Ming Lun Tseng

Plasmon inducing effects for enhanced photoelectrochemical water splitting: X-ray absorption approach to electronic structures.

Artificial photosynthesis using semiconductors has been investigated for more than three decades for the purpose of transferring solar energy into chemical fuels. Numerous studies have revealed that the introduction of plasmonic materials into photochemical reaction can substantially enhance the photo response to the solar splitting of water. Until recently, few systematic studies have provided clear evidence concerning how plasmon excitation and which factor dominates the solar splitting of water in photovoltaic devices. This work demonstrates the effects of plasmons upon an Au nanostructure-ZnO nanorods array as a photoanode. Several strategies have been successfully adopted to reveal the mutually independent contributions of various plasmonic effects under solar irradiation. These have clarified that the coupling of hot electrons that are formed by plasmons and the electromagnetic field can effectively increase the probability of a photochemical reaction in the splitting of water. These findings support a new approach to investigating localized plasmon-induced effects and charge separation in photoelectrochemical processes, and solar water splitting was used herein as platform to explore mechanisms of enhancement of surface plasmon resonance.

Ultrafast all-optical switching via coherent modulation of metamaterial absorption

We report on the demonstration of a femtosecond all-optical modulator providing, without nonlinearity and therefore at arbitrarily low intensity, ultrafast light-by-light control. The device engages the coherent interaction of optical waves on a metamaterial nanostructure only 30 nm thick to efficiently control absorption of near-infrared (750–1040 nm) femtosecond pulses, providing switching contrast ratios approaching 3:1 with a modulation bandwidth in excess of 2 THz. The functional paradigm illustrated here opens the path to a broad family of meta-devices for ultrafast optical data processing in coherent networks.

Plasmonic ZnO/Ag Embedded Structures as Collecting Layers for Photogenerating Electrons in Solar Hydrogen Generation Photoelectrodes

A new fabrication strategy in which Ag plasmonics are embedded in the interface between ZnO nanorods and a conducting substrate is experimentally demonstrated using a femtosecond-laser (fs-laser)-induced plasmonic ZnO/Ag photoelectrodes. This fs-laser fabrication technique can be applied to generate patternable plasmonic nanostructures for improving their effectiveness in hydrogen generation. Plasmonic ZnO/Ag nanostructure photoelectrodes show an increase in the photocurrent of a ZnO nanorod photoelectrodes by higher than 85% at 0.5 V. Both localized surface plasmon resonance in metal nanoparticles and plasmon polaritons propagating at the metal/semiconductor interface are available for improving the capture of sunlight and collecting charge carriers. Furthermore, in-situ X-ray absorption spectroscopy is performed to monitor the plasmonic-generating electromagnetic field upon the interface between ZnO/Ag nanostructures. This can reveal induced vacancies on the conduction band of ZnO, which allow effective separation of charge carriers and improves the efficiency of hydrogen generation. Plasmon-induced effects enhance the photoresponse simultaneously, by improving optical absorbance and facilitating the separation of charge carriers.

Laser-induced phase transitions of Ge 2 Sb 2 Te 5 thin films used in optical and electronic data storage and in thermal lithography

Amorphous thin films of Ge(2)Sb(2)Te(5), sputter-deposited on a ZnS-SiO(2) dielectric layer, are investigated for the purpose of understanding the structural phase-transitions that occur under the influence of tightly-focused laser beams. Selective chemical etching of recorded marks in conjunction with optical, atomic force, and electron microscopy as well as local electron diffraction analysis are used to discern the complex structural features created under a broad range of laser powers and pulse durations. Clarifying the nature of phase transitions associated with laser-recorded marks in chalcogenide Ge(2)Sb(2)Te(5) thin films provides useful information for reversible optical and electronic data storage, as well as for phase-change (thermal) lithography.

Fabrication of phase-change chalcogenide Ge2Sb2Te5 patterns by laser-induced forward transfer.

Femtosecond laser pulses are focused on a thin film of Ge2Sb2Te5 phase-change material, and the transfer of the illuminated material to a nearby substrate is investigated. The size, shape, and phase-state of the fabricated pattern can be effectively controlled by the laser fluence and by the thickness of the Ge2Sb2Te5 film. Results show multi-level electrical and optical reflection states of the fabricated patterns, which may provide a simple and efficient foundation for patterning future phase-change devices.

Fast fabrication of a Ag nanostructure substrate using the femtosecond laser for broad-band and tunable plasmonic enhancement.

Using a femtosecond laser, we have transformed the laser-direct-writing technique into a highly efficient method that can process AgO(x) thin films into Ag nanostructures at a fast scanning rate of 2000 μm(2)/min. The processed AgO(x) thin films exhibit broad-band enhancement of optical absorption and effectively function as active SERS substrates. Probing of the plasmonic hotspots with dyed polymer beads indicates that these hotspots are uniformly distributed over the treated area.

Local electrical characterization of laser-recorded phase-change marks on amorphous Ge2Sb2Te5 thin films.

Amorphous thin films of Ge(2)Sb(2)Te(5), sputter-deposited on a thin-film gold electrode, are investigated for the purpose of understanding the local electrical conductivity of recorded marks under the influence of focused laser beam. Being amorphous, the as-deposited chalcogenide films have negligible electrical conductivity. With the aid of a focused laser beam, however, we have written on these films micron-sized crystalline marks, ablated holes surrounded by crystalline rings, and other multi-ring structures containing both amorphous and crystalline zones. Within these structures, nano-scale regions of superior local conductivity have been mapped and probed using our high-resolution, high-sensitivity conductive-tip atomic force microscope (C-AFM). Scanning electron microscopy and energy-dispersive spectrometry have also been used to clarify the origins of high conductivity in and around the recorded marks. When the Ge(2)Sb(2)Te(5) layer is sufficiently thin, and when laser crystallization/ablation is used to define long isolated crystalline stripes on the samples, we find the C-AFM-based method of extracting information from the recorded marks to be superior to other forms of microscopy for this particular class of materials. Given the tremendous potential of chalcogenides as the leading media candidates for high-density memories, local electrical characterization of marks recorded on as-deposited amorphous Ge(2)Sb(2)Te(5) films provides useful information for furthering research and development efforts in this important area of modern technology.

Three‐Dimensional Plasmonic Micro Projector for Light Manipulation

photovoltaics, [ 5 ] super-resolution imaging, [ 6 ] and various twodimensional plasmonic lens. [ 7 ] Besides, using nanostructures to project SPP plane waves into the adjacent free space is also an important issue. The interactions of plasmonic nanostructure on SPP wave involve not only the in-plane behavior, but also out-of-plane scattering which is captured as the far-fi eld radiated light. [ 8 ] A few theoretical approaches to convert the confi ned surface plasmons into radiated waves have been proposed. [ 9 ] It is highly desirable to extend the application range of plasmonic devices into the domain of three-dimensional light manipulation. [ 10 ] Recently, three-dimensional focusing and diverging of SPP waves by a quarter circular structure composed of gold (Au) nanobumps were studied. [ 11 ] The forward and backward scattering from individual Au nanobump are observed above and below Au surface, respectively. Hence, the Au nanobumps confer additional three-dimensional propagating wave vectors ( k x , k y , k z ) on SPP wave for departing from surface. Therefore, it is possible to manipulate the three-dimensional plasmonic scattering into specifi c geometry by arranging the Au nanobumps, which is schematically depicted in Figure 1 a. In this paper, we manipulate the scattering of SPP waves by various plasmonic structures composed of arranged nanobumps on a gold thin fi lm. Upon controlling the geometry of the plasmonic structures, the height, position, and pattern of scattered light can be modifi ed as desired. It provides a simple and effi cient way to project a specifi c light pattern into free space, and demonstrate the capability of three-dimensional light manipulation.

Two-Dimensional Active Tuning of an Aluminum Plasmonic Array for Full-Spectrum Response

Color pixels composed of plasmonic nanostructures provide a highly promising approach for new display technologies, capable of vivid, robust coloration and incorporating the use of low-cost plasmonic materials, such as aluminum. Here we report a plasmonic device that can be tuned continuously across the entire visible spectrum, based on integrating a square array of aluminum nanostructures into an elastomeric substrate. By stretching the substrate in either of its two dimensions, the period and therefore the scattering color can be modified to the blue or the red of the at-rest structure, spanning the entire visible spectrum. The unique two-dimensional design of this structure enables active mechanical color tuning, under gentle elastic modulation with no more than 35% strain. We also demonstrate active image switching with this structure. This design strategy has the potential to open the door for next-generation flexible photonic devices for a wide variety of visible-light applications.

Fabrication of three-dimensional plasmonic cavity by femtosecond laser-induced forward transfer

We fabricated a three-dimensional five-layered plasmonic resonant cavity by low-cost, efficient and high-throughput femtosecond laser-induced forward transfer (fs-LIFT) technique. The fabricated cavity was characterized by optical measurements, showing two different cavity modes within the measured wavelength region which is in good agreement with numerical simulations. The mode volume corresponding to each resonance is found to be squeezed over 10(4) smaller than the cube of incident wavelength. This property may facilitate many applications in integrated optics, optical nonlinearities, and luminescence enhancement, etc.