In recent years, MECSELs did undergo rapid progress. The developments in the field will be summarized and discussed, and a complete overview on the state of the art will be given. The most important progress, like the radical design simplification, double-side pumping and power scaling capabilities play a major role. Also, the main aspects of the design of active region membranes will be reviewed with respect to flexible pumping possibilities enabled by the absence of a DBR and the substrate.

We report advances in the development of THz quantum-cascade metasurface VECSELs intended for use as local oscillators in terahertz heterodyne receiver instruments for astrophysical investigation of the interstellar medium. First, by using a patch-based amplifying metasurface we obtain QC-VECSEL lasing with milliwatt output power at 4.6 THz with reduced power consumption less than 1 W. Second, we report the phase locking of a QC-VECSEL at 3.4 THz to a microwave reference using a Schottky diode mixer. Finally, we report efforts and challenges to scale down the lasing frequency of the VECSELs to 1.9 THz.

Membrane-external-cavity surface-emitting lasers (MECSELs) consist of an epitaxial active region directly bonded to at least one transparent heatspreader with external cavity mirrors for feedback. This structure enables significant flexibility in the emission wavelength and yields a standalone laser gain medium amenable to enhanced power scaling via optimized thermal management. We outline a 4” wafer-scale manufacturing process for dual-SiC-heatspreader (SiC/epi/SiC) gain chips, incorporating external dielectric coatings and metallization for intimate mounting to a heatsink. Our process leverages low-temperature wafer bonding in concert with traditional deposition, lithography, and etching steps, allows hundreds of MECSEL gain chips to be simultaneously produced.

For wavelengths deviating from 1µm, a SESAM can be limited by the underlying DBR design. Using substrate-removal, it is possible to isolate the active region and circumvent restrictions due to a DBR. Only lately, the dynamics of SESAM mode-locked VECSELs, producing different multi-pulse emission, was investigated by several groups and motivated numerical modeling of mode-locking VECSELs. We present the fabrication, characterization and mode-locking of our red-emitting AlGaInP active regions. This includes a comparison of mode-locking dynamics of DBR-free saturable absorber and SESAM-based mode-locking approaches in the red spectral range and the nonlinear reflectivity of a set of GaInP QW SESAMs.

We present our first flip-chip processed, modelocked InGaSb vertical external-cavity surface-emitting laser (VECSEL) in the 2-µm wavelength range. The optically pumped VECSEL is passively modelocked with a semiconductor saturable absorber mirror (SESAM) resulting in 2.7-ps pulses with 89-mW average output power at a center wavelength of 2063 nm and a pulse repetition rate of 1.96 GHz. The standing-wave V-shaped cavity is formed by a 1% output coupler, a VECSEL gain chip as a folding cavity mirror, and a SESAM as the end mirror. An intracavity 1.5-mm silicon plate at Brewster’s angle was used for dispersion compensation and a fixed polarization.

We present the characterization under intracavity conditions of semiconductor saturable absorber mirrors (SESAMs) operating from 2 – 2.4 µm. The precise nonlinear reflectivity and recovery time measurement setups reveal excellent parameters of our in-house grown 2.05 and 2.4 µm SESAMs. Low saturation fluences around 4 µJ/cm2, percent range modulation depth, extremely low non-saturable losses and fast recovery within a few tens of picoseconds have been measured for all SESAMs. Using these SESAMs output power records in modelocking of vertical external-cavity surface-emitting lasers (VECSELs), thin-disk Ho:YAG lasers at 2 µm wavelength and Cr:ZnS lasers at 2.4 µm are achieved.

We report on the performance of MECSELs based on a non-resonant gain structure in respect with the operating lasing emission wavelength at 800 nm. Preliminary observations reveal an output power of 1.1 Watt and a 20 nm tuning range.

We report record performance and full gain characterization of an optically pumped continuous wave InGaSb VECSEL operating around 2 µm. Our flip-chip processed and backside-cooled VECSEL performs similar to intracavity heatspreader cooling and obtained even similar thermal resistance of only 3.45 KW-1 and an average output power of >800 mW. Our gain characterization setups can cover a wavelength range from 1.9 µm to 3 µm. Linear and nonlinear gain characteristics are measured as a function of wavelength, fluence, pump power and heatsink temperature. 2-µm VECSEL chips with different strategies in heat management are compared and theoretical predictions are in good agreement.

The key limiting factor for output power scaling of VECSELs is the thermal resistance of the structure owing to the DBR thickness and the heat conductivity of the semiconductor materials. We have successfully fabricated a flip-chip processed GaSb/AlAs0.08Sb0.92 hybrid DBR for 2 µm with only 7.5 mirror pairs and a 100-nm gold layer. The hybrid DBR reaches a high reflectivity >99.5% with a reduced total thickness of 2.3 µm. The measured spectral reflectivity of the hybrid DBR reveals a clear gold layer and matches theoretical simulations. High power 2-µm VECSEL development with the presented hybrid-mirror structure is under way.

We review the progress of commercial single-frequency VECSELs for quantum technology applications. VECSELs are ideal laser sources for applications requiring low-noise watt-level single-frequency light with excellent beam quality. In particular, we present features of lasers addressing the need for spectroscopy, laser cooling and trapping for selected atoms and molecules, such as Be, Cd, Sr and Yb. Power scaling abilities in single-frequency operation are also discussed.

With their high finesse external cavities, VECSELs provide intrinsically low frequency noise, class-A operation with broad spectral coverage, making them ideal candidates for quantum technology applications, such as atomic trapping. Subsequently, the excess frequency noise observed in VECSELs originates from pump noise, environmental noise, and mechanical instabilities. For conventional diode-pumped solid-state lasers monolithic formats, such as non-planar-ring-oscillators, have provided an effective solution to reducing environmental and technical noise. Here we present a monolithic-cavity class-A VECSEL based on a 25 mm fused silica right-angle prism. We present the cavity design and initial characteristics of the device operating at frequencies required for integration with Sr atomic clocks.