Diamond mirrors for high-power continuous-wave lasers.

High-power continuous-wave (CW) lasers are used in a variety of areas including industry, medicine, communications, and defense. Yet, conventional optics, which are based on multi-layer coatings, are damaged when illuminated by high-power CW laser light, primarily due to thermal loading. This hampers the effectiveness, restricts the scope and utility, and raises the cost and complexity of high-power CW laser applications. Here we demonstrate monolithic and highly reflective mirrors that operate under high-power CW laser irradiation without damage. In contrast to conventional mirrors, ours are realized by etching nanostructures into the surface of single-crystal diamond, a material with exceptional optical and thermal properties. We measure reflectivities of greater than 98% and demonstrate damage-free operation using 10 kW of CW laser light at 1070 nm, focused to a spot of 750 µm diameter. In contrast, we observe damage to a conventional dielectric mirror when illuminated by the same beam. Our results initiate a new category of optics that operate under extreme conditions, which has potential to improve or create new applications of high-power lasers.

Physical Origin of Early Failure for Contaminated Optics.

Laser-Induced optical breakdown often occurs unexpectedly at optical intensities far lower than those predicted by ultra-short pulse laser experiments, and is usually attributed to contamination. To determine the physical mechanism, optical coatings were contaminated with carbon and steel microparticles and stressed using a 17 kW continuous-wave laser. Breakdown occurred at intensity levels many orders of magnitude lower than expected in clean, pristine materials. Damage thresholds were found to strongly follow the bandgap energy of the film. A thermal model incorporating the particle absorption, interface heat transfer, and free carrier absorption was developed, and it explains the observed data, indicating that surface contamination heated by the laser thermally generates free carriers in the films. The observed bandgap dependence is in direct contrast to the behavior observed for clean samples under continuous wave and long-pulse illumination, and, unexpectedly, has similarities to ultra-short pulse breakdown for clean samples, albeit with a substantially different physical mechanism.

Laser acceleration of absorbing particles.

A Yb-doped fiber laser is used to accelerate and evaporate absorbing particles in air. Optical intensities of 1MW/cm2 and 2MW/cm2 illuminate stainless steel particles. These particles are accelerated to velocities of tens of meters per second before evaporating within a few tenths of a millisecond. Position measurements are made using direct imaging with a high-speed camera. A fundamental system of coupled differential equations to track particle momentum, velocity, mass, radius, temperature, vapor opacity, and temperature distribution is developed and shown to accurately model the trajectories and lifetimes of laser heated particles. Atoms evaporating from the particle impart momentum to the larger particle, which accelerates until it is slowed by drag forces. Heat transfer within the evaporating particles is dominated by radiation diffusion, a process that usually only dominates in astrophysical objects, for example in the photospheres of stars.