Probing the ultrafast gain and refractive index dynamics of a VECSEL

• Published in: Applied physics letters 2021-11, Vol.119 (19)
• Main Authors: Kriso, C., Bergmeier, T., Giannini, N., Albrecht, A. R., Sheik-Bahae, M., Benis, S., Faryadras, S., Van Stryland, E. W., Hagan, D. J., Koch, M., Mette, G., Rahimi-Iman, A.
• Format: Article
• Language: English
• Subjects: Applied physics; Laser mode locking; Laser modes; Lenses; Phase modulation; Refractivity; Semiconductor lasers; Short pulses; Surface emitting lasers
• ISSN: 0003-6951; 1077-3118
• DOI: 10.1063/5.0061346

Typically, strong gain saturation and gain dynamics play a crucial role in semiconductor laser mode-locking. While there have been several investigations of the ultrafast gain dynamics in vertical-external-cavity surface-emitting lasers (VECSELs), little is known about the associated refractive index changes. Yet, such refractive index changes do not only have a profound impact on the pulse formation process leading to self-phase modulation, which needs to be compensated by dispersion, but they are also of particular relevance for assessing the feasibility of Kerr-lens mode-locking of VECSELs. Here, we measure both refractive index as well as gain dynamics of a VECSEL chip using the ultrafast beam deflection method. We find that, in contrast to the gain dynamics, the refractive index dynamics is dominated by an instantaneous (∼100 fs) and a very slow component (∼100 ps). The time-resolved measurement of nonlinear refraction allows us to predict a pulse-length dependent, effective nonlinear refractive index n 2 , eff, which is shown to be negative and on the order of 10 − 16 m 2 / W for short pulse lengths (∼100 fs). It becomes positive for large excitation fluences and large pulse lengths (few ps). These results agree with some previous reports of self-mode-locked VECSELs for which the cavity design and pulse properties determine sign and strength of the nonlinear refractive index when assuming Kerr-lens mode-locking.