In recent years there has been significant interest in broadband, omnidirectional antireflection (AR) nanostructures that minimize Fresnel reflection due to index (impedance) mismatch at an optical interface ^{[1] [2]} .  The reflection can be suppressed by using adiabatic impedance matching implemented as an intermediate material with gradually varying index in the direction of surface normal. Subwavelength patterning is an effective method to implement such a gradient index (GRIN) surface.  However, these recent studies have been mostly restricted to planar surfaces.  Diffractive optical elements such as diffraction gratings, Fresnel zone plates, and holographic optics also suffer from Fresnel reflection losses evidenced as undesirable reflection orders.  Recently, we proposed a new class of GRIN diffractive optics that is capable of suppressing such reflection losses ^[3] .  Using the same GRIN principles, we can demonstrate diffractive elements where the reflected energy can be suppressed.

The proposed concept of the nanostructured GRIN grating is illustrated in Figure 1, where subwavelength tapered nanostructures with period p are integrated on both the ridge and groove of the grating.  Top-view and cross-section micrographs of the fabricated GRIN grating in silicon substrate are depicted in Figure 2. The grating has a period Λ of 5 mm, and the subwavelength cone-shaped pillars have nominal base diameter of 150 nm. The fabricated structure resembles a grating with nano-engineered surfaces. A cross-sectional micrograph is shown in Figure 2(b), where the cone heights on the ridge and groove are 650 and 600 nm, respectively. Some point defects characteristic of nanosphere self-assembly used in the fabrication process can be observed.  Broadband characterization of the fabricated structure indicated suppression by at least two orders of magnitude in the reflected orders of the GRIN grating over a large range of incident angles up to 60º.

1. P. Lalanne and G. M. Morris, “Antireflection behavior of silicon subwavelength periodic structures for visible light,” Nanotechnology, vol. 8, pp. 53-56, Oct. 1997.
2. Y. Kanamori, M. Sasaki, and K. Hane, “Broadband antireflection gratings fabricated upon silicon substrates,” Opt. Lett., vol. 24, no. 20, pp. 1422-1424, Oct. 1999.
3. C.-H. Chang, L. Waller, and G. Barbastathis, “Design and optimization of broadband wide-angle antireflection structures for binary diffractive optics,” Opt. Lett., vol. 35, no. 7, pp. 907-909, April 2010.

Selective detection of explosive compounds is critical for national defense and homeland security. Nitroaromatic compounds pose a particular threat; 2,4,6-trinitrotoluene (TNT), for example, is an inexpensive and readily available component of fifteen of the most widely used blends ^[1] . Existing methods to detect explosives include biosensors ^[2] , electrochemical sensors  ^[3] and fiber optic ^[4] sensors. Devices utilizing chromatography ^[5] and Raman spectroscopy ^[6] are used for the same purpose. However, sensors using the aforementioned techniques require complicated sensing and readout components; moreover, they are comparatively large in size and consume significant amounts of power during operation. In this work we describe the fabrication and demonstration of a chemical sensor capable of detecting nitroaromatic explosives in air. The aim of this work is the development of a simple sensor that has the unique features of micro-scale dimensions, simple and inexpensive fabrication, and low power consumption. It consists of a nano-patterned conductive metal line placed on top of a patterned responsive polymer, poly(4-vinylpyridine) (P4VP), as shown in Figure 1. Due to polymer-solvent interactions, P4VP swells when it encounters the target analyte, producing a large stress. Detection takes place by monitoring the change in device resistance as the metal nano line deforms or fractures when P4VP swells and transfers mechanical stress.

Fabricated devices were tested for their response to nitroaromatic exposure using a previously described system((W. E Tenhaeff, L. D. McIntosh, and K. K. Gleason, “Synthesis of poly(4-vinylpyridine) thin films by initiated chemical vapor deposition (iCVD) for selective nanotrench-based sensing of nitroaromatics,” Advanced Functional Materials, vol. 20, no. 7, pp. 1144-1151, 2010.)) ^[7] . Test devices were located on a cooled stage within a flow cell; swelling responses of P4VP films were measured via in situ interferometry. Figure 2 illustrates the change in device resistance for a 200-nm-thick, 5-μm-wide P4VP line intersected by a 100-nm-thick, 300-nm-wide Au line sensor upon exposure to 500 ppm of nitrobenzene. The concentration was increased to 650 ppm at t=15mins. The change in resistance corresponds well to the calculated change in exposure concentration. A permanent increase (8.5%) in resistance is clearly observed as the result of permanent deformation and micro-cracks; this change is large enough to be easily detected.

1. S. J. Toal, and W. C. Trogler, “Polymer sensors for nitroaromatic explosives detection,” Journal of Materials Chemistry, vol. 16, no. 28, pp. 2871-2883, Apr. 2006.
2. R. M. Wadkins, J. P. Golden, L. M. Pritsiolas, F., and S. Ligler, “Detection of multiple toxic agents using a planar array immunosensor,” Biosensors and Bioelectronics, vol. 13, no. 3, p. 407, 1998.
3. K. Masunaga, K. Hayama, T. Onodera, K. Hayashi, N. Miura, K. Matsumoto, and K. Toko, “Detection of aromatic nitro compounds with electrode polarization controlling sensor,” Sens. Actuators B, vol. 108, nos. 1-2, p. 427, 2005.
4. R. A. Ogert, L. C. Shriver-Lake, and F. S. Ligler, “Toxin detection using a fiber-optic-based biosensor,” in Proc. SPIE, 1993, vol. 1885, Mar. 1993, p. 11-17.
5. A. Hilmi, J. H. T. Luong, and A. L. Nguyen, “Determination of explosives in soil and ground water by liquid chromatography–amperometric detection,” Journal of Chromatogr. A, vol. 844, nos. 1-2, p. 97, 1999.
6. I. R. Lewis, N. W. Daniel Jr., N. C. Chaffin, P. R. Griffiths, and M. W. Tungol, “Raman spectroscopic studies of explosive materials: towards a fieldable explosives detector,” Spectrochimica Acta Part A, vol. 5, no. 12, p. 1985, 1995.
7. W. J. Arora, W. E. Tenhaeff, K. K. Gleason, and G. Barbastathis., “Integration of reactive polymeric nanofilms into a low-power electromechanical switch for selective chemical sensing,” Journal of Microelectromechical Systems, vol. 18, no. 1, pp. 97-102, 2009.

Luneburg lens is a gradient index (GRIN) element ^[1] known to produce diffraction-limited focus at the lens edge opposite to an incident plane wave. Despite its usefulness in applications such as radar systems, omnireflectors, or integrated optics, implementing the Luneburg lens in optical frequencies due to the difficulty in producing the desired GRIN profile. In this work, we describe the design and fabrication of a Luneburg lens for operation at near infrared optical frequencies using subwavelength aperiodic nanostructures.

The Luneburg lens is designed using a dielectric periodic square lattice of circular silicon rods with spatially varying parameters and subwavelength features. If the variation in the structure is gradual enough to be considered periodic within the adiabatic length scale, local dispersion relations can be analyzed through established photonic crystal theory ^[2] , and Hamiltonian optics can be used to analyze and design the propagation of light with adequate accuracy ^{[3] [4]} .  Using the developed algorithm less computational power is needed, and it allows for convenient structure optimization.

We designed a Luneburg lens with lattice constant of λ/6 at operating wavelength λ = 1.55 µm and fabricated a planar (2D) implementation using silicon-on-insulator (SOI) substrate.  The structure was patterned using electron-beam lithography and transferred into the device layer using reactive ion etching.  Although this structure suffers slightly from structure anisotropy, through design and optimization we were able to obtain a geometrical waist diameter calculated as λ/3 at the lens focus, as depicted in Figure 1. The fabricated structure, shown in Figure 2, has minimum feature size of around 90 nm, which can be readily achieved by the resolution limits of our lithographic approach. The fabricated lens is being characterized using scanning near-field optical microscope, and initial results demonstrate tight focusing of light.

1. R. K. Luneburg, Mathematical Theory of Optics, Berkeley, CA: University of California Press, 1964.
2. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals Molding the Flow of Light, Princeton, NJ: Princeton University Press, 2008.
3. P. S. J. Russell and T. A. Birks, “Hamiltonian optics of nonuniform photonic crystals,” J. Lightwave Technol., vol. 17, pp. 1982-1988, Nov. 1999.
4. Y. Jiao, S. Fan, and D. A. B. Miller, “Designing for beam propagation in periodic and nonperiodic photonic nanostructures: Extended Hamiltonian method,” Phys Rev E, vol. 70, pp. 036612, Sep. 1999.