Fundamental Carrier-Envelope Phase Noise Limitations during Pulse Formation and Detection

The difference between the positions of the maximum peak of the carrier wave of a laser pulse and the maximum of its intensity envelope is termed carrier-envelope phase (CEP). In the last decades, the control and stabilization of this parameter has greatly improved, which enables many applications in research fields that rely on CEP-stable pulses such as attosecond science and optical frequency metrology. Further progress in these fields depends strongly on minimizing the CEP noise that restricts stabilization performance. While the CEP of most high repetition-rate low-energy laser oscillators has been stabilized to a remarkable precision, some types of oscillators show extensive noise that inhibits precise stabilization. The CEP stabilization performance of low repetition-rate high peak-power amplified laser systems also remains limited by noise, which is believed to stem mainly from the CEP detection process. In this thesis, the origins of the CEP noise within four oscillators as well as the noise induced by the measurement of the CEP of amplified pulses are investigated. In the first part, the properties of the CEP noise of one Ti:sapphire oscillator and three different fiber oscillators are extracted by analyzing the unstabilized CEP traces by means of time-resolved correlation analysis of carrier-envelope amplitude and phase noise as well as by methods that reveal the underlying statistical noise properties. In the second part, investigations into the origin of CEP noise induced by the measurement of the CEP of amplified pulses are conducted by comparing several different CEP detection designs that are based on f -2 f interferometry. These detection setups differ in the employed sources of spectral broadening as well as frequency doubling media, both necessary steps to measure the CEP. The results in both parts of this thesis show that white quantum noise dominates most CEP measurements. In one particular fiber oscillator, the strong white noise is found to be a result of a correlating mechanism within the employed SESAM. During amplifier CEP detection, the CEP noise is found to be originating only to a marginal degree from the number of photons that are detected during the measurement, which excludes shot noise as a limiting source. Instead, the analysis reveals that the origin of the observed strong white noise can be interpreted as a loss of coherence during detection. This type of coherence is termed here intra-pulse coherence and describes the phase transfer within f -2 f interferometry. Its degradation is a result of amplitude-to-phase coupling during the spectral broadening process that leads to pulse-to-pulse fluctuations of the phases at the edges of the extended spectrum. Numerical simulations support the concept of intra-pulse coherence degradation and show that the degradation is substantially stronger during plasma-driven spectral broadening as compared to self-phase modulation-dominated spectral broadening. This difference in degradation also explains the much stronger CEP noise typically observed in amplified systems as compared to oscillators, as the former typically rely on filamentation-based and hence plasma-dominated spectral broadening for CEP detection. The concept of intra-pulse coherence constitutes a novel measure to assess the suitability of a spectral broadening mechanism for application in active as well as in passive CEP stabilization schemes and provides new strategies to reduce the impact of the CEP detection on the overall stabilization performance of most lasers.Diese Arbeit beschäftigt sich mit der Identifizierung und Minimierung fundamentaler Rauschquellen, die zu einer Limitierung des erreichbaren Carrier-Envelope Phasen (CEP) Jitters führen. Die Carrier-Envelope Phase beschreibt die Differenz zwischen dem Maximum der Trägerwelle und dem Scheitelpunkt der Intensitätseinhüllenden. In den letzten Jahrzehnten hat sich die Kontrolle und Stabilisierung der CEP deutlich verbessert, was zu einem schnellen Fortschritt in Forschungsfeldern geführt hat, bei denen CEP-stabile Pulse notwendig sind. Diese Forschungsfelder umfassen die Attosekundenforschung und optische Frequenzmetrologie. Weitere Entwicklungen in diesen Feldern hängt stark von der Minimierung von CEP Rauschen ab, welches die CEP Stabilisierung stark beeinträchtigt. Obwohl die CEP der Pulse der meisten Laseroszillatoren mit hohen Repetitionsraten äußerst genau stabilisiert werden kann, existieren einige Laseroszillatoren bei denen starke Rauschquellen eine Stabilisierung verhindern oder stark einschränken. Des Weiteren zeigen vor Allem verstärkte System mit niedrigen Repetitionsraten und hohen Spitzenleistungen eine Beschränkung der CEP Stabilisierung aufgrund von Rauschen, dass vermutlich zum großen Teil durch den Detektionsprozess entsteht. In dieser Arbeit ist der Ursprung von CEP Rauschen in vier unterschiedlichen Laseroszillatoren sowie während der Detektion der CEP von verstärkten Systemen untersucht worden. Im ersten Teil wurden die Eigenschaften des CEP Rauschens eines Ti:Saphir-basierten Oszillators und drei verschiedener Faserlaser analysiert. Hierzu wurde das Rauschen unter anderem mittels zeitaufgelöster Korrelationsanalyse von Carrier-Envelope Amplituden- und Phasenrauschen sowie mittels Methoden, die die statistischen Eigenschaften des Rauschens offenlegen, analysiert. Im zweiten Teil der Arbeit wurde das Rauschen untersucht, welches durch den Messprozess der CEP von verstärkten Pulsen mittels f -2 f Interferometrie entsteht. Experimentell wurden hierzu vier unterschiedliche Detektionsanordnungen verwendet, die sich durch die Nutzung unterschiedlicher nichtlinearer Prozesse zum Erzeugen der spektralen Verbreiterung sowie zur Erzeugung der zweiten Harmonischen unterscheiden. Die Ergebnisse in beiden Teilen der Arbeit zeigen dominierendes weißes Quantenrauschen in den meisten CEP Messungen. In einem bestimmten Faserlaser, in dem besonders starkes weißes Rauschen vorlag, konnte der Ursprung einerWechselwirkung innerhalb des verwendeten halbleiterbasierten sättigbaren Absorbers zugeordnet werden. Bei der Detektion der CEP bei verstärkten Systemen wurde hingegen gezeigt, dass niedrige Photonenzahlen und damit Schrotrauschen nur zum kleinen Teil für die starken weißen Rauschanteile verantwortlich gemacht werden kann. Stattdessen kann die Ursache des starken Rauschens einem Verlust von Kohärenz zugeordnet werden. Diese Art von Kohärenz ist hier mit intra-Puls Kohärenz bezeichnet und beschreibt den Phasentransfer innerhalb der Detektion mittels f -2 f Interferometrie. Der Verlust von intra-Puls Kohärenz ist eine Folge von Amplituden-zu-Phasen Koppelung während der spektralen Verbreiterung. Von Puls zu Puls führt dies zu Fluktuationen der Phase an beiden Rändern der erzeugten spektralen Verbreiterung. Numerische Simulationen unterstützen das Konzept der intra-Puls Kohärenz und zeigen auf, dass die Degradation bedeutend stärker bei plasmadominierten Prozessen ausfällt als im Vergleich zu spektraler Verbreiterung mittels Selbstphasenmodulation. Dieser unterschiedlich starke Verlust der intra-Puls Kohärenz erklärt das deutlich höhere Rauschniveau in verstärkten Systemen im Vergleich zu Oszillatoren, da verstärkte Systeme plasmadominierte Prozesse zur spektralen Verbreiterung nutzen. Das Konzept der intra-Puls Kohärenz stellt ein neues Maß zur Einschätzung einer Methode zur spektralen Verbreiterung für eine bestimmte Anwendung dar, die sowohl in aktiven sowie passiven CEP Stabilisierungen von Lasern eine Rolle spielt. Es ermöglicht somit neue Strategien, um den Einfluss der Detektion auf die CEP Stabilisierung der meisten Laser zu senken

n-Type Rear Junction Solar Cells with Locally Contacted Al-Alloyed Emitter

This diploma thesis was focused on enhancing the rear side performance of the improved PhosTop solar cell concept by means of dielectric rear side passivation and reduction of the highly doped emitter area. Stack systems and passivation layers were applied on lowly doped n-type silicon bulk and highly doped aluminium p-type emitters in order to reduce the effective rear surface recombination velocity and hence improve the open-circuit voltage. Furthermore, using a dielectric rear passivation leads to an improved internal reflection on the rear side, which results in a short-circuit current density gain. Three different solar cell concepts were realized.Besides the improved PhosTop solar cell, which is representing the reference, the Al-LARE (Aluminium - Locally Alloyed Rear Emitter) solar cell featuring a passivated n-type bulk and locally alloyed emitter is presented. Furthermore, the FALCON (Full Area Locally CONtacted emitter) solar cell is realized, which exhibits a full area alloyed and etched back emitter that is passivated and locally contacted.n-type silicon substrates were passivated using different layers and stacks of which Al2O3/SiNA-SiNx, SiNA-SiNx and CT-SiNx passivation performed best after a firing step and featured effective lifetimes of up to 9.5 ms. In the contrary to highly doped n-type silicon, the SiO2/SiNA-SiNx passivation on n-type substrates showed a severe firing instability for temperatures above 800 celsius.The characterization of the emitter formed by three different aluminium pastes revealed very low emitter saturation current densities in the range of 150 to 180 fA/cm2, showing further no influence on the set-firing peak temperature. The passivation of the etched back emitter was not found to be on a satisfactory level on samples that featured an etched back emitter, which is thicker than 1.5 µm, being below the passivation due to the field-effect of the non-etched back emitter. Compared to the fully alloyed and metallized rear side of the PhosTop solar cell, noimprovement of the rear could be made by using different passivation layers and stacks.Al-LARE solar cells were simulated using PC2D, a novel two-dimensional simulation tool. The simulation predicted a possible short-circuit current density gain of 0.5 mA/cm2 for an emitter and contact width of 100 µm and emitter spacing of 200 to 300 µm. This is in good agreement with a followed emitter width and spacing variation that was carried out on 5x5 cm2 Al-LARE solar cells. The emitter width and spacing resulted in the conclusion that the highest values are obtained for a minimum of 100 µm emitter width and an emitter spacing of 300 - 400 µm (depending on the emitter width).Large-area Al-LARE solar cells featuring an emitter width of 100 µm and an emitter spacing of 300 µm were further fabricated and analysed. The best performing Al-LARE solar cell that was passivated by SiO2/SiNA-SiNx, reached an efficiency of 17 %. Furthermore, a maximum short-circuit current density gain of 0.45 mA/cm2 compared to jointly fabricated PhosTop solar cells was found for a Al-LARE solar cell passivated by a Al2O3/SiNA-SiNx on the rear side. This solar cell concept was found to be basically limited by extremely high values of j02 in combination with an in some cases slightly increased series resistance due to contact formation problems at the rear side. Furthermore, the passivation quality of the Al2O3/SiNA-SiNx passivated rear was the only passivation able to compensate diffusion losses to the emitter and hence to sustain a comparable IQE plateau to the PhosTop solar cell.Finally, FALCON solar cells were fabricated that feature an etched back 2 µm deep, screenprinted full area aluminium alloyed passivated emitter. Two different process sequences were carried out, allowing the rear side of one experiment to be passivated by a SiO2/SiNA-SiNx stack. An overall short-circuit current density gain, similar to the Al-LARE solar cells, of 0.5 mA/cm2 was found for the best performing SiO2/SiNA-SiNx FALCON solar cell. This is only half of the short-circuit current density gain that was expected from the simulation. This is probably caused by a discrepancy of the assumed reflection difference between the unpassivated and passivated rear side for the simulation and the difference for real solar cells. Furthermore, a strong discrepancy of in some cases almost 20 mV was found between the simulated and actually measured Voc. This discrepancy can be attributed to a lower passivation quality and hence the rear SRV in the fabricated experiments compared to the simulation and increased j02.The best performing FALCON solar cell was achieved by passivating the rear using a SiO2/SiNA-SiNx stack that resulted in an efficiency of 18.9 %. Especially for low performing FALCON solar cells, a high reduction in FF and hence in efficiency was found to be due to an increased series resistance reaching approx. 1 Ohm cm2 and in some cases, an extremely high j02. j02 in combination with a not improved or even increased j01 compared to the non-passivated rear of the PhosTop solar cell, resulted in a moderate to strongly reduced Voc, as well.The main advantage of the FALCON compared to the Al-LARE solar cell is the full area etched-back and passivated emitter that leads to a constant plateau in the IQE in the visible light range and hence allows a higher jsc compared to the decreased plateau of the Al-LARE solar cell. Furthermore, this full area emitter can lead to a much lower j02 that is found for FALCON solar cells compared to the Al-LARE solar cells.In conclusion, since Al-LARE solar cells are mainly limited due to an extremely high j02, this diploma thesis suggests that unless improvements can be made, the increased fabrication effort is not justified, since a maximum obtained efficiency of 17 % is much lower than the 19.4 % of the improved PhosTop solar cell, while the latter is much easier to fabricate.The FALCON solar cell concept has a higher potential, since it is mainly limited due to process parameters such as unfilled line contacts on the rear that result in an increased series resistance.Furthermore, the passivation on the etched-back emitter needs to be further increased. For an industrial implementation of this solar cell concept the needed processing steps for fabrication have to be reduced