Abstract-We report on terahertz frequency-domain spectroscopy (THz-FDS) experiments in which we measure charge carrier dynamics and excitations of thin-film superconducting systems at low temperatures in the THz spectral range. The characteristics of the set-up and the experimental procedures are described comprehensively. We discuss the single-particle density of states and a theory of electrodynamic absorption and optical conductivity of conventional superconductors. We present the experimental performance of the setup at low temperatures for a broad spectral range from 3 to 38 cm -1 (0.1 -1.1 THz) by the example of ultra-thin films of weakly disordered superconductors niobium nitride (NbN) and tantalum nitride (TaN) with different values of critical temperatures Tc. Furthermore, we analyze and interpret our experimental data within the framework of conventional Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. By and large, we find the properties of our NbN and TaN thin films to be well described by the theory. Our results on NbN resemble tendencies towards anomalous behavior of the ratio 2∆(0)/kBTc as a function of Tc.Index Terms-Frequency-domain THz spectroscopy, superconducting thin films, BCS theory, density of states of a superconductor, optical conductivity of superconductors, superconductorinsulator transition, TaN, NbN.

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We report a distributed Bragg reflector-free semiconductor disc laser which emits 10 W continuous wave output power at a wavelength of 1007 nm when pumped with 40 W at 808 nm, focused into a 230 μm diameter spot on the gain chip. By introducing a birefringent filter plate in the laser cavity the wavelength could be tuned from 995 to 1020 nm. The laser consisted of a gain chip located at the beam waist of a linear concentric resonator with an output coupling of 2.15%. The gain chip consists of a 1.574-μm-thick resonant periodic gain structure, with ten In 0.13 Ga 0.87 As quantum wells embedded in strain-compensating GaAs 0.94 P 0.06 barrier layers, van der Waals bonded to a silicon carbide intra-cavity heat spreader.Introduction: Semiconductor disc laser (SDLs), also called vertical external cavity surface emitting lasers [1,2], are a flexible semiconductor laser technology which provides high beam quality and high power at a range of wavelengths [3][4][5]. The use of optical pumping allows optimised design of the semiconductor multi-layer stack for optical properties and the external cavity allows the use of intra-cavity elements such as birefringent filters, frequency doubling crystals and semiconductor saturable absorber mirrors [6]. However, the distributed Bragg reflector (DBR) in SDLs adds significantly to the growth time and complexity and limits the thermal performance. In contrast, recently introduced DBR-free SDL technology [7][8][9] offers a potential solution to these limitations. DBR-free SDLs on diamond, emitting 6 W continuous wave (CW) output power at 1055 nm have been demonstrated in [8].Here, we report a DBR-free SDL emitting 10.1 W continuous wave output power at 1007 nm with an incident pump power of 40 W using a silicon carbide (SiC) intra-cavity heat spreader. We demonstrate wavelength tuning, reaching a maximum power of 0.7 W between 995 and 1020 nm by introducing birefringent filter into the cavity.

Self-mode-locking has become an emerging path to the generation of ultrashort pulses with vertical-external-cavity surface-emitting lasers. In our work, a strong Kerr nonlinearity that is so far assumed to give rise to mode-locked operation is evidenced and a strong nonlinearity enhancement by the microcavity is revealed. We present wavelength-dependent measurements of the nonlinear absorption and nonlinear-refractive-index change in a gain chip using the Z-scan technique. We report negative nonlinear refraction up to 1.5⋅10 -11 cm 2 /W in magnitude in the (InGa)As/Ga(AsP) material system close to the laser design wavelength, which can lead to Kerr lensing. We show that by changing the angle of incidence of the probe beam with respect to the gain chip, the Kerr nonlinearity can be wavelength-tuned, shifting with the microcavity resonance.Such findings may ultimately lead to novel concepts with regard to tailored self-mode-locking behavior achievable by peculiar Kerr-lens chip designs for cost-effective, robust and compact fspulsed semiconductor lasers.The Kerr effect is at the basis of many important device concepts like all-optical switching 1 , optical limiting 2 and soliton mode-locking of lasers and microresonators 3,4 . The capability to accurately measure and model the nonlinear refractive index changes associated with the Kerr effect is crucial for improved device operation, where a specifically tailored nonlinear refractive index is required, e.g. for the intensity-dependent Kerr lensing. Several measurement schemes have been developed in the past for the characterization of the nonlinear refractive index 5-7 , with the Z-scan technique 8 being undoubtedly the most prominent one due to its simplicity and high sensitivity. This method continues to be of high experimental value with the rise of novel material classes like graphene and other two-dimensional semiconductors which often exhibit a very strong refractive nonlinearity 9-11 .Kerr-lens mode-locked Ti:Sapphire lasers have dominated the field of ultra-short high-power modelocked lasers since their initial discovery nearly three decades ago 12,13 . In these lasers, the intensitydependent refractive index of the gain crystal leads to self-focusing of the laser beam for high intensities. When part of the continuous-wave (cw) beam profile is suppressed by inserting a slit into C. Kriso et al. (2018) 2

We demonstrate an optically pumped semiconductor disk laser (OP-SDL) using InP quantum dots (QDs) as active material fabricated by metal-organic vapor-phase epitaxy. The QDs are grown within [(Al0.1Ga0.9)0.52In0.48]0.5P0.5 (abbr. Al0.1GaInP) barriers in order to achieve an emission wavelength around 655 nm. We present optical investigations of the active region showing typical QD behavior like blue shift with increasing excitation power and single emission lines, which show anti-bunching in an intensity auto-correlation measurement. We report maximum output powers of the OP-SDL of 1.39 W at low emission wavelength of ∼654 nm with a slope efficiency of ηdiff=25.4 %.