Enhanced Efficiency of Light-Trapping Nanoantenna Arrays for Thin Film Solar Cells

We suggest a novel concept of efficient light-trapping structures for thin-film solar cells based on arrays of planar nanoantennas operating far from plasmonic resonances. The operation principle of our structures relies on the excitation of chessboard-like collective modes of the nanoantenna arrays with the field localized between the neighboring metal elements. We demonstrated theoretically substantial enhancement of solar-cell short-circuit current by the designed light-trapping structure in the whole spectrum range of the solar-cell operation compared to conventional structures employing anti-reflecting coating. Our approach provides a general background for a design of different types of efficient broadband light-trapping structures for thin-film solar-cell technologically compatible with large-area thin-film fabrication techniques

Recent Trends in Plasmonic Nanowire Solar Cells

Light trapping is crucial for low-cost and highly efficient nanowire (NW) solar cells (SCs). In order to increase the light absorption through the NWSCs, plasmonic materials can be incorporated inside or above the NW design. In this regard, two novel designs of plasmonic NWSCs are reported and analyzed using 3D finite difference time domain method. The geometrical parameters of the reported designs are studied to improve their electrical and optical efficiencies. The ultimate and power conversion efficiencies (PCE) are used to quantify the conversion efficiency of the light into electricity. The first design relies on funnel shaped SiNWs with plasmonic core while the cylindrical NWs of the second design are decorated by Ag diamond shaped. The calculated ultimate efficiency and PCE of the plasmonic funnel design are equal to 44% and 18.9%, respectively with an enhancement of 43.3 % over its cylindrical NWs counterpart. This enhancement can be explained by the coupling between the three optical modes, supported by the upper cylinder, lower cone and plasmonic material. Moreover, the cylindrical SiNWs decorated by Ag diamond offer an ultimate efficiency and short-circuit current density of 25.7%, and 21.03 mA∕cm2, respectively with an improvement of 63% over the conventional cylindrical SiNWs

Plasmonic hybrid terahertz photomixer of graphene nanoantenna and nanowires

Due to their attractive properties, silver nanowires (Ag-NWs) are newly used as nanoelectrodes in continuous wave (CW) THz photomixer. However, since these nanowires have small contact area, the nanowires fill factor in the photomixer active region is low, which leads to reduce the nanowires conductivity. In this work, we proposed to add graphene nanoantenna array as nanoelectrodes to the silver nanowires-based photomixer to improve the conductivity. In addition, the graphene nanoantenna array and the silver nanowires form new hybrid nanoelectrodes for the CW-THz photomixer leading to improve the device conversion efficiency by the plasmonic effect. Two types of graphene nanoantenna array are proposed in two separate photomixer configurations. These are the graphene nanodisk (GND) array and the graphene bow-tie nanoantenna (GNA) array. The photomixer active region is simulated using the computer simulation technology (CST) Studio Suite® for three optical wavelengths: 780 nm, 810 nm, and 850 nm. From the results, we found that the electric field in the active region is enhanced by 4.2 and 4.8 times for the aforementioned configurations, respectively. We also showed that the THz output power can be enhanced by 310 and 530 times, respectively

Engineering aperiodic spiral order for photonic-plasmonic device applications

Thesis (Ph.D.)--Boston UniversityDeterministic arrays of metal (i.e., Au) nanoparticles and dielectric nanopillars (i.e., Si and SiN) arranged in aperiodic spiral geometries (Vogel's spirals) are proposed as a novel platform for engineering enhanced photonic-plasmonic coupling and increased light-matter interaction over broad frequency and angular spectra for planar optical devices. Vogel's spirals lack both translational and orientational symmetry in real space, while displaying continuous circular symmetry (i.e., rotational symmetry of infinite order) in reciprocal Fourier space. The novel regime of "circular multiple light scattering" in finite-size deterministic structures will be investigated. The distinctive geometrical structure of Vogel spirals will be studied by a multifractal analysis, Fourier-Bessel decomposition, and Delaunay tessellation methods, leading to spiral structure optimization for novel localized optical states with broadband fluctuations in their photonic mode density. Experimentally, a number of designed passive and active spiral structures will be fabricated and characterized using dark-field optical spectroscopy, ellipsometry, and Fourier space imaging. Polarization-insensitive planar omnidirectional diffraction will be demonstrated and engineered over a large and controllable range of frequencies. Device applications to enhanced LEDs, novel lasers, and thin-film solar cells with enhanced absorption will be specifically targeted. Additionally, using Vogel spirals we investigate the direct (i.e. free space) generation of optical vortices, with well-defined and controllable values of orbital angular momentum, paving the way to the engineering and control of novel types of phase discontinuities (i.e., phase dislocation loops) in compact, chip-scale optical devices. Finally, we report on the design, modeling, and experimental demonstration of array-enhanced nanoantennas for polarization-controlled multispectral nanofocusing, nanoantennas for resonant near-field optical concentration of radiation to individual nanowires, and aperiodic double resonance surface enhanced Raman scattering substrates

Surface plasmon enhanced photodetectors based on internal photoemission

Surface plasmon photodetectors are of broad interest. They are promising for several applications including telecommunications, photovoltaic solar cells, photocatalysis, color-sensitive detection, and sensing, as they can provide highly enhanced fields and strong confinement (to subwavelength scales). Such photodetectors typically combine a nanometallic structure that supports surface plasmons with a photodetection structure based on internal photoemission or electron-hole pair creation. Photodetector architectures are highly varied, including waveguides, gratings, nanoparticles, nanoislands, or nanoantennas. We review the operating principles behind surface plasmon photodetectors based on the internal photoelectric effect, and we survey and compare the most recent and leading edge concepts reported in the literature

New Trends in Energy Harvesting from Earth Long-Wave Infrared Emission

A review, even if not exhaustive, on the current technologies able to harvest energy from Earth's thermal infrared emission is reported. In particular, we discuss the role of the rectenna system on transforming the thermal energy, provided by the Sun and reemitted from the Earth, in electricity. The operating principles, efficiency limits, system design considerations, and possible technological implementations are illustrated. Peculiar features of THz and IR antennas, such as physical properties and antenna parameters, are provided. Moreover, some design guidelines for isolated antenna, rectifying diode, and antenna coupled to rectifying diode are exploited

Electrically Small Particles for Energy Harvesting in the Infrared and Microwave Regimes

Harnessing energy from clean and sustainable resources is of crucial importance to our planet. Several attempts through different technologies have been pursued to achieve efficient and sustainable energy production systems. However, having systems with a high energy harvesting efficiency and at the same time low energy production cost are challenging with the existing technologies. In this research, several novel structures based on electrically small particles are proposed for harvesting the microwave and infrared energy efficiently. First, a proof of concept demonstrates a metamaterial unit cell's ability to harness the ambient electromagnetic energy. A split-ring resonator (SRR) representing the metamaterial unit cell is designed at a microwave frequency (5.8 GHz) and then fabricated by using printed circuit board technology to prove this concept. A bow-tie antenna, operating at the above frequency, is also designed to show the power efficiency improvement achieved by utilizing the SRR. More than 37% of power efficiency is achieved using SRRs-based structure compared to the 13% of the bow-tie antenna. A new efficiency term is also proposed to take into account the size reduction and efficiency advancement resulting from SRR structures. To this end, two comparable arrays of SRRs and bow-tie antennas are made. Power efficiency of 63.2% and 15.3% for the SRRs and bow-tie arrays, respectively, are achieved. Another structure composed of an ensemble of electrically small resonators for harvesting microwave energy is presented. A flower-like structure composed of four electrically small SRRs arranged in a cruciate pattern, each with a maximum dimension of less than ʎo/10, is shown to achieve more than 43% microwave-to-alternating current (AC) conversion efficiency at 5.67 GHz. Even- and odd-mode currents are realized in the proposed harvester to improve the efficiency and concurrently reduce the dielectric loss in the substrate. An experimental validation is conducted to prove the harvesting capability. To extend the work to operate at the far-infrared regime, a novel structure based on electrically small resonators is proposed for harvesting the infrared energy and yielding more than 80% harvesting efficiency. The dispersion effects of the dielectric and conductor materials of the resonators are taken into account by applying the Drude model. A new scheme to channel the infrared waves from an array of SRRs is proposed, whereby a wide-bandwidth collector is utilized by employing this new channeling concept. With the same pattern of the flower-like harvester operating in microwave regime, a new structure composed of electrically small SRRs, each of whose greatest length is less than ʎo/21, is proven to achieve more than 85% of power harvesting efficiency at 0.348 THz. Furthermore, the infrared energy harvesters are fabricated using nano-fabrication tools. At last, the infrared harvesters are experimentally validated with the numerical findings using THz time-domain spectroscopy (THz-TDS).1 yea

GaAs nanowires: from doping to plasmonic hybrid devices

Semiconductor nanowires (NWs) are filamentary crystals with the diameter ranging from few tens up to few hundreds of nanometers. In the last 20 years, they have been intensively studied for the prospects that their unique quasi-one dimensional shape offers to both fundamental and applied science. More recently particular attention has been dedicated to use NWs as building blocks for nano-electronic devices. In this thesis we investigate the electro-optical properties of NWs in order to put some light on the mechanisms governing the electrical transport and the light coupling between NWs and metal nanostructures. We investigate Be and C doping in GaAs NWs synthesized by Molecular Beam Epitaxy (MBE). We obtain a doping control over a large range of densities and we identify a new in situ incorporation path. Since strong surface impurity scattering in III-V materials degrade the electronic performances, we grew NWs passivated with an AlGaAs layer and we investigate their properties. The NW passivation allows for the increase of the electron mean free path by a factor of almost 10. In addition, we designed AlGaAs/GaAs modulation doping NWs. The modulation doping structure allows for the to enhancement of the NW electron mobility revealing excellent properties for the realization of nano-electronic devices. We calculate the electron distribution in the modulation doped NWs and we observe a six-fold symmetry with six 1D electron channels when the carrier concentration is high, while for low concentrations, electrons are delocalized in the GaAs NW core. Thanks to their special interaction with light, semiconductor NWs have opened new avenues in photonics, quantum optics and solar energy harvesting. Here, we design a new system composed of a NW and an array of nanoantennas. Initially, we successfully demonstrated the plasmonic coupling between NWs and nanoantennas, observing an electric field enhancement in the NW as a function of the nanoantenna's gap distance. This finding represented an initial step toward the development of coupled nano-structures for the realization of a new generation of solar cells, detectors and non linear optical devices. Near field coupling was also used between a NW and Yagi-Uda antennas to obtain directional emission. In particular the precise tuning of the Yagi-Uda dimensions and positions leads a strong variation of the NW emission, being able to change this from backward to forward. One of the major challenges for NWs full technological deployment has been their strong polarization dependence in light absorption and emission. Here, we demonstrate that a hybrid structure formed by GaAs NWs with a highly dense array of bow-tie antennas is able to modify the polarization response of a NW. As a result, the increase in light absorption for transverse polarized light changes the NW polarization response, including the inversion of the polarization response. We calculated that the absorption of transverse polarized light can be enhanced up to 15 times. We also fabricate several electrical devices proving our calculated predictions

Development of nanostencil lithography and its applications for plasmonics and vibrational biospectroscopy

Thesis (Ph.D.)--Boston UniversityDevelopment of low cost nanolithography tools for precisely creating a variety of nanostructure shapes and arrangements in a high-throughput fashion is crucial for next generation biophotonic technologies. Although existing lithography techniques offer tremendous design flexibility, they have major drawbacks such as low-throughput and fabrication complexity. In addition the demand for the systematic fabrication of sub-100 nm structures on flexible, stretchable, non-planar nanoelectronic/photonic systems and multi-functional materials has fueled the research for innovative fabrication methods in recent years. This thesis research investigates a novel lithography approach for fabrication of engineered plasmonic nanostructures and metamaterials operating at visible and infrared wavelengths: The technique is called Nanostencil Lithography (NSL) and relies on direct deposition of materials through nanoapertures on a stencil. NSL enables high throughput fabrication of engineered antenna arrays with optical qualities similar to the ones fabricated by standard electron beam lithography. Moreover, nanostencils can be reused multiple times to fabricate series of plasmonic nanoantenna arrays with identical optical responses enabling high throughput manufacturing. Using nanostencils, very precise nanostructures could be fabricated with 10 nm accuracy. Furthermore, this technique has flexibility and resolution to create complex plasmonic nanostructure arrays on the substrates that are difficult to work with e-beam and ion beam lithography tools. Combining plasmonics with polymeric materials, biocompatible surfaces or curvilinear and non-planar objects enable unique optical applications since they can preserve normal device operation under large strain. In this work, mechanically tunable flexible optical materials and spectroscopy probes integrated on fiber surfaces that could be used for a wide range of applications are demonstrated. Finally, the first application of NSL fabricated low cost infrared nanoantenna arrays for plasmonically enhanced vibrational biospectroscopy is presented. Detection of immunologically important protein monolayers with thickness as small as 3 nm, and antibody assays are demonstrated using nanoantenna arrays fabricated with reusable nanostencils. The results presented indicate that nanostencillithography is a promising method for reducing the nano manufacturing cost while enhancing the performance of biospectroscopy tools for biology and medicine. As a single step and low cost nanofabrication technique, NSL could facilitate the manufacturing of biophotonic technologies for real-world applications

Efficient Antennas for Terahertz and Optical Frequencies.

The inherent low conductivity of metals and low skin depth at terahertz (THz) frequencies and above decrease antennas radiation efficiency at such high frequencies. In addition to the high surface impedance, due to the small dimensions of metallic antennas at THz frequencies and above can further reduce the radiation efficiency. Nevertheless, attention has been drawn to metallic antennas that are designed to operate at optical frequencies due to their ability to create substantial field confinement and enhancement at their terminals. In this thesis, a highly conductive nanomaterial, Bundled Carbon Nanotubes (BCNTs) is examined to overcome the low efficiency of metallic antenna at THz frequencies. Due to their axial current, BCNTs are modeled by a 2D anisotropic resistive sheet having a tensor surface resistivity. Using a numerical method, the radiation efficiency of antennas consisting of BCNTs and gold are compared and it is concluded that BCNTs should be packed up about 1,000 times more than the current density of BCNTs to outperform gold at 2 THz. Efficient nanoantennas near infrared (IR) frequencies are also studied to enhance the performance of uncooled IR detectors and thermophotovoltaics (TPVs) power transducers. A gold bowtie dipole antenna topology loaded with a low bandgap indium gallium arsenide antimonide (InGaAsSb) p-n junction is investigated for this purpose. Through optimized arrangements, it is shown that a large array of flexible load bowtie nanoantennas can produce an efficient TPV system that can absorb 95% of the incident power. Similarly, a focal-plane array of nano-bowtie antennas used as an uncooled IR detector is demonstrated to enhance the sensitivity of the detector by a factor equal to the field enhancement factor, approximately 23 when compared to a detector made from a thick layer of the same material, InGaAsSb. Finally, a more advanced antenna topology using a cross tapered-bowtie antenna for detecting circularly polarized (CP) IR signals is designed and its perfect CP property is verified experimentally for the microwave range. A conceptual full‐Stoke's vector polarimetric imager using focal planar arrays of the nanoantennas with vertical, horizontal, 45°-tilted, and right-hand circular polarization is proposed.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107316/1/sangjo_1.pd