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[EN] We explore the use of a beamforming method intended for large-area scanning in optical-resolution photoacoustic microscopy. It has been evaluated in a experimental setup that comprises a low-cost laser diode and a phase array with a 128-elements linear probe. Three different beamforming strategies are discussed: no-beamforming, static beamforming and dynamic beamforming. The method has been tested in gelatine-based phantoms as well as ex-vivo organs. Results show that, compared with the other two, dynamic beamforming increases up to 15dB and homogenizes signal-to-noise ratio (SNR) along images of roughly 1 cm2. The method and system presented here could be the baseline for more advanced array-based systems that leverage the low-cost laser sources for clinical applications. ; This research has been supported by the Spanish Ministry of Science, Innovation and Universities through grant Juan de la Cierva - Incorporacion (IJC2018-037897-I), and program Proyectos I+D+i 2019 (PID2019-111436RB-C22). Action co-financed by the European Union through the Programa Operativo del Fondo Europeo de Desarrollo Regional (FEDER) of the Comunitat Valenciana 2014-2020 (IDIFEDER/2018/022). A.C. received financial support from Generalitat Valenciana and Universitat Politecnica de Val ` encia through the grants APOSTD/2018/229 and program PAID-10-19, respectively. ; Cebrecos, A.; García-Garrigós, JJ.; Descals, A.; Jimenez, N.; Benlloch Baviera, JM.; Camarena Femenia, F. (2020). Dynamic beamforming for large area scan in array-based photoacoustic microscopy. IEEE. 1-4. https://doi.org/10.1109/IUS46767.2020.9251519 ; S ; 1 ; 4

The external quantum efficiency of perovskite light-emitting diodes (PeLEDs) has advanced quickly during the past few years. However, under pulsed operation, an operation mode which is important for display and visible light communication, the performance of PeLEDs changes a lot and requires in-depth understanding to facilitate these applications. Here, we report the response of PeLEDs under pulsed operation in the range of 10 Hz to 20 kHz. Beyond transient effects in the low frequencies, we find that for higher frequencies (>500 Hz) the transient electroluminescence intensity depends strongly on the duty cycle. This feature is much more pronounced and of different origin than that in conventional LEDs. We rationalise our experimental observations using a mathematical model and assign these features to the effect of mobile ionic charges in the perovskite. Our work also provides important implications for the operation of PeLEDs under the steady state, where accumulation of mobile ions at the interfaces could be beneficial for high electroluminescence yields but harmful for the long-term stability. ; Funding: ERC Starting Grants [717026, 851676]; Swedish Energy Agency EnergimyndighetenSwedish Energy Agency [48758-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

This work was supported by the European Metrological Research Programme EMRP under IND 14. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. N.K. Metzger acknowledges support from the EPSRC Centre for Innovative Manufacturing in Laser-based Production Processes, funded through EPSRC grant EP/K030884/1. ; A comparison between two methods of timing jitter calculation is presented. The integral method utilizes spectral area of the single side-band (SSB) phase noise spectrum to calculate root mean square (rms) timing jitter. In contrast the harmonic analysis exploits the uppermost noise power in high harmonics to retrieve timing fluctuation. The results obtained show that a consistent timing jitter of 1.2 ps is found by the integral method and harmonic analysis in gain-switched laser diodes with an external cavity scheme. A comparison of the two approaches in noise measurement of a diode-pumped Yb:KY(WO4)2 passively mode-locked laser is also shown in which both techniques give 2 ps rms timing jitter. ; Publisher PDF ; Peer reviewed

The EU Horizon 2020 project CAMART2 is acknowledged for partly supporting the project, and the Ion Technology Centre, ITC, in Sweden is acknowledged for ion beam analysis (ERDA). ; Pulsed laser ablation is used to form high-quality silicon-doped β-Ga2O3 films on sapphire by alternatively depositing Ga2O3 and Si from two separate sources. X-ray analysis reveals a single crystallinity with a full width at half maximum for the rocking curve around the (−201) reflection peak of 1.6°. Silicon doping concentration is determined by elastic recoil detection analysis (ERDA), and the best electrical performance is reached at a Si concentration of about 1 × 1020 cm−3, using optimized deposition parameters. It is found that a high crystalline quality and a mobility of about 2.9 cm2 (V s)−1 can be achieved by depositing Si and Ga2O3 from two separate sources. Two types of Schottky contacts are fabricated: one with a pure Pt and one with a PtOx composition. Electrical results from these structures are also presented. ; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

This paper deals with pulsed laser welding of aluminum using an Nd:YAG laser with wavelength 1.06 μm. Technically pure aluminum (95.50 wt. %) was used as the welded material. Eighteen welds (penetration passes) were fabricated in the experiment. Optical microscopy was used to assess the influence of changes in the parameters of the pulsed laser on the quality and geometry of the penetration passes of aluminum and on the hardness measurement through the interface of the welds. The results show that the geometry of the penetration passes was influenced above all by the position of the beam focus.

Among the several applications of laser sources, some requires kilometers range propagation in the atmosphere : telemetry, guidance system or active imagery. High pulse energy improves the range of the system, but may cause permanent blindness to an observer's eyes. Hence, these applications must use laser beam which wavelength are located in the eye-safe region, ideally at the local minimum of the atmosphere absorption (1550-1650 nm). Such laser sources are already commercially available, but are not suited for the demanding military needs : compacity, electrical consumption, performance and large operating temperature range (-40°C/+60°C).My work aims to develop a laser source filling these specifications. Thanks to the collaboration with the industrial partners Fibercryst and Cilas, it focuses on the design of a compact, efficient, directly diode-pumped Er3+:YAG single cristal fiber laser for military applications.With a homemade numerical simulation of a passively Q-switched Er3+:YAG laser source, many laser emitters are experimentally designed and compared. Further studies around saturable absorbers allowed sensible improvements of the output pulse energy.This work, whose results may already be commercially interesting, may lead to new technics and architectures of erbium doped solid-state laser for better prototypes. ; Parmi les nombreuses applications des sources laser, certaines nécessitent une propagation du faisceau dans l'atmosphère sur plusieurs kilomètres : télémétrie, désignation, ou encore imagerie active. Pour éviter tout risque oculaire tout en proposant de plus grandes portées, ces applications doivent faire appel à des sources laser émettant une longueur d'onde dans la gamme à sécurité oculaire autour d'un minimum local d'absorption de l'atmosphère (1550-1650 nm). De telles sources existent déjà commercialement, mais ne répondent pas aux exigences militaires de compacité, de consommation électrique, de performance et de fonctionnement sur une large gamme de température (-40°C/+60°C).Mes travaux de thèse tentent d'apporter une réponse à l'ensemble de ces exigences. Avec l'aide des partenaires industriels Fibercryst et Cilas, ils portent sur la réalisation de sources laser compactes et efficaces à fibre cristalline Er3+:YAG directement pompée par une diode laser pour des applications militaires.A l'aide d'un algorithme de simulation d'un laser Er3+:YAG déclenché passivement élaboré et affiné durant la thèse, plusieurs sources sont réalisées expérimentalement. L'étude autour des absorbants saturables pour le déclenchement passif a permis une amélioration notable des caractéristiques du faisceau.Ces recherches, dont les résultats peuvent déjà présenter un certain intérêt commercial, ouvrent la voie vers de nouvelles techniques et architectures autour des sources laser à cristaux dopés aux ions erbium pour la conception de futurs prototypes plus performants.

Among the several applications of laser sources, some requires kilometers range propagation in the atmosphere : telemetry, guidance system or active imagery. High pulse energy improves the range of the system, but may cause permanent blindness to an observer's eyes. Hence, these applications must use laser beam which wavelength are located in the eye-safe region, ideally at the local minimum of the atmosphere absorption (1550-1650 nm). Such laser sources are already commercially available, but are not suited for the demanding military needs : compacity, electrical consumption, performance and large operating temperature range (-40°C/+60°C).My work aims to develop a laser source filling these specifications. Thanks to the collaboration with the industrial partners Fibercryst and Cilas, it focuses on the design of a compact, efficient, directly diode-pumped Er3+:YAG single cristal fiber laser for military applications.With a homemade numerical simulation of a passively Q-switched Er3+:YAG laser source, many laser emitters are experimentally designed and compared. Further studies around saturable absorbers allowed sensible improvements of the output pulse energy.This work, whose results may already be commercially interesting, may lead to new technics and architectures of erbium doped solid-state laser for better prototypes. ; Parmi les nombreuses applications des sources laser, certaines nécessitent une propagation du faisceau dans l'atmosphère sur plusieurs kilomètres : télémétrie, désignation, ou encore imagerie active. Pour éviter tout risque oculaire tout en proposant de plus grandes portées, ces applications doivent faire appel à des sources laser émettant une longueur d'onde dans la gamme à sécurité oculaire autour d'un minimum local d'absorption de l'atmosphère (1550-1650 nm). De telles sources existent déjà commercialement, mais ne répondent pas aux exigences militaires de compacité, de consommation électrique, de performance et de fonctionnement sur une large gamme de température (-40°C/+60°C).Mes travaux ...

Group III-nitride is a remarkable material system to make highly efficient and high-power optoelectronics and electronic devices because of the unique electrical, physical, chemical and structural properties it offers. In particular, InGaN-based blue Laser Diodes (LDs) have been successfully employed in a variety of applications ranging from biomedical and military devices to scientific instrumentation and consumer electronics. Recently their use in highly efficient Solid State Lighting (SSL) has been proposed because of their superior beam quality and higher efficiency at high input power density. Tremendous advances in research of GaN semi-polar and non-polar crystallographic planes have led both LEDs and LDs grown on these non-basal planes to rival with, and with the promise to outperform, their equivalent c-plane counterparts. However, still many issues need to be addressed, both related to material growth and device fabrication, including a lack of conventional wet etching techniques. GaN and its alloys with InN and AlN have proven resistant essentially to all known standard wet etching techniques, and the predominant etching methods rely on chlorine-based dry etching (RIE). These introduce sub-surface damage which can degrade the electrical properties of the epitaxial structure and reduce the reliability and lifetime of the final device. Such reasons and the limited effectiveness of passivation techniques have so far suggested to etch the LD ridges before the active region, although it is well-known that this can badly affect the device performance, especially in narrow stripe width LDs, because the gain guiding obtained in the planar configuration is weak and the low index step and high lateral current leakage result in devices with threshold current density higher than devices whose ridge is etched beyond the active region. Moreover, undercut etching of III-nitride layers has proven even more challenging, with limitations in control of the lateral etch distance. In this dissertation it is presented the first nitride blue edge emitting LD with a photoelectrochemical etched current aperture (CA-LD) into the device active region.Photoelectrochemical etching (PECE) has emerged as a powerful wet etching technique for III-nitride compounds. Beyond the advantages of wet etching technique, PECE offers bandgap selectivity, which is particularly desirable because it allows more freedom in designing new and advanced devices with higher performances. In the first part of this thesis a review of PECE is presented, and it is shown how it can be used to achieve a selective and controllable deep undercut of the active region of LEDs and LDs, in particular the selective PECE of MQW active region of (10-10) m-plane and (20-2-1) plane structures is reported.In the second part of this thesis, the fabrication flow process of the CA-LD is described. The performance of these devices is compared with that of shallow etched ridge LDs with a nominally identical epitaxial structure and active region width and it is experimentally shown that the CA-LD design has superior performance. CW operation of a (20-2-1) CA-LD with a 1.5 µm wide active region is demonstrated.Finally, in the third and last part of this thesis, the CA-LD performance is discussed in more details, in particular, an analysis of optical scattering losses caused by the rough edges of the remnant PEC etched active region is presented.

Catastrophic optical damage (COD) of high power laser diodes is a crucial factor limiting ultra high power lasers. The understanding of the COD process is essential to improve the endurance of the high power laser diodes. COD is observed as a process in which the active part of the laser diode is destroyed, forming characteristic defects, the so called dark line defects (DLDs). The DLDs are formed by arrays of dislocations generated during the laser operation. Local heating associated with non-radiative recombination is assumed to be at the origin of the COD process. A summary of the methods used to assess the COD, both in real time and post-mortem is presented. The main approaches developed in recent years to model the heat transport in the laser structures under non homogeneous temperature distribution are overviewed. Special emphasis is paid to the impact of the low dimensionality of QWs in two physical properties playing a major role in the COD process, namely, thermal conductivity and mechanical strength. A discussion about the impact of the nanoscale in both physical properties is presented. Finally, we summarize the main issues of the thermomechanical modelling of COD. Within this model the COD is launched when the local thermal stresses generated around the heat source overcome the yield stress of the active zone of the laser. The thermal runaway is related to the sharp decrease of the thermal conductivity once the onset of plasticity has been reached in the active zone of the laser. ; Junta de Castilla y León (Projects VA081U16 and VA283P18) ; Spanish Government (ENE 2014-56069-C4-4-R, ENE 2017-89561-C4-3-R, FPU programme 14/00916).

The modern military industry demands the most innovative and high-quality metals and alloys, as well as processing technologies. Iron and its alloys are a common material used for different types of weapons and military equipment. Lasers have been widely used in the metal surface processing, but the changes that occur on laser treated metal surfaces have not yet been sufficiently investigated. A study of morphological and chemical changes on the metal surface induced by Nd:YAG laser treatment in ambient air is presented. A pulsed Nd:YAG laser (1064 nm, Gaussian spatial profile, FWHM 8 ns, energy up to 525 mJ, beam diameter of 1-10 mm and fluence up to 3.5 J/cm 2 ) was used. Micro-morphological analyses of sample surfaces before and after laser treatment process were performed by optical and SEM microscopies. EDX method was used for chemical analyses and Spectra colorimeter for investigation of induced color changes. The laser irradiation effects were studied as a function of two laser parameters, number of laser pulses and laser fluencies, around and over the damage threshold. The results show that there are significant differences depending on these laser parameters.