Experimental analysis of spatial states in broad-area vertical-cavity surface-emitting lasers

In dieser Arbeit wird die spontane Musterbildung in der Emission von oberflächen-emittierenden Halbleiterlasern (englisch "Vertical-cavity surface-emitting lasers", kurz VCSEL) untersucht. Dabei handelt es sich um VCSEL mit großer quadratischer (30 30 µm² und 40 40 µm²) und runder (80 µm Durchmesser) Apertur. Diese Laser neigen aufgrund ihrer sehr hohen Fresnelzahl zu der Emission von stark divergenten Transversalmoden, welche sich in Form von Intensitätsmodulationen des Laserstrahls zeigen. Die Ergebnisse der Arbeit umfassen eine quantitative Untersuchung der Abhängigkeit der Musterlängenskalen von den Betriebsparametern, eine Erklärung der Entstehung der Emissionsmuster und deren komplexer Polarisationsverteilung, sowie eine Untersuchung der Kontrolle der Mustereigenschaften durch Rückkopplung

Coupling of polarization and spatial degrees of freedom of highly divergent emission in broad-area square vertical-cavity surface-emitting lasers

The polarization of highly divergent modes of broad-area square vertical-cavity surface-emitting lasers is shown to be only marginally affected by material anisotropies but determined by an interplay of the polarization properties of the Bragg cavity mirrors and of the transverse boundary conditions. This leads to a locking of the polarization direction to the boundaries and its indeterminacy for wave vectors oriented along the diagonal. We point out a non-Poissonian character of nearest-neighbor frequency spacing distribution and the impossibility of single-wave number solutions

Влияние способа регулирования на экономичность работы забойного турбонасоса

Lasers as production tools offer several advantages, which are especially relevant for the production of solar cells. The contactless and localized nature of the energy deposition allows new processes, such as laser selective emitter doping, laser ablation of dielectric coatings and via drilling for back contact cell concepts. A critical factor is the selection of suitable laser sources and parameters in a manner that adapts the laser process to the requirements of the material, the process nature and the solar cell properties. In this paper three laser processes are investigated with the goal to identify the most suitable laser source

Polarization properties in the transition from below to above lasing threshold in broad-area vertical-cavity surface-emitting lasers

For highly divergent emission of broad-area vertical-cavity surface-emitting lasers (VCSELs) a rotation of the polarization direction by up to 90 degrees occurs when the pump rate approaches the lasing threshold. Well below threshold the polarization is parallel to the direction of the transverse wave vector and is determined by the transmissive properties of the Bragg reflectors that form the cavity mirrors. In contrast, near-threshold and above-threshold emission is more affected by the reflective properties of the reflectors and is predominantly perpendicular to the direction of transverse wave vectors. Two qualitatively different types of polarization transition are demonstrated: an abrupt transition, where the light polarization vanishes at the point of the transition, and a smooth one, where it is significantly nonzero during the transition

Length scales and polarization properties of annular standing waves in circular broad-area vertical-cavity surface-emitting lasers

It is shown that the 'flower-like' emission patterns along the perimeter of broad-area circular vertical-cavity surface-emitting lasers can be interpreted in a first approximation as annular standing waves. Their modulation period follows the dispersion relation of tilted plane waves for a plano-planar cavity. For high enough divergence angle, they show a very peculiar polarization behavior of radial polarization in near field and azimuthal polarization in far field. The polarization selection is qualitatively explained by the breaking of the isotropy between s- and p-components for waves propagating off axis in the Bragg reflectors

Проектування рудних шахт. Матеріали методичного забезпечення для практичних занять студентів спеціальності 7 (8).05030101 Розробка родовищ та видобування корисних копалин

Розглянуто теоретичні засади ключових питань у проектуванні рудних шахт. Надано рекомендації до розв’язування типових практичних завдань з проектування та розробки рудних родовищ. Наведено критерії оцінювання ре- зультатів виконання практичних робіт. Матеріали орієнтовані на активацію ви- конавчого етапу навчальної діяльності студентів. Призначено для самостійної та аудиторної роботи студентів за освітньо- професійною програмою підготовки спеціалістів і магістрів спеціальності 7 (8).05030101 «Розробка родовищ та видобування корисних копалин» під час підготовки до модульних контрольних заходів за результатами практичних за- нять з нормативної дисципліни «Проектування рудних шахт»

Porous Nickel Nano‐Foam by Femtosecond Laser Structuring for Supercapacitor Application

A focused femtosecond laser beam was scanned across a nickel electrode in a line pattern with different line distances to generate a large electrochemical surface area for charge storage. During the laser structuring process, small metal particles were generated and sintered to a porous foam‐like structure, the so‐called laser‐induced nano‐foam (LINF), which strongly adheres to the substrate surface. The structuring was carried out in argon atmosphere, in order to prevent oxidation of the LINF structure during the structuring process. The topography of the LINF was investigated by scanning electron microscopy and laser scanning microscopy. The electrochemical surface area of the electrodes was determined by cyclic voltammetry based on the charging of the double‐layer. The total capacity of the nickel LINF electrodes was measured by galvanostatic charge‐discharge to test their capability for supercapacitor applications. The surface area enlargement and therefore the total capacity increases with decreasing line distance. The LINF structure provides a surface area enlargement up to a factor of 1600 and a total capacity up to 2 C cm−2

Control of the spatial emission structure of broad-area vertical-cavity surface-emitting lasers by feedback

The wave number of transverse spatial structures in broad-area vertical-cavity surface-emitting lasers (VCSELs) is controlled via frequency-selective feedback from an external self-imaging cavity in a broad range of wave numbers and emission frequencies. The selected states follow the dispersion curves of the free-running laser. A control range of about 2.5 µm−1 in spatial frequency space and 2.5 nm in emission wavelength was obtained for square VCSELs and of about 3 µm−1 and 8 nm for circular VCSELs having a different dispersion curve. By spatial filtering in Fourier space, the shape of the structures can also be controlled to some extent. It is argued that the feedback techniques are useful to 'probe' emission states of the free-running laser