Claudia Wu

Bio: Claudia Wu is an academic researcher from Harvard University. The author has contributed to research in topics: Laser & Silicon. The author has an hindex of 9, co-authored 11 publications receiving 1753 citations.

Microstructuring of silicon with femtosecond laser pulses

Abstract: We report that silicon surfaces develop an array of sharp conical spikes when irradiated with 500 laser pulses of 100-fs duration, 10-kJ/m2 fluence in 500-Torr SF6 or Cl2. The spikes are up to 40-μm tall, and taper to about 1-μm diam at the tip. Irradiation of silicon surfaces in N2, Ne, or vacuum creates structured surfaces, but does not create sharp conical spikes.

Near-unity below-band-gap absorption by microstructured silicon

Abstract: We increased the absorptance of light by silicon to approximately 90% from the near ultraviolet (0.25 μm) to the near infrared (2.5 μm) by surface microstructuring using laser-chemical etching. The remarkable absorptance most likely comes from a high density of impurities and structural defects in the silicon lattice, enhanced by surface texturing. Microstructured avalanche photodiodes show significant enhancement of below-band-gap photocurrent generation at 1.06 and 1.31 μm, indicating promise for use in infrared photodetectors.

Femtosecond laser-induced formation of spikes on silicon

Abstract: We find that silicon surfaces develop arrays of sharp conical spikes when irradiated with 500-fs laser pulses in SF6. The height of the spikes decreases with increasing pulse duration or decreasing laser fluence, and scales nonlinearly with the average separation between spikes. The spikes have the same crystallographic orientation as bulk silicon and always point along the incident direction of laser pulses. The base of the spikes has an asymmetric shape and its orientation is determined by the laser polarization. Our data suggest that both laser ablation and laser-induced chemical etching of silicon are involved in the formation of the spikes.

Visible luminescence from silicon surfaces microstructured in air

Abstract: We report visible luminescence from SiOx formed by microstructuring silicon surfaces with femtosecond laser pulses in air. Incorporation of oxygen into the silicon lattice occurs only where the laser beam strikes the surface. Laser microstructuring therefore offers the possibility of writing submicrometer luminescent features without lithographic masks. The amount of oxygen incorporated into the silicon surface depends on the laser fluence; the peak wavelength of the primary luminescence band varies between 540 and 630 nm and depends on the number of laser shots. Upon annealing, the intensity of the primary luminescence band increases significantly without any change in the luminescence peak wavelength, suggesting that the luminescence comes from defects rather than quantum confinement.

Systems and methods for light absorption and field emission using microstructured silicon

Abstract: Methods and systems for absorbing infrared light, and for emitting current are described. A sample, such as a sample containing mainly silicon, is microstructured by at least one laser pulse to produce cone-like structures on the exposed surface. Such microstructuring enhances the infrared absorbing, and current emission properties of the sample.

Visible-light-enhanced catalytic oxidation reactions on plasmonic silver nanostructures

Abstract: Catalysis plays a critical role in chemical conversion, energy production and pollution mitigation. High activation barriers associated with rate-limiting elementary steps require most commercial heterogeneous catalytic reactions to be run at relatively high temperatures, which compromises energy efficiency and the long-term stability of the catalyst. Here we show that plasmonic nanostructures of silver can concurrently use low-intensity visible light (on the order of solar intensity) and thermal energy to drive catalytic oxidation reactions--such as ethylene epoxidation, CO oxidation, and NH₃ oxidation--at lower temperatures than their conventional counterparts that use only thermal stimulus. Based on kinetic isotope experiments and density functional calculations, we postulate that excited plasmons on the silver surface act to populate O₂ antibonding orbitals and so form a transient negative-ion state, which thereby facilitates the rate-limiting O₂-dissociation reaction. The results could assist the design of catalytic processes that are more energy efficient and robust than current processes.

Broadband polarization-independent resonant light absorption using ultrathin plasmonic super absorbers

Abstract: Resonant plasmonic and metamaterial structures allow for control of fundamental optical processes such as absorption, emission and refraction at the nanoscale. Considerable recent research has focused on energy absorption processes, and plasmonic nanostructures have been shown to enhance the performance of photovoltaic and thermophotovoltaic cells. Although reducing metallic losses is a widely sought goal in nanophotonics, the design of nanostructured 'black' super absorbers from materials comprising only lossless dielectric materials and highly reflective noble metals represents a new research direction. Here we demonstrate an ultrathin (260 nm) plasmonic super absorber consisting of a metal–insulator–metal stack with a nanostructured top silver film composed of crossed trapezoidal arrays. Our super absorber yields broadband and polarization-independent resonant light absorption over the entire visible spectrum (400–700 nm) with an average measured absorption of 0.71 and simulated absorption of 0.85. Proposed nanostructured absorbers open a path to realize ultrathin black metamaterials based on resonant absorption.

Anti-reflective coatings: A critical, in-depth review

Abstract: Anti-reflective coatings (ARCs) have evolved into highly effective reflectance and glare reducing components for various optical and opto-electrical equipments. Extensive research in optical and biological reflectance minimization as well as the emergence of nanotechnology over the years has contributed to the enhancement of ARCs in a major way. In this study the prime objective is to give a comprehensive idea of the ARCs right from their inception, as they were originally conceptualized by the pioneers and lay down the basic concepts and strategies adopted to minimize reflectance. The different types of ARCs are also described in greater detail and the state-of-the-art fabrication techniques have been fully illustrated. The inspiration that ARCs derive from nature (‘biomimetics’) has been an area of major research and is discussed at length. The various materials that have been reportedly used in fabricating the ARCs have also been brought into sharp focus. An account of application of ARCs on solar cells and modules, contemporary research and associated challenges are presented in the end to facilitate a universal understanding of the ARCs and encourage future research.

Ultrafast lasers—reliable tools for advanced materials processing

Abstract: The unique characteristics of ultrafast lasers, such as picosecond and femtosecond lasers, have opened up new avenues in materials processing that employ ultrashort pulse widths and extremely high peak intensities. Thus, ultrafast lasers are currently used widely for both fundamental research and practical applications. This review describes the characteristics of ultrafast laser processing and the recent advancements and applications of both surface and volume processing. Surface processing includes micromachining, micro- and nanostructuring, and nanoablation, while volume processing includes two-photon polymerization and three-dimensional (3D) processing within transparent materials. Commercial and industrial applications of ultrafast laser processing are also introduced, and a summary of the technology with future outlooks are also given. Scientists in Asia have reviewed the role of ultrafast lasers in materials processing. Koji Sugioka from RIKEN in Japan and Ya Cheng from the Shanghai Institute of Optics and Fine Mechanics in China describe how femtosecond and picosecond lasers can be used to perform useful tasks in both surface and volume processing. Such lasers can cut, drill and ablate a variety of materials with high precision, including metals, semiconductors, ceramics and glasses. They can also polymerize organic materials that contain a suitable photosensitizer and can three-dimensionally process inside transparent materials such as glass, and are already being used to fabricate medical stents, repair photomasks, drill ink-jet nozzles and pattern solar cells. The researchers also explain the characteristics of such lasers and the interaction of ultrashort, intense pulses of light with matter.