Quantum Dot and Quantum Well Semiconductor Disk Lasers
QD semiconductors are very promising materials for fabrication of optically pumped semiconductor disk lasers (OP-SDLs) also known as Vertical-External-Cavity Surface-Emitting Lasers (VECSELs), which are able to offer a combination of desirable laser properties – high output power and diffraction limited beam quality – the feature that is hardly achievable with other types of semiconductor lasers. QD structures embedded in this type of laser allowed a number of advantages compared with the widely used quantum well (QW) structures.
Our group has demonstrated QD-based VECSELs with CW output power exceeding 8 W at 1040 nm and 7 W at 1180 nm, which are the highest reported to date. The use of QD gain material also allowed the extension of InGaAs based SDLs emission wavelength to 1 - 1.3 µm. Other attractive features of SDLs emerge from the external cavity configuration capabilities used in combination with the broad gain bandwidth of QD materials. For example, wavelength tunability up to 60 nm around 1040 nm, 70 nm around 1180 nm, and 25 nm around 1260 nm in continuous wave QD SDLs was demonstrated. Intracavity second harmonic generation in QD SDLs was used to reach green (514 nm), orange (590 nm) and red (624 nm) spectral regions with output power of 2W, 2.5W and 0.33W, respectively.
Recently, the QD-VECSELs have also attracted considerable attention in mode-locking experiments using QW and QD semiconductor saturable absorber mirrors (SESAMs). In this way, these compelling sources can be used for nonlinear microscopy, as they combine key features such as excellent beam quality, output power, short pulse durations, amplitude stability, and can be made to operate at a large set of wavelengths, well-matched to key two-photon excited fluorescence (TPEF) dyes, while maintaining simplicity and ease of operation. In addition, the repetition rate of these lasers can be adjusted into the 100’s of MHz to 1 GHz, resulting in a compact laser cavity, but still working in a range where the peak power can be reasonable without requiring excessively large average powers. QD based SESAMs were used in number of mode-locked SDLs. They typically feature fast recovery times of hundreds of fs and allow carrier confinement in all three directions. The use of QD SESAMs in mode-locked SDLs enabled the improvements in specifications such as repetition rate, wavelength coverage, pulse fluence requirements, temperature stability and relaxed cavity design requirements. QD based mode-locked SDL with average output power of more than 1 W and sub-picosecond pulse duration was recently demonstrated.
Our group has also demonstrated number of SESAM-free mode-locked QD and QW SDLs, including a self-mode-locked QW-VECSEL with a record-high 6.8 kW peak power. This quite young technique, even though the mechanism behind the effect has yet not been well understood, is capable of enabling mode-locked operation for different gain media as well as cavity configurations. Pulses with duration of 930 fs at 210 MHz repetition rate at 985 nm with an average output power of 1.5 W and a record-high 6.8 kW peak power were demonstrated. Taking into account that SESAM technology is not well developed for wavelengths above 1.1 µm, the self-mode-locking technology demonstrates the advantage by allowing the development of ultrafast VECSELs with output power ranging from a few to tens of Watts in the short-wavelength IR and mid-IR spectral range without use of a SESAM and with peak powers not constrained by the limits of intracavity fluencies and non-saturable losses on a SESAM. Such a laser source potentially can find an application in laser micromachining and surface texturing field where femtosecond high-power lasers are required to produce repeatable, precise features down to 10 µm in size, like holes and grooves on surfaces or structures for use in microfluidic devices and a lab-on-a-chip.
This work was supported by €14.7M FP7-IP FAST-DOT (http://www.fast-dot.eu) project (2008-2012).