Shakeeb Bin Hasan

Deep-subwavelength plasmonic nanoresonators exploiting extreme coupling.

A metal-insulator-metal (MIM) waveguide is a canonical structure used in many functional plasmonic devices. Recently, research on nanoresonantors made from finite, that is, truncated, MIM waveguides attracted a considerable deal of interest motivated by the promise for many applications. However, most suggested nanoresonators do not reach a deep-subwavelength domain. With ordinary fabrication techniques the dielectric spacers usually remain fairly thick, that is, in the order of tens of nanometers. This prevents the wavevector of the guided surface plasmon polariton to strongly deviate from the light line. Here, we will show that the exploitation of an extreme coupling regime, which appears for only a few nanometers thick dielectric spacer, can lift this limitation. By taking advantage of atomic layer deposition we fabricated and characterized exemplarily deep-subwavelength perfect absorbers. Our results are fully supported by numerical simulations and analytical considerations. Our work provides impetus on many fields of nanoscience and will foster various applications in high-impact areas such as metamaterials, light harvesting, and sensing or the fabrication of quantum-plasmonic devices.

Second-order nonlinear frequency conversion processes in plasmonic slot waveguides

We present a theoretical study on the possibilities to achieve nonlinear frequency conversion in plasmonic slot waveguides having a χ(2) nonlinear medium as its core. Second-harmonic generation is used as a referential process to discuss the possibilities in achieving strong nonlinear interactions. We show that geometrical dispersion allows for the possibility of modal phase matching without resorting to other mechanisms like birefringence phase matching or periodic poling of the nonlinear medium. We disclose that in strongly dissipative systems two effects, the damping of individual modes and the phase-matching condition, have to be carefully balanced to assure an efficient energy conversion. Besides second-harmonic generation, emphasis is put on exploring the application of potentially more importance: the parametric amplification in the waveguide with the purpose to enhance its propagation length.

Shape manipulation of ion irradiated Ag nanoparticles embedded in lithium niobate

Spherical silver nanoparticles were prepared by means of ion beam synthesis in lithium niobate. The embedded nanoparticles were then irradiated with energetic (84)Kr and (197)Au ions, resulting in different electronic energy losses between 8.1 and 27.5 keV nm(-1) in the top layer of the samples. Due to the high electronic energy losses of the irradiating ions, molten ion tracks are formed inside the lithium niobate in which the elongated Ag nanoparticles are formed. This process is strongly dependent on the initial particle size and leads to a broad aspect ratio distribution. Extinction spectra of the samples feature the extinction maximum with shoulders on either side. While the maximum is caused by numerous remaining spherical nanoparticles, the shoulders can be attributed to elongated particles. The latter could be verified by COMSOL simulations. The extinction spectra are thus a superposition of the spectra of all individual particles.

Impedance generalization for plasmonic waveguides beyond the lumped circuit model

We analytically derive a rigorous expression for the relative impedance ratio between two photonic structures based on their electromagnetic interaction. Our approach generalizes the physical meaning of the impedance to a measure for the reciprocity-based overlap of eigenmodes. The consistence with known cases in the radiofrequency and optical domain is shown. The analysis reveals where the applicability of simple circuit parameters ends and how the impedance can be interpreted beyond this point. We illustrate our approach by successfully describing a Bragg reflector that terminates an insulator-metal-insulator plasmonic waveguide in the near-infrared by our mpedance concept.

Method for enhancing the resolution of holographic displays

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP

Reflectivity calculated for a three-dimensional silicon photonic band gap crystal with finite support

We study numerically the reflectivity of three-dimensional (3D) photonic crystals with a complete 3D photonic band gap. We employ the finite element method to study crystals with the cubic diamondlike inverse woodpile structure. The high-index backbone has a dielectric function similar to silicon. We study crystals with a range of thicknesses up to ten unit cells (L≤10c). The crystals are surrounded by vacuum, and have a finite support as in experiments. The polarization-resolved reflectivity spectra reveal Fabry-Perot fringes related to standing waves in the finite crystal, as well as broad stop bands with nearly 100% reflectivity, even for thin crystals. The frequency ranges of the stop bands change little with angle of incidence, which is plausible since the stop bands are part of the 3D band gap. Moreover, this result supports the previous assertion that intense reflection peaks measured with a large numerical aperture provide a faithful signature of the 3D photonic band gap. For p-polarized waves, we observe an intriguing hybridization between the Fabry-Perot resonances and the Brewster angle that remains to be observed in experiments. From the strong reflectivity peaks, it is inferred that the maximum reflectivity observed in experiments is not limited by finite size. The frequency ranges of the stop bands agree very well with stop gaps in the photonic band structure that pertain to infinite and perfect crystals. The angle-dependent reflectivity spectra provide an improved interpretation of the reflectivity measurements performed with a certain numerical aperture and a new insight in the crystal structure, namely unequal pore radii in X and Z directions. The Bragg attenuation lengths LB are found to be smaller by a factor 6 to 9 than earlier estimates that are based on the width of the stop band. Hence, crystals with a thickness of 12 unit cells studied in experiments are in the thick crystal limit (LLB). Our reflectivity calculations suggest that the 3D silicon photonic band gap crystals are interesting candidates for back reflectors in a solar cell in order to enhance the photovoltaic efficiency.

Resonant coupling of dielectric waveguides with plasmonic metaatoms

Metamaterials promise the possibility to tailor the propagation properties of light at the nano-scale. With this contribution we explore the possibility to combine the concept of metamaterials with integrated optics. We investigate a system consisting of a one-dimensional array of double cut-wires (two very thin gold sheets separated by a dielectric spacer) placed on top of a dielectric slab waveguide, which supports only the fundamental TE and TM mode in the near infrared spectral region around 1550 nm. Strong coupling of the waveguide modes to the plasmonic eigenmodes of the double cut-wire is achieved via the longitudinal component of the electric field, being relatively large for an asymmetric refractive index profile. By tuning the length of the double cutwires, we can tune the spectral position of the occurring hybrid resonance. We will show by rigorous calculations, that the resonance is anti-symmetric and hence produces artificial magnetism at optical frequencies in this simple scheme. To further explore the physics of the system, we investigate the dispersion relation of a periodic array of double cut-wires with varying lattice periods. The slab waveguide mode leads to a coupling of the individual plasmonic nanostructures. We find that for short lattice periods the dispersion closely resembles that of the slab waveguide. However when the Bragg frequency approaches the plasmonic resonance frequency, a strong interaction takes place and leads to a back-bending of the dispersion relation with regions of negative group velocity near to the band edge while an avoided crossing of both resonances takes place.