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Membrane external-cavity surface-emitting lasers (MECSELs) represent a cutting-edge approach in pushing the performance boundaries of vertically emitting semiconductor lasers. The fundamental concept of employing an extremely thin gain membrane, spanning from hundreds of nanometers to a few micrometers in thickness and sandwiched between transparent heat spreaders, introduces novel opportunities through uniform double-sided optical pumping and enhanced heat dissipation from the active region. Additionally, these advantages of MECSELs facilitate more intricate band gap engineering possibilities for the active region by integrating multiple types of quantum wells (QWs) into a single laser gain structure. This work introduces a novel design strategy for laser gain structures incorporating various QW types. The objective is to achieve broad-spectrum gain with relatively high-power operation and potentially a flat spectral tuning range. Our design focuses on ensuring sufficient gain across a wide wavelength span, achieving uniform pump absorption, and limiting carrier mobility between different quantum well types during laser operation. We demonstrate a full-width half-maximum (FWHM) tuning range exceeding 70 nm (equivalent to more than 21.7 THz) with over 125 mW of output power across this entire tuning range at room temperature., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

High-power and narrow-linewidth laser light is a vital tool for atomic physics, being used for example in laser cooling and trapping and precision spectroscopy. Here we produce Watt-level laser radiation at 457.75 nm and 460.86 nm of respective relevance for the cooling transitions of cadmium and strontium atoms. This is achieved via the frequency doubling of a kHz-linewidth vertical-external-cavity surface-emitting laser (VECSEL), which is based on a novel gain chip design enabling lasing at > 2 W in the 915-928 nm region. Following an additional doubling stage, spectroscopy of the 1 S 0  →  1 P 1 cadmium transition at 228.87 nm is performed on an atomic beam, with all the transitions from all eight natural isotopes observed in a single continuous sweep of more than 4 GHz in the deep ultraviolet. The absolute value of the transition frequency of 114 Cd and the isotope shifts relative to this transition are determined, with values for some of these shifts provided for the first time.

Several passivation techniques are developed and compared in terms of their ability to preserve the optical properties of close-to-surface InAs/GaAs quantum dots (QDs). In particular, the influence of N-passivation by hydrazine chemical treatment, N-passivation by hydrazine followed by atomic layer deposition (ALD) of AlO x and use of AlN x deposited by plasma-enhanced ALD are reported. The effectiveness of the passivation is benchmarked by measuring the emission linewidths and decay rates of photo-carriers for the near-surface QDs. All three passivation mechanisms resulted in reducing the oxidation of Ga and As atoms at the GaAs surface and consequently in enhancing the room-temperature photoluminescence (PL) intensity. However, long-term stability of the passivation effect is exhibited only by the hydrazine + AlO x process and more significantly by the AlN x method. Moreover, in contrast to the results obtained from hydrazine-based methods, the AlN x passivation strongly reduces the spectral diffusion of the QD exciton lines caused by charge fluctuations at the GaAs surface. The AlN x passivation is found to reduce the surface recombination velocity by three orders of magnitude (corresponding to an increase of room-temperature PL signal by ∼1030 times). The reduction of surface recombination velocity is demonstrated on surface-sensitive GaAs (100) and the passivating effect is stable for more than one year. This effective method of passivation, coupled with its stability in time, is extremely promising for practical device applications such as quantum light sources based on InAs/GaAs QDs positioned in small-volume photonic cavities and hence in the proximity of GaAs-air interface.

We report the design, growth, and characterization of an AlGaInP-based VECSEL, designed to be optically-pumped with an inexpensive high power blue InGaN diode laser, for emission around 689 nm. Up to 140 mW output power is achieved in a circularly-symmetric single transverse (TEM 00 ) and single longitudinal mode, tunable from 683 to 693 nm. With intensity stabilization of the pump diode and frequency-stabilization of the VECSEL resonator to a reference cavity via the Pound-Drever-Hall technique, we measure the power spectral density of the VECSEL frequency noise, reporting sub-kHz linewidth at 689 nm. The VECSEL relative intensity noise (RIN) is <-130 dBc/Hz for all frequencies above 100 kHz. This compact laser system is suitable for use in quantum technologies, particularly those based on laser-cooled and trapped strontium atoms.

Quantum dot solar cells are promising for next-generation photovoltaics owing to their potential for improved device efficiency related to bandgap tailoring and quantum confinement of charge carriers. Yet implementing effective photon management to increase the absorptivity of the quantum dots is instrumental. To this end, the performance of thin-film InAs/GaAs quantum dot solar cells with planar and structured back reflectors is reported. The experimental thin-film solar cells with planar reflectors exhibited a bandgap-voltage offset of 0.3 V with an open circuit voltage of 0.884 V, which is one of the highest values reported for quantum dot solar cells grown by molecular beam epitaxy to our knowledge. Using measured external quantum efficiency and current-voltage characteristics, we parametrize a simulation model that was used to design an advanced reflector with diffractive pyramidal gratings revealing a 12-fold increase of the photocurrent generation in the quantum dot layers.

We studied and compared single-side pumping (SSP) and double-side pumping (DSP) of a semiconductor membrane external-cavity surface-emitting laser (MECSEL). The MECSEL-active region was based on an AlGaAs quantum well structure embedded between two silicon carbide (SiC) wafer pieces that were used as transparent intra-cavity (IC) heat spreaders creating a symmetrical cooling environment. The gain structure targeted emission at 780 nm, a wavelength region that is important for many applications, and where the development of high-brightness high-power laser sources is gaining more momentum. By DSP at 20°C heat sink temperature, we could reduce the laser threshold from 0.79 to 0.69 W of absorbed pump power, while the maximum output power was increased from 3.13 to 3.22 W. The differential efficiency was improved from 31.9% to 34.4%, which represents a record value for SiC-cooled vertically emitting semiconductor lasers. The improvements are enabled by a reduced thermal resistance of the gain element by 9% compared to SSP. The beam quality was measured to be M 2 <1.09. Finally, we demonstrate a maximum tuning range from 767 to 811 nm. This wavelength range was not addressed by any MECSEL or vertical external-cavity surface-emitting laser device before and extends the available wavelengths for semiconductor based high-quality beam and high-power laser sources to a wavelength window relevant for quantum technology, spectroscopy, or medicine.

An optically pumped vertical-external-cavity surface-emitting laser (VECSEL) for direct emission in the 740-790 nm wavelength region is reported. The gain structure is based on 12 AlGaAs quantum wells. We demonstrate wavelength tuning between 747 nm and 788 nm and free-running operation with a maximum power of 4.24 W (pump power limited) for a heat sink temperature of 14°C. This laser system addresses a spectral gap not currently covered by VECSEL technology and represents the most powerful VECSEL reported within the 7XX-nm wavelength region.

Distributed feedback lasers with laterally coupled ridge-waveguide surface gratings having the protrusions placed alternately on the lateral sides of the ridge are demonstrated. This configuration enables easier-to-fabricate wider trenches than in the gratings with protrusions placed symmetrically on both sides of the ridge. The design strategy and coupling coefficient calculations are discussed. The output characteristics of fabricated lasers show lower threshold currents and higher slope efficiencies for devices with first-order alternating gratings than for those with third-order symmetric gratings having comparable grating trench widths and similar coupling coefficients.

We demonstrate 1.5 W of output power at the wavelength of 750 nm by intracavity frequency doubling a wafer-fused semiconductor disk laser diode-pumped at 980 nm. An optical-to-optical efficiency of 8.3% was achieved using a bismuth borate crystal. The wavelength of the doubled emission could be tuned from 720 to 764 nm with an intracavity birefringent plate. The beam quality parameter M 2 of the laser output was measured to be below 1.5 at all pump powers. The laser is a promising tool for biomedical applications that can take advantage of the large penetration depth of light in tissue in the 700-800 nm spectral range.

A high-efficiency optically pumped vertical-external-cavity surface-emitting laser emitting 20 W at a wavelength around 588 nm is demonstrated. The semiconductor gain chip emitted at a fundamental wavelength around 1170-1180 nm and the laser employed a V-shaped cavity. The yellow spectral range was achieved by intra-cavity frequency doubling using a LBO crystal. The laser could be tuned over a bandwidth of ~26 nm while exhibiting watt-level output powers. The maximum conversion efficiency from absorbed pump power to yellow output was 28% for continuous wave operation. The VECSEL's output could be modulated to generate optical pulses with duration down to 570 ns by directly modulating the pump laser. The high-power pulse operation is a key feature for astrophysics and medical applications while at the same time enables higher slope efficiency than continuous wave operation owing to decreased heating.

A picosecond GaInP/AlGaInP/GaAs vertical external-cavity surface-emitting laser (VECSEL) at 675 nm is reported. The laser is mode-locked with a GaInP/AlGaInP/GaAs saturable absorber mirror and emitted ~5.1 ps pulses at a 973 MHz repetition rate and an average power of 45 mW. To our knowledge, this is the first demonstration of a passively mode-locked VECSEL emitting fundamental laser radiation at the visible part of the spectrum.

A high-power dual-wavelength AlGaInAs / GaAs laser operating in a vertical external-cavity surface emitting geometry, grown by molecular beam epitaxy, is reported. The active regions of the laser are separated by an optical long-wave-pass filter to prevent absorption of short-wavelength radiation in the long-wavelength gain area. The maximum output power achieved at 15 degrees C was 0.75 W at lambda approximately 966 nm and 1.38 W at lambda approximately 1047 nm for the pump power of 21.2 W.