Fig. 1. Schematic view of experimental setup. ‘A’ is NSOM cantilever, ‘B’ is microscope objective, and ‘C’ is metal-dielectric nanolaminate.

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1. Introduction

In general, propagating waves, after passing through an aperture or pinhole, will spread out and bend from straight line propagation due to diffraction. Contrary to this classical view, several recent papers have predicted by theoretical models and simulations that diffraction can be suppressed for light propagating through properly designed structures to control light diffraction. This phenomenon is referred to by many names in the literature, such as, self-collimation, [1] self-guiding, [2] supercollimation, [3] diffraction-suppressed, [4] or non-diffractive [5] propagation. Some of these designs use 2D or 3D photonic crystals (PC) but more interesting to us due to the relative ease of fabrication are simulations which predict diffraction suppression in periodic MD nanolaminates. [4] A practical application of this phenomenon could be high resolution imaging, for example, on an endoscope, a MD nanolaminate coating on a light detection array would not only function as a protection for the sensitive electronics, but also enhance the resolution of images. Another potential application is bio-inspired [6]; such as nested rings of MD nanolaminates with independent control of the dielectric thickness or refractive index to enable variable focus in a metamaterial system. There has been considerable work done on the phenomenon of the ‘super lens’. [7] Also previous work has shown imaging performance of silver nanolaminates for lithography.⁸ Rakish, et al quantified the amount of diffraction from the light scattered in a direction normal to the direction of propagation through a 2D PC.³ In this paper, we report a direct sampling of the propagating wavefront by scanning NSOM probe to produce a cross-section of the light propagated from a sub-wavelength aperture. Our experimental result demonstrates that the diffraction of propagating light can be inhibited through MD nanolaminate compared to air.

2. Experimental setup

In the model presented by Feng and Elson [4], light emanates from an ‘H’-shaped object and propagates through a periodic series of metal and dielectric layers. Their simulations showed that the propagating light can be diffraction-enhanced or diffraction-suppressed depending on the layer thickness and material properties. To allow comparison with the Feng and Elson model, for our experimental demonstration, we fabricated apertures in the shape of the letter, ‘H’. A laser light passing through a single-mode fiber was focused onto the aperture from below with a microscope objective. We compare the light propagation up through metal-dielectric layers versus through air. The distribution of light intensity above the aperture was sampled using NSOM tip in collection mode. The NSOM tip is micromachined silicon cantilever with a circular cross-section aperture. A WiTec AlphaSNOM system was used to perform NSOM Collection Mode Scanning. Our object is an ‘H’-shaped aperture positioned within the Rayleigh range of focused beam. The divergence of the beam size is very small within the Rayleigh range, and thus the beam can be considered to be collimated. The wavelength of the incident beam is 488 nm Argon line. The Rayleigh range can be estimated by observing the intensity variation while moving sample stage along z-direction. The Rayleigh range is approximately 610 nm.

 

Fig. 2. (a). AFM image of positive photoresist surface patterned using NSOM lithography. Scan size is 10 µm×10 µm. (b) Tapping Mode AFM of APC/Ag/APC/Ag film over aperture. Scan size is 10 µm×10 µm. (c) SEM of APC/Ag/APC/Ag/APC/Ag/APC cross-section. The overall thickness of the nanolaminate is measured between the two horizontal green lines. Scale bar is 200 nm.

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The light passing through the sample was collected with NSOM cantilever sensor (<100 nm aperture) tip and detected with a photomultiplier tube (Hamamatsu). In our NSOM, the tip and objective are fixed and the sample is scanned in the x and y direction. This allows the sample to receive uniform illumination over the entire scan area. Under software control, the NSOM tip could be scanned at the surface or at an arbitrary distance above the surface. Typical scan parameters were 4 sec/line, 512 points/line, and 512 lines/image. The scan stage has a repeatability of ±2 nm in the z direction. The micrometer for positioning the scan plane for scanning in air above the aperture has a repeatability of ±5 nm.

Our object can be considered as 3 separate apertures, namely the left upright, center crossbar, and right upright. Typical dimensions of the light column emerging from each aperture were measured by NSOM. The length of the left upright, center crossbar, and right upright were 6.2 µm, 1.0 µm, and 5.9 µm, respectively. The respective widths were 0.60 µm, 0.20 µm, and 0.61µm. In the absence of the MD nanolaminate, normal diffraction occurs and one would expect to observe constructive and destructive interference patterns above a critical propagation distance. With the MD nanolaminate, the photonic bands modify the way of light diffraction. With a proper engineering of the band structure, diffraction can be slowed down significantly, and hence a better resolution image can be achieved than propagating through the air of the same distance.

 

Fig. 3. NSOM collection mode image of an ‘H’-shaped aperture through a titanium film. Aperture is illuminated with 488 nm light from below at the focal plane of microscope objective. Scan size is 10 µm×10 µm.

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To verify the concept of non-diffractive imagery, we have fabricated laterally continuous silver layers alternated with glassy polymer films in which the layer thicknesses are on the order of 20 nm and 95 nm, respectively. Figure 2(c) shows a cross-section of a nanolaminate film of 4 APC and 3 Ag layers with total thickness measured in the SEM of 441 nm.

We used NSOM in both fabrication and imaging processes. Figure 1 is the schematic showing the experimental setup of the imaging system. A focussed beam coming out of the microscope was incident from the bottom onto the aperture “H” and the MD thin film. The light leaving the MD nanolaminate was coupled to the hollow aluminum-coated circular aperture of the NSOM Si-cantilever probe and then decoupled to propagate to the detection optics. The microscope stage is scanned to construct the image.

The H-pattern apertures were fabricated using NSOM lithography [9] and glacing angle e-beam metal deposition as described in detail previously. [10] Briefly, NSOM lithography was used to pattern a positive tone resist. Then an e-beam metal evaporator system was used to deposit 180 nm of Titanium at an angle of 75° between the substrate surface normal and the direction of travel of the metal atoms. The resolution of NSOM lithography is limited by the size of the NSOM tip.

 

Fig. 4. Evidence for inhibition of light diffraction. (a) NSOM collection mode image scanned in air, 500 nm above aperture; (b) scanned at surface of metal-dielectric stack, 560 nm from aperture. Scan size is 10 µm×10 µm.

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Fig. 5. Series of NSOM scans as a function of propagation distance from the aperture. All images are 10 µm×10 µm.

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In our NSOM, the aperture is approximately 100 nm diameter. Glancing angle deposition is susceptible to introduction of imperfections due to non-uniform deposition of metal. The thickness of metal deposited is proportional to the angle between normal to the substrate and the vector joining the crucible to the arrival point. [11] The trenches written into the surface of the positive resist were typically 290 nm wide and 110 nm deep, Fig. 2(a). After glancing angle deposition of opaque Ti layer, the actual aperture width can be seen by collection mode NSOM (Fig. 3) at the surface. Metal-Dielectric nanolaminates were made of silver-polycarbonate multilayers. Silver layer was formed by sputtering from a silver target. The polycarbonate layers were made by spin coating amorphous polycarbonate (APC). To prevent silver oxidation, samples were stored under nitrogen except during measurements.

Fabrication defects can affect the propagation of the light through the MD nanolaminates. Roughness in the metal films can cause local variation in the propagation distance between metal layers. Other defects from the fabrication, such as, cracks, buckling, pinholes, dust particles, phase separated impurities, can spread and otherwise degrade the propagating light. In our fabrication technique, there is also the possibility of transference of fabrication defects from layer to layer. However, we were able to obtain flat layers over many of the apertures that we fabricated, as can be seen in the AFM scan of a silver layer, Fig. 2(b). The continuity of the silver layers is critical to obtain a good image. The film roughness and thickness was characterized with a Digital Instruments DI3100 AFM system operating in Tapping Mode. Sputtering of Ag on APC gave high quality films with the roughness of the silver layers 0.3 nm RMS and thickness of 20 nm. It was found that high quality APC films could be made by spin coating with RMS roughness 0.2 nm, thickness of 95 nm, and <1 pinhole per 10 µm×10 µm area.

3. Results and discussion

Scanning of the NSOM probe at a height of 500 nm above the surface shows that the light propagating through air is diffracted as evidenced by the blurred appearance of the image, Fig. 4(a). Conversely, scanning on a Ag/APC multilayer stack of 560 nm cumulative thickness exhibits much less diffraction, Fig. 4(b). In Fig. 5, we show a series of images scanned as a function of propagation distance from the aperture. In each case, the images scanned over the MD nanolaminate are sharper than the corresponding scan in air. We have calculated the full width half maximum of the normalized integrated light intensity across the left upright aperture for each image in Fig. 5 (Table 1).

Table 1. Comparison of diffusion in each image of Fig. 5. quantified by width of normalized light intensity.

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In this work, we have demonstrated, as a proof-of-principle, slowed diffraction for light propagating through metal-dielectric stack, but the light intensity was considerably diminished. The diminished light intensity was due to absorbance. Utilizing plasmon resonance effect, the light transmission can be enhanced for certain critical film thickness and light wavelength. Presently, we are studying the effect of wavelength and film thickness and will present those results in a future paper.