Novel semiconductor lasers attractive for UV-visible applications

Abstract

Frequency doubling of an optically pumped vertical-external-cavity surface-emitting laser enables a new multiwatt tunable yellow-orange light source.Compact, tunable, high-power semiconductor lasers, particularly those covering the UV and visible regions of the spectrum, are needed for many military and commercial applications. Yellow-orange sources are particularly desirable for sodium guide-star lasers, quantum computing, and medical applications. Semiconductor lasers in the visible region typically use one of two approaches: growth of a direct-band material with the proper energy gap, or frequency conversion of a near-IR source. Optically pumped vertical-external-cavity surfaceemitting lasers (VECSELs) are strong, low-cost candidates to meet these critical features. In addition, easy access to the laser intracavity allows for new features, such as frequency doubling (also known as second-harmonic generation, SHG), linewidth narrowing, or quality switching ('Q switching'), the production of a pulsed output beam. Compared to electrically pumped semiconductor lasers, optically pumped VECSELs have several advantages. For example, optical pumping is an easy way to achieve uniform pumping over a large area. In addition, using a barrier-pumping scheme eliminates the sensitivity to the pump wavelength and allows the use of cheap, readily available pump sources. Since no doping is required, cavity loss can be minimized.The VECSEL gain structure can be accurately designed using a many-body microscopic quantum-design tool to maximize pump absorption and improve both modal gain and high-power operation. 1 Combined with 3D optical/thermal optimization and gain-subcavity detuning, this allows for development of multiwatt sources emitting at various wavelengths. Appropriate choices for the external mirror and cavity length allow VECSELs to operate in the fundamental transverse mode (TEM 00 ) with high efficiency. By inserting a nonlinear crystal into the laser cavity, where the circulating power is 100 times stronger than the output power, efficient SHG can be achieved. Our team has successfully developed a wide range of highpower VECSELs with many desirable features, including largerange wavelength tuning, narrow linewidth, and high beam quality at various near-IR and visible wavelengths. [2][3][4] To create a yellow-orange VECSEL, we grew a structure consisting of ten repeats of highly compressive strained indium-gallium arsenide (InGaAs) quantum wells. Each quantum well is surrounded by gallium-arsenide-phosphide straincompensation layers and GaAs barriers. The thickness and composition of each layer is optimized such that the quantum wells are positioned at the antinodes of the cavity standing wave to provide resonant periodic gain (RPG) in the active region. 5 A high-reflectivity distributed-Bragg reflector (DBR)-stack made of 21 pairs of aluminum-gallium arsenide/aluminum arsenide is Continued on next page