Keng-Te Lin

Silicon-based broadband antenna for high responsivity and polarization-insensitive photodetection at telecommunication wavelengths

Although the concept of using local surface plasmon resonance based nanoantenna for photodetection well below the semiconductor band edge has been demonstrated previously, the nature of local surface plasmon resonance based devices cannot meet many requirements of photodetection applications. Here we propose the concept of deep-trench/thin-metal (DTTM) active antenna that take advantage of surface plasmon resonance phenomena, three-dimensional cavity effects, and large-area metal/semiconductor junctions to effectively generate and collect hot electrons arising from plasmon decay and, thereby, increase photocurrent. The DTTM-based devices exhibited superior photoelectron conversion ability and high degrees of detection linearity under infrared light of both low and high intensity. Therefore, these DTTM-based devices have the attractive properties of high responsivity, extremely low power consumption, and polarization-insensitive detection over a broad bandwidth, suggesting great potential for use in photodetection and on-chip Si photonics in many applications of telecommunication fields.

Romantic Story or Raman Scattering? Rose Petals as Ecofriendly, Low-Cost Substrates for Ultrasensitive Surface-Enhanced Raman Scattering

In this Article, we present a facile approach for the preparation of ecofriendly substrates, based on common rose petals, for ultrasensitive surface-enhanced Raman scattering (SERS). The hydrophobic concentrating effect of the rose petals allows us to concentrate metal nanoparticle (NP) aggregates and analytes onto their surfaces. From a systematic investigation of the SERS performance when using upper and lower epidermises as substrates, we find that the lower epidermis, with its quasi-three-dimensional (quasi-3D) nanofold structure, is the superior biotemplate for SERS applications. The metal NPs and analytes are both closely packed in the quasi-3D structure of the lower epidermis, thereby enhancing the Raman signals dramatically within the depth of focus (DOF) of the Raman optical system. We have also found the effect of the pigment of the petals on the SERS performance. With the novel petal-based substrate, the SERS measurements reveal a detection limit for rhodamine 6G below the femtomolar regime (10(-15) M), with high reproducibility. Moreover, when we employ an upside-down drying process, the unique effect of the Wenzal state of the hydrophobic petal surface further concentrate the analytes and enhanced the SERS signals. Rose petals are green, natural materials that appear to have great potential for use in biosensors and biophotonics.

Transparent, Broadband, Flexible, and Bifacial-Operable Photodetectors Containing a Large-Area Graphene–Gold Oxide Heterojunction

In this study, we combine graphene with gold oxide (AuOx), a transparent and high-work-function electrode material, to achieve a high-efficient, low-bias, large-area, flexible, transparent, broadband, and bifacial-operable photodetector. The photodetector operates through hot electrons being generated in the graphene and charge separation occurring at the AuOx-graphene heterojunction. The large-area graphene covering the AuOx electrode efficiently prevented reduction of its surface; it also acted as a square-centimeter-scale active area for light harvesting and photodetection. Our graphene/AuOx photodetector displays high responsivity under low-intensity light illumination, demonstrating picowatt sensitivity in the ultraviolet regime and nanowatt sensitivity in the infrared regime for optical telecommunication. In addition, this photodetector not only exhibited broadband (from UV to IR) high responsivity-3300 A W(-1) at 310 nm (UV), 58 A W(-1) at 500 nm (visible), and 9 A W(-1) at 1550 nm (IR)-but also required only a low applied bias (0.1 V). The hot-carrier-assisted photoresponse was excellent, especially in the short-wavelength regime. In addition, the graphene/AuOx photodetector exhibited great flexibility and stability. Moreover, such vertical heterojunction-based graphene/AuOx photodetectors should be compatible with other transparent optoelectronic devices, suggesting applications in flexible and wearable optoelectronic technologies.

Optimizing surface plasmon resonance effects on finger electrodes to enhance the efficiency of silicon-based solar cells

In this paper we demonstrate a technique for improving the conversion efficiency in conventional silicon solar cells by using surface plasmon resonance (SPR) effects to harvest incident light energy over metal finger electrodes. According to three-dimensional finite-difference time-domain (3D-FDTD) analysis, incident light covering a broad bandwidth of the solar spectrum can be transmitted through metallic hole array structures. Although the light absorption region beneath the metal finger electrodes cannot generate a photocurrent, in this study, we employ the extraordinary transmission (EOT) phenomenon, due to SPR effects, to dramatically increase the degree of light harvesting below the metal electrodes and, thereby, improve the efficiency of the entire solar cell. Experimental data reveal that the excess photocurrent density was approximately 190% of the normal current density of a standard solar cell. Therefore, the negative effect of covering the absorption area with opaque metal finger electrodes can be minimized or eliminated completely by taking advantage of the SPR effect of the metal electrodes.

Ultrahigh-sensitivity CdS photoconductors with instant response and ultralow power consumption for detection in low-light environments

In this study we describe a low-cost cadmium sulfide (CdS) photoconductor that behaves as a highly sensitive and rapidly responding detector toward low-intensity light. Through the observation of TEM images and analysis of micro-Raman spectra, the degree of crystallization of CdS films increased and their dislocation defects were removed effectively after treatment with several shots from a KrF excimer laser. Such laser treatment of CdS photoconductors could be conducted in air and completed within a few seconds. At a very low bias voltage of 1 mV, the laser-treated CdS device provided a record high responsivity of 7200 A W−1 and a detectivity of 1015 Jones. In addition, at a normal bias voltage of 1 V, it displayed an extremely high responsivity of 7 × 106 A W−1 and a detectivity of 6 × 1016 Jones. The measured response time of the laser-annealed CdS device from the dark to illumination at 10−2 fW μm−2 was only 40 ms—much faster than the shutter speed or exposure time required for a professional digital camera for such low-light image detection. Accordingly, KrF laser annealing is a simple and rapid process that can significantly enhance the low-light detection properties of CdS, a commercial photoconductor. Our strategy proposed herein appears to hold great potential for ultralow-light image detection with ultralow power consumption.

White-Light-Induced Collective Heating of Gold Nanocomposite/Bombyx mori Silk Thin Films with Ultrahigh Broadband Absorbance

This paper describes a systematic investigation of the phenomenon of white-light-induced heating in silk fibroin films embedded with gold nanoparticles (Au NPs). The Au NPs functioned to develop an ultrahigh broadband absorber, allowing white light to be used as a source for photothermal generation. With an increase of the Au content in the composite films, the absorbance was enhanced significantly around the localized surface plasmon resonance (LSPR) wavelength, while non-LSPR wavelengths were also increased dramatically. The greater amount of absorbed light increased the rate of photoheating. The optimized composite film exhibited ultrahigh absorbances of approximately 95% over the spectral range from 350 to 750 nm, with moderate absorbances (>60%) at longer wavelengths (750-1000 nm). As a result, the composite film absorbed almost all of the incident light and, accordingly, converted this optical energy to local heat. Therefore, significant temperature increases (ca. 100 °C) were readily obtained when we irradiated the composite film under a light-emitting diode or halogen lamp. Moreover, such composite films displayed linear light-to-heat responses with respect to the light intensity, as well as great photothermal stability. A broadband absorptive film coated on a simple Al/Si Schottky diode displayed a linear, significant, stable photo-thermo-electronic effect in response to varying the light intensity.

Nanocrystallized CdS beneath the surface of a photoconductor for detection of UV light with picowatt sensitivity.

In this study, we demonstrated that the improvement of detection capability of cadmium sulfide (CdS) photoconductors in the ultraviolet (UV) regime is much larger than that in the visible regime, suggesting that the deep UV laser-treated CdS devices are very suitable for low-light detection in the UV regime. We determined that a nanocrystallized CdS photoconductor can behave as a picowatt-sensitive detector in the UV regime after ultra-shallow-region crystallization of the CdS film upon a single shot from a KrF laser. Photoluminescence and Raman spectra revealed that laser treatment increased the degree of crystallization of the CdS and led to the effective removal of defects in the region of a few tens nanometers beneath the surface of CdS, confirming the result by the transmission electron microscopy (TEM) images. Optical simulations suggested that UV light was almost completely absorbed in the shallow region beneath the surface of the CdS films, consistent with the observed region that underwent major crystal structure transformation. Accordingly, we noted a dramatic enhancement in responsivity of the CdS devices in the UV regime. Under a low bias voltage (1 mV), the treated CdS device provided a high responsivity of 74.7 A W(-1) and a detectivity of 1.0×10(14) Jones under illumination with a power density of 1.9 nW cm(-2). Even when the power of the UV irradiation on the device was only 3.5 pW, the device exhibited extremely high responsivity (7.3×10(5) A W(-1)) and detectivity (3.5×10(16) Jones) under a bias voltage of 1 V. Therefore, the strategy proposed in this study appears to have great potential for application in the development of CdS photoconductors for picowatt-level detection of UV light with low power consumption.

Nondestructive Characterization of the Structural Quality and Thickness of Large-Area Graphene on Various Substrates

We demonstrate an inspection technique, based on only one ellipsometric parameter, Ψ, of spectroscopic ellipsometry (SE), for the rapid, simultaneous identification of both the structural quality and thicknesses of large-area graphene films. The measured Ψ spectra are strongly affected by changes in the out-of-plane absorption coefficients (αTM); they are also correlated to the ratio of the intensities of the D and G bands in Raman spectra of graphene films. In addition, the electronic transition state of graphene within the UV regime assists the characterization of the structural quality. We also demonstrated that the intensities and shifts of the signals in Ψ spectra allow clear identification of the structural qualities and thicknesses, respectively, of graphene films. Moreover, this Ψ-based method can be further applied to graphene films coated on various substrates. In addition, mapping of the values of Ψ is a very convenient and useful means of rapidly characterizing both the structural quality and thickness of 2D materials at local areas. Therefore, this Ψ-based characterization method has great potential for application in the mass production of devices based on large-area graphene.

Plasmonics-Based Multifunctional Electrodes for Low-Power-Consumption Compact Color-Image Sensors

High pixel density, efficient color splitting, a compact structure, superior quantum efficiency, and low power consumption are all important features for contemporary color-image sensors. In this study, we developed a surface plasmonics-based color-image sensor displaying a high photoelectric response, a microlens-free structure, and a zero-bias working voltage. Our compact sensor comprised only (i) a multifunctional electrode based on a single-layer structured aluminum (Al) film and (ii) an underlying silicon (Si) substrate. This approach significantly simplifies the device structure and fabrication processes; for example, the red, green, and blue color pixels can be prepared simultaneously in a single lithography step. Moreover, such Schottky-based plasmonic electrodes perform multiple functions, including color splitting, optical-to-electrical signal conversion, and photogenerated carrier collection for color-image detection. Our multifunctional, electrode-based device could also avoid the interference phenomenon that degrades the color-splitting spectra found in conventional color-image sensors. Furthermore, the device took advantage of the near-field surface plasmonic effect around the Al-Si junction to enhance the optical absorption of Si, resulting in a significant photoelectric current output even under low-light surroundings and zero bias voltage. These plasmonic Schottky-based color-image devices could convert a photocurrent directly into a photovoltage and provided sufficient voltage output for color-image detection even under a light intensity of only several femtowatts per square micrometer. Unlike conventional color image devices, using voltage as the output signal decreases the area of the periphery read-out circuit because it does not require a current-to-voltage conversion capacitor or its related circuit. Therefore, this strategy has great potential for direct integration with complementary metal-oxide-semiconductor (CMOS)-compatible circuit design, increasing the pixel density of imaging sensors developed using mature Si-based technology.

Filter-free, junctionless structures for color sensing

A simple structure, efficient color splitting, sufficient output of electrical signals, and low power consumption are the important characteristics of contemporary devices for color sensing. In this study, we developed filter-free, junctionless structures that exhibited a superior photo-thermo-electrical response under a low bias voltage and a short response time in milliseconds. Although our compact sensor had a simple single-layer trench-like aluminum (Al) structure, it could perform multiple functions, including light harvesting, color-selective absorption, photo-thermo-electrical transformation, and the ability to collect photoinduced differences in electrical signals. This device exploited near-field surface plasmon resonance and cavity effects to enhance the intensity of the electric field and the color-selective absorption, ultimately resulting in significant current signals in its structured Al film. This strategy significantly simplifies not only the components of the color sensor but also its fabrication; for example, red, green, and blue color detection devices could be prepared simultaneously through a single lithography, etching, and deposition step. With its ability to provide functional filter-free, junctionless structures, this strategy has great potential for the production of devices that operate on different kinds of substrates, thereby bridging various applications of color sensing technologies.