Alessio Perrone

This article reviews the state of the art of laser ablation and deposition techniques applied so far to more than 50 elements, including metals, metalloids and lanthanides, yielding a wide variety of compounds in the form of thin films. Laser deposition processes have been performed in high-vacuum (HV) reactors at pressure values ranging between 10⁻¹ and 10⁻⁵ Pa, namely pulsed laser deposition (PLD), or, under different reactive gas ambient (O₂, N₂, CH₄, NH₃ and many others), so-called reactive pulsed laser deposition (RPLD), with the aim to form thin films with desirable chemical compositions. While a few metals have not been deposited as pure metallic films because they have no immediate technological interest, others, like alkali and alkaline earth metals, cannot be deposited in pure metallic form due to their very strong reactivity with oxygen, water vapor and hydrogen molecules which are always present, even in ultra-high-vacuum (UHV) systems, at pressure values of 10⁻⁵–10⁻¹⁰ Pa. Furthermore, elements of the Mendeleev periodic table with an atomic number higher than 88, such as actinides and synthetic elements, are dangerous to handle and deposit in the form of thin films due to their high radioactivity; therefore, they are excluded from this review. The inclusion of the non-metal thin films of carbon (C) and related chemical compounds prepared by PLD and RPLD in the present review is justified by the extensive research and the numerous scientific articles reported in the field. All the results obtained by PLD and RPLD techniques so far are discussed and presented in tabular format to guide the reader.

This paper serves as a comprehensive and practical resource to guide researchers in selecting suitable metals for use as photocathodes in radio-frequency (RF) and superconducting radio-frequency (SRF) electron guns. It offers an in-depth review of bulk and thin-film metals commonly employed in many applications. The investigation includes the photoemission, optical, chemical, mechanical, and physical properties of metallic materials used in photocathodes, with a particular focus on key performance parameters such as quantum efficiency, operational lifetime, chemical inertness, thermal emittance, response time, dark current, and work function. In addition to these primary attributes, this study examines essential parameters such as surface roughness, morphology, injector compatibility, manufacturing techniques, and the impact of chemical environmental factors on overall performance. The aim is to provide researchers with detailed insights to make well-informed decisions on materials and device selection. The holistic approach of this work associates, in tabular format, all photo-emissive, optical, mechanical, physical, and chemical properties of bulk and thin-film metallic photocathodes with experimental data, aspiring to provide unique tools for maximizing the effectiveness of laser cleaning treatment.

This paper presents a comprehensive investigation into the quantum efficiency (QE) of metallic photocathodes used in modern high-performance radio frequency (RF) and superconducting radio frequency (SRF) guns. The study specifically examines how laser cleaning treatment impacts the QE of these photocathodes, providing detailed insights into their performance and potential improvements for accelerator applications, and assesses the chemical and environmental factors affecting the surface composition of metallic laser-photocathodes used in modern high-performance radio frequency (RF) and superconducting radio frequency (SRF) electron guns. This paper overviews the photocathode rejuvenation effects of laser cleaning treatment. Laser cleaning removes the oxides and hydrides responsible for the deterioration of photocathodes, increases the photoelectron emission quantum efficiency (QE) and extends the operational lifetime of high-brightness electron injectors. QE enhancement is analyzed with the aim of parametric cleaning process optimization. This study excludes semiconductor and thermionic cathodes, focusing solely on the widely used bulk and thin-film photocathodes of Cu, Mg, Y, Pb and Nb. Laser cleaning enhancement of QE in Cu from 5 × 10⁻⁵ to 1.2 × 10⁻⁴, in Mg from 5.0 × 10⁻⁴ to 1.8 × 10⁻³, in Y from 10⁻⁵ to 3.3 × 10⁻⁴, in Pb from 3 × 10⁻⁵ to 8 × 10⁻⁵, and in Nb from 2.1 × 10⁻⁷ to 2.5 × 10⁻⁵ is demonstrated. The analysis concludes with a specialized practical guide for improving photocathode efficacy and lifetime in RF and SRF guns.

This paper presents an innovative exploration of advanced configurations for enhancing the efficiency of metallic and superconducting photocathodes (MPs and SCPs) produced via pulsed laser deposition (PLD). These photocathodes are critical for driving next-generation free-electron lasers (FELs) and plasma-based accelerators, both of which demand electron sources with improved quantum efficiency (QE) and electrical properties. Our approach compares three distinct photocathode configurations, namely: conventional, hybrid, and non-conventional, focusing on recent innovations. Hybrid MPs integrate a thin, high-performance, photo-emissive film, often yttrium or magnesium, positioned centrally on the copper flange of the photo-injector. For hybrid SCPs, a thin film of lead is used, offering a higher quantum efficiency than niobium bulk. This study also introduces non-conventional configurations, such as yttrium and lead disks partially coated with copper and niobium films, respectively. These designs utilize the unique properties of each material to achieve enhanced photoemission and long-term stability. The novelty of this approach lies in leveraging the advantages of bulk photoemission materials like yttrium and lead, while maintaining the electrical compatibility and durability required for integration into RF cavities. The findings highlight the potential of these configurations to significantly outperform traditional photocathodes, offering higher QE and extended operational lifetimes. This comparative analysis provides new insights into the fabrication of high-efficiency photocathodes, setting the foundation for future advancements in electron source technologies.

In view of the demand for high-quality thermal neutron detectors, boron films have recently attracted widespread research interest because of their special properties. In this work, we report on the deposition of boron films on silicon substrates by sub-picosecond pulsed laser deposition (PLD) at room temperature. Particular emphasis was placed on the investigation of the effect of the laser energy density (fluence) on the ablation process of the target material, as well as on the morphological properties of the resulting films. In addition, based on the study of the ablation and deposition rates as a function of the fluence, the ablation/deposition mechanisms are discussed. We show that well-adherent and stable boron films, with good quality surfaces revealing a good surface flatness and absence of cracks, can be obtained by means of the PLD technique, which proves to be a reliable and reproducible method for the fabrication of thick boron coatings that are suitable for neutron detection technology.

Copper thin films are intended to serve as a cover layer of photocathodes that are deposited by ablating copper targets in a high vacuum by temporally clean 600 fs laser pulses at 248 nm. The extremely forward-peaked plume produced by the ultrashort UV pulses of high-energy contrast ensures fast film growth. The deposition rate, defined as peak thickness per number of pulses, rises from 0.03 to 0.11 nm/pulse with an increasing ablated area while keeping the pulse energy constant. The material distribution over the surface-to-be-coated can also effectively be controlled by tuning the dimensions of the ablated area: surface patterning from airbrush-like to broad strokes is available. The well-adhering films of uniform surface morphology consist of densely packed lentil-like particles of several hundred nm in diameter and several ten nm in height. Task-optimized ultrashort UV laser deposition is thereby an effective approach for the production of thin film patterns of predetermined geometry, serving e.g., as critical parts of photocathodes.

Pulsed laser deposition on 3-dimensional micro-objects of complex morphology is demonstrated by the paradigmatic growth of cellulose and polymer/Y₃Al₅O₁₂:Ce phosphor composite nanolayers. Congruent materials transfer is a result of multicomponent ablation performed by relatively low fluence (−2) ArF excimer laser pulses (λ = 193 nm). Films grown on optical and engineering components, having a thickness from ~50 nm to more than ~300 nm, are durable, well adherent and maintain the structural and functional properties of the parent solids. The results verify the unique capabilities of deep-ultraviolet pulsed laser deposition of novel functional nanostructures on arbitrary surface morphologies and highlight its potential in future 3-dimensional nanotechnologies.

We present the results of our investigation on metallic films as suitable photocathodes for the production of intense electron beams in radio frequency (RF) photoinjector guns. Pulsed laser ablation deposition (PLAD) technique was used for growing Mg and Y thin films onto Si and Cu substrates in high vacuum of 10 ⁻⁵ Pa and at room temperature. The morphology, the structure, the thickness and the contamination level of thin films surface deposited on Si were deduced by using different diagnostic techniques. Thin films deposited on Cu have been tested as photocathodes in an ultrahigh vacuum photodiode chamber at 10 ⁻⁷ Pa. Relatively high‐quantum efficiencies have been obtained for the deposited films, comparable to those of corresponding bulks. Samples could stay for several months in open air before being tested in a photodiode cell. The deposition process and the role of the gas absorption on the photocathode surface and its influence on the photoelectron performances are also presented and discussed. Copyright © 2010 John Wiley & Sons, Ltd.

The formation of a columnar structure, different expansion directions of the luminous (monomers) and obscure (droplets) plume species, and gradual deflection of the luminous plume are found to be interrelated in special experiments on pulsed laser deposition of Si. The obscure species always expands in the direction of incidence, which leads to a characteristic skewed angular distribution of the deposition rate. These observations are explained by the change of the microscopic mechanism of laser ablation from monomer evaporation at low local fluences to phase explosion at higher local fluences.