Femtosecond laser micromachining can be used either to remove materials or to change a material's properties, and can be applied to both absorptive and transparent substances. Over the past decade, this technique has been used in a broad range of applications, from waveguide fabrication to cell ablation. This review describes the physical mechanisms and the main experimental parameters involved in the femtosecond laser micromachining of transparent materials, and important emerging applications of the technology. Interactions between laser and matter are fascinating and have found a wide range of applications. This article gives an overview of the fundamental physical mechanisms in the processing of transparent materials using ultrafast lasers, as well as important emerging applications of the technology.

Femtosecond laser micromachining in transparent materials

Femtosecond filamentation in transparent media

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.

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

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.

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

In nanopore analytics, individual molecules pass through a single nanopore giving rise to detectable temporary blockades in ionic pore current. Reflecting its simplicity, nanopore analytics has gained popularity and can be conducted with natural protein as well as man-made polymeric and inorganic pores. The spectrum of detectable analytes ranges from nucleic acids, peptides, proteins, and biomolecular complexes to organic polymers and small molecules. Apart from being an analytical tool, nanopores have developed into a general platform technology to investigate the biophysics, physicochemistry, and chemistry of individual molecules (critical review, 310 references).

Nanopore analytics: sensing of single molecules

Laser induced formation of CO2 and desorption of O2 are initiated with femtosecond and picosecond laser excitation of a Pt(111) surface prepared with coadsorbed CO and O2 at 90 K. The nonlinear fluence dependent reaction yields were measured for 267, 400, and 800 nm wavelengths, and for pulse durations from 80 fs to 3.6 ps. Two-pulse correlation experiments measuring total O2 desorption yield versus time delay between 80 fs pulses show a 0.9 ps HWHM central peak and a slower 0.1 ns time-scale. At 267 nm the relative yields of O2 and CO2 are found to depend on fluence. Comparison of results at different wavelengths and pulsewidths shows that nonthermalized surface electrons play a role in the laser-induced surface chemistry.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Femtosecond laser activation of surface reactions

We develop a theory for Fano resonance tuning in dual-mode high-contrast gratings (HCGs). Compact analytical formulas of tuning sensitivity are derived and verified numerically, and are in good agreement with reported experiments. We show that the resonance tuning in HCGs, containing cooperative contribution from two propagating modes, is fundamentally different from that in single-mode microresonators. Our theory reveals the important role of the higher-order mode, which can possess large modal dispersion, especially in the long-wavelength limit beyond the cutoff of slab waveguides, to enable large tuning sensitivity. Our findings will simplify the design and optimization of active and passive tuning in HCG resonators.

Mechanism and sensitivity of Fano resonance tuning in high-contrast gratings.

Near-IR femtosecond laser ablation of monolayer tungsten diselenide WSe 2 on sapphire is demonstrated. The ablation threshold is found low (below 10 mJ/cm2), due to its low dimensionality and weak coupling to substrates. Substrate-dependent ablation threshold is predicted and will be discussed.

Femtosecond-Laser Ablation of Monolayer Tungsten Diselenide (WSe2) on Sapphire

Reducing uncertainty of wettability measurements through controlled sessile droplet deposition: Current issues

We study the performance limit of single-mode operation in low-loss large-V-number planar waveguide lasers, with transverse spatial hole burning as the dominating mechanism for transverse-mode competition. By introducing normalized variables, we develop a simple semianalytical model to describe the universal output characteristics of single-mode emission before the onset of high-order modes, which can be easily scaled to a wide range of laser configuration parameters. Our model is validated using exact numerical solutions, which show applicability beyond that of the low-loss approximation. Our analysis establishes a minimum criterion of the loss ratio between the fundamental and first higher-order mode for a robust single-mode operation. This criterion is much weaker than that of conventional wisdom based on the pure wave-propagation argument, which we attribute to a resonator enhancement effect. Our universal outcome can be denormalized to establish the relationship between single-mode extraction efficiency and optimum output coupling in a multimode laser with arbitrary modal loss ratios over a wide range of single-pass unsaturated gain and loss.

Tsing-Hua Her

Papers

Femtosecond laser activation of surface reactions

Mechanism and sensitivity of Fano resonance tuning in high-contrast gratings.

Femtosecond-Laser Ablation of Monolayer Tungsten Diselenide (WSe2) on Sapphire

Reducing uncertainty of wettability measurements through controlled sessile droplet deposition: Current issues

Universal Output Characteristics of Single-Mode Operation in Low-Loss Large- V -Number Multimode Waveguide Lasers with Transverse Spatial Hole Burning