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To better understand the large country-level heterogeneity found with respect to post-conflict economic recovery, this paper examines the potential role and impact of UN peacekeeping operations (PKOs) using a dataset which includes key information on civil conflicts and UN PKOs spanning 124 developing economies from 1990 to 2018. Analytical results provide evidence in support of PKOs’ positive impact on post-conflict economic recovery. Recovery growth (defined as a real GDP growth above peacetime growth) is found to only occur when a PKO is deployed and the relationship is stronger for, and likely driven by, the so-called transformative PKOs. Across robustness tests results imply that PKOs are, on average, associated with recovery rates of growth between 2 to 4 percentage points, and thus that peacekeeping could be an important factor in reducing and eliminating conflict-attributable macroeconomic losses. Foreign direct investments (FDI) and official development assistance (ODA) are explored as two potential channels that could help explain the growth results. While results for FDI are inconclusive, ODA results imply that there is a strong association between periods of recovery and ODA (as a percentage of GDP) when recovery periods coincide with the presence of a PKO, and again much stronger for transformative PKOs.

The fifteenth topical meeting on Optical Interference Coatings (OIC) was held June 19–24, 2022 in Whistler, British Columbia, Canada. This feature issue of Applied Optics collects selected papers presented during this conference. The OIC topical meeting takes place every three years and is a pivotal event for the international community working in the field of optical interference coatings. The conference provides premier opportunities for attendees to share knowledge on their new advances in research and development and to build bridges for further collaborations. The meeting topics cover a broad range, from fundamental research, coating design theory, new materials, deposition and characterization technologies to a vast catalogue of applications including green technologies, aerospace, gravitational wave detection, communication, optical instruments, consumer electronics, high power and ultra-fast lasers, and many others.

Modern laser systems have paved the way for spaceborne laser applications such as Earth's surface and atmosphere monitoring. Well known technologies like Nd:YAG lasers are often employed; however, they do not always comply with all the different requirements for space missions. High optical efficiencies and tunable wavelength, which are desirable for many applications, can be reconciled with a simple laser design employing Alexandrite crystals. Horizon 2020 project presented here discuss the results on the development of alexandrite laser crystal treatment prior to coating deposition, as well as future plans on crystal interference coating deposition for LIDT improvement.

Dielectric cavity mirrors for storage-ring free-electron lasers (FELs) in the VUV range have to overcome several challenges including resistance to synchrotron radiation and long-term stability in a contaminated vacuum. While oxide materials are not suitable below 190 nm, pure fluoride materials are not stable in this challenging environment. It has not been possible until now to generate lasing below 176 nm in a storage ring FEL. Duke University/TUNL and Laser Zentrum Hannover have successfully developed new strategies and produced mirrors with a high reflectivity of 95 %, which enabled FEL lasing between 168,6 and 176,7 nm.

Hybrid integrated photonics open up new application perspectives due to compact size and the shift to cost-efficient components. Therefore, integration of optical and electro-optical functionalities into photonic chips has recently attracted great interest. Research has been directed towards miniaturization of demanding spectral transfer properties for individual applications. However, it remains challenging to implement highly complex transmission and reflection characteristics with few additional process steps. In this contribution, we report on our advancement in the field of optical thin-film coating fabrication, which enables a manufacturing process comparable to die assembly in electronics. We have combined a sacrificial-substrate approach with the production of miniaturized optical thin-film coatings by ion-beam sputtering. The concept is applicable to high precision coatings with more than 130 individual layers and adding up to over 26 µm total film thickness. Segmentation down to sizes of 25 μm x 25 μm pieces is realized by laser cutting of the coating. By completely removing the substrate afterwards, we achieve a freestanding thin-film and thus minimized thickness. Our measurements indicate no general performance loss compared to coatings on glass substrates. Additionally, the substrates refractive index and absorption do not have to be considered in the multilayer-coating design. Therefore, the design can be optimized and matched to the refractive index of specific waveguides on the chip. Furthermore, we demonstrate the compatibility to releasable transfer tape. With this, we aim for enabling a high-volume feed of miniaturized thin-film filters to an automated assembly process.

Third harmonic generation (THG) from multiple thin layer structures has proven its capability to reach conversion efficiencies up to the percentage level [1] , becoming an interesting alternative to the cascaded third harmonic generation. However, such multi-layer structures need to be carefully optimized by computer algorithms. Here, we present a study of a single amorphous Hf x Al y O z layer with varying thickness. The advantage of ternary mixture materials such as Hf x Al y O z is the tunability of the bandgap, which directly influences the nonlinear susceptibility χ (3) . In comparison to previous works, where only layers of specific thickness or theoretical models were studied [2] , [3] , we present both, continuous experimental measurements and theoretical studies of the same sample.

In the course of continuous innovations in photonic integration optical components with ever decreasing size and increasing performance are gaining of importance in a wide range of applications. In addition, a trend towards shorter wavelengths in modern photonic systems can be observed, and frequency conversion units are commonly employed add-on modules for most laser types. Especially considering conversion to the third harmonic (TH) wavelength, present concepts are based on two second-order nonlinear processes involving two conversion crystals. Even though corresponding modules are commercially available, there is a need for simpler solutions that can be further scaled down and handled in automated assembly systems. One possibility is direct third-harmonic generation (THG) in a third-order process [1] . However compared to second-order processes, phase matching imposes a certain challenge here, because the refractive index differences between the fundamental and third harmonics are significantly larger for typical conversion materials.

We manufactured and tested ion beam sputtered antireflection coatings for 355 nm using several defect mitigation strategies to improve the damage resistance of laser optics for LIDAR satellite missions. The first approach to avoid the damage precursors in the film is based on a secondary ion source. The goal is to remove the particles that settle on the surface of the growing film. A second strategy is to avoid the damage precursors in the ultraviolet wavelength range by laser conditioning of the thin films during the coating process. A laser beam at the application wavelength scans the deposited layer on the sample during deposition still in vacuum. Optical coatings produced with both technologies show a significantly increased damage threshold when tested in ramped raster scan tests.

Both low stress and high laser induced damage threshold are required in the development of high power large laser facilities. IBS and ALD films have potential in such applications in aspects of film thickness control and high LIDT, but their severe stresses cause an obstacle towards the widely application. The origin of compressive stress of IBS coating and tensile stress of ALD coating is analyzed with a model based on intermolecular potential. An anti-reflection coating having compensated stress is designed and fabricated with ALD Al2O3 and IBS SiO2. The LIDT of the anti-reflection coating as well as the single layers are tested at wavelength 532 nm. A transmittance of 96.5% at single surface and a stress as low as 38 MPa are achieved. The LIDT of the anti-reflection coating is limited by the additional defects introduced at the film interfaces. This design takes advantage of the power resistance of IBS/ALD coatings and their contrary stresses, providing a concept towards the application in high power laser systems.

Spaceborne Earth observation based on laser instruments provides new technologies to monitor the atmosphere or our planet’s surface. Space-qualified Alexandrite laser crystals show convincing properties as a laser-active medium in high power laser systems for space-based missions, e.g. the wavelength tunability and the excellent material properties, such as high thermal conductivity and a good breaking strength. Therefore, the Horizon 2020 project GALACTIC was initiated to realize space-qualified, high-quality coated Alexandrite crystals relying on a purely European-based supply chain. The project consortium will push the development of Alexandrite crystals and coatings within the EU from the current Technology Readiness Level (TRL) 4 up to TRL 6. The Horizon 2020 project GALACTIC, which will be presented, has been initiated to realize space qualified high quality, high performance coated Alexandrite crystals rest on a purely European-based supply chain. The project consortium, consisting of Optomaterials S.r.l., Altechna Coatings and the Laser Zentrum Hannover e.V., provides the necessary expertise in the fields of laser crystal growth, optical coatings and laser systems to push the development of high quality, high damage threshold Alexandrite coatings within the EU up to TRL 6. In order to reach the GALACTIC goals, different technology areas will be addressed. On the one hand, Optomaterials S.r.l. as an experienced optical crystal manufacturer will continuously refine the crystal growth process and improve the raw crystals as well as the cutting and polishing quality step-by-step. As an experienced company in optical coatings production for space applications, Altechna Coatings will secondly develop high quality, low loss, high damage threshold coatings specifically tailored for application onto Alexandrite laser crystals. The superior performance of the coated crystals will then be proven in demonstrator laser systems. The Laser Zentrum Hannover e.V. has the required knowledge to set up laser demonstrator prototypes, whose specifications mimic typical earth observation laser source requirements. Finally, the GALACTIC consortium will work closely together to characterize and qualify the developed coated laser crystals. By collaborating with the European space industry and deriving the detailed requirements from typical earth observation space missions, a detailed TRL 6 components test plan will prove the TRL 6 qualification of the developed coated laser crystals. This will conclude the development process and will enhance the European non-dependence status of the Alexandrite laser crystal market. At the conference, we will present the project and its consortium as well as detailed strategies to enable a non-dependence of Europe on coated Alexandrite laser crystal technology and enabling long term European availability of this technology. This project has received funding from the European Union's Horizon 2020 research and innovation programme under Grant Agreement No 870427.

By analyzing the third harmonic generation from gradient layers of the amor- phous dielectric ternary mixture material HfxAlyOz we are able to derive the third order nonlinear susceptibility of the material.

Epsilon-near-zero-materials and their unique properties are key to successful integration and miniaturization of optical components. Novel concepts, which promise significant progress in this field of research, such as optical switches and thin film electro-optical modulators, are possible, when the electrical and optical properties of ENZ-materials are carefully exploited. To achieve a greater understanding of these properties, this contribution investigates the electrical conductivity, optical transmittance as well as losses of thin indium tin oxide films and links them to their LIDT at various ultrashort pulse durations.

Spaceborne LIDAR systems with high power laser optics enable advanced understanding of the climate change. However, nano-scaled particles originating from film deposition processes reduce the laser-induced damage threshold. A secondary ion source is used to remove the particles that settle on the surface of the growing film. A second option is to remove precursors by laser irradiation before they are covered with additional layers. To verify the impact on the defect mitigation, laser-induced damage threshold tests and raster scan tests were conducted. This work has been carried under the ESA contract AO 1-8683/16/NL/BJ activity with name "Particle mitigation in high power laser optics".

We demonstrate the mitigation of nano-sized damage precursors during the manufacturing of IBS-coatings using bombardment with a secondary ion source and laser conditioning with a nanosecond-pulsed laser. Optical coatings show a significantly increased damage threshold.

Manufacturing of optical coatings is about the precise deposition of material and its control on the nanometer scale. This has to be achieved for a stack of multiple layers with a total thickness that can reach several micrometer. In addition, this has to be maintained for the complete functional area. In this contribution we demonstrate a process capable of delivering these accuracies not only to flat but also to curved surfaces.

High throughput of microelectronic fabrication demands high performance CaF2 laser optics. Based on ArF laser-induced fluence-dependent absorption, we derived one-photon/two-photon absorption, and surface/subsurface absorption of CaF2 windows.

The development of advanced laser systems has paved the way for spaceborne laser applications like Earth’s atmosphere and surface monitoring. Prominent technologies like Nd:YAG are often exploited, however they have proven a challenge to combine the different requirements for space missions. High optical efficiencies and tunable wavelength, which are desirable for many applications, can be reconciled with a simple laser design employing Alexandrite crystals. Horizon 2020 project presented here focuses on the development of high quality and high performance coated Alexandrite crystals along with the construction of a laboratory prototype Alexandrite laser system with specifications suitable for Earth observation missions.

Indoor temperatures in apartment blocks are often indirectly controlled by the outdoor temperature using a feedforward control loop, in which the radiator supply temperature is a function of the outdoor temperature. However, this control principle cannot take into account heat gains or losses caused by tenants, electrical appliances, the sun, air leakage, etc., which may result in uneven indoor temperatures, overheating, airing and increased energy use. This can be partly addressed by using thermostatic valves on each radiator. A municipal housing company in Sweden that uses individual metering and billing (IBM) of space heating costs based on measurements of indoor temperatures in all rooms of each apartment has been studied. This article presents and evaluates a project in which these measurements were used for feedback control. The aim of the study was to evaluate the principle which is based on using the actual indoor temperatures. An existing feedforward control of the heating system with thermostatic valves was enhanced by a correction of the supply temperature. The magnitude of the correction was proportional to the difference between the actual mean indoor temperature of the apartments and the set-point temperature. The enhanced control resulted in more constant indoor temperatures, i.e. they were less dependent on the outdoor temperature. The results support the conclusion that the evaluated method would be promising to apply in multi-family buildings. The introduction of the enhanced control method provided valuable experience and awareness of influencing factors if it were to be implemented on a larger scale.

Atomic layer deposition (ALD) facilitates the deposition of coatings with precise thickness, high surface conformity, structural uniformity, and nodular-free structure, which are properties desired in high-power laser coatings. ALD was studied to produce uniform and stable Al2O3 and HfO2 single layers and was employed to produce anti-reflection coatings for the harmonics (1ω, 2ω, 3ω, and 4ω) of the Nd:YAG laser. In order to qualify the ALD films for high-power laser applications, the band gap energy, absorption, and element content of single layers were characterized. The damage tests of anti-reflection coatings were carried out with a laser system operated at 1ω, 2ω, 3ω, and 4ω, respectively. The damage mechanism was discussed by analyzing the damage morphology and electric field intensity difference. ALD coatings exhibit stable growth rates, low absorption, and rather high laser-induced damage threshold (LIDT). The LIDT is limited by HfO2 as the employed high-index material. These properties indicate the high versatility of ALD films for applications in high-power coatings.

Carriers in liner shipping markets frequently make public announcements of general rate increase (GRI) intentions, based on which EU authorities have concerns as to whether this harms market competition. This paper aims to empirically investigate how well the GRI system works from an industrial competition perspective, which will indirectly indicate whether carriers are able to manipulate spot rates following GRI announcements. Taking the Far East–North Europe trade between 2009 and 2013 as an example, the paper first reveals the gradual increase of GRI frequency and size, which reflects carriers’ attempts to restore profitability against overcapacity. However, out of all the GRI events only 28.6% were observed to be successful. Since these GRI successes must be the results of either price collusion (if any) and/or normal rate change by carriers in response to fundamental market developments, the effective collusion, if it exists, is actually lower than 28.6%. Next, we identify eight factors influencing GRI successes. To further assess their impact, we applied an ordered logit regression analysis, which, based on four of the factors involved, yields good predictability for GRI success. The four factors, in sequence of explanation power, are the total capacity of GRI carriers, the idling fleet size, the spot rate level, and the average ship-loading factor. Clearly the latter three factors are market fundamentals, which are unlikely to be influenced by an individual carrier in the short term. In actual fact, the conclusion reached is that there is little evidence that carriers can manipulate and distort spot rates through GRIs.

Thin film growth of ${\textrm{TiO}}_2$TiO2 by physical vapor deposition processes is simulated in the Virtual Coater framework resulting in virtual thin films. The simulations are carried out for artificial, simplified deposition conditions as well as for conditions representing a real coating process. The study focuses on porous films which exhibit a significant anisotropy regarding the atomistic structure and consequently, to the index of refraction. A method how to determine the effective anisotropic index of refraction of virtual thin films by the effective medium theory is developed. The simulation applies both, classical molecular dynamics as well as kinetic Monte Carlo calculations, and finally the properties of the virtual films are compared to experimentally grown films especially analyzing the birefringence in the evaluation.

We explore a technique to precisely engineer the uniformity of a coating on non-flat substrates. This allows deposition of complex coatings such as edge filters by limiting the effects of spectral shift and angular spread.

In this paper, the theoretical foundation of quantized nanolaminates is explained and the dependence of the optical band gap on the quantum well thickness is derived. The manufacturing is investigated applying molecular dynamics simulation.

With respect to the material dispersion, the phase matching is difficult to realize for a direct THG. On the basis of thin film stacks, an approach is demonstrated on the theoretical and experimental level.

An overview on limiting mechanisms for the interaction of pulsed laser radiation with dielectric multi-layer structures and some recent concepts how to go beyond the state-of-the-art.

The general architecture of frequency tripling mirrors with optimized conversion is discussed for different design criteria. The conversion and bandwidth of the mirrors are strong functions of the number of layers.

The application of novel quantized nanolaminates delivers more flexible application and allows to optimize the properties. In the paper, the manufacturing of nanolaminates using different deposition techniques and material combinations are discussed.

Interference coatings for nonlinear optical applications, mainly third harmonic generation, is discussed. An inverse relationship between bandwidth and efficiency was found. Challenges arise from undesired processes that occur at high intensities.

A vertical temperature stratification normally exists in rooms during the heating season in cold climates. An expression of the gradient in apartments heated by hydronic radiator heating systems with exhaust ventilation has earlier been developed assuming a dependency of the outdoor temperature. The expression was used by a public real estate owner when re-calculating measured indoor temperature at 2.1 m above floor to 1.2 m above floor representing the occupancy zone and used for individual metering and billing of space heating cost. To validate the suggested expression temperature measurements have been made at four heights in living rooms in apartments built in the 70’s. The heights includes 0.0, 0.1, 1.1 and 1.7 m above floor. The theoretical expression has been compared to the full-scale measurements and in general the expression overestimates the vertical temperature gradient. The measured gradients are generally very low. The thermal comfort in the aspect of vertical temperature gradient is good for the studied period.

In order to enhance the reliability and performance of space-based Lidar systems, it is desirable to increase the damage resistance of ultraviolet antireflective coatings. For laser pulses with nanosecond pulse duration, laser-induced damage is known to be triggered by nano-sized defects embedded in the optical coating. In this work, we demonstrate the mitigation of damage precursors during the manufacturing of ion-beam sputtered (IBS) coatings using two approaches: ion bombardment with a secondary ion source and laser irradiation with a nanosecond-pulsed laser. Optical coatings produced with both technologies show a significantly increased damage threshold when tested in large-area raster scans.

We have investigated the suitability of atomic layer deposition (ALD) for SiO2 optical coatings and applied it to broadband antireflective multilayers in combination with HfO2 as the high refractive index material. SiO2 thin films were successfully grown using tris[dimethylamino] silane (3DMAS), bis[diethylamino] silane (BDEAS) with plasma activated oxygen as precursors, and the AP-LTO 330 precursor with ozone, respectively. The amorphous SiO2 films show very low optical losses within a spectral range of 200 nm to 1100 nm. Laser calorimetric measurements show absorption losses of 300 nm thick SiO2 films of about 1.5 parts per million at a wavelength of 1064 nm. The films are optically homogeneous and possess a good scalability of film thickness. The film surface porosity - which correlates to a shift in the transmittance spectra under vacuum and air conditions - has been suppressed by optimized plasma parameters or Al2O3 sealing layers.

In this paper, the theoretical foundation of quantizing nanolaminates is explained, and the dependence of the optical band gap on quantum-well thickness is demonstrated. The production is investigated by applying molecular dynamics growth simulation and by correlating the results with layers deposited by ion beam sputtering and atomic layer deposition. The properties of manufactured nanolaminates are then compared to the theoretical behavior, and good agreement is found.

Anmeldes af Lars Jensen