This guest editorial summarizes the Special Section on High Power Laser Ablation.

In the not so distant future, the laser ablation propulsion rockets powered by ground-based high average power laser will challenge the existing techniques of accessing space based on chemical propulsion. Chemical rocket propulsion, which made access to space possible to begin with and continues to be the only technology in use to that end, cannot, however, adequately serve the emerged and quickly growing launch-to-orbit needs of the pico-nano-micro-satellites niche markets. No further technological improvements in chemical rocket propulsion can change its well-known inherent limitations and disadvantages. The ongoing and imminent developments in the satellites’ markets as well as the timeless fundamental need in robust low-cost ground-to-orbit logistics urgently call for adopting a new game-changing technological paradigm for accessing space. By elaborating and expanding on the potentially key role of the laser ablation propulsion launch system in enabling the affordable and safe access to space, this paper logically continues the author’s previous work “Optimizing access to space: ground-to-orbit logistics framework (GOLF)” published in 2017 in the New Space journal.

A ground-based laser system for space debris cleaning requires pulse power well above the critical power for self-focusing in the atmosphere. Self-focusing results in beam quality degradation and is detrimental for the system operation. We demonstrate that, for the relevant laser parameters, when the thickness of the atmosphere is much less than the focusing length (that is, of the orbit scale), the beam transit through the atmosphere produces the phase distortion only. The model thus developed is in very good agreement with numerical modeling. This implies that, by using phase mask or adaptive optics, it may be possible to eliminate almost completely the impact of self-focusing effects in the atmosphere on the laser beam propagation.

Small space debris objects of even a few centimeters can cause severe damage to satellites. Powerful lasers are often proposed for pushing small debris by laser-ablative recoil toward an orbit where atmospheric burn-up yields their remediation. We analyze whether laser-ablative momentum generation is safe and reliable concerning predictability of momentum and accumulation of heat at the target. With hydrodynamic simulations on laser ablation of aluminum as the prevalent debris material, we study laser parameter dependencies of thermomechanical coupling. The results serve as configuration for raytracing-based Monte Carlo simulations on imparted momentum and heat of randomly shaped fragments within a Gaussian laser spot. Orbit modification and heating are analyzed exemplarily under repetitive laser irradiation. Short wavelengths are advantageous, yielding momentum coupling up to ∼40  mNs  /  kJ, and thermal coupling can be minimized to 7% of the pulse energy using short-laser pulses. Random target orientation yields a momentum uncertainty of 86% and the thrust angle exhibits 40% scatter around 45 deg. Moreover, laser pointing errors at least redouble the uncertainty in momentum prediction. Due to heat accumulation of a few Kelvin per pulse, their number is restricted to allow for intermediate cooldown. Momentum scatter requires a sound collision analysis for conceivable trajectory modifications.

We present the formation of micro- and nanostructures on glass surfaces by 15-ns 193-nm ArF laser irradiation during the laser irradiation. The effect of laser fluence and pulse number on the micro- and nanostructure formation is studied. The scanning electron microscopy images of the irradiated surfaces show that the shape and density of the microstructures change with different laser parameters. Our results show that the microstructure formation is a characteristic feature of the ablation threshold of the glass surface at 15-ns ArF laser irradiation. The ordered microstructures are obtained with 500 pulses at 350  mJ  /  cm2 laser fluence. Also, the nanostructures can be formed at subthreshold laser fluence. The micro-Raman analysis of the glass surfaces before and after the laser irradiation shows that no phase change has occurred during surface structuring. The obtained compact and ordered micro- and nanostructures or multiscale structured surface can be applied in photonics and surface science.

One of the main figures of merit in laser-ablative propulsion is the specific impulse, Isp, defined as the impulse per unit weight of fuel, and it is related to the exhaust velocity, ve, by the acceleration of gravity, Isp  =  ve  /  g. Being a key magnitude, Isp needs to be accurately determined. It is usually inferred from other measurable quantities: the impulse coupling coefficient, Cm, defined as the ratio of the target momentum produced to the incident laser pulse energy, and Q  *  , the laser energy consumed per unit weight of ablated target material. Thus, Isp is calculated as Isp  =  CmQ  *    /  g. However, single pulse ablated mass leading to Q  *   is in the nanogram scale and cannot be directly measured by weighting the targets. So, mass loss measurements are performed by analyzing the volumes of the craters produced by a large number of laser pulses. These procedures lead to larger than desired uncertainties in the Isp values. On the other hand, more precise measurements of Isp can be carried out from the direct measurement of the exhaust velocity of the ejected particles by interferometric methods. In this work, a system based on a Nomarsky interferometer has been set up for the time-resolved diagnostic in the nanometric scale of laser ablation plumes. The performance of the implemented system was first validated by measuring the Isp produced by aluminum targets and solid propellants based on metal/salt mixtures. The Cm dependence on laser parameters and binary composition of these propellants have been determined in previous works with a torsion pendulum and a piezoelectric sensor. Once the interferometer performance is characterized, the Isp produced by solid propellants composed of metal (Zn) and metal oxides (ZnO) matrices doped with nanoparticles of different materials is determined and compared.

This paper presents the method of fabricating a frequency-selective surface (FSS) filter for terahertz frequency by short- and ultrashort laser ablation process. The FSS consists of a capacitive screen made up of the copper metallic structure supported by a Teflon dielectric substrate. The dielectric properties of the Teflon substrate at terahertz frequencies were evaluated using THz-time-domain spectroscopy technique. The numerical simulation of the designed structure was performed using Computer Simulation Technology microwave studio software with unit cell configurations and the resonance was observed at 0.135 THz. The desired metallic microstructure was created in copper using an 8-ns solid-state Nd: YAG and 150 fs Ti:sapphire laser operating at the second harmonic wavelength of 532 and 800 nm, respectively. The proposed structure provides polarization-independent operation. The experimental verification of the fabricated FSS using femtosecond ablation process was done using THz-TDS technique, and it is observed that the measured data well match with the simulated result.