Plasmonic structures for enhancement of semiconductor infrared detectors.

Plazmonika je jedna od oblasti nauke koje se u današnje vreme eksplozivno razvijaju. Ona je posvecena elektromagnetici nanokompozitnih metamaterijala koji podržavaju rezonanciju površinskih plazmona polaritona (surface plasmons polaritons, SPP). SPP predstavljaju hibridne ekscitacije nastale sprezanjem elektromagnetnih talasa sa oscilacijama slobodnih nosilaca naelektrisanja na razdvojnim površima izmeðu dva materijala sa razlicitim znakovima relativne dielektricne permitivnosti, npr. provodnika i dielektrika. Posledica ovakvog sprezanja je izmeðu ostalog lokalizacija elektromagnetnog zracenja na podtalasnom nivou, osobina plazmonskih struktura koja je našla veliku primenu u spektroskopiji, integrisanoj optici, senzorici itd. Jedna od znacajnih primena plazmonske lokalizacije je u oblasti u fotodetekcije, pre svega za poboljšanje performansi solarnih celija. Najveci problem proširenja primene plazmonike u fotodetekciji na drugu oblast od interesa, infracrvene (IC) detektore, predstavlja cinjenica da je plazmonska ucestanost vecine provodnika (metala) u ultraljubicastom ili vidljivom delu spektra. Brojne tehnološki pogodne tehnike koje su dale izuzetne rezultate za poboljšanje solarnih celija ostale su zbog toga bez primene u IC oblasti. Ova disertacija se prevashodno bavi proširenjem primenljivosti plazmonike na srednjetalasnu i dugotalasnu infracrvenu oblast i metodama prevazilaženja ogranicenja koje postavljaju sami materijali. U tu svrhu razmatrana su dva pristupa. Jedan od njih podrazumeva upotrebu submikrometarskih cestica od provodnog materijala. Funkcionalnost u IC oblasti postiže se kombinacijom izbora pogodnijeg materijala cestica (elektroprovodni opticki providni oksid umesto metala) i imerzije cestica u dielektrik visokog indeksa prelamanja. Drugi pristup podrazumeva korišcenje tankih metalnih slojeva sa ureðenom matricom apertura koji omogucuju pomeranje spektralne zavisnosti prema crvenom delu spektra menjanjem iskljucivo geometrijskih parametara matrice apertura. Oba pristupa nude mogucnost prakticno proizvoljnog podešavanja frekvencije plazmonske rezonancije i time njenu upotrebu za IC detektore. Analiza ova dva pristupa raðena je numerickim simulacijama, primenom metode konacnih elemenata. Uticaj na performanse infracrvenih detektora odreðivan je kombinovanjem rezultata numerickih modelovanja sa analitickim modelom IC detektora...Plasmonics is one of the fastest growing fields in the contemporary science. Plasmonics studies properties of nanocomposite metamaterials which support surface plasmon polariton (SPP) resonance. SPP are formed by coupling electromagnetic waves with free charge carrier oscillations at an interface between materials with different signs of their relative permittivity i.e. conductor and dielectric. One of the results of this coupling is localization of electromagnetic radiation on subwavelength scale, property of plasmonic structures that has found practical use in the fields of spectroscopy, integrated optics, sensors, etc. One of the principal applications of light localization is in the field of photodetection, primarily for the enhancement of solar cells. The main problem with any attempt to apply plasmonics for photodetector enhancement at longer wavelengths, i.e. for infrared (IR) detectors, is that the plasmon resonance frequency of most conductive materials (metals) is in the ultraviolet or visible part of the spectrum. Because of this many convenient methods yielding excellent results for plasmonic enhancement of solar cells have not been utilized in the infrared. The main goal of this dissertation is bringing plasmonic enhancement of semiconductor photodetectors to medium and long wavelength infrared parts of the spectrum by overcoming limitations imposed by material properties. To achieve this two approaches are considered and analyzed. The first approach implies the use of submicrometer conductive particles. A sufficient shift of plasmonic resonance to the infrared is achieved by both a suitable choice of the particle material (transparent conductive oxides – TCO instead of metal) and by immersion of the particles in dielectric with a large index of refraction. The second approach is based on using thin metallic films with 2D array of holes drilled through them, where redshifting is achieved by tuning the geometrical properties of the hole array. It is shown that both approaches allow one to achieve practically arbitrary positioning of plasmonic resonance in the infrared. The finite element method was used for numerical simulations of the analyzed structures. A combination of the results of numerical modeling with the analytical results for the IR detectors was used to determine the effects of the plasmonic enhancement..

Plasmonic enhancement of light trapping in photodetectors

We consider the possibility to use plasmonics to enhance light trapping in such semiconductor detectors as solar cells and infrared detectors for night vision. Plasmonic structures can transform propagating electromagnetic waves into evanescent waves with the local density of states vastly increased within subwavelength volumes compared to the free space, thus surpassing the conventional methods for photon management. We show how one may utilize plasmonic nanoparticles both to squeeze the optical field into the active region and to increase the optical path by Mie scattering, apply ordered plasmonic nanocomposites (subwavelength plasmonic crystals or plasmonic metamaterials), or design nanoantennas to maximize absorption within the detector. We show that many approaches used for solar cells can be also utilized in infrared range if different redshifting strategies are applied

Copper-Nickel heterometallic multilayer composites for plasmonic applications

Plasmonics and optical metamaterials offer possibilities for numerous applications in different fields, from transformation optics and chemical sensing to merging the beneficial properties of electronic and optical circuits. Crucial for their function are interfaces between materials of which one has to exhibit negative value of relative dielectric permittivity due to the existence of free electron plasma. However, the choice of convenient materials is rather limited and their performance is severely impaired by strong absorption losses. This is the reason why alternative plasmonic media are currently of an increasing interest. In this contribution we consider one such medium, the heterometallic multilayer consisting of copper and nickel. Copper is an excellent plasmonic material, but needs protection against surface oxidation, a role fulfilled by nickel layers which simultaneously form interfaces supporting surface waves. We describe our proposed heterometallics and consider their electromagnetic properties and experimental fabrication. Ab initio numerical simulations were done using the finite element method for Cu-Ni multilayers on a copper substrate. Laminate composite structures of alternately electrodeposited nanocrystalline Ni and Cu films on cold-rolled polycrystalline copper substrates were fabricated. Highly-densified parallel interfaces can be obtained by depositing layers at a very narrow spacing. Our results show that Cu-Ni pairs are a viable alternative to conventional plasmonic media, while the electrodeposition approach offers acceptable structural and electromagnetic parameters with large area and good uniformity at a low cost

Aluminum-based fishnets with complex aperture shapes

Plasmonic structures ensure electromagnetic field localization and concentration within subwavelength volumes, thus enabling numerous practical applications. Structuring at levels much below the operating wavelength can be used to obtain novel electromagnetic modes, an example being the designer plasmon structures where one utilizes ordered arrays of subwavelength holes to mimic the behavior of metals in different frequency ranges and obtain the designer (spoof) plasmons, surface waves with a resonant frequency tailorable by design in a wide range. In this paper we consider structures with aperturebased pattern that use aluminum for the negative permittivity part instead of the usual gold or silver. We experimentally investigate patterns with complex aperture shapes which ensure richer modal behavior of the structures, including a possibility to customize their frequency dispersion and to obtain tailorable deeply subwavelength electromagnetic field hotspots. To this purpose we utilize the proximity effects (field concentration in deep subwavelength gaps) and the edge effects (localization on sharp corners). We conclude that a new degree of design freedom is obtained when utilizing aluminum-based aperture arrays and that the proximity and edge effects at a deep subwavelength level can be used to enhance the effects of spoof plasmons. It is important at that to take into account the influence of native oxide of aluminum that appears in practical structures, as well as the increased absorption losses

Field localization control in aperture-based plasmonics by Boolean superposition of primitive forms at deep subwavelength scale

Aperture-based nanoplasmonics deals with an important class of structures using surface plasmons polaritons at subwavelength hole arrays to control propagation and localization of electromagnetic fields [1]. Such structures include extraordinary optical transmission arrays [2], fishnet-based metamaterials [3] as well as different other metasurface with applicability in sensing and detection, waveguiding, etc. [4]. Sensing: plasmonics, fiber sensors, interferometers Contributed papers 171 The electromagnetic field localization in aperture-based plasmonic structures can be controled by modifying of the structure geometry at the deep subwavelength scale [5-6]. In this contribution we define subwavelength primitive objects that are combined in Boolean manner by applying logical operations like AND or OR to them to generate complex-shaped apertures and thus modify the subwavelength unit cell geometry. Generally, any arbitrary shape may be presented as superposition of a number of primitive forms (corresponding to a series expansion). The approach can be used to generate field hotspots in a controled manner and redistribute field within a unit cell. Boolean operations applied to aperture shapes not only ensure “fine tuning” of scattering characteristics, but also the redesign of spectral characteristics of nanohole-based metasurfaces, owing to control over field concentration and the appearance of strong field nonlocalities. We designed our designer (spoof) plasmon structures [7] with deep subwavelength modifications, simulated their scattering parameters by the finite element method and fabricated the experimental samples for the mid-wavelength infrared range using the conventional silicon-based planar technologies with silver as plasmonic material. The shapes were obtained by overlapping (Boolean OR) square primitive objects. We show that one can use a set of primitive shapes readily produced by the existing lithographic equipment to generate strong field nonlocalities without increasing the complexity of the system or requiring finer resolutions. Actually the simplest situation would be to simply shift the same photolithographic mask and repeat the already used pattern. Owing to the redistribution of spectral characteristics and their structural tuning, the present approach can be used for multispectral operation of plasmonic chemical or biological sensors

Readout beam coupling strategies for plasmonic chemical or biological sesnors

Plasmonic devices are among the most sensitive contemporary sensors of chemical or biological analytes, in some cases even reaching single molecule sensitivity. These are refractometric devices making use of extreme light concentrations and offering real-time, label-free operation. For their proper operation one needs efficient coupling between the propagating interrogation beam and bound surface plasmons polaritons (SPP) whose wave vector is much larger. An external prism in Kretchmann or Otto configuration can be used for such coupling, or some kind of diffraction coupler or fiber-based endfire coupler. In this contribution we investigate theoretically and experimentally possibilities to fabricate sensor structures that simultaneously exhibit plasmonic properties and ensure diffraction-based coupling. To this purpose we fabricated different micrometer-sized two-dimensional metal-dielectric arrays that can match wave vectors of propagating beams and of SPP and at the same time show plasmonic properties tunable by design. We investigated structures functioning in reflection or in transmission mode, with gold or aluminum as the basic material and with deep subwavelength details. Our structures can be made much more compact than the conventional ones, thus being convenient for monolithic on-chip integration with light source and detector and offering a larger degree of design freedom for multianalyte CORN sensing

EULAR Points to Consider for the use of imaging to guide interventional procedures in patients with rheumatic and musculoskeletal diseases (RMDs)

OBJECTIVES: To develop evidence-based Points to Consider (PtC) for the use of imaging modalities to guide interventional procedures in patients with rheumatic and musculoskeletal diseases (RMDs). METHODS: European Alliance of Associations for Rheumatology (EULAR) standardised operating procedures were followed. A systematic literature review was conducted to retrieve data on the role of imaging modalities including ultrasound (US), fluoroscopy, MRI, CT and fusion imaging to guide interventional procedures. Based on evidence and expert opinion, the task force (25 participants consisting of physicians, healthcare professionals and patients from 11 countries) developed PtC, with consensus obtained through voting. The final level of agreement was provided anonymously. RESULTS: A total of three overarching principles and six specific PtC were formulated. The task force recommends preference of imaging over palpation to guide targeted interventional procedures at peripheral joints, periarticular musculoskeletal structures, nerves and the spine. While US is the favoured imaging technique for peripheral joints and nerves, the choice of the imaging method for the spine and sacroiliac joints has to be individualised according to the target, procedure, expertise, availability and radiation exposure. All imaging guided interventions should be performed by a trained specialist using appropriate operational procedures, settings and assistance by technical personnel. CONCLUSION: These are the first EULAR PtC to provide guidance on the role of imaging to guide interventional procedures in patients with RMDs

Discovertebral (Andersson) lesions in severe ankylosing spondylitis: a study using MRI and conventional radiography

The objective of this study is to investigate the prevalence of Andersson lesions (AL) in ankylosing spondylitis (AS) patients who will start anti-tumor necrosis factor (TNF) treatment. Radiographs and magnetic resonance imaging (MRI) of the spine were performed before therapy with anti-TNF. ALs were defined as discovertebral endplate destructions on MRI, associated with bone marrow edema and fat replacement or sclerosis, a decreased signal on T1, enhancement after contrast administration (gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA)), and increased signal on T2 and short tau inversion recovery (STIR). Additionally, conventional radiography showed a fracture line, irregular endplates, and increased sclerosis of adjacent vertebral bodies. Fifty-six AS patients were included, 68% males, mean age of 43 years, and mean disease duration of 11 years. The mean bath ankylosing spondylitis disease activity index was 6.4, and 24% of all patients had ankylosis. Only one patient showed a discovertebral abnormality with bone marrow edema of more than 50% of the vertebral bodies adjacent to the intervertebral disk of T7/T8 and T9/T10, a hypodense signal area on T1, and a high signal on STIR. Irregular endplates were depicted, and T1 after Gd-DTPA demonstrated high signal intensity around the disk margins. However, no fracture line was visible on conventional radiology, and therefore, this case was not considered to be an AL. No AL was detected in our AS patients, who were candidates for anti-TNF treatment. One patient showed a discovertebral abnormality on MRI, without a fracture line on conventional radiology. The relative small proportion of patients with a long-established disease might explain this finding for, particularly, an ankylosed spine is prone to develop an AL

Discovertebral (Andersson) lesions of the spine in ankylosing spondylitis revisited

A well-known complication in patients with ankylosing spondylitis (AS) is the development of localised vertebral or discovertebral lesions of the spine, which was first described by Andersson in 1937. Since then, many different terms are used in literature to refer to these localised lesions of the spine, including the eponym ‘Andersson lesion’ (AL). The use of different terms reflects an ongoing debate on the exact aetiology of the AL. In the current study, we performed an extensive review of the literature in order to align communication on aetiology, diagnosis and management between treating physicians. AL may result from inflammation or (stress-) fractures of the complete ankylosed spine. There is no evidence for an infectious origin. Regardless of the exact aetiology, a final common pathway exists, in which mechanical stresses prevent the lesion from fusion and provoke the development of pseudarthrosis. The diagnosis of AL is established on conventional radiography, but computed tomography and magnetic resonance imaging both provide additional information. There is no indication for a diagnostic biopsy. Surgical instrumentation and fusion is considered the principle management in symptomatic AL that fails to resolve from a conservative treatment. We advise to use the term Andersson lesion for these spinal lesions in patients with AS

The genetic architecture of the human cerebral cortex

The cerebral cortex underlies our complex cognitive capabilities, yet little is known about the specific genetic loci that influence human cortical structure. To identify genetic variants that affect cortical structure, we conducted a genome-wide association meta-analysis of brain magnetic resonance imaging data from 51,665 individuals. We analyzed the surface area and average thickness of the whole cortex and 34 regions with known functional specializations. We identified 199 significant loci and found significant enrichment for loci influencing total surface area within regulatory elements that are active during prenatal cortical development, supporting the radial unit hypothesis. Loci that affect regional surface area cluster near genes in Wnt signaling pathways, which influence progenitor expansion and areal identity. Variation in cortical structure is genetically correlated with cognitive function, Parkinson's disease, insomnia, depression, neuroticism, and attention deficit hyperactivity disorder