Browsing by Author "Zheng, Bob"

Now showing 1 - 4 of 4
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    A room-temperature mid-infrared photodetector for on-chip molecular vibrational spectroscopy
    (AIP Publishing, 2018) Zheng, Bob; Zhao, Hangqi; Cerjan, Ben; Yazdi, Sadegh; Ringe, Emilie; Nordlander, Peter; Halas, Naomi J.; Laboratory for Nanophotonics
    Infrared (IR) photodetection is of major scientific and technical interest since virtually all molecules exhibit characteristic vibrational modes in the mid-infrared region of the spectrum, giving rise to molecular spectroscopy and chemical imaging in this wavelength range. High-resolution IR spectroscopies, such as Fourier Transform IR spectroscopy, typically require large, bulky optical measurement systems and expensive photodetector components. Here, we present a high-responsivity photodetector for the mid-IR spectral region which operates at room temperature. Fabricated from silicon and aluminum, the photodetection mechanism is based on free carrier absorption, giving rise to a photoresponse rivalling commercially available cooled IR photodetectors. We demonstrate that infrared spectra of molecules deposited on this detector can be obtained by a direct electrical read-out. This work could pave the way for simple, fully integrated chemical sensors and other applications, such as chemical imaging, which would benefit from the combination of mid-IR detection, room-temperature operation, and ultracompact portability.
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    Fully Integrated CMOS-Compatible Photodetector with Intrinsic Gain and Red-Green-Blue Color Selectivity
    (2014-04-21) Zheng, Bob; Halas, Naomi J.; Nordlander, Peter J.; Link, Stephan
    Currently, image sensors are hybrid devices, combining semiconductor photodiodes with off-chip color filters of different materials to convert wavelength-selected light into useful photocurrent. Here we demonstrate a fully integrated, metal-semiconductor-metal (MSM) photodetector and plasmonic color filter fabricated entirely from Aluminum and silicon designed to detect light in selected wavelength bands across the visible spectrum. The device produces photocurrent gain by carrier accumulation, while exploiting the evanescent field of the surface plasmon for both wavelength selectivity and photocurrent enhancement. With a maximum responsivity of 12.54 A/W and a full-width-half-maximum (FWHM) spectral selectivity of ~100nm, this high performance photodetector has potential for immediate applications in color-selective low-light imaging and high pixel density imaging sensors.
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    High Chromaticity Aluminum Plasmonic Pixels for Active Liquid Crystal Displays
    (American Chemical Society, 2016) Olson, Jana; Manjavacas, Alejandro; Basu, Tiyash; Huang, Da; Schlather, Andrea E.; Zheng, Bob; Halas, Naomi; Nordlander, Peter; Link, Stephan; Laboratory for Nanophotonics
    Chromatic devices such as flat panel displays could, in principle, be substantially improved by incorporating aluminum plasmonic nanostructures instead of conventional chromophores that are susceptible to photobleaching. In nanostructure form, aluminum is capable of producing colors that span the visible region of the spectrum while contributing exceptional robustness, low cost, and streamlined manufacturability compatible with semiconductor manufacturing technology. However, individual aluminum nanostructures alone lack the vivid chromaticity of currently available chromophores because of the strong damping of the aluminum plasmon resonance in the visible region of the spectrum. In recent work, we showed that pixels formed by periodic arrays of Al nanostructures yield far more vivid coloration than the individual nanostructures. This progress was achieved by exploiting far-field diffractive coupling, which significantly suppresses the scattering response on the long-wavelength side of plasmonic pixel resonances. In the present work, we show that by utilizing another collective coupling effect, Fano interference, it is possible to substantially narrow theﾠshort-wavelengthﾠside of the pixel spectral response. Together, these two complementary effects provide unprecedented control of plasmonic pixel spectral line shape, resulting in aluminum pixels with far more vivid, monochromatic coloration across the entire RGB color gamut than previously attainable. We further demonstrate that pixels designed in this manner can be used directly as switchable elements in liquid crystal displays and determine the minimum and optimal numbers of nanorods required in an array to achieve good color quality and intensity.
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    Hot Carrier Generation in Metallic Nanostructures: Mechanisms and Novel Devices
    (2016-04-21) Zheng, Bob; Halas, Naomi J
    Hot carrier generation in metallic nanostructures offers a potential route to circumventing thermodynamic efficiencies of traditional light-harvesting devices and structures. However, previous experimental realizations of hot electron devices have shown low photo-conversion efficiencies. Several theoretical works have sought to understand the fundamental processes behind hot carrier generation and explore routes toward increasing the carrier generation efficiency. In this thesis, we discriminate between hot carrier generation from interband transitions and surface plasmons by comparing photocurrent generation in Schottky and ohmic devices. By comparing the functional form of the two types of photocurrent generation, we show that hot carrier generation in metallic nanostructures obeys the field intensity inside the metallic nanostructure, paving the way towards more efficient plasmon-induced hot carrier devices. Next, I focus on plasmonic photodetectors for the mid-IR spectral region, a technologically and scientifically important spectral region where molecular vibrational resonances exist. Despite the significance of the mid-IR, the low energy of mid-IR photons poses significant challenges for efficient photodetection and light emission. We circumvent the limitations of traditional mid-IR photodetectors by exploiting hot carrier generation in metals and demonstrate a novel uncooled CMOS-compatible photodetector for the middle wave infrared (mid-IR) spectral region.