Hot electrons rapidly dissipate their extra free energy, typically into heat. This is the origin of the Shockley-Queisser efficiency limit of the single junction solar cells. An even faster mechanism of electron-plasmon scattering is available in metals. We demonstrate by detailed simulations, that an ultra-thin solar cell with a composite metamaterial/plasmonic collector could yield efficiency exceeding the Shockley-Quasar limit. The composite collector has a double function: firstly, it is designed to participate in efficiently trapping light, and secondly, it is a plasmonic resonator tuned to absorb the energy of hot electrons, thus protecting them from phonon losses.
The optical properties of periodic and nonperiodic arrays of aligned multiwalled carbon nanotubes are presented.
Experimental analysis indicates a complex optical response that is attributed to both the individual carbon nanotube
scatterers and also to the array ensembles. These studies indicate that by controlling the geometry and spacing of the
arrays, it is possible to create structures that respond very strongly to specific wavelengths or bands of wavelengths.
Structures such as these may form the basis for numerous applications in energy conversion. This would include highly
efficient solar energy conversion as well as sensitive, finely tuned detectors that can respond to predetermined
wavelength bands ranging from the ultraviolet to the infrared region. Experimental, theoretical and modeled results are
discussed as they apply to these applications. Challenges and issues are discussed.
Vertically-aligned carbon nanotubes/nanofibers grown on various substrates by a direct-current plasma-enhanced chemical vapor deposition method have been shown experimentally to function as classical low-loss dipole antenna arrays at optical frequencies. Two fundamental antenna effects, e.g., the polarization effect and length matching effect, directly observed on large-scale CNT arrays in visible frequency range, hold them promising for industry-level fabrication of devices including linear/beam-splitting polarizers, solar energy converters, THz demodulators, etc., some of which will, however, require or prefer a flexible and/or transparent conducting substrate to be compatible for multi-level integration and low-cost manufacturing process. A low-energy dark discharge fabrication technique is therefore devised which successfully yields CNT antennas directly on polyimide films and transparent conducting oxides (ITO, ZnO) with the absence of a buffer layer.
Large-scale, two-dimensional arrays of periodic particles were prepared by nanosphere lithography. We modified the fabrication technique based on a self-assembly of latex particles on water surface in order to improve mask quality and size. Modifications of particles arrangement in an array were also practicable by using double-layered masks and mask transfer method. Such particle arrays were used for catalytic growth of aligned carbon nanotubes and ZnO nanorods with various configurations, length, and diameter. These exhibit interesting phenomena - antenna effects, photonic bandgap behavior, subwavelength lensing, and enhanced field emission. Therefore, they can be used in variety of future optoelectronic devices, such as THz and IR detectors.
Spectroscopic observations are presented for carbon nanotubes grown on silicon and quartz substrates in a hexagonal honeycomb configuration using self-assembly nanosphere lithography and plasma enhanced chemical vapor deposition method. A white light source is used as an incident beam and light reflected from the surface of the carbon nanotubes results in a distinctive signature in the reflected spectrum. A comparison of non-periodic arrays and periodic arrays of
carbon nanotubes show that the reflectance signature is only observed when the carbon nanotubes are oriented in a periodic array. Further observations regarding the light antenna effect observed in nonperiodic arrays are also reported. Theoretical curves show good agreement to experimentally observed phenomena. The unique optical properties of the arrays combined with the excellent mechanical and electrical properties of carbon nanotubes indicate that these materials may find many uses in the field of optoelectronics.
Carbon nanotubes were grown on silicon and quartz substrates in a honeycomb configuration using self-assembly nanosphere lithography and plasma enhanced chemical vapor deposition methods. Photonic nanoarrays were fabricated with varying spacing and carbon nanotube height. Both periodic and nonperiodic arrays were produced and evaluated. Optical properties of the arrays were studied and related to array geometry. Three dimensional diffraction maps were created that reveal the manner in which the nanoarrays interact with visible light. The unique optical properties of the arrays combined with the excellent mechanical and electrical properties of carbon nanotubes indicates that these materials may find many uses in the field of optoelectronics.
Carbon nanotubes (CNT) have been grown in a honeycomb configuration on silicon substrates using nanosphere self-assembly and plasma enhanced chemical vapor deposition. The optical properties of the arrays were also studied. Diffraction efficiency was found to be a function of the wavelength, angle of incidence and state of polarization of incident light. The unique optical properties of the arrays combined with the excellent mechanical and electrical properties of carbon nanotubes indicates that these materials may find many uses in the field of optoelectronics. In addition to their optical properties, periodic CNT arrays have a host of other unique electromagnetic and mechanical properties that may be exploited for numerous applications. Polarization measurements indicate that the intensity of both the diffracted light and diffusely scattered light is dependent on wavelength and angle of incidence. These arrays not only reflect and diffract light, but can also have a photonic band gap in, or around, the visible frequency range. The precise frequency location and size of this gap can be controlled by the structural and material parameters of the arrays.
We propose her a class of quantum well structures designed to achieve a coherent generation of THz radiation through a plasma instability. This can be achieved, without population inversion, if a dynamical inhomogeneity is built into the active region of the structure. We show, through self- consistent calculation of the non-equilibrium steady state, that such structures can be inherently unstable against growing charge fluctuations under a variety of conditions, including lack of population inversion. Preliminary calculations of the I-V characteristics of such structures are in good agreement with experimental results.
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