Interview

Abstract

Dr Fedor Zubov of St Petersburg Academic University in Russia talks about the work behind the paper ‘Suppression of sublinearity of light–current curve in 850 nm quantum well laser with asymmetric barrier layers’ page 1106. Dr Fedor Zubov My research relates to the development of modern injection lasers and my current interests include diode lasers of novel design – lasers with asymmetric barriers (LABs). LAB-concept was proposed in 2003 by a theorist, a member of our research team and co-author of our Letter - Levon Asryan, a professor at Virginia Tech. In such lasers, special, thin (several nanometers) asymmetric barrier layers (ABLs) on both sides of the active region block undesired transport of charge carriers, suppressing parasitic recombination of charge carriers in the waveguide. After almost 10 years our research team has realised this concept for the first time. It was a part of my Ph.D. research that I carried out in the Nanophotonics Lab under supervision of Prof. A.E. Zhukov. Recently, in 2013, the scientific community celebrated the 50th anniversary of semiconductor laser invention and now there is no sphere where they are not used. Among the most significant applications are data transmission, material processing and various medical applications. Today, diode lasers continue to be intensively investigated, new promising spectral ranges are being developed, and lasers of nondiode type are being displaced by them. In respect to application of our short-wave 0.85 µm LABs, the most interesting application seems to be high-power laser diodes used for optical pumping of nondiode lasers. Here, the problem of parasitic recombination is especially challenging. Our previous calculations have shown that the implementation of ABLs should improve device power characteristics owing to a decrease of current component related to parasitic recombination. Actually, within this work we are confirming this thesis experimentally. Our LABs, in comparison to a reference laser of traditional design, have demonstrated a significant improvement of the LCC linearity, so that the maximal output power was reached with 60% less pumping. The concept of asymmetric barrier lasers is universal, in the sense that it can be applied to diode lasers of different material systems and active region types. We have shown that suppression of parasitic recombination due to ABLs allows enhancement of main device characteristics: threshold current, efficiency, output optical power and temperature stability. This approach does not complicate the technology. In fact, temperature stability improvement would significantly simplify diode laser temperature-stabilisation systems. Such devices will provide a basis for new applications of injection lasers under elevated temperature conditions. A previous approach for suppression of parasitic recombination used a thinner waveguide in combination with a larger bandgap, resulting in the increase of vertical beam divergence and the decrease of COMD threshold. The attributes that should be improved to meet toughening application requirements are the lasing power, efficiency, reliability, brightness and cost. If we consider a 100 µm aperture single emitter for continuous wave room temperature (RT) operation, the present demand is 20 W. In the last 10 years the annual growth was 15% per year, and an achievement of this magnitude in the next 10 years seems to be realistic. Much more challenging are future diode laser pump systems for fusion energy installations. To achieve the required ∼100 GW pump power, the system should consist of ∼200 Mbars with 500W/bar at $0.01/W price. This could be achieved by using uncooled diodes, while maintaining the high efficiency and output power. I believe, in this regard, our lasers of ABL-design with improved temperature stability and power characteristics look very attractive. One part of our research is the development of whispering gallery mode microcavity quantum-dot lasers. These tiny (∼1-10 µm) semiconductor lasers are promising light sources for application in photonic ICs, biological sensing and imaging. Recently, we demonstrated RT lasing in the 1.3 µm wavelength range for microlasers as small as 1 µm, achieved under optical pumping. Our current research is the study of lasing under electrical pumping in such lasers. Another research area is the development of novel waveguide designs. We are targeting the realisation of symmetrical far-field radiation profiles with low divergence, which is necessary for effective coupling of laser emission to optical fibres. The current structure uses additional tunnel-coupled passive waveguide, suppressing high-order optical modes.