Publications by authors named "Debashis Chanda"

34 Publications

Organic Non-Wettable Superhydrophobic Fullerite Films.

Adv Mater 2021 Jun 17:e2102108. Epub 2021 Jun 17.

NanoScience Technology Center, University of Central Florida, Orlando, FL, 32826, USA.

A long-standing quest in material science has been the development of non-wettable superhydrophobic films based on a single organic material, without the requirement of fluorination or silane treatment. Here, such films and coatings, which are developed using colloidal gels of fullerite C and C nanocrystals, are described. It is illustrated that despite the high surface energy of these van der Waals molecular crystals their gelation can create films having self-affine fractal surfaces with multiscale roughness. Water droplets on resulting surfaces of fullerite films bead like a pearl resting in a Fakir state with contact angle exceeding 150°. The films are extremely water repellent and non-wettable; when submerged in water they stay dry up to 3 h even at a water depth of two feet and exhibit the plastron effect. A series of experiments are presented to provide comprehensive inspection of water droplet dynamics on these films. These include rolling, bouncing, squeezing, freezing, melting, evaporating; along with acidic and alkaline tests. Non-wettable films of such materials are unique as fullerites get photosensitized instantaneously generating extremely high yields (≈100%) of singlet oxygen ( O ) that can destroy viruses and bacteria; thereby enabling their use in rheology, water purification, and medicinal devices.
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http://dx.doi.org/10.1002/adma.202102108DOI Listing
June 2021

Scalable Van der Waals Two-Dimensional PtTe Layers Integrated onto Silicon for Efficient Near-to-Mid Infrared Photodetection.

ACS Appl Mater Interfaces 2021 Apr 23;13(13):15542-15550. Epub 2021 Mar 23.

NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826, United States.

In recent years, there has been increasing interest in leveraging two-dimensional (2D) van der Waals (vdW) crystals for infrared (IR) photodetection, exploiting their unusual optoelectrical properties. Some 2D vdW materials with small band gap energies such as graphene and black phosphorus have been explored as stand-alone IR responsive layers in photodetectors. However, the devices incorporating these IR-sensitive 2D layers often exhibited poor performances owing to their preparation issues such as limited scalability and air instability. Herein, we explored wafer-scale 2D platinum ditelluride (PtTe) layers for near-to-mid IR photodetection by directly growing them onto silicon (Si) wafers. 2D PtTe/Si heterojunctions exhibited wavelength- and intensity-dependent high photocurrents in a spectral range of ∼1-7 μm, significantly outperforming stand-alone 2D PtTe layers. The observed superiority is attributed to their excellent Schottky junction characteristics accompanying suppressed carrier recombination as well as optical absorbance competition between 2D PtTe layers and Si. The direct and scalable growth of 2D PtTe layers was further extended to demonstrate mechanically flexible IR photodetectors.
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http://dx.doi.org/10.1021/acsami.1c03512DOI Listing
April 2021

Self-assembled plasmonics for angle-independent structural color displays with actively addressed black states.

Proc Natl Acad Sci U S A 2020 Jun 3;117(24):13350-13358. Epub 2020 Jun 3.

Department of Physics, University of Central Florida, Orlando, FL 32816;

Nanostructured plasmonic materials can lead to the extremely compact pixels and color filters needed for next-generation displays by interacting with light at fundamentally small length scales. However, previous demonstrations suffer from severe angle sensitivity, lack of saturated color, and absence of black/gray states and/or are impractical to integrate with actively addressed electronics. Here, we report a vivid self-assembled nanostructured system which overcomes these challenges via the multidimensional hybridization of plasmonic resonances. By exploiting the thin-film growth mechanisms of aluminum during ultrahigh vacuum physical vapor deposition, dense arrays of particles are created in near-field proximity to a mirror. The sub-10-nm gaps between adjacent particles and mirror lead to strong multidimensional coupling of localized plasmonic modes, resulting in a singular resonance with negligible angular dispersion and ∼98% absorption of incident light at a desired wavelength. The process is compatible with arbitrarily structured substrates and can produce wafer-scale, diffusive, angle-independent, and flexible plasmonic materials. We then demonstrate the unique capabilities of the strongly coupled plasmonic system via integration with an actively addressed reflective liquid crystal display with control over black states. The hybrid display is readily programmed to display images and video.
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http://dx.doi.org/10.1073/pnas.2001435117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7306820PMC
June 2020

Dirac plasmon-assisted asymmetric hot carrier generation for room-temperature infrared detection.

Nat Commun 2019 Aug 2;10(1):3498. Epub 2019 Aug 2.

Department of Physics, University of Central Florida, Orlando, FL, 32816, USA.

Due to the low photon energy, detection of infrared photons is challenging at room temperature. Thermoelectric effect offers an alternative mechanism bypassing material bandgap restriction. In this article, we demonstrate an asymmetric plasmon-induced hot-carrier Seebeck photodetection scheme at room temperature that exhibits a remarkable responsivity of 2900 VW, detectivity of 1.1 × 10 Jones along with a fast response of ~100 ns in the technologically relevant 8-12 µm band. This is achieved by engineering the asymmetric electronic environment of the generated hot carriers on chemical vapor deposition grown large area nanopatterned monolayer graphene, which leads to a temperature gradient of 4.7 K across the device terminals for an incident power of 155 nW, thereby enhancing the photo-thermoelectric voltage by manifold compared to previous reports. The results presented outline a strategy for uncooled, tunable, and multispectral infrared detection.
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http://dx.doi.org/10.1038/s41467-019-11458-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6677812PMC
August 2019

Buckling and twisting of advanced materials into morphable 3D mesostructures.

Proc Natl Acad Sci U S A 2019 07 19;116(27):13239-13248. Epub 2019 Jun 19.

Center for Bio-Integrated Electronics, Northwestern University, Evanston, IL 60208;

Recently developed methods in mechanically guided assembly provide deterministic access to wide-ranging classes of complex, 3D structures in high-performance functional materials, with characteristic length scales that can range from nanometers to centimeters. These processes exploit stress relaxation in prestretched elastomeric platforms to affect transformation of 2D precursors into 3D shapes by in- and out-of-plane translational displacements. This paper introduces a scheme for introducing local twisting deformations into this process, thereby providing access to 3D mesostructures that have strong, local levels of chirality and other previously inaccessible geometrical features. Here, elastomeric assembly platforms segmented into interconnected, rotatable units generate in-plane torques imposed through bonding sites at engineered locations across the 2D precursors during the process of stress relaxation. Nearly 2 dozen examples illustrate the ideas through a diverse variety of 3D structures, including those with designs inspired by the ancient arts of origami/kirigami and with layouts that can morph into different shapes. A mechanically tunable, multilayered chiral 3D metamaterial configured for operation in the terahertz regime serves as an application example guided by finite-element analysis and electromagnetic modeling.
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http://dx.doi.org/10.1073/pnas.1901193116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6613082PMC
July 2019

Novel liquid crystal photonic devices enabled by two-photon polymerization [Invited].

Opt Express 2019 Apr;27(8):11472-11491

In addition to displays, liquid crystals (LCs) have also found widespread applications in photonic devices, such as adaptive lens, adaptive optics, and sensors, because of their responses to electric field, temperature, and light. As the fabrication technique advances, more sophisticated devices can be designed and created. In this review, we report recent advances of two-photon polymerization-based direct-laser writing enabled LC devices. Firstly, we describe the basic working principle of two-photon polymerization. With this powerful fabrication technique, we can generate anchoring energy by surface morphology to align LC directors on different form factors. To prove this concept, we demonstrate LC alignment on planar, curvilinear surfaces as well as in three-dimensional volumes. Based on the results, we further propose a novel, ultra-broadband, twisted-nematic diffractive waveplate that can potentially be fulfilled by this technique. Next, we briefly discuss the current status of direct-laser writing on LC reactive mesogens and its potential applications. Finally, we present two design challenges: fabrication yield and polymer relaxation/deformation, remaining to be overcome.
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http://dx.doi.org/10.1364/OE.27.011472DOI Listing
April 2019

Wireless, battery-free optoelectronic systems as subdermal implants for local tissue oximetry.

Sci Adv 2019 03 8;5(3):eaaw0873. Epub 2019 Mar 8.

Departments of Materials Science and Engineering, Biomedical Engineering, Neurological Surgery, Chemistry, Mechanical Engineering, Electrical Engineering and Computer Science, Simpson Querrey Institute and Feinberg Medical School, Center for Bio-Integrated Electronics, Northwestern University, Evanston, IL 60208, USA.

Monitoring regional tissue oxygenation in animal models and potentially in human subjects can yield insights into the underlying mechanisms of local O-mediated physiological processes and provide diagnostic and therapeutic guidance for relevant disease states. Existing technologies for tissue oxygenation assessments involve some combination of disadvantages in requirements for physical tethers, anesthetics, and special apparatus, often with confounding effects on the natural behaviors of test subjects. This work introduces an entirely wireless and fully implantable platform incorporating (i) microscale optoelectronics for continuous sensing of local hemoglobin dynamics and (ii) advanced designs in continuous, wireless power delivery and data output for tether-free operation. These features support in vivo, highly localized tissue oximetry at sites of interest, including deep brain regions of mice, on untethered, awake animal models. The results create many opportunities for studying various O-mediated processes in naturally behaving subjects, with implications in biomedical research and clinical practice.
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http://dx.doi.org/10.1126/sciadv.aaw0873DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6408152PMC
March 2019

Multi-spectral frequency selective mid-infrared microbolometers.

Opt Express 2018 Dec;26(25):32931-32940

Frequency selective detection of low energy photons is a scientific challenge using natural materials. A hypothetical surface which functions like a light funnel with very low thermal mass in order to enhance photon collection and suppress background thermal noise is the ideal solution to address both low temperature and frequency selective detection limitations of present detection systems. Here, we present a cavity-coupled quasi-three dimensional plasmonic crystal which induces impedance matching to the free space giving rise to extraordinary transmission through the sub-wavelength aperture array like a "light funnel" in coupling low energy incident photons resulting in frequency selective perfect (~100%) absorption of the incident radiation and zero back reflection. The peak wavelength of absorption of the incident light is almost independent of the angle of incidence and remains within 20% of its maximum (100%) up to θ≤45˚. This perfect absorption results from the incident light-driven localized edge "micro-plasma" currents on the lossy metallic surfaces. The wide-angle light funneling is validated with experimental measurements. Further, a super-lattice based electronic biasing circuit converts the absorbed narrow linewidth (/< 0.075) photon energy inside the sub-wavelength thick film (< λ/100) to voltage output with high signal to noise ratio close to the theoretical limit. Such artificial plasmonic surfaces enable flexible scaling of light funneling response to any wavelength range by simple dimensional changes paving the path towards room temperature frequency selective low energy photon detection.
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http://dx.doi.org/10.1364/OE.26.032931DOI Listing
December 2018

Cavity-induced hybrid plasmon excitation for perfect infrared absorption.

Opt Lett 2018 Dec;43(24):6001-6004

Photonic microcavity coupling of a subwavelength hole-disk array, a two-element metal/dielectric composite structure with enhanced extraordinary transmission, leads to 100% coupling of incident light to the cavity system and subsequent absorption. This light-funneling process arises from the temporal and spatial coupling of the broadband localized surface plasmon resonance on the coupled hole-disk array and the photonic modes of the optical cavity, which induces spectral narrowing of the perfect absorption of light. A simple nanoimprint lithography-based large-area fabrication process paves the path towards practical implementation of plasmonic cavity-based devices and sensors.
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http://dx.doi.org/10.1364/OL.43.006001DOI Listing
December 2018

Enzyme-Free Plasmonic Biosensor for Direct Detection of Neurotransmitter Dopamine from Whole Blood.

Nano Lett 2019 01 17;19(1):449-454. Epub 2018 Dec 17.

NanoScience Technology Center , University of Central Florida , Orlando , Florida 32826 , United States.

Complex biological fluids without pretreatment, separation, or purification impose stringent limitations on the practical deployment of label-free plasmonic biosensors for advanced assays needed in point of care applications. In this work, we present an enzyme-free plasmonic neurotransmitter dopamine biosensor integrated with a microfluidic plasma separator. This integrated device allows the in-line separation of plasma directly from the bloodstream and channels it to the active detection area, where inorganic cerium oxide nanoparticles function as local selective dopamine binding sites through strong surface redox reaction. A thorough understanding and engineering of the nanoparticles is carried out to maximize its dopamine sensitivity and selectivity. We obtain detection of dopamine at 100 fM concentration in simulated body fluid and 1 nM directly from blood without any prior sample preparation. The detection selectivity is found to be at least five-times higher compared to the common interfering species. This demonstration shows the feasibility of the practical implementation of the proposed plasmonic system in detection of variety of biomarkers directly from the complex biological fluids.
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http://dx.doi.org/10.1021/acs.nanolett.8b04253DOI Listing
January 2019

Wide Angle Dynamically Tunable Enhanced Infrared Absorption on Large-Area Nanopatterned Graphene.

ACS Nano 2019 Jan 11;13(1):421-428. Epub 2018 Dec 11.

Department of Physics , University of Central Florida , Orlando , Florida 32816 , United States.

Enhancing light-matter interaction by exciting Dirac plasmons on nanopatterned monolayer graphene is an efficient route to achieve high infrared absorption. Here, we designed and fabricated hexagonal planar arrays of nanoholes and nanodisks with and without optical cavity to excite Dirac plasmons on patterned graphene and investigate the role of plasmon lifetime, extinction cross-section, incident light polarization, angle of incident light, and pattern dimensions on the light-absorption spectra. By incorporating a high-k AlO layer as the gate dielectric for dynamic electrostatic tuning of the Fermi level, we demonstrate peak absorptions of 60% and 90% for the nanohole and nanodisk patterns, respectively, in the atmospheric transparent 8-12 μm infrared imaging band with high spectral tunability. Finally, we theoretically and experimentally demonstrate angular dependence of both s- and p-polarized light absorption in monolayer graphene. Our results showcase the practical usability of low carrier mobility CVD-grown graphene for wide angle infrared absorption, which is suitable for next-generation optoelectronic devices such as photodetectors, optical switches, modulators, etc.
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http://dx.doi.org/10.1021/acsnano.8b06601DOI Listing
January 2019

Covert infrared image encoding through imprinted plasmonic cavities.

Light Sci Appl 2018 21;7:93. Epub 2018 Nov 21.

1Department of Physics, University of Central Florida, 4111 Libra Drive, Physical Sciences Bldg. 430, Orlando, FL 32816 USA.

Functional surfaces that can control light across the electromagnetic spectrum are highly desirable. Plasmonic nanostructures can assume this role by exhibiting dimension-tunable resonances that span multiple electromagnetic regimes. However, changing these structural parameters often impacts the higher-order resonances and spectral features in lower-wavelength domains. In this study, we discuss a cavity-coupled plasmonic system with resonances that are tunable across the 3-5 or 8-14 μm infrared bands while retaining near-invariant spectral properties in the visible domain. This result is accomplished by regime-dependent resonance mechanisms and their dependence on independent structural parameters. Through the identification and constraint of key parameters, we demonstrate multispectral data encoding, where images, viewable in the infrared spectral domain, appear as uniform areas of color in the visible domain-effectively hiding the information. Fabricated by large area nanoimprint lithography and compatible with flexible surfaces, the proposed system can produce multifunctional coatings for thermal management, camouflage, and anti-counterfeiting.
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http://dx.doi.org/10.1038/s41377-018-0095-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6249251PMC
November 2018

Switchable Pancharatnam-Berry microlens array with nano-imprinted liquid crystal alignment.

Opt Lett 2018 Oct;43(20):5062-5065

We report a rapid nano-imprinting technique to pattern the liquid crystal alignment of a Pancharatnam-Berry phase microlens array. Through implementing a single-side aligned cell, we demonstrate a switchable microlens array with fast response time and low operation voltage. Further investigation of focusing property as well as imaging capability ensure the good quality of the microlens array. Besides planar structures, this method is also promising for patterning liquid crystal alignment on curvilinear surfaces.
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http://dx.doi.org/10.1364/OL.43.005062DOI Listing
October 2018

Adaptive liquid crystal microlens array enabled by two-photon polymerization.

Opt Express 2018 Aug;26(16):21184-21193

A tunable-focus liquid crystal microlens array is demonstrated and characterized. Using two-photon polymerization based direct-laser writing, a polymerized microlens array is fabricated on one substrate. Such a microlens array creates inhomogeneous electric field distribution and homogeneous-like liquid-crystal alignment, simultaneously. The phase profile and thus the focal length can be tuned dynamically by the applied voltage. We also further investigate the focusing property and the imaging capability of the fabricated sample. Using the adaptive microlens array as an example, we demonstrate that directly forming a curvilinear surface with liquid-crystal alignment is feasible. In addition to adaptive lens, this direct-laser writing method is also a powerful tool for making other tunable photonic devices.
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http://dx.doi.org/10.1364/OE.26.021184DOI Listing
August 2018

Superchiral Light Generation on Degenerate Achiral Surfaces.

Phys Rev Lett 2018 Mar;120(13):137601

CREOL, College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USA.

A novel route of superchiral near-field generation is demonstrated based on geometrically achiral systems supporting degenerate and spatially superimposed plasmonic modes. Such systems generate a single-handed chiral near field with simultaneous zero far-field circular dichroism. The phenomenon is theoretically elucidated with a rotating dipole model, which predicts a uniform single-handed chiral near field that flips handedness solely by reversing the handedness of the source. This property allows detection of pure background free molecular chirality through near-field light-matter interaction, which is experimentally demonstrated in the precise identification of both handedness of a chiral molecule on a single substrate with about four orders of magnitude enhancement in detection sensitivity compared to its conventional volumetric counterpart.
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http://dx.doi.org/10.1103/PhysRevLett.120.137601DOI Listing
March 2018

Wireless optoelectronic photometers for monitoring neuronal dynamics in the deep brain.

Proc Natl Acad Sci U S A 2018 02 29;115(7):E1374-E1383. Epub 2018 Jan 29.

Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801;

Capabilities for recording neural activity in behaving mammals have greatly expanded our understanding of brain function. Some of the most sophisticated approaches use light delivered by an implanted fiber-optic cable to optically excite genetically encoded calcium indicators and to record the resulting changes in fluorescence. Physical constraints induced by the cables and the bulk, size, and weight of the associated fixtures complicate studies on natural behaviors, including social interactions and movements in environments that include obstacles, housings, and other complex features. Here, we introduce a wireless, injectable fluorescence photometer that integrates a miniaturized light source and a photodetector on a flexible, needle-shaped polymer support, suitable for injection into the deep brain at sites of interest. The ultrathin geometry and compliant mechanics of these probes allow minimally invasive implantation and stable chronic operation. In vivo studies in freely moving animals demonstrate that this technology allows high-fidelity recording of calcium fluorescence in the deep brain, with measurement characteristics that match or exceed those associated with fiber photometry systems. The resulting capabilities in optical recordings of neuronal dynamics in untethered, freely moving animals have potential for widespread applications in neuroscience research.
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http://dx.doi.org/10.1073/pnas.1718721115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5816195PMC
February 2018

Two-photon polymerization enabled multi-layer liquid crystal phase modulator.

Sci Rep 2017 11 24;7(1):16260. Epub 2017 Nov 24.

CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, Florida, 32816, USA.

The performance of liquid crystal (LC) spatial light modulators depends critically on the amount of cumulative phase change. However, for regular phase modulators, a large phase change comes with a slow time response penalty. A multi-layer liquid crystal (LC) spatial light modulator offers a large phase change while keeping fast response time due to the decoupling between phase change and time response through engineered sub-micron scaffold. Here, we demonstrate specially designed 2- and 3-layer LC cells which can achieve 4 times and 7 times faster response time than that of conventional single-layer LC phase modulator of equivalent thickness, respectively. A versatile two-photon laser lithography is employed for LC cell scaffolding to accurately verify theoretical predictions with experimental measurements.
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http://dx.doi.org/10.1038/s41598-017-16596-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5701147PMC
November 2017

Actively addressed single pixel full-colour plasmonic display.

Nat Commun 2017 05 10;8:15209. Epub 2017 May 10.

Department of Physics, University of Central Florida, 4111 Libra Drive, Physical Sciences Bldg. 430, Orlando, Florida 32816, USA.

Dynamic, colour-changing surfaces have many applications including displays, wearables and active camouflage. Plasmonic nanostructures can fill this role by having the advantages of ultra-small pixels, high reflectivity and post-fabrication tuning through control of the surrounding media. However, previous reports of post-fabrication tuning have yet to cover a full red-green-blue (RGB) colour basis set with a single nanostructure of singular dimensions. Here, we report a method which greatly advances this tuning and demonstrates a liquid crystal-plasmonic system that covers the full RGB colour basis set, only as a function of voltage. This is accomplished through a surface morphology-induced, polarization-dependent plasmonic resonance and a combination of bulk and surface liquid crystal effects that manifest at different voltages. We further demonstrate the system's compatibility with existing LCD technology by integrating it with a commercially available thin-film-transistor array. The imprinted surface interfaces readily with computers to display images as well as video.
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http://dx.doi.org/10.1038/ncomms15209DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5436230PMC
May 2017

Hybrid cavity-coupled plasmonic biosensors for low concentration, label-free and selective biomolecular detection.

Opt Express 2016 Oct;24(22):25785-25796

Simple optical techniques that can accurately and selectively identify organic and inorganic material in a reproducible manner are of paramount importance in biological sensing applications. In this work, we demonstrate that a nanoimprinted plasmonic pattern with locked-in dimensions supports sharp deterministic hybrid resonances when coupled with an optical cavity suitable for high sensitive surface detection. The surface sensing property of this hybrid system is quantified by precise atomic layer growth of aluminum oxide using the atomic layer deposition technique. The analyte specific sensing ability is demonstrated in the detection of two dissimilar analytes, inorganic amine-coated iron oxide nanoparticles and organic streptavidin protein. Femto to nanomolar detection limits were achieved with the proposed coupled plasmonic system based on the versatile and robust soft nanoimprinting technique, which promises practical low cost biosensors.
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http://dx.doi.org/10.1364/OE.24.025785DOI Listing
October 2016

Unified Electromagnetic-Electronic Design of Light Trapping Silicon Solar Cells.

Sci Rep 2016 08 8;6:31013. Epub 2016 Aug 8.

Department of Physics, University of Central Florida, 4111 Libra Drive, Physical Sciences Bldg. 430, Orlando, Florida 32816, USA.

A three-dimensional unified electromagnetic-electronic model is developed in conjunction with a light trapping scheme in order to predict and maximize combined electron-photon harvesting in ultrathin crystalline silicon solar cells. The comparison between a bare and light trapping cell shows significant enhancement in photon absorption and electron collection. The model further demonstrates that in order to achieve high energy conversion efficiency, charge separation must be optimized through control of the doping profile and surface passivation. Despite having a larger number of surface defect states caused by the surface patterning in light trapping cells, we show that the higher charge carrier generation and collection in this design compensates the absorption and recombination losses and ultimately results in an increase in energy conversion efficiency. The fundamental physics behind this specific design approach is validated through its application to a 3 μm thick functional light trapping solar cell which shows 192% efficiency enhancement with respect to the bare cell of same thickness. Such a unified design approach will pave the path towards achieving the well-known Shockley-Queisser (SQ) limit for c-Si in thin-film (<30 μm) geometries.
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http://dx.doi.org/10.1038/srep31013DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4976384PMC
August 2016

Multi-spectral infrared spectroscopy for robust plastic identification.

Appl Opt 2015 Aug;54(24):7396-405

The identification and classification of plastics plays an important role in waste management and recycling processes. Present electrical and optical sorting techniques lack the required resolution for accurate identification in a high throughput manner for a diverse set of plastics commonly found in municipal waste. In this work a multi-spectral infrared spectroscopic technique is employed to construct a unique fingerprint library of 12 plastic resin groups that are commonly encountered in municipal waste. We test the proposed method in a blind plastic identification experiment, which shows excellent unbiased identification accuracy. This simple optical technique in combination with the multi-spectral library will enable high throughput and accurate detection of various plastics from recovered solid waste.
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http://dx.doi.org/10.1364/AO.54.007396DOI Listing
August 2015

Polarization-independent actively tunable colour generation on imprinted plasmonic surfaces.

Nat Commun 2015 Jun 11;6:7337. Epub 2015 Jun 11.

1] Department of Physics, University of Central Florida, 4111 Libra Drive, Physical Sciences Building 430, Orlando, Florida 32816, USA [2] NanoScience Technology Center, University of Central Florida, 12424 Research Parkway Suite 400, Orlando, Florida 32826, USA [3] CREOL, The College of Optics and Photonics, University of Central Florida, 4304 Scorpius Street, Orlando, Florida 32816, USA.

Structural colour arising from nanostructured metallic surfaces offers many benefits compared to conventional pigmentation based display technologies, such as increased resolution and scalability of their optical response with structure dimensions. However, once these structures are fabricated their optical characteristics remain static, limiting their potential application. Here, by using a specially designed nanostructured plasmonic surface in conjunction with high birefringence liquid crystals, we demonstrate a tunable polarization-independent reflective surface where the colour of the surface is changed as a function of applied voltage. A large range of colour tunability is achieved over previous reports by utilizing an engineered surface which allows full liquid crystal reorientation while maximizing the overlap between plasmonic fields and liquid crystal. In combination with imprinted structures of varying periods, a full range of colours spanning the entire visible spectrum is achieved, paving the way towards dynamic pixels for reflective displays.
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http://dx.doi.org/10.1038/ncomms8337DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4490413PMC
June 2015

Hybrid coupling mechanism in a system supporting high order diffraction, plasmonic, and cavity resonances.

Phys Rev Lett 2014 Dec 29;113(26):263902. Epub 2014 Dec 29.

CREOL, College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USA and NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826, USA and Department of Physics, University of Central Florida, Orlando, Florida 32816, USA.

The interactions between plasmonic and photonic modes of a cavity-coupled plasmonic crystal are studied in diffraction and diffractionless regimes, which lead us to the understanding of coherent interactions between electron plasma, higher order cavity, and diffraction modes. The strong interaction between plasmonic and photonic modes is shown to enhance as well as suppress surface plasmon resonance based on cavity phase relation. Numerical and analytical approaches are developed to accurately explain the physics of the interactions evident in their characteristic dispersion graphs. Further experimental measurements confirm the theoretical predictions.
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http://dx.doi.org/10.1103/PhysRevLett.113.263902DOI Listing
December 2014

Nanoimprinting techniques for large-area three-dimensional negative index metamaterials with operation in the visible and telecom bands.

ACS Nano 2014 Jun 21;8(6):5535-42. Epub 2014 Apr 21.

NanoScience Technology Center and ‡College of Optics and Photonics (CREOL), University of Central Florida , Orlando, Florida 32826, United States.

We report advances in materials, designs, and fabrication schemes for large-area negative index metamaterials (NIMs) in multilayer "fishnet" layouts that offer negative index behavior at wavelengths into the visible regime. A simple nanoimprinting scheme capable of implementation using standard, widely available tools followed by a subtractive, physical liftoff step provides an enabling route for the fabrication. Computational analysis of reflection and transmission measurements suggests that the resulting structures offer negative index of refraction that spans both the visible wavelength range (529-720 nm) and the telecommunication band (1.35-1.6 μm). The data reveal that these large (>75 cm(2)) imprinted NIMs have predictable behaviors, good spatial uniformity in properties, and figures of merit as high as 4.3 in the visible range.
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http://dx.doi.org/10.1021/nn5015775DOI Listing
June 2014

Mechanisms of enhanced optical absorption for ultrathin silicon solar microcells with an integrated nanostructured backside reflector.

ACS Appl Mater Interfaces 2013 May 29;5(10):4239-46. Epub 2013 Apr 29.

Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.

This paper investigates mechanisms of enhanced light absorption exhibited by ultrathin Si solar microcells integrated with a periodically nanostructured, semitransparent metallic reflector. This backside reflector comprises periodic nanoscale relief features formed by soft-imprint lithography with a thin (~35 nm) coating of Au. The work shows that microcells placed in direct contact above the nanostructured reflector's surface creates Fabry-Pérot cavities, which traps impinging light inside the Si slab via the excitation of cavity modes. Experimental measurements show that the short-circuit current and efficiency values for devices incorporating this thin, semitransparent backside reflector outperform similar Si microcells integrated with a planar thick (~300 nm) opaque mirror by ~10-15% because of enhanced absorption. Computational modeling that is supported by experimental measurements reveal that the dominant methods of enhancement stem from a complex interplay between backside diffraction/scattering and Fabry-Pérot resonances. These same data demonstrate that plasmonic interactions contribute minimally to the optical enhancements seen.
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http://dx.doi.org/10.1021/am400408gDOI Listing
May 2013

Coherent stitching of light in multilayered diffractive optical elements.

Opt Express 2012 Oct;20(21):23960-70

Department of Electrical and Computer Engineering and Institute for Optical Sciences, University of Toronto, Toronto, Ontario, M5S 3G4, Canada.

Diffractive optical elements serve an important function in many dynamic and static optical systems. Multilayered diffractive elements offer powerful opportunity to harness both phase and amplitude modulation for benefits in diffraction efficiency and beam shaping. However, multilayered combinations have been difficult to fabricate and provide only weak diffraction for phase gratings with low refractive index contrast. Femtosecond laser writing of finely-pitched multilayer volume gratings was optimized in bulk fused silica. We identify and quantify an optimum layer-to-layer separation according to Talbot self-imaging planes and present systematic experimental validation of this new approach to enhance otherwise weakly diffracting volume gratings.
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http://dx.doi.org/10.1364/OE.20.023960DOI Listing
October 2012

Porosity control in metal-assisted chemical etching of degenerately doped silicon nanowires.

Nanotechnology 2012 Aug 10;23(30):305304. Epub 2012 Jul 10.

Department of Electrical and Computer Engineering, University of Illinois, Urbana Champaign, IL 61801, USA.

We report the fabrication of degenerately doped silicon (Si) nanowires of different aspect ratios using a simple, low-cost and effective technique that involves metal-assisted chemical etching (MacEtch) combined with soft lithography or thermal dewetting metal patterning. We demonstrate sub-micron diameter Si nanowire arrays with aspect ratios as high as 180:1, and present the challenges in producing solid nanowires using MacEtch as the doping level increases in both p- and n-type Si. We report a systematic reduction in the porosity of these nanowires by adjusting the etching solution composition and temperature. We found that the porosity decreases from top to bottom along the axial direction and increases with etching time. With a MacEtch solution that has a high [HF]:[H(2)O(2)] ratio and low temperature, it is possible to form completely solid nanowires with aspect ratios of less than approximately 10:1. However, further etching to produce longer wires renders the top portion of the nanowires porous.
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http://dx.doi.org/10.1088/0957-4484/23/30/305304DOI Listing
August 2012

Formation of high aspect ratio GaAs nanostructures with metal-assisted chemical etching.

Nano Lett 2011 Dec 21;11(12):5259-63. Epub 2011 Nov 21.

Department of Electrical and Computer Engineering, University of Illinois, Urbana, Illinois 61801, United States.

Periodic high aspect ratio GaAs nanopillars with widths in the range of 500-1000 nm are produced by metal-assisted chemical etching (MacEtch) using n-type (100) GaAs substrates and Au catalyst films patterned with soft lithography. Depending on the etchant concentration and etching temperature, GaAs nanowires with either vertical or undulating sidewalls are formed with an etch rate of 1-2 μm/min. The realization of high aspect ratio III-V nanostructure arrays by wet etching can potentially transform the fabrication of a variety of optoelectronic device structures including distributed Bragg reflector (DBR) and distributed feedback (DFB) semiconductor lasers, where the surface grating is currently fabricated by dry etching.
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http://dx.doi.org/10.1021/nl202708dDOI Listing
December 2011

Coupling of plasmonic and optical cavity modes in quasi-three-dimensional plasmonic crystals.

Nat Commun 2011 Sep 20;2:479. Epub 2011 Sep 20.

Departments of Materials Science and Engineering, Beckman Institute, and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, USA.

The field of plasmonics has emerged as an interesting area for fundamental studies, with important application possibilities in miniaturized photonic components. Plasmonic crystals are of particular relevance because of large field enhancements and extraordinary transmission that arise from plasmonic interactions between periodic arrays of metallic elements. Here we report methods to enhance and modify the plasmonic resonances in such structures by strongly coupling them to optical modes of Fabry-Perot type cavities. First, we illustrate a type of plasmonic, narrow-band (~15 nm), high-contrast (>20 dB) absorber and an opto-fluidic modulator based on this component. Second, we use optimized samples as substrates to achieve strong amplification (>350%) and modulation (>4×) of surface-enhanced Raman scattering from surface-bound monolayers. Cavity-coupling strategies appear to be useful not only in these two examples, but also in applications of plasmonics for optoelectronics, photovoltaics and related technologies.
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http://dx.doi.org/10.1038/ncomms1487DOI Listing
September 2011

Large-area flexible 3D optical negative index metamaterial formed by nanotransfer printing.

Nat Nanotechnol 2011 Jun 5;6(7):402-7. Epub 2011 Jun 5.

Department of Materials Science, Beckman Institute, and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.

Negative-index metamaterials (NIMs) are engineered structures with optical properties that cannot be obtained in naturally occurring materials. Recent work has demonstrated that focused ion beam and layer-by-layer electron-beam lithography can be used to pattern the necessary nanoscale features over small areas (hundreds of µm(2)) for metamaterials with three-dimensional layouts and interesting characteristics, including negative-index behaviour in the optical regime. A key challenge is in the fabrication of such three-dimensional NIMs with sizes and at throughputs necessary for many realistic applications (including lenses, resonators and other photonic components). We report a simple printing approach capable of forming large-area, high-quality NIMs with three-dimensional, multilayer formats. Here, a silicon wafer with deep, nanoscale patterns of surface relief serves as a reusable stamp. Blanket deposition of alternating layers of silver and magnesium fluoride onto such a stamp represents a process for 'inking' it with thick, multilayer assemblies. Transfer printing this ink material onto rigid or flexible substrates completes the fabrication in a high-throughput manner. Experimental measurements and simulation results show that macroscale, three-dimensional NIMs (>75 cm(2)) nano-manufactured in this way exhibit a strong, negative index of refraction in the near-infrared spectral range, with excellent figures of merit.
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http://dx.doi.org/10.1038/nnano.2011.82DOI Listing
June 2011