae together to yiel a flat response was also investigate. Other novel properties such as enhance transmittance behavior, an Woo, s anomaly associate with SP resonance were observe. The SP resonate positions for both types of pattern were also foun out from the FTIR scanning an able to check with referring formula. In there, some eviations exist ue to variation of ielectric constant. The enhance factor of the negative array excees that of the positive one, although the surface of the former is covere with more gol particles. Therefore the collective localize electric fiel, enhance by Au nano particles, can combine with the surface plasmon to increase transmittance. Therefore further investigation is neee, particularly in the fiel istribute over the conuctor in both types of LHM array. By the way, the FTIR measurement can fast evaluate the optical properties of the pattern itself. The substrate effect will fully eliminate while the substrate was aopte for backgroun. Acknowlegement: The authors woul like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research uner Contract No. NSC E Conclusion Complimentary LHM patterns were fabricate to inclue Au nano particles by stanar IC processes. More than 25 Basinet s effects were observe over a wie range of wavelengths. Interestingly, some complementary waveforms References: [1] A. K. Sarychev, et.al., Resonance transmittance through a metal film with subwavelength holes, IEEE J. Quantum Electronics, 38, 7, 2002, pp [2] T. W. Ebbesen, et.al., Extraorinary optical

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