Browsing by Subject "Broad-band"

Now showing 1 - 3 of 3
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    Photonic bandgap narrowing in conical hollow core Bragg fibers
    (AIP Publishing, 2014) Ozturk, F. E.; Yildirim, A.; Kanik, M.; Bayındır, Mehmet
    We report the photonic bandgap engineering of Bragg fibers by controlling the thickness profile of the fiber during the thermal drawing. Conical hollow core Bragg fibers were produced by thermal drawing under a rapidly alternating load, which was applied by introducing steep changes to the fiber drawing speed. In conventional cylindrical Bragg fibers, light is guided by omnidirectional reflections from interior dielectric mirrors with a single quarter wave stack period. In conical fibers, the diameter reduction introduced a gradient of the quarter wave stack period along the length of the fiber. Therefore, the light guided within the fiber encountered slightly smaller dielectric layer thicknesses at each reflection, resulting in a progressive blueshift of the reflectance spectrum. As the reflectance spectrum shifts, longer wavelengths of the initial bandgap cease to be omnidirectionally reflected and exit through the cladding, which narrows the photonic bandgap. A narrow transmission bandwidth is particularly desirable in hollow waveguide mid-infrared sensing schemes, where broadband light is coupled to the fiber and the analyte vapor is introduced into the hollow core to measure infrared absorption. We carried out sensing simulations using the absorption spectrum of isopropyl alcohol vapor to demonstrate the importance of narrow bandgap fibers in chemical sensing applications.
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    Plasmonic light-sensitive skins of nanocrystal monolayers
    (IOP Publishing, 2013) Akhavan, S.; Gungor, K.; Mutlugun, E.; Demir, Hilmi Volkan
    We report plasmonically coupled light-sensitive skins of nanocrystal monolayers that exhibit sensitivity enhancement and spectral range extension with plasmonic nanostructures embedded in their photosensitive nanocrystal platforms. The deposited plasmonic silver nanoparticles of the device increase the optical absorption of a CdTe nanocrystal monolayer incorporated in the device. Controlled separation of these metallic nanoparticles in the vicinity of semiconductor nanocrystals enables optimization of the photovoltage buildup in the proposed nanostructure platform. The enhancement factor was found to depend on the excitation wavelength. We observed broadband sensitivity improvement (across 400-650 nm), with a 2.6-fold enhancement factor around the localized plasmon resonance peak. The simulation results were found to agree well with the experimental data. Such plasmonically enhanced nanocrystal skins hold great promise for large-area UV/visible sensing applications.
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    Ultra-low-cost broad-band near-infrared silicon photodetectors based on hot electrons
    (2015) Nazirzadeh, Mohammad Amin
    Silicon is at the heart of all of the end-user digital devices such as smart phones, laptops, and wearable technologies. It is the holy grail for the largescale production of semiconductor devices since start of the semiconductor era due to its relatively good electrical, mechanical and chemical properties. Silicon’s mediocre optical properties also make it an acceptable material for energy harvesting and ultraviolet photodetection applications. But its relatively large bandgap (1.12 eV ) makes it infrared blind. So Silicon photodetectors fail to detect infrared light using traditional techniques. Hence, an all-Silicon solution is of interest for low-cost civil applications like telecommunication and imaging. Silicon based Schottky junction is a promising candidate for infrared photodetection. Internal photoemission is the main mechanism of photodetection in the Schottky junctions. Incident photons elevate the kinetic energy of the electrons in the metal so that the energetic electrons can jump over the Schottky barrier or tunnel through it. Carefully designed metal contact of the Schottky junction can, at the same time, give rise to hot electron generation through plasmon resonances. Here we introduce ultra-low-cost broad-band near-infrared Silicon photodetectors with a study over types of metal and nanostructures and fabrication techniques. The devices exhibit photoresponsivity as high as 2 mA/W and 600 µA/W at 1300 nm and 1550 nm wavelengths, and can see beyond 2000 nm wavelengths. Their dark current density is as low as 50 pA/µm2 . Simplicity and scalability of fabrication in this type of structures make them the most cost effective infrared detectors due to lack of expensive fabrication steps such as sub-micron lithography and high temperature epitaxial growth techniques.