Xuejun Lu

All ink-jet-printed carbon nanotube thin-film transistor on a polyimide substrate with an ultrahigh operating frequency of over 5 GHz

We report a flexible carbon nanotube (CNT) thin-film transistor (TFT) fabricated solely by ink-jet printing technology. The TFT is top gate configured, consisting of source and drain electrodes, a carrier transport layer based on an ultrapure, high-density (>1000 CNTs/μm2) CNT thin film, an ion-gel gate dielectric layer, and a poly(3,4-ethylenedioxythiophene) top gate electrode. All the TFT elements are ink-jet printed at room temperature on a polyimide substrate without involving any photolithography patterning or surface pretreatment steps. This CNT-TFT exhibits a high operating frequency of over 5 GHz and an on-off ratio of over 100. Such an all-ink-jet-printed process eliminates the need for lithography, vacuum processing, and metallization procedures and thus provides a promising technology for low-cost, high-throughput fabrication of large-area high-speed flexible electronic circuits on virtually any desired flexible substrate.

Temperature-dependent photoresponsivity and high-temperature (190K) operation of a quantum dot infrared photodetector

In this letter, a longwave infrared (LWIR) InAs–InGaAs quantum dot infrared photodetector with a peak detection wavelength of 9.9μm is reported. A large photoresponsivity of 2.5A∕W and a high peak specific photodetectivity D* of 1.1×108cmHz1∕2∕W were obtained at the operating temperature of 190K. The QDIP showed a strong temperature-dependent photoresponsivity over the temperature range from 78to190K. This effect is shown to be attributable to temperature-dependent electron capture probability.

Polymeric waveguide prism-based electro-optic beam deflector

Beam steering devices without moving parts are highly desir- able for their potential application in emerging optical technologies such as holographic optical storage systems, all optical networks, and optical switches. We demonstrate a thin-film waveguide beam deflector device that consists of an electro-optic prism array within a polymer waveguide. An electrode structure defines the prism array within the planar wave- guide. The deflection efficiency of 28 mrad/kV and the maximum deflec- tion angle of 68.4 mrad at 6300 V are obtained for this demonstration device. Further optimization of electrode-field poling and processing is likely to improve these results by at least an order of magnitude.

Polymeric electro-optic modulator based on 1×2 Y-fed directional coupler

We have demonstrated a polymeric electro-optic modulator based on a 1×2 Y-fed directional waveguide coupler. The symmetric geometry of the 1×2 Y-fed directional coupler provided the modulator unique characteristics of intrinsic 3 dB operating point and two complementary output ends. A low switching voltage of 3.6 V and a high extinction ratio of 26 dB were obtained with the modulator operating at a wavelength of 1.34 μm. The modulator was fabricated with a novel electro-optic polymer that was synthesized from polyurethane cross-linking with a chromophore.

Inkjet-Printed Two-Dimensional Phased-Array Antenna on a Flexible Substrate

In this letter, we present a two-dimensional 2-bit 4 × 4 phased-array antenna on a flexible Kapton substrate fabricated using inkjet printing. Printed carbon nanotube thin-film transistors (CNT-TFTs) form the switching elements in the phase-shifting network. A multilayer interconnection scheme has been utilized to fully package the subsystem and enable convenient access of the control lines to the 64 CNT-TFTs. By appropriately controlling the on and off states of the TFT switches using a computer mainframe, beam steering of a 5-GHz RF signal at four steering angles of θ = 0^°, φ = 0^°; θ = 14.5^°, φ = 0^°; θ = 20.7^°, φ = -45^°; and θ = 34^°, φ = -26.5^° are experimentally demonstrated. The insertion loss and the power consumption by the switch array are measured to be 8.17 dB and 11.2 mW, respectively .

Voltage tunable two-color InAs∕GaAs quantum dot infrared photodetector

We report a bias voltage tunable two-color InAs/GaAs quantum dot infrared photodetector working under the normal incidence infared irradiation. The two-color detection of our device is realized by combining a photovoltaic and a photoconductive response by bias voltage tuning. The photovoltaic response is attributed to the transition of electron from the ground state to a high continuum state. The photoconductive response arises from the transition of electron from the ground state to the wetting layer state through the barrier via Fowler-Nordheim tunneling evidenced by a broad feature of the photocurrent peak on the high energy side

Optimizing light absorption in quantum dot infrared photodetectors by tuning surface confinement of surface plasmonic waves

In this paper, we measured the transmission of the 2DSHA surface plasmonic structures and its variation with the hole diameters a of the 2DSHA structures. The relationship between the transmission and the hole diameters a is found to be different from the prediction of Bethes diffraction theorem. We also found that the photocurrent of the quantum dot (QD) infrared photodetectors (QDIPs) with different QD active layer thicknesses show different dependence on the hole diameters a of the 2DSHA structures. The photocurrent of the QDIPs with 10 active QD layers (10-QDIPs) saturates and starts to decrease as the hole diameter a is larger than 1.6 µm, whereas that of the QDIPs with 20 active QD layers (20-QDIPs) increases linearly with the hole diameter. The difference in the hole-diameter dependence of the 10-QDIPs and the 20-QDIPs is attributed to the variation of the near-field spreading in the vertical (surface-normal) direction due to the change in the hole diameters. An over 6 time (6×) photocurrent enhancement is obtained by optimizing the hole diameter of the 2DSHA surface plasmonic structure.

Surface plasmonic resonance induced near-field vectors and their contribution to quantum dot infrared photodetector enhancement

In this paper, we analyse surface plasmonic resonance (SPR) induced near-field electric-field vector distribution in the quantum dot (QD) region and determine their roles in quantum dot infrared photodetector (QDIP) enhancement. SPR can be excited in metallic two-dimensional subwavelength hole arrays (2DSHAs) when illuminated at resonant wavelengths. The SPR induced near-field vectors (Ez, Ex and Ey) and their distributions and overlaps with the QD active region are simulated. A long-wave infrared (LWIR) QDIP is fabricated with the 2DSHA plasmonic structure to experimentally measure the SPR enhancement spectrum and compare it with the near-field vector components and their distribution in QDs. We found that QDIP enhancement is closely related to the near-field intensity overlap integral in the QD region. The large near-field overlap integral corresponds to high QDIP enhancement. Such near-field overlap integral dependent plasmonic enhancement is attributed to the interaction of and the electric-dipole interaction in QDs.

Surface plasmonic enhanced polarimetric longwave infrared photodetection with band pass spectral filtering

In this paper, we report a surface plasmonic enhanced polarimetric longwave infrared (LWIR) photodetector. Polarization-selective detection of LWIR incidence with different polarizations is achieved at different plasmonic resonant modes. Band-pass spectral filtering is also provided at the plasmonic resonant modes by the plasmonic enhancement. The extinction ratio (ER) of the polarimetric detection and its limiting factor is discussed.

A modulation-doped longwave infrared quantum dot photodetector with high photoresponsivity

An InAs–InGaAs quantum dot (QD) longwave infrared (LWIR) photodetector (QDIP) with a peak wavelength of 8.2 µm is presented. The QDIP has modulation-doped QD active layers. At 77 K, the QDIP showed high photoresponsivities of 5.4 A W−1 and 3.6 A W−1 at biases of −0.8 V and 0.6 V, respectively. A peak photodetectivity of 7.8 × 109 cm Hz1/2 W−1 was obtained at 77 K in an IR detector compatible package.