Fabrication and optical characterization of nano-hole arrays in gold and gold/palladium films on glass

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1 107 Fabrication and optical characterization of nano-hole arrays in gold and gold/palladium films on glass O M Piciu 1 *, M W Docter 2, M C van der Krogt 3, Y Garini 4, I T Young 2, P M Sarro 1, and A Bossche 1 1 DIMES/EW1, Delft University of Technology, Delft, The Netherlands 2 Imaging Science and Technology, Delft University of Technology, Delft, The Netherlands 3 Kavli Institute of Nanoscience, Delft University of Technology, Delft, The Netherlands 4 Department of Physics, Bar-Ilan University, Israel The manuscript was received on 20 March 2007 and was accepted after revision for publication on 31 January DOI: / JNN102 Abstract: This paper reports on the improved fabrication process and the optical characterization of different nano-hole arrays in thin metal films that are to be integrated into a novel atto-litre titre plate device for high-speed molecular analysis, such as DNA hybridizations and protein immunoassays. The optical detection is based on the enhanced optical transmission that was recently discovered when light passes through periodically distributed sub-wavelength apertures in optically thick metal films. The transmitted light has also small angular diffraction and well-defined spectral features. Using electron-beam (e-beam) lithography (EBL) and lift-off technique, various array structures with hole diameters ranging between 100 nm and 200 nm and different pitches were fabricated in a 200-nm thick layer of gold (Au), palladium (Pd), and gold/palladium (Au/Pd ¼ 60/40) alloy on glass. Introducing Pd to Au, the grain size of the material is decreased, getting a more well-defined shape of the holes. The transmitted spectrum was measured through periodically and randomly distributed nano-holes in Au. Transmitted spectra were compared as well through similar subwavelength hole arrays in Au, Pd, and Au/Pd alloy. Moreover, the fluorescence of Rhodamine G6 (0.05 mm) was measured when using the transmitted light through periodical cavities in Au as the illumination source. It reveals a nine-fold increase in the fluorescent signal. Keywords: sub-wavelength hole arrays, enhanced light transmission, electron-beam lithography, lift-off technique 1 INTRODUCTION An important part of the worldwide technological research work done in the last decade has been directed towards advanced devices for biochemical analyses. These applications require low-cost tools and have the advantage that reduced amounts of reagents and samples are being used with high-speed reactions and optimized parameter detection. The miniaturization of the system, which is an important parameter for such devices, is often limited by the optical spatial resolution of the system. *Corresponding author: Laboratory of Electronic Instrumentation, Department of Micro-electronics, Faculty of Elec. Eng, Maths, & Comp. Sci., Delft University of Technology, Mekelweg 4, room , Delft, 2628 CD, The Netherlands. O.M.Piciu@tudelft.nl The extraordinary optical transmission (EOT) of light described in 1998 by Ebbesen et al. [1], concerning sub-wavelength periodical cavities in metal films, has generated considerable experimental and theoretical activity in the past years. The most important effects of EOT, i.e. small angular diffraction, spectral selection, and enhanced light transmission ([1, 2]), and their origin are still being studied. Nevertheless, it has been identified as a promising optical detection device, stimulating investigation into a large range of applications in optics, photonics, and biosensors [3, 4]. The atto-litre titre plate for high-speed molecular analyses, which uses illumination of small volumes based on the above-mentioned optical effects, is the focus of the research presented in this paper. The basis of the device is an optical, periodic, subwavelength hole array in a thin metal film, where JNN102 Ó IMechE 2008 Proc. IMechE Vol. 222 Part N: J. Nanoengineering and Nanosystems

2 108 O M Piciu, M W Docter, M C van der Krogt, Y Garini, I T Young, P M Sarro, and A Bossche each hole serves as a reaction chamber. Several groups are currently working with various biochemical experiments using nano-hole arrays. They use different fabrication methods, such as focused ion beam (FIB) milling [5], soft-imprinting lithography (SIL) [6], phase-shifting photolithography (PSP), and electron-beam deposition (EBD) [7], or selfassembling on pre-patterned macro-porous films [8]. Nevertheless, these methods have their drawbacks, such as a longer etching time and re-deposition of metal back on the sample surface for the FIB, requirements of pre-fabricated molds or masters for nano-imprinting, or isotropic shape of the holes in case of the self-assembling techniques. A different approach was considered and improved in this paper, i.e. making use of EBL, direct reactive ion etching (RIE), electron-gun (e-gun) deposition, and lift-off procedure, which gives a fast, stable, and reproducible process for creating nano-hole arrays with controllable hole periodicity and shape. 1.1 Device description The main part of the atto-litre titre plate device consists of an optical, periodic, sub-wavelength hole array in a thin metal film. Each hole represents an individual biochemical reaction chamber (Fig. 1). The holes are first filled (individually or as subarrays) with a solution containing certain molecules necessary for detection, such as DNA fragments, antibodies, or antigens. These molecules bind to a metal substrate through a sulphur metal chemical bond. In the next step, a transparent plate with embedded nano-channels covers the holes. Via these channels, the sample containing the target molecules is transported to the holes and a chemical recognition between the target molecules and the molecules bound to the Au substrate takes place. Illumination is done by a collimated light beam with a wavelength that matches the peak transmission of the arrays. The detection can be made in two different ways: 1. fluorochrome-labelled molecules inside the holes are excited and their transmission is detected; 2. aggregated molecules inside the holes will change the optical properties of transmitted light. In both cases, the light is collected through a conventional microscope objective lens with a chargecoupled device (CCD) camera. The intensity of the emitted or transmitted light will be correlated with the concentration of the target molecules. In this way, the atto-litre titre plate will support different types of molecule recognition and be suitable for a range of applications, such as gene expression analyses, treatment monitoring, and diagnostics. 1.2 Background The EOT phenomenon used for this optical detection is not yet well understood. It is explained in the literature by multiple theories; one is based on photon plasmon interaction [1], and a second one on composite diffracted evanescent waves (CDEWs) [9]. Nevertheless, when a periodic sub-wavelength structure (such as a set of holes or slits) is embedded in an optically thick metal film, the transmitted light exhibits three main features: 1. the transmitted light has a well-defined spectrum; Fig. 1 Schematic diagram of the atto-litre titre plate device and sensor Proc. IMechE Vol. 222 Part N: J. Nanoengineering and Nanosystems JNN102 Ó IMechE 2008

3 Fabrication and optical characterization of nano-hole arrays the intensity of the transmitted light is larger than the amount of light impinging through the holes it is much larger than the classical prediction that is proportional to the (diameter/ wavelength) 4 [10]; 3. the exit beam has a small dispersion angle of about 3 [2], which is in contrast with the wide angle predicted by diffraction theory [10]. The device is a titre plate with up to several hundred million reaction wells per square centimetre of chip area, instead of the 200 wells per square centimetre of chip area found in the recently developed nano-litre titre plates [11]. The proposed method allows one to make a parallel detection through 10 4 holes receiving a large amount of information simultaneously. Another advantage of this approach is the possible use of different molecules in each hole (or in each square unit) in order to realize parallel, real-time analyses in a single experiment. dose and to overcome issues such as proximity effects and overexposure. Using an acceleration voltage of 100 kv and an aperture of 400 mm, the following settings have been determined: with a beam step size (BSS) of 5 nm and an estimated spot size of 8 nm, a dose of 4400 mc/cm 2 for 100-nm structures, and a dose of 2400 mc/cm 2 for 150-nm structures; squaredots with periodic or random distribution have been patterned into the e-beam sensitive resist. After the exposure, the development is done in Microposit MF322 pure solution (Rohm and Haas), for one minute, and the samples are then rinsed in DI water and spin-dried. The next process step is to transfer the dot-array pattern, through RIE into the hard-baked 2 TECHNOLOGY 2.1 Nano-cavities in gold layer on glass A lift-off technique is used to prepare the nanocavities in an Au layer on glass. Fused silica samples (20 20 mm 2 ) are first ultrasonically cleaned in fuming nitric acid (HNO per cent), de-ionized (DI) water, and isopropyl alcohol (IPA), each time for two minutes, and finally spin-dried. Next, 20-nm thick chromium (Cr) film is sputtered on top of the substrate, using an Alliance Concept AC450 machine at a deposition rate of 20 nm/min. This Cr layer serves as a reflective layer to facilitate the focusing of an electron-beam pattern generator (EBPG) and is also used as a conductive layer for discharge during the e-beam exposure. A bi-layer resist scheme is applied on top of the Cr. The bottom layer is HPR- 504 (from Olin), which is an organic, positive-tone resist for near ultraviolet (NUV) lithography. The top layer is hydrogen silsesquioxane (HSQ ¼ FOx-12 from Dow Corning), which is an inorganic, negativetone resist for high-resolution EBL [12, 13]. First, the photo-resist is spin-coated at 5000 rpm for 55 seconds on a Fairchild spinner, after using a primer (hexamethyldisilazane (HMDS)) to enhance its adhesion to the metal. Next, it is hard baked at 100 C, 200 C, and 250 C, respectively, each for two minutes, and then over-coated with the HSQ resist, at 6000 rpm for 45 seconds, and baked at 150 C and 220 C, again for two minutes each. The e-beam resist has a thickness of 150 nm and the total thickness of the bi-layer is 1.1 mm. Different exposure tests have been performed, using a Leica EBPG 5000þ, in order to find the correct Fig. 2 Schematic diagram of the fabrication steps: (a) double layer resist scheme; (b) e-beam exposure and development; (c) RIE of the HPR-504 resist; (d) metal e-gun evaporation; (e) HPR-504 lift-off JNN102 Ó IMechE 2008 Proc. IMechE Vol. 222 Part N: J. Nanoengineering and Nanosystems

4 110 O M Piciu, M W Docter, M C van der Krogt, Y Garini, I T Young, P M Sarro, and A Bossche HPR-504 photo-resist, using O 2 plasma, with a Leybold-Heraeus Z401S parallel plate type RIE reactor. The process sequence is schematically depicted in Fig. 2. It shows that, first, pillars are formed at the positions of the later holes. The pillars are removed during the lift-off procedure. The etching rate of the resist is 30 nm/min at 20 sccm O 2 flow, RF power of 20 W and 1.4 mbar pressure. The process is real-time controlled via an interferometer (Sofie Instruments). After obtaining the pillars, each sample is electron-gun evaporated with 200 nm of Au, using a Leybold-Heraeus L560 evaporator. In the first tests, the Au is deposited at an evaporation rate of 0.5 nm/s. Due to surface roughening caused by introduction of small gold spheres, the evaporation rate is decreased to 0.1 nm/s in order to obtain a much smoother surface. In the last step of the process, the pillars are liftedoff in fuming HNO 3 at 40 C for six minutes, followed by an ultrasonic cleaning for three minutes in the same conditions. Finally, the samples are rinsed with IPA and spin-dried. Inspection of the samples is performed with a FEI/Philips XL30SFEG scanning electron microscope (SEM). Using this technique, square-hole arrays in Au with the hole size between 100 nm and 200 nm, various pitches, and a depth of 200 nm have been fabricated (e.g. see Fig. 3(a)). Randomly distributed square holes in similar Au layer with the same hole size as the previous sample, and with the same filling factors (e.g. see Fig. 3(b)), have also been fabricated. 2.2 Nano-cavities in gold/palladium (Au/Pd : 60/40) on glass The next arrays are manufactured in an Au/Pd (60/40) alloy. Introducing Pd to Au leads to smaller grains compared to Au itself. The e-beam evaporation of the alloy can be performed at a higher deposition rate, without introducing the small spheres that are formed when depositing Au at the same rate. The high-resolution SEM images are shown in Fig. 4, supporting the presence of smaller grains of this alloy Fig. 3 SEM pictures of the arrays fabricated in 200 nm thickness of Au: (a) square-hole array, with a hole size of 160 nm and pitch of 750 nm; (b) randomly distributed square-holes, with a hole size of 160 nm and the same filling factor as (a) Fig. 4 SEM pictures of the arrays in (a) Au and (b) Au/Pd revealing a smaller grain size of Au/Pd Proc. IMechE Vol. 222 Part N: J. Nanoengineering and Nanosystems JNN102 Ó IMechE 2008

5 Fabrication and optical characterization of nano-hole arrays 111 Round hole arrays with square distribution, a hole diameter between 150 nm and 200 nm, various pitches, and a depth of 200 nm have been obtained (e.g. see Fig. 5). Through these changes, a few functional improvements were made, such as better, well-defined shapes of the holes, an improved focus of the beam on the sample surface, resulting in a more accurate exposure, an optimal aspect ratio of the pillars, and 40 per cent higher efficiency of the lift-off procedure. 3 RESULTS Fig. 5 SEM picture of circular holes with square distribution, in 200 nm of Au/Pd, with a hole size of 175 nm and pitch of 1050 nm compared to pure Au. A smaller grain size leads to a more well-defined shape when using lift-off for fabrication. The general lines of the process are the same as for the nano-cavities in Au, but with some improvements. The thickness of the Cr layer is increased to 30 nm. This modification gives a more constant height measurement during the EBPG exposure and, therefore, an improved focus of the beam at the sample surface, which results in a more accurate exposure, and, moreover, it increases the lithography reproducibility. Also, the HPR-504 thickness is decreased to 670 nm, reducing the resist etching time and avoiding problems, such as bending of the pillars, caused by the high aspect ratio between their height and their cross-sectional area. After different exposure tests, the following settings have been determined to pattern round-shaped dots with a periodic square distribution into the e-beam resist: an acceleration voltage of 100 kv, an aperture of 400 mm, a BSS of 5 nm, and a dose of 1500 mc/cm 2 for 175-nm structures. The pattern is again transferred to the photo-resist and a 200-nm layer of Au/Pd (60/40) is electron-gun evaporated on top of the pillars at an evaporation rate of 0.25 nm/s. The lift-off procedure has also been improved. The samples are now oriented upside-down in order to reduce contamination of removed pillars on the array exit. They are, first, etched for 60 seconds in the HNO 3 (100 per cent) at room temperature, using an ultrasonic bath. The Cr layer on the bottom of the holes is removed by dipping the samples for 60 seconds in a solution of potassium hydroxide (KOH) and potassium ferricyanide (K 3 [Fe(CN) 6 ]), which has an etching rate of 40 nm/min. After rinsing with DI water, the samples are cleaned with IPA and spin-dried. 3.1 Measurements Before integrating the Au or the Au/Pd hole arrays into an atto-litre titre plate device as reaction chambers, some characterization of the structures has to be done in order to decide which arrays are best suited for the optical detection method. Following, the light transmission and the fluorescent enhancement are described. To optically characterize the sub-wavelength hole arrays, the intensity of white light through the fabricated configurations is measured. Collimated illumination, which means perpendicular incidence on the array, is used in an upright optical microscope (Leica DM-RXA), similar to the optical characterizations in [14]. The detection is done with a Spectra- Cube (Applied Spectral Imaging), which provides both spectral and spatial information. This allows us to calculate the transmission by dividing the obtained transmission spectrum corrected for the background light by the spectrum of the light source. The measurements are done in the spectral range between 450 nm and 850 nm for which the sensitivity of the system is optimal. The transmissions through periodically and randomly distributed nano-holes in Au are measured for different pitches and hole diameters. An example of the recorded spectra is depicted in Fig. 6, for hole-size of 160 nm and pitch of 750 nm. Separately, but in a similar manner, the transmission is measured through squarely distributed nanoholes in Au, Pd, and Au/Pd films on glass, in order to verify the influence of Pd within the alloy and to decide if, from the optical point of view, the Au/Pd arrays are suitable to be integrated into the atto-litre titre plate device. Obtained spectra for hole size of 180 nm and pitch of 800 nm, in 200 nm thick metal films are shown in Fig. 7. In a second set of measurements, the influence of the holes periodicity on the fluorescence in/above the arrays is determined. An Olympus BX-FM upright microscope is configured with an argon-ion laser excitation source (Reliant, Laser-physics), with a wavelength JNN102 Ó IMechE 2008 Proc. IMechE Vol. 222 Part N: J. Nanoengineering and Nanosystems

6 112 O M Piciu, M W Docter, M C van der Krogt, Y Garini, I T Young, P M Sarro, and A Bossche Fig. 6 Spectra of the transmitted light through the square distribution (hole size of 160 nm with 750-nm pitch) and random distribution (hole size of 160 nm, the same filling factor as the square-hole arrays) of nano-holes in 200-nm Au film on glass, corrected for the background light intensity [a. u.] Fig. 7 Au Au/Pd Pd wavelength [nm] Spectra of the transmitted light through the square distribution (hole size of 180 nm with 800-nm pitch) of round-hole arrays in 200-nm films of Au, Au/Pd, and Pd respectively, corrected for the background light of 514 nm for illumination, and a 100 dry lens with a numerical aperture (NA) of Square holes distributed arrays in Au film are coated with Rhodamine G6 (FLUKA), 0.05 mm concentration. An overview of the sample and the measurements is given in Fig. 8. The light intensity is measured through a hole array or bare glass (provided by the marker), uncoated and coated with a fluorescent solution of Rhodamine (emission 555 nm), with and without a filter (Chroma Technology Corp., excitation 540 nm, emission 560 nm). The intensity of the detected signal can improve if the laser wavelength, the optimal transmission wavelength, and the excitation peak are better matched. The results are indicated per unit area and an average over 50 images is used. Fluorescent images are similar to the ones published elsewhere [15]. The results are compensated for the use of neutral density (ND) filter, exposure time, and illumination light. For each location, marked by A, B, C, and D in Fig. 8, the ratio R between the emission and excitation signal is calculated. The ratios for areas A and B are expected to be zero, because no fluorescent molecules are present. Nevertheless, some values in these locations appear, possibly due to the glass autofluorescence. To derive the enhancement of the transmission through the arrays compared to the signal through bare glass, the ratios of area C and D are compared after compensating for the autofluorescence. When the ratio of area A is denoted as RA, the enhancement is calculated by (RD RB)/ (RC RA), which has a value of 9. This means that, indeed, the use of arrays gives a higher signal compared to the bare glass. 3.2 Discussions Enhanced transmission, small angular diffraction, and the spectral selection of light passing through an array of sub-wavelength apertures in optically thick metal films have been described as EOT [1, 2]. Proc. IMechE Vol. 222 Part N: J. Nanoengineering and Nanosystems JNN102 Ó IMechE 2008

7 Fabrication and optical characterization of nano-hole arrays 113 Fig. 8 Overview of the sample used to determine the fluorescent enhancement due to use of an array of square holes with square distribution in Au layer on glass (the hole size is 160 nm and the pitch is 750 nm). Several areas can be discriminated; the marker (area A and C) and the array (area B and D). Some are bare (A and B), while the others have Rhodamine on top (area C and D). First, the ratios R, given by dividing the observed emission intensity (with l > 550 nm) by the excitation intensity (with all l), are calculated for the different areas. R is a measure for which part of the illumination turned into fluorescence. The overall enhancement is (RD RB)/(RC RA), which is the amount of fluorescence coming from the array (RD) and marker (RC) after correction for any auto-fluorescence (RA, RB) The transmitted light through the randomly and periodically distributed holes in Au on glass when using the same filling factor (the same number of holes of the same size/unit area) has been compared. The intensity of the light through the periodically distributed holes is, in the less optimal conditions, more than a factor of three higher than through randomly distributed holes (Fig. 6). Furthermore, the spectra of the light transmitted through the hole arrays in Au show a series of peaks at wavelengths between 450 nm and 850 nm, proving the coupling of the surface waves with the photons. The intensity of these peaks in a periodic array can be up to 25 times higher compared to the transmission through the random array. The calculation for zero incidence and the Bloch wave modes, as given in reference [1], is realized. The calculated peak positions (for a pitch of 750 nm in Au, for the air side) are 564 nm for the diagonal [ 1, 1] modes and 767 nm for the orthogonal [ 1, 0] and [0, 1] modes. Those peaks are consistent with the data and within the experimental error (10 per cent, [1]); the peaks depicted in Fig. 6 are being situated at 619 nm and 786 nm. The peak at 521 nm is explained by the direct recombination of conduction band electrons below Fermi energy with holes in the d-band [16]. Furthermore, it has been observed that when using an alloy (60/40) of Au and Pd, the shape of the holes is better defined during the lift-off procedure because of the small grains of this alloy compared to pure Au. Nevertheless, the nano-hole arrays in Au/Pd and Au have to be optically compared, in order to choose the ones that give a higher intensity of the transmission. In Fig. 7, the spectra of the light through hole arrays made in 200 nm thick Au, Au/Pd alloy, and Pd are depicted. Similar to the previous sample, the positions of the peaks have been calculated for a pitch of 800 nm, and they are as follows: for the Au array, 601 nm for the diagonal [ 1, 1] modes, and 815 nm for the orthogonal [ 1, 0] and [0, 1] modes. The calculations are consistent with the measurements, and the spectrum of Au array plotted in Fig. 7 presents peaks at 628 nm and 833 nm. Calculated peak for [ 1, 0] and [0, 1] modes for Au/Pd (60/40) alloy dielectric constant is considered as a linear sum of the dielectric constants of Au and Pd appears at 710 nm, and might be observed in the spectrum at around 750 nm, almost 80 nm left-shifted from the 833 nm Au peak. The second Au/Pd peak can not be calculated because of the multiple answers given by the specificity of the dielectric constant. Nevertheless, in Fig. 7, for the 628-nm peak of the Au array, a peak might be observed in the Au/Pd array spectrum at 554 nm, also around 80 nm left-shifted. As it appears from the three plotted spectra, Pd introduces to the Au/Pd alloy an overall blue shift and a decrease in the total transmitted intensity (almost four times for certain peaks). To optically characterize these arrays in terms of fluorescence, the enhanced fluorescence transmission has been determined. One observes that the intensity of the fluorescent light through these periodical sub-wavelength apertures is enhanced by a factor of nine compared to the fluorescence through the bare glass. JNN102 Ó IMechE 2008 Proc. IMechE Vol. 222 Part N: J. Nanoengineering and Nanosystems

8 114 O M Piciu, M W Docter, M C van der Krogt, Y Garini, I T Young, P M Sarro, and A Bossche 4 CONCLUSIONS An improved manufacturing procedure was used for fabricating different nano-hole arrays with subwavelength hole sizes in Au and Au/Pd alloy (60/40) on glass. By increasing the thickness of Cr, a more accurate e-beam exposure was obtained, resulting in a more repeatable production method. Moreover, a decreased thickness of the photo-resist gave a better aspect ratio of the pillars, avoiding problems such as their bending and shading during the metal deposition process. A higher efficiency of the lift-off technique was also achieved, together with a better defined shape of the holes. In conclusion, the improvements led to a more stable and reproducible nano-hole arrays fabrication procedure. A negative-tone e-beam resist, EBL, and lift-off technique have been used. Randomly distributed holes have also been fabricated for the optical characterization of the arrays in Au, using the same filling factors and the same technique. Analysis of the Au samples showed an enhanced transmission of light (at least three times and, for one wavelength, even 25 times higher) for the periodically distributed holes versus the randomly distributed holes. Comparing the transmission spectra of hole arrays with similar characteristics in Au, Au/Pd (60/40) alloy, and Pd, one observes that Pd introduces to the alloy samples an almost 80-nm shift to lower wavelengths and a decrease in the overall intensity, for certain peaks the transmission being almost four times lower than for Au. In the second part of the measurement, the nanohole arrays are coated with Rhodamine solution. From the calculated ratios, the intensity of the fluorescent light above/through these periodical subwavelength apertures is enhanced by a factor of nine compared to the fluorescence through bare glass. The measured enhanced transmission indicates that the Au nano-hole arrays remain the best potential candidate to be further integrated into a novel atto-litre titre plate device for high-speed molecular analysis. ACKNOWLEDGMENTS This work was partially financially supported by the Dutch Government as part of the NanoNed and MicroNed programme, under the auspices of the Stichting voor de Technische Wetenschappen (STW), and by TNO and Cyttron Consortium. It was conducted in the Nanofacility clean-room of the Kavli Institute of NanoScience, the Delft Institute for Micro-Electronics and Submicron Technology (DIMES), and the Department of Imaging Science and Technology, Delft University of Technology, The Netherlands. REFERENCES 1 Ebbesen, T. W. et al. Extraordinary optical transmission through sub-wavelength hole arrays. Nature, 1998, 391, Lezec, H. J. et al. Beaming light from a sub-wavelength aperture. Science, 2002, 297, Thio, T. et al. Strongly enhanced optical transmission through subwavelength holes in metal films. Physica B, 2000, 279, Sambles, R. More than transparent. Nature, 1998, 391, De Leebeeck, A. et al. On-chip surface based detection with nanohole arrays. Anal. Chem., 2007, 79(11), Malyarchuk, V. et al. High performance plasmonic crystal sensor formed by soft nanoimprint lithography. Optic Express, 2005, 13(15), Kwak, E. et al. Surface plasmon standing waves in large-area subwavelength hole arrays. Nano Letters, 2005, 5(10), Abdelsalam, M. E. et al. Preparation of arrays of isolated spherical cavities by self-assembly of polystyrene spheres on self-assembled pre-patterned macroporous films. Adv. Mater., 2004, 16(1), Lezec, H. J. and Thio, T. Diffracted evanescent wave model for enhanced and suppressed optical transmission through subwavelength hole arrays. Optic Express, 2004, 12(16), Bethe, H. A. Theory of diffraction by small holes. Phys. Rev., 1944, 66, Seidel, M., Dankbar, D. M., and Gauglitz, G. A miniaturized heterogeneous fluorescence immunoassay on gold-coated nano-titer plates. Anal. Bioanal. Chem., 2004, 379, Word, M. J., Adesida, I., and Berger, P. R. Nanometerperiod gratings in hydrogen silsesquioxane fabricated by electron beam lithography. J. Vac. Sci. Technol. B, 2003, 21(6), L12 L van Delft, F. C. M. J. M. et al. Hydrogen silesquioxane/ novolak bilayer resist for high aspect ratio nanoscale e-beam lithography. J. Vac. Sci. Technol. B, 2000, 18(6), Docter, M. W. et al. Measuring the wavelengthdependent divergence of transmission through subwavelength hole-arrays by spectral imaging. Optic Express, 2006, 14, Docter, M. W. et al. Structured illumination microscopy using extraordinary transmission through subwavelength hole-arrays. J. Nanophotonics, 2007, 1, Mooradian, A. Photoluminescence of metals. Phys. Rev. Lett., 1969, 22, Proc. IMechE Vol. 222 Part N: J. Nanoengineering and Nanosystems JNN102 Ó IMechE 2008