Fundamental analysis and optimization of barrier-pumped GaAs-based VECSELs
The first laser, built from a flash lamp pumped ruby, was reported in 1960 by T. H. Maiman . Its demonstration was a great success in an emergent research field. In the end of the same year, the demonstration of the more popular helium neon laser followed, which is still found in many laboratorie...
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|The first laser, built from a flash lamp pumped ruby, was reported in 1960 by T. H. Maiman . Its demonstration was a great success in an emergent research field. In the end of the same year, the demonstration of the more popular helium neon laser followed, which is still found in many laboratories or used as a practical laser model in lectures . Still in the 60’s, also attention is drawn to semiconductor lasers. Pioneering work was performed by H. Krömer and Z. I. Alferov who obtained the Nobel prize for the development of the double heterostructure diode lasers . Then, in the 70’s, it was realized that the semiconductor lasers could be significantly improved, if quantum wells (QWs) are employed as gain media. However, the underlying physical mechanisms were not well known and subject of ongoing research. Especially, the growth of QWs and the therewith connected development of the epitaxy was a challenge. The next milestone in the development of semiconductor lasers was accompanied by the research on epitaxy techniques. In 1975, the first optically pumped QW laser was demonstrated by J. P. van der Ziel et al. . The laser gain region comprises 50 GaAs/(AlGa)As QWs and had to be cooled to a temperature of 15°K in order to achieve threshold with pump intensities of 36 kW/cm². First electrical pumped devices were also demonstrated at the end of the 70’s. For instance, in 1979, room temperature operation was reported with a single QW as gain medium and with a threshold current of 2 kA/cm² by Tsang et al. .
To date, QW lasers have been steadily improved concerning the thresholds, output powers, power consumption, and also concerning the range of accessible emission wavelength. Laser operation has been demonstrated from the ultraviolet, to the optical, near- and mid-infrared wavelength regime. In particular, diode lasers have become a mass product and are found in many everyday life’s electronics. For example, they are used for sensors in computer mice, barcode scanners, CD, DVD or Blu-ray disk drives, and smartphones . However, the most important application today is their utilization as transmitters in fiber-optic communications, which satisfies the need for the transmission of high data volumes. The by far widest spread diode laser is the vertical-cavity surface-emitting laser (VCSEL). In 2014 it was estimated that the number of sold VCSELs, since its invention in the late 80’s, has passed the one billion mark .
The term VCSEL is related to its basic operation principle and its differentiation to edge emitting diode lasers. In an edge-emitter, the laser resonator is formed by inherently existing edges of the cleaved semiconductor structure. Consequently, the directionality of the laser is in the plane of the QWs and perpendicular to these edges. In contrast, the VCSEL comprises monolithically grown high reflective laser mirrors, which form a laser cavity perpendicular, or vertical, to the QW planes. The laser light is emitted from the surface instead from the edges.
This thesis is dedicated to a very similar kind of semiconductor laser, namely the vertical-external-cavity surface-emitting laser (VECSEL). In comparison to a VCSEL, one of the monolithic laser mirrors is removed and replaced by an external mirror. Moreover, VECSELs are optically pumped, resulting in a scheme which is similar to other solid state disk lasers. Accordingly, the VECSEL is also often referred to as semiconductor disk laser (SDL), or optical-pumped semiconductor laser (OPSL) [8, 9].
Although VECSELs are also commercially available, the market is not comparable to the above-mentioned scale in case of VCSELs. The reasons are essentially higher manufacturing costs and more specific fields of application. Instead, a VECSEL can provide a unique, highly specialized laser source, optimized for a desired application. Since the first demonstration of the VECSEL in 1997 by Kuznetsov et al. , several reviews and text books have been published, which summarize the achieved results in these fields [8, 9, 11–14]. Owing to the external cavity, it combines the great wavelength versatility of semiconductor lasers with outstanding properties of other solid state lasers. Examples are their high beam quality with almost ideal circular beam profile, or a low intensity noise. Moreover, the intra-cavity elements can be used to manipulate the VECSELs operation mode. Birefringent filters can be applied to force single-frequency operation, saturable absorbing mirrors for mode-locking, or nonlinear crystals for highly efficient intra-cavity frequency conversion. Selected highlights of these results will also be presented at the relevant sections in the course of this thesis.
As mentioned, a VECSEL is usually optically pumped. Depending on the application, the requirement of an additional pump source in comparison to electrical pumped diode lasers is not necessarily a disadvantage. A VECSEL can also be regarded as a converter between the pump light and the actually emitted VECSEL light. This kind of conversion cannot only involve the above-mentioned features, like a rectification of the beam quality or intensity noise of a pump laser, mode-locking or single-frequency operation. More importantly, also the emission wavelength of the pump laser can be converted. This enables the application of a pump device, which is not necessarily bound to a specific wavelength, but cost-efficient. A mature and cost-efficient laser technology is for instance provided by fiber-coupled GaAs/Al(GaAs) laser diodes with emission wavelength at 808 nm and which is used for most VECSEL devices.
However, to obtain an efficient device, a strong absorption of the pump light is required, which is not provided by the absorption of the thin quantum wells. Instead, a high absorption can be provided by the barriers which enclose the QWs. This concept is called barrier-pumping, accordingly, and turned out to be very effective. At room temperature operation optical input to output efficiencies close to 60 % are achieved . The opposite concept, namely “in-well” pumping, involves critically reduced laser efficiencies and, thus, is less attractive and has been studied to a smaller extent [8, 16–18].
In the present thesis, 808 nm barrier-pumped VECSELs on GaAs-substrates are investigated. A QW design for these devices is the well-explored (GaIn)As/GaAs system. The functionality, physics and capabilities of these devices are introduced in chapter 2. In fundamental operation, i.e. without intra-cavity frequency conversion, the accessible wavelength range with this material system reaches from 920 nm to 1.2 μm [15, 19]. However, close to the borders of this range, the output powers are significantly impaired due to fundamental limitations. At 1 μm, the most powerful VECSELs have been reported, so far. Output powers in excess of 100 W could be achieved [20, 21]. In contrast, a maximum output power of 12 W is achieved at 920 nm  and an output power of 50 W at 1180 nm . Interestingly, there are no reports of (GaIn)As/GaAs VECSELs emitting either below 920 nm, or beyond 1.2 μm. The interest in efficient devices in the mentioned wavelength range, is primarily driven by highly efficient intra-cavity frequency-doubling, which gives access to Watt level output powers in the visible range. So far, output powers in the order of 20 W can be achieved with VECSELs emitting green and yellow wavelength [22, 23]. Nevertheless, due to the restriction of the fundamental emission from the (GaIn)As/GaAs QWs, there is still a lack of high-power devices in the blue and red. In this thesis, it is investigated how VECSELs can be optimized to provide more powerful devices in the future.
VECSELs from three regimes within the mentioned wavelength range are investigated in chapters 3 – 6. A mature 1 μm emitting sample used is to demonstrate the experimental methods for fundamental studies on VECSELs (chapter 3). The methods comprise the evaluation of laser power curves and spectra, detailed structural studies using photoluminescence and reflectance measurements, modal gain studies, and also thermal resistance analysis. Such complete study of a 1 μm sample yields a reference which enables detailed comparisons to the samples at other wavelengths, also applying other design concepts (chapters 5 and 6). Accordingly, these studies will also be carried out for all other samples throughout the chapters 4 – 6.
One key parameter in VECSELs is the so called detuning (cf. chapter 2). Due to its importance, its influence is discussed and studied in chapter 4, also by means of a mature 1 μm emitting sample. The knowledge of its impact on the VECSEL’s performance will also help to identify or exclude performance limitations in chapters 5 and 6.
Chapter 5 deals with the short-wavelength limitation of barrier-pumped GaAs-based VECSEL structures around 920 nm. It is discussed that the shallow QW depth is a factor which fundamentally limits the material gain, as charge carriers can be thermally reemitted from the QWs into the barriers. Possible QW designs for emission wavelength between 920 nm – 950 nm are discussed. The performances and properties of VECSELs with the discussed designs are studied and compared to the 1 μm emitting reference sample.
The other border of accessible wavelength with the (GaIn)As/GaAs system is at the wavelength of about 1.2 μm. Indeed, an excellent confinement potential is found here, but it is the crystal strain which sets stringent limitations to the growth of the QWs. An alternative QW design on GaAs substrates and for the emission at 1.2 μm and beyond is provided by a type-II QW. In such a QW, electrons and holes are spatially separated. If designed appropriately, their recombination happens across the material interfaces which causes a reduced transition energy in comparison to the materials band gaps. Although diode lasers based on type-II QW designs have already been realized and studied, this concept is not explored yet for the application in VECSELs. Instead, other approaches have been followed in the past, such as (GaIn)(NAs)/GaAs QWs or QDs. In chapter 6, the approach with type-II QWs is studied by means of the (GaIn)As/Ga(AsSb)/GaAs system. The design is discussed in detail and preliminary photoluminescence studies are carried out to evaluate the potential for the use as gain medium. Afterwards, the first type-II VECSEL is demonstrated and studied by the methods from chapter 3.
Overall, this thesis presents novel design concepts to increase the already stunning wavelength range of VECSELs even further.