Compact optical filter for dual-wavelength fluorescence-spectrometry based on enhanced transmission through metallic nano-slit array

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1 Appl Phys B ( : DOI /s y Compact optical filter for dual-wavelength fluorescence-spectrometry based on enhanced transmission through metallic nano-slit array X. Hu L. Zhan Y. Xia Received: 6 August 2008 / Revised version: 10 November 2008 / Published online: 15 January 2009 Springer-Verlag 2009 Abstract A novel optical filter based on enhanced transmission through metallic nano-slit is proposed for dualwavelength fluorescence-spectrometry. A special structure, sampled-period slit array, is utilized to meet the requirement of dual-wavelength transmission in this system. Structure parameters on the transmission property are analyzed by means of Fourier transformation. With the features both to enhance the fluorescence generation and to enhance light transmission, in addition with the feasibility for miniaturization, integration on one chip, and mass production, the proposed filters are promising for the realization of dual-wavelength fluorescence-spectrometry in micro-totalanalysis-system. PACS Dj Bz Mf 1 Introduction Fluorescence spectrometry has become a key technique for micro-total-analysis-system (mtass an attractive concept for researches in the fields of biology, diagnostics, and chemistry, which can integrate the capabilities of entire laboratories onto compact devices consisting of microchips and other micro-fabricated elements [1]. However, two key problems, integratable low-noise optical detectors and optical filter [2], block its development in mtass. Traditional optical filters for the fluorescence detection can be classified into two categories: interference filters that are always formed by X. Hu ( L. Zhan Y. Xia Institute of Optics and Photonics, Department of Physics, Shanghai Jiao Tong University, Shanghai , China huxiao@sjtu.edu.cn Fax: multiple-layer film and absorption filters that contain special absorption substances [2]. To push mtass into reality, it is urgent to develop optical filters that may be miniaturized and integrated easily on one chip. Surface plasmon on metallic film has been extensively used in spectroscopy technology because it can generate extremely strong electromagnetic field near the surface, which greatly enhances the spectral signals [3]. Since the discovery of enhanced optical transmission through metallic film with periodic subwavelength structure [4], its enhancements both on spectral signal and optical transmission attract much interest on spectroscopy [5 7]. In simple terms, enhanced transmission effect indicates the light transmits through subwavelength holes or slits without scattered out of incident light [4, 8] at certain resonant wavelengths. The experiments show that the intensity of transmitted light is even much stronger than the incident light at the area of holes or slits [4, 8]. It has been proved that the surface plasmon polaritons (SPPs at the metal surface play a key role on such extraordinary optical transmission [8]. This effect happens at the wavelength where the incident light resonantly couple with the SPPs through the structure periodicity. This paper proposes a novel concept of optical filter using the perforated metallic film, which meets the dualwavelength fluorescence spectrometry in mtass. It is well known that dual-wavelength fluorescence spectrometry is a powerful tool to study quantitatively the substance in turbid samples [9]. For example, it is used to determine the contents of epinephrine (E and norepinephrine (NE in catecholamines (CAs, whose content in blood is closely related to many diseases such as parkinsonism, pheochromocytoma, and melancholia [10]. Another attractive example is to study the distribution of DNA and RNA synthesis during the cell cycle [11, 12]. In some cases, it is used to eliminate environment impact in measurement. For example, it is used to

2 630 X. Hu et al. eliminate the disturbance of size and depth of the fluorescent lesion and intensity of incident light when image protease activity in vivo [13]. The dual-wavelength spectroscopy requires optical filters with two desired transmission windows that overlap the two studied spectral peaks. Our study shows that by adjusting the structure parameters, the proposed optical filter can satisfy the above requirement and fulfill dualwavelength fluorescence spectrometry in mtass. 2 Theory and discussion In dual-wavelength fluorescence-spectrometry, the fluorescence intensities at two specific wavelengths are alternatively measured. The ratio of these two fluorescence intensities is analyzed to obtain the quantity of the detected substances. For certain substances, it is important to choose the two proper wavelengths. For example, the fluorescence of the substance A in Fig. 1 overlaps with that of undesired substance B. Usually, the detecting wavelength of A is chosen at the location of strong fluorescence, for example, at the fluorescence peak wavelength 1. Another wavelength Fig. 1 Schematic view of dual-wavelength fluorescence-spectrometry 2 is chosen at the location where the fluorescence intensity of B equals that at 1, and the fluorescence of A is zero, i.e., I B ( 2 = I B ( 1 and I A ( 2 = 0. The intensity difference of two signals presents the information of the substance A. To meet the requirement of dual-wavelength fluorescence-spectrometry in mtass, the filter transmission peaks must be centered right at these two required wavelengths. As illustrated in Fig. 2a, for a sub-wavelength slit array with the periodicity a, the phase matching condition to achieve enhanced transmission is [4, 14] k sp = k ± m G, (1 where k =(2π/sin θ is the in-plane wavevector component of incident light for normal incident, θ = 0, G is the reciprocal vectors ( G =2π/a, k sp is the propagation constant of SPPs, which can be solved by the equation [8] k sp = n eff k = εm ε s ε m + ε s k, (2 where ε m and ε s are the dielectric constants of the metal film and the substrate, respectively, and n eff = k sp /k is the effective index of SPPs. From (1 and (2 only one structure parameter a determines the resonant transmission wavelengths for a periodicstructure metal film. Once a is given, all the resonant peaks are fixed. However, for dual-wavelength spectroscopy, the transmission for two specific wavelengths is required. This is not always the case that the two resonant maxima just overlap the two required specific wavelengths. To exploit the application of the extraordinary optical transmission in dual-wavelength fluorescence spectrometry, a sampled-period slit array is proposed to overcome the problem. Figure 2b illustrates the scheme of the structure. The whole array is divided into N identical sample periods with length p. In each period, only the segment with Fig. 2 Schematic view of the periodic slit arrays (aand sampled-period slit arrays (b

3 Compact optical filter for dual-wavelength fluorescence- spectrometry based on enhanced transmission 631 the length l is occupied by the slits with periodicity a, and the left part is imperforated. The sampling rate is R = l/p. Compared with the regular structure illustrated in Fig. 2a, this structure is a nesting of two sets of periodic arrays with the periodicity a and p, respectively, or it can be considered as a periodic structure with periodicity a modulated by another periodic structure with longer periodicity p. The structure can be expressed by n(z = S(z n r (z [ ( z = comb p [ n comb rect ( z a ( ] z l rect ( z d ], (3 where, S(z represents the refractive index of sampling period with periodicity p, n r (z for the index of array with periodicity a, and d is the slit width. n is the index difference between metal film and the substance filled in slits, comb and rect are the comb function and the rectangular function, and denotes the convolution. According to (1, if the slit array has the period a = 0 /n eff, the transmission for the incident wavelength 0 is enhanced. The transmission spectrum F(1/ can be obtained by transforming the function n(z/n eff into the Fourier space F(1/ = F r (1/ F s (1/ ( = apldn 4 eff [comb n neff a [ ( ( neff p neff l comb sinc sinc ( ] neff d ]. (4 In (4, the first item comb( n effa sinc( n effd is the Fourier transformation of the uniform slit array n r (z, in which comb function describes the position of resonant wavelengths, and sinc function describes the envelope of the resonant peaks. The second item comb( n effp sinc( n effl is the Fourier transformation of the sampling period S(z. Equation (4 demonstrates that the sampled-period structure s spectrum is the convolution of the spectrum of the original periodic array and the sampling period. The wavelengths of resonant peak in sampled-period structure s spectrum are determined by comb(n eff a/ and comb(n eff p/, i.e., by two structure parameters a and p. Two independent variables may satisfy the requirement of dual-wavelength transmission. Comb function can be considered as a sequence of δ functions, and the spectrum of a function will shift a distance of x 0 if it is convoluted with δ(x 0. Considering the convolution of the first-order resonant peak of the uniform slit array n r (z with the sampling period S(z in (4, the interval between two successive resonant peaks can be estimated as = 2 m /n effp, (5 where m = ( /2, the average of the two resonant maxima, and = 2 1, the spacing of the two resonant maxima. As an illustration, optical filter is designed for dualwavelength spectrometry to image protease activity in vivo. Two fluorochromes are used, one is protease-activatable whose fluorescence reflects the observed enzyme, and the second one is protease resistant which is bound to the substrate and serves as an internal standard. The ratio of the two fluorochromes fluorescence can present the concentration of the enzyme, which is independent of size, depth of the fluorescent lesion, and intensity of incident light [13]. Here, we suppose that the two fluorochromes are Cy5.5 and Cy7, as in [13], whose emission peaks are 694 nm and 777 nm, respectively. a should be 456 nm by (1 with = 694 nm, p is estimated to be 4308 nm by (5, = 83 nm, and m = ( /2 = 736 nm. Besides, the coupled mode theory [14] is employed to verify the result, which shows that the value of calculated with (5 deviates a little from the supposed value. By adjusting the value of p, theresult can be obtained as shown by the solid line in Fig. 3, which is calculated with a = 455 nm and p = 4150 nm. These resonant transmission peaks coincide with the emission maxima of Cy nm and Cy7 777 nm. To estimate the fabrication tolerance of the periodic slit arrays, we study the impact of the errors of parameters p and a on the resonant maxima, which is shown in Fig. 4. The figure demonstrates that the shift of m and the error Fig. 3 Calculated transmission spectrum using couple mode theory (a = 455 nm, p = 4150 nm, solid line a = 0, dashed line a = 50 nm, dot line a = 20 nm, dash-dot line a = 10 nm

4 632 X. Hu et al. Fig. 4 Shift of resonant peaks vs error of periodicity parameter a and p. (a Shift of m vs error of a at different wavelength. (b Error of vs error of p for different required ( m = 400 nm. (cerrorof vs error of p at different wavelength m ( = 100 nm of are linearly dependent on errors of a and p, respectively, and the deviation rate is different for different wavelength. Higher precision is required for a than that for p. To investigate the impact of the random error on fabrication, the coupled mode theory is used to calculate the transmission spectrum with same structure parameters in Fig. 3, but each period has a random error in the range of a ± a ( a is the random error of the periodicity. We calculated many times for the same a. As the ultimate case, the result with the biggest resonant shift is shown in Fig. 3. The figure shows that the random error of the structure periodicity leads to flattened transmission peaks. For a structure with a = 455 nm, a random error a = 20 nm impacts little on its application in spectroscopy. Electron beam lithography (EBL and focused ion beam (FIB are two possible approaches to fabricate the proposed filter. Usually, EBL has a resolution limit of 10 nm [15], which is high enough to satisfy the tolerance. The FIB technique, in which the sample is milled by focused ion bombardment with nominal beam diameter of 5 nm, provides even higher resolution. Compared with FIB, EBL involves several steps but is particularly useful for large-scale manufacture. Another important case to be considered is the thickness of the dielectric media between the observed molecules and the metal film. In the vicinity of metal, the fluorescence rate of molecules becomes a function of the distance between the molecule and the metal surface. For the molecules separated at a distance s from the metal surface, its fluorescence intensity I s can be calculated by [16] [ ( I 4 ] 1 s s0 = 1 +, (6 I s where I denotes the fluorescence intensity at infinite separation distance, i.e., in the absence of any metallic surface, s 0 is called the Förster radius and gives the distance at which the fluorescence intensity decreases by a factor of 2 compared to the unquenched state. Typical values for dipolar coupling are s 0 = 5 7 nm. From the formula the fluores-

5 Compact optical filter for dual-wavelength fluorescence- spectrometry based on enhanced transmission 633 cence of molecules in direct contact with the metal is completely quenched. Therefore we have to position the molecules within some distance of the metal surface so that the quenching effect is avoided while the plasmon field is strong enough. In contrast to the quenching phenomenon, which is effective only for molecules being near the surface within about nm, the exponential decay of the surface plasmon field results in a reduction of excitation probability only on a length scale of several hundred nm [17]. Therefore, if the thickness of the dielectric layer between the metal film and the observed molecules is well designed, say about 20 nm, it is enormous to gain the detection sensitivity by exploiting the enhanced optical field of a resonantly excited surface plasmon mode as the illumination light source without paying the price that the quenching mechanism to the metal would cost. It should be noted that the periodic structure-induced SPPs result in the enhancements both for the emission of molecular fluorescence and for the transmission of this emission [17, 18]. The strong electromagnetic fields induced by SPPs make the molecules to irradiate higher intensity than in the case of light impinging on the surface. This prominent enhancement is attractive for fluorescence spectroscopy [17, 18]. Moreover, the periodic-structure array enables the metal film to transmit most of the generated fluorescence. Theory demonstrates that near 100% transmittance may be obtained for this emission if the structure parameters such as film thickness and the slit width are optimized [19]. The experiment also shows that the transmittance could be up to two times of the fractional aperture area [4]. The proposed filter takes advantage of the two enhancements of metal film, which leads to a much improved sensitivity. Besides, these compact filters are easy to be miniaturized and to be integrated on one chip. All of these properties make the proposed filter a promising candidate for dual wavelength spectrometry in mtass. 3 Conclusion The emphasis in this work has focused on the design of dual-band optical filter for the application in dual wavelength fluorescence spectrometry in mtass. The transmission property of the sampled-period slit arrays on the metal film has been presented using Fourier transmission. The utility of this special milled metal film may lead to spectacular performance for quantitatively sample detection in mtass. Compared with traditional filter, fluorescence is enhanced by the metal film, and it is easily to be pre-designed and adapt to be integrated on single chip, the fabrication tolerance can be achieved with conventional fabrication methods. The sampled slit array milled on metal film studied in this work would be a promising approach for the implement of dual-wavelength fluorescence spectrometry in mtass. Acknowledgements The authors acknowledge the support from the National Natural Science Foundation of China of the grant , the Program for New Century Excellent Talents in University of China, and the SMC Young Star scientist Program of Shanghai Jiao Tong University. References 1. A. Manz, N. Graber, H.M. Widmer, Sens. Actuators B 1, 244 ( M. Dandin, P. Abshire, E. Smela, Lab. Chip 7, 955 ( G.C. Schatz, Acc. Chem. Res. 17, 370 ( T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, P.A. Wolff, Nature 391, 667 ( Y. Liu, J. Bishop, L. Williams, S. Blair, J. Herron, Nanotechnology 15, 1368 ( A.G. Brolo, R. Gordon, B. Leathem, K.L. Kavanagh, Langmuir 20, 4813 ( J. Dintinger,S.Klein, T.W. Ebbesen, Adv. Mater.18, 1267 ( C. Genet, T.W. Ebbesen, Nature (London 445, 39 ( C. John Cowles, J. Opt. Soc. Am. 55, 690 ( Y. Zhao, F. Su, C. Wang, M. Jang, J. Bai, C. Chen, X. Zhang, J. Hebei Univ. 28, 63 (in Chinese 11. Z. Darzynkiewicz, D.P. Evenson, L. Staiano-Coico, T.K. Sharpless, M.R. Melamed, J. Cell Physiol. 100, 425 ( H.M. Shapiro, Practical Flow Cytometry (Liss, New York, M.F. Kircher, R. Weissleder, L. Josephson, Bioconjugate Chem. 15, 242 ( Z. Fan, L. Zhan, X. Hu, Y. Xia, Opt. Commun. 281, 5467 ( A. Broers, A. Hoole, J. Ryan, Microelectron. Eng. 32, 131 ( H. Kuhn, D. Möbius, H. Bücher, in Physical Methods of Chemistry, ed. by A. Weissberger, B.W. Rossiter (Wiley Interscience, New York, 1972, Part III B, Chap T. Liebermann, W. Knoll, Colloids Surf. A 171, 115 ( F. Yu, B. Persson, S. Lofas, W. Knoll, J. Am. Chem. Soc. 126, 8902 ( S. Astilean, P. Lalanne, M. Palamaru, Opt. Commun. 175, 265 (2000